Nature, structure and strength of the acidic sites of amorphous silica alumina=an IR and NMR study
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结构力学(structural mechanics)Structural mechanics is a branch of solid mechanics, which mainly studies the laws of force and force transfer of engineering structures, and how to optimize the structure. The so-called engineering structure refers to the system that can bear and transfer the external load, including the rod, plate, shell and their combinations, such as aircraft fuselage and wing, bridge, roof truss and load-bearing wall.The task is to study structural mechanics in engineering structure under external load stress, strain and displacement law; analysis of different forms and different materials of engineering structure, analysis method and calculation formula for engineering design; engineering structure subjected to external forces and transfer; research and development of new engineering structure.The natural structure observed in nature, such as roots, stems and leaves of plants, animal bones, egg shell, can find their strength and stiffness related not only to material, but also closely related with their shape, many engineering structures are natural structures created out of inspiration. The structure design should not only consider the strength and stiffness of the structure, but also to do material saving and light weight. The weight is particularly important for some projects, such as aircraft weight can make the aircraft flight range, rising quickly, high speed and low energy consumption.A brief history of structural mechanicsHumans began to manufacture all kinds of artifacts in ancienttimes, such as houses, boats and bow, musical instruments, these are simple structure. With the progress of society, people for structural design patterns and the strength and stiffness of the structure has been gradually recognized, and accumulated experience, which is reflected in the brilliant achievements of ancient buildings, such as Egypt, Pyramid, China the Great Wall, Zhaozhou Anji bridge, Beijing the Imperial Palace. Despite the presence of mechanics in these structures in knowledge, but did not form a discipline.As far as the basic principles and methods are concerned, structural mechanics is developed simultaneously with theoretical mechanics and material mechanics. Therefore, structural mechanics is integrated with theoretical mechanics and material mechanics in the initial stage of development. By the early nineteenth Century, due to the development of industry, people began to design a variety of large-scale engineering structures, the design of these structures should be more accurate analysis and calculation. Therefore, the analysis theory and analysis method of engineering structure began to be independent. By the middle of the nineteenth Century, structural mechanics began to become an independent discipline.Many computational theories and methods of structural mechanics appeared in the nineteenth Century. At France in 1826 proposed a normal method for solving statically indeterminate structure problems. From 1830s onwards, due to the bridge by train, not only need to consider the bridge under static load problems, must also be considered to withstand the dynamic load, because the bridge span increases, the metal truss structure.In the decades since 1847, scholars have studied the force analysis of statically determinate truss structures by means of graphic method and analytic method, which laid the foundation of truss theory. In 1864, Maxwell established the unit load method and the displacement reciprocal theorem, and calculated the displacement of the truss by the unit load method. Thus, scholars finally got the method to understand the statically indeterminate problem.After the establishment of the basic theory, the new structure and its corresponding theory have been continuously developed while solving the problems of the original structure. From the late nineteenth Century to the early twentieth Century, scholars carried out a lot of mechanical research on the ship structure, and studied the dynamic theory of the beam under the moving load, as well as the problems of free vibration and forced vibration.In the early twentieth Century, the development of Aeronautical Engineering promoted the stress and deformation analysis of thin-walled structures and stiffened plates and shells, and studied the stability problems. At the same time, bridges and buildings started to use a large number of reinforced concrete materials, this requires scientists to study systematically the steel structure, the displacement method was founded in Germany in 1914 of the Dixon, for solving the problem of rigid frame and continuous beam etc.. Later, in the 20~30 century, some simple calculation methods were put forward for the complex statically indeterminate bar structures, so that the general designers could master and use them.By 1920s, people have put forward the idea of honeycomb sandwich structure. According to the concept of limit state of structure, scholars have come up with a new design and calculation theory for beams, plates and frames on elastic foundations. The mechanical problems of structures subjected to various dynamic loads (especially the action of earthquakes) have been studied in many aspects, such as experiment and theory. With the development of structural mechanics, fatigue problems, fracture problems and composite structure problems have entered the field of structural mechanics.In the middle of the twentieth Century, the advent of electronic computers and finite element methods made it possible to make complex calculations of large structures, thus bringing the level of research and application of structural mechanics to a new level.The discipline system of structural mechanicsThe general structural mechanics according to the different nature and its research object is the static structure, structural dynamics, theory, structure, fracture and fatigue theory of rod structure theory, theory of thin-walled structure and overall structure theory etc..Structural statics is the first branch of structural mechanics. It mainly studies the elastic-plastic deformation and stress state of engineering structures under static loads, and the structural optimization problems. Static load refers to the load that does not change with time, the load that changesslowly, and also can be regarded as static load approximately. Structural statics is the basis of other branches of structural mechanics.Structural dynamics is a branch of study on the response and performance of engineering structures under dynamic loads. Dynamic load refers to the load that changes with time. Under dynamic load, the stress, strain and displacement in the structure must be the function of time. Because of the time factor, the research content of structural dynamics is generally more complex than that of structural statics.The theory of structural stability is the branch of study on the stability of Engineering structures. Slender and thin structures are widely used in modern engineering, such as thin rods, thin plates and thin shells. When they are compressed, they will lose stability (wrinkling or buckling) when the internal stress is less than the yield limit, that is to say, the structure produces too large deformation, thus reducing and even completely losing the bearing capacity. Large deformation also affects other requirements of structural design, such as aerodynamic performance of aircraft. The most important content of structural stability theory is to determine the critical buckling load of structures.Structural fracture and fatigue theory is the study of engineering structures are inevitable because of internal crack, crack under external load expansion caused by fracture, caused by fatigue failure would be expanded subject in smaller amplitude under alternating load. Now, the research history of fracture and fatigue is not long and imperfect, but the theoryof fracture and fatigue is developing very fast.In structural mechanics, theoretical and experimental studies on various engineering structures, based on the research object also formed some research fields, which are the main truss structure theory, the theory of thin-walled structures and the overall structure of the theory of three categories. The whole structure is made of raw materials, machined by mechanical milling or by chemical etching. It is especially suitable for some boundary conditions and is often used as variable thickness structure. With the development of science and technology, many new structures emerge, such as sandwich structure and composite structure appearing in the middle of twentieth Century.The research methods of structural mechanics mainly include three kinds of analysis, experimental research, theoretical analysis and calculation of Engineering structure. In the structural design and research, these three aspects are often alternate and complement each other.The use analysis is in the structure use process, carries on the analysis, the comparison and the summary to the structure appears, this is easy and reliable one kind of research method. The use analysis plays an important role in the evaluation and improvement of structure. The newly designed structures also need to be used to test the performance.The experimental research can provide an important basis for the identification of structure, which is also the main means to test and develop the theory and calculation method ofstructural mechanics. The experimental research can be divided into three types: model experiment, real structural component experiment and real structure experiment. For example,Aircraft ground failure test, flight test and vehicle collision test, etc..The mechanical structure usually takes more manpower, material and financial resources, so only to a limited degree, especially in the early stages of the structural design, generally rely on theoretical analysis and calculation of the structural components.In the field of solid mechanics, provides the basic knowledge necessary for the development of material mechanics, structural mechanics, elastic mechanics and plastic mechanics is the theoretical basis of structural mechanics, structural mechanics is also combined with other physical disciplines form many interdisciplinary, such as fluid elastic force etc..Structural mechanics is an ancient discipline, and it is also a rapidly developing subject. A large number of new engineering materials and new engineering structures have provided new research contents and new requirements for structural mechanics. The development of computer provides a powerful computational tool for structural mechanics. On the other hand, structural mechanics also plays an important role in the development of mathematics and other subjects. The emergence and development of the finite element method is closely related to the study of structural mechanics.。
2021春期1378管理英语3网上形考任务(单元自测1至8)试题及答案2021春期1378管理英语3网上形考任务(单元自测1至8)试题及答案单元自测1 形考任务试题及答案题目为随机,用查找功能(Ctrl+F)搜索题目 [题目]—— No, sorry, I didn't know the rule about punching in. [答案]Didn't you punch in this morning, Wendy? [题目]— I suppose there'll be a lot of arguments. — [答案]I should imagine so. [题目]— I wish you suess in your career. — [答案]The same to you. [题目]— I'm leaving for Shanghai tomorrow. — [答案]Have a pleasant trip! [题目]— Who should be responsible for the aident?— The boss, not the workers.They just carried out the order. [答案]as told [题目]——We have three major divisions: Operations, Sales and Marketing, Finance and Administration. [答案]How many departments do we have? [题目]A bus driver the safety of his passengers. [答案]is responsible for [题目]A number of boys absent some time during the term. [答案]have been [题目]He will write to me as soon as he home. [答案] returns [题目]I am your early ing. [答案]looking forward to [题目]Leave the reference books behind, you won't be able to think independently. [答案]or [题目]My friend invited me the art club, and I aepted it with pleasure. [答案]to join [题目]The key suessful implementation is clearly municating the strategy to the whole pany. [答案]to [题目]The programme givesstudents the to learn more about global warming. [答案]opportunity [题目]This is the man last night. [答案]whom I saw 二、翻译为下面的句子选择正确的翻译(每题10分)。
英文回答:Given the crystal structure and the strength of the chemical keys,we can see why aluminium trioxide has a melting point higher than magnesium oxide。
The crystal structure of aluminium trioxide is more closely organized, consisting of Al2O3 crystal cells, and the aluminium ion and oxygen ion displays a close six—bit structure, which is more robust and requires greater energy to ovee their interaction and melt。
In contrast, the crystal structure of magnesium oxide consists of the Mgo unit crystal cells, with magnesium ions and oxygen ions having eight—part structures, which are looser and have relativelylow melting points。
This conclusion is an important guide for the development of relevant policies and routes, requiring that the physical characteristics of the material be fully taken into account and that appropriate measures be taken to ensure its rational application and development。
美丽与实力并存英语作文Title: The Symbiosis of Beauty and Strength。
In the realm of human existence, the interplay between beauty and strength has long been a subject of fascination and contemplation. From ancient philosophies to modern discourses, the dynamic relationship between these two attributes has shaped cultures, influenced perceptions, and driven aspirations. In this essay, we delve into the intricacies of this symbiotic relationship, exploring how beauty and strength coexist, complement each other, and contribute to the holistic perception of individuals and societies.First and foremost, it is essential to acknowledge that beauty and strength manifest in multifaceted dimensions, transcending mere physical appearances or raw physical power. Beauty encompasses not only outward aesthetics but also inner qualities such as grace, compassion, and intellect. Similarly, strength extends beyond muscularprowess to include resilience, determination, and moral fortitude. Thus, the convergence of beauty and strength occurs not just at the surface level but also at the depths of character and essence.One of the most evident arenas where beauty and strength converge is in the realm of nature. The majestic mountains, with their rugged peaks and serene valleys, exemplify the harmonious blend of aesthetic appeal and formidable resilience. Likewise, the delicate petals of a flower conceal the robustness of its roots, which anchor it firmly to the earth and sustain its existence through adversity. Nature, in its infinite wisdom, demonstratesthat beauty and strength are not disparate qualities but rather integral aspects of the same continuum.In human society, the synthesis of beauty and strength finds expression in various forms, ranging from individuals to institutions. Consider the archetype of a leader whose charisma captivates hearts while their unwavering resolve inspires action. Such leaders embody the fusion of charm and courage, leveraging their magnetic appeal to galvanizecollective efforts toward noble pursuits. Similarly, worksof art that endure the test of time possess an allure that transcends aesthetics, resonating with audiences across generations through the enduring strength of their themes and messages.Moreover, the convergence of beauty and strength is exemplified in the realm of personal development.Individuals who cultivate inner beauty through self-awareness, empathy, and integrity invariably radiate a magnetic charisma that transcends physical attributes. Simultaneously, those who nurture mental and emotional resilience find themselves better equipped to navigatelife's challenges with grace and fortitude. Thus, the journey toward self-actualization entails not only enhancing external appearances but also cultivating theinner virtues that sustain and empower us in the face of adversity.In the societal context, the symbiosis of beauty and strength is reflected in cultural norms, values, and institutions. Cultures that celebrate diversity, creativity,and inclusivity harness the collective strength of their members while enriching the tapestry of human experience with vibrant expressions of beauty. Likewise, institutions founded on principles of justice, equality, and compassion wield their influence with authority and integrity, embodying the fusion of moral beauty and institutional strength.However, it is crucial to acknowledge that theinterplay between beauty and strength is not always harmonious. At times, societal pressures and distorted perceptions can create false dichotomies, pitting beauty against strength in a superficial contest of value and worth. In such instances, individuals may find themselves torn between conforming to external standards of attractiveness or embracing their intrinsic qualities of resilience and authenticity. Thus, fostering a culture that celebrates the diversity of human potential and recognizes the inherent beauty in strength is essential for nurturing holistic well-being and collective flourishing.In conclusion, the symbiosis of beauty and strength isa profound phenomenon that permeates every aspect of human existence. Whether observed in nature, expressed through individuals, or embodied in societal constructs, the convergence of these two attributes enriches our lives with depth, meaning, and resilience. By embracing the inherent beauty in strength and the enduring strength in beauty, we embark on a journey of self-discovery and collective evolution that transcends superficial distinctions and celebrates the holistic essence of human potential.。
介绍长城英语作文3年级The Great Wall of China is one of the most famous landmarks in the world. It is a symbol of Chinese culture and history. The Great Wall stretches over 13,000 miles across northern China. It was built to protect China from invasions and attacks from nomadic tribes. The wall is not just one continuous structure; it consists of various walls, trenches, and natural barriers.The construction of the Great Wall began over 2,000 years ago, during the reign of the first emperor of China, Qin Shi Huang. He connected several walls built by earlier dynasties to create a unified defense system. Over the centuries, different dynasties continued to build and expand the wall, adding their unique designs and materials. The most famous sections of the wall were built during the Ming Dynasty(1368-1644). These sections are made of bricks and stones, making them more durable and stronger.The Great Wall is not just a wall; it is a marvel of engineering. The builders faced many challenges, including rough terrain, extreme weather, and limited tools. They used local materials, such as earth, wood, bricks, and stones, depending on what was available in each region. The wall includes watchtowers and military barracks, which were used by soldiers to keep watch for enemies. The watchtowers served as lookout points and communication stations, allowing messages to be sent quickly across long distances.Visiting the Great Wall is an unforgettable experience. Tourists from around the world come to see this incredible structure. There are many sections of the wall that people can visit, such as Badaling, Mutianyu, and Jinshanling. Badaling is the most popular section because it is easily accessible and well-preserved. Many tourists prefer to hikealong the wall, enjoying the stunning views of thesurrounding mountains and valleys.The Great Wall is not just a historical site. Itrepresents the spirit and determination of the Chinese people. The wall took thousands of workers many years to complete. While building the wall, many faced hardships, and some even lost their lives. The wall stands as a symbol of their sacrifices and hard work.Over the years, the Great Wall has become a UNESCO World Heritage Site and is recognized as one of the New Seven Wonders of the World. This recognition helps promote the significance of preserving this historical monument forfuture generations. However, the wall faces challenges suchas natural erosion, weathering, and human activities. Efforts are being made to protect and restore the wall. Local governments and organizations are working together to raiseawareness about the importance of preserving this cultural treasure.The Great Wall is also a source of inspiration for artists, writers, and filmmakers. It has been featured in many movies and literature, showcasing its grandeur and historical significance. The wall serves as a reminder of the strength of human spirit and the importance of protecting our cultural heritage.As I walked along the Great Wall, I imagined the soldiers who once patrolled its lengths, keeping watch over the land.I could almost hear their voices echoing through the mountains as they defended their homes. It filled me with a sense of wonder to think about the history that unfolded along this remarkable structure.The Great Wall of China is a testament to the ingenuity and perseverance of the Chinese people throughout history. It is more than just a wall; it is a symbol of unity, strength,and resilience. As I learned about its history and significance, I felt a deep appreciation for this incredible achievement. I hope that one day everyone has the opportunity to visit and experience the Great Wall for themselves.In conclusion, the Great Wall of China is a magnificent structure rich in history and culture. It stands as a reminder of the past and represents the spirit of the Chinese people. Visiting the Great Wall is a journey through time, allowing us to connect with history, nature, and our shared human experience. It is a place where we can reflect on the achievements of those who came before us and the importance of preserving our world for future generations.Whether you are an adventurer, a history enthusiast, or simply someone who enjoys beautiful landscapes, the Great Wall offers something for everyone. It is an experience that stays with you long after you leave, inspiring you to learn more about the history and culture of this amazing world. Iwill always cherish my visit to the Great Wall of China, andI believe that it is a place that should be preserved, protected, and respected for all time.Every step I took along the wall was filled with awe and admiration. The views were breathtaking, and the feeling of walking on a structure that has stood for centuries is truly humbling. As I stood on the Great Wall and looked out at the rolling hills and valleys, I realized that I was part of something much larger than myself. I was honoring the legacyof those who came before me and helping to preserve it for those who will come after.In the end, the Great Wall of China is more than just a historical landmark. It is a symbol of perseverance, strength, and the human spirit. It teaches us about the importance of unity in the face of adversity and the need to protect our cultural heritage. I hope that everyone can experience the wonder of the Great Wall at least once in their lifetime, asit is a journey that truly expands our horizons and deepens our understanding of the world around us.。
介绍长城的英语作文演讲稿The Great Wall is a magnificent structure that stretches across the northern part of China. It is one of the most famous landmarks in the world, attracting millions of tourists every year. The Great Wall is not just a physical barrier, but also a symbol of China's rich history and cultural heritage.The Great Wall is an engineering marvel. It was built over several centuries, starting from the 7th century BC. The wall was constructed using a variety of materials, including bricks, stones, and earth. It stretches for over 13,000 miles, making it the longest man-made structure in the world. The wall was built to protect China from invasions and attacks from nomadic tribes in the north.Walking along the Great Wall is an awe-inspiring experience. As you climb up the steep steps and look out over the vast landscape, you can't help but feel a sense of wonder and admiration. The wall snakes its way throughmountains, valleys, and deserts, creating a breathtaking panorama. It is truly a testament to the ingenuity and determination of the ancient Chinese people.The Great Wall is not just a physical barrier, but also a cultural symbol. It represents the unity and strength of the Chinese people. The wall has witnessed countlessbattles and conflicts throughout history, but it has always stood strong. It is a reminder of the resilience and perseverance of the Chinese nation.In addition to its historical significance, the Great Wall is also a tourist attraction. Visitors from all over the world come to marvel at its grandeur and explore its ancient secrets. There are various sections of the wallthat are open to the public, each offering a unique experience. Some sections are well-preserved, while others are in ruins, giving visitors a glimpse into the past.The Great Wall is a symbol of China's rich cultural heritage. It is a testament to the ingenuity and determination of the ancient Chinese people. Walking alongthe wall, one can't help but feel a sense of awe and admiration. It is a reminder of the resilience and strength of the Chinese nation. Whether you are a history enthusiast or simply a curious traveler, the Great Wall is a must-visit destination. So come and explore this magnificent structure, and immerse yourself in the history and beauty of China.。
大自然的力量的英文The Power of NatureNature is a force to be reckoned with. From the raging waters of a river to the destructive power of a tornado, the natural world reminds us of its strength every day. But despite its overwhelming power, nature also brings us beauty, wonder, and a sense of tranquility.One of the most awe-inspiring displays of nature's power is a thunderstorm. The lightning, thunder, and rain are all evidence of the energy that is released during a storm. The crackling of the lightning bolt can be heard for miles, and the rain can come down in sheets, flooding streets and causing damage. But as dangerous as thunderstorms can be, they are also a reminder of the incredible power of nature.Another example of nature's strength is the ocean. The waves that crash against the shore are a reminder that the ocean is always in motion, never still. The currents and tides can change in an instant, and the power of the water can be overwhelming. But, like a thunderstorm, the ocean also brings us beauty. The rhythm of the waves, the sound of the seagulls, and the salty smell of the air all combine to create an unforgettable experience.And then there are the natural disasters that strike with little warning. Hurricanes, tornadoes, earthquakes, and wildfires can all leave a trail of destruction in their wake. But even in the aftermath of such disasters, there is a sense of awe at the power of nature. Despite the destruction, there is also a sense of resilience and rebirth as life begins to return to the affected areas.In the end, the power of nature is a reminder that we are just a small part of a much larger world. It is a force that cannot be tamed or controlled, but it is also a source of inspiration and wonder. We must respect its power and work to protect the natural world, but we must also embrace its beauty and seek to understand it better.。
法理学笔记------司法考试、法学专业最新笔记系列(The notes of jurisprudence ---- the latest notes series of judicialexamination and law major)The overall impression should be examined through a pass and a question.Three point two sevenThe change of the history of legal thought.Three point two eightRelated content of Sociology of law.Six point one three1., the characteristics of legal methods and legal thinking:Use a way of reasoning rather than simply solve the problem of violence. The force of the country is to protect law enforcement backing, is normally ready just in case. Law is the combination of "reason" and "force"It must be based on legal reasoning, judging and solving legal problems. Law, not morals, habits, etc, is the basis of argument. Law is the balancer that regulates social conflictThe determination and shall be conducted in accordance with relevant legal procedure. Procedure precedes justice2. Marx's basic views on the nature of lawThe nature of a law is a multi-level structure, and the nature of the law at different levels is intrinsically linked together, which constitutes the essence of law. The essential hierarchy of law is manifested as:The essence of moral law for the performance of the formal law, national law, which is enacted by the state, the fingering is recognized and guaranteed by the state force of the implementation of the official code of conduct. The formal method embodied in law always by public authorities in accordance with certain procedures formulated or approved authority; always rely on the power mechanism to ensure the realization of formal method based on formal; always forms to be published.The essence of the law reflects the class nature of law, namely, in class society, the will of the state law reflects the will of the ruling class is actually. This is because only the ruling class has the power to raise the will of this class to the will of the state by virtue of the intermediary of the state, and no other class can do it.The essence of the method was demonstrated by material restriction, namely the content of the law is restricted by social existence, the contents of the will of the ruling class is the content of the law is not baseless, nor the inherent class, but is decided from certain material production of social common interests and needs of the class sexual selection.In summary, lawmakers not to create the law, but only in the legal representation, is objective existence in the social life including the relations of production, class relations, kinship, social relations and the corresponding social norms and social needs of national law, and be protected by the authority of the state. Therefore, the essence of law is the unity of state will, class nature and social material restriction.The essence of the two law as the initial manifestation of formal law, official, also known as national law; secondly the essence of law reflects the class nature of law; the essence of law embodied as law material restriction, fingering the content is restricted by certain social factors, but also by some agency will determine the final material life the conditions, historical traditions, customs, national structure and international environment condition is relative to the social and material conditions for the minor factors in social factors.3., the force of the law is not equal to pure violence, and the coercive force of the law is based on statutory coercive measures and sanctions. Moreover, in the process of enforcement of law, it is the moral power of law to guarantee its function constantly in the process of law enforcement.The normative function of the 4. law includes five kinds: guideline, evaluation, education, prediction and compulsion.The social function of the 5. law involves three fields of economic life, political life and ideological and cultural life,at the same time, it involves two directions of political function and social function. The social function of law is to establish and maintain certain political order and social public order.Limitations of the 6. law:The law is based on society, so it can not go beyond the needs of social development. The law may lag behind or emerge loopholes when social reality changes before the law.Law is only one of the social norms. It is not the only way to adjust social relations. The law should be restricted by other social norms, social conditions and social environment.The scope of the law is not unlimited, and some social relations are not suitable to be adjusted by law, such as friendship, love relations, etc..Limitations of the law itself, such as limitations of language ability.The difference between the value judgment and the fact judgment of the 7. law:The direction of judgment is different. The value judgment of law takes subject as orientation and scales with the difference of subject. The judgment of law is based on the existing legal system and not by the will of man.The dimensions of judgment are different. The value of the lawjudge mark has a strong personal subjectivity, influenced by emotion, attitude, to determine the main interests and needs; the fact judgment will remain neutral and value neutrality, influence as far as possible to eliminate all kinds of subjective and objective factors.The methods of judgment are different.The value judgment is a normative judgment, "the law should concern how problem; method of judgment of facts by descriptive judgment, pay attention to" the law is actually how problem.The authenticity of the judgment is different. The authenticity of the value judgment of law depends on the degree of agreement between the value relationship between the principal and the object, and the value goal of different times is different, which is the result of social selection after the historical test. The truth of the judgment of law depends on whether it conforms to the object truth.8., in general, freedom is at the top of the value of law; justice is the externalization of freedom; it is the standard of restricting other values under freedom; order must be bound by the standards of freedom and justice.Justice can become a standard of value judgement is independent of law, can be used to measure the law is "good" or "bad law".9., the difference between legal principles and legal rules:In the content, the provisions of legal rules are clear, as themain body to set specific patterns of behavior and the legal consequences of specific provisions; legal principle is more general and vague, do not set specific behavior patterns, only put forward general requirements or standards.In the application, the rule of law is only applicable to a certain type of behavior; legal principle is the value criterion of a certain behavior, a legal department or all legal system are common, with the macroscopic guidance.In the mode of application, the rule of law is to "all or nothing" approach to the case; rules when applicable, either valid or invalid, or is it being followed, or is in violation of. The principle of law has different intensity, and the principle is more flexible when applicable. When the two principles conflict in specific cases, the judge must balance the strength of the case according to the specific circumstances of the case and the relevant background.Generally speaking, the rule of law should be expressed by legal provisions, but in special cases, the rules of law may also be carried and expressed by precedent or custom.10., the content of the legal right is actually the three elements of the right - freedom, claim and right of appeal. The right of freedom is the foundation, the right of claim is the content of the entity, and the right of action is the guarantee.11., the jurisprudence of the case is not the origin of our national laws, nor is the habit of the origin of our national laws, but the habit of being recognized by the state as acustomary law has become the unofficial source of our national laws.12., from a legal point of view, should we affirm or deny the retroactive effect of the new law?Law is the norm of people's behavior, which is used to direct people's behavior. According to the law, a person makes an arrangement, decides, or does not do something, in order to anticipate the possible effect of the action. Thus, the law provides people with the expected effect of behavior. But this kind of behavior is expected to be achieved. On the one hand, the law is required to be stable and not frequently changed; on the other hand, the law is required to be public and its content is well known. In this way, people can direct their behavior according to the law. And the law that people can know can only be the law of his behavior at that time, the person behavior can not predict the change of the law at that time, and how can change. In this way, when the behavior is made, the evaluation of behavior can only be based on the behavior of the effective law, and can not be changed by the behavior of the new law to evaluate. That is to say, the state can not be used to guide the legal norms today yesterday's people's behavior, not because people previously act now appears to be illegal and bring them to justice, namely the negative law retroactivity, behavior evaluation "old old method". Otherwise, it will be unfair to the actors. So, in principle, "from the old."".However, the provisions of the new law are sometimes more beneficial to the perpetrator. If the new law is applied, it will not harm the perpetrator, but it will enhance the interestsof the party, and it will not arouse the unfairness of the party. In this way, we can affirm the retroactive effect of the new law and apply the new law retroactively to the evaluation of "old times". This is a special case of a lighter case.13., the principle of concurrence of legal liabilities:As for the concurrence of legal responsibilities among different legal departments, they should be dealt with in general. If the relatively minor legal liabilities have been investigated, the heavier legal liabilities shall be taken into consideration. In practice, the concurrence of legal liabilities refers to the concurrence of civil tort liability and liability for breach of contract.14., the difference between the two major legal systems:In the characteristics of legal thinking mode, civil law belongs to the deductive thinking, and the common law belongs to the inductive thinking, focus on analogical reasoning.In the origin of law, civil law is the official source of French law, common law and statutory law and case law are the formal source of law.In the classification of law,Civil law countries generally divide the public law and private law as the basis of legal classification, while the common law system is the basic classification of common law and equity law.During the proceedings, civil law and canon law program to belong to the inquisitorial system of litigation, the common law is the adversary system.In the compilation of code, the main development stages of civil law are representative of the code, especially in modern times, civil law countries have carried out large-scale codification activities. The common law system carried out a great deal of legislative activities in the Tudor Dynasty, and the number of laws enacted in modern times has also increased, but on the whole, it is not inclined to make systematic codification. In addition, there are still many differences between the two parties in the court system, legal concept, legal application, technical and legal concepts.Civil law system and Anglo American law system:Characteristics of continental law system:Comprehensive inheritance of the law of the Rome. Codification, legal standardization, abstraction and generalization.Make clear the division of labor between legislation and judicature, emphasize the authority of making laws, and generally do not recognize the function of law making by judges.Law plays an important role in promoting the development of law. Generally speaking, trial procedure is adopted in litigation procedure.Characteristics of common law system:Take Britain as the center, based on the common law of england. Take precedent law as the main form and follow the precedent.The change is relatively slow and conservative, reflecting the habit of "backward looking".In the development of law, the judge has prominent function. The system is complex and lacks systematicness.The focus on the procedure of litigation centralized doctrine".15. rule by lawOne meaningA country under the rule of law is the first concept used in german. The early rule of law refers to a particular form of state in medieval Europe, especially the German Empire, which was then considered a guardian of peace and order". The modern sense of the rule of law, is the product of the German bourgeois constitutionalism movement, its basic meaning is the state power, especially the administrative power must be exercised in accordance with the law, so that the rule of law is sometimes called the rule of law government.Two basic conditionsThe protection of human rights by law to restrict the abuse of public power. The law of governance.The establishment of tort and restrict the power of state power by the constitution.We give extensive civil rights. The establishment of the judicial principle generally, such as the independence of the judiciary.The general relationship between the 16. law and human rightsThe human rights are the legal evaluation standard. Human rights are the source of law. Do not reflect the demand of human rights law is not good law, never have legal and contributed to the legal order; and the law embodies the spirit of human rights and content, is a good law in general, is a manifestation of social progress law.The law is the embodiment of human rights and protection. The realization of human rights depends on the confirmation and protection of the law. Without the legal recognition, declaration and protection of human rights, human rights can only stay in the proper state of moral rights, or often face the risk of being infringed and can not be relieved. Physical fitness is often materialized in the form of legal rights. On the basis of human rights and legal rights, only by obtaining the most basic human rights can the general rights be converted into legal rights; on the other hand, the legal rights are the embodiment and guarantee of human rights. The existence of human rights only in the form of legal rights has practical significance, and basic human rights must be legalized. The legalization of human rights is restricted by the economic and cultural development of a country.。
英语作文有科学精神The Scientific Spirit in English Essay WritingThe essence of scientific spirit lies in a rigorous, inquisitive, and objective approach to understanding the world. When it comes to writing English essays, this spiritis not only applicable but also essential for producing high-quality, well-reasoned, and persuasive texts. Here's how the scientific spirit can be infused into the process of English essay writing.Firstly, the scientific spirit demands clarity and precision in language. English essay writing should strivefor clear expression, avoiding ambiguity and vagueness. This means choosing the right words, constructing sentences that are easy to follow, and organizing ideas logically. Just as a scientist would use precise terminology to describe a phenomenon, a writer should use exact language to conveytheir thoughts.Secondly, critical thinking is a cornerstone of the scientific method and is equally important in essay writing.A good essay is not just a collection of facts; it is an analysis that evaluates different viewpoints, supports claims with evidence, and anticipates counterarguments. This process mirrors the scientific approach of forming hypotheses,testing them, and revising them based on new data.Thirdly, objectivity is key in both scientific research and academic writing. An essay should present a balanced view, acknowledging alternative perspectives and biases. This requires a writer to distance their personal feelings fromthe argument and to base their conclusions on logical reasoning and empirical evidence, much like a scientist would rely on data and peer review.Moreover, the scientific spirit encourages a thorough and systematic approach to research. When writing an essay, this means conducting comprehensive literature reviews, citing sources accurately, and ensuring that all references are properly formatted. This diligence in research and citationis not just a matter of academic integrity but also a way to build a solid foundation for the essay's arguments.Lastly, the iterative nature of science is mirrored inthe process of revising and editing an essay. Just as a scientist refines their theories over time, a writer shouldbe willing to revise their work multiple times. This involves critically assessing the structure, coherence, and strengthof the arguments, and making necessary adjustments to improve the overall quality of the essay.In conclusion, the scientific spirit, with its emphasison clarity, critical thinking, objectivity, thorough research, and iterative improvement, is a valuable guide for anyone looking to write compelling and persuasive English essays. By adopting these principles, writers can produce work that isnot only academically sound but also engaging and informative for the reader.。
The Importance and Appreciation of OurBody PartsOur bodies are intricate and amazing creations, each part playing a vital role in our daily lives. From the soles of our feet to the crown of our heads, every inch of our bodies is a testament to the wonders of nature and evolution. In this essay, I will delve into the importance and my appreciation for some of the key body parts, exploring their functionalities, beauty, and significance in our overall well-being.Let's begin with the feet, the unsung heroes of our bodies. Our feet bear the weight of our entire bodies, allowing us to traverse the world and explore its wonders. The intricate structure of the feet, with bones, muscles, tendons, and ligaments working in harmony, enables us to walk, run, jump, and dance. Without healthy feet, our ability to move freely and independently would be severely compromised.Moving up, we come to the legs, powerful levers that propel us forward. The thighs and calves, packed with muscle, provide the strength and endurance needed forphysical activities. The knees, hips, and ankles are joints that allow for a range of motion, enabling us to bend, squat, and kick. The legs are not only essential for locomotion but also contribute to our overall fitness and well-being.The torso, the central hub of our bodies, houses vital organs such as the heart, lungs, and liver. The ribcage protects these organs, while the spine provides structural support and allows for flexibility. The abdominal muscles, often admired for their aesthetic appeal, play a crucial role in maintaining posture and stability. A strong core is essential for good balance and the prevention of injuries. The arms and hands are our tools for manipulation and expression. The shoulders, elbows, and wrists are joints that enable a wide range of motion, allowing us to reach, grasp, and manipulate objects. The hands, with their intricate network of bones, tendons, and muscles, are incredibly dexterous, capable of performing tasks as delicate as threading a needle or as powerful as lifting heavy weights.The head, the seat of our intellect and emotions, is perhaps the most complex and fascinating part of the body. The brain, housed within the skull, controls all bodily functions, processing information, generating thoughts, and regulating emotions. The eyes, windows to the soul, allow us to see the world and interpret visual cues. The ears, with their intricate structures, enable us to hear and interpret sound. The nose, mouth, and throat are responsible for smelling, tasting, and breathing, essential for our survival.Beyond their functionalities, I also appreciate the beauty and uniqueness of each body part. The curves of the hips, the strength of the thighs, the delicacy of the wrists, and the expressiveness of the face - all contribute to the overall aesthetic of the human body. Each body part is unique, reflecting the individuality and diversity of human beings.In conclusion, our bodies are intricate and remarkable, each part playing a vital role in our existence. We should cherish and care for our bodies, treating them with respect and gratitude. By understanding the importance andfunctionality of each body part, we can better appreciate the wonders of the human body and the miracle of life itself.**身体部位。
Nature,Structure and Strength of the Acidic Sites of Amorphous Silica Alumina:An IR and NMR StudyG.Cre´peau,†V.Montouillout,†,‡A.Vimont,†L.Mariey,†,§T.Cseri,|and F.Mauge´*,†Laboratoire de Catalyse et Spectrochimie,UMR CNRS,ENSICAEN,Uni V ersity of Caen,14050Caen Cedex,France,Centre de Recherche sur les Mate´riaux a`Haute Tempe´rature,UPR CNRS4212,45071Orle´ans Cedex,France,Equipe de Recherche en Physico-chimie et Biotechnologies,EA3914,IUT-UFR de Sciences,14032Caen Cedex,France,and Institut Franc¸ais du Pe´trole,IFP-Lyon,Direction Catalyse et Se´paration,De´partement Catalyse He´te´roge`ne,69390Vernaison,FranceRecei V ed:April11,2006;In Final Form:June8,2006IR spectroscopy of probe molecules(pyridine,2,6-dimethylpyridine,and CO)as well as high-resolution solidstate NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphoussilica alumina(ASA).ASA samples are heterogeneous compounds that contain a silica alumina mixed phaseas well as aluminum clusters and pure silica zones.The distribution of various forms depends both on thepreparation method and on the Si/Al ratio.Formation of mixed phase leads to the creation of acidic hydroxylgroups of various strength,up to that present in dealuminated HY zeolite.Detailed spectroscopic analysesshow that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom intetrahedral environment.The strength of the acidity of the OH species of ASA could be modified both by thelocation of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in thevicinity of silanol group.Cogelification of high silica-containing ASA appears as the best mean to preparehomogeneous amorphous aluminosilicate,which exhibits the strongest Brønsted acidity.IntroductionDevelopments of fuel market as well as specifications for more environmental-friendly products lead refiners to invest in processes for heavy oil conversion.Among them,hydrocracking is a key process that produces selectively high quality fuels with low sulfur content.1The Hydrocracking catalyst is a bifunctional solid,which combines hydro-dehydrogenation and acidic function.The hydro-dehydrogenation component is usually a sulfide phase,while the acidic component is generally a zeolite or an amorphous silica alumina(ASA).To obtain a good selectivity toward gas oil and kerosene,the acidity developed by hydrocracking catalysts should be moderate and significantly lower than that of zeolites.Nevertheless,too weak acidic properties lead to a catalyst of low activity.Consequently, balanced acidity is crucial to obtain high conversion and appropriate selectivity.The use of oxides presenting moderate acidic properties as ASA is preferred to maximize middle distillate products. Recently,important developments in the synthesis of this type of oxide such as the synthesis of ASA via the sol-gel method or coprecipitation method have been reported,2,3or of alumi-nosilicates with controlled mesoporosity or micro-mesoporosity4-7 have been reported.However,the adjustment of these new preparations to hydrocracking application requires an accurate control of their acidic properties.8Therefore,the aim of this work is to determine the nature, strength,concentration and environment of acidic sites present in different ASA samples that can be used as support of the hydrocracking catalyst.Several papers reported the analysis of acidic properties of ASA or related materials using adsorption of probe molecule followed by IR spectroscopy.9,10The first papers dealing with the characterization of the surface acidic sites of ASA appeared in the70s.11,12In these studies,nature of the acidic sites as well as their evolution versus Si/Al ratio was examined using pyridine or CD3CN chemisorption.These papers show that the formation of the mixed silica-alumina phase leads to the creation of acidic OH groups able to protonate pyridine.More recently,using CO adsorbed at low temperature,a detailed analysis of the nature and the strength of the acidic sites of ASA prepared via different routes was reported.3,13,14These studies confirm the presence of some highly acidic OH groups on ASA.However,even with the benefit of the improved signal-to-noise ratio of the last generation of the Fourier transform infrared(FTIR)spectrom-eters,papers report for high-silica-containing ASA samples only the presence of one main OH band located at a wavenumber equivalent to that of SiOH groups of silica,i.e.,a very weakly acidic material.Thus,although several structures were proposed in the literature to account for this feature,13,15the exact origin of the strong Brønsted acidity of ASA is still under discus-sion.IR spectroscopy is a tool of choice to characterize the acidic properties of oxides,particularly when using different basic probe molecules,which present complementary chemical and spectroscopic properties.Hence in this work,we will study adsorption of(i)pyridine,the most widely used probe on ASA, (ii)2,6-dimethylpyridine(DMP),which presents strong affinity for weak Brønsted acid sites,and(iii)carbon monoxide,which, when adsorbed at low temperature,is particularly suited to probe the strength of acidic OH groups.*To whom correspondence may be addressed.E-mail:francoise.mauge@ensicaen.fr.†Laboratoire de Catalyse et Spectrochimie.‡Centre de Recherche sur les Mate´riaux a`Haute Tempe´rature.§Equipe de Recherche en Physico-chimie et Biotechnologies.|Institut Franc¸ais du Pe´trole.15172J.Phys.Chem.B2006,110,15172-1518510.1021/jp062252d CCC:$33.50©2006American Chemical SocietyPublished on Web07/15/2006High-resolution solid-state NMR is another well-established method to probe the local environment of both aluminum and proton atoms and provide important information about the solid acidity of zeolites and ASA materials.In particular,1H magic angle spinning(MAS)NMR with the development of double-resonance NMR,which combines the high-resolution1H MAS experiment with the reintroduction of the dipolar coupling,has become a relatively popular tool to identify the different OH groups present in the materials.In this work,we used a27Al MAS experiment to probe the environment of aluminum atoms, and1H{27Al}TRAPDOR16(transfer of population in double resonance)experiment to assign the different proton lines on the basis of their vicinity with Al centers.The combined use of double resonance NMR experiments and IR spectroscopy appears as a unique mean to specify the environment of protonic species in ASA.Experimental Section2.1.Catalysts.The ASA samples studied in the present work are either prepared at the Institut Franc¸ais du Pe´trole or commercial materials.They are named SiXAlY where X and Y are respectively the weight percentage in SiO2and in Al2O3 (Table1).Si88Al12and Si94Al6were prepared by cogelifica-tion using sodium metasilicate and aluminum sulfate as precur-sors.Sulfuric acid was added to the solution to adjust the pH. The synthesis was performed at293K.After cogelification, the samples were washed,dried at393K,and calcined in dry air at823K.Si30Al70and Si40Al60,which were supplied by Condea,were prepared by impregnation of alumina with silica gel.The specific surface areas were determined by the Brunauer-Emmett-Teller(BET)method.Characteristics of the studied ASA are summarized in Table1.2.2.NMR.High-resolution solid-state NMR experiments were conducted at9.4T on a Bruker AVANCE400spectrom-eter,operating at400.33and104.5MHz for1H and27Al, respectively.27Al MAS NMR spectra of hydrated samples were acquired with a MAS frequency of15kHz after a single,short pulse,ensuring quantitative excitation of the central transition. For1H experiments,the samples previously dehydrated at700 K for4h,were placed inside a4-mm MAS NMR rotor,under vacuum,using a home-built device,described elsewhere.17,18 The1H MAS spectra were recorded using a rotor-synchronized Hahn-echo pulse sequence in order to remove the background signal arising from residual probehead protons.The spinning rate was fixed to10kHz and the evolution period to600µs. The1H nutation frequency was set to59kHz and the recycling delay to30s to avoid no saturation.Similar1H excitation conditions were used for1H{27Al}TRAPDOR experiment,16 combined with a continuous irradiation on the quadrupolar27-Al nuclei during the first evolution period of the Hahn-echo sequence(i.e.,600µs).The27Al nutation frequency was set to 70kHz to satisfy adiabaticity conditions.The1H and27Al chemical shifts were referenced relative to TMS and Al(NO3)3 1M,respectively.2.3.IR Spectroscopy.The ASA samples were grounded,and pressed into self-supported wafers(disk of ca.5mg cm-2).The samples were activated in situ in the IR cell.The activation treatment consists of a ramp up to a given temperature(423, 573,or723K)under primary vacuum.When the activation temperature was reached,the cell was pumped under secondary vacuum down to a residual pressure of5×10-6Torr.For the three activation temperatures,the heating time was kept constant, i.e.,the heating rates were different:V)2°C/min for T act) 423K,V)3°C/min for T act)573K,and V)3.5°C/min for T act)723K.Catalyst acidity was characterized by IR spectroscopy using pyridine,DMP,and CO as probe molecules.Two different IR cells were used depending on the temperature of adsorption probe molecule.For pyridine and DMP experiments,a con-ventional glass cell was used,while for CO experiments,a low-temperature cell was chosen.The latter IR cell is equipped with CaF2windows and presents double walls with a space for cooling agent(liquid nitrogen in this work).After activation,1Torr of pyridine or of DMP was introduced in the cell where the wafer was maintained at323K(or373K for DMP).An adsorption temperature higher than ambient temperature was used to ensure a good diffusion of the probe molecule through the catalyst pellet.After adsorption of the probe molecule,the sample is then evacuated at increasing temperatures,up to the temperature of sample activation.For CO-adsorption experiments,the activated sample was first cooled to100K under vacuum.Then,small doses of CO were introduced and IR spectra were recorded for each dose.The spectra presented in this paper are all normalized to a constant mass of wafer(m)10mg for a disk diameter of16 mm).The IR spectra following adsorption of probe molecule correspond to difference spectra,i.e.,spectra of the samples taken in the presence of probe molecule after subtracting the spectrum before probe molecule adsorption.The IR spectra were recorded with a FTIR Magna spectrom-eter from Thermo Optek.To obtain high-quality spectra (particularly in the zone4000-3000cm-1),the spectrometer was equipped with a XT-KBr beam splitter and with a MCT/A detector.The spectra were scanned using256scans with a resolution of4cm-1.Results and Discussion3.1.NMR Characterization.3.1.1.Al Environment.Figure 1shows the27Al MAS NMR spectra of the ASA samples.The spectrum of the Si94Al6sample,containing the lower amount of aluminum shows a main signal centered at53.4ppm and a weaker,broader signal at around1ppm,respectively,attributed to4-fold(Al IV)and6-fold(Al VI)oxygen-coordinated aluminum. This suggests that,in this sample,aluminum atoms are almost exclusively in a tetrahedral environment.The signal is Gaussian like and does not exhibit a specific quadrupolar line shape.It is characteristic of a quite symmetric environment for the aluminum.The spectra of the two samples obtained by impregnation,Si40Al60and Si30Al70,are quite similar.They are composed of two main peaks with maximum at around60 and5ppm,attributed to aluminum in tetrahedral and octahedral environment.These spectra are similar to that ofγ-alumina.TABLE1:Characteristics of the Studied Samplesname wt%SiO2wt%Al2O3preparation method Ssp(m2/g)γ-Al2O30100273Si30Al703070impregnation of alumina with silica gel339Si40Al604060impregnation of alumina with silica gel324Si88Al128812cogelification449Si94Al6946cogelification488Acidic Sites of ASA J.Phys.Chem.B,Vol.110,No.31,200615173However,the nonzero intensity between the two main lines indicates the presence of a third,weaker component,not present in alumina.The signals exhibit an asymmetric line shape with a steep low-field edge and a trailing high-field edge,charac-teristic of distorted environment.Finally the spectrum of the Si88Al12sample,obtained by cogelification,exhibits clearly three components with isotropic chemical shift in the range of 60-70,30-40,and 0-10ppm.The signals at 60-70and 0-10ppm are unambiguously assigned to Al IV and Al VI species.19According to the literature,20the third signal (30-40ppm)is ascribed to a pentacoordinated aluminum center.This type of environment is commonly observed in oxides obtained by sol -gel routes.19All the spectra have been simulated using Dmfit software 21to extract a precise isotropic chemical shift,mean quadrupolar coupling as well as the relative intensity of the various components.The results are summarized in Table 2.Despite the different synthesis routes used,the results indicate that the fraction of 6-fold coordinated aluminum increased with the amount of alumina.In fact,the Al IV /Al VI ratio is maximum at low Al concentration.Moreover,the comparison between the spectra of γ-Al 2O 3and those of impregnated samples indicates that these samples (Si40Al60,Si30Al70)are mainly composed of a core of alumina surrounded by silica layer.By contrast,the presence of an important amount of penta and tetracoordi-nated aluminum in the Si88Al12obtained by cogelification suggests a better incorporation of aluminum in the silica network.3.1.2.Proton Environments.1H MAS spectral resolution improves with the dehydration process thus allowing thedistinction between different hydroxyl groups.Moreover,it is possible to discriminate the protons adjacent to aluminum from other OH groups using double resonance experiments,such as 1H {27Al }TRAPDOR sequence,based on the analysis of the effect of dipolar 1H/27Al interaction on MAS proton spectra.The 1H MAS NMR spectrum of the Si40Al60sample,previously outgassed at 700K for 4h,is presented in Figure 2.In addition to the main peak at 1.8ppm,attributed to silanol groups,22a weak shoulder can be observed at about 2.8ppm.No signal at 7ppm characteristic of Brønsted acid sites Si-(OH)Al was detected.23The assignment of the weak signal at 0.8ppm is not straightforward.Indeed,according to the literature it can be attributed to either a SiOH group interacting with the oxygen of the framework 24or to a AlOH species of pure γ-alumina.25The environment of the two types of proton is probed using 1H {27Al }TRAPDOR sequence.In this experiment,the use of continuous-wave 27Al irradiation during 1H signal acquisition results in a loss of intensity of resonances attributed to proton in close spatial proximity with aluminum (at about 5Åor less).The TRAPDOR experience (dotted line in Figure 2)shows that the irradiation of 27Al leads to the disappearance of the shoulder centered at 2.8ppm,while the main peak at 1.8ppm is not affected.One may thus conclude that a small fraction of hydroxyl groups is close to aluminum atoms,while the majority of proton species are silanol groups,distant from aluminum.3.1.3.Conclusion.The 27Al NMR experiments clearly evidence the difference in aluminum local environment induced by the synthesis conditions.In the ASA prepared by impregna-tion of alumina by silica gel (Si30Al70,Si40Al60),the environment of aluminum atoms remains very close to that of pure alumina,in variance with the ASA prepared by cogelifi-cation (Si88Al12,Si94Al6),where indication of mixed phase was detected.At least,two types of OH groups can be distinguished on the ASA surface:silanol groups,as observed in pure silica,and hydroxyl groups close to aluminumatoms.Figure 1.27Al MAS NMR spectra of ASA (B0)9.4T,rotation 14kHz).TABLE 2:NMR Parameters Characteristics of the Different Aluminum Environment in ASA SamplesAl IVAl VAl VIsample δiso (ppm)C Q (MHz)%δiso (ppm)C Q (MHz)%δiso (ppm)C Q (MHz)%Si94Al655 2.3847 6.716Si88Al1258 4.34034 4.7155 4.945Si40Al3072 5.43030 2.8412 4.866Si60Al40725.431602.84124.865Figure 2.1H {27Al }TRAPDOR experiment of Si40Al60(B0)9.4T,rotation 10kHz).The full and the dotted lines correspond,respectively,to 1H MAS spectrum acquired without and with 27Al irradiation.15174J.Phys.Chem.B,Vol.110,No.31,2006Cre ´peau et al.3.2.IR Characterization.The effects of the composition,preparation method and activation treatment on the acidic sites of ASA samples were examined by IR spectroscopy.The present study focuses on the ASA Si30Al70,Si40Al60,and Si88Al12solids.3.2.1.Hydroxyl Groups.Figure 3presents the OH stretching zone of the different ASA after activation under vacuum at 423,573,and 723K.For the three samples,an asymmetric and sharp band at ∼3748cm -1characteristic of silanol groups is de-tected.26The zoom on the ν(OH)band of ASA activated at 723K shows that the SiOH band shifts from 3746up to 3749cm -1with increasing Si content.When activation is performed at 423K,a broad envelope,which extends from 3700to 3300cm -1,can also be detected.This envelope completely disappears on activation at 723K,indicating the presence of silanol groups in interaction with water molecules at low activation temper-ature.Indeed,the molecular water (band at 1640cm -1)is completely eliminated only for activation temperatures higher than 573K.It is worth noting that,even for samples containing 60and 70wt %of alumina,no bands characteristic of alumina ν(OH)vibrations are detected (absence of bands at 3790-80,3775-70,3730-25,and 3700-3675cm -1).In the literature,the absence of hydroxyl group characteristic of alumina has already been reported on ASA in the case of samples containing less than 50wt %of alumina.27For ASA samples containing 75wt %of Al 2O 3,Cairon et al.detected a shoulder at 3725cm -1and a relatively large envelope centered at 3700cm -1.28In our case,the absence of Al -OH band,even in the spectrum of the Si30Al70sample,indicates a lower concentration of surface Al atoms probably due to the preparation method.Indeed,the high alumina-containing ASA of the present study were prepared by impregnation of alumina by silica gel,whereas the samples studied by Cairon et al.28were obtained by cogelification.The concentration of silanol groups can be assessed using values of molar extinction coefficient of silanol band, (SiO -H)about 3µmol -1cm,previously determined by different authors.29-31Thus,the amount of silanol groups per square nanometer of oxide was calculated.Figure 4reports silanol group concentrations vs silica content for the three ASA studied and compares these values to that obtained for a pure silica (SiO 2Degussa,S BET )450m 2/g).From the comparison between the number of silanol per square nanometer and the silica content measured by chemical analysis,one can assess the surface concentration in silica.In general,the concentration of silanol groups increases with silica content,as expected from the band assignment.However,a more detailed analysis shows that,on Si88Al12,the concentration of silanol groups corresponds to that expected from chemical analysis,indicating a similar Sicomposition on the surface and in the core of the particles.By contrast,on Si30Al70and Si40Al60samples,the Si composition was higher than that expected,indicating an enrichment in silica on the surface.Such differences can be related to the synthesis methods since Si30Al70and Si40Al60samples are prepared by impregnation of alumina with silica gel while Si88Al12is obtained via cogelification.3.2.2.Pyridine Adsorption.Introduction.Pyridine is the most commonly used probe molecule to assess the acidic sites 10and one of the first molecules used to characterize the acidic properties of ASAs.11,12Pyridine presents the advantage of giving rise to very distinct vibrationnal bands according to its mode of interaction with the surface.For instance,wavenumbers of ν8a and ν19b bands are commonly used to discriminate pyridine coordinated to Lewis acid sites (LAS),protonated on Brønsted acid sites (BAS),or in interaction via a H bond.32,33Regarding the site strength,the wavenumber of the ν8a band is a good indicator of the Lewis strength,whereas no information about the BAS acidic strength can be drawn from the wave-numbers of ν8a and ν19b bands specific of Brønsted acid sites.Such an information can,nevertheless,be obtained by study of pyridine thermodesorption.Nature of Acidic Sites.Figure 5presents evolution of the pyridine vibration bands vs thermodesorption temperature for Si40Al60activated at 573K.The other ASA samples show qualitatively very similar spectra.Three interaction modes of pyridine can be distinguished:Protonation (PyH +),characterized by bands at 1640,1547,and 1492cm -1.From pyridine thermodesorption,one can note that the major fraction of the pyridinum species is elimi-nated on evacuation at 473-523K.However some OH species still retain pyridine at temperature as high as 573K,indicating the existence of a small amount of very strongly acidic OHgroups.Figure 3.(A)IR spectra of the ASA activated at different temperatures.(B)Enlargement of the ν(OH)zone for the ASA activated at 723K.Figure 4.Evolution of the silanol group number per square nanometer with silica amount for the ASA samples and silica activated at 723K.Acidic Sites of ASA J.Phys.Chem.B,Vol.110,No.31,200615175Hydrogen bond (PyH),characterized by bands at 1597,1492,and 1451cm -1.The band at 1597cm -1completely disappears following evacuation at 373K,confirming its attribution to weakly interacting species.Coordination (PyL),characterized by bands at 1622,1615(sh.),1578,1492,and 1451cm -1.After evacuation up to 423K,the shoulder at 1615cm -1disappears,whereas the band at 1451cm -1shifts to 1458cm -1.These features indicate the presence of weak LAS.By contrast,the bands at 1622and at 1458cm -1are still clearly observed even after evacuation at 573K indicating the presence of very strong LAS.It should be mentioned that,when pyridine is evacuated at temperatures higher than 473K,no variation in the position of the bands at 1622and 1458cm -1is observed.This is in variance with what is generally observed on pure γ-alumina where these two bands shift progressively from 1623to 1626cm -1and from 1451to 1456cm -1when the temperature of evacuation of pyridine increases from 373up to 573K.34The absence of shift in the position of the LAS bands when pyridine coverage decreases can be an indication of a very homogeneous environ-ment of the strong Lewis acid sites on ASA or a weaker lateral interaction effect between adsorbed pyridine molecules due to a smaller LAS density on ASA compared to Al 2O 3.It is also worth noting that it is not possible to specify which OH groups are responsible for the protonation of pyridine by analyzing the OH group region.Indeed,OH groups are alsoperturbed by pyridine coordinated species,and thus even afterthermodesorption at 573K,these species are still detected on ASA.The effect of the chemical composition on the ASA acidic sites is presented in Figure 6.From pyridine adsorption spectra,the results show the higher the silica content,the larger the amount of BAS (1640and 1547cm -1).Conversely,the increase of silica percentage decreases the amount of LAS (1622and 1451cm -1).It should be mentioned that weak LAS (shoulder at 1615cm -1)are specifically diminished on the high silica sample.Activation temperature changes the nature of ASA acidic sites.As expected,the higher the activation temperature,the larger the amount of LAS (spectra not shown).Conversely,the increase of the activation temperature decreases the amount of BAS.3.2.3.DMP Adsorption.Introduction.The use of methyl-substituted pyridine such as 2,6dimethylpyridine (DMP or lutidine)to probe the acidic properties of alumina was reported for the first time by Kno ¨zinger et al.35,36DMP is more basic (p K B )7.3)than pyridine (p K B )8.8),due to the presence of the two methyl groups in the R position.Consequently,DMP can protonate on more weakly acidic OH groups compared to pyridine.37Protonation of the DMP is clearly evidenced by the appearance of ν8a and ν8b bands in the 1650-1625-cm -1zone (Table 3),as well as by the appearance of a δ(NH)band at 1270cm -1.With respect to the bands in the zone 1618-1603cm -1,some confusion appears in the literature about their assignment.Corma et al.38ascribed the band detected at 1618cm -1on γ-alumina to DMP protonated on weak Brønsted acidic sites,whereas Matulewicz et al.39and Morterra et al.40attributed it to DMP coordinated to Lewis acidic sites.The attribution of this band to DMP in interaction with weak BAS appears,however,inconsistent with several experimental observations.Indeed,the band at 1618cm -1:(i)is more clearly visible on samples activated at high temperature,(ii)is no longer observed on alumina following H 2O or H 2S adsorption,41and (iii)is still detected after DMP evacuation at temperature as high as 573K.Thus,the band at 1618cm -1should be assigned to the ν8a vibration of DMP coordinated to strong Lewis acid sites of alumina,i.e.,on Al 3+in incomplete tetrahedral coordinance.On alumina,DMP adsorption also leads to the appearance of a band at 1603-1606cm -1that was assigned by Morterra et al.40to DMP coordinated to weaker LASs.It should be mentioned that H bonding of DMP with silanol groups also gives rise to a band at such a wavenumber (1604cm -1).Therefore,contrary to previous reports,38DMP can interact with various acidic sites of the surface,including with LAS.DMP is particularly well suited to distinguish the BAS from the LAS (Table 3).Difficulties arise for discriminating the nature of weak acidic sites.DMP signal is sensitive to the LAS strength.Recently,we evidenced that the DMPH +signal can also monitor the strength of the BAS.42Thus,the wavenumbers of ν8a and ν-(NH)bands of DMPH +vary with the strength of Brønsted acidity:the lower the ν8a wavenumber and the higher the ν-(NH)wavenumber,the stronger the OH acidity.Nature of Acidic Sites.Figure 7A presents spectra of DMP adsorbed on Si40Al60activated at 573K in the 1700-1550-cm -1region,i.e.,the most characteristic zone of variousDMPFigure 5.IR spectra of pyridine adsorbed on Si40Al60activated at 573K and pyridine adsorbed at 323K and evacuated at:(a)298K,(b)323K,(c)373K,(d)423K,(e)473K,(f)523K,(g)573K.Figure 6.IR spectra of pyridine evacuated at 373K on the ASA samples activated at 573K.TABLE 3:2,6-DMP Vibration Frequency According to Its Mode of Adsorption (cm -1)vibration modesDMP physisorptionDMP ‚‚‚H DMP-L (coordination)DMPH +(protonation)ν8a 159516001618-16031650-1640ν8b158015801573163015176J.Phys.Chem.B,Vol.110,No.31,2006Cre ´peau et al.adsorption modes.On the ASA samples,three interaction modes of DMP can be distinguished:Protonation (DMPH +),characterized by the ν8a and ν8b bands at ∼1650-1640and 1630cm -1and by the δ(NH)band at 1270cm -1.Coordination (DMPL),characterized by the band at ∼1618cm -1and by a part of the band at 1583cm -1Hydrogen bonding (DMPH ‚‚‚H)or interaction with the weak LASs,characterized by the band at 1604cm -1and by a part of the band at 1583cm -1.Figure 7A,which presents the effect of DMP thermodesorp-tion,shows that the bands at 1604and 1583cm -1preferentially disappear,as expected from their assignment to DMP in weak interactions with the surface (H-bond or weak LAS).The band at 1618cm -1resists to increasing the evacuation temperature up to 573K,in agreement with its attribution to DMP coordinated to the strong LAS.Thermodesorption leads also to a progressive decrease of the intensity of the two bands specific of protonated DMP species.However,these two bands are still observed after pumping at 573K indicating the presence of some strong BAS on the ASA surface.In addition,one can note a shift of the νa DMPH +band when evacuation temperature increases.To more accurately analyze the temperature of elimination of each DMP species,difference spectra for two successive temperatures of evacuation were also calculated (Figure 7B).Thus on this figure,the negative bands characterize the species that are eliminated in the considered temperature range.The shift of the bands characteristic of DMPH +species with evacuation temperature is more clearly visible.For mild evacuation temperatures (between 373and 423K),DMPH +species,which are eliminated,give rise to bands at 1651and 1630cm -1(see negative bands).When the evacuation temper-ature increases,the ν8a band of eliminated protonated DMP species shifts progressively down to 1644cm -1,while the wavenumber of ν8b vibration remains constant (∼1630cm -1).These observations are in agreement with our previous study,42since the lower the ν8a frequency,the stronger the BAS site.The present work indicates that the ν8a band of DMPH +formed on ASA contains several components and,accordingly,only DMP in interaction with the strongest acidic OH groups resists to the highest temperatures of evacuation.Figure 8compares the spectra of DMP adsorbed on Si40Al60and Si88Al12after DMP evacuation at 573K.The two samples present bands characteristic of DMP coordinated to strong LASs (ν8a band at 1618cm -1)and of DMPH +(bands in the zone 1650-1630cm -1).The absence of H-bond DMP is consistent with the high temperature of evacuation (573K).Detailedcomparison of the two DMP spectra reveals some differences in the DMPH +envelope.The high-frequency part of the DMPH +envelope on Si88Al12does not extend up to wave-number as high as on Si40Al60.This difference in frequency reflects the difference in acidic strength of ASA OH groups.The Si88Al12solid exhibits stronger BASs.In summary,DMP confirms the presence of BAS and strong LAS on the ASA,as detected by pyridine.Moreover,the wavenumber of DMPH +bands indicates that Si88Al12exhibits a larger concentration of strong Brønsted acidic sites than low silica-containing ASA samples.3.2.4.Carbon Monoxide Adsorption.Introduction.Carbon monoxide,a weak base,is a very powerful probe molecule.It can coordinate on Lewis acid sites,and if adsorbed at low temperature (T ≈100K),it can also form a H bond with acidic OH groups.Depending on its interaction mode,wavenumbers of ν(CO)bands can be directly related to the acidic strength of Lewis acid sites or hydroxyl groups.The vibration frequency of CO in interaction with LAS can reach 2230cm -1for very strong LAS and shifts down to 2157cm -1for very weak LAS.For CO in interaction with acidic OH groups,the ν(CO)frequency range is located between 2180and 2152cm -1depending on the OH acidic strength.Thus,discriminating between CO in interaction with LAS from CO in interaction with acidic OH groups is not always staightforward from analyzing the CO stretching frequency zone.A parallel study of ν(CO)and ν(OH)zone is often necessary to distinguish between CO interaction via coordination or via H bonding.CO in interaction with OH groups of metal oxides or zeolites leads to the formation of H-bond and the extent of ν(OH)band perturbation,∆ν(OH/CO),is proportional to the OH group acidity.As reported by Maache 43and Cairon,13correlation between ν(CO)frequencies and ∆ν(OH/CO)values couldbeFigure 7.(A)IR spectra of DMP adsorbed on Si40Al60activated at 573K;DMP adsorbed at 373K and evacuated at:(a)298K,(b)323K,(c)373K,(d)423K,(e)473K,(f)523K,(g)573K.(B)Difference between spectra corresponding to two successive temperatures ofevacuation.Figure 8.IR spectra of DMP evacuated at 573K on Si40Al60and Si88Al12activated at 573K.Acidic Sites of ASA J.Phys.Chem.B,Vol.110,No.31,200615177。