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Use of Vacuum for the Stabilization of Dry Sand Slopes

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Use of Vacuum for the Stabilization of Dry Sand SlopesB.Bate,A.M.ASCE 1;and L.M.Zhang,M.ASCE 2Abstract:Vacuum is proposed as a means for rescuing soil slopes showing signs of impending failure.Two aspects associated with the pro-posed method were studied;namely,the theory of air flow through dry soils and the effectiveness of vacuum for enhancing the stability of soil slopes.A model test device was developed,and two series of tests were carried out using this device.One was a series of tests on pore-air pres-sure distributions in dry sand slopes,and the second series involved dry sand slope stability tests.The results revealed that a vacuum (negative pore-air pressure)even as small as 20.4kPa signi ficantly increased the stability of the model slopes with dimensions of 0.930.530.28m (length 3width 3height).The pore-air pressure distributions in the model slopes were simulated using a finite-element partial differential equation solver,FlexPDE .Fick ’s law and mass conservation were used to formulate the air flow through dry soils.Good agreement was achieved between the experiment results and the numerical simulations.A computer routine,called Slope-Air,was developed for slope stability analysis using Bishop ’s simpli fied method and considering the pore-air pressure distributions in the slope.The calculated factors of safety of the model slopes at failure were consistent with the results of the model slope stability tests.DOI:10.1061/(ASCE)GT.1943-5606.0000731.©2013American Society of Civil Engineers .CE Database subject headings:Air flow;Slopes;Slope stability;Vacuum;Seepage;Sand (soil type).Author keywords:Air flow;Slopes;Slope stability;Stabilization;Vacuum;Seepage.IntroductionMany analyses in geotechnical engineering assume that the soil is fully saturated or that the pore air in the soil is fully connected to the atmosphere,and in this way the gauge pore-air pressure is zero.Such analyses may not be applicable to a range of practical problems where the pore-air pressure is not equal to atmospheric pressure,such as vacuum stabilization,methane hydrate exploration,and soil vapor extraction.In response to elevated pore-air pressures in soil,the net normal stress decreases.Accordingly,the shear strength and stiffness of the soil can be signi ficantly reduced (Wheeler et al.1991;Fredlund and Rahardjo 1993;Duffy et al.1994).Consequently,the short-term stability of slopes or the load capacity of foundations will be de-creased,while the slope deformations will be larger.Fredlund and Rahardjo (1993)estimated the pore-air pressures in a dam at the end of construction and analyzed the stability of the dam.It was found that ignoring the in fluence of pore-air pressure could result in a 15e 34%overestimate of the factor of safety and thus lead to an unsafe design.Previous studies have emphasized the adverse effects of pore-air pressure on the stability and deformation of soil masses.Yet,it is possible to apply pore-air pressure in a positive way.In fact,vacuum technology has been used successfully to improve soft soil foundations(Harvey 1997;Shang et al.1998;Chu et al.2000;Tang and Shang 2000).Vacuum technology was also reported to have been applied to rock slopes at a surface mining site to enhance the slope stability (Brawner and Cavers 1986),as well as to horizontal drains to stabilize landslides (Arutiunian 1983;Arutjunyan 1988;Pakalnis et al.1983a ;Brawner 2003).Early applications of the vacuum technique in slope stability,however,did not gain enough attention,and have not led to further employment of vacuum techniques in recent decades.The reason may be attributed to the heterogeneous nature of the ground-water system and the soil stratum,the insuf ficient techniques of sealing,and the complex field operation,which make it hard to dis-tinguish the effect of vacuum from other means,that is,local stress relief by a dragline in a tar sand mine (Pakalnis et al.1983b ).However,because the vacuum consolidation technique and the development of geomembrane liners have become more mature in recent years (Thevanayagam et al.1994),vacuum-assisted slope stabilization has promising applications.In this study,the mechanisms of vacuum technology in the sta-bilization of dry soil slopes were investigated,and vacuum tech-nology was proposed for the emergency rescue of active landslides.The concept involves a proposed vacuum rescue system that applies a vacuum to an enclosed soil mass (e.g.,cut slope,fill slope,or fill behind a retaining wall).The enclosure can be formed by existing protection layers (e.g.,shotcrete,chunan,masonry,or concrete walls)or a geomembrane that can be quickly installed in the field.Once prefailure signs are diagnosed on a slope or a retaining wall,the vacuum rescue system can be mobilized immediately to stabilize the slope or retaining wall and to avert disastrous consequences.The vacuum technique provides temporal support of the slope,although conventional approaches to slope improvement,such as slope modi fication,earth buttresses,and restraining structures (Mitchell 1981;Turner and Schuster 1996)are still needed.A preliminary study was undertaken to better understand the pore-air pressure distributions caused by external vacuum points and the effect of applied vacuum on slope stability.This preliminary study had the following objectives:(1)to perform numerical analysis of1Assistant Professor,Dept.of Civil,Architectural,and Environmental Engineering,Missouri Univ.of Science and Technology (formerly Univ.of Missouri-Rolla),Rolla,MO 65409(corresponding author).E-mail:bate@ 2Professor,Dept.of Civil and Environmental Engineering,The Hong Kong Univ.of Science and Technology,Clear Water Bay,Hong Kong.E-mail:cezhangl@ust.hkNote.This manuscript was submitted on May 2,2011;approved on March 13,2012;published online on March 15,2012.Discussion period open until June 1,2013;separate discussions must be submitted for in-dividual papers.This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering ,Vol.139,No.1,January 1,2013.©ASCE,ISSN 1090-0241/2013/1-143e 151/$25.00.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .two-dimensional (2D)steady-state air flow through dry sand,(2)to investigate the in fluence of vacuum on the stability of soil slopes and to quantify the in fluence using a limit equilibrium method,and (3)to conduct model tests to verify the numerical analyses and estimate the effectiveness of vacuum stabilization.Theories of air flow through dry soils,mechanisms of using vacuum for soil slope stabilization,and slope stability analysis that consider the in fluence of pore-air pres-sures were studied first.Then a series of model tests was conducted to investigate steady-state flow of air in dry sand slopes and the sta-bility of dry sand slopes subjected to vacuum.Finally,2D numerical analyses were conducted to simulate the air flow in the models,and the slope stability analyses were performed to calculate the factors of safety of the model slopes.Numerical Simulation Scheme Theory of Airflow in Dry SoilsThe rate of air flow through soil can be related to the air pressure gradient according to Fick ’s law (Fredlund and Rahardjo 1993)∂m∂t ¼2k a ∂u a ∂zð1Þwhere ∂m =∂t 5mass of air flowing through a unit area of air-filled pore space in unit time;k a 5air coef ficient of permeability;u a 5pore-air pressure;∂u a =∂z 5pore-air pressure gradient through the soil along the z -direction;and g 5gravitational constant.An ex-perimental study of air flow by Blight (1971)has shown that Fick ’s law gives a satisfactory approximation of test results.The air coef ficient of permeability of a dry soil is usually much larger than its water coef ficient of permeability (Fredlund and Rahardjo 1993;Ba-Te 2005).Therefore,under constant flux con-ditions,steady-state air flow can be reached in a relative short time in dry soil.The following assumptions were made:(1)the soil is nondeformable or there is no change in total stresses,(2)the soil is homogeneous and isotropic,and (3)the air flow through the dry soil only involves single-phase flow.Based on the conservation of mass and Fick ’s law,the 2D air flow continuity equation can be expressed as∂2ua ∂xþ∂2u a∂z ¼0ð2Þwhere x and z 5coordinates.Eq.(2)was used as the governing equation for numerical simulation of pore-air pressure distributions in model soil slopes.Also,a finite-element partial differential equation solver,FlexPDE (PDE Solutions Inc.2011),was used to obtain a solution.Mechanisms for Use of Vacuum in Soil Slope StabilizationVacuum can help stabilize a soil slope by two mechanisms,namely,an increase in the shear strength of soil and the acceleration of drainage.The shear strength of an unsaturated soil can be expressed in terms of two independent stress-state variables ðs 2u a Þand ðu a 2u w Þas follows (Fredlund et al.1978):t f ¼c 9þðs 2u a Þf tan f 9þðu a 2u w Þf tan f bð3Þwhere t f 5shear strength;c 95effective cohesion intercept;ðs 2u a Þf 5net normal stress at failure;u a 5pore-air pressure;u w 5pore-water pressure;f 95effective angle of shearing resistance;f b 5angle indicating the rate of change in shear strength relative to the change in matric suction ðu a 2u w Þf .Eq.(3)is known as the extended Mohr-Coulomb failure criterion.In vacuum stabilization,there is no increase in total stresses (Leong et al.2000).However,when vacuum (i.e.,negative pore-air pressure)is applied to a soil mass,the net normal stress ðs 2u a Þincreases.As a result,in the case of the vacuum-stabilized dry sand slopes where the suction term does not contribute,this leads to an increase in the shear strength t f ,as shown in Eq.(3).The shear-strength increment will increase the stability of the slopes against shallow failure.In the case of vacuum-stabilized unsaturated soil slopes,in addition to the increase in the net normal stress,ðu a 2u w Þwill also increase because of the accelerated drainage of the pore water by vacuum.In the matric suction range that vacuum can create (,100kPa),it is most likely that the friction angle f b associated with matric suction is not zero (Fredlund et al.1978;Vanapalli et al.1996).As a result,the shear strength of the soil will increase.Furthermore,the shear strength can also be enhanced by possible density increase as a result of the compression of the soil under vacuum pressure.Slope Stability Analysis Considering Pore-Air Pressure EffectsFredlund and Rahardjo (1993)derived a general limit equilibrium method for calculating the factor of safety of soil slopes,including the effect of pore-air pressure.In a general limit equilibrium for-mulation,the factor of safety is de fined as “the factor by which the cohesion and the coef ficient of friction must be divided to drive the slope to a limit equilibrium state ”(Duncan 1996).The slip surface can usually be assumed to be circular,with little inaccuracy,unless there are geological controls that constrain the slip surface to a noncircular shape (Duncan 1996).Adopting assumptions made by Bishop (1955),the resultant interslice shear forces can be ignored.The Fredlund-Rahardjo formulation (Fredlund and Rahardjo 1993)can then be simpli fied asFS ¼Pi 5ni ¼1(c i 9b i R þ"N i 2u wib i tan f b i tan f i 92u ai b i 12tan f bitan f i9!#R tan f i9)Pi 5ni ¼1W i x ið4ÞwhereN i ¼W i 2c i 9b i sin a i FS þu ai b i sin a i FS tan f i 92tan f bi þu wi b i sin a i FS tan f b icos a i þÀsin a i tan f i 9ÁFSð5Þwhere n 5number of slices;FS 5factor of safety;R 5radius for a circular slip surface;W i 5total weight of the i th slice with a base sloping distance of b i ;N i 5total normal force on the base of the i th slice;u ai and u wi 5pore-air and pore-water pressures at the base of the i th slice;a i 5angle between the tangent to the center of the baseof the i th slice and the horizontal;c i 9,f i 9,and f bi 5effective cohesion intercept,effective angle of shearing resistance and angle related to the change in matric suction at the base of the i th slice,respectively;and x i 5horizontal distance from the centerline of the i th slice to the center of rotation.The angle f b begins to deviate from the effective friction angle as the soil desaturates at suctions greater than the air-entry value (Vanapalli et al.1996).As the soil suction reaches the residual suction,f b appears to approach a value close to zero.For the dry clean sand,D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .the residual suction must have been exceeded.Therefore,f b is set to zero.Pore-water pressures in the dry sand are also set as zero.In this study,a computer routine called Slope-Air (Ba-Te 2005)was developed to analyze the stability of unsaturated soil slopes,by which distributed pore-air and pore-water pressures could be con-sidered simultaneously.The method of slices,as shown in Eqs.(4)and (5),was used,and circular slip surfaces were assumed for uniform soil.The results of pore-air pressure distribution,calculated using FlexPDE ,are mapped onto the soil slope pro file.The pore-air pressures along a slip surface can be used to calculate the factor of safety along that surface.A grid-and-radius scheme was adopted to find the location of the center of the critical slip surface and the radius corresponding to the minimum factor of safety.An effective peak friction angle of 34°(corresponding to the relative density of 75%)was selected based on Cai ’s (2001)triaxial test results.Materials and Experimental Setup of Model Slope TestsModel Testing DevicesA rotatable box was designed and built for the experimental study.The main components are a rotatable acrylic model box (1.230.530.4m,length 3width 3height),a stand on which the box could rotate,a vacuum supply system,and measuring devices.The model soil slope was prepared in the box.Dry sand was used to form the model and simulate the steady-state condition of the vacuum drainage of a slope.The omission of capillary water in the model box singled out the effect of vacuum on net normal stress and got rid of the effect of matric suction.A latex membrane was applied to cover the top of the slope to hold the vacuum.During testing,negative pore-air pressures were applied to the model soil slope.The negative air pressure was supplied by a vac-uum pump and controlled by pressure regulators.The box could be rotated and fixed at any angle between 0and 40°.2D air flow con-ditions were simulated by using perforated strips as vacuum inlets and outlets.Air pressure measuring tips were installed inside the model soil slope through predrilled holes that were aligned in two rows in the acrylic wall.A digital pressure switch (SMC series ZSE40F)was used to display the pore-air pressure.The resolution of this vacuum gauge was 0.1kPa.Two video cameras were installed orthogonal to each other to record the failure of the soil slope.Soil Properties and Sample PreparationUniform Leighton Buzzard sand (Fraction E)was used to form the model slope.This soil was fine and uniform uncrushed silica sand whose grain size ranged between 0.09and 0.15mm (Table 1).A dry pluviation method was used to prepare the model sand slope.Two main factors affect the density of the sand;namely,mass pouring rate and pouring height (Takemura et al.1998;Ueno 1998).The pouring rate was carefully controlled using a scoop.Calibration between the free fall height and the relative density was conducted at four dif-ferent free-fall heights.The average density values from those test results are plotted in Fig.1.A free-fall distance of 790mm was selected for final model preparation.The corresponding relative density of the sand was 75%.Based on Cai ’s (2001)triaxial test results,the extrapolated peak friction angle of the sand at a relative density of 75%was approximately 34°.This was also assumed to be the effective angle of internal friction,f 9,for Eqs.(4)and (5).The critical state friction angle of this sand was 29.7°(Cai 2001).The prepared rectangular trapezoidal model slope was 0.9m in length and 0.28m in height,with an initial slope angle of 18.4°.Pore-Air Pressure Distribution TestsPore-air pressure distribution tests were carried out at the boundary conditions as shown in Fig.2and Table 2.These boundary con-ditions differed in the magnitude of the applied vacuum,the loca-tions of the vacuum sources,and the location of the vent to the atmosphere.Readings were taken after the pore-air pressures had reached a steady state (normally less than 1min).Slope Stability TestsFirst,a reference model Test T1,in which the slope was exposed to the atmosphere,was brought to failure by rotating the model box.Then,one slope stability test was performed with a uniformly dis-tributed vacuum inside the soil slope (T2),followed by two model slope tests with nonuniform pore-air pressure distributions (T3and T4).During Tests T2and T3,a suitable vacuum pressure was first applied to the model.After the steady-state pore-air pressure dis-tributions had been measured,the models were tilted slowly in stages to failure or to the maximum slope angle (58°).The slope angle increment in each stage was approximately 5°.In model Test T4,a large vacuum (∼15kPa)was first applied to the model.The model was then raised to a slope angle of 45°in stages,and after that,Table 1.Properties of Leighton Buzzard Sand (Fraction E)(data from Cai 2001;Haigh et al.2000)PropertyValue Speci fic gravity,G s2.650Uniformity coef ficient,C u1.300Maximum dry density (kg/m 3)1,590.000Minimum dry density (kg/m 3)1,320.000Minimum void ratio 0.667Maximum void ratio 1.008Coef ficient of waterpermeability at e 50:72ðm =s Þ0.98031024Critical friction angle,f crit ðdegrees Þ29.700Effective friction angle,f 9ðdegrees Þ34.000Effective cohesion,c 9ðkPa Þ0.000Grain size D 10(mm)0.095Grain size D 50(mm)0.140Grain size D 60(mm)0.150Fig.1.Calibration of the relationship between free-fall height and relative density of dry Leighton Buzzard sand (Fraction E)D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .the magnitude of the applied vacuum was reduced in stages until the slope failed.The test conditions for the four model slope stability tests are summarized in Table 2.Results and Discussions Pore-Air Pressure DistributionThe 2D flow condition was checked by measuring the air pressures at the 2D coordinate of (0.51,0)and three different positions normal to the 2D plane (Fig.2).The results of pore-air pressure measure-ments are shown in Fig.3.The differences among three pressure values at three positions normal to the 2D plane were 0.1kPa for Test A1and 0kPa for Tests A2,A3,and T3.The 2D flow condition,created by the perforated strip vacuum source,was validated.Figs.3(a and b)show the pore-air pressure distributions at source vacuums of 24.0and 21.55kPa,respectively.The boundary conditions were the same.The resulting pressure contours were similar in shape and different in magnitude.The magnitude of air pressure was proportional to the magnitude of the applied vacuum.This behavior occurred because Eq.(2)is a linear partial differentialequation.As long as the boundary conditions do not change,the computed steady-state pressure at any point will be linearly pro-portional to the source pressure.Fig.3also indicates that the measured air pressure values were generally in good agreement with the calculated ones.Fick ’s law is capable of describing the steady-state air flow through the dry homogeneous sand.A few mismatches between the measured and calculated air pressure results were observed.The largest mismatch was at co-ordinate (0.28,0.05)in Test A1.The measured vacuum was 21.3kPa,while the calculated one was between 21.6and 21.7kPa.The problem may be associated with connections of the tubings.However,the difference of all of the rest of the measurements was no larger than 0.2kPa.This difference was primarily caused by the limited resolution of the digital vacuum gauge,which was 0.1kPa.The resolution was of high precision for normal usage.However,the vacuum in the soil slopes at failure was less than 0.6kPa.A higher precision vacuum gauge is recommended for future research.Slope StabilityThe inclination of the model slope was represented by both the angle between the base of the slope and the horizon (BSA)and theangleFig.2.Model soil slope dimensions and points of air pressure source and measurementTable 2.Pore-Air Pressure Distribution Tests and Model Slope Stability Test Schemes Model slope test numberAir pressure distribution (kPa)Slope angle at failure(degrees)a bcdeAir pressure Test A124.0Open to the atmosphere Sealed boundary Sealed boundary Sealed boundary —Air pressure Test A221.55Open to the atmosphereSealed boundarySealed boundarySealed boundary —Air pressure Test A3Sealed boundary Sealed boundary 20.620.6Open to the atmosphere—Slope stability Test T1Sealed boundary Open to the atmosphereOpen to the atmosphereOpen to the atmosphereSealed boundary 34Slope stability Test T2Sealed boundary 20.4Sealed boundary Sealed boundary Sealed boundary 58Slope stability Test T3Open to the atmosphere20.5Sealed boundarySealed boundarySealed boundary 58Slope stability Test T4Sealed boundarySealed boundary20.320.3Open to the atmosphere45Note:The locations of points a,b,c,d,and e are shown in Fig.2.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .between the slope surface and the horizon (SSA).The SSA before failure was the sum of the BSA before failure and the initial slope angle (18.4°).In Test T1,the slope was stable until the BSA reached 15.6°(i.e.,the SSA reached 34°).When the box was lifted higher,the sand on the slope surface started to slide down until the SSA was restored to 34°[Fig.4(a)].The slip surface was essentially planar and shallow throughout the process.In Test T2,a uniformly distributed vacuum of 20.4kPa was applied to the slope and maintained using a latex membrane.At a BSA of 15.6°(SSA of 34°),the model slope was stable.When the BSA was further increased,the slope remained stable until the BSA reached 39.6°(SSA at 58°).At an SSA of 58°,the soil on the crest settled by 25mm [Fig.4(b)],but no signs of sliding failure were pared with the reference test,T1,the stability of model Slope T2was greatly enhanced by the applied uniform vacuum,even though the magnitude of the vacuum was only 20.4kPa.The pore-air pressure distribution in Test T3is shown in Fig.3(c).As the model was tilted,there was no appreciable slope movement at a BSA of 15.6°(SSA at 34°).As the BSA reached 35.6°(SSA at 54°),the soil on the crest started to settle.As the BSA reached 39.6°,a large amount of soil on the crest moved downward toward the toe of the slope,as shown in [Fig.4(c)].The stability enhancement effect of the nonuniformly distributed vacuum was again signi ficant.The other model,T4,was also subjected to nonuniformly dis-tributed vacuum.There were two vacuum sources on the slope,as shown in Fig.3(d),and the average vacuum of the two sources was taken as the magnitude of the applied vacuum.The slope was initially subjected to a large vacuum of 210kPa and was tilted to a BSA of 26.6°(or an SSA of 45°).Afterward,the vacuums at the two sources were gradually reduced.The vacuum distribution shortly before the onset of failure is shown in Fig.3(d),where the average applied vacuum was 20.6kPa.When the average applied vacuum was further reduced to 20.3kPa,a rupture surface starting from the crest developed rapidly.The sand in the lower middle of the slope moved toward the toe,and the soil near the crest settled substantially.Retrogressing slope failure was recorded.Within 5min,the slope surface had become parallel to the box base.Fig.4(d)shows a circular slip surface observed during the failure progress.Failures in dry cohesionless slopes are generally planar.Under the applied vacuum,the dry sand in the models exhibited an apparent cohesion (i.e.,a shear-strength increment because of the increased net normal stress).Therefore,failure along a circular shear zone could develop.After the failure occurred,the pre-embedded strips and tips deviated from the original locations,and the readings were no longer reliable.The stabilizing effect of the nonuniform vacuum in this model was also evident.Another practical implication that can be drawn from this research is that vacuum should be applied to the potential slip failure zone to maximize its effect.In summary,an applied vacuum,whether uniformly distributed or nonuniformly distributed,greatly enhanced the stability of the model slopes.Slope Stability AnalysisThe stability of model slopes of various inclinations,as shown in Table 2,was analyzed using a self-coded computer program,Slope-Air.In model Test T1,the calculated factor of safety was 1.06at an SSA of 32°and 1.0at an SSA of 34°,and the predicted slip surface was planar and cut the slope surface.In model slope Tests T2,T3,and T4,the centers of possible slip surfaces were plotted in the region to the upper left of the slope (Fig.5).The minimum factor of safety corresponding to each center was determined by varying the radius of the possible slip surface.Then,the contours of the factor of safety at each grid point in the region were plotted,as shown in Figs.5(a e c).The critical slip surfaces corresponding to the minimum factor of safety were also plotted.The computed relationships between the factor of safety and the SSA for Models T1,T2,and T3are shown in Fig.6.With a particular pore-air pressure distribution,the factor of safety decreased with the SSA.At a particular SSA,the factors of safety of the slopes stabilized by vacuum were predicted to be much higher than those at zero pore-air pressure.When the SSA reached 34°,a slope without vacuum stabilization would fail,and the factor of safety was equal to unity.However,when a uniform vacuum of 20.4kPa was applied,the slope would not fail until an SSA larger than 45°was reached.Similarly,when the nonuniform vacuum field of T3was applied [Fig.3(c)],the model predicted that the stability of the slope would be greatly enhanced and the slope would not fail until the SSA was close to 50°.The calculated factors of safety for model Tests T2and T3at failure (SSA of 58°)were 0.91and 0.90,respectively.TheFig.3.Pore-air pressure distributions in (a)Test A1;(b)Test A2;(c)Test T3;(d)Test A3D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .critical slip surfaces are shown in Figs.5(a and b).The calculated failure patterns and the locations of the slip surfaces were similar to those seen in the model tests [Figs.4(b and c)].However,the calculated factors of safety were slightly smaller than 1.0.Side friction and restraint from the latex membrane were two possible reasons,as subsequently discussed.In Test T4,a slope was initially subjected to a large vacuum and tilted to an SSA of 45°.Failure of the slope was then obtained by reducing the applied vacuum in stages.The computed relationship between the average applied vacuum at the two sources and the factor of safety is shown in Fig.7.As expected,the factor of safety decreased as the vacuum decreased.At the onset of failure of the slope,the measured average applied vacuum was 20.3kPa.Because of the rapid development of failure,the pore-air pressure distribution in the slope at the applied vacuum level could not be measured.As a result,the calculated pore-air pressure distribution corresponding to the measured applied vacuum (20.3kPa)was used in the slope stability analysis.The calculated pressures were essentially pro-portional to those shown in Fig.3(d).The calculated factor of safety at failure was 1.01,which was consistent with the test results.Slope-Air predicted deep slip surfaces,which were close to the toe of the slopes (Tests T2,T3,and T4).The model test results agreed with this prediction in general.However,the failure surface was slightly shallower than predicted,especially in the case of Test T4(Fig.5).Two factors led to this observation;namely,the vacuum-induced side friction and the membrane effects.The vacuum-induced side friction was most signi ficant at the toe and in the crest region,where the height of the soil column was small.When the soil was about to move,tensile stresses of the membrane developed near the toe,while relatively smaller tensile stresses developed near the crestbecause of the geometry of the soil slope.Therefore,the soil in the toe area was restrained,and the slip surface was slightly shallower.Boundary Effects of Model Slope TestsTo account for side boundary effects in the model slope tests,a three-dimensional slope stability analysis was conducted on each of the test models (T2,T3,and T4).A closed-form solution developed by Gens et al.(1988)for cohesive soils was adopted as the starting point of this analysis.The solution assumed that the model side boundaries were vertical surfaces,which was the case in this study.The three-dimensional factor of safety ðF 3Þas a function of the 2D factor of safety ðF 2Þis expressed asF 3¼F 2241þ2M E ÀA b QCÁÂR ÂL 35ð6Þwhere M E 5first moment of area of each side plane of slip soil aboutthe center of the circular slip surface;L 5length of the slip surface in the transverse direction (0.5m in the model tests);R 5radius of theslip surface;and ðA b QCÞ5length of the circular slip surface.Results of calculations using Eq.(6)indicated that the factors of safety from the three-dimensional analysis were 7.57,7.65,and 7.44%higher than those from the 2D analysis for model slope Tests T2,T3,and T4,respectively.The side boundary effects were consistent for Tests T2,T3,and T4,and yielded an average of 7.5%increase in factors ofsafety.Fig.4.Failure of model slope:(a)Test T1,angle between the base of the slope and the horizon 515.6°;inset:transverse view;(b)Test T2,angle between the base of the slope and the horizon 539.6°;(c)Test T3,angle between the base of the slope and the horizon 539.6°;(d)Test T4,angle between the base of the slope and the horizon 526.6°;note the circular slip surface in T4D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S I C H U A N U N I VE R S I T Y o n 11/17/13. C o p y r i g h t A S C E .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .。

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Exercises
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III. Translation 3. Birth records and interviews with the attending staff will be able to show if during delivery there was an excessive use of vacuum extraction, or improper use of forceps. 在检查新生儿出生记录和对护理人员所做回访 中，我们能发现在分娩中是否存在有过度使用 胎头吸引器或者产钳使用不当的情况。
ateq f620 快速入门指南说明书

ateq f620 快速入门指南说明书

Quick Start Guide2 /02Table of contents Safety advisory / WarrantyGood practices and safety instructionsAir quality requirementsPreambleATEQ F620, a universal leak testerLeak testPrinciple of a cycleYour ATEQ F620Front panelConnectors on the back panel (with all options) Power supply connectorsDigital linksAnalog outputsDigital inputs/outputsPneumatic connectorsPneumatics configurationUser interfaceOverviewKeysDisplayStarting upPower upPreparing a programModifying a parameterSelecting a programStarting and stopping current cycleUser adjustmentsOptions of the menus SpecificationsCharacteristicsQSG_F620.283.00_EN_01 / 2019-02-20QSG_F620.283.00_EN_01 / 2019-02-203 /46ATEQ Manufacturer Plants - Measurement Solution, Global Leader.We continuously work on improving our products. This is why information contained in this manual, the device and the technical specifications may be modified without prior notification.Pictures and figures in this manual are non-contractual.Safety advisory / Warranty4 /46Safety recommendationsIf the device is supplied with 100 / 240 V AC, it is mandatory to connect it to the ground witha good link to the ground, to protect against electric hazard or electrocution.It is dangerous to change the status of the outputs.They can control power actuators or other equipment (mechanical, pneumatic, hydraulic,electrical or other) which can cause serious personal injury and damage to surrounding material.For safety and quality measurement reasons, it is important, before powering on the device,to ensure that it is air supplied with a minimum operating pressure (0.6 MPa (87 PSI) ± 15%).Recommendations for the test environmentKeep the test area as clean as possible.Recommendations for operatorsATEQ recommends that the operators who use the devices have training and a level ofqualification that correspond to the job to perform.General recommendations—Read the user manual before using the device.—All electrical connections to the device must be equipped with safety systems (fuses, circuit breakers, etc.) adapted to the needs and in accordance with the applicablestandards and rules.—To avoid electromagnetic interference, electrical connections to the device must be shorter than 2 meters.—Power supply plug must be grounded.—Disconnect the device from the mains before performing any maintenance work.—Shut off the compressed air supply when working on the pneumatic assembly.—Do not open a connected device.—Avoid splashing water on the device.ATEQ is at your disposal for any information concerning the use of the device undermaximum safety conditions.We draw your attention to the fact that ATEQ cannot be held responsible for anyaccident related to a misuse of the measuring instrument, the workstation or non-compliance of the installation with safety rules.In addition, ATEQ declines any responsibility for the calibration or the fitting of theirinstruments that is not done by ATEQ.ATEQ also declines any responsibility for any modification (program, mechanical orelectrical) of the device done without their written consent.QSG_F620.283.00_EN_01 / 2019-02-20QSG_F620.283.00_EN_01 / 2019-02-205 / 46The air supplied into the device must be clean and dry. Even though the device is provided with a filter, the presence of dust, oil or impurities may cause malfunction.Air quality requirements according to ISO standard 8573The air must be clean and dry.The presence of impurities, oil or humidity in the air may cause deterioration which will not be covered by the warranty.When the instrument is working in vacuum conditions, impurities must be prevented from being drawn into its internal components.For this purpose we strongly recommend that a suitable airtight filter is installed between the part under test and the instrument.ATEQ recommends the following characteristics for the air supplied into the device.Recommended additional equipmentATEQ recommends the installation of this additional equipment: —Air dryer to provide dry air at less than - 40°C dew point —25 micron and 1/100 micron double filter.Preamble6 /46ATEQ F620 is a leak detector that tests the airtightness of parts.ATEQ F620 can memorise 128 different test programs.QSG_F620.283.00_EN_01 / 2019-02-20QSG_F620.283.00_EN_01 / 2019-02-207 / 46Direct measurement principleThe part under test 3 and the reference part 5 are filled to an identical pressure.A differential sensor 4 measures the pressure variation between the part under test 3 and the reference part 5. In some applications, the reference part can be replaced by a cap.1 Device2 Pressure sensor3 Part under test4 Differential pressure sensor5 Reference partDesensitized testThis mode is used for the measurement of large leaks, when the reject level required isabove the full scale of the differential sensor.The test pressure is applied to the input of the test part 3.The measurement is performed by the pressure sensor 2.1 Device2 Pressure sensor3 Part under test4 Cap on the reference connectorOther types of test are available in option (Burst test, Volume, Operator...).The measurement cycle is made of 4 main phases: fill, stabilization, test, dumping.8 /460Waiting phase1Fill phase2Stabilization phase3 Test4 DumpingQSG_F620.283.00_EN_01 / 2019-02-20Your ATEQ F6209 /46 The user interface is located on the front panel.1 Display2Cycle keys3Navigation keys4USB connectors5Quick connector6Mechanical RegulatorFor more information, refer to User interfaceQSG_F620.283.00_EN_01 / 2019-02-2010 /46QSG_F620.283.00_EN_01 / 2019-02-2011 /46 * These connectors are not operational. They are provided for future development of ourdevices.12 /46The device can be connected to an external power supply (24 V DC - 2 A) or provided with an internal power supply (100 / 240 V AC) (option).External supply24 V DC connector (J7)The device can be connected to a 24 V DC - 2 A power supply through a M12 4 pins type connector.24 V DC on the relay board connector (J11) (option)The device can also be connected to a 24 V DC - 2 A power supply through J11 connector on the relay board.Apply 24 V DC to the pin 2 or 4.13 /46Internal supply only100 / 240 V AC connector (J7) (option)The device can be connected to a 100 / 240 V AC power supply (option).This connector has a ON/OFF button.It is mandatory to connect the device to the ground with a good link to the ground, to protectagainst electric hazard or electrocution.1 ON 0 OFF14 /46PC USB connectors (on front face)USB connectors can be used for connecting miscellaneous compatible USB devices.The USB connectors are located under the rubber cover 1 (see figure).1Rubber cover2USB connector to PC3USB connector to USB keyDo not connect two USB devices at the same time.Do not use a cable longer than 2 m.Push the rubber cover 1 slightly forward for an easy access to USB connectors 2 and 3.Only use this connection for temporary communication. Connection to a PC cannot be usedpermanently because the communication can be disconnected by the PC.Printer RS232 connector / Modbus (option) or Profibus (option)(J12)RS232 - SubD 9 pins male connector (printer)RS232 for printer, bar code reader, PC connection.15 / 46RS232 - SubD 9 pins female connector (Profibus) optionProfibus: SubD 9 pins female connector.Devicenet connectors (J5) (J6) (option)M12 type connector - 5 pins male connector (J5) (Devicenet input)For connection to others ATEQ devices.M12 type connector - 5 pins female connector (J6) (Devicenet output)For connection to others ATEQ devices.Profinet connectors (J5 + J6) (option)M12 D coded type connector - 4 pins female connector (J5 + J6)16 /46Standard connection Ethernet TCP / IP protocol.—Ethernet IP—Profinet—Ethercat (J5 = Input J6 = Output).M12 type connector - 8 pins female connector (J1)17 /4618 /46The 24V DC power supply for the digital inputs can be provided by 2 means:—The internal power supply of the device (0.3A max)—An external power supply provided by the customer.Inputs default mode is PNP. NPN mode is available on request.Relay board connector (J11) (option)Characteristics—Inputs• Activation: + 24 V DC.—Outputs• Dry contacts• 60 V AC / DC max - 200 mA max.19 /46The device can be energized through the J11 connector of the relay board (except if internal supply option):0 V to the pin 1624 V DC to the pin 2 or 4.Program selection extension connector (J10) (option)The J10 connector is an extension of the J11 connector that enables the selection of 128 programs.Characteristics—Inputs•Activation: + 24 V DC.20 /46Program selection (J11 and J10)The connectors J11 and J10 (option) enable you to select a program from digital inputs.Combinations of connector pins to activate for program selection* X is equal to 0 or 1 in function of the program number.21 /46Valve codes and auxiliary outputs board connector (J9) (option)Characteristics—Outputs:• 24 V DC - 100 mA max per output. —Inputs:•Activation: + 24 V DC.22 /46Pneumatic connectors used to connect the part under test are located on the back panel of the device.Pneumatic supplyThe pneumatic supply has to meet specific requirements recommended by ATEQ. Refer to Good practices and safety instructions section.A specific filter may be necessary.The air is supplied via the filter located on the back panel of the device.Metal air filterThe metal filter is used for 1 MPa(145 PSI) range.The maximum pressure admissible is1.2 MPa (174 PSI).Plastic air filterThe plastic filter is used for 0.5 MPa(72.5 PSI) range (direct and indirectmodes) or 2 MPa (290 PSI) range (forpilot valves input).The maximum pressure admissible is690 kPa (100 PSI).23 /46Quick connector (on front face) (option)Use this function to check the calibration.As this connector is part of the measurement circuit, all its connections must be air tight.Test and reference outputsThe outputs enables parts to be connected (test and reference)1 Test connector2 Reference connector3 Not used4 Exhaust output5 Pressurization outputMetallic fitting available for test (1) and reference (2) connectors: —2.7/4 mm —3/5 mm —4/6 mm —6/8 mmDifferential sealed part connectors (option) (V1 and V2)External volume (closed tube) connection.Metallic fitting available for V1 and V2 connectors: —2.7/4 mm24 /46Other inputs / outputsThe outputs enables parts to be connected (test and reference)1Pilot pressure input or test pressureinput (according configuration)2Test pressure output (indirect mode)(from 0.5 to 2 MPa (72.5 to 290 PSI)according configuration)3Pneumatic input or output (accordingconfiguration)4Exhaust output (indirect mode)Pneumatic output 0.6 MPa (87 PSI) (option)A and B: automatic connectors option. Theseconnectors are used to drive pneumaticcaps on the part under test.Air supply input for optionsInstant fitting: 6 mm diameter—Vacuum input for vacuum range—External regulated pressure (accordingconfiguration)Calibration check by volume variation connector (option) (V)External volume (closed tube) connection.25 /46 Direct mode - Low rangePressure: from 0 to 500 kPa (72.5 PSI)ConnectionsDirect mode - Medium rangeMaximum pressure: 1 MPa (145 PSI)26 /46ConnectionsDirect mode - High rangeMaximum pressure: 2 MPa (290 PSI)27 /46ConnectionsIndirect modeMaximum pressure: 1 MPa (145 PSI)28 /46Connections29 /46Direct mode - Sealed part testPressure: from 0 to 500 kPa (72.5 PSI)This configuration can be used for test of small test part volumes.Protect volumes and pipes from air blowing and temperature variations.ConnectionsDirect mode - Sealed part differential volume testPressure: from 0 to 500 kPa (72.5 PSI)30 /46ConnectionsDirect mode - Option test check by pressure dropPressure: from 0 to 500 kPa (72.5 PSI)31 /46ConnectionsUser interface32 /46The user interface comprises a display and user keys located on the front panel.1 Display2Cycle keys3Navigation keysKEYSCycle keysThe cycle keys are used to start and to stop a measurement cycle.33 /46Navigation keysThe navigation keys are used to select menus/options and change parameter values.Smart keySmart key is a programmable key that provides direct access to a function selected bythe user.This key is programmable through the MAIN MENU screen:MAIN MENU > CONFIGURATION > MISCELLANEOUS > SMART KEYDISPLAYThe device uses 4 main screens.The Program screenUse the Program screen to select a test program.1Current program name (here NAME )2 Current program number (here 001)3 Test type (here LEAK TEST )Access at start-up of the instrument or by pressing several times Esc .34 /46The Measurement cycle screenThe Measurement cycle screen displays the different values of the current test (or last one).1Test pressure measurement2Test result or step phase3Test reject value4Vertical line test result5Remaining time of the current phaseor ready status6Leak measurement7Measurement unit8Current program9Vertical line test resultA star (*) can be displayed after the measurement unit 7 when the standard conditions function is validated.Refer to the Reference Manual.The MAIN MENU screenThe MAIN MENU screen gives access to different sections for managing the device and the test parameters.Access: from the Program screen, press .MAIN MENUSPE CYCLEPARAMETERSCONFIGURATIONSERVICERESUL TSUSB35 / 46Starting up1. Make sure that all the necessary connections are in place.Electrical: such as power supply, inputs/outputs Pneumatic: including line pressure supply2. Power up your device.When power-up is completed, the Program screen is displayed with last program used on screen.Use this procedure to configure a new test program.On the MAIN MENU screen:ACCESSING THE PARAMETERS1.Select PARAMETERSusing the up/downkeys and press .MAIN MENUPARAMETERS CONFIGURATIONSERVICE RESUL The program list is displayed.SELECTING A PROGRAM NUMBER 2. Select the program to configure and press.A list of the available measurement types is displayed: —LEAK TEST type—BLOCKAGE type (option)—DESENSITIZED TEST type (option) —OPERATOR type (option) —BURST TEST type (option) —VOLUME type (option)36 /46CONFIGURING THE ASSOCIATED MEASUREMENTS3.Select a measurement type and press .The parameters of the selected measurement type aredisplayed.4. Define the measurement cycle parameters.See: Modifying a parameter.Use this procedure to complete the test program setup.The protection of the parameters is configurable. If the icon is displayed at the bottom of the screen, you must insert the USB unlocking device or enter a password before modifying a parameter.On the PARAMETERS screen of the program (see: Preparing a program):1. Press up/down to select the parameter tomodify, and press.An arrow is displayed on the right of the parameterbeing modified.2. Use the up/down up/down keysto modify the parameter value, and press tovalidate.The arrowreturns to the left of the modified parameter.3. Repeat these steps until all parameters are set.4. To return to the MAIN MENU screen, press Escas many times as necessary.37 / 46If necessary, you can select another program.1. Pressup/down.Use the front panel keys to start/stop a measurement cycle.With the desired program displayed on the Program screen:STARTING A MEASUREMENT CYCLE 1. Press Start.The cycle phases of the program are successively displayed:FILLSTABILISATION TEST DUMPAt the end of the cycle, the results are displayed and READY appears at the bottom right of the screen.During the measurement cycle, you may press to access the MAIN MENU screen and set parameters for a next measurement cycle.STOPPING A CYCLE2. Press Reset to immediately stop the current measurement cycle and return to the Program screen.User adjustments38 /46Different menus are accessible on the MAIN MENU screen.For more information, refer to the Reference Manual.SPE CYCLE menuUse this menu to carry out specific procedures necessary to ensure the proper operationof specific measurement cycles (for example, adjustment of pressure regulator).Some parameters are displayed when specific functions are activated.39 /46TO START SPECIAL CYCLES...1. On the SPECIAL CYCLE MENU screen, select acycle, and press to validate.2. PressStart to start the cycle.3. To stop the current cycle pressReset .PARAMETERS menuUse this menu to configure the measurement cycle associated to each test program.Default parameters of the LEAKtype testsSome parameters are displayed when specific functions are activated.40 /46Additional functions41 /46Some functions are available depending on software version.42 /46CONFIGURATION menuUse this menu to configure your ATEQdevice.Start on the instrument frontpanel. Programs can only be started from the instrumentConfiguration of the assigned function to theSmart key43 /46SERVICE menuUse this menu to do the maintenance of your device (status check, internal tests...).RESULTS menuIn this section, manage measurements results.44 /46USB menuThis section describes save and restore parameters on an external USB device.Specifications45 /46 Technical characteristics of the device.Main characteristics46 /46。

英语高能怎么说

英语高能怎么说

英语高能怎么说高能的英语怎么说energetichigh-energyhigh grade energy例句Since the guides are usually made from materials having high-energy surfaces, wetting by polar lubricants usually is satisfactory由于导纱杆一般都是用具有高能表面的材料制备，一经极性润滑剂润滑，往往即达到饱和状态。

With the higher olefins, stabilization of the energetic intermediates occurs more easily在较高的烯烃情况下，高能的中间物稳定作用更易出现。

collision between high energy proton and nucleus高能质子同核的碰撞high energy nucleus - nucleus collisions高能核-核碰撞John is tall enough to reach the ceiling.约翰个子高能摸到天花板Refined Measurement and Signal Analysis Techniques in Vacuum and Plasma-Filled High Power Microwave Sources; Final technical优化的测量和信号分析技术在真空和等离子体填充的高能微波源方面的应用The process of photoluminescence refers to the radioative recombination of electronhole pairs generated by shining high energy light on a crystal光致发光过程就是高能量光子照射到晶体上之后，晶体所产生的电子一空穴对的辐射复合过程。

The field of heterogeneous catalysis中文翻译

The field of heterogeneous catalysis中文翻译

Rational Design of Low-Temperature Hydrogenation Catalysts:Theoretical Predictions and Experimental Verification低温加氢催化剂的设计: 理论与实践The field of heterogeneous catalysis, specifically catalysis on bimetallic alloys, has seen many advances over the past few decades. One of the main goals of the catalysis industry is to develop new materials that have novel catalytic properties. Bimetallic catalysts, which often show electronic and chemical properties that are distinctly different from those of the parent metals, offer the opportunity to design new catalytic materials with enhanced activity, selectivity, and stability [1-2] . Currently bimetallic catalysts are widely utilized in many heterogeneous catalysis [3] and electro-catalysis [4] applications.In order to understand the origins of the novel catalytic properties, bimetallic surfaces have been the subject of many experimental and theoretical studies, as summarized in several reviews [5-7] . It is now well established that bimetallic surfaces often show novel properties that are not present on either of the parent metal surfaces [5-16] . The modification effect is especially important when the admetal coverage is in the sub monolayer to monolayer regime. However, it is difficult to know a priori how the electronic and chemical properties of a particular bimetallic surface will be modified relative to the parent metals. For this reason, the study of bimetallic surfaces in the field of catalysis has gained considerable interest. There are two critical factors that contribute to the modification of the electronic and chemical properties of a metal in a bimetallic surface. First, the formation of the hetero-atom bonds changes the electronic environment of the metal surface, giving rise to modifications of its electronic structure through the ligand effect. Second, the geometry of the bimetallic structure is typically different from that of the parent metals, e.g. the average metal-metal bond lengths change. This lattice mismatch leads to the strain effect that is known to modify the electronic structure of the metal through changes in orbital overlap [16] . While studies on model bimetallic surfaces provide fundamental insights into the novel properties, in an industrially relevant supported catalyst the active metal will be present in the form of nanoparticles. As shown in Fig.1, research efforts in our group involve three parallel approaches, with the goals being to bridge the“materials gap”and“pressure gap”between fundamental surface science studies and real world catalysis. In the current review we will utilize hydrogenation reactions as examples to demonstrate how the utilization of these three parallel approaches can lead to the rational design of bimetallic catalysts with novel low-temperature hydrogenation activities.Catalytic hydrogenations are among the most commonly practiced catalytic processes, ranging from common steps in organic synthesis, to batch processes in pharmaceutical production, to stabilization of edible oils, and to petroleum upgrading processes. Because hydrogenation reactions are typically exothermic, it is advantageous to carry out these reactions at low temperatures. In the current review we will first use the hydrogenation of cyclohexene to demonstrate the feasibility of increasing the low-temperature hydrogenation activity by reducing the binding energies of atomic hydrogen and cyclohexene, which can be achieved by designing bimetallic surfaces with specific surface structures. We will then discuss several other types of hydrogenation reactions to further illustrate the advantages of bimetallic catalysts in terms of both hydrogenation activity and selectivity.1 Structures of bimetallic surfacesIn the current review we will focus mainly on bimetallic surfaces by depositing one monolayer ofa 3d transition metal on either a Pt(111) single crystal or a polycrystalline Pt substrate. As shown in Fig.2, monolayer bimetallic surfaces can have three ideal configurations: a surface 3d-Pt-Pt(111) configuration, where the 3d monolayer grows epitaxially on the surface of the Pt substrate; an intermixed configuration, where the 3d atoms reside in the first two Pt layers to some varying degree; and the unique subsurface Pt-3d-Pt(111) configuration, where the first layer is comprised of Pt atoms and the second layer is occupied with the 3d metals.Procedures for the preparation of bimetallic surface structures under ultra-high vacuum (UHV) conditions have been described in detail previously [5] . For example, the Ni/Pt(111) bimetallic surfaces have been characterized using a wide range of experimental techniques and DFT modeling [17] . When Ni is deposited with the Pt(111) surface held at 300 K, Ni atoms stay on the top-most layer to produce the Ni-Pt-Pt(111) surface configuration. If this surface is subsequently heated to 600 K, or if the monolayer deposition of Ni occurs with the Pt(111) substrate held at 600 K, most of the Ni atoms diffuse into the subsurface region to produce the Pt-Ni-Pt(111) subsurface structure. Similar surface and subsurface structures have been obtained for several other 3d metals on the Pt(111) substrate [5,18] .The ab initio calculations in the current review were performed using the Vienna Ab initio Simulation Package (V ASP) version 4.6 [19-20] . The monolayer bimetallic systems were modeled on the closed-packed Pt(111) substrate. The PW91 functional was used within the generalized gradient approximation with an energy cutoff on the basis set of 396 eV. The bimetallic systems were modeled using a periodic 2*2 or 3*3 unit cell with four metal layers, with the slabs being separated by 6 equivalent layers of vacuum in the epitaxial direction. The top two layers were allowed to relax to the lowest energy configuration while the third and fourth layers were frozen at the bulk Pt-Pt distance. More details about the DFT modeling procedures on monolayer bimetallic surfaces can be found in a recent review [5] .2 Low-temperature hydrogenation of cyclohexeneCyclohexene is used as a probe molecule to study the hydrogenation because cyclic hydrocarbons are important reaction intermediates in many refinery and petrochemical processes, in addition to serving as building blocks for many chemicals produced in the chemical industry. Furthermore, cyclohexene has several competitive reaction pathways, including decomposition, dehydrogenation, disproportionation (self-hydrogenation), and hydrogenation. Comparative studies of these reaction pathways provide an opportunity to determine how the hydrogenation activity and selectivity are affected by the formation of bimetallic surfaces.2.1 DFT and experimental studies on single crystal surfacesOne hypothesis for promoting the low-temperature hydrogenation of alkene is that both reactants, atomic hydrogen and alkene, should bond relatively weakly on the catalyst surface to facilitate the hydrogenation steps. DFT calculations were performed to estimate the values of hydrogen binding energy (HBE) on several 3d-Pt-Pt(111) and Pt-3d-Pt(111) surfaces, as shown in Fig.3A [18] . Fig.3A reveals that HBE is related to the position of the surface d-band center with respect to the Fermi level, in agreement with the trend observed in previous studies for other surfaces [5] . In general, the addition of a 3d metal surface layer on Pt(111)moves the d-band center closer to the Fermi level as compared to the bulk 3d metals. This is primarily due to the tensile strain induced by the Pt lattice as the ligand effect is the weakest between late transition metal over layers and the Pt(111) substrate [17] . Conversely, subsurface 3d metals shift the surface d-band center of Pt away from the Fermi level as compared to that of Pt(111), mainly due to the electronic interactionof Pt and the subsurface 3d atoms [17] . The comparison in Fig.3A demonstrates that HBE typically follows the trend of 3d-Pt-Pt(111)>Pt(111)> Pt-3d-Pt(111). In addition, the nearly linear correlation between HBE and the surface d-band center should enable one to predict HBE on other bimetallic surfaces based on the extensive database of d-band center values for many bimetallic surfaces [5] .In addition to the trend in the correlation of HBE with surface d-band center, the binding energies of unsaturated hydrocarbons, such as cyclohexene, follows the same trend as HBE. As shown in Fig.3B, DFT calculations reveal that the Pt-3d-Pt(111) subsurface structures bond to cyclohexene more weakly than Pt(111) and the corresponding 3d-Pt-Pt(111) surface structures [18] . For example, DFT results indicate that both cyclohexene and atomic hydrogen are more weakly bonded on Pt-Ni-Pt(111) than on Ni-Pt-Pt(111), Pt(111) and Ni(111), suggesting that the subsurface Pt-Ni-Pt(111) structure should be more effective in the hydrogenation of cyclohexene than the surface structure. This has been confirmed experimentally by comparing the hydrogenation activity of cyclohexene using temperature programmed desorption (TPD), as shown in Fig.4 [18] . As illustrated in the TPD peak area of the cyclohexane product, the subsurface Pt-Ni-Pt (111) structure shows the highest hydrogenation yield, with the desorption peak centered at a very low temperature of 203 K. Similar bimetallic surface structure can also be produced by depositing one monolayer of Pt on a Ni(111) substrate, which also possesses the novel low-temperature pathway for cyclohexene hydrogenation [21] .The trend in the DFT calculations in Fig.3B also shows that the binding energy of cyclohexene on Pt-Co-Pt(111) and Pt-Fe-Pt(111) is even weaker than that on Pt-Ni-Pt(111). Although this might suggest that the former two surfaces would be more active toward the hydrogenation than Pt-Ni-Pt(111), one should keep in mind that the adsorption of cyclohexene needs to be strong enough for the hydrogenation to take place. One would therefore expect to observe a volcano relationship for the hydrogenation activity as the d-band center moves further away from the Fermi level, i.e., when the adsorption of cyclohexene becomes too weak for the hydrogenation to occur. This is verified experimentally in the results shown in Fig.5. The hydrogenation yield from TPD measurements is the highest on Pt-Ni-Pt(111), but starts to decrease on the Pt-Co-Pt(111) and Pt-Fe-Pt(111) surfaces, where the binding of cyclohexene becomes too weak for hydrogenation to occur. On the other side of the volcano curve, the binding energies of cyclohexene on the 3d-Pt-Pt(111) surfaces are too strong, preventing the effective hydrogenation of cyclohexene [18] 2.2 Polycrystalline bimetallic surfacesIndustrial catalysts are often supported nanoparticles of varying shape and size. Polycrystalline bimetallic films provide a potential way to bridge the “materials gap”between single crystal surfaces and supported catalysts. As illustrated in Fig.6, it is possible to assume that the surface chemistry of the nanoparticle should be dominated primarily by the first few atomic layers. It is also reasonable to assume that the chemistry of the individual crystal facets on the nanoparticle (primarily (111) and (100) for an FCC nanoparticle) can be approximated by their respective single crystal extension [22] .With these assumptions we have investigated the chemical properties of 3d-Pt bimetallic structures prepared on a polycrystalline Pt film that contained mainly the (111) and (100) facets. Similar to Pt(111), monolayer Ni was deposit on a Pt foil at room temperature to produce the Ni-Pt-Pt surface structure, followed by annealing to higher temperatures to obtain the Pt-Ni-Pt subsurface structure [22] . The TPD results of the hydrogenation of cyclohexene are shown inFig.7. Similar to the corresponding single crystal surfaces, the subsurface Pt-Ni-Pt polycrystalline structure shows significantly higher hydrogenation activity than that from the polycrystalline Pt and Ni surfaces. These results confirm the assumption that the trend observed on single crystal bimetallic surfaces can be extended to the polycrystalline counterparts.2.3 Thermodynamic stability of bimetallic surfaces under hydrogenation conditionsBefore extending the surface science results to supported catalysts, it is important to verify that the desirable Pt-Ni-Pt subsurface structure is the thermodynamically preferred configuration under hydrogenation conditions. As demonstrated in several recent studies, including single crystal surfaces [23-24] , polycrystalline films [22,25] and supported catalysts [26] , the thermodynamically preferred Pt-Ni bimetallic structure is directly related to the chemical environment present on the surface. Fig.8 shows the DFT predicted potential for segregation for a 3d metal atom to segregate from the subsurface to the surface of Pt(111). These values were calculated for the environments of vacuum, and with 0.5 monolayer (ML) atomic hydrogen and 0.5 ML atomic oxygen, using procedures described previously [24] . The thermodynamic potential for segregation is defined as follows:where ΔE seg is the thermodynamic potential for segregation per Pt-3d pair, E A/3d-Pt-Pt is the total energy for the surface configuration with adsorbate A, E A/Pt-3d-Pt is the total energy for the subsurface configuration with adsorbate A, and M is the total number of Pt-3d pairs per unit cell. As defined in a previous publication [22] , a positive ΔE seg value indicates that the subsurface Pt-3d-Pt is more stable. The DFT results in Fig.8 predict that for the reducing environment of vacuum and 0.5 ML atomic hydrogen, the subsurface configuration is thermodynamically preferred, whereas in 0.5 ML atomic oxygen the surface configuration is preferred. There is a nearly linear trend between ΔE seg and the difference in d-band, ΔƐd , which leads to a generalized equation in predicting the thermodynamic stability of a wide range of bimetallic surfaces [24] . Because the environment of hydrogenation reactions is similar to that of the reducing environment, with the bimetallic surface being partially covered by hydrogen, the results in Fig.8 suggest that the desirable subsurface Pt-Ni-Pt configuration should be thermodynamically stable, making it possible to extend model surfaces to supported catalysts for hydrogenation reactions.2.4 Synthesis and evaluation of supported catalystsSupported monometallic Pt and bimetallic Ni-Pt and Co-Pt catalysts were synthesized on γ-Al 2 O 3 using the incipient wetness method [27-28] . The catalysts were characterized using a variety of techniques, including extended X-ray absorption fine structure (EXAFS). The utilization of EXAFS is critical in these studies because it provides direct information on the extent of bimetallic bond formation based on the coordination numbers of Ni-Pt and Co-Pt under in-situ reaction conditions. As summarized in Table 1, the detection of the Ni-Pt and Co-Pt nearest neighbors confirms that bimetallic bonds are indeed produced on the supported catalysts [28] . The supported catalysts were evaluated using both batch and flow reactors to determine the reaction kinetics of the hydrogenation of cyclohexene at a low temperature of 303 K [28] . Fig.9 shows the production of cyclohexane from cyclohexene on Pt/γ-Al 2 O 3 , Co-Pt/γ-Al 2 O 3 , and Ni-Pt/γ-Al 2 O 3 , using a batch reactor equipped with Fourier transform infrared (FTIR)spectroscopy. The solid lines are fittings using the Langmuir-Hinshelwood model, resulting in a rate constant of 1.7, 21 and 24 min -1 for supported Pt, Co-Pt, and Ni-Pt, respectively [28] . The trend observed in the rate constant of cyclohexene hydrogenation is consistent with that from the single crystal surfaces for the same reaction, Ni-Pt>Co-Pt>Pt, as shown earlier in the volcano curve in Fig.5. The observation of the similar trend between model surfaces and supported catalysts provides an important demonstration of the rational design of bimetallic catalysts from combined theoretical and experimental approaches.3 Research opportunities in bimetallic catalysis3.1 Low-temperature hydrogenation reactionsWe have applied similar combined approaches for the design of bimetallic catalysts for the low-temperature hydrogenation of several types of hydrocarbon molecules. Below we will provide several examples of hydrogenation reactions that are of both fundamental and practical importance.Hydrogenation of acrolein. Studies of the selective hydrogenation of unsaturated aldehydes, such as αβ-unsaturated aldehydes, have been of growing interest for the production of fine chemical sand pharmaceutical precursors [29] .The hydrogenation of the C=C and/or C=O bonds in unsaturated aldehydes offers the possibility to improve both the hydrogenation activity and selectivity through the formation of bimetallic surfaces. Using the hydrogenation of acrolein as a probe reaction, we demonstrated that the selective hydrogenation of the C=O bond can be achieved through the formation of the subsurface Pt-Ni-Pt(111) and Pt-Co-Pt(111) bimetallic structures [30-31] .Hydrogenation of benzene.The hydrogenation of benzene to cyclohexane is of significant importance in the petroleum industry and for environmental protection. The process of benzene hydrogenation has been utilized commercially for the production of cyclohexane, which is one of the key intermediates in the synthesis of Nylon-6 and Nylon-66 [32] . We have identified Co-Pt bimetallic catalysts as promising materials for the hydrogenation of benzene at a relatively low temperature of 343 K [28,33] . For example, Table 2 summarizes the batch and flow reactor results of benzene hydrogenation on several Co-based bimetallic catalysts. The Co-Pt catalyst shows the highest rate constant and lowest activation barrier for the hydrogenation of benzene, which is consistent with the relatively weak binding energies of atomic hydrogen and benzene from DFT calculations [33] . In addition, the catalyst support also plays a role in controlling the hydrogenation activity of Co-Pt catalysts [34]Selective hydrogenation of acetylene in ethylene. The selective hydrogenation of acetylene in the presence of ethylene is an important reaction because acetylene poisons the catalysts in ethylene polymerization reactions [35-36] . By supporting Pd-Ag bimetallic catalysts on ion-exchanged β-zeolites, we observed a synergistic effect that led to a higher selectivity for acetylene hydrogenation in the presence of excess ethylene [37] .The increase in the hydrogenation selectivity is attributed to a combination of an enhanced π-cation interaction between acetylene and zeolite at low temperatures and the ability of the Pd-Ag bimetallic catalysts to perform hydrogenation at such low temperatures [37] .3.2 Reducing bulk Pt in bimetallic catalysts with metal carbidesAs demonstrated in Figs.3-5, the subsurface Pt-Ni-Pt structure is desirable to enhance the activity and selectivity of the hydrogenation of unsaturated hydrocarbons. However, if elevated temperatures are required for reactions, the subsurface Ni atoms start to diffuse into bulk Pt,leaving a monometallic Pt surface and therefore the disappearance of the enhanced bimetallic hydrogenation activity [17,22] . In addition, as shown in Fig.8, adsorbates such as oxygen can cause the subsurface Ni atoms to segregate to the surface, forming the Ni-Pt-Pt surface that is not active for hydrogenation reactions. One idea to overcome such inherent instability of Pt-Ni-Pt is to replace the bulk Pt with an alternative substrate, such as transition metal carbides that often show catalytic properties similar to Pt [38-44] . We have explored the utilization of tungsten monocarbide (WC) to produce the Pt-Ni-WC structure [45] . As WC has been shown to be an effective diffusion barrier layer [46] , thermal deactivation due to Ni diffusion will be alleviated. Furthermore, it is also possible that the WC substrate would anchor Ni by the formation of W—Ni or C—Ni bonds to prevent its segregation to the surface in an oxygen-rich environment. Fig.10 shows a comparison of the hydrogenation of cyclohexene from Pt-Ni-Pt and Pt-Ni-WC surfaces [45] . The Pi-Ni-WC surface shows higher hydrogenation activity, which is consistent with parallel DFT calculations [45] . The promising results in Fig.10 suggest the possibility to synthesize a more active and stable Pt-Ni-WC hydrogenation catalyst with much lower loading of Pt than that in Pt-Ni-Pt.3.3 Production of hydrogen using bimetallic catalystsAll examples presented above are for hydrogenation reactions, which can be classified as hydrogen-consuming reactions and require catalysts to bond to atomic hydrogen and adsorbates relatively weakly. Using the surface d-band center argument in Fig.3, these reactions are preferred on bimetallic catalysts with surface d-band center away from the Fermi level, such as the Pt-3d-Pt subsurface structures. In contrast, for hydrogen-producing reactions, the desirable catalysts should be those that bond to hydrogen and adsorbates more strongly, i.e., with d-band center closer to the Fermi level, such as the 3d-Pt-Pt surface structures. This hypothesis has been confirmed experimentally in our recent studies for the production of H 2 from the reforming of biomass derived molecules, including ethanol, ethylene glycol and glycerol on the Ni-Pt-Pt(111) surface [47] . Similarly, the Ni-Pt-Pt(111) surface also shows a very high activity for H 2 production from the decomposition of ammonia [48] .We have also demonstrated that coking of the catalyst surfaces, a common deactivation mechanism in dehydrogenation reactions, can be reduced by the formation of bimetallic surfaces [49] . These results further demonstrate the possibility of designing bimetallic catalysts from combined theoretical predictions and experimental verification.4 ConclusionsThe field of catalysis is undergoing a revolution in the selection process of catalytic materials, from the traditional“trial-And-error”method to the“rational design”approach, with the latter requiring atomic level understanding of the catalyst structures and reaction mechanisms. In the current review we utilized several hydrogenation reactions to demonstrate the importance of combining theoretical and experimental approaches for designing bimetallic structures with desirable catalytic properties. In addition, the examples also illustrated the possibility to bridge the “materials gap”and “pressure gap”between fundamental studies on single crystal surfaces and catalytic evaluation of supported catalysts. Similar approaches can be adopted for the rational design of bimetallic catalysts beyond hydrogenation reactions.多相催化领域，特别是在催化作用双金属合金，在过去的几十年里看到了许多进展。