Mechanistic Aspects of the H2-SCR of NO on a Novel Pt/MgO-CeO2Catalyst
Costas N.Costa and Angelos M.Efstathiou*
Department of Chemistry,Heterogeneous Catalysis Laboratory,Uni V ersity of Cyprus,
P.O.Box20537,CY1678,Nicosia,Cyprus
Recei V ed:August2,2006;In Final Form:December6,2006
Steady State Isotopic Transient Kinetic Analysis(SSITKA)coupled with Temperature-Programmed Surface
Reaction(TPSR)experiments,using on line Mass Spectroscopy(MS)and in situ DRIFTS have been performed
to study essential mechanistic aspects of the selective catalytic reduction of NO by H2under strongly oxidizing
conditions(H2-SCR)at140°C over a novel0.1wt%Pt/MgO-CeO2catalyst for which patents have been
recently obtained.The nitrogen paths of reaction from NO to N2and N2O gas products were probed by
following the14NO/H2/O2f15NO/H2/O2switch(SSITKA-MS and SSITKA-DRIFTS)at1bar total pressure.
It was found that the N-pathways of reaction involve two different in structure active chemisorbed NO x species,
one present on the MgO and the other one on the CeO2support surface.The amount of these acti V e NO x
intermediate species formed was found to be14.4μmol/g,corresponding to a surface coverage ofθ)3.1
(based on Pt metal surface)in agreement with the SSITKA-DRIFTS results.A large fraction of it(87.5%)
was found to participate in the reaction path for N2formation,in harmony with the high N2selectivity(82%)
exhibited by this catalyst.Inactive adsorbed NO x species were also found to accumulate on both Pt and
support(MgO and CeO2).The mechanism of reaction must involve a H-spillover from the Pt metal to the
support surface(location of active NO x species).It was proven via the NO/H2/16O2f NO/H2/18O2(SSITKA-
MS)experiment that gaseous O2does not participate in the reaction path of N2O formation.
Introduction
Nitrogen oxides(NO x)are mainly derived from the combus-tion of fossil fuels such as coal and petroleum in power plants and many industrial sites in which thermal energy is produced, and also from the use of gasoline and diesel in transportation vehicles.To meet strict emission regulations,removal of NO x from the flue gas of stationary polluting sources such as power plants,industrial boilers,gas turbines,biomass incinerators,and gasifiers is required.1Selective catalytic reduction of NO x with ammonia under strongly oxidizing conditions(NH3-SCR)was first introduced in1973.1,2Since then,NH3-SCR is a widely practiced NO x control technology in many combustion facilities with a total capacity of more than70GW in both Japan and Germany.1,3However,many problems are encountered in the use of NH3-SCR technology,3,4namely catalyst deterioration, NH3-slip(emissions of unreacted toxic ammonia),ash odor,air heaters fouling,and a high running cost.
Reduction of NO x by hydrocarbons under strongly oxidizing conditions(HC-SCR)has been extensively studied as recently reviewed.5However,severe catalyst deactivation has not yet made possible the development of an industrial HC-SCR technology,6and more importantly,no suitable catalytic system has been yet found at temperatures lower than160°C.7In addition,the excess of hydrocarbon required ends up polluting the atmospheric air with hydrocarbon that must be burned,but it will produce more CO2.8Today’s emerging needs for the development and use of green technologies and,in particular, those leading to reducing emissions of CO2(a greenhouse gas contributing to global warming)to the atmosphere,along with the problems faced by the use of NH3-SCR,3,4demand the finding of appropriate non-carbon-containing reducing agents for the catalytic elimination of NO x from industrial flue gas streams,and preferably at low temperatures(T<160°C)for reduced running and investment costs.
Today,there are considerable efforts worldwide for moving toward a hydrogen economy,since hydrogen is being largely considered as an ideal energy carrier for the development of a sustainable energy system to overcome the energy supply shortage,respect the environment,and reduce CO2emissions.9 Therefore,hydrogen availability in the industrial sector is expected to further increase,whereas hydrogen cost is expected to be reduced in the coming years.Hydrogen was reported to be very efficient in the catalytic conversion of NO into N2,10-12 and it could potentially be used to reduce NO x emissions.Until full transition to a hydrogen economy and zero emissions of greenhouse gases are achieved,the selective catalytic reduction of NO by H2in the presence of strongly oxidizing conditions (H2-SCR)might be considered as a breakthrough NO x control technology in favor of the present NH3-SCR.
Recent works from our laboratory13-15have demonstrated that remarkably high N2selectivity values(S N
2
)80-92%)for the H2-SCR of NO in the100-250°C range can be obtained over Pt supported on mixed oxidic and perovskite-type materials (e.g.,Pt/MgO-CeO2,Pt/La-Sr-Ce-Fe-O,Pt/La-Ce-Mn-O).In addition,a very wide temperature window of operation and a positive effect of5vol%of H2O present in the feed stream on N2yield have been observed.13-15It was shown that the catalytic behavior of the above-mentioned supported-Pt catalysts appears to be the best ever reported16-26for the H2-SCR of NO in the presence of5vol%of H2O and/or20-40 ppm of SO2in the feed stream at T<200°C.For the Pt/MgO-CeO2catalytic system,patents have been recently obtained.27
*Address correspondence to this author.Phone:+35722892776.
Fax:+35722892801.E-mail:efstath@ucy.ac.cy.
3010J.Phys.Chem.C2007,111,3010-3020
10.1021/jp064952o CCC:$37.00?2007American Chemical Society
Published on Web01/26/2007
A detailed study of the catalytic performance of the present Pt/ MgO-CeO2for the low-temperature H2-SCR of NO has been reported.28
Steady State Isotopic Transient Kinetic Analysis(SSITKA)29-32 has long been documented and widely accepted as the most powerful technique in performing in situ mechanistic studies for heterogeneous gas-solid catalytic reactions,especially at1 bar or higher pressures.SSITKA consists of following the composition of the outlet of a reactor initially at steady state when one of the reactants is suddenly replaced by the same molecular species but with one of its atoms replaced by one of its stable isotopes.In particular,SSITKA experiments on de-NO x reactions have been published for NO/NH3/O2over V2O5/ TiO2,32,33NO/CH4/O2over Pd/TiO234and CaO-La2O3sys-tems,35NO/C3H6/O2over Pt/SiO2,36,37NO/H2over Pt/SiO2,38,39 and NO/H2/O2over Pt/La-Ce-Mn-O40and Pt/SiO240,41cata-lysts.In the latter work,41for the first time the concentration of adsorbed active intermediate NO x species formed during H2-SCR and aspects of the reaction pathways of N2and N2O formation were obtained.However,no experimental evidence was provided about the chemical structure of the active NO x or the existence of likely spectator adsorbed NO x species.Such information was recently reported for the H2-SCR of NO over Pt/La-Ce-Mn-O and Pt/SiO2catalysts.40
In the present work,SSITKA experiments with the use of both15NO and18O2stable isotopes were conducted with in situ Mass Spectrometry and Diffuse Reflectance Infrared Fourier Transform Spectroscopy(DRIFTS)on a0.1wt%Pt/MgO-CeO2catalyst to better understand its remarkable catalytic performance27,28by determining(a)the chemical structure of adsorbed acti V e and inacti V e(spectator)NO x species,(b)the surface coverage of active NO x under H2-SCR working reaction conditions,(c)the location(Pt or support)of active NO x species, and(d)the reactivity toward hydrogen gas of the various adsorbed NO x species formed during H2-SCR. Experimental Section
Catalyst Preparation and Characterization.The Mg-Ce-O solid(50wt%MgO-50wt%CeO2)was prepared by using the sol-gel method following the experimental procedures reported by Balakrishnan et al.42and after using Mg(EtO)2and Ce(NO3)3·H2O(Aldrich)as precursors of Mg and Ce,respec-tively.Selective pre-sulfation of the MgO-CeO2support was carried out that dramatically enhanced catalyst stability in the presence of20-40ppm SO2in the feed stream.27,28The0.1wt %Pt/MgO-CeO2was prepared by the incipient wetness impregnation method with use of H2Pt(IV)Cl6(Aldrich).After impregnation and drying overnight at120°C,the catalyst was calcined in air at600°C for2h.The crystal structure of the prepared Mg-Ce-O solid was checked by X-ray diffraction (XRD)analysis(SIEMENS Diffract500system,Cu K R radiation(λ)1.5418?)).
High-Resolution Transmission Electron Microscopy(HR-TEM)studies(JEOL1010electron microscope operated at an acceleration voltage of80kV)were performed on the fresh solid catalyst after calcination in air at600°C for2h followed by reduction in H2at300°C for2h.A5mg powder sample was dispersed in1mL of ethanol/water mixture(1:1)and kept in an ultrasonic bath for5h.The sample was then deposited on a carbon-covered copper grid and dried at25°C.These measure-ments allowed direct estimation of the mean Pt particle size and details of the morphology of support phases.Pt dispersion and mean particle size were also determined by H2chemisorp-tion according to the following procedure,and taking into account the observations made by Gatica et al.43After calcina-tion in a20%O2/He gas mixture at600°C for2h,the catalyst was reduced in H2(1bar)at300°C for2h.The feed was then changed to He and the temperature was increased to500°C in He flow until no hydrogen desorption was observed.A possible H-spillover that might have taken place at300°C was eliminated by the latter procedure.The reactor was then quickly cooled in He flow to25°C and the feed was changed to a1%H2/He gas mixture for30min.The feed was then changed back to He and kept at25°C for10min before the temperature of the catalyst was increased to600°C( )30deg/min)to carry out a TPD experiment.From the amount of hydrogen desorbed,the amount of Pt in the sample,and assuming H/Pt s)1:1,the dispersion of Pt was estimated.
Transient Mass Spectrometry.Transient isotopic and tem-perature-programmed surface reaction(TPSR)experiments were conducted in a specially designed transient gas flow system and a quartz microreactor that have been described.44,45For transient experiments,0.15g of catalyst was used and the total flow rate was30NmL/min.Chemical analysis of the gas effluent stream of reactor during transient experiments was performed with an on line quadrupole mass spectrometer(Omnistar,Balzers).The latter was equipped with a fast response inlet capillary/leak valve (SVI050,Balzers)and data acquisition systems.The gaseous response signals obtained by mass spectrometry were calibrated against standard gas mixtures.
Before any measurements were taken,the supported-Pt catalyst was calcined in5%O2/He for2h at600°C followed by reduction in10%H2/He gas mixture at300°C for2h.The feed was then switched to pure He at300°C for15min and the reactor was cooled in the appropriate temperature of the experiment to be followed.Table1describes the necessary sequence of steps performed for each kind of isotopic transient experiment conducted.The last step is that during which measurements by on line mass spectrometer were recorded. SSITKA(Steady State Isotopic Transient Kinetic Analysis) experiments involved the switch of the NO/H2/O2/Ar/He feed to an equivalent in isotopic composition15NO/H2/O2/He or NO/ H2/18O2/He feed gas mixture after steady state was achieved. The argon(Ar)gas was used as a tracer,the decay of which was used to monitor the gas phase hold-up of the system.32 Following the switch14NO/H2/O2/Ar/He f15NO/H2/O2/He,the mass numbers(m/z)2,28,29,30,31,32,40,44,and45used for H2,14N2,14N15N,14NO,15NO,O2,Ar,14N2O,and14N15NO, respectively,were continuously monitored.In the case of the switch NO/H2/16O2/Ar/He f NO/H2/18O2/He,the mass numbers (m/z)2,30,32,34,36,40,44,and46used for H2,NO,16O2, 16O18O,18O2,Ar,N216O,and N218O,respectively,were continu-
TABLE1:Sequential Step Changes of Gas Flow during Transient Isotopic Experiments Conducted on the0.1wt%Pt/
MgO-CeO2Catalyst
experiment
code sequence of step changes of gas flow over the catalyst sample
A14NO/H2/O2/Ar/He(30min,140°C)f15NO/H2/O2/He(140°C,t)(SSITKA experiment)
B14NO/H2/O2/Ar/He(30min,140°C)f15NO/He(15min,140°C)f He(5min,140o C)f
cool quickly to room temperature f TPSR in10%H2/He
C NO/H2/16O2/Ar/He(30min,140°C)f NO/H2/18O2/He(140°C,t)(SSITKA experiment) Mechanistic Studies of the H2-SCR of NO J.Phys.Chem.C,Vol.111,No.7,20073011
ously monitored.Details of the mass spectrometry analysis of the reactor gas effluent have been reported.13,44It should be noted that neither NH3nor any NO2gas product was measured under the present H2-SCR of NO reaction conditions.
The reaction mixture contained0.25%NO,1%H2,5%O2, and He as balance gas.In the case that Ar gas was used as a tracer in the reaction mixture,1%He was replaced with1% Ar.The15NO/He gas mixture used in Expt.B(Table1) contained0.25%15NO gas,and it was prepared by using1mol %of15NO/He isotopic mixture(ISOTEC,https://www.doczj.com/doc/c73445267.html,A).The NO/ H2/18O2/He gas mixture used in the SSITKA experiments(Expt. C,Table1)contained2mol%18O2isotope gas,and it was prepared by using a3mol%18O2isotopic gas mixture (ISOTEC,https://www.doczj.com/doc/c73445267.html,A,97atom%18O).
In Situ Transient DRIFTS.Diffuse Reflectance Infrared Fourier Transform(DRIFT)spectra were recorded on a Perkin-Elmer GX FTIR spectrophotometer at a resolution of1cm-1 and with high-temperature/high-pressure temperature control-lable DRIFTS cells(Spectra Tech and Harrick Scientific) equipped with ZnSe IR windows.About30mg of catalyst sample in powder form were used for each experiment.The total flow rate was kept constant at50NmL/min.Before any DRIFTS experiments were conducted,the supported-Pt catalyst was pretreated in5%O2/Ar for2h at600°C followed by10% H2/Ar gas mixture at300°C for2h.The feed was then switched to pure Ar at300°C for15min,and the sample was cooled to the appropriate temperature of the experiment to follow.For FTIR single-beam background subtraction,the spectrum of the solid catalyst(after H2reduction at300°C)was taken in Ar flow at the appropriate temperature.DRIFTS spectra were collected at the rate of1scan/s and at1cm-1resolution in the 3000-800cm-1range.The averaged spectrum(50spectra were collected)was then recorded.Spectra were analyzed by using the instrument’s Spectrum for Windows software.
Results
Catalyst Characterization.The Pt dispersion,D(%)((μmol Pt s/μmol Pt)×100),of the investigated0.1wt%Pt/MgO-
CeO2fresh catalyst was found to be90%(4.6μmol Pt s/g of catalyst)with a mean particle size of about1.2nm(after using the following relationship:d Pt(nm))1.02/D).46A direct measurement of the Pt mean particle size and morphology was obtained through HRTEM images(Figure1a).A mean Pt size of about1.6nm was estimated for the used catalyst(after H2-SCR of NO in the100-400°C range)in good agreement with the H2chemisorption results obtained for the fresh catalyst. Figure1b shows HRTEM images of the MgO-CeO2solid support in which the interface formed between MgO and CeO2 crystallites is apparent.The composition of the latter two metal oxides was probed via X-ray diffraction analyses of the Mg-Ce-O support.The result of Figure1b is important since new NO x adsorption and catalytic sites might have been created along the interface of the two oxides as compared to those found on the surface of MgO and CeO2single crystallites.As will be discussed later on,acti V e intermediate NO x species are formed during H2-SCR of NO on the MgO-CeO2support within the Pt-support interface region.
In Situ DRIFTS Studies.The composition of the H2-SCR of NO feed gas mixture consisted of0.25vol%of NO,1vol %of H2,5vol%of O2,and Ar as a carrier gas.The assignment of the IR absorption bands was made based on the literature47-69 but also on experimental evidence obtained in the present work. Table2reports the chemical structures and absorption IR bands (N-O stretching mode)of various adsorbed NO x species assigned to the observed DRIFTS spectra of the present work and others reported in the literature.47-69
Figure2shows IR bands in the2300-1300cm-1range recorded after30min of H2-SCR of NO at140°C over the Pt/MgO-CeO2catalyst.Four main IR bands were observed. The band recorded at2220cm-1is assigned to adsorbed nitrosyl (NO+)coadsorbed with a nitrate NO3-species on adjacent metal cation-oxygen anion site-pair of support.This assignment is based on reported studies of NO and NO2chemisorption on MgO and similar metal oxides,and also on experimental results of this work to be presented next.The band at2220cm-1is characteristic of gaseous N2O or of a weakly adsorbed nitrous oxide species.63However,the former possibility is excluded since after passing a mixture of250ppm N2O/Ar through the DRIFTS cell in the absence of a catalyst,no IR band at2220 cm-1was observed.The value of250ppm of N2O corresponds to the composition for complete conversion of NO in the reaction mixture,and considering20%N2O selectivity at140°C in the presence of catalyst in the DRIFTS cell;note that the NO conversion measured at the outlet of the DRIFTS cell was only15%.In addition,the IR band at2220cm-1was not seen when the same mixture of250ppm N2O/Ar was passed through the DRIFTS cell with the catalyst sample in place at140°C, neither the IR band at about1285cm-1due to the bending mode of chemisorbed N2O on metal oxide surfaces.
50
Figure1.(a)HRTEM images of0.1wt%Pt/MgO-CeO2solid catalyst in which the well-dispersed Pt crystals(mean diameter of1.6 nm)are shown.(b)HRTEM images of0.1wt%Pt/MgO-CeO2solid where the interface formed between MgO and CeO2crystals by the sol-gel synthesis method is shown.
3012J.Phys.Chem.C,Vol.111,No.7,2007Costa and Efstathiou
The IR band in the 2000-1900cm -1range corresponds to NO δ+-Pt species.50,51Deconvolution (Gaussian peak shape)of the IR band observed in the 1750-1500cm -1range (see inset graph,Figure 2)results in three IR bands centered at 1670,1620,and 1580cm -1.The IR band assignments in the 1750-1500cm -1range to be discussed next were based on the following
elements:(a)the low Pt loading used in the present catalyst (0.1wt %),(b)the range of IR bands in which the various kinds of NO x species appear (see Table 2),(c)the integral band intensities and band widths shown in the inset of Figure 2a,(d)the support chemical composition,and (e)experimental results obtained on a 0.1wt %Pt/SiO 2catalyst.The IR band at 1670cm -1corresponds to bridged or bent NO on Pt (NO δ+-Pt),50,51that recorded at 1620cm -1to bidentate nitrates on Pt 52,53but also to molecularly adsorbed water (bending mode),54-56and the IR band at 1580cm -1to bidentate nitrates on the support.63,66The strong IR band at 1400cm -1is assigned to nitritos adsorbed species on the support 63(see Table 2).
Figure 3presents IR bands obtained after 30min on the 15NO/H 2/O 2
reaction mixture (Figure 3,solid line)and also after 30min on the 14NO/H 2/O 2reaction mixture (Figure 3,dotted line)in separate experiments in the DRIFTS cell conducted over a 0.1wt %Pt/SiO 2catalyst.Silica was used as a support to minimize NO x chemisorption on the support alone;no IR bands of chemisorbed NO (NO/H 2/O 2feed mixture)were observed on SiO 2alone.In the case of use of the 14NO/H 2/O 2reaction mixture,the obtained IR spectrum in the 1720-1360cm -1range was best deconvoluted with three IR bands.The first band centered at 1620cm -1corresponds to adsorbed bidentate (bridged)nitrates and water on Pt,52-58while the other two bands (1545and 1475cm -1)correspond to either monodentate nitrates or bridged NO on step and defect sites on the Pt surface.57-60The former assignment seems more likely given the presence of 5vol %of O 2in the feed stream and the small Pt particles (d ≈1.2-1.5nm).The formation of any bidentate nitrates (1545cm -1)at the Pt -support interface,however,cannot be excluded.On the other hand,in the case of use of the isotopic reaction mixture (15NO/H 2/O 2),the recorded IR spectrum was best deconvoluted after invoking four IR bands centered at 1620,1590,1505,and 1440cm -1.The IR band appearing at 1590cm -1is due to the adsorbed bidentate nitrate on Pt,the latter appearing at 1620cm -1under the 14NO/H 2/O 2feed mixture and now showing the 15N -O isotopic shift.Similarly,the bands at 1505and 1440cm -1are ascribed to monodentate nitrates or bridged NO on Pt defect sites as previously discussed.Thus,based on the results of Figure 3it is clearly illustrated that the band centered at 1620cm -1is due both to adsorbed water and bidentate nitrates on Pt,while other adsorbed NO x species on Pt can be easily measured over the low-loading 0.1wt %Pt supported on a relatively inert support.The relatively strong absorbance signal of the various adsorbed NO x species formed on Pt obtained with the FTIR spectrometer and DRIFTS cell (Harrick Scientific)used in the present work should be noted.
TABLE 2:Chemical Structures and Absorption Bands (N -O Stretching Mode)of Various Adsorbed Ν x Species on Supported-Pt Catalysts 47-
69
a
M )metal cation on the support surface.b See also Scheme
1.
Figure 2.In situ DRIFTS spectra recorded over the 0.1wt %Pt/MgO -CeO 2catalyst after 30min of H 2-SCR of NO at 140°C.Inset graph:Deconvolution of the DRIFTS spectrum recorded in the 1750-1500cm -1range.Feed gas composition:H 2)1.0vol %,NO )0.25vol %,O 2)5vol %,Ar as balance
gas.
Figure 3.In situ DRIFTS spectra recorded over the 0.1wt %Pt/SiO 2catalyst after 30min of 15NO/H 2/O 2/Ar reaction (-)and after 30min of 14NO/H 2/O 2/Ar reaction (---)at 140°C.Feed composition:H 2)1.0vol %,14NO or 15NO )0.25vol %,O 2)5vol %,Ar as balance gas.
Mechanistic Studies of the H 2-SCR of NO
J.Phys.Chem.C,Vol.111,No.7,20073013
We have recently reported 28that the increase of Pt loading from 1.0to 2.0wt %resulted in a decrease of specific integral reaction rate (per gram of catalyst)of NO conversion.At 140°C,the 0.1and 1.0wt %Pt/Mg -Ce -O catalysts showed similar specific integral reaction rates,where the latter catalyst exhibits four times higher number of Pt s /g cat .A similar experiment to that presented in Figure 2was conducted with 1.0wt %Pt/Mg -Ce -O catalyst.The IR bands at 1670and 1620cm -1were significantly increased.On the basis of these results,it is illustrated again that the IR bands observed at 1670and 1620cm -1are indeed due to Pt (see also Figure 3).
The chemical structure of the acti V e adsorbed NO x species participating in the reaction path of the H 2-SCR of NO at 140°C was determined by SSITKA-DRIFTS experiments.Initially,DRIFTS spectra were recorded after 30min of reaction in 14NO/H 2/O 2.The feed stream was then switched to the equivalent isotopic 15NO/H 2/O 2gas mixture,and DRIFTS spectra were recorded after 30min of reaction.Figure 4shows DRIFTS spectra recorded over the Pt/Mg -Ce -O catalyst before (-)and 30min after (---)the switch 14NO/H 2/O 2/He f 15NO/H 2/O 2/He was made at 140°C.The IR bands that shifted to lower wavenumbers after the isotopic switch (15N -O vs 14N -O stretching vibrational mode)correspond to acti V e adsorbed intermediate NO x species formed during the NO/H 2/O 2reaction that eventually lead to N 2and N 2O gas products.However,the likelihood for the presence of inacti V e but exchangeable 14NO x species with gaseous 15NO cannot be excluded.The shift (～30-50cm -1)of the IR bands shown in Figure 4(dotted vertical line segments)is consistent with the values predicted by the theoretical relationship between the vibrational frequency and the reduced mass of a two-atom bonding model,63and to the experimental work of Beutel et al.70detailing the observed isotopic shifts of adsorbed NO x species over Cu/ZSM-5upon its exposure to labeled 15NO.
After appropriate band deconvolutions similar to those shown in Figures 2and 3,only two IR bands had shifted to lower wavenumbers (Figure 4).The first band recorded at 2220and shifted to 2170cm -1corresponds to nitrosyl (NO +)species coadsorbed with a nitrate NO 3-species on adjacent metal cation -oxygen anion site-pair of support,as previously noted.The second IR band recorded at 1580cm -1and shifted to 1550cm -1is attributed to bidentate nitrates also formed on the support,as previously discussed.Therefore,the two acti V e intermediate NO x species formed on the Pt/Mg -Ce -O catalyst during H 2-SCR of NO are both present on the support.It should be noted that a completely different result was obtained in the case of Pt/SiO 2,40where adsorbed nitrosyls and unidentate
nitrates on Pt were found to be the acti V e NO x species formed during H 2-SCR of NO.
Due to the chemical composition of support that consists of two metal oxide phases (MgO and CeO 2),it is of fundamental importance to know the location of the two active NO x species besides their chemical structure (Figure 4).Figure 5presents SSITKA-DRIFTS results obtained over the 0.1wt %Pt/MgO (Figure 5a)and 0.1wt %Pt/CeO 2(Figure 5b)catalysts,the supports of which were prepared by the sol -gel method in a similar way as the mixed oxide Mg -Ce -O support (see the Experimental Section).It is illustrated that the active NO x species associated with the IR band at 2220cm -1is present on both catalysts,but only on Pt/CeO 2does this species appear to be a true acti V e NO x intermediate species since it exhibited the isotopic IR band shift (Figure 5b).Similarly,the second active NO x species associated with the 1580cm -1IR band is considered to be formed on Pt/MgO,according to the results of Figures 4and 5a.
To identify whether the active adsorbed NO x species formed during the H 2-SCR of NO interact re V ersibly with gaseous NO (exchangeable NO x species),the following experiment has been designed.After 30min of 14NO/H 2/O 2/Ar reaction at 140°C,the reaction feed was switched to 15NO/Ar flow and the DRIFTS spectrum was recorded after 15min.The purpose of the latter switch was to stop the reaction and simultaneously allow the exchange of re V ersibly chemisorbed NO x species with 15NO of the gas phase.Figure 6shows DRIFTS spectra recorded over the Pt/Mg -Ce -O catalyst 30min after reaction (14NO/H 2/O 2/Ar)and 15min after the switch 14NO/H 2/O 2/Ar f 15NO/Ar was made at 140°C.The IR bands shifted to lower wavenum-bers (after appropriate band deconvolution was performed),following the isotopic switch,correspond to re V ersibly chemi-sorbed NO x species formed during the H 2-SCR of NO.After deconvolution of the IR band in the 2280-2140cm -1range,it was found that the original IR band at 2220cm -1was shifted to 2170cm -1.As previously mentioned,this IR band
corre-
Figure 4.In situ DRIFTS spectra recorded over the 0.1wt %Pt/MgO -CeO 2catalyst after 30min of 14NO/H 2/O 2/Ar reaction (-)at 140°C,and after 30min following the isotopic switch 14NO/H 2/O 2/Ar f 15NO/H 2/O 2/Ar (---)at 140°C.Feed composition:H 2)1.0vol %,NO )0.25vol %,O 2)5vol %,Ar as balance
gas.
Figure 5.In situ DRIFTS spectra recorded after 30min of 14NO/H 2/O 2/Ar reaction (-)at 140°C,and after 30min following the isotopic switch 14NO/H 2/O 2/Ar f 15NO/H 2/O 2/Ar (---)at 140°C over the 0.1wt %Pt/MgO (a)and 0.1wt %Pt/CeO 2catalysts.Feed composition:H 2)1.0vol %,NO )0.25vol %,O 2)5vol %,Ar as balance gas.
3014J.Phys.Chem.C,Vol.111,No.7,2007Costa and Efstathiou
sponds to nitrosyl coadsorbed with a nitrate NO 3-species on adjacent metal cation -oxygen anion sites of support.Therefore,it can be said that among the two acti V e intermediate NO x species which are formed on Pt/Mg -Ce -O during H 2-SCR of NO (Figure 4),only nitrosyl coadsorbed with a nitrate NO 3-species on adjacent metal cation -oxygen anion sites can be considered as a reversibly chemisorbed species.This exchange route will be further discussed below.In the case of Pt/SiO 240among the two acti V e intermediate NO x species formed,only nitrosyls on Pt were considered as re V ersibly chemisorbed species .
Band Assignment in the 2250-2100cm -1Range.The N -O stretching frequency,which is at 1876cm -1for gaseous NO,can shift markedly depending on the kind of bonding.62If an electron is lost,nitrosonium ion,NO +,is formed with an N -O stretching frequency in the 2300-2200cm -1range (NO +salts).62Numerous studies clearly indicate that such a high-frequency IR band cannot be obtained after exposing a metal oxide surface to NO(g)alone.63In addition,an IR band in the 2200-1980cm -1range was assigned to the formation of NO 2+adsorbed species after exposing a metal oxide surface to NO 2-(g)alone.61Recently,theoretical 71,72and experimental 64studies have revealed a strong cooperative NO x adsorption effect that can lead to the formation of adsorbed adjacent NO x species of different structure on a metal cation -oxygen anion pair adsorp-tion site.For example,according to Scheme 1,two NO 2molecules can lead to the formation of a nitrate (NO 3-)ion binding on Mg 2+,and an adjacent nitrosonium ion (NO +)binding on an oxygen anion species.Between these two NO x adsorbed species there is an electronic interaction that determines the N -O bonds vibrational stretching frequencies.
The formation of various adsorbed NO x species upon NO 2interaction with the Pt/MgO -CeO 2catalyst in the presence of
5%O 2and 0.25%NO at 140°C is presented in Figure 7,parts a and b,respectively.As is clearly shown,no IR band at 2220cm -1was observed as opposed to the case of H 2-SCR of NO (Figures 2and 4-6).The IR band with the highest wavenumber observed at 2095cm -1is due to NO 2δ+or NO δ+on a metal cation of support (Table 2),where the presence of NO in the mixture of NO 2/O 2does not produce different in structure NO x adsorbed species (compare parts a and b of Figure 7).The presence of 1%H 2in the NO 2/O 2mixture seems to affect only the intensity of the IR band at 1495cm -1(compare parts a and b of Figure 7,dotted-line curves).According to Figure 7a,the concentration of NO 2in the NO 2/O 2mixture affects significantly the kinds and population of adsorbed NO x in the 1700-1000cm -1range.
On the basis of the experimental results of Figure 7,it is suggested that the IR band at 2220cm -1formed under the NO/H 2/O 2reaction conditions (Figures 2and 4-6)is the result of population of adjacent NO x adsorbed species as depicted in Scheme 1under a favorable catalyst surface state.The shift of IR band from 2095(Figure 7)to 2220cm -1(Figures 2and 4-6)is the result of the strengthening of the N -O bond in the NO +species in the NO x cooperative chemisorption effect (Scheme 1).Other adsorbed species (e.g.,-OH)are expected to affect the electron transfer between the two NO x adsorbates,thus determining the strength of the N -O bond in the NO +species.
The surface coverage of NO x species formed during the H 2-SCR of NO and which are re V ersibly chemisorbed (exchange-able )on the catalyst surface was determined by a combination of an isotopic exchange (use of 15NO)followed by H 2temperature-programmed surface reaction (Expt.B,Table 1).Figure 8shows the transient response curves of 14NO and 14N 15N obtained over the Pt/Mg -Ce -O catalyst under the H 2-TPSR step.A broad 14NO peak is observed in the 40-250°C range,which practically consists of two peaks.On the contrary,a rather narrow 14N 15N peak was obtained at 115°C with a
small
Figure 6.In situ DRIFTS spectra recorded over the 0.1wt %Pt/MgO -CeO 2catalyst after 30min of 14NO/H 2/O 2/Ar reaction (-)and after 15min following the switch 14NO/H 2/O 2/Ar f 15NO/Ar (---)at 140°C.Feed composition:H 2)1.0vol %,NO )0.25vol %,O 2)5vol %,Ar as balance gas.Isotopic feed composition:15NO )0.25vol %,Ar as balance gas.
SCHEME 1:(A)Possible Reaction Path for the
Formation of Coadsorbed Active NO x Species Formed during NO/H 2/O 2(H 2-SCR)Reaction on the 0.1wt %Pt/MgO -CeO 2Catalyst and (B)Reversible Interaction of Gaseous NO with Adsorbed Nitrosyl NO +on the Ceria
Surface
Figure 7.In situ DRIFTS spectra recorded over the 0.1wt %Pt/MgO -CeO 2catalyst after 30min exposure to (a)0.05%NO 2/5%O 2/He (-)and 0.25%NO 2/5%O 2/He (---)gas mixtures and (b)0.25%NO 2/0.25%NO/5%O 2/He (-)and 0.25%NO 2/1%H 2/O 2/He (---)gas mixtures at 140°C.
Mechanistic Studies of the H 2-SCR of NO J.Phys.Chem.C,Vol.111,No.7,20073015
shoulder at the rising part of it.Significantly different H 2-TPSR profiles were obtained in the case of Pt/SiO 240,where 15NO and 14N 15N were observed.The formation of 14N 15N(g)(Figure 8)strongly suggests that N 2production via reduction of adsorbed NO x by H 2requires the interaction of two different in structure adsorbed NO x ,one of which is reversibly chemisorbed on the catalyst surface,while the other one is not.It is suggested that these species are those depicted in Scheme 1A,NO 3-and NO +,with the latter species being the exchangeable one as depicted in Scheme 1B.The amounts of produced gaseous species (Figure 8)and the equivalent amount of adsorbed NO x are given in Table 3(Expt.B).
Transient Mass Spectrometry.The surface coverage of the acti V e adsorbed intermediate NO x species found in the N-pathway of the H 2-SCR of NO at 140°C,and which are responsible for the production of N 2and N 2O,was determined by SSITKA experiments (use of 15NO)with use of on line mass spectrometry.Figure 9presents transient isotopic response curves of N 2(a)and N 2O (b)obtained after the switch 14NO/H 2/O 2/Ar/He f 15NO/H 2/O 2/He was made at 140°C (Expt.A,Table 1)over the Pt/Mg -Ce -O catalyst.The results are expressed in terms of the dimensionless concentration Z ,which is the fraction of the ultimate change (giving Z )0)as a function of time.Thus,Z is defined by
where subscripts 0and ∞refer to the values of y (mole fraction)just before (t )0)and long after the isotopic switch (t f ∞).The decay of Ar gas concentration shown in Figure 9was used to monitor the gas phase hold-up of the system.29,32,73As seen in Figure 9,the only 14N-containing isotopic products formed were 14N 15N (Figure 9a)and 14N 15NO (Figure 9b).Very little 14N 2
isotopic gas was practically observed.The amount of acti V e NO x species that participates in the reaction path to form N 2and N 2O was calculated by integrating the corresponding transient response curves of 14N 15N and 14N 15NO,and that of 14N 2
with respect to the Ar curve.This amount was found to be 14.4μmol N/g.It was also found that 87.5%of this amount (12.6μmol N/g)led to the formation of N 2,whereas 12.5%(1.85μmol N/g)led to the formation of N 2O.A significantly smaller amount of acti V e NO x species (3.05μmol N/g)was found in the case of Pt/SiO 240compared with the present Pt/Mg -Ce -O catalyst.Only 68.4%(2.08μmol N/g)of this amount was found to lead to N 2,while the remaining amount led to N 2O (0.96μmol N/g).The amount of acti V e NO x species calculated based on the SSITKA-MS results for the present 0.1wt %Pt/Mg -Ce -O (Figure 9)and 0.1wt %Pt/SiO 2catalysts 40
is given in Table 3.The last column of Table 3gives also the amount of total adsorbed NO x species in terms of surface coverage,θ,the latter quantity estimated based on the exposed surface Pt atoms (μmol Pt s /g).For adsorbed NO x on support sites the parameter θhas no physical meaning.However,values of θgreater than unity point out that part of the estimated acti V e NO x cannot reside on Pt.
To gather information for the role of gaseous oxygen present in the reaction feed (NO/H 2/O 2)on the mechanism of H 2-SCR,similar SSITKA-MS experiments with the use of 18O 2were conducted.Figure 10presents transient response curves of N 216O and Ar obtained on Pt/Mg -Ce -O catalyst after the switch NO/H 2/16O 2/Ar/He f NO/H 2/18O 2/He was made at 140°C (Expt.C,Table 1).As is clearly seen in Figure 10,the concentration of N 216O produced does not change after the isotopic switch,while at the same time no evolution of N 218O was noticed.A completely different result was obtained in the case of Pt/SiO 2,40where the concentration of N 216O was reduced after the
isotopic
Figure 8.Transient response curves of 14NO and 14N 15N gases obtained during H 2-TPSR (10%H 2/He flow)on the 0.1wt %Pt/MgO -CeO 2catalyst according to the sequence of steps described in Expt.B of Table 1.Q H 2/He )30cm 3/min; )30deg/min.
Z (t ))
(y (t )-y ∞)(y 0-y ∞)
(1)
Figure 9.Transient response curves of 14Ν2,14Ν15Ν,Ar (a)and 14Ν2 ,14Ν15Ν ,Ar (b)obtained following the isotopic switch 14NO/H 2/O 2/Ar/He (30min)f 15NO/H 2/O 2/He (t )at 140°C (Expt.A,Table 1)over the 0.1wt %Pt/MgO -CeO 2catalyst.Feed gas composition:H 2)1.0%,NO )0.25%,O 2)5%,Ar )1%,He as balance
gas.
Figure 10.Transient response curves of Ν216O and Ar obtained following the isotopic switch NO/H 2/16O 2/Ar/He f NO/H 2/18O 2/He at 140°C (Expt.C,Table 1)over the 0.1wt %Pt/MgO -CeO 2catalyst.No N 218O gas formation was observed.Feed gas composition:H 2)1.0vol %,NO )0.25vol %,O 2)2vol %,Ar )1vol %,He as balance gas.
3016J.Phys.Chem.C,Vol.111,No.7,2007Costa and Efstathiou
switch,while a continuous evolution of N218O was noticed.The latter results strongly indicate that gaseous O2participates toward N2O formation in the H2-SCR of NO in the case of Pt/SiO2but not in the case of Pt/MgO-CeO2catalyst.These results reveal a strong support effect on the oxygen pathway of reaction toward N2O formation.
Discussion
The purpose of this work was to study essential mechanistic aspects of the H2-SCR of NO over the0.1wt%Pt/MgO-CeO2solid that exhibits remarkable catalytic behavior in the 120-200°C range,27,28namely a nitrogen reaction selectivity of83%,and NO conversions between80%and97%(GHSV )80000h-1)in the presence of5vol%H2O and20-45ppm
of SO2in the feed as compared with a conventional0.1wt% Pt/SiO2catalyst.14The results of the present work provide for the first time intrinsic reasons for explaining the strong support effect exhibited in the catalytic behavior of supported-Pt solids for the present H2-SCR of NO,and contribute to the enhance-ment of our understanding for further improvements of N2 selectivity needed for certain practical applications of the H2-SCR of NO at T<200°C.
The differences in the DRIFTS spectra(Figures2and4-6), SSITKA response curves(Figures9and10),and H2-TPSR profiles(Figure8)obtained over the present Pt/MgO-CeO2and Pt/SiO240catalyst formulations reflect the strong dependence of chemical structure and reactivity of adsorbed NO x on support chemical composition.The in situ SSITKA-DRIFTS(use of 15NO)experiments(Figures2and4-6)have demonstrated that on the Pt/MgO-CeO2catalyst surface,there exist two acti V e NO x intermediate species.The chemical structures of these species are nitrosyls(NO+)coadsorbed with a nitrate NO3-species on adjacent metal cation-oxygen anion sites of CeO2 support(IR band at2220cm-1),which are able to exchange with gaseous NO(reversibly chemisorbed NO x species),and bidentate(bridged)nitrates on the MgO support(IR band at 1540cm-1),which are irreversibly chemisorbed at140°C.The concentration of these two species determined by SSITKA-MS experiments(Figure9)was found to be14.4μmol/g(θ)3.1). Other adsorbed NO x species were formed during H2-SCR of NO at140°C but it was found not to participate(spectator species)in the formation of N2and N2O.The chemical structure of these NO x species has been presented in the Result section. It is noted that none of the inactive(spectator)NO x species were found to be a reversibly chemisorbed species.The amount of inactive NO x accumulated on the catalyst surface after30min of H2-SCR of NO was determined by subtracting the amount of active NO x(Expt.A,Table3)from the amount of NO x determined by H2-TPSR(Figure8),and it was found to be0.4μmol/g(θ)0.1).
It is very important to point out the significance of the results of DRIFTS isotopic experiment presented in Figure6.It has been shown that the two adsorbed NO x species formed under reaction conditions and which gave the isotopic shift in their corresponding IR bands are indeed two acti V e intermediate NO x species and not inactive ones,which simply exchanged with gaseous15NO.In the case of Pt/SiO2,40the two acti V e NO x intermediate species observed were of different chemical structure:nitrosyls(IR band at1900cm-1)and unidentate nitrates(IR band at1480cm-1)formed on Pt.The first one was found to be reversibly chemisorbed,while the second one was found to be irreversibly chemisorbed.
The shape of the14NO transient response curve observed in Figure8during H2-TPSR reflects the decomposition/desorption kinetics of the nonexchangeable with gaseous15NO nitrate species formed on Pt and support during H2-SCR at140°C (Figure6).The shoulder(practically a second peak after having deconvoluted the14NO curve,Figure8)observed in the rising part of the14NO peak centered at187°C implies the presence of another nonexchangeable NO x adsorbed species,in harmony with the isotopic DRIFTS results of Figure6.In the case of Pt/SiO2,40a more complex transient response curve of15NO was reported,suggesting that more than one kind of Pt-NOδ+ nitrosyl adsorbed species has been exchanged with gaseous 15NO,where the nonexchangeable adsorbed NO x were coupled with exchangeable NO x species under hydrogen to form14N15N and H2O.In the case of Pt/La0.5Ce0.5MnO3catalyst,4014N2O was formed due to the reduction of two nonexchangeable NO x species,whereas N2formation was due to the reduction of two different in chemical structure NO x from which only one was exchangeable.The same is true for the present Pt/MgO-CeO2 catalyst(Scheme1,Figure8).
The amount of N-containing adsorbed intermediate species measured during H2-TPSR agrees with the amount of acti V e NO x measured during SSITKA(Figure9,Table3).In the former experiment,reduction of NO x resulted only in the formation of N2,whereas under SSITKA,N2O was also formed.This difference is the result of the different kinetics of NO x reduction to N2in the presence of10%H2/He(Figure8)and0.25%NO/ 1%H2/5%O2/He(Figure9)gas mixtures.In the former case, about40%of the total adsorbed NO x formed under H2-SCR of NO had desorbed as molecular NO.As mentioned in our earlier transient work,14an assisted-hydrogen effect on the lowering of Pt-NO x bond strength might be expected as reported in the case of supported-Rh catalysts.74Such an effect might also be considered to occur between M-NO x species(M)metal cation on the support)and adjacent-OH species on the support.The concentration of the-OH species is enhanced by a hydrogen spillover process necessary for the reduction of M-NO x species on the support as will be discussed next.The rate of this process is expected to increase with H2partial pressure,in harmony with the largely positive order(1.3)of H2-SCR of NO with respect to H2gas.75
Scheme2depicts the essential mechanistic features of the H2-SCR of NO over the0.1wt%Pt/MgO-CeO2catalyst at 140°C based mainly on the results of this work.The SSITKA-MS study that allowed quantitative determination of the active NO x species(Table3)strongly suggests that these species must
TABLE3:Amount of N-Containing Species Desorbed(μmols/g cat)during Various Kinds of Transient Isotopic Experiments over Pt/MgO-CeO2and Pt/SiO2Catalysts
catalyst sample
Expt.
(Table1)14N15N14N15NO14N214NO
total adsorbed NO x
species(μmol/g)
0.1wt%Pt/MgO-CeO2A 5.350.920.9314.4(θ)3.10)
B 3.57.814.8(θ)3.18)
0.1wt%Pt/SiO2a A 1.040.48 3.04(θ)0.65)
B 1.0 1.4 3.4(θ)0.74)
a From ref40.
Mechanistic Studies of the H2-SCR of NO J.Phys.Chem.C,Vol.111,No.7,20073017
reside within the metal -support interface region,in proximity to the Pt clusters.Reduction of these acti V e NO x requires the spillover of atomic hydrogen formed on Pt as indicated in Scheme 2.Such a mechanism was recently supported by experimental results of NO x reduction by hydrogen in the case of Pd/MnO x -CeO 2catalyst.24Note the similarity of the latter support with the present support composition of MgO -CeO 2.As indicated by the SSITKA experiments (Figure 9),the interaction of the two active NO x species favors the formation of N 2over N 2O,in harmony with the high N 2reaction selectivity values obtained for the present catalytic system.27,28,75It might be speculated whether a fast NO x back spillover from the support to the Pt surface followed by its reduction by H atoms on Pt could be considered as alternative mechanistic steps of the H 2-SCR of NO (Scheme 2).These ideas seem rather unfavorable considering the low temperature of reaction and the binding strength of adsorbed NO x on the support.
Maunula et al.76have studied the NO/H 2reaction mechanism over a 0.2wt %Pt/CeO 2-Al 2O 3catalyst.An acti V e intermediate NO x species formed was reported to be likely the (NO)2dimer,designated as *NO....NO*.This can lead either to adsorbed N 2O and atomic oxygen,or to N 2and atomic oxygen.In the present work,adjacent NO x species of the same or different chemical structure,located not on Pt but instead on the support,lead to the formation of N 2and N 2O by the aid of atomic H formed on Pt.
Burch et al.36,37,41have studied the mechanism of the NO/H 2reaction and H 2-SCR of NO over Pt/SiO 2by SSITKA ex-periments.The authors suggested that formation of N 2in an extent of 85%was the result of the interaction of two dif-ferent NO x species ,while that of N 2O was the result of two similar in nature NO x species .41It should be noted that neither the chemical structure of the acti V e NO x species nor the chemical structure and composition of inacti V e NO x (spectator species)have been determined.However,several aspects of the mech-anism of N 2formation over the Pt/SiO 2catalyst suggested by Burch et al.41find strong support from the results of the present work and those previously reported by us,40and these are as follows.
(a)The main route of N 2formation involves the interaction of two different in chemical structure active NO x species.Their structure was identified for the first time in our previous work 40(nitrosyls and unidentate nitrates adsorbed on Pt).
(b)One of the two NO x species required for the formation of N 2has been suggested 41to be a weakly chemisorbed NO (named NO preads ).In our previous work,40this species was identified as the nitrosyl on Pt that easily exchanges with gaseous 15NO.The second NO x species,named NO ′ads by Burch et al.,41provides a reduced NH x (x )0-2)species that recombines with NO preads to form N 2.This second NO x species is the unidentate nitrate as identified in our previous work.40
(c)The surface coverage of the acti V e NO x intermediate species that lead to N 2formation is considerably larger than that leading to N 2O.
(d)Pt surface is not fully co V ered with N-containing species .Burch et al.41proposed that NO-derived spectator species can be stable under reaction conditions and this has been revealed for the first time in our previous work.40The bidentate nitrate on Pt (IR band at 1620cm -1)was not an active NO x intermediate species.
In the present work,the transient evolution of 14N 15NO isotopic species with time on stream (Figure 9b)appears very similar to that of 14N 15N (Figure 9a).In the work of Burch et al.41it was shown that N 2O isotopic species were evolved first compared to the corresponding N 2ones.This difference is largely the result of the different kinetics and mechanism operated in the Pt/SiO 2and Pt/MgO -CeO 2catalyst surfaces which are also influenced by the different reaction conditions used in the two studies.
The 18O 2SSITKA-MS experiments (Figure 10)performed over the Pt/MgO -CeO 2catalyst provided important mechanistic information about the oxygen pathway of the N 2O formation in the H 2-SCR of NO.The results obtained show that gaseous O 2does not participate in the formation of N 2O.A completely different result was reported by Costa and Efstathiou for Pt/SiO 2and Pt/La 0.5Ce 0.5MnO 3catalysts,40where gaseous O 2was found to participate in the formation of N 2O.It was shown that participation of gaseous O 2was more substantial on Pt/SiO 2than Pt/La 0.5Ce 0.5MnO 3catalyst.In particular,it was found that on Pt/SiO 2about 17%of the total oxygen (O atoms)participat-ing in the reaction mechanism of N 2O formation originated from the gas-phase oxygen,whereas the rest of it originated from the NO reactant molecule.On the contrary,the respective percentage over the Pt/La 0.5Ce 0.5MnO 3catalyst was found to be twice as small.As previously mentioned,the acti V e precursor intermediate NO x species that lead to the formation of N 2and N 2O during H 2-SCR of NO over Pt/SiO 2are Pt -N δ+and Pt -NO 3.40The latter species is formed after oxidation of adsorbed NO on Pt.40,59,60On the contrary,the acti V e precursor inter-mediate NO x species that lead to the formation of N 2and N 2O over the present Pt/MgO -CeO 2catalyst are formed on MgO and CeO 2support surfaces (Figures 4and 5,Scheme 2).It is important to state here that formation of NO 2on the present catalyst (Scheme 2)is selectively reduced to N 2as implied by the isotopic results of Figure 10.
Shibata et al.26have recently reported on the H 2-SCR of NO over supported-Pt catalysts (1wt %Pt)in the low-temperature range of 75-250°C and using a feed gas of 0.1%NO,0.5%H 2,and 6.7%O 2in He.On the basis of the in situ FTIR reaction studies conducted at 75°C,combined with some transient titration measurements of adsorbed species,the authors con-cluded that NH 4+adsorbed species on acidic supports (e.g.,MFI,H-BEA,H-Y zeolites)were responsible for the reduction of NO on support surface sites.The authors suggested that NH 3is formed by reduction of NO with H 2on Pt,which then is readsorbed on the Bro ¨nsted acid sites of support to form NH 4+species.A criticism on this mechanism may come from the fact that such a strong suggestion for the above-referenced H 2-SCR mechanism was not based on SSITKA-DRIFTS studies from which one probes only true acti V e reaction intermediate species under reaction conditions.The reactivity of preadsorbed NH 3toward the reactant mixture of NO/O 2cannot be considered as an appropriate experiment to draw the conclusion that NH 4+is a true active intermediate species for the H 2-SCR of NO.In the present SSITKA-DRIFTS experiments (Figures 2and 4-6)
SCHEME 2:Illustration of the Bifunctional Catalysis Taking Place on the Pt/MgO -CeO 2Catalyst
a
a
Active adsorbed NO x intermediate species are formed near the interface between Pt nanoparticles and the support surface of MgO and CeO 2.
3018J.Phys.Chem.C,Vol.111,No.7,2007Costa and Efstathiou
no evidence was provided for the formation of an NH4+ adsorbed acti V e intermediate species over the Pt/MgO-CeO2 catalyst.However,the formation of a very small surface coverage of NH x species that could not be detected by the present DRIFTS studies cannot be excluded.
According to the results of the present work and the discussion offered previously,important factors that seem to control the H2-SCR of NO activity and N2selectivity are the following:(a)the different in chemical nature sites of support which influence the structure of adsorbed NO x and their binding energy(thermal stability),(b)the Pt oxidation state that influences H2chemisorption,and in turn the H-spillover from Pt to the support surface(across the metal-support interface), and the surface coverage of active NO x species,(c)the reactant feed composition that also determines the surface coverage of the active precursor NO x species,and(d)the Pt particle size. For the present Pt/MgO-CeO2system,27,28a decrease in the NO conversion rate by an order of magnitude was observed by increasing the Pt particle size from1.3to3.5nm.On the contrary,only a small decrease in N2selectivity was obtained. In the case of0.1wt%Pt/MgO,our recent work28showed much higher conversions and N2selectivity values than the1.0 wt%Pt/MgO catalyst studied by Shibata et al.,26where the Pt particle size was larger than that of0.1wt%Pt/MgO,a result that confirms the strong dependence of H2-SCR of NO performance on the Pt particle size.
Conclusions
The following conclusions can be derived from the results of the present work:
1.The15NO SSITKA-DRIFTS combined with H2-TPSR following15NO isotopic exchange have illustrated that the N-pathway from gaseous NO toΝ2formation in the H2-SCR of NO at140°C over the Pt/MgO-CeO2catalyst passes through two different in chemical structure NO x adsorbed precursor intermediates.
2.It was shown that the two acti V e NO x species are formed within the metal-support interphase region,whereas inacti V e NO x are populated on both the metal and support surfaces during H2-SCR of NO at140°C.
3.The N-pathway from gaseous NO to N2O gas product in the H2-SCR of NO at140°C includes the same active NO x precursor intermediate species that lead also to N2gas product.
4.The mechanism of the H2-SCR of NO over the0.1wt% Pt/MgO-CeO2catalyst must include a H-spillo V er process from the Pt metal to the MgO-CeO2support surface.
5.The surface pool of active NO x that eventually leads to the formation of N2is found to be significantly larger than the surface pool of active NO x that leads to the formation of N2O.
6.The18O2SSITKA-MS experiments have shown that the H2-SCR of NO toward N2O formation on Pt/MgO-CeO2does not involve the participation of gaseous oxygen. Acknowledgment.The financial support of the Research Committee of the University of Cyprus through a competitive research grant is gratefully acknowledged.The authors express their sincere thanks to Professor J.L.G.Fierro(ICP/CSIC Madrid,Spain)for the HRTEM measurements. References and Notes
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