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Al2O3 catalysts in the reduction of nitric oxide by methane
Applied Catalysis A: General, 97 (1993) Lll-L17 Elsevier Science Publishers B.V., Amsterdam
Lll
APCAT A 2486
Influence of metal particle size and effect of gold addition on the activity and selectivity of Pt/Al,O, catalysts in the reduction of nitric oxide by methane M.C. Demicheli, L.C. Hoang, J.C. M&r&o and J. Barbier LACCO (CNRS: URA DO350), UniversitB de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers (France)
and M. Pinabiau-Carlier Direction des Etudes et Techniques Nouvelles, Centre d%tudes et de Recherches sur les Sciences et Techniques Appliqukes, Gas de France, 361 avenue du Prbident WiLson, BP 33, 93211 La Plaine St. Denis (France) (Received 27 November 1992, revised manuscriptreceived 27 January 1993)
Ahatract
The catalytic reduction of nitric oxide by methane over alumina supportedplatinum and Pt-Au catalysts was investigatedusing a gas mixture of 2 vol.-% NO and CH, in helium at 250-350°C. With monometallic platinum catalysts,the specific reaction rate was found to increase with increasingparticle sizes from 1.5 to 15 nm, indicating that the reaction can occur preferentiallyon large platinum planes.This resultwasconfirmed throughaddition of gold selectivelydepositedon the edgesand comers of platinum particles.The selectivitypattern obtained suggeststhat the formation of stronglyadsorbed species (NO:- ) is favouredon small platinum particles of higher oxidation state. Keywords: gold; metal particle size; methane;nitric oxide, platinum-alumina; reduction
INTRODUCTION
The reduction of NO is an important part of the catalytic control of exhaust emissions in both mobile and stationary sources. Although many kinetic and mechanistic studies have appeared on the catalytic reduction of NO with HP, CO and NH3 [11,much less work has been done on the reaction with hydrocarbons [ 21. Correspondence to: Dr. M.C. Demicheli, LACCO (CNRS: URA D0350), UniversitA de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers, France.
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -L17
As far as the structure sensitivity of these reactions is concerned, Otto and Yao [3] observed that, for the reduction of NO with hydrogen, the reaction rate and the activation energy depended upon the degree of dispersion of Group VIII noble metals. With both rhodium and platinum supported on y-A1203, they found pronounced changes in the reaction parameters with metal loading [ 31. Recently, Oh and Eickel [ 41 have reported that, for the reduction of NO by CO on rhodium catalysts, the specific reaction rate increased drastically with increasing particle sixes from 10 to 670 A. However, changing the support material had little effect [4]. With regard to the mechanism of NO reduction, all steps leading to NO activation (adsorption, surface reactions, etc.) could be compared whatever the reductant used. The purpose of the present work is to establish the structure sensitivity of the reduction of NO by methane on various Pt/A1203 catalysts with different metallic dispersions. Some catalysts were modified by gold deposition obtained by reduction of Au3+ with preadsorbed hydrogen on a platinum parent catalyst. In this type of preparation, gold is preferentially deposited in decoration on low coordination sites, such as edges and comers [ 5,6]. These bimetallic P&Au were used as model catalysts for the discussion on the structure sensitivity of the catalytic reduction of NO by methane on platinum. RESULTS AND DISCUSSION
Platinum catalysts were prepared by impregnation of a-alumina (100 m2 g-l) samples with the required quantities of a H2PtC1, solution (8 g Pt/l) to obtain 1,2 and 10% platinum in the final catalysts. Water was evaporated at 70’ C, and the resulting slurries dried at 110 ’ C overnight. The precursors were reduced in flowing hydrogen at 5OO”C, then passivated with 1% oxygen in nitrogen at ambient temperature. Finally the catalysts were sintered in the same gas mixture under carefully adjusted time/temperature conditions [ 71, in order to obtain catalysts wit the same number of accessible platinum atoms on a weight basis. Chemical analyses of the catalysts are shown in Table 1. To eliminate chlorine, a sample of the non-sintered 1% platinum catalyst was heated to 500” C under 10% water in hydrogen for 16 h ‘[ Ptl (D ) in Table 11. Even after this treatment the residual chlorine was only reduced to 50%. Ptl (D ) was used for catalytic modification. The catalyst was introduced in a suitable reactor [ 81 and heated up to 300 ’ C in flowing hydrogen. After cooling down to ambient temperature, the solid was immersed in water and hydrogen was replaced by nitrogen to eliminate all solved and reversibly .adsorbed hydrogen. Then a nitrogen-outgassed solution of HAuCl, was introduced. The following surface redox reactions were assumed to occur: Pt-H=Pt+H++e[AuCl,]-+3e-=Au+4Cl-
M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
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TABLE 1 Chemical analysis of the catalysts Catalyst
Ptl (S) Pt2(s) PtlO(S) Ptl(D) PTl-Au1 Ptl-Au3
Metal loading ( % ) Pt
AU
0.92 2.0 9.2 0.7 0.68 0.68
0 0 0 0 0.032 0.150
Cl (%I
0.4 0.4 0.4 0.2 0.35 0.35
The AuClr concentration in the solution was set to fit with fractions of a gold monolayer on the platinum particles. After 3 h of reaction the catalyst was washed thoroughly with demineralized water, and dried at 120’ C under a nitrogen flow for 1 day. The gold contents of these catalyst are given in Table 1. Blank experiments using alumina alone showed no gold deposition or homogeneous complex decomposition. The activity tests were performed in a flow reactor: a mixture of 2% NO and 2% CH, in He was admitted over the prereduced catalyst at a contact time of 0.6 g s cme3, while the temperature was raised by regular increments. With all the catalysts, a reasonably steady state was obtained after 1 h of reaction at a given temperature, and little change in activity or selectivity was observed even after one day-on-stream. The effluent was analysed with a gas chromatograph, equipped with a Porapak (Q+N) packed column which made it possible to obtain separate peaks for HP, Nz, CO, NO, CH4, COz, N20 and H,O. Hz and CO were formed on Pt/A1203 catalysts, only above 350°C. NH3 was not analysed. Even so, its concentration must be very low, as the nitrogen balance for the reaction at 35O”C, was good to within 5%. The metal accessibility ( Act ) was measured by hydrogen chemisorption and HZ/O, titration at ambient temperature in a static volumetry apparatus. The irreversibly adsorbed hydrogen uptake (IH), and the oxygen (OT) and hydroheld the ratios consumed in successive titrations gen (HT) IH : OT: HT = 1: 1.5 : 3. With monometallic platinum catalysts this corresponds to adsorption stoichiometries Pt-H = Pt-0 = 1 [ 91. The mean particle size was determined by transmission electron microscopy (TEM ) . Assuming that gold does not chemisorb hydrogen or oxygen [lo], the ratio of gold atoms to exposed platinumatoms was worked out. All these values are listed in Table 2. Fig. 1 shows NO conversion versus temperature for the platinum catalysts: the lower the metallic dispersion, the higher the specific activity. On the other hand, Fig. 2 shows that N,O selectivity [calculated as N20/ (NxO + N,) ] in-
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
TABLE 2 Surface parameters of the catalysts Catalyst
Act (%)
Mean particle size (nm)
Au/% (atomic ratio)
PTl(S) Pt2(s) Ptl-(S) Ptl (D) Ptl-Au1 Ptl-Au3
57 30 6 66 51 36
1.5 3.2 15 _
0 0 0 0 0.07 0.33
220
240
260
280
300
320
340 T. “C
360
Fig. 1. Effect of temperature on the NO conversion in the NO-CHI reaction on Pt/A1203 catalysts. PNo = PC- = 2 vol.-%; contact time = 0.6 g s cmm3. (cl) Rl(S), (+) PQ(S), (H) PtlO(S).
creases with temperature at relatively low conversions. To explain this behavior, Hu [ 111 proposed the following reaction scheme:
Initially, all reactions are relatively fast, but accumulation of poison (presumably NO,’ species formed by reaction of NO with 0’ ions adsorbed on the surface) retards reactions (2) and (3) more than reaction (l), which results in an increase of the N20/N2 ratio as the reaction proceeds [ 111. Interestingly, an abrupt change of selectivity to pure nitrogen occurs at a temperature (conversion) value which depends on the catalyst structure (Fig. 2). A similar effect has been observed in the reduction of NO with H2 on platinum [3] and ruthenium [ 121, and it was claimed that in the presence of NO, nitrous oxide
L15
M.C. Demicheli 1 Appl. Catal. A 97 (1993) Lll -Ll7
220
240
260
280
300
320 340 T, “C
360
Fig. 2. Effect of temperature on the N20 selectivity in the NO-CH, reaction on Pt/A1,OB catalysts. PNo=Pcti ~2 vol.-%; contact time=0.6 g s cme3. Symbols as in Fig. 1.
is deplaced from the active sites. To account for the structure sensitivity of the reaction, it has been suggested that a higher oxidation state may be expected for platinum in a dispersed phase [ 31. Therefore, the formation of poisoning species like NO; could be associated with that higher binding energy of oxygen on small platinum particles, in the present case. As a result, a sustained production of NzO via path 1 in Hu’s scheme can be expected on well-dispersed platinum catalysts. Thus, the selectivity pattern in Fig. 2 appears linked to the thermodynamic stability of strongly adsorbed species formed by NO oxidation. In the reduction of NO with CO or H2 on noble metals it has been claimed that, either the NO dissociation itself, or the reaction the resulting adsorbed oxygen atoms with CO or H adsorbed can be rate determining [ 2,3]. In contrast, in the reduction of NO by methane on platinum the rate limiting step is probably the dissociative adsorption of methane [ 11,131. For the reduction of NO with CH, or CD, at 3OO”C, it was observed that CH, reacted 1.8 faster than CD4 on a Pt/Al,O, catalyst. Such a “kinetic isotope effect” strongly suggests that cleavage of the C-H bonds must occur in the rate determining step of the reaction [ 11,131. A similar mechanism was proposed for Rh/A1203 [ 21. For the reaction of methane over metals, Frennet [ 141 has reported the sequence Rh = Pd> Re > MO = W > Ni, Ta. At present, we have some evidence that the rate of NO reduction by methane follows the sequence Pt > Pd> Rh > Ni. This slight difference could be due to the ability of NO to partially oxidize the metallic surface. As regards methane adsorption, previous investigators [ 15-171 have proposed that dissociative adsorption of methane requires an ensemble of 5-7 metallic atoms. By a statistical model, Martin [ 181 has demonstrated that the larger the metallic particle, the higher the probability of finding large ensemble sites. All these considerations are in agreement
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
TABLE 3 Average parameters of the reaction Catalyst
Temp. of half NO conversion ( ’ C )
Temp. of full NO conversion ( oC )
Temp. of selectivity breaking ( oC )
ptl(S)
330 294 275 327 325 325
340 308 290 350 340 340
340 318 309 350 345 346
PtB(s) PtlO(S) Ptl (D) Ptl-Au1 Ptl-Au3
with the observed higher specific activity of larger platinum particles for the reduction of NO by methane. Table 2 shows that for Pt-Au catalysts the number of exposed platinum atoms decreases with the amount of added gold, in line with recent results on the adsorption of Hz, 0, and CO on Pt-Au/SiO, reported by Balakrishnan et al. [lo]. A decrease in the platinum accessibility with the addition of gold was also observed by Galvagno and Parravano [ 191. Concerning the stability of the metal deposits, it was pointed out in a previous study [5] that the surface composition of the P&Au catalysts, prepared by the described redox technique, was insensitive to oxidative or reducing treatments. The curves of NO conversion and of N,O selectivity versus temperature were, for Pt-Au catalysts, similar to the corresponding characteristics of platinum catalysts (Table 3). Thus, the effect of the addition of gold on the activity and selectivity of the NO-CH, reaction is very small, and far from being a deactivating one. The microstructure of the Pt-Au catalysts is not known, and many factors will contribute to determine the nature of the resulting active centers following gold modification. Under certain preparation conditions, gold has a dispersive effect on the platinum ensemble [lo], which for Pt-Au/A1,OB catalyst could be magnified due to the close proximity of platinum and gold [ 201. In the case of Pt-Au bimetallic catalysts prepared by surface redox reactions using, as reducer of Au3+ ions, preadsorbed hydrogen on platinum particles; it was suggested that gold is deposited preferably on low coordination platinum sites, while the high coordination sites remain free from gold deposition [ 5,6]. The comparable catalytic properties of platinum and Pt-Au modified catalysts can be explained by assuming that gold is deposited on inactive platinum sites. In conclusion, the reduction of NO by methane, involving relatively large ensemble sites, occurs preferentially on flat planes exhibited by large platinum particles.
M.C. Demicheli f Appl. Catal. A 97 (1993) Lll -Ll7
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REFERENCES
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
B. Harrison, M. Wyatt and K.C. Gough, Catalysis (London), 5 (1982) 127. J.R. Hardee and J.W. Hightower, J. Catal., 86 (1984) 137. K. Otto and H.C. Yao, J. Catal., 66 (1980) 229. S.H. Oh and C.C. Eickel, J. Catal., 128 (1991) 526. R. Melendrez, G. Del Angel, P. Marecot and J. Barbier, XIII Simposio Iberoamericano de Catalisis, Segovia, Spain, 6-10 July, 1992. J.M. Dominguez, P. Marecot and J. Barbier, in preparation. A. Fakche, Thesis, Poitien University (1991). C. Montassier, J.C. Men&o, J. Moukolo, J. Naja, L.C. Hoang and J. Barbier, J. Mol. Catal., 70 (1991) 645. J.E. Benson and M. Boudart, J. Catal., 4 (1965) 705. Balakrishnan, A. Sachdev and B.E. Koel, J. Catal., 121 (1990) 441. Y.-H. Hu, Thesis, Rice University (1975). K. Otto and M. Shelef, Z. Phys. Chem., 85 (1973) 308. Y.-H. Hu, D. Van L&burg and J.W. Hightower, Kinet. Katal., 18 (1977) 545. A. Frennet, Catal. Rev. Sci. Eng., 10 (1974) 37. G.A. Martin and B. Imelik, Surf. Sci., 42 (1974) 157. A. Frennet, G. Lienard, G. Crucq and L. Degols, J. Catal., 53 (1978) 150. H.F. Leach, C. Mirodatos and D.A. Whan, J. Catal., 63 (1980) 138. G.A. Martin, Catal. Rev. Sci. Eng., 30 (1988) 519. S. Galvagno and G. Parravano, J. Catal., 55 (1979) 272. H.C. De Jongste and V. Ponec, J. Catal., 64 (1980) 228.
Lll
APCAT A 2486
Influence of metal particle size and effect of gold addition on the activity and selectivity of Pt/Al,O, catalysts in the reduction of nitric oxide by methane M.C. Demicheli, L.C. Hoang, J.C. M&r&o and J. Barbier LACCO (CNRS: URA DO350), UniversitB de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers (France)
and M. Pinabiau-Carlier Direction des Etudes et Techniques Nouvelles, Centre d%tudes et de Recherches sur les Sciences et Techniques Appliqukes, Gas de France, 361 avenue du Prbident WiLson, BP 33, 93211 La Plaine St. Denis (France) (Received 27 November 1992, revised manuscriptreceived 27 January 1993)
Ahatract
The catalytic reduction of nitric oxide by methane over alumina supportedplatinum and Pt-Au catalysts was investigatedusing a gas mixture of 2 vol.-% NO and CH, in helium at 250-350°C. With monometallic platinum catalysts,the specific reaction rate was found to increase with increasingparticle sizes from 1.5 to 15 nm, indicating that the reaction can occur preferentiallyon large platinum planes.This resultwasconfirmed throughaddition of gold selectivelydepositedon the edgesand comers of platinum particles.The selectivitypattern obtained suggeststhat the formation of stronglyadsorbed species (NO:- ) is favouredon small platinum particles of higher oxidation state. Keywords: gold; metal particle size; methane;nitric oxide, platinum-alumina; reduction
INTRODUCTION
The reduction of NO is an important part of the catalytic control of exhaust emissions in both mobile and stationary sources. Although many kinetic and mechanistic studies have appeared on the catalytic reduction of NO with HP, CO and NH3 [11,much less work has been done on the reaction with hydrocarbons [ 21. Correspondence to: Dr. M.C. Demicheli, LACCO (CNRS: URA D0350), UniversitA de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers, France.
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -L17
As far as the structure sensitivity of these reactions is concerned, Otto and Yao [3] observed that, for the reduction of NO with hydrogen, the reaction rate and the activation energy depended upon the degree of dispersion of Group VIII noble metals. With both rhodium and platinum supported on y-A1203, they found pronounced changes in the reaction parameters with metal loading [ 31. Recently, Oh and Eickel [ 41 have reported that, for the reduction of NO by CO on rhodium catalysts, the specific reaction rate increased drastically with increasing particle sixes from 10 to 670 A. However, changing the support material had little effect [4]. With regard to the mechanism of NO reduction, all steps leading to NO activation (adsorption, surface reactions, etc.) could be compared whatever the reductant used. The purpose of the present work is to establish the structure sensitivity of the reduction of NO by methane on various Pt/A1203 catalysts with different metallic dispersions. Some catalysts were modified by gold deposition obtained by reduction of Au3+ with preadsorbed hydrogen on a platinum parent catalyst. In this type of preparation, gold is preferentially deposited in decoration on low coordination sites, such as edges and comers [ 5,6]. These bimetallic P&Au were used as model catalysts for the discussion on the structure sensitivity of the catalytic reduction of NO by methane on platinum. RESULTS AND DISCUSSION
Platinum catalysts were prepared by impregnation of a-alumina (100 m2 g-l) samples with the required quantities of a H2PtC1, solution (8 g Pt/l) to obtain 1,2 and 10% platinum in the final catalysts. Water was evaporated at 70’ C, and the resulting slurries dried at 110 ’ C overnight. The precursors were reduced in flowing hydrogen at 5OO”C, then passivated with 1% oxygen in nitrogen at ambient temperature. Finally the catalysts were sintered in the same gas mixture under carefully adjusted time/temperature conditions [ 71, in order to obtain catalysts wit the same number of accessible platinum atoms on a weight basis. Chemical analyses of the catalysts are shown in Table 1. To eliminate chlorine, a sample of the non-sintered 1% platinum catalyst was heated to 500” C under 10% water in hydrogen for 16 h ‘[ Ptl (D ) in Table 11. Even after this treatment the residual chlorine was only reduced to 50%. Ptl (D ) was used for catalytic modification. The catalyst was introduced in a suitable reactor [ 81 and heated up to 300 ’ C in flowing hydrogen. After cooling down to ambient temperature, the solid was immersed in water and hydrogen was replaced by nitrogen to eliminate all solved and reversibly .adsorbed hydrogen. Then a nitrogen-outgassed solution of HAuCl, was introduced. The following surface redox reactions were assumed to occur: Pt-H=Pt+H++e[AuCl,]-+3e-=Au+4Cl-
M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
L13
TABLE 1 Chemical analysis of the catalysts Catalyst
Ptl (S) Pt2(s) PtlO(S) Ptl(D) PTl-Au1 Ptl-Au3
Metal loading ( % ) Pt
AU
0.92 2.0 9.2 0.7 0.68 0.68
0 0 0 0 0.032 0.150
Cl (%I
0.4 0.4 0.4 0.2 0.35 0.35
The AuClr concentration in the solution was set to fit with fractions of a gold monolayer on the platinum particles. After 3 h of reaction the catalyst was washed thoroughly with demineralized water, and dried at 120’ C under a nitrogen flow for 1 day. The gold contents of these catalyst are given in Table 1. Blank experiments using alumina alone showed no gold deposition or homogeneous complex decomposition. The activity tests were performed in a flow reactor: a mixture of 2% NO and 2% CH, in He was admitted over the prereduced catalyst at a contact time of 0.6 g s cme3, while the temperature was raised by regular increments. With all the catalysts, a reasonably steady state was obtained after 1 h of reaction at a given temperature, and little change in activity or selectivity was observed even after one day-on-stream. The effluent was analysed with a gas chromatograph, equipped with a Porapak (Q+N) packed column which made it possible to obtain separate peaks for HP, Nz, CO, NO, CH4, COz, N20 and H,O. Hz and CO were formed on Pt/A1203 catalysts, only above 350°C. NH3 was not analysed. Even so, its concentration must be very low, as the nitrogen balance for the reaction at 35O”C, was good to within 5%. The metal accessibility ( Act ) was measured by hydrogen chemisorption and HZ/O, titration at ambient temperature in a static volumetry apparatus. The irreversibly adsorbed hydrogen uptake (IH), and the oxygen (OT) and hydroheld the ratios consumed in successive titrations gen (HT) IH : OT: HT = 1: 1.5 : 3. With monometallic platinum catalysts this corresponds to adsorption stoichiometries Pt-H = Pt-0 = 1 [ 91. The mean particle size was determined by transmission electron microscopy (TEM ) . Assuming that gold does not chemisorb hydrogen or oxygen [lo], the ratio of gold atoms to exposed platinumatoms was worked out. All these values are listed in Table 2. Fig. 1 shows NO conversion versus temperature for the platinum catalysts: the lower the metallic dispersion, the higher the specific activity. On the other hand, Fig. 2 shows that N,O selectivity [calculated as N20/ (NxO + N,) ] in-
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
TABLE 2 Surface parameters of the catalysts Catalyst
Act (%)
Mean particle size (nm)
Au/% (atomic ratio)
PTl(S) Pt2(s) Ptl-(S) Ptl (D) Ptl-Au1 Ptl-Au3
57 30 6 66 51 36
1.5 3.2 15 _
0 0 0 0 0.07 0.33
220
240
260
280
300
320
340 T. “C
360
Fig. 1. Effect of temperature on the NO conversion in the NO-CHI reaction on Pt/A1203 catalysts. PNo = PC- = 2 vol.-%; contact time = 0.6 g s cmm3. (cl) Rl(S), (+) PQ(S), (H) PtlO(S).
creases with temperature at relatively low conversions. To explain this behavior, Hu [ 111 proposed the following reaction scheme:
Initially, all reactions are relatively fast, but accumulation of poison (presumably NO,’ species formed by reaction of NO with 0’ ions adsorbed on the surface) retards reactions (2) and (3) more than reaction (l), which results in an increase of the N20/N2 ratio as the reaction proceeds [ 111. Interestingly, an abrupt change of selectivity to pure nitrogen occurs at a temperature (conversion) value which depends on the catalyst structure (Fig. 2). A similar effect has been observed in the reduction of NO with H2 on platinum [3] and ruthenium [ 121, and it was claimed that in the presence of NO, nitrous oxide
L15
M.C. Demicheli 1 Appl. Catal. A 97 (1993) Lll -Ll7
220
240
260
280
300
320 340 T, “C
360
Fig. 2. Effect of temperature on the N20 selectivity in the NO-CH, reaction on Pt/A1,OB catalysts. PNo=Pcti ~2 vol.-%; contact time=0.6 g s cme3. Symbols as in Fig. 1.
is deplaced from the active sites. To account for the structure sensitivity of the reaction, it has been suggested that a higher oxidation state may be expected for platinum in a dispersed phase [ 31. Therefore, the formation of poisoning species like NO; could be associated with that higher binding energy of oxygen on small platinum particles, in the present case. As a result, a sustained production of NzO via path 1 in Hu’s scheme can be expected on well-dispersed platinum catalysts. Thus, the selectivity pattern in Fig. 2 appears linked to the thermodynamic stability of strongly adsorbed species formed by NO oxidation. In the reduction of NO with CO or H2 on noble metals it has been claimed that, either the NO dissociation itself, or the reaction the resulting adsorbed oxygen atoms with CO or H adsorbed can be rate determining [ 2,3]. In contrast, in the reduction of NO by methane on platinum the rate limiting step is probably the dissociative adsorption of methane [ 11,131. For the reduction of NO with CH, or CD, at 3OO”C, it was observed that CH, reacted 1.8 faster than CD4 on a Pt/Al,O, catalyst. Such a “kinetic isotope effect” strongly suggests that cleavage of the C-H bonds must occur in the rate determining step of the reaction [ 11,131. A similar mechanism was proposed for Rh/A1203 [ 21. For the reaction of methane over metals, Frennet [ 141 has reported the sequence Rh = Pd> Re > MO = W > Ni, Ta. At present, we have some evidence that the rate of NO reduction by methane follows the sequence Pt > Pd> Rh > Ni. This slight difference could be due to the ability of NO to partially oxidize the metallic surface. As regards methane adsorption, previous investigators [ 15-171 have proposed that dissociative adsorption of methane requires an ensemble of 5-7 metallic atoms. By a statistical model, Martin [ 181 has demonstrated that the larger the metallic particle, the higher the probability of finding large ensemble sites. All these considerations are in agreement
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M.C. Demicheli / Appl. Catal. A 97 (1993) Lll -Ll7
TABLE 3 Average parameters of the reaction Catalyst
Temp. of half NO conversion ( ’ C )
Temp. of full NO conversion ( oC )
Temp. of selectivity breaking ( oC )
ptl(S)
330 294 275 327 325 325
340 308 290 350 340 340
340 318 309 350 345 346
PtB(s) PtlO(S) Ptl (D) Ptl-Au1 Ptl-Au3
with the observed higher specific activity of larger platinum particles for the reduction of NO by methane. Table 2 shows that for Pt-Au catalysts the number of exposed platinum atoms decreases with the amount of added gold, in line with recent results on the adsorption of Hz, 0, and CO on Pt-Au/SiO, reported by Balakrishnan et al. [lo]. A decrease in the platinum accessibility with the addition of gold was also observed by Galvagno and Parravano [ 191. Concerning the stability of the metal deposits, it was pointed out in a previous study [5] that the surface composition of the P&Au catalysts, prepared by the described redox technique, was insensitive to oxidative or reducing treatments. The curves of NO conversion and of N,O selectivity versus temperature were, for Pt-Au catalysts, similar to the corresponding characteristics of platinum catalysts (Table 3). Thus, the effect of the addition of gold on the activity and selectivity of the NO-CH, reaction is very small, and far from being a deactivating one. The microstructure of the Pt-Au catalysts is not known, and many factors will contribute to determine the nature of the resulting active centers following gold modification. Under certain preparation conditions, gold has a dispersive effect on the platinum ensemble [lo], which for Pt-Au/A1,OB catalyst could be magnified due to the close proximity of platinum and gold [ 201. In the case of Pt-Au bimetallic catalysts prepared by surface redox reactions using, as reducer of Au3+ ions, preadsorbed hydrogen on platinum particles; it was suggested that gold is deposited preferably on low coordination platinum sites, while the high coordination sites remain free from gold deposition [ 5,6]. The comparable catalytic properties of platinum and Pt-Au modified catalysts can be explained by assuming that gold is deposited on inactive platinum sites. In conclusion, the reduction of NO by methane, involving relatively large ensemble sites, occurs preferentially on flat planes exhibited by large platinum particles.
M.C. Demicheli f Appl. Catal. A 97 (1993) Lll -Ll7
L17
REFERENCES
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
B. Harrison, M. Wyatt and K.C. Gough, Catalysis (London), 5 (1982) 127. J.R. Hardee and J.W. Hightower, J. Catal., 86 (1984) 137. K. Otto and H.C. Yao, J. Catal., 66 (1980) 229. S.H. Oh and C.C. Eickel, J. Catal., 128 (1991) 526. R. Melendrez, G. Del Angel, P. Marecot and J. Barbier, XIII Simposio Iberoamericano de Catalisis, Segovia, Spain, 6-10 July, 1992. J.M. Dominguez, P. Marecot and J. Barbier, in preparation. A. Fakche, Thesis, Poitien University (1991). C. Montassier, J.C. Men&o, J. Moukolo, J. Naja, L.C. Hoang and J. Barbier, J. Mol. Catal., 70 (1991) 645. J.E. Benson and M. Boudart, J. Catal., 4 (1965) 705. Balakrishnan, A. Sachdev and B.E. Koel, J. Catal., 121 (1990) 441. Y.-H. Hu, Thesis, Rice University (1975). K. Otto and M. Shelef, Z. Phys. Chem., 85 (1973) 308. Y.-H. Hu, D. Van L&burg and J.W. Hightower, Kinet. Katal., 18 (1977) 545. A. Frennet, Catal. Rev. Sci. Eng., 10 (1974) 37. G.A. Martin and B. Imelik, Surf. Sci., 42 (1974) 157. A. Frennet, G. Lienard, G. Crucq and L. Degols, J. Catal., 53 (1978) 150. H.F. Leach, C. Mirodatos and D.A. Whan, J. Catal., 63 (1980) 138. G.A. Martin, Catal. Rev. Sci. Eng., 30 (1988) 519. S. Galvagno and G. Parravano, J. Catal., 55 (1979) 272. H.C. De Jongste and V. Ponec, J. Catal., 64 (1980) 228.