Mechanism and site requirements for activation and chemical conversion of methane on supported Pt clusters and turnover rate comparisons among noble metals

被引:355
作者
Wei, JM [1 ]
Iglesia, E [1 ]
机构
[1] Univ Calif Berkeley, Dept Chem Engn, Berkeley, CA 94720 USA
关键词
D O I
10.1021/jp036985z
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Isotopic tracer and kinetic studies are used to probe the identity and reversibility of elementary steps required for H2O and CO2 reforming of CH4 on supported Pt clusters and to demonstrate a rigorous kinetic and mechanistic equivalence for CO2 and H2O reforming, CH4 decomposition, and water-gas shift reactions. Reforming rates are exclusively limited by C-H bond activation on essentially uncovered Pt crystallite surfaces and unaffected by the concentration or reactivity of co-reactants (H2O, CO2). Kinetic isotopic effects are consistent with the sole kinetic relevance of C-H bond activation (k(H)/k(D) = 1.58-1.77 at 873 K); these isotope effects and measured activation energies are similar for H2O reforming, CO2 reforming, and CH4 decomposition reactions. CH4/CD4 cross exchange rates are much smaller than the rate of methane chemical conversion in CO2 and H2O reforming reactions; thus, C-H bond activation steps are irreversible, except as required by the approach to equilibrium for the overall reforming reaction. Reactions of (CH4)-C-12/(CO2)-C-12/(CO)-C-13 mixtures led to identical C-13 fractions in CO and CO2, indicating that CO2 activation is quasi-equilibrated and kinetically irrelevant. Binomial water and dihydrogen isotopomer distributions during reactions of CH4/ CO2/D-2 mixtures indicate that these products form in quasi-equilibrated steps. Turnover rates for H2O and CO2 reforming and CH4 decomposition increased with increasing Pt dispersion, suggesting that coordinative unsaturated surface Pt atoms, prevalent in small crystallites, are more reactive than Pt atoms in low-index surfaces for C-H bond activation. Pt dispersion but not turnover rates were influenced by the identity of the support (ZrO2, gamma-Al2O3, ZrO2-CeO2)- Similar CO oxidation rates were measured before and after CH4 reactions, indicating that Pt dispersion is not affected by unreactive deposits or sintering during catalysis. These mechanistic conclusions and metal dispersion effects appear to apply generally to CH4 reactions on Group VIII metals, but the surface reactivity of Pt clusters in C-H bond activation reactions is greater than for similar size clusters of other metals. Turnover rates are compared here, for the first time, for most catalytically important noble metals (Rh, Ir, Pt, Ru) as a function of their metal dispersion on several supports. These turnover rates rigorously exclude transport and thermodynamic artifacts and provide a direct comparison of the reactivity of noble metal clusters for catalytic reactions of CH4 on materials and at conditions relevant to industrial practice.
引用
收藏
页码:4094 / 4103
页数:10
相关论文
共 69 条
[21]  
Dumesic J.A., 1993, MICROKINETICS HETERO
[22]   MOLECULAR-BEAM INVESTIGATION OF CATALYTIC-OXIDATION OF CO ON PD (111) [J].
ENGEL, T ;
ERTL, G .
JOURNAL OF CHEMICAL PHYSICS, 1978, 69 (03) :1267-1281
[23]  
ERDOHELYI A, 1991, STUD SURF SCI CATAL, V107, P525
[24]   SURFACE-STRUCTURES OF NORMAL PARAFFINS AND CYCLOHEXANE MONOLAYERS AND THIN CRYSTALS GROWN ON (111) CRYSTAL-FACE OF PLATINUM - LOW-ENERGY ELECTRON-DIFFRACTION STUDY [J].
FIRMENT, LE ;
SOMORJAI, GA .
JOURNAL OF CHEMICAL PHYSICS, 1977, 66 (07) :2901-2913
[25]   The dynamics of the dissociative adsorption of methane on Pt(533) [J].
Gee, AT ;
Hayden, BE ;
Mormiche, C ;
Kleyn, AW ;
Riedmüller, B .
JOURNAL OF CHEMICAL PHYSICS, 2003, 118 (07) :3334-3341
[26]   STEPS IN CH4 OXIDATION ON PT AND RH SURFACES - HIGH-TEMPERATURE REACTOR SIMULATIONS [J].
HICKMAN, DA ;
SCHMIDT, LD .
AICHE JOURNAL, 1993, 39 (07) :1164-1177
[27]   QUANTIFICATION OF THE SELECTIVE ACTIVATION OF C-H BONDS IN SHORT-CHAIN ALKANES - THE REACTIVITY OF ETHANE, PROPANE, ISOBUTANE, N-BUTANE, AND NEOPENTANE ON IR(111) [J].
JOHNSON, DF ;
WEINBERG, WH .
JOURNAL OF CHEMICAL PHYSICS, 1995, 103 (13) :5833-5847
[28]   QUANTIFICATION OF THE INFLUENCE OF SURFACE-STRUCTURE ON C-H BOND ACTIVATION BY IRIDIUM AND PLATINUM [J].
JOHNSON, DF ;
WEINBERG, WH .
SCIENCE, 1993, 261 (5117) :76-78
[29]   THE REACTION OF CO2 WITH CH4 TO SYNTHESIZE H-2 AND CO OVER NICKEL-LOADED Y-ZEOLITES [J].
KIM, GJ ;
CHO, DS ;
KIM, KH ;
KIM, JH .
CATALYSIS LETTERS, 1994, 28 (01) :41-52
[30]   Structure sensitivity of methane dissociation on palladium single crystal surfaces [J].
Klier, K ;
Hess, JS ;
Herman, RG .
JOURNAL OF CHEMICAL PHYSICS, 1997, 107 (10) :4033-4043