(561f) Active Site Requirements and Kinetic Consequences of Reactive Oxygen Atoms During CH4 Reforming and Combustion On Pt Clusters

The catalytic conversion of methane to transportation fuel or chemicals is of practical significance. Commercially viable gas-to-liquid processes convert natural gas (primarily CH₄) via steam or autothermal reforming reactions which produce CO/H₂ mixtures as feedstock for downstream synthesis steps. Reforming kinetics are well established on Group VIII (Ni, Pt, Rh, Ir, and Ru) metal clusters, which proceed via a kinetically-coupled CH₄ decomposition-oxidation cycle [1-2]. CH₄ activation involves kinetically-relevant C-H bond dissociation on uncovered metal surfaces to form chemisorbed CH3 and H species; this step is coupled with the oxidation step in which chemisorbed O* and OH* species, formed from H₂O or CO₂ co-reactants, scavenge CH_x species to complete catalytic cycles. The reversible activation of these co-reactants is fast compared with C-H bond activation steps and surfaces remain essentially uncovered by reactive or unreactive intermediates during CH₄ reforming. CO/CO₂ and H₂/H₂O rations are determined by the water-gas-shift thermodynamics. The use of O₂ as co-reactant, in contrast to CO₂ and H₂O reformings that are limited solely by the activation of C-H bonds in CH₄, promotes CH₄ turnover rates and leads to rates higher than with H₂O or CO₂ co-reactants. Herein, we describe the role of O₂ in CH₄-O2 reaction on Pt clusters (1.5-30 nm) based on kinetics and isotopic probe reaction measurements under strict kinetic control at relevant operating conditions. We also probe the molecular level details of the reaction using density functional theory (DFT) computations; specifically, we describe the roles of chemisorbed oxygen and oxygen vacancies and the influences of oxygen binding energies on the transition state structures, reaction barriers, and energies during the kinetically-relevant C-H bond dissociation steps.

At all residence times and CH₄/O₂ conversions, Pt clusters give CO/CO₂ ratios below 0.02 (873 K) leading to detectable O₂ in the effluent stream. Competitive ¹³CH₄-¹²CO-O₂ oxidation reactions show O₂ selectivities towards ¹²CO that are two orders of magnitude higher than ¹³CH₄. These evidences suggest that the direct CH₄ partial oxidation is not possible at the molecular scale; CO and H₂ are thus form from reactions of CH₄ with combustion products (H₂O or CO₂) via reforming pathways. We focus here on combustion pathways, because this reaction is the only one detected with CH₄- O₂ reactant mixtures. The combustion reaction (on Pt clusters) proceeds via multiple kinetic regimes, each of which can be characterized with distinct kinetic equations and rate parameters that are determined by the coverage and reactivity of the chemisorbed O*. CH₄ combustion turnover rates shifted from linear dependence on P_CH4¹P_O2⁰ to P_CH4²/P_O2 as the O₂ / CH₄ ratios decreased. At even lower O₂/CH₄ ratios, rates became proportional to P_CH4⁰P_O2¹. CO and H2 only formed upon the complete consumption of the limiting O₂ reactant following reforming kinetics. The seemingly complicated results, when plotting the CH₄ conversion rates (as pseudo first-order rate constants, rCH4/PCH4) with O₂ / CH₄4 ratios, converge onto a universal correlation of rates and reactant pressures. This is because the O₂/CH₄ ratios determined the relative ratios of chemisorbed oxygen (O*) and oxygen vacancies (*) during CH₄ conversion which in turn determined the turnover rates and the associated operating kinetic regimes. Chemisorbed O* and * form oxygen-oxygen (O*-O*) and oxygen-oxygen vacancy (O*-*) site-pairs which assist the kinetically-relevant activation of C-H bonds in CH₄ at much higher rates than uncovered metal sites present during reforming. The O* and * ratios on Pt surfaces during reactions are determined by the surface oxygen chemical potential (expressed as virtual oxygen pressure, (O₂)_v). We describe a rigorous way to correlate the (O₂)_v to the gas phase composition and the kinetically-relevant rate constants during oxidation reactions; we show that the (O₂)_v is a strict function of O₂/CH₄ ratios and therefore the appropriate descriptor which allows us to predict catalytic activity and the thermodynamically stable state of the metal clusters during reaction.

The catalytic functions of chemisorbed oxygen (O*) and vacancies (*) on the kinetically-relevant C-H activation path are examined using ab initio quantum chemical treatments on closed-packed metal surfaces and clusters in cubooctahedral constructions (201 atoms). Weakly bound O* at near saturation coverage, together with a vicinal vacancy (*), formed O*-* site-pairs which assist the initial C-H bond dissociation to form hydroxyl and methyl intermediates. The C-H activation barriers on these site-pairs decreased as the O* becomes weakly bound and acquired a more basic character, consistent with prevalent Brønsted-Evans-Polanyi (BEP) relations of activation barriers and reaction energy. We probe the effects of O* basicity experimentally by varying the Pt cluster sizes and thus varying the average coordination numbers of metal atoms residing on the cluster surface. Large Pt clusters, with weakly bound O* species, are more active than the smaller clusters at regimes where O* are involved in assisting the C-H bond dissociation. The trend is reversed from the reported structure-sensitivity effects for reforming reactions where the under-coordinated atoms located on surface defects are most reactive. Similar kinetic investigations were carried out on Rh, Ru, and Pd clusters. The elementary steps remain identical, except that the CH₄-O₂ reaction proceeds almost exclusively on O* and OH* saturated surfaces on Rh, Pd and Ru, because these metals bind O* and OH* more strongly, and thus the concentration of oxygen vacancies (*) are lower than that on Pt at any given O₂/CH₄ ratios. The O*-* site pairs only become available on these metals at much higher temperatures or lower O₂/ CH₄ ratios.

We illustrated here, using the case of CH₄-O₂ reaction, that kinetic dependences and product selectivity are not properties of named reactions (combustion, reformings, or partial oxidation). These reactions proceed via identical catalytic cycles consisting of a closed sequence of elementary steps. Different rates and selectivities are detected because the identity of the kinetically-relevant step varies according to (O₂)_v which in turn lead to the observed combustion, apparent partial oxidation, or reforming products in the reactor effluent.

References

1. J. Wei and E. Iglesia, J. Phys. Chem. 108, 4094 (2004).

2. J. Wei and E. Iglesia, J. Catal. 225, 116 (2004).

Acknowledgement

This study was supported by BP as part of the Methane Conversion Cooperative Research Program.

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