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Density functional theory (DFT) and infrared spectroscopy results are combined with mechanism-based rate equations to assess the structure and thermodynamics of chemisorbed CO (CO") and its activation during Fischer-Tropsch synthesis (FTS). CO" binding becomes weaker with increasing coverage on Ru(0001) and Ru201 clusters, but such decreases in binding energy occur at higher coverages on Ru201 clusters than on Ru(0001) surfaces (CO"/Ru = 1.55 to 0.75); such differences appear to reflect weaker repulsive interactions on the curved surfaces prevalent on small Ru201 clusters. Ru201 clusters achieve stable supramonolayer coverages (CO"/Ru > 1) by forming geminal dicarbonyls at low-coordination corner/edge atoms. CO" infrared spectra on Ru/SiO2 (~7 nm diameter) detect mobile adlayers that anneal into denser structures at saturation. Mechanism-based FTS rate equations give activation energies that reflect the CO"-saturated surfaces prevalent during catalysis. DFT-derived barriers show that CO" predominantly reacts at (111) terraces via H-assisted reactions, consistent with measured effects of H2 and CO pressures and cluster size effects on rates and O-rejection selectivities. Barriers are much higher for unassisted CO" dissociation on (111) terraces and low-coordination atoms, including step-edge sites previously proposed as active sites for CO" dissociation during FTS. DFT-derived barriers indicate that unassisted CO" dissociation is irreversible, making such steps inconsistent with measured rates. The modest activation barriers of H-assisted CO" dissociation paths remove a requirement for special low-coordination sites for unassisted CO" activation, which is inconsistent with higher rates on larger clusters. These conclusions seem generally applicable to Co, Fe, and Ru catalysts, which show similar FTS rate equations and cluster size effects. This study also demonstrates the feasibility and relevance of DFT treatments on the curved and crowded cluster surfaces where catalysis occurs. [ABSTRACT FROM AUTHOR]

Abstract: Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O* reactions with CO and CH4, during CH4–O2 catalysis on Pt and Rh clusters. CO2 and H2O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O*) are depleted from cluster surfaces. Oxygen selectivities, measured using 12CO–13CH4–O2 reactants, increase with O2/CO ratio and O* coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O* reacts much faster with CO than with CH4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O* to produce CO2 at any residence time required for detectable extents of CH4 conversion. O* selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8nm diameter, 201 Pt atoms) at low O* coverages, which are prevalent at low O2 pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O* atoms (on *–O* site pairs). CH4 oxidation occurs via kinetically relevant C–H bond activation on *–* site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH4, forming a three-centered HC3–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH3 and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH3 * binding energies. Ensemble-averaged O* selectivities increase linearly with O2/CO ratios, which define the O* coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O2 dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33nm) at all temperatures (573–1273K) relevant for CH4–O2 reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O* selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in 12CO–13CH4–O2 mixtures, consistent with lower CO/CO2 ratios measured by varying the residence time and O2/CH4 ratio independently in CH4–O2 reactions. These mechanistic assessments and theoretical treatments for O* selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH4 to CO (and H2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H2) are predominantly formed upon complete O2 depletion from the sequential reforming steps. [Copyright &y& Elsevier]

A combined experimental and theoretical comparative study of the hydrodeoxygenation (HDO) of anisole was conducted over Pt, Ru, and Fe metals. In the experimental part, an inert silica support was used to directly compare the catalytic activity and selectivity of the three metals at 375 °C under H 2 flow at atmospheric pressure. In parallel, for density functional theory (DFT) calculations the close-packed Pt(1 1 1), Ru(0 0 0 1), and Fe(1 1 0) surfaces were employed to compare the possible mechanisms on these metals. It was observed that over Pt/SiO 2 and Ru/SiO 2 catalysts, both phenol and benzene were the major products in a phenol/benzene ratio that decreased with the level of conversion. By contrast, over the Fe/SiO 2 catalyst, no phenol formation was detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the CH 3 side group occurs before the C O bond breaking. This removal of H atoms from the CH 3 group facilitates the activation of the aliphatic C alkyl O bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals are related to the reactivity of this surface phenoxy. That is, over Pt(1 1 1) and Ru(0 0 0 1) the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. This result is in agreement with the experiments over Pt/SiO 2 and Ru/SiO 2 catalysts. Over these catalysts, both phenol and benzene are major products, with the selectivity to benzene increasing with conversion at the expense of phenol. In contrast, over the Fe(1 1 0) surface, the strong metal oxophilicity makes the direct cleavage of the C O bond in the surface phenoxy easier than hydrogenation to phenol. Thus, it is predicted that phenol is not formed over iron, but only benzene should be observed as HDO product at all conversion levels, which is also in agreement with the experimental observations. [ABSTRACT FROM AUTHOR]