Structure-insensitive reactions over kinetics

Kinetics of Structure Insensitive Reactions Over Clean Single Crystal Surfaces... 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Recently, FT synthesis reactions were shown to be independent of metal dispersion on Si02-supported catalysts with 6-22% cobalt dispersion (103). Turnover rates remained nearly constant (1.8-2.7 x 10 s ) over the entire dispersion range. Dispersion effects on reaction kinetics and product distributions were not reported. These tests were performed at very low reactant pressures (3 kPa CO, 9 kPa H2), conditions that prevent the formation of higher hydrocarbons and lead to methane with high selectivity and to CO hydrogenation turnover rates 10 times smaller than those obtained at normal FT synthesis conditions and reported here. These low reactant pressures also lead to kinetics that become positive order in CO pressure. Thus, the reported structure insensitivity (103) may agree only coincidentally with the similar conclusions that we reach here on the basis of our results for the synthesis of higher hydrocarbons on Co. 

The CO + O2 reaction also appears to be structurally insensitive over Ru, since kinetic data for Ru(0001) and silica-supported Ru are fairly similar (94). The highest activity occurs when the Ru is covered with a monolayer of oxygen. Since the reaction is, instead, inhibited by an oxygen-covered surface for the case of Rh, the higher activity of Ru compared to Rh at medium-pressure conditions could be explained (94). 

The methanation reaction (3H2 + CO — CH4 + H20) has been thoroughly studied by Goodman and co-workers (4, 5, 71, 96) over Ni single crystals. Since the specific rates, activation energies, and pressure dependencies are very similar over Ni(100), Ni(lll), and AI203-supported Ni, the reaction is structure insensitive (71, 96). Transient kinetic studies at medium pressures combined with postreaction AES analysis on Ni(100) have identified a carbidic form of adsorbed carbon as the reaction intermediate, and graphitic carbon as a poison formed at higher temperatures (71, 96). 

Kinetic measurements on a Pt catalyst showed no dependence on the size of the crystallites. On an inert Si02 support the catalyst turnover number remained virtually constant over the particle size range 2-1000 nm that is, the reaction is structure-insensitive. With a Ti02 support, the TON was increased by a factor of 500 following high-temperature reduction (Table 5-39). 

The oxidation of CO by either 02 or NO was studied by Peden et al. and Oh et al. over Rh, Pd, Pt,and Ir single crystals (90-92). The CO + 02 reaction was relatively insensitive to the atomic structure of the surface, and the specific activities and kinetic parameters agreed for both crystal surfaces and for alumina-supported catalysts. The Rh surfaces deactivated at high 02 pressures due to the formation of a near-surface oxide (91, 92). On the other hand, the CO + NO reaction was very sensitive to... 

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