Butane hydrogenolysis over single-crystal rhodium catalysts

Abstract

Hydrogenolysis of n-butane has been studied over the (1 10) and ( 1 11) surfaces of Rhodium. On Rh( 1 lo), the products of the hydrogenolysis consist of methane > ethane > propane. The hydrogenolysis reaction exhibits a good fit to Arrhenius behaviour for reaction temperatures up to 500 K. At higher temperatures the reaction rate tends to 'roll over' due to insufficient surface coverage of hydrogen. The 'roll over' affects product distribution, yielding more complete hydrogenolysis to methane. The behaviour is qualitatively similar on Rh( 11 l ) , with 'roll over' occurring at a lower temperature, namely 475 K. However, the hydrogenolysis is more selective on Rh( 11 l), yielding 50 mol O/O ethane. The high ethane selectivity seen in previous work on Ir( 110) is not seen on the Rh( 110) surface presumably because the Rh surface does not exhibit the (1 x 2) 'missing-row' reconstruction that is stable on Ir( 110) surfaces under reaction conditions. The hydrogenolysis selectivity of the Rh single-crystal surfaces correlates well with supported Rh metal particles subjected to oxidation-reduction cycling.

Chemical and Nanostructural Aspects of the Preparation and Characterisation of Ceria and Ceria-Based Mixed Oxide-Supported Metal Catalysts

“…Diagrams in Figure 4.7 correspond to the latter step, high temperature TPD. This experimental protocol is close to that applied in a number of TPD-H 2 studies on different Rh/Ce0 2 (124,166,204,222,380) and Pt/Ce0 2 (135) catalysts.…”

n-Butane hydrogenolysis as a probe of surface sites in rhodium metal particles: correlation with single crystals

“…As Rh particle size is increased, hydrogenolysis selectivity approaches that of Rh-(111). Preoxidation of the supported Rh at 773 K causes the ethane selectivity to drop to near 30% and a corresponding increase in hydrogenolysis activity of approximately 4 times. The increased activity and altered hydrogenolysis selectivity can be restored by annealing in H2 at 773 K. These changes in activity and selectivity are reversible and are caused by surface restructuring that occurs on the Rh surfaces due to volume increase upon oxidation and formation of rough surfaces when the catalyst is reduced under mild conditions.…”

Correlations between Surface Science Models and “Real-World” Catalysts

“…The formation of a metallacycle pentane is sterically forbidden on the Ir(111) surface. 88 Numerous studies have addressed the hydrogenolysis of n-butane on Rh 93 and Pt; 94 neopentane on Ir 88 and Pt; 94 n-hexane on Pt 95 and Au-Pt; 96,97 n-heptane on Pt; 98 cyclopropane on Ni, 99 Mo, 100 Pt, 4 and Ir; 101 methylcyclopropane on Ir 101 and Pt; 102 and cyclohexane on Pt 103 and Ru. 104 Altogether this large inventory of single-crystal reaction data shows very convincingly that alkane hydrogenolysis, in general, is structure sensitive with the more open surfaces exhibiting considerably greater propensity for breaking C-C bonds.…”