Methane Dissociation on High and Low Indices Rh Surfaces

Abstract

We present DFT calculations of the energetics of the elementary reaction steps in the dehydrogenation of CH 4 to C on extended Rh(111) and Rh(211) surfaces. The results are compared to the energetics for the same reactions on a planar (111) surface and the edge atoms that are shared between two (111) facets of a nanorod model. The adsorption energies between comparable surfaces of the extended and nanorod models are very similar. Only C adsorbs significantly stronger on the planar surface of the nanorod model than on the extended (111) surface due to the involvement of more reactive edge atoms. Also, the reaction energies between the two types of surface models are very similar. The small differences in reaction and activation energies are largely due to small geometrical differences. In all cases, CH dissociation has the highest activation barrier. However, dissociative CH 4 adsorption is rate controlling under typical steam reforming conditions because of the entropy loss associated with methane adsorption. The barrier for CH 4 dissociation significantly decreases with a decrease of the coordination number of the surface metal atoms. Accordingly, the corrugated surfaces are predicted to be more reactive for methane dissociation than the planar ones.

Methane dissociation on Ni (100), Ni (111), and Ni (553): A comparative density functional theory study

“…The barrier is lower by 0.23 eV when compared to that obtained for the Ni (1 1 1) flat surface. As expected, the presence of step sites has a strong influence on the activation energy [72][73][74][75]. This must be attributed to the strong bonding of CH 3 at the step edge.…”

Ethylene formation by methane dehydrogenation and C–C coupling reaction on a stoichiometric IrO2 (110) surface – a density functional theory investigation

“…This observation is strikingly different with the process on almost all transition metal surfaces where the CH dissociation is the rate-determining step. [60][61][62][63][64] Based on the calculated results, it can be inferred that CH 2 is a kinetically significant intermediate and is the most abundant species on the IrO 2 (110) surface. Therefore, by controlling the reaction conditions at mild temperature, it could be possible to block CH 4 dehydrogenation after the first two dehydrogenation reactions and achieve selective dehydrogenation to CH 2 .…”

Low temperature steam reforming of methane: A combined isotopic and microkinetic study

“…and Blaylock et al[36], while an experimentally reported value of 715 kJ/mol was reported by Isett et al[76] for the same facet. On the other hand for Rh, in the work of Bunnik et al[74] a value of 705 kJ/mol was reported for Rh(111), whereas for the same surface adsorption energies as high as 775, 813 and 825.0 kJ/mol were calculated by Zhang et al[77], van Grootel et al[78] and Wang et al[79] respectively. Based on the overall satisfactory agreement described above, there is confidence that also the values of the calculated activation energies through the UBI-QEP framework lie within physically realistic limits and represent the surface energetics reasonably.There is a trend identified in the estimated chemisorption heats, wherein most of those of CHx * species are larger for Rh in comparison to Ni suggesting a stronger binding for all these intermediates.…”