Methanation surface structure sensitivity

In the C02 reforming of methane, carbon formation can occur via two possible pathways CH4 decomposition and CO disproportionation (the Boudouard reaction). Carbon formation by CH4 decomposition is a structure-sensitive reaction (158,159). Specifically, the Ni(100) and Ni(110) surfaces are more active in the decomposition of CH4 to carbon than the Ni(lll) surface (158). The CO disproportionation,... 

Characterization of the Surfaces of Catalysts Measurements of the Density of Surface Faces for High Surface Area Supports. - It has always been a tenet of theories of catalysis that certain reactions will proceed at different rates on different surface planes of the same crystal. Experiments with metal single crystals have vindicated this view by showing that the rate of hydrogenolysis of ethane on a nickel surface will vary from one plane to another. In contrast the rate of methanation remains constant for the same planes.4 Because of this structure sensitivity of catalytic processes there is a requirement for methods of determining the number of each of the different planes which a catalyst and its support may expose at their surfaces. Electron microscopy studies of 5nm Pt particles supported upon graphite show them to be cubo-octahedra with surfaces bound by (111) and (100) planes.5 Similar studies of Pd and Pt prepared by evaporation reveal square pyramids of size 60-200 A bounded by incomplete (111) faces.6... 

This section will address the following problems loss of matter in the methane dimeriza-ticm catalysts, the role of sintering, the loss of surface area, and finally, structure sensitivity of some of the methane-oxidative coupling catalysts. 

The structure sensitivity of the methanation reaction is shown in Fig. 18. In general the TOF decreases for increasing FE for all the metals studied. The decrease in TOF for particles smaller than 5 nm is usually explained in terms of the decrease in surface concentration of ensembles of sufficient size to dissociate adsorbed CO, but why this effect should persist out to 100 nm for some metals is little discussed. 

Hydrogenolysis reactions over Ir single crystals and supported catalysts have also been shown to be structure sensitive [M, M and M] In particular, it was found that the reactivity tracked the concentration of low-coordination surface sites. Figure A3.10.22 shows ethane selectivity (selectivity is reported here because both ethane and methane are products of butane cracking) for -butane hydrogenolysis over Ir(l 11) and the reconstructed surface Ir(l 10)-(1 x 2), as well as two supported Ir catalysts. There are clear selectivity differences between the two Ir surfaces, with Ir(l 10)-(1 x 2) having approximately three times the ethane selectivity of Ir(l 11). There is also a similarity seen between the ethane selectivity on small Ir particles and Ir (110)-(1 X 2), and between the ethane selectivity on large Ir particles and Ir(l 11). 

Andersson MP, Abild-Pedersen F, Remediakis IN, Bligaard T, Jones G, Engbaek J, Lytken O, Horch S, Nielsen JH, Sehested J, Rostrap-Nielsen JR, Nprskov JK, Choikendorff I. Structure sensitivity of the methanation reaction H -induced CO dissociation on nickel surfaces. J Catal 2008 255 6. 

---