Paper SS1-TuM5
H Induced CO Dissociation on Nickel Surfaces

Tuesday, October 16, 2007, 9:20 am, Room 608

The dissociation of CO is both a model test reaction on clean single crystals and a relevant reaction step for industrial methanation. Much attention has therefore been dedicated to understanding the details of the mechanism. First of all, there seems to be a lack of consensus on whether CO can dissociate on Nickel at low temperatures (400 K) in a UHV experiment. In this work we have used a Ni(14,13,13) crystal which is mis-cut in such a way that there is an atomic step for each 26 terrace atoms. We show that CO (which has carefully been cleaned from Ni(CO)₄) does not dissociate unless the temperature is larger than 400 K and there are atomic steps present on the surface. If the steps are blocked by sulfur no dissociation could be measured under UHV conditions. The activation energy for dissociation is found to be 1.6 eV, which is slightly larger than that for desorption. This observation is found to corroborate with DFT calculations indicating a barrier for dissociation of 1.7 eV. This is, however, in contrast with the activation energy of 1.07 eV and 1.01 eV observed for the methanation reaction on single crystals and real supported catalysts, respectively. Furthermore, surprisingly small prefactors for those reactions have been reported in both cases. By investigating the possible pathways using DFT, a much lower barrier for CO dissociation of 1.08 eV was identified when considering a COH intermediate. By incorporating this intermediate in the reaction pathway, the low activation energy and low prefactors can be explained in terms of a COH intermediate dissociating on step sites both in the single crystal experiments and on the nanoparticles constituting the real catalysts. In the latter case the conclusion is further confirmed by investigations of the relation between the particle size and the catalytic rate, where a strong deviation from a simple surface area effect is found.