from first principles PP-I-1

KL-1The detailed mechanism of chain growth depends on the structure of the catalyst. We willdiscuss several possibilities.We find that the optimum chain-growth probability not only requires a step-edge type ofreaction center, but also a site that has a dual reaction center. In the latter case CO dissociationand chain growth do not interfere. A necessary condition is that Cads is generated initially ina four fold site and upon hydrogen addition moves to a three fold site.Whereas overall kinetics according to the single site model is limited by chain termination asslow step, in the dual site model the chain growth reaction has to be the slower elementaryreaction step. This is in line with the general observation that chain growth parameters aredifferent for the formation of short hydrocarbon chains versus the longer ones. Since particlesize and shape will affect the relative stability of reactive centers, these results have importantconsequences for particle size dependence.We predict low chain growth probabilities on the terraces, where formation of intermediateCH x species is relatively slow. The dominant chain growth sites will be the step-edge typecenters with efficient CH x formation rates. When one compares the relative consumption ratesof different metals it is important to establish whether comparisons are made for surfaces withCO dissociation as the slow step versus the rate of chain growth termination or the elementaryrate of chain growth. In the comparison of Ru and Co, the terraces are found to be morereactive for Ru, but the step edge sites give the higher selectivity for Co.References:[1] R.A.van Santen, E. van Steen, I. Ciobica, M.M. Ghouri, Adv. Catalysis, 54, 127 (2011).[2] R.A.van Santen, M.M. Ghouri, S. Shetty, E. J. M. Hensen, Catal. Sci. and Technol, 1, 891 (2011)16