Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CH(x) (x = 0-3) byproducts of methanol decomposition. To explore this problem computationally, we applied our recently developed approach to model realistically metal catalyst particles as moderately large three-dimensional crystallites. We present here the first results of this advanced approach where we comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CH(x) species are formed, we carried out density functional calculations of C-O bond scission in methanol and various dehydrogenated intermediates (CH3O, CH2OH, CH2O, CHO, CO), deposited on the cuboctahedron model particle Pd79. We calculated the lowest activation barriers, approximately 130 kJ mol(-1), of C-O bond breaking and the most favorable thermodynamics for the adsorbed species CH3O and CH2OH which feature a C-O single bond. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH3 and CH2 preferentially adsorb at edge sites of nanoparticles, we rationalize experimental data on catalyst poisoning.