Theoretical investigation of conversion between different C2Hx species over Pd–Ag/Pd(100) surface alloys: influence on the selectivity and transformation of carbonaceous species
Full reaction pathways for acetylene hydrogenation on model catalysts are important for understanding the influence of ethylene selectivity and the formation of carbonaceous species. Herein, we report detailed hydrogenation and dehydrogenation reaction pathways for acetylene on the Pd(100) surface and its surface alloys with Ag atoms using first principles calculations. All adsorption species with two C atoms were taken into account. Thirteen corresponding reaction barriers were compared to analyse the reaction activity and selectivity. The selectivity for ethylene of pure Pd(100) surface was not suitable owing to the low reaction barrier for converting CHCH2 species to CHCH3 species and the high reaction barrier for ethylene formation. When the surface Ag concentration was 50%, in which all surface Pd atoms were separated, the ethylene selectivity was enhanced owing to the raised reaction barrier for converting CHCH2 to CHCH3. By analysing the formation of carbonaceous species and their hydrogenation reactions, it was found that, when carbonaceous species were formed on the catalyst, they were difficult to convert back to acetylene and ethylene molecules when hydrogen was supplied. Therefore, the Pd–Ag(100) facet catalyst was not beneficial for the movement of carbon deposits, with the low selectivity mainly due to hydrogenation of the carbonaceous species rather than over-hydrogenation of ethylene. These results will significantly influence current understanding of morphology designations in Pd–Ag bimetallic nanocatalysts.