Alloying effect on the C-C coupling reactions in acetylene hydrogenation by palladium-coinage metal alloys, a DFT study and microkinetic modeling

1,3-butadiene produced during acetylene hydrogenation is detrimental for catalysts. Combining density functional calculations and microkinetic modeling, we examined the formation of 1,3-butadiene from the coupling of various of C2 species on Pd(111) and PdM(111)(M = Cu, Ag, Au). It is demonstrated that the adsorption energies on PdM(111) linearly correlate with the ones on Pd(111). Transition state scaling (TSS) relations (linear relationships between the adsorption energies in the initial/final states and in the transition states) exist for both hydrogenation and coupling reactions. Microkinetic modeling reveals that introducing coinage metals into Pd promotes the selectivity of CH2CH2 and significantly hinders the formation 1,3-butadiene. PdAu shows the best ability to suppress the green oil formation and the catalyst deactivation. CHCH and CHCH2 are the main fragments for the C-C coupling on PdM(111), and CHCH + CHCH2 to CHCHCHCH2 and 2CHCH(2) to CH2CHCHCH2 are the main coupling reactions leading to 1,3-butadiene on PdM(111). High temperature and high H-2 to CHCH ratio are favorable for inhibition of the green oil formation.