Theoretical Investigation Of Acetylene Conversion On Pt And Rh Surfaces

Understanding the chemistry of acetylene on transition metal surfaces has attracted much attention because it presents a prototype to study many aspects related to the reactivity of olefins and serves as a benchmark for the study of more complex reactions involving higher alkenes and aromatics.In this study, the catalyzed performaces of two metals for acetylene conversion to ethylidyne are studied by using density functional theory; the catalyted conversion mechanism is obtained, which is may used to guide the catalysts design in this reaction.Firstly, the conversion of acetylene on Pt(111) is systematically studied. The molecule of acetylene adsorbs weakly over the Pt(111) surface in the configurations designated as di-σ,→2→2 andÏ€adsorptions. Most intermediates of C2Hx (x = 1– 4) are inclined to adsorb to the surface via a saturated sp3 configuration of carbon atoms with the lost H atoms replaced by the metals. DFT calculations and kinetic analysis suggest that without preadsorbed H on surface, conversion of acetylene to ethylidyne occurs via the initial disproportionation of acetylene forming the adsorbed vinyl, which further dehydrogenate to vinylidene to provide the surface available H atoms; the H atoms would recombine vinylidene to ethylidyne (C₂H₂→CHCH₂→CCH₂→CCH₃); when the coadsorbed H presented, the acetylene would hydrogenation to vinyl initially, then the vinyl dehydrogenate to vinylidene and last hydrogenation to vinylidyne (C₂H₂→CHCH₂→CCH₂→CCH₃). Hydrogenation of vinyl to ethylene involves a high activation barrier indicating it is hard to hydrogenation vinyl to ethylene.Then, acetylene hydrogenation conversion into ethylidyne on Rh(111) has been explored by DFT slab calculation and microkinetic modeling. Acethylene could form one adsorption configurations designated asη²Î·² adsorptions on fcc and hcp sites of Rh(111); ethylene could form two adsorption configurations designated as di-→and→adsorptions; ethyl, vinyl, vinylidene, ethylidyne, and ethylidene prefer the saturated sp³ configuration of both carbon atoms with the lost H atoms replaced by the metal atoms. The three-step conversion pathways on Rh(111), i.e., acetylene→vinyl→vinylidene→ethylidyne, is the most feasible, in which the vinylidene hydrogenation is the rate-limiting step. The pathway through ethylidene intermediate, ethylene→vinyl→ethylidene→ethylidyne, is impracticable because it has a conversion rate at least 10⁴ times lower than the most favorable path at the real reaction conditions. Conversion involving direct isomerizations is also unlikely to be important due to the very high energy barriers involved.