H2 adsorption and H/D exchange on Au/TS-1 and Au/S-1 catalysts

Abstract

We report a combined quantum-mechanics/molecular-mechanics (QM/MM) analysis of H2 dissociation and hydrogen–deuterium (H/D) exchange on four potential active sites inside the TS-1 pores: (1) Au3/T6–Ti-non-defect, (2) Au3/T6–Si-non-defect, (3) Au3/T6–Ti-defect, and (4) Au3/T6–Si-defect. We provide full kinetic and thermodynamic data calculated at standard conditions (298.15 K, 1 atm) for Eley–Rideal mechanisms on these sites. The H/D exchange on Au3/TS-1 occurs in two steps: (1) first H2 dissociation on Au3/TS-1 to form H–Au3–H species and (2) D2 (or second H2) attack on these H–Au3–H species to form HD. The energetics of the first H2 dissociation step is site-sensitive (with respect to Au sites), while that of the D2 (or second H2) addition step is not site-sensitive. We found that two different mechanisms for the second step are both kinetically and thermodynamically favorable. The most favorable mechanism (ΔE act ~ 28 kcal/mol) involves an attack of D2 on both the H atoms in the H–Au3–H intermediate, and two HD molecules are formed simultaneously. The first H2 dissociation step is almost thermoneutral and the D2 (or second H2) addition step is somewhat exothermic. A comparison of the pure QM and QM/MM calculations on Au3/TS-1 suggests that the formation of the H–Au3–H species inside the TS-1 pores becomes thermodynamically more favorable due to the long-range interactions. The activation energies for the first H2 dissociation step (19–24 kcal/mol) are lower than those for the D2 (or second H2) addition step (28–31 kcal/mol). Therefore, the increase in the HD formation rate with temperature is likely to be stronger than the increase in the H–Au3–H formation rate. On the basis of the calculated activation energies and the reaction thermochemistry, we predict a viable Eley–Rideal H/D exchange pathway that may operate at or above 573 K. We also found potential H/D exchange channels on bare TS-1 (without Au3) where gas-phase D2 (or second H2) attacks the Ti–OH or Si–OH groups (of defect sites) and exchanges one of the D atoms to form HD and Ti–OD or Si–OD groups.