(254h) A Density Functional Theory Study of Reaction of H2 and O2 to Form H2O2 on Gas-Phase Au-Alloy Clusters
In situ production of H2O2 by direct oxidation of H2 by air or O2 could significantly improve the economics for commercial use of this selective oxidant. Progress in this area, for liquid phase reactions, has recently been reported for supported Au, Pd, and Au-Pd alloys by Hutchings and co-workers[1,2]. For vapor phase reactions, Goodman and co-workers have used inelastic neutron scattering (INS) to demonstrate formation of OOH and H2O2 species from H2 and O2 on Au/TiO2 catalysts. Indirect evidence for activity of Au/Ti sites inside TS-1 pores (5.5 Å) reported by Yap et al. and Taylor et al. suggests that small Au clusters containing only a few atoms are potentially important catalytic sites for in situ oxidant (probably H2O2) synthesis in direct propylene epoxidation using H2 and O2. Due to experimental limitations in probing few-atom clusters, we have employed DFT calculations to show the viability of H2O2 formation pathways on small gas-phase Au clusters[6,7]. Motivated by the reported success of the Au-Pd alloy catalysts, we have now extended our calculations to gas phase Au-alloy dimers and trimers of Cu-Au, Ag-Au, Au-Pd, and Au-Pt and carried out full thermochemistry (298.15 K, 1 atm) which we report here. The main steps in our pathway are similar to those we have found over pure Au clusters: (1) O2 adsorption on the cluster (M), (2) first H2 addition to form H-M-OOH species, (3) second H2 addition to form H2O2 adsorbed on H-M-H, (4) H2O2 desorption, and (5) cycle-closure steps. In general, either (1) or (2) is the rate determining step (RDS). Although the activation barriers are not very high, the DE (and DG) for the first H2 addition step is positive on Cu-Au and Ag-Au clusters indicating thermodynamically unfavorable formation of hydroperoxy (OOH) species. Furthermore, the OOH formation is more unfavorable on Cu-Au and Ag-Au dimers (DE > +17 kcal/mol) than on the corresponding trimers (0 DE DE = -14 kcal/mol), but the second H2 addition to form H2O2 is slightly unfavorable (DE = +4 kcal/mol) on that cluster. On the other hand, formation of OOH and H2O2 species is both thermodynamically and kinetically favorable on most of the Au-Pd and Au-Pt dimers/trimers, with OOH formation slightly unfavorable only on PdAu2 (DE = +2 kcal/mol) and Pd2Au (DE = +4 kcal/mol). Interestingly, while the activation barriers for the OOH formation are lower on Au-Pt clusters than that on Au-Pd clusters, we predict the reverse trend for the H2O2 formation step. We conclude that supported AuPd and AuPt nanoclusters would be interesting candidates for experimental testing and are considering support effects in on-going computational work. References (1) Landon, P.; Collier, P. J.; Carley, A. F.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917. (2) Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. J. Catal. 2005, 236, 69. (3) Sivadinarayana, C.; Choudhary, T. V.; Daemen, L. L.; Eckert, J.; Goodman, D. W. J. Am. Chem. Soc. 2004, 126, 38. (4) Yap, N.; Andres, R. P.; Delgass, W. N. J. Catal. 2004, 226, 156. (5) Taylor, B.; Lauterbach, J.; Delgass, W. N. Appl. Catal. A 2005, 291, 188. (6) Wells, D. H.; Delgass, W. N.; Thomson, K. T. J. Catal. 2004, 225, 69. (7) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. B 2005, 109, 22392. (8) Joshi, A. M.; Delgass, W. N.; Thomson, K. T., Manuscript in Preparation.