Density Functional Theory (DFT) study of reaction pathways on gold and gold -alloy nanoclusters

Ajay Madhav Joshi, Purdue University

Abstract

Production of propylene oxide (PO) in a single step with no side products has been a long-sought industrial target. While a H2O2/TS-1 based route to PO appears imminent, due to cost associated with H2O 2, researchers have also focused on direct propylene epoxidation using H2 and O2 over Au/Ti catalysts. Indirect evidence for activity of Au/Ti sites inside TS-1 pores (5.5 Å) suggests that small Au clusters are potentially important catalytic sites. Our DFT calculations predict that H2O2 formation is viable on small Au clusters (Au3 and Au5). The main steps in the reaction pathway are: (1) O2 adsorption on the cluster (M), (2) first H2 addition to form H-M-OOH species, (3) second H2 addition to form H2O2/H-M-H, (4) H2O2 desorption, and (5) cycle-closure steps. In the case of Au-alloy clusters, the ΔG0 for the first H2 addition step is positive on Au-Cu and Au-Ag clusters indicating thermodynamically unfavorable formation of hydroperoxy (OOH) species. Interestingly, formation of OOH and H2O2 species is favorable on Pd3, AuPd, and on almost all Au-Pt clusters. An optimum H2 efficiency is likely on Au-Pd clusters, while water formation is likely to be dominant on Au-Pt clusters. It has been speculated that Au/TS-1 catalysts may operate by a “sequential” pathway: (1) H2O2 formation on Au and (2) propylene epoxidation on Ti. To examine this possibility we performed QM/MM calculations on both non-defect and defect Ti-sites with and without Au clusters adsorbed on them. An H2O2 formation pathway similar to that found on gas-phase Au clusters operates on Au3/Ti sites. H2O 2 attacks on Ti-defect sites to form Ti-OOH species and propylene subsequently reacts with these Ti-OOH species to form PO. However, Au clusters are likely to adsorb near Ti-defect sites. Interestingly, we predict that the “sequential” propylene epoxidation pathway is kinetically unfavorable on the Au3/Ti-defect site due to a very high activation barrier for Ti-OOH formation. In fact, interplay between DFT calculations and reaction kinetic studies suggests that the actual epoxidation mechanism is “simultaneous”. The rate determining step in this mechanism involves attack of propylene (adsorbed on Au-Ti interface site) on H-Au-OOH to make PO.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Physical chemistry|Chemical engineering|Chemistry

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