Mechanistic study of the extraordinary catalytic activity of nanogold-TiO₂ systems

Gold nanoparticles dispersed on metal-oxide surfaces have attracted extensive interests due to their exceptional catalytic activity to many important chemical reactions. However, large-scale applications of nanoscale gold (nanogold) catalysts in the industry are still in challenge due in part to the incomplete understanding of the origin of their high catalytic activity in various operating conditions. To gain deeper insights into the nanogold catalysis, particularly at the molecular level, we have conducted comprehensive computational research on the catalytic mechanism of the CO oxidation on a variety of gold clusters (Au_n) supported on rutile TiO₂ (110) surfaces (here denoted as nanogold-TiO₂ systems) by using state-of-the-art saddle-point-searching algorithm and the Born-Oppenheimer molecular dynamics (BOMD) simulation method within the framework of the density functional theory.

One open issue in the field of nanogold catalysis concerns the location of the most active zone. In our real-time BOMD simulation, we directly observe the formation of an OCOO complex at the Au-TiO₂ interface, from which we uncover the dual-perimeter-site (DPS) mechanism. The follow-up saddle-point-searching computation confirms the favorability of the DPS mechanism as it entails much lower energy barriers compared to the CO oxidation on the surface sites of gold clusters. We, therefore, conclude that the interfacial region is a critical active zone and mainly responsible for the high catalytic activity of nanogold-TiO₂ systems.

The active role of the lattice oxygen on the TiO₂ support is another crucial issue which has been in debate for decades due to the lack of the direct experimental evidence. Our BOMD simulation, for the first time, provides a direct simulation proof of the generation of the oxygen vacancy at the gold-TiO₂ interface during CO oxidation, thus proving the active role of the lattice oxygen. The formation of the oxygen vacancy at the gold-TiO₂ interface also demonstrates the feasibility of a controversial mechanism, Mars-van Krevelen (Mv-K) mechanism, in nanogold-TiO ₂ systems. The saddle-point-searching computation confirms the more likelihood of the Mv-K mechanism over the DPS mechanism for the CO oxidation catalyzed by fluxional Au clusters on TiO₂ surfaces.

Furthermore, we have provided deeper insights on the size- and shape-dependence of catalytic activity by a systematical study of the CO oxidation on a series of Aun/TiO₂ (n=1-4, 7, 16-20) systems and ‘magic-number’ Au clusters (e.g., Au147 and Au309) with the identified reaction mechanisms. For small-sized Aun clusters (n < 10), the reaction barriers are mainly responsible for the difference in catalytic activity, whereas, for larger Aun clusters (n ≥ 16), the O₂/CO ratio of adsorption energy becomes more important. Our computational results also suggest the hollow-cage Au18, which was reported as the most active gold clusters in experiments for the MgO-supported Aun clusters (n?20), is most active to CO oxidation among the TiO₂-supported Au clusters we studied.