(720b) Identifying the Active Site for the Water-Gas Shift Reaction over TiO2 Supported Pt Catalysts: Single Pt Atoms Versus Pt Clusters

Authors: 
Heyden, A., University of South Carolina
Ammal, S. C., University of South Carolina
Atomically dispersed noble metal catalysts on support surfaces offer maximum utilization of expensive noble metals that have potential applications in a variety of industrial chemical reactions. Fabrication of stable single atom catalysts has, especially in the case of Pt, always been a challenging task because of their high mobility that can lead to sintering under realistic reaction conditions. A few recent studies reported successful synthesis of single Pt atom catalysts on various supports which also showed excellent activity for CO oxidation and water-gas shift (WGS) at low temperatures. In spite of these advances, the exact nature and function of the active sites in these highly active catalysts is still a matter of debate and disagreements have been reported regarding the active site structures being single metal atoms or small metal nanoparticles. In this work, using a combination of DFT and microkinetic modeling techniques, we investigated the WGS activity of Pt catalysts supported on TiO2 (110) considering different active sites at a Pt cluster/TiO2 interface and a positively charged single Pt atom. Our goal is to determine under which conditions a specific active site is responsible for the majority of the observed experimental behavior.

We find that positively charged single Pt atoms, if stabilized on a TiO2 surface, can be as active or even more active as nanoclusters for the WGS at low temperatures (< 573 K). Corner interface Pt sites become most active at temperatures above 573 K. These results agree with the experimental report by Flytzani-Stephanopoulos group (J. Am. Chem. Soc. 2015, 137, 3470) that positively charged single Pt atoms on TiO2 are active for the WGS reaction. In a contradictory report, Stair and coworkers (Science 2015, 350, 189) claimed that the active site is the Pt nanoparticle and not the single Pt atoms based on the fact that they observed the disappearance of only the Pt nanoparticle related CO peak/spectrum and no change in the Pt single atom related CO peak during WGS reaction at a temperature of up to 300 °C. In our single atom model, one CO remains on the Pt atom throughout the catalytic cycle which can explain the non-disappearance of the Pt single atom related CO peak observed by the Stair group. In fact, this strongly adsorbed CO reduces the adsorption strength of the second CO molecule on Pt which reacts easily with the neighboring oxygen atom to form CO2. In other words, the support, one CO and one H atom act as ligands on the positively charged Pt atom. Overall, we find that the redox pathway operates on different types of active sites at the Pt/TiO2 (110) interface and oxygen vacancies on the TiO2 surface play a significant role in WGS activity.