Developing relationships between catalyst active site geometry and performance is necessary for the design of optimal catalysts. Model single crystal catalysts, under ultra-high vacuum conditions, as well as more realistic size or shape controlled metal nanoparticle systems have been utilized to elucidate these insights in the past. However, it has also been shown that difficult to detect isolated metal atoms on oxide supports can play an unexpected role in governing the reactivity of heterogeneous metal catalysts. Significant effort has recently been dedicated to understanding atomic scale catalyst characteristics that control selectivity in the reduction of CO2 by H2 to produce higher valued carbon-based products. Studies examining reactivity and selectivity trends for CO2 reduction by H2 over transition metal catalysts have identified particle size as a main characteristic controlling catalytic performance. In addition, it has been shown that larger particle sizes are less stable under CO2 reduction reaction conditions than small particles, in opposition to typically observed instability associated with smaller metal catalytic particles. Direct evidence relating these particle size dependent phenomena in CO2 reduction catalysis to the nature of active sites controlling catalytic functionality is still lacking. Here we explore catalytic CO2 reduction by H2 at stoichiometric methanation and H2-lean CO2:H2 feed ratios over various weight loadings of Rh on TiO2 supports. We observe a strong correlation between the reverse water gas shift turn over frequency (TOF) and the concentration of isolated Rh atomic sites on the TiO2 support (Rhiso) and a concurrent correlation between the methanation TOF and the concentration of Rh sites on the surface of Rh nanoparticles (RhNP). We also found that larger particles disperse under reaction conditions to form Rhiso sites and that the dispersion process controls the instability of catalytic selectivity. The results strongly suggest that Rhiso sites on the TiO2 support are active sites almost exclusively for CO2 reduction to CO, whereas RhNP sites are active sites almost exclusively for CO2 reduction to methane. This study provides critical information for the design of selective CO2 reduction catalysts and highlights the importance of thorough analysis of particle geometry dependent catalytic reactivity, where other factors, such as isolated site concentration, may actually be affecting catalytic performance.

Isolated metal active site concentration controls catalytic CO2 reduction selectivity

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.