CO2 reduction by H2 on heterogeneous catalysts is an important class of reactions. However, atomic scale details of structure-function relationships are still poorly understood. Particularly, it has been suggested that metal particle size plays a unique role in controlling the stability of CO2 hydrogenation catalysts and the distribution of active sites, which dictates reactivity and selectivity. These studies often have not considered the possible role of isolated metal active sites. Here, we utilize probe molecule diffuse reflectance infrared Fourier transform spectroscopy with known site-specific extinction coefficients to quantify the fraction of Rh sites residing as single-atom isolated sites (Rhiso), as well as Rh sites on the surface of Rh nanoparticles (RhNP) for a series of TiO2 supported Rh catalysts. Strong correlations were observed between the catalytic reverse water gas shift (CO production) turn over frequency (TOF) and the fraction of Rhiso sites and between catalytic methanation (CH4 production) TOF and the fraction of RhNP sites. Differences in the reactivity of Rhiso and RhNP sites for CO2 reduction were further substantiated by selectively leeching Rh nanoparticles from TiO2 leaving behind almost exclusively Rhiso sites and a significant decrease in almost exclusively CH4 production. Furthermore, it will be shown that H2 reduction induced oxide overlayer formation on Nb2O5 supported Rh catalysts allows for selective poisoning of Rh nanoparticles leaving only exposed Rhiso sites and high selectivity for CO production in CO2 reduction by H2. This work demonstrates that isolated atoms and nanoparticles of the same metal on the same support can exhibit uniquely different selectivity in competing parallel reaction pathways. In addition. it is shown that Rh weight loading and dynamic support effects allow approaches to tune the concentration of exposed active sites that exist as isolated Rh atoms.

Isolated Rh atoms on oxide supports as CO<sub>2</sub> reduction catalysts

251st National Meeting (2016)

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Isolated Rh atoms on oxide supports as CO2 reduction catalysts

Isolated Rh atoms on oxide supports as CO₂ reduction catalysts