The Role of Isolated Metal Atoms and Support Effects in CO2 Reduction Catalysis by Rh

In this work, we coupled a variety of characterization techniques with an array of reactivity experiments to gain the requisite fundamentals for designing optimal heterogeneous catalysts. The main emphasis was to investigate the effect of Rh catalyst site geometry and adsorbate stabilized metal-support interactions on CO2 reduction selectivity.Much effort has been performed in the catalysis field identifying active catalytic sites by developing structure-function relationships, however, the role of isolated catalyst sites has often been overlooked due to difficulty in characterizing these single atom catalysts. The hypothesis of our first study was that isolated Rh catalytic sites play a role in driving CO2 reduction chemical pathways. Through development of a quantitative FTIR technique, fractions of Rh existing in both isolated catalytic (Rhiso) site geometries and surface sites on Rh NPs (RhNP) were measured and related with kinetic data to identify the site-specific reactivity of both site types. It was established that Rhiso sites drive CO production and RhNP sites drive CH4 production nearly exclusively. Additionally, interesting dynamic changes in catalyst behavior were observed in CO2 reduction conditions where CH4 production decreased, yet CO production increased during time-on-stream. This transformation was investigated by coupling the following in-situ techniques: FTIR, ESTEM, EXAFS and XANES, with a battery of reactivity experiments for understanding both the underlying mechanism of this transformation and how to exploit this change in pathway reactivity to control selectivity. Reaction conditions and Rh weight loading were optimized to attain a 90% switch in selectivity between producing CO and CH4 at the same conditions before and after treating the catalyst with a high CO2:H2 ratio. It was determined that the dramatic change in catalyst reactivity was from the TiO2 support forming a permeable overlayer on the RhNP sites, which caused them to behave catalytically like a more noble metal and produce CO. Although similar strong metal-support interactions (SMSI) have been thoroughly studied for controlling CO2 reduction selectivity, unlike in this work, the overlayers formed are usually hindered by either being impermeable to reactants, therefore killing off reactivity completely or unstable in CO2 reduction reaction conditions