The ever-evolving active site: transformation of single atoms to extended structures during the Rh-catalyzed reverse...

At low temperatures (<400 °C), single atoms of Rh supported on rutile TiO₂ (rTiO₂) are responsible for the formation of CO during the reverse water gas shift (RWGS), while methane production is associated with the Rh-TiO₂ interface due to the observed correlation between methane formation rates and the volume-averaged Rh nanoparticle diameter. As the temperature is increased to >540 °C, there is a notable increase in CO selectivity as the methane production rates tend towards zero. The time to reach zero depends on the temperature but is independent of the initial Rh structure (single atoms and/or nanoparticles), which is controlled by the catalyst preparation method (wetness impregnation versus colloidal nanoparticles). At 600 °C and >4 h time on stream, the catalytic behaviour becomes completely agnostic to the initial Rh structure as well as weight loading, and the catalysts are highly selective for the RWGS reaction. Post-reaction HR-TEM image analysis confirms Rh nanoparticles crystallize/order during the reaction; at 400 °C, most of the Rh particles are disordered, while at 600 °C, they are more ordered (i.e., there is the development of defined faceting). Infrared spectroscopy of CO adsorption on Rh nanoparticles confirms the appearance of defined facets after annealing in nitrogen at high temperatures. Annealing the Rh/rTiO₂ catalysts prior to the RWGS reaction demonstrates the structural transformation of Rh depends only on time and temperature and not on reactant or product fugacity. Sites responsible for stabilizing Rh single atoms are no longer competent at higher temperatures, enabling single atom integration into existent nanoparticles. As the reaction temperature is increased to temperatures >540 °C, the dominant Rh structure for CO production evolves from single atoms to extended surfaces.