Significant recent interest has been placed on identifying and exploiting the unique reactivity of atomically dispersed active metal sites on oxide supports. Generally it has been found that the synthesis of atomically dispersed active species with high concentration, and stabilization against agglomeration under reaction conditions, is difficult. Focus has been placed primarily on the excellent reactivity of these active sites in reactions where activity is the crucial descriptor of performance. In this talk we will discuss the reactivity of atomically dispersed Rh active sites on TiO2 supports in the reduction of CO2 by H2. It will be shown that these species are highly stable under reaction conditions and exhibit different selectivity than Rh nanoparticles for CO2 reduction.1,2 In addition, environmental conditions were identified that enable reversible tuning of the catalyst morphology in situ, resulting in catalysts with high concentrations of atomically dispersed Rh active sites as the only exposed species.
Catalytic CO2 reduction by H2 at stoichiometric methanation and H2-lean CO2:H2 feed ratios was explored over various weight loadings of Rh on TiO2 supports. The concentration of atomically dispersed, or isolated Rh active sites, and Rh atoms sitting at the surface of Rh nanoparticles were quantified using probe molecule Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), coupled with known extinction coefficients. 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). 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. We also found that at high CO2:H2 ratios methane-producing RhNP sites convert to CO-producing Rhiso sites, thereby controlling the instability of catalytic selectivity with time on stream. This process was augmented to completely convert the catalyst into a state with either exclusively exposed RhNP or Rhiso sites, allowing tuning of selectivity from almost completely methane to completely CO depending on the state of the catalyst. In addition, we determined Rhiso sites produced from the high CO2:H2 treatment were very stable under reaction conditions without adding any promoter species.
This study provides critical information towards the design of atomically dispersed Rh catalysts that exhibit excellent stability under reaction conditions and unique reduction properties compared to Rh nanoparticles based active sites.
(1) Matsubu, J. C.; Yang, V. N.; Christopher, P. J. Am. Chem. Soc. 2015, DOI: 10.1021/ja5128133.
(2) Matsubu, J. C.; Christopher, P. (In Preparation) 2015