(678d) Catalytic Reactivity and in Situ Synthesis of Stable TiO2 Supported Rh Isolated Sites

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 (Rh_iso) and a concurrent correlation between the methanation TOF and the concentration of Rh sites on the surface of Rh nanoparticles (Rh_NP). The results strongly suggest that Rh_iso sites on the TiO₂ support are active sites almost exclusively for CO₂ reduction to CO, whereas Rh_NP sites are active sites almost exclusively for CO₂ reduction to methane. We also found that at high CO₂:H₂ ratios methane-producing Rh_NP sites convert to CO-producing Rh_iso 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 Rh_NP or Rh_iso sites, allowing tuning of selectivity from almost completely methane to completely CO depending on the state of the catalyst. In addition, we determined Rh_iso 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