(449d) CO2 Hydrogenation over Rhodium Cluster Catalyst Nucleated within a Manganese Oxide Framework

As global concerns regarding carbon mitigation escalate, there is a growing focus on methods that can convert carbon dioxide into valuable products. One such method, CO₂ hydrogenation, not only helps mitigate CO₂ emissions but also yields valuable chemicals like methane and carbon monoxide. However, due to the stability of CO₂, this reaction requires catalysts with high activity, selectivity, and stability, often necessitating high pressure and temperature for efficient thermal catalysis. Various metals, including cobalt, nickel, and rhodium, have been explored for CO₂ hydrogenation. Rhodium shows high reactivity compared to other transition metal catalysts, enabling operation at lower temperatures and pressures. Moreover, to isolate metal centers, metal oxide frameworks (MnOx) offer a promising means for enhancing catalytic performance, providing a tunable structure, and facilitating the incorporation of secondary elements to improve catalytic activity. However, there is a lack of a detailed study for understanding the structure-function relationship for CO₂hydrogenation over highly controlled rhodium sites embedded in metal oxide frameworks.

In this study, we examined the effect of framework on CO₂ hydrogenation with two different MnOxs: octahedral layered structure (OL1) and octahedral molecular sieve structure (OMS2). Additionally, we introduced three secondary elements (V, Zn, and Na) to investigate their influence of electron withdrawing, donating, and traditional promotional effects. Our findings indicate that Rh-Na-OL1 exhibited the highest methane selectivity, particularly around 250 °C. OL1 generally shows higher selectivity compared to OMS2, while OMS2 demonstrated better stability. To characterize the catalysts, we employed temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray absorption fine structure (XAFS) analysis. These techniques revealed that rhodium initially existed in the Rh³⁺ state, transitioning towards Rh¹⁺-Rh⁰ during CO₂ hydrogenation. We are able to understand the structure-function relationship for Rh catalysts, and the kinetic and mechanistic influences of secondary metal additions in CO₂hydrogenation.