Carbon dioxide (CO₂) is the main contributor to the greenhouse effect, driving global warming and climate change. To limit warming to 1.5 C and achieve net-zero CO₂ emissions by 2050, it is essential to develop industrial processes that produce clean fuels from CO₂. One promising approach is to convert CO₂ into valuable chemicals.

This study compares the effectiveness of Ru-CeO₂ and Ru-CeO₂-TiO₂ catalysts for the conversion of CO₂ to methane. Despite the lower Ru content, the TiO₂-containing catalysts show significantly higher catalytic activity. To understand this fact, in situ X-ray absorption measurements have been carried out on the Ru K-edge and Ce L₃-edge. XAS measurements show that Ru-CeO₂ systems exhibit strong oxidation-reduction abilities, with Ru being reduced by H₂ and CO₂/H₂ and oxidized by CO₂. However, in Ru-CeO₂-TiO₂ catalysts, strong interactions between CeO₂ and TiO₂ cause irreversible Ru reduction and large formation of Ce³⁺ centers in the presence of hydrogen, which enhances the catalytic performance.

The analysis of the Ce L₃-edge revealed that the Ru-CeO₂ systems predominantly exhibit Ce⁴⁺, which was observed to undergo partial reduction under both H₂and H₂/CO₂ atmospheres, followed by its re-oxidation under CO₂. However, the Ru-CeO₂-TiO₂ catalysts demonstrate a substantially higher amount of Ce³⁺ compared to the Ru-CeO₂ sample under all conditions (H₂, CO₂, H₂/CO₂), with the amount of Ce³⁺ further increasing under atmospheres with H₂ (H₂ and H₂/CO₂).

Therefore, the XAS findings indicate that the presence of TiO₂ in the catalysts stabilizes the metallic state of Ru, which remains in this state during the methanation reaction. Moreover, TiO₂ promotes the formation of Ce³⁺, enhancing the catalysts' reactivity. This effect is attributed to TiO₂ facilitating an electronic transfer at the interface and perturbing the regular fluorite geometry of ceria, thus promoting the presence of Ce³⁺. A trend that is in agreement with previous studies for CeO₂/TiO₂(110) model systems. The presence of Ce³⁺ significantly impacts the catalytic properties of the sample, aiding in the oxidation-reduction of Ce and stabilizing Ru. Consequently, the presence of reduced cerium plays a crucial role in determining the surface chemistry of the catalyst, crucial for efficiently converting CO₂ into methane.

Dr. Jorge Moncada

Research Associate, Brookhaven National Laboratory

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