The catalytic properties of Rh/CeO2 catalysts for the Sabatier reaction - A study of the influence of Rh particle size and the inclusion of Sm- and Fe dopants in the ceria

Abstract

The demand for fuel increases and efforts are being made to develop renewable alternatives such as biogas and hydrogen gas. However, these have low energy densities but this can be improved by transforming and storing the energy in C-H bonds in other products. One such product is methane, which can be produced through the methanation of carbon dioxide. However, the high stability of carbon dioxide makes a catalyst necessary for the transformation. In this study, ceria supported rhodium, i.e., 1-3 wt% Rh/CeO2, where both components have shown promising properties for this reaction, was studied. The aim was to study how Rh particle size affects the catalytic properties, but also to compare obtained results with Rh/Ce1-xSmxO2-δ, Rh/Ce1-xFexO2-δ and Ce1-xRhxO2-δ from another study to see the effects of dopants and the inclusion of Rh in the CeO2. The Rh/CeO2 was synthesized using dry impregnation followed by freeze-drying and calcination. Used characterization methods were XRF, BET and CO chemisorption, with DRIFTS as a complement. The reaction was run using a fixed bed reactor at four different temperatures: 150, 250, 350 and 400 °C. The results showed a specific surface area in the interval 123-133 m2/g sample and a decrease in Rh particle size with higher loads of Rh on the ceria. Out of the Rh/CeO2 samples, the one with the smallest Rh particle size showed the highest reaction rate, while the one with the biggest Rh particle size showed the highest selectivity. It was not possible to directly determine whether the Rh was best used for impregnation or as a dopant due to the reaction rate being higher for Rh/CeO2 but the selectivity being higher for Ce1-xRhxO2-δ. However, Rh/Ce1-xSmxO2-δ turned out to be the most efficient catalyst for this reaction, both in regard of activity as well as selectivity. A Fe dopant generally decreased the catalytic properties. This study gave an insight to the impact of Rh particle size and the use of dopants, but more experiments need to be done to deem the results statistically significant for more realistic conditions with varying of reaction conditions. Further experiments also need to be done to study more catalytic properties than what was covered in this work.