On the Function of Ceria Supported Rhodium Catalysts for Methanation of Carbon Dioxide

Licentiate thesis, 2020

Technologies for energy harvesting of sustainable sources such as solar and wind lack an inherent energy buffer. As such, storing excess energy in chemical bonds, for example that of hydrogen (H2), is a desirable complementary concept. While hydrogen gas is easily cycled and applied in electrical systems, it brings high costs for long termstorage and transportation. The technical solutions introduce complications in terms of limited equipment lifetime and when applied in combustion systems also safety concerns. An alternative for these applications is to further convert the hydrogen into hydrocarbons such as alkanes and alcohols. Through CO2 hydrogenation, methane can be produced and used in current infrastructure solutions due to it being the major component also in natural gas. While typically produced using nickel based catalysts, other metals such as ruthenium and rhodium have shown promise, of which Rh/CeO2 is one such catalyst. While kinetic studies exist of said catalyst, the reaction mechanism is unknown, which hampers further development. This work aims towards clarifying the catalyst structure-function relationship as well as the important surface reactions. The ambition is to facilitate the start of fundamental research approach, which can later be developed to achieve a more complete understanding, allowing for tuning the important properties of said catalyst.

In this work, Rh/CeO2 catalysts were prepared by incipient wetness impregnation and studied in situ during CO2 hydrogenation in regards to its structural behaviour, by the use of high-energy X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy, as well as the surface reactions using diffuse reflectance infrared spectroscopy. As signals from processes and adsorbates that truly participate in the reaction may be hidden by spectator signals, these studies were performed transiently as to enhance the response of active species while decreasing that of spectators.

It was revealed that CeO2 is active during reaction conditions, possibly partaking in a cycle of formation and healing of oxygen vacancies during the reaction cycle as evidenced by the cycling of Ce4+ and Ce3+. Furthermore, while predominantly reduced, a RhOx phase was observed, showing the strong metal support interaction of Rh and CeO2. Regarding surface reactions, several carbonyl species (b-CO, h-CO, m-CO) were shown to be active on the catalyst, as well as some carbonates (b-CO3, p-CO3) and formate (b-HCOO) species.

While the complete pathway need more experimental data to be determined, the activity of the carbonyl species suggests that the reaction follows a carbon monoxide based pathway such as the carbide pathway.