X-ray Spectroscopy and Microscopy of Cobalt and Nickel in CoMoS and CoNiMoS Hydrodesulfurization Catalysts

In recent years, there has been growing interest in understanding the catalysis background for the hydrodesulfurization (HDS) reaction. Strict regulations have been introduced surrounding the production of ultra-low-sulfur fuels (<10 ppm), and as a result, the HDS catalyst has become a favorite topic for analysis. During the HDS hydrotreating reaction, sulfur is cleaved from the hydrocarbons by the catalyst and converted to H2S. The HDS catalysts are generally composed of the bimetal sulfides CoMo, NiMo, and trimetallic CoNiMo supported on alumina. Preparing and developing catalysts with higher activity and stability can improve the HDS process and is thus especially important for the petroleum industry. Unraveling the effect of the spin state and the local symmetry of metals in the active phase can explain their catalytic performances under various conditions. Hard X-ray spectroscopies (XAS and RIXS) can provide unique insights into the electronic and geometric structure, combined with element, symmetry, and spin sensitivity. In the first part of the RIXS process, a core electron (hv1) is excited into the localized unoccupied density of states. Consequently, a 3p electron (hv2) is emitted to fill the core-hole. Due to the large penetration depth of the hard X-rays (E>5000 eV), RIXS probes the bulk properties of the sample and is compatible for reactions under extreme conditions, such as catalytic reactions.