Role of Citric Acid in Preparing Highly Active CoMo/Al₂O₃ Catalyst: From Aqueous Impregnation Solution to Active Site Formation

Abstract

In this work, the role of citric acid (CA) in preparing CoMo/Al 2 O 3 catalyst was investigated from aqueous impregnation solution to catalytic active site formation by laser Raman, UV–vis, in situ IR and mass spectrometry characterizations. COS and CO were employed as probe molecules to illustrate the formation of active site during and after catalyst sulfidation, respectively. Citric acid mainly coordinates with Mo, resulting in the presence of Mo–CA complex, which consequently enlarge the temperature window of MoS 2 formation. Parallels between in situ IR and activity testing suggest that citric acid addition increases the concentration of CoMoS active site without significantly changing its intrinsic activity. It is concluded that the essential role of CA in preparing CoMo/Al 2 O 3 catalyst is mainly ascribed to a tuning effect on the 2D morphology of MoS 2 slabs, resulting in MoS 2 exposes more S-edge to accommodate a cobalt atom to form more CoMoS sites.