Mechanism of the deactivation of Hopcalite catalysts studied by XPS, ISS, and other techniques

Abstract

A detailed study of the thermal deactivation of a commercial Hopcalite catalyst, “CuMn 2O 4,” has been made by a multitechnique catalyst characterization approach using XPS, ISS, XRD, SEM, TA, BET, and activity testing. It is shown for the first time that the deactivation is due to segregation of the potassium component to the surface of the catalyst. The driving force of the segregation is the amorphous-to-spinel phase change at >773 K. Deactivation in N 2 or O 2 at 823 K resulted in a restructuring of the outer surface zone of the catalyst with Cu being enhanced relative to Mn. The deactivation and the concomitant phase change are accompanied by a change in the dominant oxidation state of Cu from Cu 2+ to Cu 1+ and of Mn from Mn 3+ to Mn 4+. The oxidation state change is associated with the formation of the spinel phase where the coupled redox reaction Cu 2+ + Mn 3+ Cu 1+ + Mn 4+ is experimentally supported in the XPS data by the chemical shifts and shake-up structure (Cu), and by multiplet splitting (Mn). Plasma chemical reoxidation has been used to reestablish Cu 2+ at the catalyst surface at a low temperature and nonequilibrium conditions. This study reveals the phenomenon of phase-change-induced segregation of a promoter when the ionic radius or the oxidation state precludes its accommodation in the lattice.