Copper-perovskite interfacial engineering to boost deNOx activity

Abstract

By adjusting the Cu doping level and calcination temperature during synthesis of a series of noble metal-free lanthanum copper manganite perovskites LaCuxMn1-xO3, we show the importance of tuning the redox chemistry for optimum steering of the catalytic activity and N2 selectivity in the catalytic reduction of NO by CO. A prerequisite is the in situ formation of an extended copper-perovskite phase boundary. The associated improvement of redox and surface properties by Cu addition leads to an optimized balance of the reduction of the catalyst in the NO+CO reaction mixture. The creation of oxygen vacancies improves the reaction kinetics and initiates the NO dissociation. Strongly dependent on the calcination temperature, the Cu/Mn ratio, the reducing agent, as well as the reaction temperature as parameters for Cu-perovskite interfacial control, surface reduction can induce partial copper exsolution, significant perovskite reduction, agglomeration of exsolved Cu particles and eventual decomposition of the catalyst structure. We show that increasing the amount of initial copper incorporated into the perovskite structure can beneficially extend the phase boundary, but exceeding the amount of copper leads to increased sintering of the copper particles at the expense of catalyst activity, especially in repeated catalytic cycles. Increasing the calcination temperature, in addition to the initial sintering of the catalyst and shifting the reaction onset temperature to higher temperatures in the first cycle, improves the reduction resistance of the catalyst. Consequently, in order to form a higher amount of phase boundary-bound active sites, stronger reducing conditions than provided by the NO+CO reaction mixture are needed. This effect can be counterbalanced by using stronger reduction agents prior to performing subsequent catalytic cycles by inducing the exsolution process.