Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2−δ Catalyst of CO Oxidation Reaction

Abstract

A Mn0.2Zr0.8O2−δ mixed oxide catalyst was synthesized via the co-precipitation method and studied in a CO oxidation reaction after different redox pretreatments. The surface and structural properties of the catalyst were studied before and after the pretreatment using XRD, XANES, XPS, and TEM techniques. Operando XRD was used to monitor the changes in the crystal structure under pretreatment and reaction conditions. The catalytic properties were found to depend on the activation procedure: reducing the CO atmosphere at 400–600 °C and the reaction mixture (O2 excess) or oxidative O2 atmosphere at 250–400 °C. A maximum catalytic effect characterized by decreasing T50 from 193 to 171 °C was observed after a reduction at 400 °C and further oxidation in the CO/O2 reaction mixture was observed at 250 °C. Operando XRD showed a reversible reduction-oxidation of Mn cations in the volume of Mn0.2Zr0.8O2−δ solid solution. XPS and TEM detected the segregation of manganese cations on the surface of the mixed oxide. TEM showed that Mn-rich regions have a structure of MnO2. The pretreatment caused the partial decomposition of the Mn0.2Zr0.8O2−δ solid solution and the formation of surface Mn-rich areas that are active in catalytic CO oxidation. In this work it was shown that the introduction of oxidation-reduction pretreatment cycles leads to an increase in catalytic activity due to changes in the origin of active states.

In Situ X-ray Diffraction as a Basic Tool to Study Oxide and Metal Oxide Catalysts

“…Typically, operando XRD is applied to elucidate the structure of active component under conditions of several processes: methane dry reforming (CH 4 + CO 2 → CO + H 2 ) [175,[204][205][206][207][208], methanol/ethanol steam reforming [209,210], the water-gas shift (WGS, CO + H 2 O → H 2 + CO 2 ) [176,201,[211][212][213][214][215][216], the reverse water-gas shift reaction (RWGS, H 2 + CO 2 → CO + H 2 O) [217][218][219], total oxidation reactions of CO and hydrocarbons, catalytic hydrogenation and dehydrogenation [220][221][222][223][224], and self-sustained reaction-rate oscillations [51,102,[225][226][227][228][229]. In all the above-mentioned applications, the phase composition and the chemical state of the catalysts are strongly dependent on the reagent/product composition in a certain reaction process.…”