Influence of the Fuel/Oxidant Ratio on the Elaboration of Binary Oxide Catalyst by a Microwave-Assisted Solution Combustion Method

Kawthar Frikha,Jamel Bouaziz,Kamel Chaari,Simona Bennici,Lionel Limousy

doi:10.3390/en13123126

Abstract

Three series of binary metal oxide catalysts containing Ni, Cu, or Co oxides were prepared, fully characterized and tested in CO oxidation. The catalysts, with a constant transition metal loading of 10 wt%, were prepared from metal nitrates and urea mixtures by a microwave-assisted solution combustion method. The RV/OV ratio, corresponding to the stoichiometry of the reactants, calculated from their elemental oxidizing and reducing valences, was varied. In order to modify the redox character of the synthesis medium from the quantity of urea; an excess of urea was used for attaining reducing conditions, while a deficit of urea shifted the medium to oxidizing conditions. Three RV/OV ratios (0.9, 1.0, and 1.1) were selected to elaborate the different binary metal oxide catalysts, nine oxide catalysts were synthesized. Then, the influence of the stoichiometry (RV/OV ratio) on the bulk and surface properties of the binary metal oxide catalysts was investigated. Similarly, the influence of the RV/OV ratio on the CO oxidation activity was discussed and the optimal value of RV/OV ratio was identified. The results show that the increasing of the RV/OV ratio from 0.9 to 1.1, particularly in Ni- and Co-containing catalysts, induces stronger metal-aluminum interactions, in the form of aluminates phases, and that are correlated to the dramatic reduction of the CO oxidation activity. The best physicochemical properties and highest catalytic activities were achieved with the catalysts prepared in redox systems stoichiometrically balanced (RV/OV = 1). The Cu-containing catalysts presented the best catalytic activities in CO oxidation.

Highlights

Metal oxides are among the most useful catalytic materials in heterogeneous redox catalysis.In this category, the active phase can consist of a single metal oxide or a combination of metal oxides.Metal oxide catalysts are applied in a wide range of catalytic reactions, including selective reduction reactions, selective oxidation reactions and total oxidation reactions [1,2,3]
These results suggest that the urea-nickel/cobalt nitrates mixtures lead to the formation of γ−alumina and the metal oxides (NiO or CoO) that further react to form MAl2 O4 in a solid state reaction
The enhancement of the RV/OV ratio did not show any benefit for the oxidation of CO on the nickel-based catalysts

Summary

Introduction

Metal oxides are among the most useful catalytic materials in heterogeneous redox catalysis.In this category, the active phase can consist of a single metal oxide or a combination of metal oxides.Metal oxide catalysts are applied in a wide range of catalytic reactions, including selective reduction reactions, selective oxidation reactions and total oxidation reactions [1,2,3]. Metal oxides are among the most useful catalytic materials in heterogeneous redox catalysis. In this category, the active phase can consist of a single metal oxide or a combination of metal oxides. Metal oxide catalysts are applied in a wide range of catalytic reactions, including selective reduction reactions, selective oxidation reactions and total oxidation reactions [1,2,3]. Transition metal oxides are very interesting in heterogeneous catalysis, due to their improved and tunable acid-base and redox properties [4,5,6,7]. The most commonly used transition metal oxide catalysts include chromium oxide, manganese dioxide, iron oxide, cobalt oxide, nickel oxide and Energies 2020, 13, 3126; doi:10.3390/en13123126 www.mdpi.com/journal/energies

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