Effect of the Chemical Composition of Mesoporous Cerium-Zirconium Oxides on the Modification with Sulfur and Gold Species and Their Application in Glycerol Oxidation

Piotr Kaminski

doi:10.3390/chemengineering1020018

Abstract

Ceria, zirconia, and mixed cerium-zirconium mesoporous oxides were synthesized and used as supports for sulfur and gold species. The materials were characterised using selected advanced techniques (Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), elemental analysis, X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), N2 adsorption, and desorption isotherms, Ultraviolet-Visible Spectroscopy (UV-vis), Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR), Temperature Programmed Reduction (TPR-H2), Thermogravimetric and Differential Thermal Analysis (TG-DTA)), which allowed for monitoring of the oxidation state of metals (cerium and gold) and the surface properties of the catalysts, in particular the concentration of the components on the surface and in the bulk of materials. The interactions between gold, sulfur, and metals from oxides were considered. The goal of this work was studied the changes in the chemical composition of materials and the oxidation states of cerium species after the modification of oxides with sulfur and gold species and the estimation of the influence of these changes on the surface properties. The chemical composition of surface affects the mobility of surface oxygen and the oxidation state of cerium, which can play the role of redox sites (e.g., Ce3+/Ce4+ species), and therefore it strongly influences the adsorption of hydrogen sulfide and then gold loading. Additionally, gold catalysts modified with sulfur species were tested in the reaction of glycerol oxidation in the liquid phase at basic conditions as the test reaction of the catalytic oxidation of organic pollutants from water.

Highlights

Ceria is an oxide characterised of high oxygen storage/transport capacity
The real molar ratio between ceria and zirconia for all of the supports was around 50% higher than the nominal molar ratio. This difference can suggest the inhomogeneous composition of materials and/or the presence of oxygen vacancies in the crystal lattice of catalysts, which can be created by the partial removal of oxygen atoms from the crystal structure
The chemical adsorption of hydrogen sulfide on the surface of ceria, zirconia, or the mixed cerium-zirconium oxides can be the effective method of modification with sulfur species

Summary

Introduction

Ceria is an oxide characterised of high oxygen storage/transport capacity. This oxide is able to release oxygen under oxygen deficient environment and quickly reoxidize under oxygen rich environment, but these processes have low effectiveness under high temperatures and reductive conditions. In the gas phase, when cerium oxide is reduced, the oxygen vacancies are refilled with air atmosphere, and a cyclic redox process can run [2], for example, in CO hydrogenation, a certain amount of ceria can remarkably enhance the reducibility of the catalyst due to the defect sites of Ce4+-O−-Ce3+ in the crystal structure. This can explain why ceria that is modified with gold, palladium, and/or copper is applied as a catalyst or a promoter in selected oxidation processes, e.g., Water Gas Shift (WGS) [3,4], methanol oxidation [5], Volatile Organic Compounds (VOCs) oxidation [6], and soot combustion [7]. The addition of zirconia stabilizes ceria and these oxides can create a ceria-zirconia solid solution in broad composition range, and ceria modification with zirconia can improve textural parameters, catalytic activity at lower temperatures, and oxygen storage/transport properties [8,9,10,11,12]

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