Titania and zirconia binary oxides as catalysts for total oxidation of ethyl acetate and methanol decomposition

Highlights

• TiO₂-ZrO₂ oxides with 2:8; 5:5 and 8:2 Zr/Ti mol ratio obtained by various methods.

• Hydrothermal procedure provides 2–7 times higher porosity than the urea one.

• The catalytic activity of binary oxides increases due to the improved texture.

• Simple regulation of catalytic behaviour by Zr/Ti ratio and preparation method.

Abstract

Template assisted hydrothermal and homogeneous precipitation with urea techniques were used for the preparation of TiO₂-ZrO₂ binary oxides with wide range variation of their composition. Low temperature nitrogen physisorption, XRD, SEM, HRTEM, XPS, UV–vis, FTIR, FTIR of adsorbed pyridine and Raman spectroscopy were used for the characterization of the obtained samples. Their catalytic properties were tested in total oxidation of ethyl acetate as a representative VOCs and methanol decomposition as hydrogen carrier. It was established that the template assisted hydrothermal technique provoked preparation of materials with good crystallinity, high BET surface area and well developed mesoporous structure, while the homogeneous precipitation with urea method facilitated the formation of more highly dispersed in wide Zr/Ti region samples. The degree of incorporation of Zr⁴⁺ ions into titania lattice could be controlled by the samples composition and the preparation method used, which revealed good opportunity for effective and simple regulation of their catalytic properties.

Introduction

The risk of energy crisis and the global problem with the environmental pollution demands development of novel clean and efficient approaches. Volatile organic compounds (VOCs) are among the most hazardous pollutants which are emitted from natural sources and various anthropogenic activities [[1], [2], [3], [4], [5], [6]]. They have significant effect on human health and natural ecosystem via depletion of stratospheric and formation of tropospheric ozone, ground level smog formation and climate changes. Compared to membrane separation, bio-filtration, thermal destruction, adsorption and condensation approaches, catalytic total oxidation of VOCs to CO₂ and water has been recognized as more economic and environmentally friendly methods for their abatement. On the other hand, fuel cell technology has been considered as perspective clean and efficient alternative of the most commonly used fossil fuels [[7], [8]]. However, the technical troubles related to the handling, storage and transport of hydrogen, especially for the on-board application, forced the interest to fuel processors, which convert a liquid fuel to hydrogen [9]. Methanol is excellent liquid hydrogen source due to its low boiling point, high volumetric energy density, easy transportation and possibility to be obtained from renewable and waste sources [[7], [8], [9], [10], [11], [12]]. Nowadays, the catalytic decomposition of methanol is reported as the simplest process for hydrogen release [9].

Up to now, significant efforts have been devoted to find active, stable and low cost catalysts for VOCs elimination and methanol decomposition. The rapid deactivation of the most preferable transition metal oxide catalysts and the need of operation at lower temperatures without additional toxic emissions demand the increase of their efficiency. This could be achieved using transition metal oxides in the nanoscale, with well developed surface area and mesoporosity or in multi-component state, where improved texture and dispersion, synergistic effects or facile electron exchange could be realized [[3], [13], [14], [15], [16], [17], [18], [19], [20]]. Among various binary transition metal oxide systems, TiO₂-ZrO₂ mixed oxides have demonstrated high activity in photocatalytic elimination of organic contaminants, both in gas and liquid phase, photocatalytic water degradation with a potential for hydrogen production, isomerization and cracking of alkanes, hydration and polymerization of alkenes and selective reduction of NO_x with hydrocarbons [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. However, despite the number of publications aimed at the preparation and application of titania-zirconia oxides, the mechanism of their activity in various catalytic processes has not been cleared enough. Many authors reported increase of the specific surface area of binary oxides due to the stabilization of amorphous Zr_xTi_1-xO₄ solid solution for the equimolar materials at temperatures below the expected for the solid state reaction [[30], [32]]. Linnik et al. [33] found formation of Zr_xTi_1-xO₄ solid solution even at lower zirconia content (up to 10 mol%). Akiyama and Nonaka [34] registered an increase in the number of oxygen vacancies in ZrO₂ after doping with titania, which could be controlled by the morphology of the binary oxides [35]. Stabilization of anatase phase was demonstrated in [36].

It was well established that the precursors and the preparation technique can strongly affect the morphology, phase composition and texture characteristics of metal oxide composites [[25], [37], [38], [39], [40]]. However to the best of our knowledge data for the preparation of ordered mesostructured TiO₂-ZrO₂ materials have been rarely reported [[25], [29], [41]]. In our previous study we demonstrated the advantages of template assisted hydrothermal and homogeneous precipitation with urea techniques for the preparation of mixed CeO₂-ZrO₂ nanostructured composites [42]. In this paper, similar synthesis methods were applied and compared for the preparation of TiO₂-ZrO₂ mixed oxides with various compositions. The relation between the preparation procedure used and the structure, texture, morphology, surface and catalytic properties of the obtained materials under different reaction conditions was the main challenge in the study. For the purpose, the samples were characterized by complex of various physicochemical techniques, such as nitrogen physisorption, XRD, SEM, HRTEM, FTIR and FTIR of adsorbed pyridine, UV–vis and Raman spectroscopy. Their catalytic behaviour was studied in methanol decomposition and ethyl acetate oxidation with a potential for environmental protection.