The activity and mechanism of uranium oxide catalysts for the oxidative destruction of volatile organic compounds

Abstract

Uranium oxide based catalysts have been investigated for the oxidative destruction of volatile organic compounds (VOCs) to carbon oxides and water. The catalysts have been tested for the destruction of a range of organic compounds at space velocities up to 70 000 h⁻¹. Destruction efficiencies greater than 99% can be achieved over the appropriate uranium based catalyst in the temperature range 300–450°C. Volatile organic compounds investigated include benzene, butylacetate, cyclohexanone, toluene, methanol, acetylene, butane, chlorobutane and chlorobenzene. The catalysts are thermally stable, destroy low concentrations and mixtures of VOCs and lifetime studies indicate that deactivation during oxidation of chlorinated VOCs did not occur. A temporal analysis of products (TAPs) reactor is used to investigate the mechanism of oxidation of VOCs by uranium oxide catalysts. Studies indicated that VOCs were oxidised directly to carbon oxides on the catalyst surface. A combination of TAP pulse experiments with oxygen present and absent in the gas phase has indicated that the lattice oxygen from the catalyst is responsible for the total oxidation activity. This has been confirmed by studies using isotopically labelled oxygen which indicates that the catalyst operates by a redox mechanism.

Introduction

Over the last several years, environmental legislation has imposed increasingly stringent limits for permitted atmospheric emission levels. In particular, the release of volatile organic compounds (VOCs) has received much attention. VOCs are a wide ranging class of chemicals derived from many sources and contain over 300 compounds as designated by the United States Environmental Protection Agency [1]. Their release has widespread environmental implications and has been linked to the increase in photochemical smog [2], the depletion in atmospheric ozone [3] and the production of ground-level ozone [4]. In addition, many VOCs are inherently toxic and/or carcinogenic. The US Clean Air Act (1990) calls for a 90% reduction in the emissions of 189 toxic chemicals, many of which are VOCs, by 1998. In 1994 it has been estimated that 706 000 t of organic pollutants were discharged to the atmosphere from the US alone [5]. Approximately 70% of these compounds can be classed as VOCs and although it cannot be determined directly, it is estimated that discharges worldwide are at least twice that of the US. In view of the magnitude of the problem presented to the chemical and processing industries, the major challenge they face is to reduce the emission of pollution without stifling economic growth.

Abatement technologies to control the release of VOCs to the environment are therefore of paramount importance. Many technologies for the treatment of VOC laden effluent have been developed, the most widely adopted being adsorption, often using carbon or zeolite type adsorbents. However, this process can generate considerable further waste as the adsorbent must usually be buried in landfill sites. The most widely adopted technique is thermal combustion or incineration, which requires temperatures in excess of 1000°C. Whilst it is a simple and often effective method of control, the high temperatures required culminate in a relatively fuel intensive technique with little control over the ultimate products. The latter is particularly problematic and can result in incomplete oxidation of the waste stream and the formation of toxic by-products such as dioxins, dibenzofurans and oxides of nitrogen, if conditions are not carefully controlled. Alternatively, heterogeneous catalytic oxidation offers many potential advantages. The use of a catalyst in the oxidative destruction of VOCs significantly lowers the process operating temperature, which is typically in the range 300–600°C. This reduction in temperature is advantageous, as economically little or no supplementary fuel is required to sustain combustion. Legislatively too, the process is no longer regarded as an incineration process, eliminating certain regulatory requirements. In addition, catalytic oxidation offers a much greater degree of control over the reaction products and can operate with dilute effluent streams (<1% VOC), which cannot be treated easily by thermal combustion. Hence, catalytic oxidation may be considered as a more appropriate method for end of pipe pollution control.

Two classes of catalysts are commonly used, these are noble metal and metal oxide based systems. A prospective catalyst must be active at relatively low temperatures and show high selectivity to carbon oxides. Ideally, the catalyst must also be able to destroy effectively low concentrations of VOCs at very high flow rates with little or no deactivation. Supported noble metal systems, primarily platinum and palladium, show high activity for the oxidation of many VOCs, with high selectivity to carbon oxide products. However, these tend to be relatively expensive and can be rapidly deactivated by the presence of chlorinated compounds, sulphur or other metals in the waste stream [6]. The second class of catalysts are metal oxides and some of the most active ones are based on copper [7], cobalt [8], chromium [9] and manganese [10]. Generally, these are less expensive than precious metals and show higher resistance to poisoning. However, for complete oxidation they are inherently less active. The development of oxide catalysts which may be used for the combustion of a wide range of volatile organic compounds presents a major challenge for future research.

This work outlines the advances made in the development of a new series of uranium oxide based catalysts and presents the results of transient studies using a temporal analysis of products (TAPs) reactor to unravel the reaction mechanism. Uranium oxide was initially selected as a catalyst for several reasons, in particular, U₃O₈ has uranium present in mixed oxidation states with a facile transition between states, and can also show a wide range of metal/oxygen stoichiometry. These are important features which are characteristic of other oxidation catalysts. Additionally uranium oxides have also shown relatively high activity for carbon monoxide oxidation [11]. Uranium oxide based catalysts have been widely used by the chemical industry for a considerable time. Well established procedures for the safe handling of these materials exist and these are primarily determined by issues of chemical toxicity. The results presented in this paper demonstrate that uranium oxides show high and stable activity for the destruction of a range of VOCs under industrially relevant flow rates and temperatures. Previously, uranium oxides have been considered by many as a burden on our environment; however, this work demonstrates that they can be effectively used to provide a solution to a major problem affecting the environment.