Reaction network for the total oxidation of toluene over CuO–CeO₂/Al₂O₃

Abstract

The total oxidation of toluene was studied over a CuO–CeO₂/γ-Al₂O₃ catalyst in a Temporal Analysis of Products (TAP) setup in the temperature range 673–923 K in the presence and absence of dioxygen and at various degrees of reduction of the catalyst. The reaction rate significantly decreases over a mildly reduced catalyst. Under vacuum and at high-temperature, mild reduction also occurs in the absence of toluene.

In the presence of dioxygen, the catalyst activity is determined by weakly bound surface lattice oxygen atoms and adsorbed oxygen species, the lifetime of which is close to 1 s. The weakly bound oxygen is highly reactive and is only found over a fully oxidized catalyst.

The formation of products containing ¹⁸O during the isotope-exchange experiments with ¹⁸O₂ indicates that both lattice and adsorbed oxygen are involved in (a) reoxidation of mildly reduced copper oxide and (b) abstraction of hydrogen atoms and scission of C–C bonds.

Isotopic labeling with C₆H₅–¹³CH₃ and C₆H₅–CD₃ indicates the following reaction paths: adsorption of toluene on the active site, containing Cu²⁺ with 4–5 adjacent surface lattice oxygen atoms; the simultaneous abstraction of H from the methyl and the phenyl group followed by the abstraction of the methyl carbon and next the destruction of the aromatic ring.

Introduction

The catalytic total oxidation of volatile organic compounds (VOCs) is generally considered to be an effective method for reducing the emission of pollutants in the environment [1], [2]. The main advantages of catalytic combustion compared with other decontamination technologies are high efficiency at a very low pollutants concentration, low energy consumption and low production of secondary pollutants, e.g., NO_x. Conventional catalysts based on noble metals supported on Al₂O₃ are successfully used to eliminate VOCs by total oxidation. Noble metal catalysts are very active, but they are costly and have low stability in the presence of chlorine compounds [1], [3]. Transition metal oxides, such as copper, cobalt, manganese, and chromium, are known to be active combustion catalysts [1], [4]. They are less active at lower temperatures but have comparable activity at high temperatures and have high catalyst loading capabilities. CuO was reported to be as effective as Pt for the incineration of n-butanol and methyl mercaptan [3]. Larsson and Andersson found excellent performance for the incineration of CO, ethyl acetate, and ethanol over CuO_x/Al₂O₃ [5], [6]. Rajesh reported that CuO/Al₂O₃ was even more active than Pt/Al₂O₃ for the complete oxidation of ethanol [7]. CuO was the most active transition-metal oxide of those tested for the catalytic incineration of toluene with γ-Al₂O₃ as support [8]. Copper promoted by ceria is known to show better catalytic performance for the complete oxidation of benzene, toluene, and p-xylene than copper only [5], [8], [9], [10].

It is generally accepted that the oxidation of VOCs (toluene, propane) over transition metal oxide catalysts occurs according to a Mars–van Krevelen type redox cycle and proceeds through nucleophilic attack of the lattice oxygen of the oxides [11], [12], [13], [14], [15], [16], [17], [18]. This mechanism includes two steps: the first step consists of reactant oxidation using the catalyst lattice oxygen, which will be replaced, and in the second step, by atoms originating from dioxygen. However, the first step in the oxidation process is not an elementary step, and the actual mechanism involves many consecutive and/or parallel steps [11], [19], [20], [21], [22]. Toluene adsorption with the aromatic ring parallel to the exposed metal oxide planes leads to the destruction of the molecule and the formation of total oxidation products. Perpendicular end-on adsorption of toluene on oxygen containing sites leads to abstraction of H-atoms from the methyl group and an adsorbed complex through a strong C–O bond. This complex is considered to be the benzaldehyde and/or other selective-products precursor or further oxidized to carbon oxides [11], [19], [20], [21], [22]. On the other hand, according to the Mars–van Krevelen mechanism, the dioxygen is only required to reoxidize the reduced surface metal centers. Dioxygen molecules are activated through an interaction with the surface of the catalyst. This activation proceeds first through a dissociative adsorption, which includes coordination, electron transfer, and dissociation, followed by incorporation into the lattice. Consequently, two possible states of oxygen are available on the surface of the catalyst. Adsorbed dioxygen species are also reported to be active in hydrocarbon oxidation catalysis [10], [11], [14], [23], [24], [25], [26]. The role and nature of the respective oxygen active species (e.g., adsorbed oxygen species acting as electrophilic oxygen and lattice “nucleophilic” oxygen) in catalytic combustion are not fully clarified. Other aspects, e.g., the nature of the active sites and the mechanism for C–H and C–C bond activation are still incompletely explained.

The elucidation of the above issues should aid in better understanding the mechanism of action of the oxide catalysts. Transient response techniques with millisecond time scale provide a powerful tool for the investigation of the reaction steps and the possible catalyst surface transformations during reaction [27]. The temporal-analysis-of-products (TAP) have been recognized as an important transient experimental method for heterogeneous catalytic reaction studies. A TAP pulse response experiment consists of injecting a very small amount of gas, typically 10¹³–10¹⁴ molecules per pulse, into a tubular fixed bed reactor that is kept under vacuum. The pressure rise in the micro-reactor is small, and the transport in the reactor, which is driven by a gas concentration gradient, is dominated by Knudsen diffusion. Well-defined Knudsen diffusion is used as a tool for measuring chemical reaction rates and obtaining kinetic parameters [27], [28], [29]. The time-dependent exit flow rate of each gas is detected by a mass spectrometer. In this study, a TAP reactor is applied as a unique transient tool to investigate the reaction network and kinetics for the catalytic total oxidation of toluene using a commercial CuO–CeO₂/γ-Al₂O₃ catalyst.