Reaction network for the total oxidation of toluene over CuO–CeO2/Al2O3
Graphical abstract
Alternating pulse and isotopic labeling experiments indicate that the catalytic total oxidation of toluene over CuO–CeO2/γ-Al2O3 consists of the following sequence: parallel adsorption of toluene on the catalyst surface, step 1; the simultaneous abstraction of H from the methyl and the phenyl group, steps 2 and 3; abstraction of the methyl carbon atom, step 4, followed by destruction of the aromatic ring, step 5. Two types of oxygen are directly involved in the oxidation of toluene: adsorbed oxygen and weakly bound surface lattice oxygen.
Highlights
► Reaction network for the total oxidation of toluene over CuO–CeO2/Al2O3. ► Transient response techniques with millisecond time scale. ► The role and nature of the respective oxygen active species. ► Life time of the reactants species on the catalyst surface. ► Isotopic labeling experiments with 18O2, C6H5–13CH3, and C6H5–CD3.
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., NOx. 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 CuOx/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.
Section snippets
Catalyst preparation and characterization
The (11.58 wt.%)CuO–(6.36 wt.%)CeO2/γ-Al2O3 catalyst is a commercial mixed metal oxide, which was synthesized via impregnation of γ-Al2O3 with Cu(NO3)2 and Ce(NO3)3 precursors, dried and calcined above 973 K. The bulk chemical composition of the tested catalysts was determined by means of inductively coupled plasma atomic emission spectrometry (ICP-AES) (IRIS Advantage system, Thermo Jarrell Ash). N2 physisorption at 77 K was applied to determine the BET specific surface area using a Gemini V
Toluene conversion to CO2 in the presence and absence of dioxygen
The outlet molar flow rate of C7H8 and CO2 obtained by performing single-pulse experiments, pulsing C7H8/Ar and C7H8/O2/Ar at 823 K over oxidized catalyst, is presented in Fig. 1a and b, respectively. The amount of C7H8 in the feed mixture was kept the same in both the experiments. A ratio of 1:9 for C7H8/O2 was taken in the latter case.
The behavior of toluene oxidation observed from Fig. 1a suggests that the reaction is carried out according to a Mars–van Krevelen mechanism [35]. Lattice oxygen
General discussion
The results of this study show that the total oxidation of toluene over the CuO–CeO2/Al2O3 catalyst is carried out by a classical redox, i.e., Mars–van Krevelen mechanism. However, adsorbed oxygen can also participate in the reaction. Active surface oxygen species with a lifetime of the order of 1 s will contribute to a higher catalytic activity, but only if dioxygen is present in the feed and the catalyst is fully oxidized. A lifetime of weakly bound oxygen species as low as 10 ms was reported
Conclusions
The catalytic cycle for the total oxidation of toluene on CuO–CeO2/Al2O3 can be summarized as follows.
Dioxygen ensures the reoxidation of the reduced catalyst according to the well-known Mars–van Krevelen mechanism. Weakly bound surface lattice oxygen atoms and adsorbed oxygen species, the lifetime of which is close to 1 s, are highly reactive and only found over a fully oxidized catalyst and in the presence of dioxygen. The total oxidation rate significantly decreases over a mildly reduced
Acknowledgments
This work was supported by a Concerted Research Action (GOA) of Ghent University and the ‘Long Term Structural Methusalem Funding by the Flemish Government’.
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