(300e) Toluene Total Oxidation Over CuO-CeO2/Al2O3 Catalyst: Nature and Role of Oxygen Species

Introduction

The catalytic total oxidation of volatile organic compounds is generally considered to be an effective method for reducing the emission of pollutants in the environment [1]. 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 (NO_x). Conventional, catalysts, based on noble metals (Pt, Pd) supported on Al₂O₃, are successfully used to eliminate VOCs by total oxidation. Transition metal oxides, such as copper, cobalt, manganese, and chromium, are known to be an active combustion catalyst [1]. They are less active at lower temperatures but show comparable activity at higher temperatures and high catalyst loading capabilities. CuO was reported equally effective as Pt for the combustion of n-butanol and methyl mercaptan [2]. Larsson and Andersson found excellent performance for the incineration of CO, ethyl acetate, and ethanol over CuO_x/Al₂O₃ [3]. Alternatively, Rajesh and Ozkan reported that CuO/Al₂O₃ was even more active than Pt/Al₂O₃ for the total oxidation of ethanol [4]. CuO was the most active transition-metal oxide of those tested for the catalytic combustion of toluene with g-Al₂O₃ as the support [5]. At the same time, copper promoted by ceria catalyst was observed to show better catalytic performance for the total oxidation of benzene, toluene and p-xylene than that of copper supported catalyst [5,6].

The mechanism of the oxidation of VOCs over transition metal oxide catalyst, were established to be Mars and Van Krevelen type redox cycles [7]. This mechanism includes two steps: the first step consists of the reactant oxidation using the catalyst lattice oxygen which will be replaced, in the second step by gaseous oxygen. On the other hand, the interaction of gas-phase oxygen with oxide catalysts is an important step in total oxidation of hydrocarbon. Oxygen 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 the incorporation into the lattice. Consequently, two possible states of oxygen are available on the surface of the catalyst. Adsorbed oxygen species are also active in hydrocarbon oxidation catalysis [7]. The respective roles and nature of the active oxygen species e.g., adsorbed oxygen species acting as electrophilic oxygen, and of lattice nucleophilic oxygen in catalytic combustion is not fully clarified and must be studied. The nature of the active sites and role of oxygen species for C?H bond activation are still incompletely explained.

Transient response techniques with millisecond time scale is a powerful tool for the investigation of the reaction steps and dynamics of the catalyst surface transformations under the influence of gaseous media. The temporal-analysis-of-products (TAP) has been recognized as an important transient experimental method for heterogeneous catalytic reaction studies. In this study, a TAP reactor is applied to investigate the nature and role of oxygen species for the catalytic total oxidation of toluene using a well-characterized CuO/CeO₂/γ-Al₂O₃ catalyst.

Experimental procedure

The 10 wt.% CuO- 5 wt.% CeO₂/Al₂O₃ catalyst is a commercial mixed metal oxide prepared via impregnation of γ-Al₂O₃ with Cu(NO₃)₂ and Ce(NO₃)₄ precursors [8]. The sharp CuO and Cu XRD diffraction lines point towards crystallites of some 100 nm in size. Alumina and ceria are an order of magnitude smaller than CuO with dimension of 7 and 5 nm respectively. The BET surface area of the catalyst amounts to 156 m²/g. The TAP experiments are performed in a micro-reactor which is placed in vacuum (10^-4 ? 10^-5 Pa) with a very small amount of reactant molecules (~10¹⁵ molecules/pulse). The quartz micro-reactor has 33 mm long and has an inner diameter of 4.75 mm. Experiments were performed by pulsing C₇H₈ with and without O₂ at various reduction degrees of the catalyst and at temperatures varying from 723K to 923K. Typically experiment carried out over 10 mg of catalyst, which corresponds to 1.4 × 10¹⁹ O atoms based on both CuO and CeO₂ (reduction CeO₂ to Ce₂O₃) present in the catalyst. Since a maximum inlet pulse intensity of ~10¹⁵ molecules/pulse is applied in all experiments, the number of reactant molecules in a pulse is always 4 orders of magnitude lower than oxygen available for reaction. Three types of pulse experiments have been performed: single-pulse, multi-pulse and alternating pulse experiments. Single-pulse experiments are used to study the interaction of several reactants with the catalyst at a predetermined state of the latter. In contrast, multi-pulse experiments are executed to alter the state of the catalyst and are often followed by a single-pulse experiment to redefine its state. In alternating pulse experiments, two different reactants are pulsed from both pulse valves with a certain delay in between the two pulses. That way, intermediates formed on the first pulse by introduction of a pump molecule can be probed on the second pulse with a suitable probe molecule. The catalyst was reduced by pulsing of carbon monoxide or toluene. The degree of the catalyst reduction is defined as the  ratio of number of oxygen atoms taken from the catalyst to the total amount of oxygen in copper oxide and CeO₂ (reduction CeO₂ to Ce₂O₃).

Results

The activation of toluene both in the presence and absence of gas-phase O₂ over a CuO?CeO₂/γ-Al₂O₃ catalyst was investigated. The yield  of CO₂ produced from single pulse experiments of C₇H₈ with (Fig. 1a) and without gas-phase oxygen (Fig. 1b) in the feed at different degrees of reduction of the catalyst. The temperatures was varying from 723K to 923K. It can be seen from Fig. 1a and b, that the yield of CO₂ obtained while pulsing toluene with O₂ in the feed, were much higher than that obtained when pulsed without O₂, at the same degrees of catalyst reduction. The reaction rate is dramatically decreased with increasing level of catalyst reduction when we used only toluene. Thus, reactivity of the copper oxides surface under experimental conditions are mostly determined by weakly bound oxygen. The effect of oxygen partial pressure on the reaction rate was also investigated. The reaction order in oxygen decreased gradually from 0.3 to 0.1 with increase in the reaction temperature from 723 to 923K. This dependence points at the involvement of two types of surface oxygen in the catalytic combustion of toluene: the surface lattice oxygen and the chemisorbed adatoms which are continuously supplied from the gas phase.

To obtain information about the participation of lattice and adsorbed oxygen pulse experiments with toluene and isotopically labeled oxygen (¹⁸O₂ ) were performed. Preferential formation of H₂¹⁶O at the beginning of the reaction (> 90% of total water formed) supports the redox mechanism (Fig.2a). The presence of 10% of H₂¹⁸O indicates that electrophilic adsorbed oxygen is also participating in the total oxidation process. Fig. 2b shows that the fraction of ¹⁸O in carbon dioxide (18% ¹⁸O) is about two times as high as in water. Alternating pulse experiments with different time delay between the pulses, showed that oxygen active species are present on the catalyst surface and has a short less than 1s life time.

Conclusion

The reactivity of the metal oxides surface  in the low and medium-temperature range is mostly determined by weakly bound oxygen forms. Removal of oxygen atom from the surface layer of metal oxides catalyst by C₇H₈/O₂ pulse generate oxygen vacancy. This oxygen vacancy is replenished by oxygen diffusion from the subsurface layer of the catalyst or during reoxidation of the mildly-reduced catalyst with O₂. Gas-phase O₂ can restore the vacancy as well as generate an adsorbed oxygen species. Those oxygen species are reactive and has less than 1s life time on the catalyst surface. On a deeply reduced catalyst the O₂ that is adsorbed on a surface incorporates into the lattice first before participating in the oxidation process.

References

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