Catalytic feasibility of Ce-doped LaCoO3 systems for chlorobenzene oxidation: An analysis of synthesis method
Graphical abstract
Ce-doped LaCoO3 perovskites catalysts prepared by two different methods (citrate and reactive grinding) present an excellent catalytic activity and stability in the chlorobenzene total combustion reaction with zero formation of by-products.
Introduction
Chlorinated volatile organic compounds (Cl-VOCs) including polychloromethanes (PCMs), polychloroethanes (PCAs) and polychloroethylenes (PCEs), are widely used as solvents, degreasing agents and a variety of commercial products.1 These contaminants are mainly considered to be persistent and resistant to biodegradation in the environment.2 Cl-VOCs have been classified as hazardous gas pollutants and were included in the list of highly harmful chemicals targeted in the emission reduction efforts of most counties due to its toxicity and carcinogenic nature. Chlorobenzene is one of the chlorinated compounds from industrial processes which is usually used as model for Cl-VOC because it is a precursor or intermediate product of polychlorinated wastes.3,4 A number of techniques have been used to eliminate chlorinated VOCs, and catalytic combustion is considered to be an efficient and low energy cost option.5,6
Vanadium based catalysts,3,7 nobel metals (Pt, Pd and Ru) supported on zeolites7,8 and transition metal oxides9, 10, 11 have been reported as catalysts used for the catalytic combustions of Cl-VOC. The use of noble metals is limited due to their high cost and low resistance to chlorine poisoning.12,13 Transition metal oxides, including cobalt, manganese, copper and chromium, not only present a better thermal stability, a strong poisoning resistance and low costs, but also enhance catalytic activity by the modification with rare earth elements.14,15 Dai et al.16 studied CeMnLa catalysts which presented a high catalytic activity, selectivity and stability at 350 °C. Yang et al.17 reported the catalytic performance of mixed transition oxides of CeMOx (M: V, Cr, Mn, Fe, Co, Ni and Cu) for Cl-VOC combustion reaction. Moreover, Chen et al.18 showed that Co–Cu–Mn mixed oxide catalysts exhibited high catalytic activity in the oxidation of VOCs. Co3O4/La–CeO2 catalyst was found to provide more adsorbed species and lattice oxygen species due to the interaction between Co3+/Co2+ and Ce4+/Ce3+ couples, which resulted in the enhancement in the catalytic activity.
Rare earth perovskites have been widely studied in Cl-VOCs oxidation due to the high structural and thermal stability that favour their use in industrial conditions (thermal shock, chorine poisoning, water vapour, etc.). Much effort has been made to enhance redox abilities and surface acidities by the substitution of A or B cations,19,20 because the oxidation state at A-site or B-site can be modified. Further, anion vacancy could be produced by charge compensation. Kieβling et al.21 have shown that rare earth perovskites presented great potential as catalysts for the destruction of chloromethanes, chloroethanes and chloroethenes.
In this work the influence of Ce doping of LaCoO3 perovskite in its physicochemical characteristics and thus in chlorobenzene combustion catalytic activity was widely studied. Additionally, the effect of the used synthesis method was analysed considering the importance of this step when large amount of catalysts must be synthesized for an industrial use. For this purpose, two methods will be employed at lab scale, i.e. the citrate and the reactive grinding methods. Both have been widely used in perovskites synthesis. The first one has been used because it provides solids with high surface area and purity.22 The second one is well known to provide perovskites with high crystallinity at lower temperatures than other common techniques. Additionally, reactive grinding does not produce liquid effluents or hazardous vapours because single oxides are the perovskite precursors in agreement with green chemistry procedures.23 The durability of catalysts was also studied in order to evaluate its possible industrial application.
Section snippets
Catalysts preparation
La1−xCexCoO3 (x = 0, 0.05, 0.1, 0.2, 0.5) perovskites were prepared by two different methods. In citrate method,24 metal nitrate solutions were added to a citric acid solution (10% in excess) and agitated during 15 min. The resulting solution was concentrated with a slow evaporation in a rota vapour at 75 °C under vacuum until a gel was obtained. This gel was dried at 100 °C overnight in a vacuum stove, producing a solid amorphous precursor. The resulting precursor was milled and calcined in
Catalytic performance in chlorobenzene combustion
The samples synthetized in this work were evaluated in the catalytic combustion of chlorobenzene. Due to the high toxicity of dioxins and the need for laboratory safety, this compound was used as a model reagent in order to predict destruction behaviour of the synthetized catalysts.3,25 The catalytic activity results are shown in Fig. 1, Fig. 2 and Table 1. Fig. 1 shows the light off curves of chlorobenzene combustion for catalysts prepared by the reactive grinding method. As can be seen,
Conclusions
In this work, a series of Ce-doped LaCoO3 perovskites catalysts were synthetized by two methods, i.e. the citrate and the reactive grinding methods. Both methods provided active catalysts in the studied reaction reaching only total combustion products, CO2, Cl2 and H2O at high reaction temperatures. The most active catalysts are those with the low substitution level (x = 0.05) synthetized by both methods. Thus, this fact results in considering the possibility of a future scale up. The reactive
Acknowledgements
The financial support from Universidad Nacional de San Luis, CONICET, ANPCyT is gratefully acknowledged.
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