Reaction kinetics and mechanism of benzene combustion over the NiMnO₃/CeO₂/Cordierite catalyst

Abstract

The kinetics of the catalytic combustion of benzene at different concentrations over the NiMnO₃/CeO₂/Cordierite catalyst were investigated to gain more insight into the catalytic reaction mechanism. A kinetic study was performed in a packed-bed tubular reactor under different conditions. The catalytic combustion kinetics were modeled using a Power-Law model and the Mars–Van Krevelen model. The results showed that the Mars–Van Krevelen kinetic model provided a significantly better fit to explain the catalytic combustion kinetics of benzene over the NiMnO₃/CeO₂/Cordierite catalyst. The results showed that the reduction reaction occurred more easily than did the oxidation reaction on the surface of the catalyst. Moreover, the values of the pre-exponential factor for the reduction steps (7.84 × 10¹¹ s⁻¹) is higher than those of the oxidation steps (1.04 × 10⁹ s⁻¹), indicating that the more is the effective collision times between the activated molecules, and the easier for chemical reaction to occur, and the degree and speed are more intense and rapid. Therefore, it can be concluded that the catalyzed surface oxidation reaction is the control step of catalytic benzene combustion. Based on this analysis of the experimental results and the assumptions of the Mars–Van Krevelen model, it was determined that the catalytic combustion of benzene over the NiMnO₃/CeO₂/Cordierite catalyst obeys the Mars–Van Krevelen mechanism. The catalytic combustion reaction occurred by the interaction between the benzene molecules and the active sites of the NiMnO₃/CeO₂/Cordierite catalyst. The catalytic oxidation of benzene involves a catalytic redox cycle of adsorption, deoxidation, desorption, oxygen supply and regeneration.

Introduction

Volatile organic compounds (VOCs) emitted from many industrial processes are an important class of air pollutants [1], [2]. These pollutants can cause many environmental problems such as toxic emissions, precursors of ozone and photochemical smog formation [3], [4]. Reducing VOC emissions is a very important environmental issue for human health. VOCs are usually eliminated by thermal oxidation, catalytic oxidation, adsorption and absorption. Among these removal methods, catalytic combustion has been recognized as one of the most promising techniques for VOC removal due to its practical applications in pollutant abatement, and this technology has been determined to be more environmentally friendly than conventional flame combustion because it is more versatile and economical in handling organic emissions. Moreover, catalytic incineration, which operates at relatively low temperatures and under controlled conditions, does not emit undesirable by-products, such as dioxins and NO_x [5], [6], [7].

In general, there are two major types of efficient catalysts for total VOC oxidation, supported noble metals and transition metal oxides. Although the former are highly active at relatively low temperatures, their application is limited due to the high price of precious metals and problems related to sintering and volatility [8], [9]. In recent years, research efforts in this field have been focused on developing new catalytic materials with low manufacturing costs that are capable of high activity at moderate temperatures [10], [11], [12]. We investigated the catalytic performance of Ni–Mn/CeO₂/Cordierite catalysts for benzene combustion in a previous work, which demonstrated that Ni–Mn mixed oxide demonstrates higher activity than the pure oxides. When the Ni/Mn molar ratio was 1:1, the temperature required for the complete conversion of benzene over the catalyst was even lower than that of a supported Pt catalyst [13]. The X-ray diffraction (XRD) and H₂-temperature programmed reduction (H₂-TPR) results supported our findings, revealing that the catalyst with the highest activity had a perovskite crystal structure and higher redox activity than the other tested catalysts.

Apart from the catalyst composition, another aspect that must be considered for catalytic combustion is the kinetic model, which represents a key element in predicting the reaction rate of VOC combustion over the catalyst [14], [15], [16]. Based on these results, a reaction mechanism is proposed that can be used as the theoretical basis for the design of high activity catalysts, providing more insight into the behavior of the catalysts. The catalytic combustion kinetics of VOCs as single oxides or simple mixed oxides over various catalysts were presented in the literature [17], [18], [19]. However, studies of kinetic models devoted to obtaining perovskite composite oxides are scarce. Therefore, the search for a suitable kinetic model that offers greater insight into the catalytic combustion reaction mechanism of VOCs over the NiMnO₃/CeO₂/Cordierite catalyst is a worthwhile effort.

Based on previous research, a kinetic study was performed in a packed-bed tubular reactor under differential conditions. The catalytic combustion kinetics were investigated by using the Power-Law and Mars–Van Krevelen models. The aim of this work is to investigate the catalytic combustion reaction kinetics and the mechanism of benzene over the NiMnO₃/CeO₂/Cordierite catalysts. Emphasis was placed on deriving kinetic expressions describing the rate of this reaction. To the best of our knowledge, this article is the first report on the kinetics and mechanism of the complete oxidation of benzene over the perovskite-type NiMnO₃/CeO₂/Cordierite catalyst.