Research paperElectric field assisted benzene oxidation over Pt-Ce-Zr nano-catalysts at low temperature
Graphical Abstract
Introduction
Volatile organic compounds (VOCs), represented by halogenated hydrocarbon, aldehydes and aromatics, etc., mainly stem from petrochemical industry, paint printing, interior decoration, rubber and plastic processing, automobile manufacturing and other industrial production processes, as well as exhaust emissions from vehicles such as automobiles and ships (Devi and Kavitha, 2013, Ferreira et al., 2004, Liotta, 2010). It is urgent to reduce VOCs release effectively due to their adverse impact on environmental and public health. At present, many technologies such as biodegradation, photocatalysis, plasma, direct oxidation and catalytic oxidation have been used for VOCs removal (Bouazza et al., 2008, Jiang et al., 2015, Kim and Shim, 2010, Mamaghani et al., 2017).
Catalytic oxidation is one of the most promising VOCs removal methods. In general, development of various catalysts is the core of this technology and has been widely investigated in recent years (Chen et al., 2018b, Cheng et al., 2018, Cheng et al., 2020, Dai et al., 2012, Deng et al., 2018, Einaga et al., 2015, Garcia et al., 2010, Li et al., 2011, Rokicińska et al., 2017, Wang et al., 2013, Xing et al., 2013). The active components of catalysts act as the main sites of efficient catalytic reaction, which is usually divided into non noble metals (Fe, Co, Ni etc.) and noble metals (Pd, Pt and Ru etc.). The advantages of transition metals lie in abundant resources and low price, which attracts much attention. Many transition metals catalysts, such as cobalt (Li et al., 2017, Ren et al., 2019, Yang et al., 2020, Zhong et al., 2020), manganese (Ni et al., 2020, Yang et al., 2017b), nickel (Li et al., 2014), titanium (Ji et al., 2018, Nguyen-Phan et al., 2009) exhibit excellent VOCs removal performance through the innovation of preparation methods and performance modification. Compared with transition metal, precious metal catalysts have excellent oxidation ability because of their unique electronic structure, but they are expensive and rare in resources (Cheng et al., 2020, Padilla et al., 2008, Zhao et al., 2011, Zhong et al., 2009, Zuo et al., 2017). Therefore, the research on precious metals usually aim to maintain high activity while reducing the amount of precious metals to obtain lower cost in practical application. Furthermore, the role of supports also counts for much, and rare earth oxides particularly true with CeO2-based catalysts, are usually taken into consideration due to the excellent oxygen transport capacity, as a result of flexible switching between Ce3+ and Ce4+, which strongly affects catalytic performance in VOCs removal (Delimaris and Ioannides, 2009, Feng et al., 2018, Wang et al., 2016, Yang et al., 2017a, Yang et al., 2017b). It is always the goal of researchers to achieve high-efficiency VOCs removal and the reaction temperature is required as low as possible. Regardless of transition metal or precious metal catalysts, however, it is hard to completely convert VOCs to CO2 below 150 °C.
Numerous studies on catalysts for VOCs removal have been carried out in the last two decades. In regard to low cost and high efficiency at low temperature, however, it is difficult to make significant progress only in catalyst design. The development of new catalytic technologies has received increasing research attention. For instance, non-equilibrium plasma catalysis (Li et al., 2020, Lu et al., 2006, Mao et al., 2018, Mustafa et al., 2018, Parastaev et al., 2018), non-Faradaic electro-chemical modification of catalytic activity (NEMCA) systems (Delucasconsuegra et al., 2007, Vayenas and Koutsodontis, 2008) and other electrochemical processes have been conducted to promote VOCs catalytic reaction at low temperature. Nevertheless, the high energy consumption or high temperature trigger limit the application of these technologies. Sekine et al. proposed a non-conventional catalytic process combining oxidation catalyst with electric field and applied this system on CH4 steam reforming, ethanol (C2H6O) degradation and reverse gas-shift reaction (Oshima et al., 2013, Oshima et al., 2014, Sekine et al., 2009, Yabe et al., 2017). Our previous works have covered the catalytic oxidation of methane over Pd-based (Li et al., 2018), Co-Mn (Li et al., 2019c), Co-Ce-Zr (Li et al., 2019b), Pd-Ce-Co (Shen et al., 2019) catalysts in the electric field and found that the electric field greatly improves the methane conversion, among which the best catalytic system with methane light off temperature (T50) as low as 225 °C, indicating the worth of investigation on this field (Li et al., 2019b). Assisted with electric field gains more possibilities to lower the temperature required by efficient conversion of some VOCs gas. Moreover, it will bring unexpected benefits to the traditional catalytic system by exploring more chemical reaction processes suitable for electric field.
Herein, we first introduce electric field into the traditional catalytic reaction process of VOCs gas benzene and significantly improve the conversion (above 90%) at low temperature (below 100 °C). Pt element and Ce-Zr composite metal oxide are selected as active components and support for this experiment, respectively. Namely, a series of m% Pt/CexZr1−xO2 (m=0.5, 0.75, 1; x = 0.25, 0.5, 0.75) catalysts are synthesized by self-propagating high-temperature synthesis (SHS) method and applied for the catalytic oxidation of benzene in electric field. The relation between structure of catalyst and catalytic performance, as well as the role of electric field in this system are investigated by various characterizations.
Section snippets
Catalysts synthesis
The Pt-Ce-Zr catalysts with different Pt loading and Ce/Zr ratio were prepared by the self-propagating combustion synthesis (SHS) method. All chemical reagents are analytical grade. Briefly, metal precursors of Ce (NO3)3·6H2O, Zr (NO3)4·5H2O and Pt (NO3)2 were dissolved in distilled water at a certain molar ratio. The stoichiometric glycine (CH2NH2COOH) was then added to the precursor mixture as fuel. After that, the mixture was stirred vigorously for 1 h and then transferred to the crucible.
Catalytic activity
The evaluation of Pt-Ce-Zr catalysts performance with different Pt loading levels and Ce/Zr molar ratios are investigated by taking the oxidation of benzene as a model reaction. The benzene conversion in/without the electric field as function of the reaction temperature is shown in Fig. 3. It can be seen that with the increase of reaction temperature, the conversion of benzene increases. A faster increase in conversion occurs above approximately 140 °C without electric field. As a note, the
Conclusion
In this study, a novel catalytic system for benzene oxidation at low temperature is constructed by combining electric field. A series of Pt-Ce-Zr nano-catalyst are synthetized by self-propagating combustion synthesis (SHS) method and the 1 wt% Pt/Ce0.75Zr0.25O2 catalyst assisted by electric field shows the best catalytic performance with 90% benzene conversion at 96.5 °C and excellent water resistance. The Pt2+ and adsorbed oxygen together form the surface active sites of the catalyst. The
CRediT authorship contribution statement
Xuteng Zhao: Writing - original draft, Writing - review & editing, Methodology. Dejun Xu: Data curation. Yinan Wang: Investigation. Zuwei Zheng: Investigation. Ke Li: Validation. Yiran Zhang: Validation. Reggie Zhan: Funding acquisition. He Lin: Conceptualization, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51676127).
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