Removal of VOCs from gas streams with double perovskite-type catalysts

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Abstract

Double perovskite-type catalysts including La2CoMnO6 and La2CuMnO6 are first evaluated for the effectiveness in removing volatile organic compounds (VOCs), and single perovskites (LaCoO3, LaMnO3, and LaCuO3) are also tested for comparison. All perovskites are tested with the gas hourly space velocity (GHSV) of 30,000 hr 1, and the temperature range of 100–600°C for C7H8 removal. Experimental results indicate that double perovskites have better activity if compared with single perovskites. Especially, toluene (C7H8) can be completely oxidized to CO2 at 300°C as La2CoMnO6 is applied. Characterization of catalysts indicates that double perovskites own unique surface properties and are of higher amounts of lattice oxygen, leading to higher activity. Additionally, apparent activation energy of 68 kJ/mol is calculated using Mars-van Krevelen model for C7H8 oxidation with La2CoMnO6 as catalyst. For durability test, both La2CoMnO6 and La2CuMnO6 maintain high C7H8 removal efficiencies of 100% and 98%, respectively, at 300°C and 30,000 hr 1, and they also show good resistance to CO2 (5%) and H2O(g) (5%) of the gas streams tested. For various VOCs including isopropyl alcohol (C3H8O), ethanal (C2H4O), and ethylene (C2H4) tested, as high as 100% efficiency could be achieved with double perovskite-type catalysts operated at 300–350°C, indicating that double perovskites are promising catalysts for VOCs removal.

Introduction

Volatile organic compounds (VOCs) are one of major air contaminants, which generally have a high vapor pressure (> 0.1 mmHg) or low boiling point (≤ 250°C) (EUR-lex. Access to European law, 2004). Some VOCs are toxic at low concentration levels and cause adverse effects on human health (Rumchev et al., 2007). Moreover, VOCs can also react with nitrogen oxides (NOx) in atmosphere to form ozone (O3) and peroxyacetyl nitrate (PAN) with solar irradiation (Roberts, 1990, Ivanova et al., 2013, Kamal et al., 2016), leading to environmental hazards. Besides, parts of VOCs are regarded as important odor-causing substances. Thus, how to effectively reduce VOCs emission has become an important issue.

Thermal catalysis is regarded as an effective way for VOCs removal because it provides oxidation route to successfully convert VOCs into carbon dioxide (CO2) and water (H2O(g)) (Ojala et al., 2015). Compared with other technologies such as adsorption, incineration, and absorption, thermal catalysis not only consumes less energy for maintaining high performance but also reveals high durability. Metal oxides including MnOx, CuO, Co3O4 and Fe2O3 have been applied as catalysts for removal of VOCs (Zhou et al., 2011, Tang et al., 2006). However, they need to be operated at a high temperature (> 350°C) for good performance in removing aromatic VOCs (Bialas et al., 2011, Aguilera et al., 2011, Białas et al., 2014). Single perovskite-type catalysts are promising for VOC removal, and they are of good potential to replace noble metals due to good activity and high thermal stability. Single perovskites can be generally described as ABO3 or A2BO4, where A-site is often a rare earth element (La, Pr, and Sm) or an alkaline (Sr and Ba) coordinating to 12 oxygen anions. Typically, the B-site is a smaller transition metal cation (Fe, Cu, Ni, Co, or Mn) occupying in oxygen framework. Previous studies indicate that single perovskite-type catalysts have good activity for VOC removal (Huang et al., 2008, Zhang et al., 2015, Alifanti et al., 2007). Catalytic properties of perovskites mainly rely on nature, oxidation states and arrangement of B-site cation (Zheng et al., 2014). In addition, activity of perovskites could be effectively enhanced by partial substitution of suitable elements at A-site or B-site (Ebbinghaus et al., 2009, Yoon et al., 2013, Chen et al., 2017), because the valence state at A-site or B-site can be varied. Further, anion vacancy could be produced by charge compensation. The perovskites modified by partial substitution have been applied for the removal of various VOCs (Huang et al., 2008, Levasseur and Kaliaguine, 2009, Hosseini et al., 2013). Interestingly, single perovskites could be further synthesized to form double perovskite-type catalysts which can simply be represented by A2B′B″O6 (Steward and Rooksby, 1951, Roy, 1954). Likewise, A-site stands for an alkaline metal or rare earth element. The B′ and B″ are transition metals and they generally present rock–salt structure at sub-lattice of B-site (Anderson et al., 1993). Double perovskites possess unique surface properties (Kobayashi et al., 1998, Teraoka et al., 1998). In structure, three-dimensional of B-site with ordered arrangement of B′O6 and B″O6 is presented alternately, and they corner-share octahedral along three directions of the crystal lattice, while cations of A-site position between the void of octahedral (Li et al., 2011b, Li et al., 2011c). It has attracted considerable attention due to specific arrangement of alternating B-site structure. For space group symmetry, structure of double perovskites can be usually varied because different sizes of B-site cations and the A-site cations cause distortion of the octahedral. Therefore, double perovskites may have more variations than single perovskites, and this greater variation may promote catalytic performance (Zheng et al., 2014). It is expected that the activity of double perovskites is higher than that of single perovskites toward VOC removal. Double perovskites have been applied for catalytic combustion of methane (Li et al., 2011b, Hu et al., 2012, Hu et al., 2015), steam reforming of methane (Tuza and Souza, 2016), partial oxidation of methane (Yin and Hong, 2009). Also, double perovskites are studied for solid oxide fuel cell (SOFC) (Sun et al., 2016, Ding et al., 2016). However, they have not been applied for VOC removal so far.

In this study, double perovskite-type catalysts including La2CoMnO6 and La2CuMnO6 are prepared and evaluated for VOC removal, because Co, Cu, and Mn are commonly considered to possess high activities for VOC removal. Also, single perovskites including LaCoO3, LaMnO3, and LaCuO3 are prepared and tested for the comparison purpose. Toluene (C7H8) is one of the important volatile organic compounds which are commonly applied in chemical processes. In addition to its wide application, C7H8 has high toxicity at a low concentration. Therefore, C7H8 is selected as the target compound in this study. Various operating parameters including operating temperature, gas hourly space velocity (GHSV), H2O(g) and CO2 contents are extensively evaluated for the effects on C7H8 removal. Additionally, the kinetics of catalytic oxidation of C7H8 are studied. Also, various VOCs including ethylene (C2H4), isopropyl alcohol (C3H8O), ethanal (C2H4O) are tested to evaluate the activities of double perovskites. All experimental tests are conducted with a lab-scale experimental setup, and the removal mechanisms will be elucidated.

Section snippets

Catalyst preparation

All perovskite-type catalysts including double perovskites (La2CoMnO6 and La2CuMnO6) and single perovskites (LaCoO3, LaMnO3, and LaCuO3) were synthesized by the Pechini-modified method (Pan et al., 2014). The 1 mol/L solutions of metal nitrates and citric acid were prepared as precursors, and the precursor solutions were mixed completely at an appropriate stoichiometry and heated to 80°C for 1 hr with stirring. For preparation of single perovskite (ABO3), the molar ratio of A: B was adjusted at

Characterization of the catalysts

XRD patterns of the perovskites prepared are shown in Fig. 2. It indicates that diffraction peaks of LaCoO3, LaMnO3, and LaCuO3 are attributed to the crystalline of single perovskites (Zhang et al., 2015, Jia et al., 2011, Touahra et al., 2016). Diffraction peaks with La2CoMnO6 and La2CuMnO6 are mainly located at 23°, 33°, 41°, 47°, 58°, 68° and 78°, which are assigned to the crystalline of double perovskite. The results indicate that perovskites including single-type and double-type are

Conclusions

Double perovskite-typed catalysts including La2CoMnO6 and La2CuMnO6 are evaluated for VOC removal, while single perovskites (LaCoO3, LaMnO3, and LaCuO3) are also tested used for comparison. All perovskites are prepared using Pechini-modified method, and show good surface properties. Especially, double perovskite-type catalysts present higher concentrations of lattice oxygen, resulting in higher activity. In terms of catalysis, double perovskites are superior to single perovskites. As La2CoMnO6

Acknowledgments

Authors would like to express their gratitude to the Ministry of Science and Technology (MOST), Republic of China (ROC) (No. 102WFA0700516), and National Central University (No. 105G910-9) for funding. Also, the authors would like to thank the part of financial support from the Industrial Technology Research Institute (No. 105G910-8).

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