Article pubs.acs.org/IECR
Au/MnOx/3DOM La0.6Sr0.4MnO3: Highly Active Nanocatalysts for the Complete Oxidation of Toluene Yang Jiang, Jiguang Deng,* Shaohua Xie, Huanggen Yang, and Hongxing Dai* Key Laboratory of Beijing on Regional Air Pollution Control, Beijing Key Laboratory of Green Catalysis and Separation, and Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China S Supporting Information *
ABSTRACT: Three-dimensionally ordered macroporous La0.6Sr0.4MnO3 (3DOM LSMO) supported manganese oxide and gold (yAu/zMnOx/3DOM LSMO; y = 1.76−6.85 wt %; z = 8 wt % (weight percent of Mn2O3)) nanocatalysts were fabricated by means of in situ poly(methyl methacrylate)-templating and gas bubble-assisted poly(vinyl alcohol)-protected reduction methods. The 3DOM LSMO support possessed a rhombohedral crystal structure and a surface area of 22−25 m2/g. MnOx and Au nanoparticles (NPs) (3.2−3.8 nm) were well dispersed on the surface of 3DOM LSMO. Catalytic performance of the samples for toluene oxidation was found to be well related to their surface adsorbed oxygen species concentrations and low-temperature reducibility. Among the as-prepared samples, 5.92Au/8MnOx/3DOM LSMO performed the best at a space velocity of 20 000 mL/(g h): the T50% and T90% (corresponding to toluene conversion of 50 and 90%) were 205 and 220 °C, respectively. The apparent activation energies (52.8−68.5 kJ/mol) obtained over the yAu/8MnOx/3DOM LSMO samples were much smaller than those (79.3−89.5 kJ/mol) obtained over the bulk LSMO supported counterparts. We believe that the excellent catalytic performance of 5.92Au/8MnOx/3DOM LSMO might be ascribed to the large surface area, high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between Au NPs or MnOx and 3DOM LSMO.
1. INTRODUCTION Emissions of volatile organic compounds (VOCs) from industrial and transportation activities can cause serious environmental problems. Therefore, strictly controlling the emissions of VOCs is meaningful for environmental protection. There are various methods to remove VOCs, among which catalytic combustion is believed to be one of the most efficient pathways.1,2 The development of high-efficiency catalysts is the key issue. Up to now, supported noble metals and single or mixed metal oxides have been used to catalyze the oxidative removal of VOCs. Among the single metal oxides, manganese oxides (MnOx) are catalytically active for the complete oxidation of propane, nhexane, benzene, and toluene.3−6 For example, Finocchio and Busca pointed out that the bulk oxygen diffusion rate of Mn3O4, Mn2O3, and MnO2 influenced the oxidation rate of propane.3 In the oxidation of n-hexane, Delmon and co-workers observed that the γ-MnO2 catalyst performed better than the 0.3 wt % Pt/TiO2 catalyst.6 After investigating the oxidation of benzene over manganese oxide octahedral molecular sieve (OMS-2) catalyst, Luo et al.4 concluded that because of the hydrophobic property and facile evolution of lattice oxygen, the OMS-2 catalyst exhibited excellent low-temperature activity and stability. In addition, Aguero et al.5 claimed that the high oxygen adsorption capacity, surface defects, and good reducibility of the MnOx/Al2O3 sample contributed to the enhancement in catalytic performance for ethanol and toluene combustion. Because of low cost, high thermal stability,7 and good antipoisoning ability, perovskite-type oxides (ABO3) have recently received great attention in the catalytic oxidation of © 2015 American Chemical Society
VOCs. However, obtaining the perovskite crystal structure requires high calcination temperature during preparation processes. The as-prepared ABO3 catalysts are usually low in surface area ( 6.05Au/3DOM LSMO (2.8 × 10−4 mmol/(gcat s)) > 6.85Au/8MnOx/3DOM LSMO (2.2 × 10−4 mmol/(gcat s)) > 1.76Au/8MnOx/3DOM LSMO (1.9 × 10−4 mmol/(gcat s)) > 4.25Au/8MnOx/3DOM LSMO (1.6 × 10−4 mmol/(gcat s)) > 5.63Au/8MnOx/bulk-LSMO (1.5 × 10−4 mmol/(gcat s)) > 8MnOx/3DOM LSMO (0.7 × 10−4 mmol/(gcat s)). To evaluate the low-temperature reducibility of the samples, we plotted the initial (where less than 25% oxygen in the sample was removed for the first reduction band) H2 consumption rate as a function of inverse temperature (Figure 5B).38,39 Apparently, the initial H2 consumption rate decreased in the order of 5.92Au/8MnOx/3DOM LSMO > 6.85Au/ 8MnOx/3DOM LSMO > 4.25Au/8MnOx/3DOM LSMO > 6.05Au/3DOM LSMO > 1.76Au/8MnOx/3DOM LSMO > 5.63Au/8MnOx/bulk-LSMO > 8MnOx/3DOM LSMO > 8MnOx/bulk-LSMO. Obviously, the changing trend in lowtemperature reducibility had a good relationship with that in catalytic performance shown below.
reduction peaks shifted to lower temperatures after Au loading. The first peak was due to the reduction of the chemically adsorbed oxygen species on the highly dispersed Au NPs (i.e., from Au−Ox to Au0) or the interface between Au NPs and LSMO support (i.e., from Mn−Ox−Au to Au0).37 For the sample with a higher Au loading, the shift toward low temperature became more pronounced. In other words, Au had a promotional effect on the reduction of manganese species. As can be seen from the reduction profiles of the 5.92Au/8MnOx/3DOM LSMO and 5.63Au/8MnOx/bulkLSMO samples, the presence of a porous structure enhanced the dispersion of Au NPs and hence the reduction of manganese species. Table 2 summarizes the quantitative analysis results of reduction peaks in the H2-TPR profiles of the samples. With the loading of Au NPs, the H2 consumption of the sample increased. Among all of the samples, the highest H 2 consumption (3.14 mmol/gcat) at low temperature (
