Catalytic Removal of Volatile Organic Compounds Over the Three-Dimensionally Ordered Mesoporous or Macroporous MOx (M = Co, Fe, Mn, Cr) Catalysts

Catalytic removal of volatile organic compounds over the three-dimensionally ordered mesoporous or macroporous MO_x (M = Co, Fe, Mn, Cr) catalysts

Hongxing Dai ^a,*, Yunsheng Xia ^a, Ruzhen Zhang ^a, Lei Zhang ^a, Jiguang Deng ^a, Yingshu Liu ^b, Kai Wang ^c, Hong He ^a, Jian Li ^a

^a College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. E-mail address: hxdai@bjut.edu.cn.

^b School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China

^c School of Resource and Safety Engineering, China University of Mining & Technology Beijing, Beijing 100083, China

Most of volatile organic compounds (VOCs) are environmental pollutants. Catalytic combustion is a good way for VOCs removal. The key issue is the availability of an effective catalyst. Recently, we have fabricated MO_x (M = Co, Fe, Mn, Cr) with a three-dimensionally (3D) or 3D ordered macroporous (3DOM) structure using nitrate of Co, Fe, Mn or Cr as metal source, and found that these materials (see Table 1) exhibited excellent catalytic activities for the combustion of formaldehyde, acetone, methanol, and toluene^[1].

The as-fabricated materials were characterized by the XRD, BET, SEM, TEM, XPS, and H₂-TPR techniques. The typical TEM and SEM images of the samples are shown in Fig. 1. It is found that the KIT-6- and SBA-16-templating derived samples possessed ordered mesoporous architectures with crystalline walls. There was formation of nanovoids within the walls of the 3DOM-structured Fe₂O₃-2 sample. The surface areas of the mesoporous MO_x (M = Co, Fe, Mn, Cr) samples were above 100 m²/g. XPS analyses revealed the copresence of Co²⁺/Co³⁺ in Co₃O₄, Fe³⁺/Fe²⁺ in Fe₂O₃, and Cr³⁺/Cr⁵⁺/Cr⁶⁺ in CrO_x. Compared to the corresponding bulk metal oxides, the macro- or mesoporous counterparts exhibited higher surface oxygen species amount and were more reducible at low temperatures. The porous materials showed much better catalytic activities (evaluated with the temperature T_50% and T_90% at VOC conversion = 50 and 90%, respectively) than the bulk counterparts in VOC combustion (Table 1). The unusual catalytic performance of the porous transition-metal oxides was related to their better low-temperature reducibility, 3D ordered mesoporous or 3DOM architecture, higher surface oxygen species concentration, and larger surface area.

Table 1. Fabrication methods, pore structures, surface areas (S), and catalytic activities of the as-prepared catalysts

Catalyst	Fabrication method and calcination conditions	Pore structure	S (m2/g)	Catalytic activitya T50%/T90% (oC)
				HCHO	acetone	methanol	toluene
bulk Co3O4	thermal decomposition; 500oC for 3 h	nonporous	10	-	-	142/-	200/-
Co3O4	KIT-6-templating; 400oC for 3 h	ordered mesopore	121	-	-	105/139	140/180
bulk Fe2O3	thermal decomposition; 500oC for 3 h	nonporous	27	-	235/-	264/-	380/-
Fe2O3-1	KIT-6-templating; 400oC for 3 h	ordered mesopore	113	-	151/208	170/204	-
Fe2O3-2	P123/PMMA-templating (Fe/P123 molar ratio = 232); 550oC for 3 h	3DOM with mesopore walls	46	-	-	-	240/288
bulk MnO2	(Beijing Chemical Reagent Co.)	nonporous	10	-	-	-	285/340
MnO2	ultrasound-aided SBA-16-templating; 450oC for 3 h	ordered mesopore	266	-	-	-	190/240
bulk Cr2O3	thermal decomposition; 500oC for 4 h	nonporous	5	152/-	142/-	164/-	190/-
CrOx-1	ultrasound-assisted KIT-6-templating; 400oC for 4 h	ordered mesopore	124	92/117	75/124	98/130	-
CrOx-2	solvent-free KIT-6-templating; 240oC for 24 h	ordered mesopore	106	-	-	-	140/234

Reaction conditions: 1000 ppm VOC and space velocity = 20,000 mL/(g h).

Figure 1. TEM and SEM images of (a) Co₃O₄, (b) Fe₂O₃-1, (c) Fe₂O₃-2, (d) MnO₂, (e) CrO_x-1 and (f) CrO_x-2

Reference

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