Catalytic oxidation of toluene over a porous Co₃O₄-supported ruthenium catalyst

Abstract

Porous Co₃O₄-MOF and Ru/Co₃O₄-MOF were prepared through a metal–organic frameworks (MOFs)-templated method, and the bulk materials resulted from precipitation method were prepared for comparison. All catalysts were tested for the catalytic oxidation of toluene in the temperature range of 150–300 °C, and Ru/Co₃O₄-MOF showed apparently higher catalytic activity and CO₂ selectivity than other materials. The influence of water vapour on the catalytic oxidation of toluene was studied at 235 °C and 245 °C, and it could be found that the inhibition effect weakened with the increase of temperature. Additionally, the test of 12 h on-stream reaction was conducted to explore the stability of Ru/Co₃O₄-MOF.

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1. Introduction

Volatile organic compounds (VOCs) emitted from industrial processes and transportation activities are considered as great contributors to air pollution and dangerous to human health.^1–3 The most common approaches for VOCs removal include thermal incineration, catalytic oxidation, biochemical methods, adsorption strategies, and other techniques.^4–9 Due to its advantages of high efficiency, low temperature, and high selectivity to harmless products, catalytic oxidation is regarded as a promising pathway to reduce VOCs emissions to meet the standards of air pollution control regulations.^10,11 In recent years, various catalysts have been used for the complete oxidation of VOCs, such as noble metals,^11–15 transition metal oxides,^16–20 and perovskite-type oxides.^21–25 Supported platinum and palladium materials are well established as efficient catalysts for complete oxidation of VOCs,^26–30 whereas their wide applications are limited due to the high costs. Transition metal oxides have been increasingly studied as the catalysts for oxidation of VOCs in single or mixed metal oxides formations,^31–33 usually showing lower catalytic activities than noble metals. Recently, they have been used as supports or additives for platinum, palladium, and gold-based catalysts in many cases.^34–37

Ruthenium catalysts have received considerable importance in CO oxidation,^38,39 deacon process,^40,41 and catalytic oxidation of VOCs.^42–44 Nevertheless, ruthenium-based materials catalyzed VOCs oxidations are far less investigated compared to other noble metals. In the reports on Ru promoted oxidation of VOCs, ethyl acetate,⁴⁵ propane,^46–48 toluene,^42,49 and chlorobenzene^50–53 could be totally oxidized within the temperature range of 175–275 °C. Noteworthy, Mitsui et al. compared the catalytic performance of CeO₂ supported noble metals in oxidation of ethyl acetate, such as Ru, Pt, Pd, and Rh, respectively, and found that Ru containing catalyst exhibited the highest catalytic activity.⁵⁴

Spinel cobalt oxide (Co₃O₄) has been well established as an intriguing material in the complete oxidation of VOCs, and its outstanding catalytic activity was mostly attributed to the mobile oxygen within the Co₃O₄ structure.^55–58 In the field of catalytic oxidation of VOCs, it is generally concluded that the efficiency of the catalyst is greatly concerned with the oxygen vacancy, redox ability, and some morphological factors such as surface area and porous structure.^59–62 Accordingly, various nanocasting strategies were developed to modify the structures of the catalysts, and Co₃O₄ with different nanostructures, such as nanoflowers, nanorods, and nanotubes, have been prepared.^58,63

In recent years, metal–organic frameworks (MOFs) have emerged as promising sacrificial templates for fabricating hierarchically porous metal oxides through solid-state thermolysis method.^64–67 As compared to other templates, MOFs exhibit the advantages of high porosity, large surface area, long-range ordering structure, and easily removable properties under controlled calcinations conditions. Through MOFs-templated method, various metal oxides have been prepared and employed in different research fields, such as spindle-like mesoporous α-Fe₂O₃ as lithium-ion battery anode materials⁶⁴ and Mn_xCo_3−xO₄ nanocages as DeNO_x catalysts.⁶⁷ Most recently, Kuang et al. have prepared novel porous Co₃O₄ nanocubes, which exhibit high specific surface area of 120.9 m² g⁻¹, showing great efficiency in gas sensing applications.⁶⁸

To the best of our knowledge, porous Co₃O₄ supported Ru catalyst for complete oxidation of VOCs has not been reported. Therefore, it is necessary to investigate the preparation and application of porous Co₃O₄ supported Ru catalysts in oxidation of VOCs, such as toluene. Herein, porous Co₃O₄ was prepared using MOF-templated strategy, and its supported Ru material was obtained through impregnation method. Bulk Co₃O₄ and its corresponding Ru catalyst were also prepared for comparison. Tests were conducted to explore their catalytic activities in complete oxidation of toluene. The catalysts were characterized by BET, XRD, TEM, and XPS to study their physicochemical properties, which correspond with their catalytic activities.

2. Experimental section

2.1. Catalyst preparation

Co-MOF (ZIF-67) and porous Co₃O₄ were synthesized following the procedure described in the literature with some modifications.⁶⁸ In a typical preparation, a 12 mL aqueous solution of Co(NO₃)₂·6H₂O (1.75 g, 6 mmol) was added to a 80 mL aqueous solution of 2-methylimidazole (21.30 g, 260 mmol), and the mixture was stirred for 6 h at room temperature. The resulting purple precipitates were collected by centrifugation, and washed with water for 3 times and alcohol for 3 times, respectively. The obtained Co-MOF (ZIF-67) was dried at 80 °C overnight. The as-prepared Co-MOF (ZIF-67) precursor was loaded in a ceramic crucible and then calcined in a muffle furnace at a ramp of 5 °C min⁻¹ from 30 °C to 325 °C and kept at this temperature for 3 h, giving a black powder, which was denoted as Co₃O₄-MOF.

Supported 1 wt% Ru/Co₃O₄-MOF catalyst was prepared by impregnation method. The as-prepared Co₃O₄-MOF support was impregnated with an aqueous solution containing an appropriate amount of Ru(NO)(NO₃)₃, and then dried at 80 °C for 6 h and calcined at 325 °C for 3 h.

Bulk Co₃O₄–B sample was prepared using precipitation method with Co(NO₃)₂·6H₂O and Na₂CO₃ as reagents.⁵⁸ The resulting precipitates were dried at 80 °C for 6 h and calcined at 325 °C for 3 h, affording bulk Co₃O₄–B. Impregnation of Co₃O₄–B with an aqueous solution containing an appropriate amount of Ru(NO)(NO₃)₃ followed by calcination at 325 °C for 3 h gave 1 wt% Ru/Co₃O₄–B.

2.2. Catalyst characterization

Physicochemical properties of the catalysts were characterized by means of techniques. X-ray diffraction (XRD) patterns of the samples were recorded on a powder diffractometer (Rigaku D/Max-RA) using Cu Kα radiation (40 kV and 120 mA). The porous texture was characterized by N₂ adsorption at 77 K in an automatic surface area and porosity analyzer (Autosorb iQ, Quantachrome). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM) were taken on a Tecnai G2 F20 S-TWIN field emission transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo escalab 250Xi X-ray using Al Kα source.

2.3. Catalytic evaluation

Catalytic activities of the catalysts were evaluated in a continuous flow fixed-bed quartz microreactor (i.d. = 4 mm) from 100 to 300 °C with 100 mg of catalyst (80–100 mesh). Toluene was introduced into reactant flow by Ar through a saturator, and its concentration in the total flow of the reactant mixture (1000 ppm toluene + 20% O₂ + Ar (balance)) was calibrated by GC through a by-pass (Fig. S1†). The total flow ratio of the reactant mixture was 100 mL min⁻¹ with the weight hourly space velocity (WHSV) of 60 000 mL g⁻¹ h⁻¹. Before the test, the sample was pretreated at 250 °C in a 20% O₂/Ar stream for 0.5 h. After the pretreatment, the reactant gas containing toluene was introduced to the reactor. Analysis of the reactants and products were performed on-line with a gas chromatography (GC 2010 Plus, Shimadzu) equipped with a flame ionization detector (FID) and the concentration of CO₂ in the outlet gas was detected by another FID with a methanizer (MTN) furnace for converting CO₂ to CH₄.

3. Results and discussion

3.1. Catalyst characterization

From the XRD patterns of the as-prepared Co₃O₄ samples in Fig. 1, it can be seen that these materials exhibited 31.4°, 36.9°, 38.2°, 44.8°, 59.4°, and 65.3° (2θ) peaks, which are attributable to standard Co₃O₄ sample (JCPDS PDF 43-1003). Any impurity phases such as Co, CoO, Co₂O₃, and Co(OH)₂ were not observed. It should be noted that all diffraction peaks of the MOFs-templated samples are apparently broaden in width and weaken in intensity, indicating that the as-prepared materials are composed of abundant nanosized crystallites. The XRD patterns of Ru/Co₃O₄-MOF showed no obvious difference from those of Co₃O₄-MOF, illustrating that Ru addition did not significantly change the formation of the nanostructure of Co₃O₄-MOF.

Fig. 1 XRD patterns of the catalysts.

The N₂ adsorption–desorption isotherms and BJH pore size distribution were performed to evaluate the specific surface area and the porosity of all the catalysts. As shown in Fig. 2, Co₃O₄-MOF and Ru/Co₃O₄-MOF presented typical type IV adsorption isotherm with a H3-type hysteresis loop at a relative pressure range of 0.8–1.0, confirming the presence of a mesoporous structure. The specific surface area and pore size distribution of Ru/Co₃O₄-MOF were similar to those of Co₃O₄-MOF (Fig. 2 and Table S1†). Obviously, MOFs-templated samples showed higher specific surface areas than the precipitation-resulted samples (Table S1†).

Fig. 2 N₂ adsorption–desorption isotherms and BJH pore size distributions of the catalysts.

Fig. 3 showed the TEM images of Co₃O₄-MOF and Ru/Co₃O₄-MOF samples. Slightly distorted nanocube was observed, retaining the initial morphology of the Co-MOFs precursors in some extent and confirming the successful preparation of expected materials. With the incorporation of Ru species, no apparent change of the nanostructure was observed for Ru/Co₃O₄-MOF. As shown in the high-resolution TEM images, 0.28 nm lattice spacing of Co₃O₄ (2 2 0) as well as 0.46 nm (1 1 1) lattice spacing were observed. For Co₃O₄, the (1 1 1) planes were considered to be inactive crystal planes since they contain only Co²⁺, which makes no contributions in the catalytic oxidation.³⁷ Ru/RuO_x nanoparticles were not observed in our HRTEM characterizations.

Fig. 3 TEM and HRTEM images of the catalysts: (a and b) Co₃O₄-MOF, (c and d) Ru/Co₃O₄-MOF.

In order to analyze the chemical states of the Ru species, XPS characterizations for the Ru-containing samples were conducted, and the XPS spectra of Ru 3d were presented in Fig. 4. It can be seen that the samples exhibited three Ru 3d_5/2 peaks at 280.8, 281.9, and 282.8 eV, which could be assigned to Ru⁰, Ru⁴⁺, and Ru⁶⁺, respectively.⁵³ Apparently, the oxidic Ru species dominate in both catalysts. The ratio of Ru⁴⁺/Ru⁶⁺ for Ru/Co₃O₄-MOF is 2.0, higher than that of Ru/Co₃O₄–B (1.36). Ru/Co surface atomic ratios of Ru/Co₃O₄-MOF and Ru/Co₃O₄–B were 0.038 and 0.013, revealing that Ru/Co₃O₄-MOF exhibits a higher Ru dispersion (Table S2†).

Fig. 4 XPS spectra of Ru 3d for Ru/Co₃O₄-MOF and Ru/Co₃O₄–B.

3.2. Catalytic studies

The catalytic efficiencies of the catalysts for the oxidation of toluene at a WHSV of 60 000 mL g⁻¹ h⁻¹ are shown in Fig. 5. It can be easily seen that Co₃O₄-MOF showed better catalytic activity than Co₃O₄–B, illustrating that the porous nanostructure facilitates the complete oxidation of toluene. Among the as-prepared catalysts, Ru/Co₃O₄-MOF gave the lowest T₉₀ of 238 °C, which is much lower than those of other catalysts, and its catalytic activity was comparable to reported 1 wt% Pd-loaded catalysts with the T₉₀ temperature range of 212–258 °C in the oxidation of toluene.^69,70 Noteworthy, Co₃O₄-MOF showed a lower T₉₀ (264 °C) than that of Ru/Co₃O₄–B (286 °C), demonstrating that the porous support itself exhibits great catalytic activity possibly due to its unique nanostructural properties which are favorable for the adsorption and diffusion of the toluene molecules.

Fig. 5 Toluene conversion as a function of reaction temperature over the catalysts under the conditions of toluene concentration = 1000 ppm, toluene/O₂ molar ratio = 1/200, and WHSV = 60 000 mL g⁻¹ h⁻¹.

CO₂ selectivity represents an important performance of a catalyst for catalytic oxidation of VOCs. Accordingly, herein CO₂ selectivity was examined for both Ru/Co₃O₄-MOF and Co₃O₄-MOF materials, and the results were illustrated in Fig. 6. Using Ru/Co₃O₄-MOF as the catalyst, CO₂ and H₂O were the only products, and CO₂ selectivity was above 99.9%, revealing that toluene could be completely oxidized over this sample. For comparison, Co₃O₄-MOF catalyst was also employed under the same conditions. Along with the increase of temperature from 200 °C to 275 °C, small amount CO was observed. The CO₂ selectivity rised from 91% to 100%, and no other organic species were observed.

Fig. 6 CO₂ selectivity at different temperatures over Ru/CO₃O₄-MOF and CO₃O₄-MOF catalysts under the conditions of toluene concentration = 1000 ppm, toluene/O₂ molar ratio = 1/200, and WHSV = 60 000 mL g⁻¹ h⁻¹.

Water vapor exists in the VOCs-contained flue gases with a percentage level. Hence, we conducted the catalytic oxidation of toluene in presence of 1.5 vol% water vapor to examine the effect of water vapor on the catalytic performance. The most effective material of Ru/Co₃O₄-MOF was utilized as the catalyst. As shown in Fig. 7, when the temperature was raised to 235 °C, the conversion decreased from 86% to 79% within the 1st hour, possibly due to the actual higher temperature on the surface of the catalyst resulted from the unbalanced heat release. Then, the conversion smoothly increased from 79% to 84% in the next 2 h. Addition of water vapor led to a sharp decrease of toluene conversion by ca. 30%. Interestingly, when water vapor was cut off, the catalyst showed apparent higher catalytic efficiency than before. It was tentatively proposed that water facilitates the dispersion of Ru/RuO₂, whereas water occupies the active sites of the catalyst leading to the inhibition effect. When abundant water was introduced to the system, the inhibition effect played a major role. When the water vapor was cut off, higher dispersion of Ru species led to an apparent promotion effect. For the reaction conducted at 245 °C, the introduction of water vapor barely influence the catalytic activity of Ru/Co₃O₄-MOF.

Fig. 7 Effect of water vapor on toluene conversion at different temperatures over Ru/Co₃O₄-MOF catalyst under the conditions of toluene concentration = 1000 ppm, toluene/O₂ molar ratio = 1/200, water vapor concentration = 1.5 vol%, and WHSV = 60 000 mL g⁻¹ h⁻¹.

To evaluate the catalytic stability, on-stream toluene oxidation experiment was performed at 230 °C and 240 °C, respectively. The results are shown in Fig. 8. When the temperature was 230 °C, the toluene conversion decreased slowly within the 1st hour. Then, the conversion showed a slightly increasing tendency, revealing that the catalytic activity became higher with time extension. When the reaction temperature was raised to 240 °C, the toluene conversion sharply increased to 95.6%. Over time, an equivalent conversion was obtained, and no significant loss in catalytic activity was detected, demonstrating that the Ru/Co₃O₄-MOF sample was catalytically durable.

Fig. 8 Toluene conversion as a function of on-stream reaction time at 230 °C and 240 °C over the Ru/Co₃O₄-MOF catalyst under the conditions of toluene concentration = 1000 ppm, toluene/O₂ molar ratio = 1/200, and WHSV = 60 000 mL g⁻¹ h⁻¹.

4. Conclusions

Porous Co₃O₄ was prepared through MOFs-templated method, and its supported ruthenium catalyst was synthesized by impregnation. These materials showed larger BET surface areas than the bulk materials resulted from precipitation method. The BJH pore diameters of Ru/Co₃O₄-MOF and Co₃O₄-MOF were 4.9 nm, revealing a mesoporous structure. Catalytic oxidation of toluene over these materials was conducted. For Ru/Co₃O₄-MOF sample, complete oxidation of toluene was observed at 240 °C, showing a highest catalytic activity. Within the temperature range of 200–275 °C, the CO₂ selectivity was above 99.9% for Ru/Co₃O₄-MOF. In the stability tests, slowly increasing toluene conversion was observed. Additionally, water vapor showed an inhibition effect for the toluene oxidation at 235 °C. However, the catalytic activity became better when water vapor was cut off. The inhibition effect almost disappeard at a relatively higher temperature of 240 °C, and the toluene conversion basically maintained at 100%, demonstrating a great water-resistance for Ru/Co₃O₄-MOF.

Acknowledgements

This work was supported by the State 863 project (no. 2012AA062803).

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Footnote

† Electronic supplementary information (ESI) available: The BET surface areas and pore diameters of the catalysts (Table S1), XPS data (Table S2), and the catalytic evaluation platform (Fig. S1). See DOI: 10.1039/c5ra07072d

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