The investigation of non-noble metal doped mesoporous cobalt oxide catalysts for the water–gas shift reaction

Abstract

In this study, we report an investigation of the low temperature water–gas shift (LT-WGS) reaction over a series of non-noble metal doped (Me = Mn, Fe, Co, and Ni) mesoporous Co₃O₄ catalysts. The effect of metal dopants on the structure and reducibility of the mesoporous Co₃O₄ oxide was examined using X-ray diffraction (XRD), N₂-adsorption/desorption isotherm measurements, and H₂-temperature programmed reduction (TPR) measurements. Experimental results revealed that among the Me-doped Co₃O₄ catalysts, Ni/Co₃O₄ demonstrated the highest catalytic performance (X_CO = 93% with 47% H₂ yield at 280 °C). The higher activity of the Ni-doped Co₃O₄ catalyst was mainly due to its smaller crystallite size (8.6 nm) and strong interaction between Co and Ni, which lead to the higher reducibility of Co₃O₄ compared to the other metal-doped Co₃O₄. To further optimize the loading of Ni- over the mesoporous Co₃O₄, a series of Ni(x%)/Co₃O₄ catalysts were prepared by varying the Ni-loading in the range of 3 to 15 wt%. Among these catalysts, 5 wt% Ni- was found to be the optimum loading, whereas higher Ni-loaded samples (10 and 15 wt%) showed a decrease in catalytic performance and hydrogen yield during the WGS reaction.

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Introduction

The water–gas shift (WGS) reaction plays a crucial role in low temperature fuel cell technology for on-board purification of hydrogen sources.¹ Industries heavily depend on hydrocarbon reforming for the production of hydrogen, which produces a mixture of H₂ and CO as the main products.^2,3 The presence of insignificant amounts of CO in the hydrogen stream can deactivate the fuel-cell electrodes because the CO tolerance level of the Pt-electrode is below 30 ppm.⁴ To overcome this problem, the reformed gas is passed through several fuel processing steps, including CO removal by means of the WGS and CO preferential oxidation (PrOx). The conventionally used low temperature (LT)-WGS catalyst composed of Cu–Zn–Al ternary oxides, converts CO into CO₂ and produces hydrogen in the temperature range of 250–400 °C.^5–7 Although the conventional catalyst (Cu–Zn–Al) exhibits a high WGS activity, it is pyrophoric and normally requires lengthy and complex activation steps before usage.^8,9

Recently, our group focused on the development of mesoporous catalysts for use in the WGS reaction.^10,11 Our previous studies demonstrated that mesoporous material provides several advantages in the WGS reaction: (1) the ordered porous structures ensure an uninterrupted diffusion of reactant and product molecules; (2) well-dispersed metal oxide over the mesoporous support provides more accessible active sites, and (3) a higher catalyst surface area is achieved than the metal oxide supported on a non-mesoporous oxide. Here, we report the successful fabrication of Mn-, Fe-, Cu- and Ni-doped mesoporous Co₃O₄ catalysts and evaluated their catalytic performance in the LT-WGS reaction. The transition metals Mn, Fe, Cu, Ni and Co were selected because of their low cost and strong redox properties. The WGS reaction is catalyzed by associated or redox mechanism, therefore a highly reducible metal oxide catalyst can facilitate the WGS reaction. Ordered mesoporous Co₃O₄ has been widely used in catalysis and has displayed excellent stability under harsh reaction conditions.¹² Co₃O₄ is a reducible oxide that has received considerable attention in catalysis due to the weaker Co–O bond strength and low hopping barrier of oxygen vacancies on the surface compared to the other 3d transition metal series.¹ The presence of oxygen vacancies promotes lattice-oxygen mobility in Co₃O₄, creating its interest as an active catalyst for CO oxidation, PrOx and WGS reactions.^13–15 The nanocasting route using silica materials as a hard template was used for the synthesis of ordered mesoporous Co₃O₄.¹⁶ The as prepared mesoporous Co₃O₄ materials showed a large surface area (∼89 m² g⁻¹) and a uniform pore size distribution (ESI Fig. S1†), which made it ideal for use as a support. As prepared Me-doped mesoporous Co₃O₄ catalysts were used in the LT-WGS reaction. The results showed that in the presence of dopants, the reducibility of Co₃O₄ has increased, which in turn affected the CO conversion and H₂ yield. Among the Me-doped mesoporous Co₃O₄ catalysts, Ni-doped mesoporous Co₃O₄ exhibited excellent catalytic performance. Further, to optimize the loading of Ni- over the mesoporous Co₃O₄, a series of Ni(x%)/Co₃O₄ catalysts were prepared by varying the Ni-loading in the range of 3 to 15 wt%. The effect of Ni-loadings on the mesoporous structure of Co₃O₄ was examined using N₂ adsorption/desorption isotherms, and those results have been correlated to the catalytic performance in the LT-WGS reaction.

Results and discussion

Effects of Mn-, Cu-, Fe-, and Ni-doped Co₃O₄

The composition and phase purity of Mn-, Fe-, Ni-, and Cu-doped mesoporous Co₃O₄ were examined by XRD and the results are shown in Fig. 1(A). Well-defined diffraction peaks indicate the crystalline nature of the samples, which can be indexed well to the face-centered cubic phase of Co₃O₄ spinel.¹⁷ No diffraction peaks related to dopants were observed in any of the Me-doped Co₃O₄ samples. This might be caused by the uniform dispersion of dopant over the mesoporous Co₃O₄ or the formation of a solid solution of dopant and Co₃O₄. The crystallite sizes of the Me-doped Co₃O₄ samples were estimated using the Debye–Scherrer equation for the full-width at half-maximum (FWHM) of the (311) reflection at 2θ = 36.7°. The results are summarized in Table 1. Mn-, Fe- and Cu-doped mesoporous Co₃O₄ samples show higher crystallite size, however Ni-doped sample exhibits decreased crystallite size compared to the pure mesoporous Co₃O₄. The reduction in crystallite size of mesoporous Co₃O₄ upon doping with Ni- could be ascribed to the loss of crystallinity or deformation in the hexagonal structure of Co₃O₄.

Fig. 1 XRD (A) and TPR (B) patterns of Me/Co₃O₄ catalysts.

Table 1 Characteristics of metal-doped mesoporous Co₃O₄

Catalyst	Surface area (m^2 g^−1)	Crystallite size of Co3O4 (nm)
Co3O4	89	11
Mn/Co3O4	52	14
Cu/Co3O4	49	13
Fe/Co3O4	65	13
Ni/Co3O4	36	8.6

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H₂-TPR experiments were used to investigate the doping effect of Mn-, Fe-, Ni-, and Cu- on the reducibility of mesoporous Co₃O₄. As shown in Fig. 1(B), pure mesoporous Co₃O₄ exhibits two broad reduction peaks. The first peak centered at 292 °C is associated with the reduction of trivalent cobalt oxide (Co₃O₄) to divalent cobalt oxide (CoO) (eqn (1)), and the second peak centered at 415 °C is associated with the subsequent reduction of divalent cobalt oxide (CoO) to metallic cobalt (Co^o) (eqn (2)).¹⁸

Co₃O₄ + H₂ → 3CoO + H₂O (1)

3CoO + 3H₂ → 3Co^o + 3H₂O (2)

All the Me-doped mesoporous Co₃O₄ catalysts show broad TPR profiles, which implies a multistage reduction of the catalyst. Mn/Co₃O₄ shows three reduction peaks around 338, 419 and 497 °C, corresponding to a reduction of Mn₃O₄, Co₃O₄ and CoO, respectively.¹⁹ The significant increase in the Co₃O₄ and CoO reduction temperature in the case of Mn/Co₃O₄ may be attributed to the formation of spinel CoMn₂O₄. Cu/Co₃O₄ catalyst shows three reduction peaks around 211, 286 and 337 °C, corresponding to the reduction of highly dispersed CuO, Co₃O₄ and CoO, respectively.¹¹ The H₂-TPR profile of Fe/Co₃O₄ exhibits three peaks around 331, 356, and 515 °C, which are likely related to the reductions of Fe₂O₃, Co₃O₄, and CoO, respectively.²⁰ Ni/Co₃O₄ displays two reduction peaks around 285 and 405 °C, which are attributed to the reduction of Co₃O₄, and a combined reduction peak due to the NiO and CoO, respectively.²¹ The reduction temperature of tri-cobalt oxide (Co₃O₄) in the prepared catalysts follows the order: Mn/Co₃O₄ (419 °C) > Fe/Co₃O₄ (356 °C) > Co₃O₄ (292 °C) > Cu/Co₃O₄ (286 °C) > Ni/Co₃O₄ (285 °C). These results imply that the presence of a second metal in the mesoporous Co₃O₄ framework lowered the reduction temperature for Co₃O₄ except for Fe/Co₃O₄ and Mn/Co₃O₄ catalysts. This might be due to the formation of CoFe₂O₄ and CoMn₂O₄. The lower reduction temperatures of tri-cobalt oxide in the case of Cu- and Ni-doped Co₃O₄ might be due to the strong interaction between Co and the doped metal. Among the Me-doped Co₃O₄ samples, Ni/Co₃O₄ shows the lowest temperature for Co₃O₄ reduction, which can be attributed to the weak Co–O bond strength caused by the strong interaction between Co and Ni. The decrease in Co–O bond strength in Co₃O₄ on doping of Ni- can lead to enhanced oxygen vacancies on the catalyst surface. These oxygen vacancies are considered as active sites for the water dissociation in WGS reaction.¹¹

Catalyst performance in the LT-WGS reaction

The catalytic performances of Me-doped Co₃O₄ catalysts were investigated in the LT-WGS reaction. The CO conversion over Me-doped Co₃O₄ catalysts as a function of reaction temperature is shown in Fig. 2(A). The results showed that Ni/Co₃O₄ possess the highest catalytic activity (>93% CO conversion at 280 °C) compared to the Mn-, Fe- and Cu-doped Co₃O₄ catalysts (<50% CO conversion at 280 °C). However, Mn/Co₃O₄ exhibited inferior activity compared to the support (mesoporous Co₃O₄) itself. This implies that the doping of mesoporous Co₃O₄ with Mn- caused a negative impact on the CO conversion. Interestingly, at 280 °C, the activity patterns of Me-doped mesoporous Co₃O₄ catalysts follow the same trend as the reducibility of Co₃O₄ in Me-doped mesoporous Co₃O₄: Ni/Co₃O₄ > Cu/Co₃O₄ > Co₃O₄ > Fe/Co₃O₄ > Mn/Co₃O₄. This result suggests that the reducibility of Co₃O₄ is crucial to attaining the high activity in the LT-WGS reaction. Fig. 2(B) depicts the hydrogen yields in the LT-WGS reaction for all the Me-doped mesoporous Co₃O₄ catalysts as a function of reaction temperature. The yield of hydrogen was negligible at 240 °C for Mn-, Fe- and Cu-doped mesoporous Co₃O₄ catalysts. However, at the same temperature, Ni/Co₃O₄ exhibited a 7% hydrogen yield and it increased with further increases of the reaction temperature. Fig. 2(B) reveals that the Ni/Co₃O₄ catalyst has the highest hydrogen yield throughout the entire temperature range among the Me-doped Co₃O₄ catalysts in LT-WGS.

Fig. 2 CO conversion (A) and hydrogen yield (B) as a function of reaction temperature over Ni/Co₃O₄ catalysts with varying Ni loading (H₂O/(CH₄ + CO + CO₂) = 2.0; GHSV = 36 027 h⁻¹).

Effect of Ni-loadings on the mesoporous Co₃O₄

Among the various catalysts screened for the LT-WGS reaction, Ni/Co₃O₄ exhibited the best catalytic performance. To investigate the optimum loading of Ni on the mesoporous Co₃O₄ in order to achieve both the highest CO conversion and H₂ yield, we prepared a series of Ni(x%)/Co₃O₄ catalysts (x = wt% of Ni) by varying the Ni-loading in the range of 3 to 15 wt%. The detailed characterization and activity results of these catalysts are discussed in the subsequent sections. Fig. 3(A) displays the XRD patterns of Ni(x%)/Co₃O₄ catalysts with various Ni-loadings. The peak positions and intensity of Co₃O₄ show no significant change upon doping with up to 10 wt% Ni-, however, the 15 wt% Ni-loaded samples show a slightly increased in the intensity of the (311) peak. The crystallite size of Ni(x%)/Co₃O₄ catalysts was estimated from (311) diffraction peak using the Scherrer formula and these results are summarized in Table 2. As shown, the crystallite size of mesoporous Co₃O₄ decreases from 11 to 8.2 nm after loading with 3 wt% Ni. A similar size is maintained up to the 10 wt% Ni-loading. However, a further increase in the Ni-loading to 15 wt% resulted in a crystallite size increase to the same value (11 nm) as that observed for pure mesoporous Co₃O₄. This suggests that excess loadings of Ni – on the mesoporous Co₃O₄ led to a decrease in Ni–Co interactions, which do not affect the crystallite size of the mesoporous cobalt. In addition, no distinct XRD peaks correspond to the metal or oxide phase of Ni- are observed up to a 10 wt% Ni-loading. This implies that the dopant (Ni-) was uniformly dispersed over the Co₃O₄ matrix up to 10 wt% loading. However, above this loading, a NiO peak starts to appear in the case of Ni(15%)/Co₃O₄ catalyst.

Fig. 3 XRD (A) and TPR (B) patterns of Ni(x%)/Co₃O₄ catalysts.

Table 2 Characteristics of Ni(x%)/Co₃O₄ catalysts

Catalyst	Surface area (m^2 g^−1)	Crystallite size of Co3O4 (nm)	Pore volume (cm^3 g^−1)	H2 consumption at 330–516 °C (cm^3 g^−1)
Co3O4	89	11	0.18	205
Ni(3%)/Co3O4	74	8.2	0.17	207
Ni(5%)/Co3O4	36	8.6	0.15	210
Ni(10%)/Co3O4	29	8.7	0.11	216
Ni(15%)/Co3O4	45	11	0.16	220

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However, above this loading, a NiO peak starts to appear in the case of Ni(15%)/Co₃O₄ catalyst. This might be caused by the segregation of the NiO from the support at higher Ni-loadings.

In order to investigate the effect of Ni-loading on the reducibility of Co₃O₄, TPR measurement was carried out using H₂. H₂-TPR profiles of Ni(x%)/Co₃O₄ catalysts are presented in Fig. 3(B). All the samples exhibit broad reduction peak profiles, which could be due to the overlapping contribution from different components in Ni(x%)/Co₃O₄. In general, two main reduction peaks can be discerned. The first reduction peak in the temperature range of 200–300 °C corresponded to the reduction of Co₃O₄ to CoO. The second peak at about 330–516 °C is attributed to the combined reduction peaks of NiO and CoO. The amount of H₂ consumed in this temperature range (330–516 °C) is measured and summarized in Table 2.

As shown, the amount of hydrogen consumed increases with increase of Ni-loading. However, with the increase of the Ni-loading from 5 to 10% and then 15%, the H₂ consumption amount does not increase to double or triple as expected. This might be due to the poor dispersion of Ni over the mesoporous Co₃O₄ on excess loadings of Ni. The reduction temperature of tri-cobalt oxide in Ni(3%)/Co₃O₄ is the same as that for the pure mesoporous Co₃O₄. However, Ni(x%)/Co₃O₄ catalysts (x = 5 to 15%) show a lower temperature (∼285 °C) for reduction of tri-cobalt oxide compared to pure mesoporous Co₃O₄. N₂-adsorption/desorption measurement was carried out to investigate the effect of Ni-loadings on the textural property of mesoporous Co₃O₄. ESI Fig. S2† displays the isotherm plots of pure mesoporous Co₃O₄ and Ni(x%)/Co₃O₄ catalysts. The pure mesoporous Co₃O₄, Ni(3%)/Co₃O₄ and Ni(5%)/Co₃O₄ exhibit a type IV isotherm with a H1-type hysteresis loop, which is characteristic of mesoporous materials. However, Ni(10%)/Co₃O₄ and Ni(15%)/Co₃O₄ catalysts showed a type III isotherm, which corresponds to the non-mesoporous materials. The formation of non-mesoporous materials is caused by the deposition of excess Ni- over the mesoporous framework of Co₃O₄ that results in pore blockage. This observation was supported by the pore volume as shown in Table 2.

Catalytic performance over Ni(x%)/Co₃O₄

The catalytic performance of the Ni(x%)/Co₃O₄ catalysts in the LT-WGS reaction is shown in Fig. 4. Notably, the un-doped mesoporous Co₃O₄ was moderately active in the LT-WGS reaction. However, doping of Ni- over the mesoporous Co₃O₄ gradually enhanced the catalytic efficiency of Ni(x%)/Co₃O₄ catalysts. The catalytic activity increased with increasing the loading amount of Ni- from 3 to 5 wt% and reached a maximum at 5 wt% of Ni-loading (X_CO = 93% at 280 °C). Further increase in the Ni-loading (10 and 15 wt%) resulted in a lower catalytic performance compared to the Ni(5%)/Co₃O₄ catalyst. The highest CO conversion of Ni(5%)/Co₃O₄ could be attributed to the mesoporous structure of the catalyst which was absent in Ni(10%)/Co₃O₄ and Ni(15%)/Co₃O₄ catalysts. Therefore, 5 wt% Ni supported on mesoporous Co₃O₄ can be considered as the optimum loading as it provided the high activity in the LT-WGS reaction. Fig. 4(B) shows the H₂ yields during the WGS reaction over the Ni(x%)/Co₃O₄ catalysts as a function of reaction temperature. It revealed that the yields of hydrogen increased after doping of Ni over the mesoporous Co₃O₄ except for Ni(15%)/Co₃O₄. Table S1† shows a comparison of reported data on CO conversion for the WGS reaction using Co based catalysts. Clearly, catalytic activity of Ni/Co₃O₄ catalyst shows higher activity in WGS reaction.

Fig. 4 (A) CO conversion and (B) hydrogen yield as a function of reaction temperature over Ni(x%)/Co₃O₄ catalysts with varying Ni loading (H₂O/(CH₄ + CO + CO₂) = 2.0; GHSV = 36 027 h⁻¹).

Among the Ni(x%)/Co₃O₄ series, the Ni(5%)/Co₃O₄ catalyst showed the highest yield of hydrogen throughout the temperature range (240 to 360 °C) compared to other Ni(x%)/Co₃O₄ catalysts. The Ni(10%)/Co₃O₄ and Ni(15%)/Co₃O₄ catalysts showed a reduction in hydrogen yield. This can be attributed to the formation of CH₄, as the bulk Ni species are the active sites for methane formation (CO + 3H₂ → CH₄ + H₂O) under WGS reaction conditions. As the formation of one mole of methane consumed 3 moles of H₂, the production of methane can decrease the H₂ yield in the output gas of WGS reaction.

To check the potential of the Ni(5%)/Co₃O₄ catalyst, a time-on-stream study was performed at 320 °C and at the GHSV of 36 201 h⁻¹. Fig. 5 shows the CO conversion as a function of time. These results demonstrate that the CO conversion decreased during the initial period of 10 h from 92 to 86%, thereafter, the catalyst maintained a stable activity up to 50 h time-on-stream. The morphology of the fresh and used Ni(5%)/Co₃O₄ catalyst was investigated by TEM, as shown in Fig. 6. The average particle sizes of the fresh and used Ni(5%)/Co₃O₄ catalyst were calculated to be 12 and 18 nm, respectively. This observation implies that the Ni(5%)/Co₃O₄ catalyst undergoes gradual particle growth during the time-on-stream study caused by catalyst sintering.

Fig. 5 CO conversion with time-on-stream over Ni(5%)/Co₃O₄ catalyst (H₂O/(CH₄ + CO + CO₂) = 2.0; T = 320 °C; GHSV = 36 027 h⁻¹).

Fig. 6 TEM images of pure mesoporous Co₃O₄ (A), fresh Ni(5%)/Co₃O₄ (B) and used Ni(5%)/Co₃O₄ (C) catalysts.

Conclusions

We have successfully synthesized Mn-, Fe-, Ni- and Cu-doped mesoporous Co₃O₄ catalysts and evaluated their catalytic efficiency in the LT-WGS reaction. Among these metal-doped mesoporous Co₃O₄ catalysts, Ni/Co₃O₄ exhibited the highest CO conversion and H₂ yield compared to the Mn/Co₃O₄, Fe/Co₃O₄ and Cu/Co₃O₄ catalysts. The higher activity of Ni/Co₃O₄ is attributed to the strong interaction between Co and Ni, which led to the easier reducibility of mesoporous Co₃O₄ support. Furthermore, loading of Ni- over the mesoporous Co₃O₄ was optimized for the improved performance in the LT-WGS reaction. The results showed that 5 wt% loading of Ni was optimum for the catalyst performance in the LT-WGS reaction. The time-on-stream study showed that Ni(5%)/Co₃O₄ catalyst maintained stable activity performance for 50 h with 85% CO conversion. This finding suggests that Ni(5%)/Co₃O₄ catalyst can be regarded as a potential catalyst in the LT-WGS reaction.

Experimental

Catalyst preparation

Mesoporous Co₃O₄ was synthesized through a nano-casting method using KIT-6 followed by a subsequent incipient wetness impregnation method of cobalt precursor in similar to that of previously reported works.²² The ordered mesoporous KIT-6, which has a three-dimensional hexagonal regular arrangement of uniform mesopores, was synthesized using an amphiphilic co-polymer according to the corresponding literature.²³ A triblock copolymer, EO₂₀PO₇₀EO₂₀ (Pluronic P123, Aldrich), was used as the structure-directing agent, and tetraethylorthosilicate (TEOS, Aldrich), was used as the silica source. P123 was dissolved completely in deionized water. A polymer solution was added to a 35 wt% HCl aqueous solution with vigorous stirring at 35 °C. Subsequently, n-butanol was added and aged for 1 h with stirring. Then, TEOS was added to the solution under stirring in order to cover the hexagonal array of micelle rods with silica. The final mixed solution was vigorously stirred for 24 h at 35 °C. The product of milky solution was transferred to an autoclave and kept for 24 h at 100 °C under static conditions. The hydrothermally-treated white solid product was filtered without washing, and subsequently dried at 100 °C for 24 h. To remove the surfactant, an ethanol/HCl solution was used with stirring for about 2 h, and the as-prepared KIT-6 powder was calcined at 550 °C for 6 h. In order to prepare mesoporous Co₃O₄, cobalt nitrate hexahydrate precursor was dissolved in deionized water, and the solution was impregnated into the pores of KIT-6 by homogenous mixing. The well-mixed gel of the cobalt precursor solution with KIT-6 powder was dried in an oven at 80 °C overnight and calcined at 550 °C for 3 h. Finally, KIT-6, used as a template of mesoporous Co₃O₄, was removed by washing with a 2 M NaOH solution to obtain template-free mesoporous Co₃O₄ metal oxides.

For preparing mesoporous Me/Co₃O₄ (Me = Mn, Fe, Cu, and Ni) catalysts, the corresponding Me-precursors were added through an impregnation method with a metal loading of 5 wt% on the mesoporous Co₃O₄. The required amount of Me-precursors was dissolved in deionized water and mixed with the Co₃O₄. Then, Me/Co₃O₄ samples were dried at 110 °C for 6 h, and calcined at 400 °C for 6 h. The Ni(x%)/Co₃O₄ catalyst series were also synthesized using the same preparation method by varying the Ni-loadings from 3 to 15 wt%. Where, x represents the Ni-loading on the mesoporous Co₃O₄ oxide.

Catalyst characterization

X-ray diffractograms of the catalysts were recorded in the 2θ range of 20–80° using a Rigaku D/MAX-IIIC diffractometer (Ni-filtered Cu-Kα radiation, 40 kV, 100 mA). The crystallite size was estimated using the Debye–Scherrer equation.^24,25 The values of the full width at half maximum (FWHM) were obtained for the peak of Co₃O₄ at 2θ = 36.8°. The BET surface area and the type of isotherm were determined by the N₂ adsorption/desorption method at 77 K using an ASAP 2010 Micromeritics. The pore size distributions of Co₃O₄ catalysts were also calculated using the BJH (Barrett–Joyner–Halenda) model from the desorption branch of the nitrogen isotherm. Hydrogen-temperature programmed reduction (H₂-TPR) experiments were conducted on an Autochem 2920 (Micromeritics). Typically, 0.1 g of sample was loaded into a quartz reactor. The H₂-TPR was performed using 10% H₂ in Ar with a heating rate of 10 °C min⁻¹, from room temperature to 800 °C. The sensitivity of the detector was calibrated by reducing a known weight of NiO. A detailed procedure for the H₂-TPR measurement was provided in a previous study.^26,27 Surface morphologies of mesoporous Co₃O₄ catalyst and used catalysts, which were previously washed with hexane solvent to remove adsorbed soluble wax components on the Co₃O₄ surfaces, were also characterized by transmission electron microscopy (TEM) using a TECNAI G2 instrument operated at 200 kV.

Catalytic activity measurement

The catalytic activities of the metal doped mesoporous cobalt oxide samples were evaluated in a fixed-bed micro-tubular quartz reactor under atmospheric pressure from 240 to 360 °C. The catalyst charge was 48 mg and reduced in 5% H₂/N₂ at 400 °C for 1 h prior to catalytic measurements. After pretreatment, the temperature was decreased to 200 °C. The simulated reformed gas consisted of 6.5 vol% CO, 7.1 vol% CO₂, 0.7 vol% CH₄, 42.4 vol% H₂, 28.7 vol% H₂O, and 14.5 vol% N₂. The feed H₂O/(CH₄ + CO + CO₂) ratio was intentionally fixed at 2.0 to avoid coke formation.^28–30 A GHSV of 36 027 h⁻¹ was used to screen the catalysts in this study. Water was fed using a syringe pump and was vaporized at 180 °C upstream of the reactor. The reformed gas was chilled, passed through a trap to condense the residual water, and then analyzed on-line using a micro-gas chromatograph (Agilent 3000).

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013R1A1A1A05007370).

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Footnote

† Electronic supplementary information (ESI) available: Pore size distribution of Co₃O₄, isotherm plots of Ni(x%)/Co₃O₄ catalysts. See DOI: 10.1039/c6ra11410e

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