Mesoporous Co₃O₄ supported Pt catalysts for low-temperature oxidation of acetylene

Abstract

Three-dimensionally ordered mesoporous Co₃O₄ (meso-Co₃O₄) and its supported Pt catalysts were synthesized for the catalytic oxidation of acetylene. All the catalysts formed mesoporous structures and possessed high surface areas of 110–122 m² g⁻¹. Meso-Co₃O₄-supported Pt catalysts exhibited highly catalytic performances, and the 0.6Pt/meso-Co₃O₄ catalyst gave the lowest temperature of 120 °C for acetylene oxidation. It was concluded that the ordered mesoporous structure, with plenty of structural defects, good low-temperature reducibility and the high concentration of active oxygen species were responsible for the excellent catalytic performance of 0.6Pt/meso-Co₃O₄.

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1. Introduction

Acetylene (C₂H₂) is mainly emitted from motor vehicle exhausts and petrochemical processing. It enhances the photochemical reaction and promotes the formation of secondary organic aerosols (SOAs) and ozone.^1–3 Catalytic oxidation is one of the most effective pathways to eliminate acetylene, in which the key issue is the development of the catalytic materials. However, as far as we know, there are few reports about catalysts for the catalytic oxidation of acetylene. Thus, it is very significant to explore a high performance catalyst for removing acetylene.

Co₃O₄ shows excellent catalytic performance and is widely used in many reactions, for example, CO oxidation,⁴ ethylene oxidation,⁵ formaldehyde oxidation (with⁶ or without⁷ noble metal promoters) and the oxidation of other organic molecules.⁸ It is commonly accepted that the activity of the catalyst is closely related to its morphology and structure. Different structures and morphologies of Co₃O₄ have been reported, including nano-rod, nano-tube, nano-sheet, nano-sphere and multiporous structures.^9,10 Three-dimensional ordered mesoporous metal oxides possess high surface area, large pore volume and controllable pore diameter, which have attracted more and more attentions for their special physicochemical properties. Bai et al.¹¹ prepared nano-Co₃O₄, 2D-Co₃O₄, and 3D-Co₃O₄ catalysts for the catalytic oxidation of formaldehyde. The result showed that 3D-Co₃O₄ catalyst exhibited the best catalytic performance for formaldehyde oxidation.

Pt particles supported on metal oxides were effective catalysts in the oxidation reaction.¹² Zhang et al.¹³ reported that Pt/TiO₂ was the most active catalyst for HCHO oxidation when 1 wt% noble metal (Pt, Rh, Pd and Au) was supported on TiO₂. Compared with metal oxide catalysts, oxide-supported Pt catalysts showed higher catalytic activity for the oxidation reaction. Tang et al.¹⁴ prepared Pt/MnO_x–CeO₂ catalysts for the oxidation of formaldehyde. The presence of Pt active sites promoted the adsorption of formaldehyde and the effective activation of oxygen molecule.

Herein, we report the synthesis of meso-Co₃O₄ and meso-Co₃O₄-supported Pt catalysts for the oxidation of acetylene. The effects of mesoporous structure, lattice defect, oxygen species and low-temperature reducibility are investigated in terms of acetylene removal.

2. Experimental

2.1. Catalyst preparation

KIT-6 was synthesized according to the strategy reported previously.¹⁵ Meso-Co₃O₄ was prepared using KIT-6 as the hard template. In a typical synthesis process of meso-Co₃O₄, 1.0 g of KIT-6 was added to a solution of Co(NO₃)₂·6H₂O in ethanol. The samples were evaporated to dryness at 60 °C. The products were calcined at 550 °C for 6 h. The hard template of KIT-6 was removed by NaOH solution (2 mol L⁻¹) at 60 °C. Orderedly meso-Co₃O₄ was obtained after centrifugation, washing with deionized water and drying at 100 °C. For comparison, the bulk-Co₃O₄ was prepared via the thermal decomposition of Co(NO₃)₂·6H₂O at 550 °C for 6 h.

The meso-Co₃O₄ and bulk-Co₃O₄ supported Pt catalysts were prepared by the means of NaBH₄ reduction. In a typical synthesis process, HPtCl₄ aqueous solution was mixed with PVP at 35 °C. Then 1 g of meso-Co₃O₄ or bulk-Co₃O₄ was added to the solution. The mixed solution was stirred for 4 h. Subsequently, NaBH₄ was added under rapidly stirring. The solid was centrifugal separation, washed with deionized water, dried at 100 °C for 8 h and calcined at 350 °C for 4 h. The final catalysts are marked as xPt/meso-Co₃O₄ or xPt/bulk-Co₃O₄ (x denotes the weight percentage of Pt element to the related catalyst, x = 0.3, 0.6 and 1.0). The content of Pt were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

2.2. Catalyst characterization

X-ray diffraction (XRD) patterns were tested by a powder meter (Bruker D8 Advance) using Cu Kα radiation (λ = 0.154056 nm), operating at 40 kV and 10 mA. Data were recorded in a 2θ of the low-angle from 0.5 to 3° and wide-angle range of 10–80°. N₂ adsorption–desorption isotherms and pore size distributions of the catalysts were tested on an automated gas sorption analyzer (Micrometrics TriStar II 3020). The samples were degassed for 4 h at 300 °C. Transmission electron microscopy (TEM) images were observed using a JEOL JEM 2100 instrument at an accelerating voltage of 200 kV. The catalysts were dispersed in ethanol, and dropped on a copper grid. Scanning electron microscopy (SEM) images measurements were obtained on a SU8010 instruments with the working voltages of 5 kV. Raman spectra were recorded on a Renishaw Laser Raman spectrometer with a laser excitation wavelength of 532 nm under room temperature. The spectra were obtained with resolution of 1 cm⁻¹ and scanned from 100 to 1000 cm⁻¹. X-ray photoelectron spectra (XPS) were recorded on an AxiosmAX spectrometer with an Al Kα X-ray source. The charge-shifted spectra were calibrated using the C 1s photoelectron signal at 284.6 eV. The data were processed by XPS-PEAK software. O₂-TPD experiments were tested on a DAS-7000 instrument. 0.05 g of samples were pretreated in the O₂ flow (30 mL min⁻¹) at 300 °C for 1 h. The samples were cooled, then the carrier gas changed to pure He. The sample was heated by a flow (30 mL min⁻¹) of pure He from 30 °C to 850 °C at a rate of 10 °C min⁻¹. H₂-TPR experiments were also tested on the above instrument. 0.05 g of samples were pretreated in the N₂ flow at 300 °C for 1 h. The samples were cooled and reduced by a flow (30 mL min⁻¹) of a 10% H₂/90% N₂ mixture from 100 to 600 °C with a heating rate of 10 °C min⁻¹. The H₂ consumptions of the reduction band were calibrated by a standard CuO (99.998%) powder.

2.3. Catalytic activity test

The catalytic activities of the catalysts were tested in the fixed bed reactor. The samples (100 mg, 40–60 mesh) were placed in the middle of the reaction tube. The concentration of gases was detected by a SP-1000 chromatographic analyzer with a TCD detector. The reaction gas consisted of 50 ppm C₂H₂, 13 v% O₂ and balance gas N₂, and the total flow rate of the reactant was 60 mL min⁻¹ (under standard condition). The gas hourly space velocity (GHSV) of the catalytic oxidation was 30 000 h⁻¹. The conversion rate of C₂H₂ was calculated by the following formula:

3. Results and discussion

3.1. Structural and textural analyses

N₂ adsorption–desorption isotherms and pore size distributions of the meso-Co₃O₄ and xPt/meso-Co₃O₄ are shown in Fig. 1a and b, respectively. All samples show type IV isotherms¹⁶ with a H1 hysteresis loop, suggesting the presence of uniform mesopores. It is consistent with the low-angle XRD results (Fig. 2B). BET surface areas, average pore diameters and pore volumes of all samples are displayed in Table 1. All catalysts possess the average pore size of maximum distribution at 5 nm. The broad peak of pore size distribution at 15 nm can be attributed to interspace of the catalysts. The pore diameters, surface areas and pore volumes of xPt/meso-Co₃O₄ do not significantly reduce. The surface areas of meso-Co₃O₄ and xPt/meso-Co₃O₄ (110–122 m² g⁻¹) are much larger than those of bulk-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ (8.1–8.4 m² g⁻¹). It indicates that most of Pt nanoparticles are dispersed on the surface of the catalysts.

Fig. 1 (a) N₂ adsorption–desorption and (b) pore size distributions of meso-Co₃O₄, 0.3Pt/meso-Co₃O₄, 0.6Pt/meso-Co₃O₄ and 1.0Pt/meso-Co₃O₄.

Fig. 2 (A) Wide-angle and (B) low-angle XRD patterns of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c) 0.6Pt/meso-Co₃O₄, (d) 1.0Pt/meso-Co₃O₄, (e) bulk-Co₃O₄ and (f) 0.6Pt/bulk-Co₃O₄.

Table 1 Textural properties of samples

Sample	Surface area (m^2 g^−1)	Average pore diameter (nm)	Pore volume (cm^3 g^−1)
Meso-Co3O4	122.0	8.4	0.38
0.3Pt/meso-Co3O4	111.5	7.9	0.33
0.6Pt/meso-Co3O4	110.9	7.8	0.32
1.0Pt/meso-Co3O4	110.0	8.2	0.33
Bulk-Co3O4	8.4	—	—
0.6Pt/bulk-Co3O4	8.1	—	—

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From the wide-angle XRD patterns in Fig. 2A, all the catalysts show diffraction peaks at 19°, 31.3°, 36.9°, 38.2°, 44.5°, 55.6°, 59.4° and 65.3° (2θ), conforming the (111), (220), (311), (222), (400), (422), (511) and (440) planes.¹⁷ It is consistent with the standard cobalt oxide spinel structure (JCPDS PDF# 43-1003). The loading of Pt particles rarely affects Co₃O₄ crystal. Fig. 2B shows the low-angle XRD patterns of the samples. The meso-Co₃O₄ and xPt/meso-CO₃O₄ catalysts exhibit (211) and (220) diffraction peaks at 2θ angle 1° and 1.14°.⁵ It indicates that the catalysts possess a three-dimensional orderedly mesoporous structure. The regularity of the mesoporous structure decreases after Pt loading.

Fig. 3 presents TEM images of the meso-Co₃O₄ and xPt/meso-Co₃O₄. All samples display orderedly mesoporous structure. Pt particles are dispersed on the surface of meso-Co₃O₄. Dispersive Pt particles may provide more active sites for the catalytic oxidation of acetylene. The loading of Pt does not change the mesoporous structure. The orderedly mesoporous structure can facilitate the transport and diffusion of reactant and product molecules. Meanwhile, such a mesoporous structure can prevent Pt particles from leaching and aggregation because of the limitation of the special channels. By making a statistic analysis on the sizes of more than 200 Pt particles in the TEM images of the 1.0Pt/meso-Co₃O₄ sample, the particle size distribution of Pt particles are shown in Fig. 4. The mean diameter of Pt particles is about 2.9 nm.

Fig. 3 TEM images of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c and d) 0.6Pt/meso-Co₃O₄ and (e and f) 1.0Pt/meso-Co₃O₄.

Fig. 4 Pt particles size distribution of 1.0Pt/meso-Co₃O₄.

Raman spectra of all the samples are shown in Fig. 5. Raman peaks at 195, 480, 518, 615 and 689 cm⁻¹ correspond to the F_2g⁽¹⁾, E_g, F_2g⁽²⁾, F_2g⁽³⁾ and A_1g symmetries of crystalline Co₃O₄, respectively.¹⁸ The peak at 689 cm⁻¹ can be ascribed to Raman vibration of Co³⁺–O²⁻.⁷ The overall enhancement of peak intensity in bulk-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ samples should be due to the influence of crystallinity and the orientation effect during the Raman test. With the loading of Pt, Raman peak at 689 cm⁻¹ shows an obvious shift to lower frequencies and the peak intensity decreases. The red shift indicates that xPt/meso-Co₃O₄ catalysts have a lattice defect or residual stress of the spinel structure.¹⁹ The existence of lattice defect is beneficial for the formation of the oxygen vacancies.²⁰ The oxygen vacancies can activate, adsorb oxygen and provide the lattice sites of oxygen migration, resulting in the formation of highly active electrophilic oxygen species.²¹ The decrease of the peak intensities is due to the decrease of Co³⁺. It can be concluded that there is an electron transfer from Pt⁰ to Co³⁺.

Fig. 5 Raman spectra of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c) 0.6Pt/meso-Co₃O₄, (d) 1.0Pt/meso-Co₃O₄, (e) 0.6Pt/bulk-Co₃O₄ and (f) bulk-Co₃O₄.

3.2. The results of XPS

XPS is performed in order to gain the information of surface element compositions, chemical states and adsorbed oxygen species of catalysts. Pt 4f XPS spectra of xPt/meso-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ are shown in Fig. 6A. The binding energies of 71.1, 74.2 and 77.3 eV can be assigned to Pt⁰, Pt²⁺ and Pt⁴⁺ species, respectively.²² This result indicates that both Pt⁰ and Pt^δ+ (Pt²⁺ and Pt⁴⁺) species exist on the surface of Co₃O₄ support. Pt^δ+ species is generally active species rather than metallic Pt in oxidation reaction.²³ Table 2 lists the relative amounts of Pt⁰, Pt²⁺ and Pt⁴⁺ species on the surface of 0.6Pt/meso-Co₃O₄, 1.0Pt/meso-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ by the means of XPS-peak-differentiation-imitating analysis. It is hard to calculate the content of the Pt species for 0.3Pt/meso-Co₃O₄. The Pt^δ+ concentrations over 0.6Pt/meso-Co₃O₄, 1.0Pt/meso-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ catalysts are 42.9%, 37.5% and 35.9%. It is attributed to the dispersive Pt particles on the Co₃O₄ and the formation of Pt–O–Co bond. Therefore, it is concluded that there is an electron transfer from Pt to Co³⁺ after the loading of Pt particles. It is consistent with the result of Raman characterization. 0.6Pt/meso-Co₃O₄ possesses higher concentrations of Pt^δ+ than 1.0Pt/meso-Co₃O₄, and thus increases the density of active oxygen species and Co²⁺ species of 0.6Pt/meso-Co₃O₄. Meanwhile, 0.6Pt/meso-Co₃O₄ shows higher concentrations of Pt^δ+ in the meso-Co₃O₄ surface than 0.6Pt/bulk-Co₃O₄. It should be due to the order mesoporous structure of meso-Co₃O₄. Co 2p shows four peaks at BE = 779.5, 780.7, 794.5 and 796.9 eV, which corresponds to Co³⁺ 2p_3/2, Co²⁺ 2p_3/2, Co³⁺ 2p_1/2 and Co²⁺ 2p_1/2 (ref. 11) in Fig. 6B. Table 3 lists the surface Co³⁺/Co²⁺ molar ratios of the catalysts. The existence of surface oxygen vacancies is further confirmed by the XPS. The surface Co³⁺/Co²⁺ molar ratios of bulk-Co₃O₄ are higher than meso-Co₃O₄. Compared to the meso-Co₃O₄ and bulk-Co₃O₄, the Co³⁺/Co²⁺ molar ratios of xPt/meso-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ catalysts decrease, indicating that the surface Co²⁺ concentration increases after Pt loading. xPt/meso-Co₃O₄ possess a higher oxygen vacancy concentration than meso-Co₃O₄. It should be due to the electron transfer from Pt to Co³⁺ through the reaction of Pt⁰ + Co³⁺ → Pt^δ+ +Co²⁺. 0.6Pt/meso-Co₃O₄ gives the minimum value of the surface Co³⁺/Co²⁺ molar ratio and the most surface oxygen vacancy. From Fig. 6C, O 1s exhibits two peaks at 529.6 and 531.0 eV. The former can be assigned to the surface lattice oxygen (O_latt), and the latter corresponds to surface adsorbed oxygen (O_ads).²⁴ It is widely recognized that the O_ads species play an important role in the deep oxidation of the organics.²⁵ Combined with the results from Table 3, the O_ads/O_latt molar ratio (0.98) of bulk-Co₃O₄ is lower than those of meso-Co₃O₄ (1.20) and of xPt/meso-Co₃O₄ (1.25–1.44), indicating that meso-Co₃O₄ possess more adsorbed oxygen species. The highest O_ads/O_latt molar ratio of 0.6Pt/meso-Co₃O₄ is 1.44, and thus 0.6Pt/meso-Co₃O₄ has the most surface absorbed oxygen species.

Fig. 6 (A) Pt 4f XPS spectra of (a) 0.3Pt/meso-Co₃O₄, (b) 0.6Pt/meso-Co₃O₄, (c) 1.0Pt/meso-Co₃O₄ and 0.6Pt/bulk-Co₃O₄. (B) O 1s and (C) Co 2p XPS spectra of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c) 0.6Pt/meso-Co₃O₄, (d) 1.0Pt/meso-Co₃O₄, (e) bulk-Co₃O₄ and (f) 0.6Pt/bulk-Co₃O₄.

Table 2 Surface states of Pt species derived from XPS analyses

Catalyst	Pt^0 (%)	Pt^2+ (%)	Pt^4+ (%)
0.6Pt/meso-Co3O4	57.1	29.7	13.2
1.0Pt/meso-Co3O4	62.5	27.4	10.1
0.6Pt/bulk-Co3O4	64.2	19.4	16.5

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Table 3 Surface compositions and apparent activation energies of the bulk-Co₃O₄, 0.6Pt/bulk-Co₃O₄, meso-Co₃O₄ and xPt/meso-Co₃O₄ samples for acetylene oxidation

Catalyst	Co^3+/Co^2+ molar ratio	Oads/Olatt molar ratio	H2 consumption (mmol g^−1)	Ea (kJ mol^−1)
Meso-Co3O4	0.69	1.20	16.1	77
0.3Pt/meso-Co3O4	0.65	1.31	15.4	50
0.6Pt/meso-Co3O4	0.60	1.44	14.6	46
1.0Pt/meso-Co3O4	0.66	1.25	15.6	48
Bulk-Co3O4	0.83	0.98	16.9	99
0.6Pt/bulk-Co3O4	0.73	1.07	16.0	68

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3.3. The results of O₂-TPD

To further study the effect of activated oxygen species for acetylene oxidation, O₂-TPD experiments are carried out. The result is shown in Fig. 7. The desorption peak that are less than 450 °C can be ascribed to surface active oxygen, such as O₂⁻ and O⁻, and the desorption temperature of bulk phase lattice oxygen are higher than 700 °C.¹¹ Compared with meso-Co₃O₄, xPt/meso-Co₃O₄ presents higher intensities of desorption peaks below 450 °C. This phenomenon indicates that xPt/meso-Co₃O₄ surface contains more abundant surface active oxygen species than meso-Co₃O₄. It is in according with the result of XPS. For 0.3Pt/meso-Co₃O₄, the desorption peak is centered at 443 °C. For 1.0Pt/meso-Co₃O₄, the desorption peak shifts to lower temperature range, starting at 300 °C and is centered at 425 °C. Among xPt/meso-Co₃O₄, 0.6Pt/meso-Co₃O₄ exhibits the lowest beginning oxygen desorption temperature and highest intensities of desorption peak. The desorption temperature and intensity of surface-active oxygen species are connected to the catalytic ability for oxidation reaction. Lower beginning oxygen desorption temperatures and larger intensities of desorption peaks lead to better catalytic ability.¹¹ It indicates that the adding of Pt is beneficial to increasing the surface activity oxygen species of xPt/meso-Co₃O₄, which can enhance the catalytic performance of acetylene oxidation.

Fig. 7 O₂-TPD profiles of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c) 0.6Pt/meso-Co₃O₄ and (d) 1.0Pt/meso-Co₃O₄.

3.4. The results of H₂-TPR

Low-temperature reducibility is an important factor in oxidation reactions. H₂-TPR experiments are carried to investigate the reducibility of the samples. The results are shown in Fig. 8. For the bulk-Co₃O₄, there are two reduction peaks at 364 °C and 473 °C. For the meso-Co₃O₄, there are two reduction peaks at 308 °C and 450 °C. The former is due to the reduction of Co₃O₄ to CoO, whereas the latter is attributed to the reduction of CoO to Co⁰. Comparing the reduction profile of bulk-Co₃O₄, meso-Co₃O₄ is more reducible at lower temperatures. Therefore, meso-Co₃O₄ with high surface area displays a more amount of reducible sites. After the adding of Pt, there is a new reduction peak at 140 °C for 0.3Pt/meso-Co₃O₄. It should be due to the reduction of Pt^δ+. Meanwhile, the reduction peaks of Co₃O₄ to CoO and CoO to Co⁰ shift to lower temperatures. For 0.6Pt/meso-Co₃O₄ and 1.0Pt/meso-Co₃O₄, the reduction peaks of Pt^δ+ to Pt⁰ and Co³⁺ to Co²⁺ are overlapped. 0.6Pt/meso-Co₃O₄ presents the lowest reduction peak at 132 °C. It indicates that the adding of Pt gives rise to the improvement in low-temperature reducibility of xPt/meso-Co₃O₄, which can enhance the catalytic performance of acetylene oxidation. Meanwhile, 0.6Pt/meso-Co₃O₄ displays the highest ability of low-temperature reducibility. By quantitatively analyzing the reduction peaks in H₂-TPR profiles, the H₂ consumption of the samples are shown in Table 3. The H₂ consumption of meso-Co₃O₄, 0.3Pt/meso-Co₃O₄, 0.6Pt/meso-Co₃O₄, 1.0Pt/meso-Co₃O₄, bulk-Co₃O₄ and 0.6Pt/bulk-Co₃O₄ is 16.1, 15.4, 14.6, 15.6, 16.9 and 16.0 mmol g⁻¹, respectively. Due to the presence of the partially overlapping reduction of Co³⁺, Co²⁺ and Pt^δ+ ions, it is hard to convert the H₂ consumption to the Co³⁺/Co²⁺ ratios and the content of the Pt^δ+. However, it can conclude that there are more amounts of oxygen vacancies in xPt/Co₃O₄ than in pure Co₃O₄. It is in good agreement with their Raman and XPS results.

Fig. 8 H₂-TPR profiles of (a) meso-Co₃O₄, (b) 0.3Pt/meso-Co₃O₄, (c) 0.6Pt/meso-Co₃O₄, (d) 1.0Pt/meso-Co₃O₄, (e) bulk-Co₃O₄ and (f) 0.6Pt/bulk-Co₃O₄.

3.5. Catalytic performance

In order to eliminate influence of diffusion, the kinetics research of acetylene oxidation is first carried out. Fig. 9a shows the reaction results for meso-Co₃O₄ catalyst at various space velocities. In the absence of mass-transfer limitations, acetylene conversion is excepted to increase by n-fold as the space velocity decrease by n-fold. Acetylene conversion ratios (Fig. 9b) suggest that meso-Co₃O₄ should decrease the influence of diffusion under 30 000 h⁻¹ space velocity. Fig. 10 shows the catalytic performances of the catalysts for the oxidation of acetylene. It is evident that the most effective catalyst for acetylene oxidation is 0.6Pt/meso-Co₃O₄, while the worst active catalyst is bulk-Co₃O₄. The order of the catalytic activity is 0.6Pt/meso-Co₃O₄ > 1.0Pt/meso-Co₃O₄ > 0.3Pt/meso-Co₃O₄ > meso-Co₃O₄ > bulk-Co₃O₄, in good agreement with the reducibility sequences of these catalysts. The bulk-Co₃O₄ catalyst displays the worst catalytic activity, and the complete conversion temperature of acetylene is at 220 °C. The meso-Co₃O₄ catalyst shows better oxidation activity, completely converting acetylene at 160 °C. Among xPt/meso-Co₃O₄ catalysts, 0.6Pt/meso-Co₃O₄ catalyst exhibits the best catalytic performance, which can completely oxidize acetylene at 120 °C. Fig. 11 shows the Arrhenius plots for acetylene oxidation over the Co₃O₄ and xPt/Co₃O₄, and the calculated apparent activation energies (E_a) are summarized in Table 3. It is shown that the E_a value for acetylene oxidation decreased in the sequence of bulk-Co₃O₄ > meso-Co₃O₄ > 0.6Pt/bulk-Co₃O₄ > 0.3Pt/meso-Co₃O₄ > 1.0Pt/meso-Co₃O₄ > 0.6Pt/meso-Co₃O₄. The apparent activation energy of 0.6Pt/meso-Co₃O₄ is the lowest. The oxidation of acetylene might be more easily over the mesoporous Co₃O₄ supported Pt catalysts. For the xPt/meso-Co₃O₄ samples, Pt particles are dispersed on the surface of meso-Co₃O₄, and more oxygen vacancies are present in xPt/meso-Co₃O₄. O_ads is a beneficial role with respect to catalytic oxidation of acetylene. To further study the effect of O_ads in acetylene oxidation. Then, a test is carried out at low conversions under a kinetically controlled regime at 60 °C. As shown in Fig. 12, the acetylene consumption rate increases with the rising of O_ads/O_latt molar ratio. That is to say, a higher O_ads concentration is beneficial for the enhancement in catalytic oxidation of acetylene.

Fig. 9 (a) Acetylene conversion as a function of reaction temperature over meso-Co₃O₄ at space velocities of 15 000 h⁻¹, 22 500 h⁻¹ and 30 000 h⁻¹ and (b) acetylene conversion ratios for 15 000 h⁻¹, 22 500 h⁻¹ and 30 000 h⁻¹ as a function of reaction temperature, using date from (a).

Fig. 10 Catalytic performance as a function of reaction temperature over bulk-Co₃O₄, 0.6Pt/bulk-Co₃O₄, meso-Co₃O₄, 0.3Pt/mso-Co₃O₄, 0.6Pt/meso-Co₃O₄ and 1.0Pt/meso-Co₃O₄ catalysts for the oxidation of C₂H₂ under the conditions: [C₂H₂] = 50 ppm, [O₂] = 13 vol%, N₂ balance and GHSV = 30 000 h⁻¹.

Fig. 11 Arrhenius plots in acetylene oxidation of meso-Co₃O₄, 0.3Pt/meso-Co₃O₄, 0.6Pt/meso-Co₃O₄, 1.0Pt/meso-Co₃O₄, bulk-Co₃O₄ and 0.6Pt/bulk-Co₃O₄. Reaction gas composition: [C₂H₂] = 50 ppm, [O₂] = 13 vol%, N₂ balance and GHSV = 30 000 h⁻¹.

Fig. 12 Acetylene consumption rates at 60 °C as a function of O_ads/O_latt molar ration of meso-Co₃O₄, 0.3Pt/meso-Co₃O₄, 0.6Pt/meso-Co₃O₄ and 1.0Pt/meso-Co₃O₄. Reaction gas composition: [C₂H₂] = 50 ppm, [O₂] = 13 vol%, N₂ balance and GHSV = 30 000 h⁻¹.

In order to examine the chemical and mechanical stability of the catalysts, the on-steam reaction experiments were carried out over the most effective 0.6Pt/meso-Co₃O₄ at 120 °C for acetylene oxidation, as shown in Fig. 13. There is no significant loss in catalytic activity. It indicates that 0.6Pt/meso-Co₃O₄ catalyst is chemical stability for acetylene oxidation. SEM images of the recovered catalyst are shown in Fig. 14. The morphology of the catalyst did not show significant change. Therefore, 0.6Pt/meso-Co₃O₄ may be a chemical and mechanical durable catalyst for acetylene oxidation under the adopted conditions.

Fig. 13 Durability test on 0.6Pt/meso-Co₃O₄ catalyst. Reaction gas composition: [C₂H₂] = 50 ppm, [O₂] = 13 vol%, N₂ balance and GHSV = 30 000 h⁻¹.

Fig. 14 SEM images of (a and b) 0.6Pt/meso-Co₃O₄ catalyst and (c and d) the used 0.6Pt/meso-Co₃O₄.

It is generally known that the porous network of catalysts can provide more contact sites and promote the diffusion of reactant and product molecules. The mesoporous structure is beneficial to enhancing the catalytic oxidation of acetylene. The big surface area of meso-Co₃O₄ (122.0 m² g⁻¹) is highly conductive to acetylene oxidation. xPt/meso-Co₃O₄ also possess the high surface area (111.0–111.5 m² g⁻¹). After the loading of Pt, xPt/meso-Co₃O₄ catalysts exhibit higher conversion at low temperature than meso-Co₃O₄ catalyst. The reaction mechanism of C₂H₄ oxidation over the Co₃O₄ has been reported.²⁶ It was reported that the oxidation of hydrocarbons on transition metal oxides follows a Mars-Van Krevelen mechanism.²⁷ The surface oxygen species will participate in hydrocarbons oxidation process and recovery by the gas-phase oxygen molecules. Thus, the reaction mechanism of acetylene oxidation over the catalysts may be related to surface oxygen species. Acetylene molecule can be absorbed on the surface of xPt/meso-Co₃O₄ to form an adsorbed intermediate. The adsorbed acetylene species are further oxidized to CO₂ and H₂O under the effect of surface active oxygen. From the results of XPS and O₂-TPD, xPt/meso-Co₃O₄ display more surface absorbed oxygen species than meso-Co₃O₄. The existence of lattice defect is beneficial for the formation of the oxygen vacancies. The oxygen vacancies can be favorable for the adsorption of oxygen and hence enhancing the catalytic oxidation of acetylene. xPt/meso-Co₃O₄ exhibit more lattice defect than meso-Co₃O₄, which can be demonstrated from the result of Raman, XPS and H₂-TPR. It has been reported that better catalytic activity is related to low-temperature reducibility. Better reducibility means that oxygen species are easily activated. From the result of H₂-TPR, xPt/meso-Co₃O₄ show better low-temperature reducibility than meso-Co₃O₄. Among all xPt/meso-Co₃O₄ catalysts, 0.6Pt/meso-Co₃O₄ catalyst exhibits the best catalytic performance, which can completely oxidize acetylene at 120 °C. 0.6Pt/meso-Co₃O₄ shows orderedly mesoporous structure, sufficient oxygen vacancies, more adsorbed oxygen species and better low-temperature reducibility. Therefore, the highest catalytic performance of 0.6Pt/meso-Co₃O₄ for the oxidation of acetylene was obtained.

4. Conclusions

The meso-Co₃O₄ with high surface area are synthesized by using the KIT-6 hard template, and xPt/meso-Co₃O₄ catalysts are prepared by the means of NaBH₄ reduction. The activity for acetylene oxidation over different catalysts decreases in the following order: 0.6Pt/meso-Co₃O₄ > 1.0Pt/meso-Co₃O₄ > 0.3Pt/meso-Co₃O₄ > meso-Co₃O₄ > bulk-Co₃O₄. 0.6Pt/meso-Co₃O₄ exhibits the highest catalytic activity of acetylene oxidation at 120 °C and good stability for at least 60 h. The excellent catalytic performance of 0.6Pt/meso-Co₃O₄ can be associated with its orderedly mesoporous structure, plenty of adsorption oxygen species and better low-temperature reducibility.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U1662103, 21673290 and 21376261); the National Hi-Tech Research and Development Program (863) of China (2015AA034603); and the China National Offshore Oil Corporation Fund (LHYJYKJSA20160002).

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