Low-temperature catalytic oxidation of vinyl chloride over Ru modified Co₃O₄ catalysts

Abstract

Ruthenium modified cobalt oxides were prepared by (1) impregnating ruthenium chloride hydrate on cobalt oxides, Ru-supported catalysts (Ru/Co₃O₄), and (2) Ru-doped catalysts (Ru–Co₃O₄) where the ruthenium ions were added to the precursor solution, by a one-step sol–gel method with cobalt nitrate. The physicochemical properties of the catalysts were characterized by ICP, BET, XRD, HR-TEM, TPR, and XPS analysis. The effects of ruthenium were studied for the total oxidation of vinyl chloride. This Ru modifier was observed to enhance oxygenate formation. The different preparation methods made contributions to the different amounts of Ru⁴⁺ on the surfaces of the catalysts while Ru⁴⁺ would be in synergy with Co²⁺ concentration, and this also changed the chemical coordination of oxygen on the surface. Dispersion of Ru oxides on the cobalt oxides surface could not only improve the catalytic activity and stability on steam, but also decrease the amount of chlorinated by-products and increase HCl selectivity.

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Introduction

Recently, several legislations concerning various pollutants have the improvement of air quality as a major objective. Volatile organic compounds (VOCs), which are emitted from heavy and widespread applications in industry, are significant atmospheric pollutants owing to their high toxicity and malodorous nature. Many VOCs, especially chlorinated VOCs such as dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloromethane, trichloroethylene, and tetrachloroethylene, require special attention because of their extreme stability.¹ One of the principal solutions to reduce the emission is the catalytic oxidation technique,² which offers the possibility of removing them from aerial effluents at low temperature with a more economical process than other technologies. In addition, with this technique, chlorinated VOCs could be completely oxidized into CO₂, H₂O, Cl₂ or HCl, which are low toxicity final products.³ In general, noble metals such as platinum, palladium, gold, and rhodium are dispersed on supports with high specific surface areas,^4–7 which are well established as efficient catalysts for VOCs oxidation.⁸ They are very reactive, causing complete oxidation, and can avoid the formation of by-products.⁹ However, it is necessary to optimize the composition of these catalysts in order to obtain lower supported content catalysts because of their high cost.¹⁰

Many researchers have certified that single or mixed transition metal oxides could have an excellent catalytic activity and stability for total oxidation of VOCs.¹¹ Also, noble metal-based catalysts perform with high specific activity, resistance to deactivation, and ability to be regenerated, without considering their expensive costs. Among these oxides, cobalt oxides catalysts have been reported to be effective with low-temperature oxidation of carbon monoxide,¹² hydrocarbons,¹³ and diesel soot.¹⁴ Active behavior of Co₃O₄ catalysts is most likely related to high bulk oxygen mobility and the easy formation of highly active oxygen species. On the other hand, ruthenium catalysts recently have received much attention because of their high activity in oxidation and reduction reactions, especially with oxidation of hydrogen chloride to chlorine in industry.¹⁵ Supported ruthenium catalysts offer good reactivity in various catalytic reactions such as methane,¹⁶ ammonia oxidation,¹⁷ CO oxidation,¹⁸ steam reforming,^19,20 and reduction of NO by methane.²¹ Another important reason is that under oxidation conditions Ru transformes to Ru dioxide showing highly desirable reactivity and stability, which decreases the cost compared to the other noble metals. Previous reports investigated these catalysts, such as Ru–CeO₂,²² Ru/Al₂O₃,²³ and Ru/TiO₂,²⁴ and showed very good activity and chemical stability for the oxidation of chlorinated hydrocarbons.

Large amounts of vinyl chloride (VC), as a model chlorinated VOC, are released in the chloro-alkali industrial process, which are very harmful to the environment and public health. In recent studies, chlorine species adsorbed on active surface sites could be removed via the Deacon reaction on RuO₂.²⁵ Therefore, the association of ruthenium and cobalt oxides, which combine the highly active oxygen species with the ability for chlorine activation, can establish a successful catalytic system in VC oxidation reactions. The catalytic performances of supported noble metals are strongly dependent on the preparation method, metal loading and, as a consequence, physicochemical properties.⁸

In this work, Ru modified Co₃O₄, including those which were doped and supported, were synthesized, characterized, and tested for VC oxidation capability. The concentration of chlorinated by-products was studied during the oxidation process. The effects from a different preparation method had an influence between the ruthenium and cobalt species, which involved Co²⁺ and Ru⁴⁺ dispersion. The relationship between ruthenium with cobalt structural features and catalytic activity was investigated. Better performance on activity and stability will provide a challenge with industrial applications for future environmental protection.

Experimental

Catalysts preparation

Pure Co₃O₄ was prepared via a conventional citrate sol–gel method. Typically, a stoichiometric amount of citric acid (CA) was added to an aqueous cobalt salt solution using Co(NO₃)₂·6H₂O as the precursor with a molar ratio of CA/Co equal to 1.1. The mixture was heated to 80 °C under magnetic stirring to completely dissolve the CA and maintained at this temperature to evaporate excess amounts of H₂O until the formation of a viscous gel. After being dried at 100 °C for 10 h in an oven, the resulting material was finally calcined at 450 °C (the heating rate was 1 °C min⁻¹) for 4 h under air atmosphere and then cooled down.

Ru-supported Co₃O₄ catalysts (noted as Ru/Co₃O₄) were prepared by an optimized impregnation method. Certain amounts of RuCl₃ as the precursor were dissolved in an ethanol aqueous solution with the ratio of ethanol to water 1 : 4, then the previously prepared Co₃O₄ was added into this solution, following evaporation at 80 °C under magnetic stirring, and then drying at 100 °C for 10 h in an oven. After that, the catalysts were calcined under the same condition as above.

Ru-doped Co₃O₄ catalysts (noted as Ru–Co₃O₄) were prepared by the same sol–gel method as that of pure Co₃O₄, where an aqueous solution contained a certain amount of Co(NO₃)₂·6H₂O and RuCl₃ as precursors.

For a comparison, Ru supported on a commercial SiO₂ was prepared by the same impregnation method as that of Ru/Co₃O₄. Additionally, for a facile description, the Ru-modified catalysts were denoted as X% Ru/Co₃O₄, X% Ru–Co₃O_4, and X% Ru/SiO₂, respectively, where X represented the weight percentage of Ru in the catalyst.

Catalysts characterization

The contents of Ru in the prepared catalysts were quantitatively determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Varian 710ES instrument (USA). Before measurement, the metal oxides were dissolved using a mixture of inorganic acids (H₂SO₄, HNO₃, and HF).

Powder X-ray diffraction (XRD) patterns of these catalysts were recorded on a Bruker AXS D8 Focus diffractometer (Germany) with CuKα (λ = 0.154056 nm) radiation at 40 kV and 40 mA. The scan range was from 10 to 80° (2θ) with a step size of 0.02° and a scanning rate of 0.6° min⁻¹. The lattice parameter and crystallite size of each catalyst were calculated according to the XRD Rietveld refinement using the Topas 4 software.

The nitrogen adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020M surface area & porosity analyzer (USA). The specific surface area (SSA) of each catalyst was obtained by the Brunauer–Emmett–Teller (BET) method.

The redox properties of these catalysts were analyzed by temperature-programmed reduction of H₂ (H₂-TPR) on a Micromeritics AutoChemII 2920 sorption instrument (USA). In a typical run, 0.10 g of the catalyst placed in a quartz reactor was pre-treated at 450 °C for 30 min with a gas flow of 3 vol% O₂/He. After cooling down to room temperature, the reducing gas (10 vol% H₂/Ar) with a flow rate of 50 mL min⁻¹ was introduced into the reactor under heating from 25 to 800 °C at a rate of 10 °C min⁻¹. The signal of H₂ uptake in the H₂-TPR was collected by a thermal conductivity detector (TCD) after a cold trap to remove the produced water.

X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Fisher ESCALAB 250Xi electron spectrometer (USA) with an AlKα (1486.6 eV) radiation source. The constant charging of the XPS spectra was corrected by C1s of adventitious carbon with binding energy of 284.8 eV.

Transmission electron microscopy (TEM) observations of the catalysts were performed on a FEI Tecnai G2 F30 microscope (USA) with STEM-EDX detector. The specimens for TEM were prepared by grinding the powdered catalyst in a mortar, dispersing it in ethanol and placing a droplet of the suspension onto a copper grid coated with a carbon film.

Catalytic testing

The catalytic reaction of VC oxidation was carried out in a quartz tubular fixed-bed flow reactor (inner diameter = 6 mm) under atmospheric pressure. In each experiment, 0.60 g of the catalyst (pellets with diameters from 0.3 to 0.45 mm), was placed in the reactor. The feed gas (120 mL min⁻¹) with VC concentration of 1000 ppm diluted by air, was passed through the catalyst bed, corresponding to a gas hourly space velocity (GHSV) of 15 000 h⁻¹. A K-type thermocouple was placed in the catalyst bed to monitor the reaction temperature heated in the range of 50–350 °C.

The gas effluent was analyzed by an online PerkinElmer Clarus 580 (USA) gas chromatograph (GC) equipped with a flame ionization detector (FID) during a stepwise increase of the reaction temperature, usually after 30 min at the selected temperature, to achieve the steady state. In addition, a PerkinElmer TurboMatrix 650 thermal desorber (USA) combined with an Agilent 5975C inert mass spectrometer (USA) was used for the trapping and qualitative determination of possible Cl-containing by-products, such as CH₂Cl–CHCl₂, CH₂Cl₂, CHCl₃, and CCl₄.

VC conversion and HCl selectivity was calculated according to the following equations:

where [VC]_in, [VC]_out, [CCl₄], [CHCl₂CH₂Cl], [CHCl₃], and [CH₂Cl₂] represent the VC concentrations in the inlet and outlet as well as the concentrations of CCl₄, CHCl₂CH₂Cl, CHCl₃, and CH₂Cl₂ in the effluent gas, respectively.

Results and discussion

The actual Ru content and the physicochemical properties of Ru modified catalysts are listed in Table 1. All catalysts presented a close weight percent to the nominal ones. The XRD patterns of all prepared catalysts are shown in Fig. 1, and crystalline phase identification is performed by referring to the standard powder diffraction file (PDF). Notably, all catalysts presented characteristic reflection peaks of cubic spinel Co₃O₄ oxides (Fd3̄m, JCPDS #42-1467) at 2θ of 19.0°, 31.2°, 36.8°, 44.8°, 59.4°, and 65.2° corresponding to the (111), (220), (311), (400), (511), and (440) crystal faces, respectively.^26,27 There were no other diffracion peaks related to CoO phase in the XRD patterns.²⁸ In addition, there was no observation of the feature patterns related to Ru oxides in the region of 25–42° in all Ru modified catalysts, which could be due to the low amounts of Ru species on these catalysts and/or the lower intensity of Ru peaks than the cobalt oxides ones. The lattice parameters of Co₃O₄ structure and the crystallite sizes of all catalysts were calculated by Topas 4.0 program and listed in Table 1. The lattice parameters of Ru-doped catalysts (0.5% Ru–Co₃O₄ and 1% Ru–Co₃O₄) increased compared with pure Co₃O₄ from 8.0844 to 8.0896 Å. This could be attributed to the substitution effect of Ru ions into cubic lattice Co₃O₄ spinel, which resulted in a lattice parameter increase after the metal incorporation in doped ones.²⁹ However, Ru-supported catalysts (0.5% Ru/Co₃O₄ and 1% Ru/Co₃O₄) gave no obvious change in parameter, about 8.8046 or 8.0847 Å, which could be due to the Ru particles dispersed on the surface of cobalt oxides without changing the lattice parameter. The crystalline size was increased from 27.8 nm on pure Co₃O₄ to ∼32 nm and ∼39 nm on Ru–Co₃O₄ and Ru/Co₃O₄, respectively. While, the BET surface areas of all catalysts were in the range of 20–29 m² g⁻¹ without any obviously difference in Table 1.

Table 1 Physico-chemical properties of the catalysts

Catalysts	Ru loading^a (wt%)	Lattice parameter^b (Å)	Crystalline size^c (nm)	SBET^d (m^2 g^−1)
a Measured by ICP-AES.b Lattice parameters calculated in Topas 4.0 program.c Crystallite sizes calculated in Topas 4.0 program.d Surface area determined from N2 isotherm.
Co3O4	—	8.0844	27.8	20
0.5% Ru–Co3O4	0.48	8.0873	32.4	26
1% Ru–Co3O4	0.98	8.0896	31.6	29
0.5% Ru/Co3O4	0.49	8.0847	39.3	24
1% Ru/Co3O4	0.97	8.0846	39.9	23

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Fig. 1 XRD patterns of all catalysts.

TEM and EDX-mapping images of 1% Ru–Co₃O₄ and 1% Ru/Co₃O₄ catalysts are shown in Fig. 2 and 3. It could be observed that both catalysts were nano-scaled particles, which showed characteristic lattice fringes of the cobalt oxide. The dark regions presented on the 1% Ru/Co₃O₄ (Fig. 2b) could be associated with formation of a RuO₂ crystalline phase on the surface, leading to a lower resolution of these lattice fringes of Co₃O₄ as compared with those on 1% Ru–Co₃O₄ (Fig. 2a). Based on analysis of the EDX-mapping images (Fig. 3a and b), the high distribution density of Ru element on the surface of 1% Ru/Co₃O₄ could be observed. Contrarily, only a trace of surface Ru species was detected on 1% Ru–Co₃O₄ with the same total quantity of Ru in bulk domains. These findings indicated that the Ru species were well dispersed on the surface of cobalt oxides in 1% Ru/Co₃O₄, while most of the Ru was preferentially doped into the crystalline lattice of cobalt oxides on 1% Ru–Co₃O₄, according to the XRD results.

Fig. 2 TEM images of 1% Ru–Co₃O₄ (a) and 1% Ru/Co₃O₄ (b).

Fig. 3 EDX-mapping of 1% Ru–Co₃O₄ (a) and 1% Ru/Co₃O₄ (b).

The redox properties of all catalysts were analyzed by H₂-TPR. As shown in Fig. 4, the H₂-TPR profiles could be generally divided into two regions, according to the varied reduction temperatures assigned to the changes of different reduction species on the catalysts. For the pure Co₃O₄ catalyst, the first reduction peak centered at 256 °C was associated with the reduction of Co³⁺ into Co²⁺, while the second peak centered at 351 °C was attributed to further reduction of Co²⁺ into metallic cobalt.³⁰ It is accepted that the reduction of ruthenium ions (Ru⁴⁺ or Ru²⁺) into metallic species (Ru⁰) usually takes place at a lower temperature in comparison with the occurrence of Co₃O₄ reduction.³¹ According to the literature, ruthenium(IV) in RuO₂ could be reduced in several steps from 97 °C to 176 °C.³² However, it was difficult to observe the separate reduced peaks assignable to ruthenium ions over the catalysts because of its relatively lower content of surface ruthenium ions. Moreover, the reducible behaviors of ruthenium, induced by their location in a Co₃O₄ lattice, probably were being overlapped with the reduction peaks of Co₃O₄ at the low temperature.³³ Notably, the reduction temperatures of various peaks maxima over all Ru modified catalysts were shifted to a lower temperature range as compared to pure Co₃O₄, indicating that the addition of ruthenium markedly facilitated the reduction of cobalt species. Especially, Ru-supported catalysts exhibited even better reducible behaviors than the Ru-doped catalysts, from 227 °C to 182 °C in 0.5% Ru catalysts, and from 186 °C to 145 °C in 1% Ru catalysts, respectively. The first reduced peaks shifted to lower temperatures, which could be associated to Ru improving the reduction of Co³⁺ into Co²⁺. The higher the quantity of ruthenium ions distributed on the surface of cobalt oxide, the greater the ability to improve reduction.³⁴ On the other hand, it is proposed that the surface RuO_x nanoparticles are preferentially reduced at low temperature than cobalt oxides.³⁵ Atomic hydrogen, which is generated by dissociation on Ru⁰ reduction nuclei, will smoothly spread to neighboring Co₃O₄ particles to accelerate their reduction.³⁶ This phenomenon, called hydrogen spillover effect, will noticeably decrease the reduction temperatures. From the results above, it could be observed that Ru had a strong interaction with the Co₃O₄ layer. Overall, the relative low-temperature reducibility of all catalysts from the highest to the lowest, which was derived from H₂-TPR analysis, followed the order of 1% Ru/Co₃O₄ > 0.5% Ru/Co₃O₄ > 1% Ru–Co₃O₄ > 0.5% Ru–Co₃O₄ > Co₃O₄.

Fig. 4 H₂-TPR profiles of all catalysts.

All catalysts were analyzed by the XPS technique in order to investigate surface element compositions, the metal oxidation states, and the nature of the oxygen species. Considering that the binding energy (BE) of Ru3d was very close to that of C1s,³⁷ the Ru oxidation states were determined by Ru3p spectra. As illustrated in Fig. 5, Ru-doped catalysts showed lower spectra intensities than the Ru-supported catalysts, verifying that low contents of residual Ru species were present on catalyst's surface, which could be attributed to Ru introduction into the Co₃O₄ framework. Additionally, Ru3p XPS spectra could be deconvoluted into three peaks located at 461.6, 463.4, and 466.3 eV, which was assigned to the metallic Ru, Ru⁴⁺, and Ru⁶⁺ species, respectively.^37,38 The atomic molar ratios of surface Ru⁴⁺ species (Ru⁴⁺/Ru) are determined and listed in Table 2.

Fig. 5 Ru3p XPS spectra of all catalysts.

Table 2 Atomic surface ratios and surface composition of the catalysts

Catalysts	Ru^4+/Ru (at%)	Co^3+/Co^2+^a (at/at)	Oads/Olatt (at%)
a Co^3+/Co^2+ = D1(Co2p3/2)/D2(Co2p3/2).
Co3O4	—	1.85	49.8
0.5%-Co3O4	25.5	1.84	47.0
1% Ru–Co3O4	38.9	1.54	44.1
0.5% Ru/Co3O4	50.5	1.47	49.8
1% Ru/Co3O4	52.1	1.26	48.8

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The order in terms of Ru⁴⁺ atomic molar ratio from the highest to the lowest was 1% Ru/Co₃O₄ (52.1%) > 0.5% Ru/Co₃O₄ (50.5%) > 1% Ru–Co₃O₄ (38.9) > 0.5% Ru–Co₃O₄ (25.5%), where the Ru-supported catalysts possessed more abundant surface Ru⁴⁺ species than the Ru-doped catalysts. In addition, the Co2p XPS spectra of all catalysts are shown in Fig. 6. For Co₃O₄ oxide, the BE value of Co2p_3/2 was around 779.8 eV with a spin-orbital splitting of 15.1 eV. Herein, the Co2p spectra of all catalysts were deconvoluted into two spin–orbit doublets, Co³⁺ (D₁) and Co²⁺ (D₂), as well as three satellite peaks (S₁, S₂, and S₃).^30,39–41 Moreover, pure Co₃O₄ presented the normal molar ratio of Co³⁺/Co²⁺ (1.85, in Table 2),⁴² while the Co³⁺/Co²⁺ ratio decreased down to the range of 1.84–1.26 over the Ru doped and supported catalysts, indicating reduction of Co³⁺ into Co²⁺ due to the interaction of Ru species with Co₃O₄ spinel structure.⁴³

Fig. 6 Co2p XPS spectra of all catalysts.

The O1s spectra of all catalysts are presented in Fig. 7, which is mainly deconvoluted into two peaks corresponding to different oxygen species on the catalysts. As previously reported, the peaks at BE of 529.9–530.2 and 531.4–531.6 eV were characteristic of lattice oxygen (O_latt, i.e., O²⁻) and surface adsorbed oxygen (O_ads, i.e., OH group, O⁻, or O₂²⁻), respectively.^44,45 The surface oxygen species O_ads/O_latt ratio was decreased from 49.8% to 44.1% in Ru-doped catalysts, whereas the Ru-supported catalysts presented nearly the same ratio with Co₃O₄ (from 49.8% to 48.8%). Besides, the binding energy of O_latt slightly increased, which also correlated to Co²⁺ concentration on the surface.⁴²

Fig. 7 O1s XPS spectra of all catalysts.

The catalytic performances of Co₃O₄ and Ru-modified Co₃O₄ catalysts for VC oxidation were evaluated. The light-off curves of VC conversion as a function of reaction temperature are shown in Fig. 8, and the T₅₀ and T₉₀ values corresponding to the temperatures at 50% and 90% of VC conversion are listed in Table 3.

Fig. 8 The light-off curves as a function of reaction temperature for VC oxidation.

Table 3 Light-off temperatures at 50% and 90% of VC conversion

Catalysts	T50^a (°C)	T90^a (°C)
a The temperature of conversion at 50% and 90%.
Co3O4	253	295
0.5% Ru–Co3O4	218	258
1% Ru–Co3O4	199	238
0.5% Ru/Co3O4	193	231
1% Ru/Co3O4	186	216

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1% Ru/SiO₂ as reference possessed the poorest catalytic activity of VC oxidation with a complete conversion of VC achieved at 400 °C. However, all of those Ru-modified Co₃O₄ catalysts exhibited higher catalytic activity for VC oxidation than the pure Co₃O₄ and reference catalysts. The catalyst of 1% Ru/Co₃O₄ was proven to be the optimum one with the lowest T₅₀ and T₉₀. These results indicated that single Ru oxide was inactive at low temperature for the oxidative destruction of VC, and Co₃O₄ was the predominant active component for the reaction over Ru-modified Co₃O₄ catalysts. Furthermore, catalytic activity according to the T₅₀ and T₉₀ values from the highest to lowest followed the order of 1% Ru/Co₃O₄ > 0.5% Ru/Co₃O₄ > 1% Ru–Co₃O₄ > 0.5% Ru–Co₃O₄ > Co₃O₄ > 1% Ru/SiO₂, which was consistent with the sequence of redox ability obtained in H₂-TPR.

According to these results, it was deduced that the Ru species not only affected the redox property of Co₃O₄, but also played a particular role in the reaction. The catalytic activity should be associated to lower temperature reducibility, which was caused by Ru dispersed on the cobalt surface, and an increased Ru⁴⁺/Ru content. Moreover, Co³⁺ was replaced by Co²⁺ or Ru⁴⁺ causing higher concentration of surface Co²⁺, which was indicative of oxygen defects, increasing the higher intrinsic activity.⁴⁶ A linear relationship between T₅₀ and Ru⁴⁺/Ru with Co³⁺/Co²⁺ is shown in Fig. 9. Thus, the catalytic activity had a positive correlation to the surface Ru⁴⁺ amount, and a negative correlation with the surface Co³⁺/Co²⁺ ratio. Compared with other supported noble metal catalysts listed in Table 4,^22,24,25 ruthenium supported cobalt oxides catalysts showed a good catalytic performance in chlorinated VOCs oxidation.

Fig. 9 The linear relationship between T₅₀ and Ru⁴⁺/Ru, Co³⁺/Co²⁺.

Table 4 Catalytic performances for chlorinated VOCs on supported noble metals catalysts reported in the literature

Catalysts	Conditions^a	Chlorinated VOCs conversion^b	Reference
a Chlorobenzene (CB), dichloromethane (DCE), vinyl chloride (VC).b The temperature of conversion at 50% and 90%.c 10% O2 and N2 balance.d CeO2 nanorods (-r), CeO2 nanocubes (-c), CeO2 nano-octahedra (-o).
1% Ru–CeO2	CB = 550 ppm	T50 = 204, T90 = 250	22
1% Ru/CeO2	GHSV = 15 000 h^−1	T50 = 217, T90 = 250
1% Ru/SBA-15		T50 = 312, T90 = 350
1% Ru/TiO2	DCM = 750 ppm^c	T50 = 235, T90 = 267	24
1% Ru/Al2O3	GHSV = 10 000 h^−1	T50 = 276, T90 = 308
0.4% Ru/CeO2-r^d	CB = 1000 ppm	T50 = 230, T90 = 275	25
0.4% Ru/CeO2-c	GHSV = 30 000 h^−1	T50 = 250, T90 = 306
0.4% Ru/CeO2-o		T50 = 320, T90 = 363
0.5% Ru/Co3O4	VC = 1000 ppm	T50 = 193, T90 = 231	This work
1% Ru/Co3O4	GHSV = 15 000 h^−1	T50 = 186, T90 = 216

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As is well known, highly chlorinated by-products can be yielded in the reaction of CVOCs oxidation.⁴⁷ Therefore, it is reasonable to provide experimental information about the selectivity to the ideal completely oxidized products (namely H₂O, CO₂, and HCl or Cl₂) and, more particularly, the concentrations of by-products. In this work, possible chlorinated organics in the effluent feed were quantitatively determined in the catalytic test. During the stepwise increased temperature oxidation process, 1,1,2-trichloroethane (CH₂Cl–CHCl₂), dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄) were the major chlorinated by-products,⁴⁸ and their concentrations as a function of reaction temperature over all prepared catalysts are shown in Fig. 10. Significant amounts of chlorinated by-products were formed at the temperature range of 160–360 °C. The concentrations of these chlorinated organics over 1% Ru/Co₃O₄ were lower than those over any other catalysts, which could be attributed to its optimum low-temperature catalytic activity in the reaction.

Fig. 10 Concentrations of chlorinated organics as a function of reaction temperature, (A) CHCl₂CH₂Cl, (B) CH₂Cl₂, (C) CHCl₃, and (D) CCl₄.

Additionally, an unexpected high amount of CH₂Cl₂ was detected, in Fig. 10B, when the VC conversion was achieved at about 95% (Fig. 8) over all other catalysts. And it quickly decreased when the reaction temperature increased. During the reaction, CH₂Cl–CHCl₂ might be produced by chlorinate-addition reaction to vinyl chloride, and then be cracked into CH₂Cl₂ and CHCl₃ by thermal decomposition. And last, CCl₄ could be formed as a result of the further chlorination of CH₂Cl₂ or CHCl₃. This phenomenon was observed between the Ru-doped and supported cobalt oxide in the distribution concentration of chlorinated by-products. Ru-supported catalysts presented a lower concentration of CH₂Cl–CHCl₂, which would form low concentrations of CH₂Cl₂ and CHCl₃. However, Ru-doped catalysts only changed the reaction activity and had a little effect on chlorinated by-products, which showed the same tendency with pure Co₃O₄. However, it seemed that RuO₂ dispersed on a surface had a better performance of restraining the chloride addition reaction of vinyl chloride.

Hydrogen chloride (HCl) selectivity was evaluated in order to understand the final product in the VC oxidation, in Fig. 11. It should be pointed out that all chlorinated compounds were dechlorinated into HCl without any chlorinated byproducts detected at higher temperature over all catalysts.⁴⁹ Because Cl₂ was not detected during the oxidation, the HCl selectivity gradually decreased with the rise of reaction temperature due to the formation of chlorinated by-products. Then, when VC conversion reached around 95%, lowest HCl selectivity was obtained, simultaneously accompanied by the formation of high amounts of chlorinated by-products. With reaction temperature continuously increased, HCl selectivity rose back to near 100%, corresponding to the diminished concentrations of these chlorinated organics. 1% Ru/Co₃O₄ exhibited the minimum change in HCl selectivity among all catalysts, which was also attributed to its best catalytic activity for VC oxidation.

Fig. 11 Selectivity (%) to HCl over all catalysts.

Moreover, the catalytic stability in long-term tests without deactivation is also important, which was evaluated over 1% Ru/Co₃O₄, as the optimum catalyst, at various reaction temperatures for 120 hours (40 hours for each specific temperature 200 °C, 220 °C, and 240 °C with the same catalyst). The corresponding profiles of VC conversion as a function of time on stream are illustrated in Fig. 12. 1% Ru/Co₃O₄ exhibited relatively poor catalytic stability at low temperature (200 °C). Indeed, VC conversion decreased from 72% to 63% during the first 5 hours, indicating the occurrence of gradual deactivation on the catalyst,⁵⁰ and finally VC conversion reached a steady value of about 55% until the end of time. However, it was very stable when the temperature reached to 220 °C or 240 °C and an even higher temperature.

Fig. 12 Lifetime test conducted on different temperature.

Meanwhile, the concentrations of those chlorinated by-products in the stability test at 220 °C were also determined and shown in Fig. 13. In spite of the presence of some fluctuations, VC conversion was achieved stably at about 95% at 220 °C during the first 40 hours test, and the formation of CHCl₃, CCl₄, and CH₂Cl₂ as the main chlorinated organics was unavoidable with the concentrations about 10, 15, and 8 ppm, respectively. However, at a higher reaction temperature of 260 °C (shown in Fig. 10), 1% Ru/Co₃O₄ showed excellent catalytic activity and stability with a complete VC conversion and an absence of chlorinated by-products formation.

Fig. 13 Lifetime test performed with 1% Ru/Co₃O₄ at 220 °C.

Conclusions

All Ru-modified Co₃O₄ presented an improved catalytic activity and HCl selectivity, decreasing undesirable chloride by-products formation, as compared to Co₃O₄ and 1% Ru/SiO₂. Moreover, it clearly demonstrated the difference between Ru-supported and doped catalysts, where the supported ones presented better catalytic performance with higher reaction activity. This high catalytic activity could be attributed to high reducibility of Co oxides, and also to the complex interplay between Ru species on the surface and Co₃O₄ phase composition. Thus, the substitution of Ru species for Co³⁺ in a spinel structure with a low amount on the surface of the supported catalysts could increase Co²⁺ concentration. High relative proportion of Co²⁺ and Ru⁴⁺, which also promoted oxygen defects or vacancies, not only increased the catalytic activity but also decreased the amounts of chlorinated by-products and increased HCl selectivity.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2016YFC0204300), National Natural Science Foundation of China (21577035), the Commission of Science and Technology of Shanghai Municipality (15DZ1205305), 111 Project (B08021), and “Région Rhône Alpes” (Coopera project 113955/2015). Thanks are also due to the China Scholarship Council for the joint PhD program with Institute de Recherches sur la Catalyse et l'Environnement de Lyon (IRCELYON), Centre National de la Recherche Scientifique (CNRS), and Université Claude Bernard Lyon 1 (UCBL1).

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