Insights into the Morphological E ff ect of Co 3 O 4 Crystallite on Catalytic Oxidation of Vinyl Chloride

Co3O4 catalysts of cube and sphere shapes were prepared by one-step hydrothermal synthesis with different controlled amounts of Co(NO3)2·6H2O and NaOH. The morphological effects on both physicochemical properties and catalytic activities of vinyl chloride oxidation were investigated by material characterization and performance evaluation. The obtained results showed that the morphology, resulting in the exposure difference of crystal planes, significantly affected the catalytic property. The catalytic activity for vinyl chloride oxidation followed a descending order of Co3O4 cube (Co3O4-c) > Co3O4 sphere (Co3O4-s) > Co3O4 commercial (Co3O4-com). The cube-shaped Co3O4 presented higher catalytic activity and stability than Co3O4 spheres despite their similar crystallographic structures as well as physicochemical and redox properties. Accordingly, the different catalytic behaviors should be attributed to a morphological effect. The Co3O4 cube with a preferential exposure of (001) plane presented higher abundance of surface Co2+ cations and adsorbed oxygen species, which acted as the active sites responsible for the improvement of its catalytic activity.


Introduction
A current global environmental issue is the atmospheric pollution arising from the emission of volatile organic compounds (VOCs), released from the combustion of fossil fuel and commercial waste exhausts such as petrochemical, industrial printing, and dry-cleaning processes.These emissions have adverse impacts on human health and the environment.As one of the promising technologies, catalytic oxidation could degrade VOC pollutants into harmless products with high selectivity.Thus, one of the most important research objectives for VOC oxidation is the preparation of highly efficient catalysts with good reaction stability.
Co 3 O 4 presents a spinel structure with an Fd-3m crystallographic space group, where oxygen atoms are arranged in a cubic close-packed matrix and the Co 3+ and Co 2+ cations are positioned in the octahedral and tetrahedral lattice sites, respectively [14,15].The Co 3 O 4 particles usually expose the lower index plane with (110), (111), and (001) planes [16], while the catalytic activity is likely related to high bulk oxygen mobility and highly active oxygen species [17,18].Recent research has established that the tunable morphology of Co 3 O 4 nanocrystals has a notable effect on CO or hydrocarbon oxidation [19], which was ascribed to the reactive preferential crystal planes with different shapes.For example, the nanorod-shaped Co 3 O 4 privileges the presence of active Co 3+ species on (110) planes, which were demonstrated to be the most active sites for CO adsorption.Thus, this nanorod structure can also catalyze the oxidation of CO at temperatures as low as -77 • C, keeping a constant stability in moist conditions [20].Furthermore, it has been demonstrated that Co 3+ species on (011) planes in nanobelt structures are more reactive than those on (001) planes in nanocubes [21].From this viewpoint, a clear clarification of the morphology-activity relationship is of great significance for the development of novel catalytic materials.However, the correlation between the morphological features and catalytic activity of Co 3 O 4 for chlorinated VOC oxidation has rarely been studied.
In this study, a Co 3 O 4 catalyst with different morphologies (cube and sphere) was prepared, characterized, and investigated for the oxidation of vinyl chloride (VC), and the morphological effect of Co 3 O 4 on both physicochemical characteristics and catalytic activities was extensively studied.

Characterization Results
The inductively coupled plasma results in Table 1 show that all catalysts presented nearly the same weight percentage of Co (approximately 70 wt.%), which agree well with the theoretical value of Co 3 O 4 .The scanning electron microscope (SEM) images of the catalysts in Figure 1 demonstrate that the morphology of Co 3 O 4 varied depending on the amount of Co(NO 3 ) 2 •6H 2 O and NaOH during the preparation process, which is in agreement with previous reports [22].Obviously, the synthesis with a lower concentration of cobalt nitrate resulted in the formation of the Co 3 O 4 -c with a particle size around 250 nm, seen in Figure 1a, while Co 3 O 4 -s could be obtained using a higher concentration of cobalt nitrate with similar particle size as Co 3 O 4 -c (Figure 1b).In contrast, Figure 1c shows that Co 3 O 4 -com presented an irregular particle size from 100 to 300 nm.In addition, the morphology of Co 3 O 4 -c was also confirmed by the transmission electron microscope (TEM) images in Figure 1d.According to the literature [23,24], the dominant exposed planes of cubic Co 3 O 4 are six (001) planes.to high bulk oxygen mobility and highly active oxygen species [17,18].Recent research has established that the tunable morphology of Co3O4 nanocrystals has a notable effect on CO or hydrocarbon oxidation [19], which was ascribed to the reactive preferential crystal planes with different shapes.For example, the nanorod-shaped Co3O4 privileges the presence of active Co 3+ species on (110) planes, which were demonstrated to be the most active sites for CO adsorption.Thus, this nanorod structure can also catalyze the oxidation of CO at temperatures as low as -77 °C, keeping a constant stability in moist conditions [20].Furthermore, it has been demonstrated that Co 3+ species on (011) planes in nanobelt structures are more reactive than those on (001) planes in nanocubes [21].
From this viewpoint, a clear clarification of the morphology-activity relationship is of great significance for the development of novel catalytic materials.However, the correlation between the morphological features and catalytic activity of Co3O4 for chlorinated VOC oxidation has rarely been studied.
In this study, a Co3O4 catalyst with different morphologies (cube and sphere) was prepared, characterized, and investigated for the oxidation of vinyl chloride (VC), and the morphological effect of Co3O4 on both physicochemical characteristics and catalytic activities was extensively studied.

Characterization Results
The inductively coupled plasma results in Table 1 show that all catalysts presented nearly the same weight percentage of Co (approximately 70 wt.%), which agree well with the theoretical value of Co3O4.The scanning electron microscope (SEM) images of the catalysts in Figure 1 demonstrate that the morphology of Co3O4 varied depending on the amount of Co(NO3)2•6H2O and NaOH during the preparation process, which is in agreement with previous reports [22].Obviously, the synthesis with a lower concentration of cobalt nitrate resulted in the formation of the Co3O4-c with a particle size around 250 nm, seen in Figure 1a, while Co3O4-s could be obtained using a higher concentration of cobalt nitrate with similar particle size as Co3O4-c (Figure 1b).In contrast, Figure 1c shows that Co3O4-com presented an irregular particle size from 100 to 300 nm.In addition, the morphology of Co3O4-c was also confirmed by the transmission electron microscope (TEM) images in Figure 1d.According to the literature [23,24], the dominant exposed planes of cubic Co3O4 are six (001) planes.The X-ray diffraction (XRD) patterns of the Co3O4 catalysts are presented in Figure 2. All diffraction peaks were indexed as the standard pure crystalline phase of Co3O4 (JCPDS file No. 42-1467) without any detection of impurities.Co3O4-c and Co3O4-s presented analogical XRD patterns with similar characteristic peaks.As seen in Table 1, the average crystallite size was estimated by applying the Scherrer equation.It was 62 nm for Co3O4-c, whereas it increased to 100 and 93 nm for Co3O4-s and Co3O4-com, respectively.Moreover, no obvious difference in the specific surface area of each catalyst was observed, which were 7, 7, and 6 m 2 •g -1 for Co3O4-c, Co3O4-s, and Co3O4-com, respectively.Chemical surface compositions and valence states were determined by X-ray photoelectron spectroscopy (XPS), and the spectra of Co 2p and O 1s are displayed.In Figure 3a, the binding energy (B.E.) value of Co 2p3/2 and Co 2p1/2 was around 780 and 795 eV, respectively [14,25].The satellite peaks at 785 eV confirmed the existence of Co 2+ species in the octahedral sites [26][27][28].Herein, the Co 2p spectra were resolved using a fitting procedure partially based on that suggested by Biesinger et al. [29].The contributions from Co 3+ and Co 2+ cations were identified at 779.5 and 781.1 eV, respectively [30].Additionally, two satellite peaks, S1 and S2, at 785.3 and 789.4 eV appeared due to electron correlations and final state effects in Co 2+ and Co 3+ cations, respectively [31].In turn, the satellite S3 was attributed to the spin orbit contributions of Co 2p1/2 and also from satellites S1 and S2 [32][33][34].
Based on the quantitative analysis results in Table 1, the Co 3+ /Co 2+ molar ratio over the Co3O4 catalysts followed an increasing order of Co3O4-c < Co3O4-s < Co3O4-com.The lower molar ratio of Co 3+ /Co 2+ over Co3O4-c indicates that higher abundance of Co 2+ cations were presented on the (001) plane of Co3O4-c, which could be a crucial factor for the catalytic oxidation of VOCs [14,35].The O 1s spectra in Figure 3b were also decomposed into two peaks.The peaks located at 529.7 and 531.2 eV could be ascribed to lattice oxygen (Olatt, i.e., O 2-) and adsorbed oxygen onto surface oxygen vacancies (Oads, i.e., O -, O2 2-, OH -, ), respectively [36,37].According to the quantitative analysis of O 1s spectra, The X-ray diffraction (XRD) patterns of the Co3O4 catalysts are presented in Figure 2. All diffraction peaks were indexed as the standard pure crystalline phase of Co3O4 (JCPDS file No. 42-1467) without any detection of impurities.Co3O4-c and Co3O4-s presented analogical XRD patterns with similar characteristic peaks.As seen in Table 1, the average crystallite size was estimated by applying the Scherrer equation.It was 62 nm for Co3O4-c, whereas it increased to 100 and 93 nm for Co3O4-s and Co3O4-com, respectively.Moreover, no obvious difference in the specific surface area of each catalyst was observed, which were 7, 7, and 6 m 2 •g -1 for Co3O4-c, Co3O4-s, and Co3O4-com, respectively.Chemical surface compositions and valence states were determined by X-ray photoelectron spectroscopy (XPS), and the spectra of Co 2p and O 1s are displayed.In Figure 3a, the binding energy (B.E.) value of Co 2p3/2 and Co 2p1/2 was around 780 and 795 eV, respectively [14,25].The satellite peaks at 785 eV confirmed the existence of Co 2+ species in the octahedral sites [26][27][28].Herein, the Co 2p spectra were resolved using a fitting procedure partially based on that suggested by Biesinger et al. [29].The contributions from Co 3+ and Co 2+ cations were identified at 779.5 and 781.1 eV, respectively [30].Additionally, two satellite peaks, S1 and S2, at 785.3 and 789.4 eV appeared due to electron correlations and final state effects in Co 2+ and Co 3+ cations, respectively [31].In turn, the satellite S3 was attributed to the spin orbit contributions of Co 2p1/2 and also from satellites S1 and S2 [32][33][34].
Based on the quantitative analysis results in Table 1, the Co 3+ /Co 2+ molar ratio over the Co3O4 catalysts followed an increasing order of Co3O4-c < Co3O4-s < Co3O4-com.The lower molar ratio of Co 3+ /Co 2+ over Co3O4-c indicates that higher abundance of Co 2+ cations were presented on the (001) plane of Co3O4-c, which could be a crucial factor for the catalytic oxidation of VOCs [14,35].The O 1s spectra in Figure 3b were also decomposed into two peaks.The peaks located at 529.7 and 531.2 eV could be ascribed to lattice oxygen (Olatt, i.e., O 2-) and adsorbed oxygen onto surface oxygen vacancies (Oads, i.e., O -, O2 2-, OH -, ), respectively [36,37].According to the quantitative analysis of O 1s spectra, Chemical surface compositions and valence states were determined by X-ray photoelectron spectroscopy (XPS), and the spectra of Co 2p and O 1s are displayed.In Figure 3a, the binding energy (B.E.) value of Co 2p 3/2 and Co 2p 1/2 was around 780 and 795 eV, respectively [14,25].The satellite peaks at 785 eV confirmed the existence of Co 2+ species in the octahedral sites [26][27][28].Herein, the Co 2p spectra were resolved using a fitting procedure partially based on that suggested by Biesinger et al. [29].The contributions from Co 3+ and Co 2+ cations were identified at 779.5 and 781.1 eV, respectively [30].Additionally, two satellite peaks, S 1 and S 2 , at 785.3 and 789.4 eV appeared due to electron correlations and final state effects in Co 2+ and Co 3+ cations, respectively [31].In turn, the satellite S 3 was attributed to the spin orbit contributions of Co 2p 1/2 and also from satellites S 1 and S 2 [32][33][34].
Based on the quantitative analysis results in Table 1, the Co 3+ /Co 2+ molar ratio over the Co 3 O 4 catalysts followed an increasing order of Co 3 O 4 -c < Co 3 O 4 -s < Co 3 O 4 -com.The lower molar ratio of Co 3+ /Co 2+ over Co 3 O 4 -c indicates that higher abundance of Co 2+ cations were presented on the (001) plane of Co 3 O 4 -c, which could be a crucial factor for the catalytic oxidation of VOCs [14,35].The O 1s spectra in Figure 3b were also decomposed into two peaks.The peaks located at 529. 7  the Oads/Olatt ratio of Co3O4-c was higher than that obtained from other catalysts (Table 1), which indicates that Co3O4-c provided more surface-active oxygen species than others.The H2-TPR profiles show that two reduction peaks were clearly observed over the Co3O4-c and Co3O4-s catalysts in Figure 4a.The two peaks at 285 and 330 °C were assigned to the reduction of Co 3+ to Co 2+ and Co 2+ to metallic cobalt, respectively [38].Comparatively, the Co3O4-com catalyst showed an overlapping peak, indicating a successive reduction behavior of Co 3+ into Co 2+ and Co 0 .Additionally, the temperature of reduction peaks maximum over Co3O4-c and Co3O4-s were relatively lower than that over Co3O4-com, suggesting that the cube-and sphere-shaped Co3O4 catalysts presented better reducibility.The low-temperature reducibility could be evaluated by the initial H2 consumption rate (less than 25% of oxygen consumption in the first reduction band of the H2-TPR profile) [39,40].Figure 4b shows the initial H2 consumption rate versus inverse temperature over the Co3O4 catalysts.The initial H2 consumption rate over the catalysts decreased in the order of Co3O4-c > Co3O4-s > Co3O4-com, indicating that Co3O4-c exhibited the highest low-temperature reducibility.This trend is in good agreement with the order of the Oads/Olatt ratio. .

Catalytic Performances for VC Oxidation
The catalytic performances of the Co3O4 catalysts were tested for the oxidation of VC, which acted as a model reaction for chlorinated VOCs.Based on the light-off curves shown in Figure 5 and the T50 and T90 values listed in Table 2, it could be found that the catalytic activity for VC over the catalysts followed Co3O4-c > Co3O4-s > Co3O4-com, which is similar to the TPR results.the Oads/Olatt ratio of Co3O4-c was higher than that obtained from other catalysts (Table 1), which indicates that Co3O4-c provided more surface-active oxygen species than others.The H2-TPR profiles show that two reduction peaks were clearly observed over the Co3O4-c and Co3O4-s catalysts in Figure 4a.The two peaks at 285 and 330 °C were assigned to the reduction of Co 3+ to Co 2+ and Co 2+ to metallic cobalt, respectively [38].Comparatively, the Co3O4-com catalyst showed an overlapping peak, indicating a successive reduction behavior of Co 3+ into Co 2+ and Co 0 .Additionally, the temperature of reduction peaks maximum over Co3O4-c and Co3O4-s were relatively lower than that over Co3O4-com, suggesting that the cube-and sphere-shaped Co3O4 catalysts presented better reducibility.The low-temperature reducibility could be evaluated by the initial H2 consumption rate (less than 25% of oxygen consumption in the first reduction band of the H2-TPR profile) [39,40].Figure 4b shows the initial H2 consumption rate versus inverse temperature over the Co3O4 catalysts.The initial H2 consumption rate over the catalysts decreased in the order of Co3O4-c > Co3O4-s > Co3O4-com, indicating that Co3O4-c exhibited the highest low-temperature reducibility.This trend is in good agreement with the order of the Oads/Olatt ratio.

Catalytic Performances for VC Oxidation
The catalytic performances of the Co3O4 catalysts were tested for the oxidation of VC, which acted as a model reaction for chlorinated VOCs.Based on the light-off curves shown in Figure 5 and the T50 and T90 values listed in Table 2, it could be found that the catalytic activity for VC over the catalysts followed Co3O4-c > Co3O4-s > Co3O4-com, which is similar to the TPR results.

Catalytic Performances for VC Oxidation
The catalytic performances of the Co 3 O 4 catalysts were tested for the oxidation of VC, which acted as a model reaction for chlorinated VOCs.Based on the light-off curves shown in Figure 5 and the T 50 and T 90 values listed in Table 2, it could be found that the catalytic activity for VC over the catalysts followed Co 3 O 4 -c > Co 3 O 4 -s > Co 3 O 4 -com, which is similar to the TPR results.

Catalysts T50 (°C) T90 (°C)
Co3O4-c 308 340 Co3O4-s 320 372 Co3O4-com 372 431 The final products in the effluent gas were water (H2O), carbon dioxide (CO2), and hydrogen chloride (HCl), which were identified by online mass spectrometry.However, during the reaction of VC oxidation, highly chlorinated by-products were yielded and quantitatively determined.The major chlorinated organics were namely 1,1,2-trichloroethane (CH2ClCHCl2), dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).As presented in Figure 6, the concentrations of the chlorinated organics over Co3O4-c or Co3O4-s were relatively lower than those over Co3O4-com catalyst, which could be owning to their higher catalytic activity.Moreover, the temperature corresponding to the maximum formation of chlorinated organics in Figure 6 indicates that Co3O4-c or Co3O4-s also presented higher chlorination activity compared with Co3O4-com.Regarding the formation mechanism, CH2ClCHCl2 might be produced from the addition reaction between VC and the surface active chlorine species.Then, it could undergo catalytic cracking on Co3O4 catalysts, resulting in the formation of CH2Cl2, CHCl3, and CCl4.The final products in the effluent gas were water (H 2 O), carbon dioxide (CO 2 ), and hydrogen chloride (HCl), which were identified by online mass spectrometry.However, during the reaction of VC oxidation, highly chlorinated by-products were yielded and quantitatively determined.The major chlorinated organics were namely 1,1,2-trichloroethane (CH 2 ClCHCl 2 ), dichloromethane (CH 2 Cl 2 ), trichloromethane (CHCl 3 ), and tetrachloromethane (CCl 4 ).As presented in Figure 6, the concentrations of the chlorinated organics over Co 3 O 4 -c or Co 3 O 4 -s were relatively lower than those over Co 3 O 4 -com catalyst, which could be owning to their higher catalytic activity.Moreover, the temperature corresponding to the maximum formation of chlorinated organics in Figure 6

Catalysts T50 (°C) T90 (°C)
Co3O4-c 308 340 Co3O4-s 320 372 Co3O4-com 372 431 The final products in the effluent gas were water (H2O), carbon dioxide (CO2), and hydrogen chloride (HCl), which were identified by online mass spectrometry.However, during the reaction of VC oxidation, highly chlorinated by-products were yielded and quantitatively determined.The major chlorinated organics were namely 1,1,2-trichloroethane (CH2ClCHCl2), dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).As presented in Figure 6, the concentrations of the chlorinated organics over Co3O4-c or Co3O4-s were relatively lower than those over Co3O4-com catalyst, which could be owning to their higher catalytic activity.Moreover, the temperature corresponding to the maximum formation of chlorinated organics in Figure 6 indicates that Co3O4-c or Co3O4-s also presented higher chlorination activity compared with Co3O4-com.Regarding the formation mechanism, CH2ClCHCl2 might be produced from the addition reaction between VC and the surface active chlorine species.Then, it could undergo catalytic cracking on Co3O4 catalysts, resulting in the formation of CH2Cl2, CHCl3, and CCl4. Figure 7 displays the selectivities of HCl and CO 2 at different reaction temperatures.At higher temperature, all VC was oxidized into HCl without the detection of other chlorinated by-products.However, the lowest HCl selectivity was obtained over Co 3 O 4 -c at the temperature range of 240 to 320 • C, which could be due to the simultaneous formation of high amounts of chlorinated by-products.The selectivity of HCl rose back to 100% with the temperature continuously increased, indicating the complete decomposition of chlorinated by-products.However, the selectivity for CO 2 always remained above 90% over the Co 3 O 4 -c and Co 3 O 4 -s catalysts beneficially due to their higher reaction activity, confirming the good carbon balance in the reaction.Figure 7 displays the selectivities of HCl and CO2 at different reaction temperatures.At higher temperature, all VC was oxidized into HCl without the detection of other chlorinated by-products.However, the lowest HCl selectivity was obtained over Co3O4-c at the temperature range of 240 to 320 °C, which could be due to the simultaneous formation of high amounts of chlorinated byproducts.The selectivity of HCl rose back to 100% with the temperature continuously increased, indicating the complete decomposition of chlorinated by-products.However, the selectivity for CO2 always remained above 90% over the Co3O4-c and Co3O4-s catalysts beneficially due to their higher reaction activity, confirming the good carbon balance in the reaction.Finally, the catalytic durability for VC oxidation in long-term experiments (i.e., 40 h) was evaluated over the Co3O4 catalysts, as shown in Figure 8. Co3O4-com exhibited relatively poor stability, with the conversion at 40% at 360 °C.Comparatively, approximately the same VC conversion of 80% was observed over the Co3O4-c and Co3O4-s catalysts at lower temperatures of 330°C and 350°C, respectively.Co3O4-s maintained stable activity during the initial 15 h but gradually deactivated with more time, giving the conversion of 60%.Ultimately, Co3O4-c presented the relatively more stable VC conversion of 80%, confirming its higher catalytic stability.Figure 7 displays the selectivities of HCl and CO2 at different reaction temperatures.At higher temperature, all VC was oxidized into HCl without the detection of other chlorinated by-products.However, the lowest HCl selectivity was obtained over Co3O4-c at the temperature range of 240 to 320 °C, which could be due to the simultaneous formation of high amounts of chlorinated byproducts.The selectivity of HCl rose back to 100% with the temperature continuously increased, indicating the complete decomposition of chlorinated by-products.However, the selectivity for CO2 always remained above 90% over the Co3O4-c and Co3O4-s catalysts beneficially due to their higher reaction activity, confirming the good carbon balance in the reaction.Finally, the catalytic durability for VC oxidation in long-term experiments (i.e., 40 h) was evaluated over the Co3O4 catalysts, as shown in Figure 8. Co3O4-com exhibited relatively poor stability, with the conversion at 40% at 360 °C.Comparatively, approximately the same VC conversion of 80% was observed over the Co3O4-c and Co3O4-s catalysts at lower temperatures of 330°C and 350°C, respectively.Co3O4-s maintained stable activity during the initial 15 h but gradually deactivated with more time, giving the conversion of 60%.Ultimately, Co3O4-c presented the relatively more stable VC conversion of 80%, confirming its higher catalytic stability.Considering all of the results above, the lower catalytic activity and durability of Co 3 O 4 -com in the reactions could be associated with its poorer low-temperature reducibility, as well as the lower surface abundance of Co 2+ and O ads species compared to those Co 3 O 4 catalysts of unique morphology.However, despite the close crystallographic structure and similar physicochemical properties, Co 3 O 4 -c presented higher catalytic activity and stability for VC oxidation than Co 3 O 4 -s, which reveals the close relation of catalytic reactivity with the morphological effect.Especially, the Co 3 O 4 -c catalyst with a preferential exposure of (001) planes possesses more Co 2+ cations on the surface of cubic structure.It was proven that the higher abundance of Co 2+ correlated well with higher oxygen mobility, thus resulting in the significant improvement of catalytic activity.

Co 3 O 4 Preparation
All chemicals throughout the experiment were analytically pure and not further purified.Co(NO 3 ) 2 •6H 2 O and NaOH were provided by Sinopharm Chemical Reagent Co., Ltd.The Co 3 O 4 catalyst was prepared by a hydrothermal process as reported in [22,23] with some modifications.Specifically, both Co(NO 3 ) 2 •6H 2 O and NaOH were dissolved in 30 mL deionized water and kept stirring for 30 min.Subsequently, the solution was transferred into a Teflon-lined stainless-steel autoclave and then treated at 180 • C for 3 h.After cooling down to room temperature, the obtained product was filtrated, washed several times with ethanol, and further dried at 60

Characterizations
Elemental analysis of Co content in the catalysts was conducted using inductively coupled plasma (ICP) on a HORIBA Jobin Yvon Activa instrument (HORIBA, Paris, France).X-ray diffraction (XRD) patterns of the catalyst were recorded by a Bruker D8 Advance A25 with Cu Kα radiation (λ = 0.154184 nm) at 50 kV and 35 mA.The nitrogen sorption was measured at 77 K on a Micromeritics ASAP 2020 apparatus after a degassing pretreatment.The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method.Temperature-programmed reduction of hydrogen (H 2 -TPR) was performed on a Micromeritics AutoChem 2920 apparatus using 10 vol% H 2 /Ar (50 mL•min −1 ) as the reducing gas.The reduction temperature increased from 50 to 800 • C at a rate of 10 • C•min −1 , and the H 2 consumption was measured by a thermal conductivity detector (TCD).X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra DLD electron spectrometer using an AlKα (1486.6 eV) radiation source.The binding energy of C 1s electron (284.6 eV) was used to calibrate the spectra.Scanning electron microscopy (SEM) was performed on a JEOL JSM-7800F microscope (JEOL, Tokyo, Japan), while transmission electron microscopy (TEM) was performed on a JEOL JEM-2100 microscope (JEOL, Tokyo, Japan).

Catalytic Tests
Catalytic oxidation of VC was performed using a fixed-bed reactor at atmospheric pressure.A total of 0.

Figure 1 . 10 Figure 1 .
Figure 1.SEM images of (a) Co 3 O 4 -c, (b) Co 3 O 4 -s, (c) Co 3 O 4 -com, and a TEM image of (d) Co 3 O 4 -c.The X-ray diffraction (XRD) patterns of the Co 3 O 4 catalysts are presented in Figure 2. All diffraction peaks were indexed as the standard pure crystalline phase of Co 3 O 4 (JCPDS file No. 42-1467) without any detection of impurities.Co 3 O 4 -c and Co 3 O 4 -s presented analogical XRD patterns with similar characteristic peaks.As seen in Table 1, the average crystallite size was estimated by applying the Scherrer equation.It was 62 nm for Co 3 O 4 -c, whereas it increased to 100 and 93 nm for Co 3 O 4 -s and Co 3 O 4 -com, respectively.Moreover, no obvious difference in the specific surface area of each catalyst was observed, which were 7, 7, and 6 m 2 •g −1 for Co 3 O 4 -c, Co 3 O 4 -s, and Co 3 O 4 -com, respectively.

Figure 4 .
Figure 4. (a) H2-TPR profiles and (b) the initial H2 consumption rate as a function of inverse temperature of the Co3O4 catalysts.

Figure 3 .
Figure 3. (a) Co 2p and (b) O 1s XPS spectra of the Co 3 O 4 catalysts.The H 2 -TPR profiles show that two reduction peaks were clearly observed over the Co 3 O 4 -c and Co 3 O 4 -s catalysts in Figure4a.The two peaks at 285 and 330 • C were assigned to the reduction of Co 3+ to Co 2+ and Co 2+ to metallic cobalt, respectively[38].Comparatively, the Co 3 O 4 -com catalyst showed an overlapping peak, indicating a successive reduction behavior of Co 3+ into Co 2+ and Co 0 .Additionally, the temperature of reduction peaks maximum over Co 3 O 4 -c and Co 3 O 4 -s were relatively lower than that over Co 3 O 4 -com, suggesting that the cube-and sphere-shaped Co 3 O 4 catalysts presented better reducibility.The low-temperature reducibility could be evaluated by the initial H 2 consumption rate (less than 25% of oxygen consumption in the first reduction band of the H 2 -TPR profile)[39,40].Figure4bshows the initial H 2 consumption rate versus inverse temperature over the Co 3 O 4 catalysts.The initial H 2 consumption rate over the catalysts decreased in the order of Co 3 O 4 -c > Co 3 O 4 -s > Co 3 O 4 -com, indicating that Co 3 O 4 -c exhibited the highest low-temperature reducibility.This trend is in good agreement with the order of the O ads /O latt ratio.

Figure 4 .
Figure 4. (a) H2-TPR profiles and (b) the initial H2 consumption rate as a function of inverse temperature of the Co3O4 catalysts.

Figure 4 .
Figure 4. (a) H 2 -TPR profiles and (b) the initial H 2 consumption rate as a function of inverse temperature of the Co 3 O 4 catalysts.

Figure 5 .
Figure 5.The light-off curves of vinyl chloride (VC) oxidation as a function of reaction temperature over the Co3O4 catalysts (reaction conditions: 1000 ppm VC, air balanced, and weight hourly space velocity (WHSV) = 48,000 mL g −1 h −1 ).

Figure 5 .
Figure 5.The light-off curves of vinyl chloride (VC) oxidation as a function of reaction temperature over the Co 3 O 4 catalysts (reaction conditions: 1000 ppm VC, air balanced, and weight hourly space velocity (WHSV) = 48,000 mL g −1 h −1 ).

10 Figure 5 .
Figure 5.The light-off curves of vinyl chloride (VC) oxidation as a function of reaction temperature over the Co3O4 catalysts (reaction conditions: 1000 ppm VC, air balanced, and weight hourly space velocity (WHSV) = 48,000 mL g −1 h −1 ).
• C. Finally, a calcination at 500 • C for 3 h in air was conducted to yield the Co 3 O 4 material.For the synthesis of Co 3 O 4 cube (Co 3 O 4 -c), the amount of Co(NO 3 ) 2 •6H 2 O and NaOH was 8.73 g and 0.30 g, respectively.A total of 17.46 g Co(NO 3 ) 2 •6H 2 O and 0.30 g NaOH were used for the preparation of Co 3 O 4 sphere (Co 3 O 4 -s).For comparison, a commercial Co 3 O 4 (Co 3 O 4 -com) purchased from Aladdin Co. Ltd was also used.

Table 1 .
The Co content, crystallite size and surface atomic ratios of the Co 3 O 4 catalysts.

Table 1 .
The Co content, crystallite size and surface atomic ratios of the Co3O4 catalysts.

Table 2 .
T50 and T90 values for the catalytic oxidation of vinyl chloride (VC) over the Co3O4 catalysts.

Table 2 .
T 50 and T 90 values for the catalytic oxidation of vinyl chloride (VC) over the Co 3 O 4 catalysts.

Table 2 .
T50 and T90 values for the catalytic oxidation of vinyl chloride (VC) over the Co3O4 catalysts.