The Influence of Residual Sodium on the Catalytic Oxidation of Propane and Toluene Over Co3o4 Catalysts

catalysts

Article The Inﬂuence of Residual Sodium on the Catalytic Oxidation of Propane and Toluene over Co3O4 Catalysts

Guangtao Chai 1,2, Weidong Zhang 1, Yanglong Guo 2,* , Jose Luis Valverde 3 and Anne Giroir-Fendler 1,*

1 Université Claude Bernard Lyon 1, Université de Lyon, CNRS, IRCELYON, 2 Avenue Albert Einstein, F-69622 Villeurbanne, France; [email protected] (G.C.); [email protected] (W.Z.) 2 Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China 3 Department of Chemical Engineering, University of Castilla La Mancha, Avda. Camilo José Cela 12, 13071 Ciudad Real, Spain; [email protected] * Correspondence: [email protected] (Y.G.); [email protected] (A.G.-F.); Tel.: +86-21-64-25-29-23 (Y.G.); +33-472-431-586 (A.G.-F.) Received: 16 July 2020; Accepted: 30 July 2020; Published: 3 August 2020

Abstract: A series of Co3O4 catalysts with different contents of residual sodium were prepared using a precipitation method with sodium carbonate as a precipitant and tested for the catalytic oxidation 1 1 of 1000 ppm propane and toluene at a weight hourly space velocity of 40,000 mL g− h− , respectively. Several techniques were used to characterize the physicochemical properties of the catalysts. Results showed that residual sodium could be partially inserted into the Co3O4 spinel lattice, inducing distortions and helping to increase the specific surface area of the Co3O4 catalysts. Meanwhile, it could negatively affect the reducibility and the oxygen mobility of the catalysts. Moreover, residual sodium had a significant influence on the catalytic activity of propane and toluene oxidation over the synthesized Co3O4 catalysts. The catalyst derived from the precursor washed three times presented the best activity for the catalytic oxidation of propane. The origin was traced to its better reducibility and higher oxygen mobility, which were responsible for the formation of active oxygen species. On the other hand, the catalyst obtained from the precursor washed two times exhibited better performance in toluene oxidation, benefitting from its more defective structure and larger specific surface area. Furthermore, the most active catalysts maintained constant performance in cycling and long-term stability tests of propane and toluene oxidation, being potentially applicable for practical applications.

Keywords: catalytic oxidation; propane; toluene; Co3O4; residual sodium

1. Introduction Volatile organic compounds (VOCs), mainly released from transportation and industrial processes, cause serious environmental pollution, and have toxic effects on human health [1,2]. They contribute to the formation of photochemical smog and further promote the production of ozone. Moreover, the degradation of VOCs is usually difficult and easily produces secondary pollutants, resulting in long-lasting pollution and impacting the ecosystem. Thus, the removal of VOCs is of great importance to protect the environment [3–6]. Over the past decades, research has focused on developing different efficient techniques to degrade VOCs. One of the most promising and economical techniques to remove these pollutants is the catalytic oxidation method, which allows VOC removal from dilute streams at a

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low temperature with high efficiency. With this method, VOCs can be completely converted into CO2 and H2O[7–11]. Noble metal (Pt, Pb, Au, Ru, etc.) dispersed on supports with high specific surface areas have long been studied because of their remarkable performance in VOC oxidation [12–15]. However, low dosages of noble metals are required while maintaining the activity due to their high price and low availability. Moreover, it is important to prevent the agglomeration of metal particles in practical applications to reduce the tendency towards deactivation and prolong the lifetime. Consequently, the use of catalysts based on transition metal (Fe, Co, Ni, Mn, Cu, Ce, etc.) oxides has been explored as an alternative approach [16–20]. 2+ 2 Spinel cobalt oxide, Co3O4, with Co surrounded by a tetrahedral O − coordination sphere and 3+ 2 Co surrounded by octahedral O − environment, has been proven to be one of the most promising and efficient materials for several catalytic reactions due to the advantages of low-temperature reducibility, high bulk oxygen mobility, and facile formation of active surface oxygen species [21–24]. The synthesis process is of great importance to Co3O4 catalysts since it could determine the physicochemical properties, such as the crystallite size, morphology, and reducibility of Co3O4 catalysts, and then affect their catalytic activity. Precipitation is one of the most widely used methods to prepare Co3O4 and other metal oxide catalysts in heterogeneous catalysis, which requires a simple set-up, short time, and can be easily scaled for practical industrial applications [25–28]. However, it should be noted that the complete removal of precipitant is difficult before and even after the calcination of catalyst precursors. Jun and co-workers [29] found that the residual sodium existed in the form of NaNO3 after calcination and it decreased the reducibility of CuO phase in the Cu/ZnO/Al2O3 catalyst. Yu and co-workers [30] revealed that the residual sodium in the catalyst precursor was unfavorable to the catalytic performance and the stability of Cu-CeO2-Al2O3 catalyst. Choya and co-workers [31] synthesized two Co3O4 samples via precipitation with and without residual sodium and found that the surface sodium decreased the Co3+/Co2+ molar ratio and mobility of active lattice oxygen species, leading to an overall negative impact on the catalytic activity of methane combustion. To the best of our knowledge, the influence of residual precipitants on the catalytic activity of Co3O4 catalysts for the oxidation of propane and toluene has not been reported yet. In this work, a series of Co3O4 catalysts were synthesized via a precipitation method using sodium carbonate as the precipitant and labeled as Co-CO3-xT, which will be described in detail in the section on catalyst preparation. The catalysts were investigated in the total oxidation of propane and toluene, respectively. All the catalysts were characterized by Thermogravimetric and Differential Scanning Calorimeter (TG-DSC), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), Raman, N2-sorption, Fourier Transform Infrared (FT-IR), and Carbon Monoxide-Temperature Programmed Reduction (CO-TPR). Propane oxidation without oxygen was evaluated as well. Moreover, cycling and long-term stability tests were carried out on the most active catalysts in the end. The aim of this work was to investigate the influence of residual sodium on the activity of Co3O4 catalysts, as well as the structure-performance relationship of the catalysts.

2. Results and Discussion

2.1. Decomposition of the Catalyst Precursors TG-DSC analyses were carried out to investigate the thermal decomposition of cobalt oxide precursors. Similar weight loss curves were observed for all the precursors, as shown in Figure1a. There was a small weight loss of ca. 2 wt.% below 200 ◦C, which was caused by the removal of physically and chemically bound water. In the temperature range of 200 to 300 ◦C, the curves showed an intense loss of mass (ca. 23 wt.%) together with an endothermic peak at around 255 ◦C due to the thermal decomposition of cobalt hydroxide carbonate in the catalyst precursors [23,32]. The endothermic peak in the DSC curves, as shown in Figure1b, shifted to higher temperatures when increasing the number Catalysts 2020, 10, 867 3 of 15 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 15 increasingof washing the steps. number Moreover, of washing no obvious steps. weightMoreover loss, no occurred obvious above weight 400 loss◦C, occurred suggesting above that 400 500 °C,◦C wassuggesting enough that for 500 the formation°C was enough of thermally for the formation stable Co3 Oof4 .thermally stable Co3O4.

0 20 2% Co-CO3-1T Co-CO3-1T 18 -4 Co-CO3-2T Co-CO3-2T 16 Co-CO3-3T Co-CO3-3T

-8 Co-CO3-5T ) 14 Co-CO3-5T -1 12 -12 23% 10 -16 8 6 Weight loss (%) -20 (mWmg DSC 4 -24 2 0 -28 50 100 150 200 250 300 350 400 450 500 550 50 100 150 200 250 300 350 400 450 500 550 (a) o (b) o Temperature ( C) Temperature ( C) Figure 1. ((a)) TG TG and and ( (b) DSC curves of the catalyst precursors.

2.2. Characterization of the Catalysts The residual residual sodium sodium content content present present in in the the cata catalystslysts was was measured measured by byICP-OES. ICP-OES. The The results results are shownare shown in Table in Table 1. 1This. This value value was was extrem extremelyely high high (2.4 (2.4 wt.%) wt.%) in sample Co-COCo-CO33-1T,-1T, whereas whereas it significantlysigniﬁcantly decreased to 0.33 wt.% after washing the catalyst precursor twice, and further dropped to 0.14 wt.% with one more washing in the sample Co-CO3-3T.-3T. In this sense, the sample Co-CO 3-5T,-5T, washed ﬁvefive times, was preparedprepared inin orderorder toto obtainobtain aa catalystcatalyst freefree ofof sodiumsodium inin itsits structure.structure.

Table 1. Results from physico-chemicalphysico-chemical characterizations.

a b b c 2 1 c c 3 1 Catalyst Na Content (wt.%) d (nm) a (Å) SSA (m g− ) Average Pore Size (nm) Vpore (cm g− ) Na Content a d b a b SSA c (m2 Average Pore Size c Vpore c (cm3 CatalystCo-CO 3-1T 2.40 19.8 8.091 28 12.9 0.085 Co-CO3-2T(wt.%) 0.33 30.8(nm) 8.088(Å) 27g−1) 11.8(nm) 0.081 g−1) Co-CO3-3T 0.14 31.3 8.088 25 10.3 0.064 Co-COCo-CO3-1T3-5T 2.40<0.1 33.0 19.8 8.0888.091 2228 8.8 12.9 0.049 0.085 a Determined by ICP-OES analysis. b Average crystallite size, lattice constant calculated from the XRD patterns. Co-CO3-2T 0.33 30.8 8.088 27 11.8 0.081 c Speciﬁc surface area obtained using Brunauer-Emmet-Teller (BET) method, average pore size, and total pore Co-COvolume3-3T obtained using0.14 Barrett-Joyner-Halenda 31.3 (BJH)8.088 method. 25 10.3 0.064

Co-CO3-5T <0.1 33.0 8.088 22 8.8 0.049 Figure2a displays the wide-angle XRD patterns of the catalysts with di fferent residual sodium a Determined by ICP-OES analysis. b Average crystallite size, lattice constant calculated from the XRD contents. Diffraction peaks at 2θ = 18.9, 31.2, 36.6, 44.9, 59.4, and 69.1◦ were observed and attributed to patterns. c Specific surface area obtained using Brunauer-Emmet-Teller (BET) method, average pore a cubic phase of spinel Co3O4 (PDF # 74-2120). These peaks could be ascribed to the (111), (220), (311), size, and total pore volume obtained using Barrett-Joyner-Halenda (BJH) method. (400), (511) and (440) planes of Co3O4. Like other studies reported [31,33], no phases associated with sodium species were observed in any of the samples, possibly due to the good dispersion and low Figure 2a displays the wide-angle XRD patterns of the catalysts with different residual sodium contents of sodium. However, the diffraction peaks became sharp and intense as the number of washing contents. Diffraction peaks at 2θ = 18.9, 31.2, 36.6, 44.9, 59.4, and 69.1° were observed and attributed steps increased, suggesting the growth of crystallite size, which was confirmed by the results calculated to a cubic phase of spinel Co3O4 (PDF # 74-2120). These peaks could be ascribed to the (111), (220), from the most intense (311) diffraction peak given by the Scherrer equation (Table1). Moreover, it could (311), (400), (511) and (440) planes of Co3O4. Like other studies reported [31,33], no phases associated be seen that the (311) diffraction peak in the XRD pattern of sample Co-CO -1T appeared at a lower with sodium species were observed in any of the samples, possibly due to 3the good dispersion and diffraction angle compared with that of other samples, as shown in Figure2b, indicating that sodium low contents of sodium. However, the diffraction peaks became sharp and intense as the number of may be introduced into the spinel lattice of Co O , inducing the structure distortion and inhibiting washing steps increased, suggesting the growth3 of 4crystallite size, which was confirmed by the results the crystallization of Co O [31]. Taking as a reference the above-mentioned six diffraction peaks, calculated from the most3 intense4 (311) diffraction peak given by the Scherrer equation (Table 1). the lattice constant of each sample was also estimated. This way, it was observed that the values of the Moreover, it could be seen that the (311) diffraction peak in the XRD pattern of sample Co-CO3-1T lattice constant in Co-Co -2T, Co-Co -3T, and Co-Co -5T were identical, while that of Co-Co -1T was appeared at a lower diffraction3 angle3 compared with3 that of other samples, as shown in Figure3 2b, larger, which could be again associated with the presence of sodium into the Co3O4 lattice. indicating that sodium may be introduced into the spinel lattice of Co3O4, inducing the structure distortion and inhibiting the crystallization of Co3O4 [31]. Taking as a reference the above-mentioned six diffraction peaks, the lattice constant of each sample was also estimated. This way, it was observed that the values of the lattice constant in Co-Co3-2T, Co-Co3-3T, and Co-Co3-5T were identical, while

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Catalyststhat of Co-Co2020, 10,3-1T 867 was larger, which could be again associated with the presence of sodium into4 ofthe 15 Co3O4 lattice.

(311)

Co-CO3-5T

Co-CO3-3T Co-CO3-3T Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity (a.u.) Co-CO -2T Co-CO3-2T 3

Co-CO -1T Co-CO3-1T 3

10 20 30 40 50 60 70 80 36 37 38 (b) (a) 2 Theta (°) 2 Theta (°) Figure 2. ((aa)) XRD XRD patterns patterns of of the the catalysts; catalysts; ( (bb)) enlargement enlargement of of the the most most intense intense diffraction diﬀraction peak (311).

The Raman spectraspectra allowallow usus toto get get information information about abou thet the lattice lattice distortion distortion of of the the catalysts catalysts (Figure (Figure3). 1 3).Five Five bands bands at 191, at 191, 474, 474, 514, 609,514, and609, 679and cm 679− werecm−1 observed,were observed, corresponding corresponding to the stretchingto the stretching modes 1 of the spinel Co O [23]. The bands at 474 and 679 cm were−1 attributed to the E and A modes, modes of the spinel3 4 Co3O4 [23]. The bands at 474 and 679 −cm were attributed to the Eg g and A1g1g modes, 1 respectively, whereas the bands at 191, 514, and 609 cm could−1 be assigned to the F modes. A mode respectively, whereas the bands at 191, 514, and 609− cm could be assigned to2g the F2g modes.1g A1g was associated with the octahedral sites, whereas the F and E modes corresponded to the vibration mode was associated with the octahedral sites, whereas2g the Fg2g and Eg modes corresponded to the of tetrahedral and octahedral sites [7,22]. It was found that the Raman bands of Co-CO -1T shifted to vibration of tetrahedral and octahedral sites [7,22]. It was found that the Raman bands3 of Co-CO3-1T shiftedlower frequencies to lower frequencies compared withcompared those ofwith other those catalysts, of other which catalysts, was due which to the was lattice due expansion to the lattice and expansionlattice distortion and lattice caused distortion by the introduction caused by the of sodium.introduction This resultof sodium. agrees This well result with theagrees XRD well analysis. with theMoreover, XRD analysis. the red shiftMoreover, was also the observed red shift in was Co-CO also3 -2T,observed which in suggests Co-CO the3-2T, defective which suggests structure the of defectivethe catalyst. structure of the catalyst.

Co-CO3-5T

Co-CO3-3T Internsity (a.u.) Internsity (a.u.) Co-CO3-2T

Co-CO3-1T

200 300 400 500 600 700 800 -1 Raman shift (cm ) Figure 3. Raman spectra of the catalysts.

Adsorption bands at 3502, 3379, 1544, 1347, 1070, 969, 837, 779, 699, and 656 cm 1 attributable Adsorption bands at 3502, 3379, 1544, 1347, 1070, 969, 837, 779, 699, and 656 cm−1 −attributable to to the stretching vibration modes of the cobalt hydroxide carbonate structure were observed in the the stretching vibration modes of the cobalt hydroxide carbonate structure were observed in the FT- FT-IR spectra of the catalyst precursors, as shown in Figure S1 [34,35]. The FT-IR adsorption spectra IR spectra of the catalyst precursors, as shown in Figure S1 [34,35]. The FT-IR adsorption spectra of of the catalysts are shown in Figure4a. Bands corresponding to the catalysts mainly occurred at the catalysts are shown in Figure 4a. Bands corresponding to the catalysts mainly occurred at 656 and 1 656 and 546 cm− , which was associated with the stretching vibration of Co–O in the Co3O4 spinel 546 cm–1, which was associated with the stretching vibration of Co–O in the Co3O4 spinel lattice. The lattice. The former could be assigned to the stretching vibration mode of Co2+–O from the tetrahedral

Catalysts 20202020,, 10,, 867x FOR PEER REVIEW 5 of 15 Catalysts 2020, 10, x FOR PEER REVIEW 5 of 15 former could be assigned to the stretching vibration mode of Co2+–O from the tetrahedral sites, sites,former whereas could be the assigned latter was to the attributed stretching to thevibration stretching mode vibration of Co2+–O of Cofrom3+–O the in tetrahedral the octahedral sites, whereas the latter was attributed to the stretching vibration of Co3+–O in the octahedral coordination coordinationwhereas the latter [32]. was One attributed can see into the Figure stretching4b that vibration the stretching of Co3+ vibration–O in the bandsoctahedral of octahedrally coordination [32]. One can see in Figure 4b that the stretching vibration bands of octahedrally coordinated Co3+ coordinated[32]. One can Co see3+ and in Figure tetrahedrally 4b that coordinated the stretchi Cong2 +vibrationshifted tobands lower of wavenumbers octahedrally whencoordinated the number Co3+ and tetrahedrally coordinated Co2+ shifted to lower wavenumbers when the number of washing steps ofand washing tetrahedrally steps of coordinated the precursor Co was2+ shifted more thanto lower one, wavenumbers which wouldindicate when the the number decrease of inwashing the strength steps of the precursor was more than one, which would indicate the decrease in the strength of the Co–O ofof thethe Co–Oprecursor bond was [36 ].more In addition, than one, some which additional would in infrareddicate the adsorption decrease bandsin the atstrength 836 and of 1356 the Co–O cm 1 bond [36]. In addition, some additional infrared adsorption bands at 836 and 1356 cm–1 were clearly− –1 werebond clearly [36]. In observed addition, in some the sample additional Co-CO infrared3-1T in adsorption Figure4c, which bands could at 836 be and assigned 1356 cm to the were stretching clearly observed in the sample Co-CO3-1T in Figure 4c, which could be assigned to the stretching vibration vibrationobserved ofin thethe undecomposedsample Co-CO3 nitrate-1T in Figure groups 4c, [37 which]. could be assigned to the stretching vibration of the undecomposed nitrate groups [37]. of the undecomposed nitrate groups [37].

Co-CO -5T Co-CO -5T 3 3 Co-CO3-5T Co-CO3-5T Co-CO3-5T Co-CO3-5T Co-CO3-3T Co-CO3-3T Co-CO3-3T Co-CO -3T Co-CO3-3T 3 Co-CO -2T Co-CO3-3T Co-CO3-2T Co-CO3-2T Co-CO3-2T Intensity (a.u.) Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity Co-CO -2T Co-CO3-2T 3 Intensity (a.u.) Intensity (a.u.) Co-CO3-2T Intensity (a.u.) Co-CO -1T Co-CO -1T 3 3 Co-CO -1T Co-CO -1T 3 Co-CO3-1T 3 Co-CO3-1T 4000 3500 3000 2500 2000 1500 1000 500 700 650 600 550 500 450 400 1800 1600 1400 1200 1000 800 4000 3500 3000 2500 2000 1500 1000 500 700 650 600 550 500 450 400 1800 1600 1400 1200 1000 800 (a) Wavenumber (cm−1) Wavenumber (cm−1) (c) Wavenumber (cm−1) -1 (b) -1 -1 (a) Wavenumber (cm ) (b) Wavenumber (cm ) (c) Wavenumber (cm ) Figure 4. (a) Full and (b,c) partial FT-IR spectra of the catalysts. FigureFigure 4.4. ((aa)) FullFull andand ((bb,,cc)) partialpartial FT-IRFT-IR spectraspectra ofof thethe catalysts.catalysts.

FigureFigure 55 showsshows the N N2-2sorption-sorption isotherms isotherms and and pore pore size size distribution distribution curves curves of the of thecatalysts. catalysts. All Figure 5 shows the N2-sorption isotherms and pore size distribution curves of the catalysts. All Allcatalysts catalysts exhibited exhibited type type IV IVisotherms isotherms with with H3 H3 hysteresis hysteresis loops loops according according to to the the International International Union catalysts exhibited type IV isotherms with H3 hysteresis loops according to the International Union of Pure and Applied Chemistry (IUPAC)(IUPAC) classiﬁcation,classification, beingbeing mesoporousmesoporous in structure.structure. The speciﬁcspecific of Pure and Applied Chemistry (IUPAC) classification, being mesoporous in structure. The specific surface areaarea (SSA),(SSA), porepore volumevolume (V (Vporepore),), andand averageaverage porepore size size of of the the catalysts catalysts are are a a function function of of (Table (Table1) surface area (SSA), pore volume (Vpore), and average pore size of the catalysts are a function of (Table the1) the content content of of residual residual sodium. sodium. Co-CO Co-CO3-1T3-1T showed showed the the largest largest SSA SSA and and V Vporepore,, asas wellwell asas thethe average 1) the content of residual sodium. Co-CO3-1T showed the largest SSA and Vpore, as well as the average pore size,size, whichwhich couldcould bebe ascribedascribed to to the the enhancement enhancement of of textural textural properties properties caused caused by by the the distortion distortion in pore size, which could be ascribed to the enhancement of textural properties caused by the distortion thein the spinel spinel lattice lattice of Co of3 CoO43.O The4. The lower lower the the residual residual sodium, sodium, the lowerthe lower the values the values of SSA of andSSA V andpore bothVpore in the spinel lattice of Co3O4. The lower the residual sodium, the lower the values of SSA and Vpore wereboth observed,were observed, suggesting suggesting that the that Co 3theO4 particlesCo3O4 particles could easily could sinter easily during sinter the during thermal the treatment thermal both were observed, suggesting that the Co3O4 particles could easily sinter during the thermal withtreatment less residualwith less sodium. residual Regarding sodium. Regarding the pore size the distributionpore size distribution of the catalysts, of the it catalysts, was found it thatwas treatment with less residual sodium. Regarding the pore size distribution of the catalysts, it was allfound samples that all exhibited samples a exhibited similar pore a similar sizeof pore ca. size 2.5 nm.of ca. However, 2.5 nm. However, sample Co-CO sample3-1T Co had-CO3 a-1T broader had a found that all samples exhibited a similar pore size of ca. 2.5 nm. However, sample Co-CO3-1T had a porebroader size pore distribution size distribution than other than samples, other which samples, might which be attributed might be to attributed the presence to the of accumulated presence of broader pore size distribution than other samples, which might be attributed to the presence of holesaccumulated between holes particles between by nano-Co particles3O 4bycrystallite nano-Co bridges3O4 crystallite [36], which bridges could, [36], in turn,which contribute could, in toturn the, accumulated holes between particles by nano-Co3O4 crystallite bridges [36], which could, in turn, increasecontribute of to the the pore increase volume. of the pore volume. contribute to the increase of the pore volume.

Co-Co -1T 12 Co-CO -5T 3 12 3 Co-Co3-1T Co-CO3-5T 50 Co-Co3-2T Co-CO3-3T , STP) , STP) 50 Co-Co3-2T Co-CO3-3T Co-Co3-3T 10 Co-CO3-2T −1 −1 , STP), , a.u.) , a.u.) 10 Co-Co3-3T Co-CO3-2T g g -1 Co-Co -5T Co-CO -1T −1 −1

3 , a.u.) 3 3 3 40

g Co-Co -5T Co-CO -1T 40 3 -1 3 3 8 nm nm 8 −1 −1 30 nm g g -1 3 30 3 6

g 6 3 20 20 4 4

10 2 dV/dD (mm 10 dV/dD (mm 2 dV/dD(mm Quantity adsorbed (cm adsorbed Quantity Quantity adsorbed (cm adsorbed Quantity

Quantity adsorbed (cm 0 0 00.0 0.2 0.4 0.6 0.8 1.0 0 0 10 20 30 40 50 0.0 0.2 0.4 0.6 0.8 1.0 0 1020304050 Relative pressure (p/p ) Pore Diameter (nm) (a) Relative pressure (p/p0 ) (b) (a) 0 (b) Pore Diameter (nm) Figure 5. (a) N2-sorption isotherms and (b) pore size distribution curves of the catalysts. FigureFigure 5.5. ((aa)N) N22-sorption isothermsisotherms andand ((b)) porepore sizesize distributiondistribution curvescurves ofof thethe catalysts.catalysts. The CO-TPR experiment allows us to evaluate the reducibility of a catalyst. The evolutions of The CO-TPR experiment allows us to evaluate the reducibility of a catalyst. The evolutions of CO2 concentration at the device outlet during the reduction tests are shown in Figure 6 and those of CO2 concentration at the device outlet during the reduction tests are shown in Figure 6 and those of

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Catalysts 2020, 10, x FOR PEER REVIEW 6 of 15 The CO-TPR experiment allows us to evaluate the reducibility of a catalyst. The evolutions of CO 2consumptionconcentration are at thedisplayed device in outlet Figure during S2. The the reductionconsumption tests of are CO shown matches in Figureperfectly6 and with those the of CO consumption are displayed in Figure S2. The consumption of CO matches perfectly with the formation of CO2, indicating that all the consumed CO molecules were oxidized into CO2 by depletion formationof the oxygen of CO species2, indicating in the catalysts. that all the As consumed reported COin Reference molecules [38], were the oxidized reduction into process CO2 by started depletion for ofall thesamples oxygen from species around in the 200 catalysts. °C and was As reported completed in Referencebelow 600 [ 38°C],, thewith reduction two main process reduction started peaks for all samples from around 200 C and was completed below 600 C, with two main reduction peaks observed. The former one in the◦ range of 200 to 350 °C was associated◦ with the reduction of Co3O4 to observed.CoO; the latter The formerone detected one in thebetween range 350 of 200°C toand 350 550◦C °C was was associated assigned with to the the reduction reduction of of CoO Co3O to4 to CoO; the latter one detected between 350 C and 550 C was assigned to the reduction of CoO to metallic Co. From the profiles of CO2 concentration,◦ one◦ could see that the samples Co-CO3-3T and metallic Co. From the profiles of CO concentration, one could see that the samples Co-CO -3T and Co-CO3-5T were the most reducible ones2 with the lowest initial reduction temperature, followed3 by Co-CO -5T were the most reducible ones with the lowest initial reduction temperature, followed by the sample3 Co-CO3-2T. Catalyst Co-CO3-1T showed the highest initial reduction temperature, which the sample Co-CO -2T. Catalyst Co-CO -1T showed the highest initial reduction temperature, which could probably be 3explained by the negative3 impact of residual sodium on the reducibility of Co3O4. couldIt was probablyreported that be explained the reactivity by the of negativesurface oxygen impact species of residual and sodiumthe mobility on the of reducibilitylattice oxygen of played Co3O4. Ita remarkable was reported role that in the the reactivity oxidation of performance surface oxygen of a species catalyst and [38,39 the], mobility which would of lattice be oxygenreflected played by the a remarkablereducibility roleof the in thecatalysts. oxidation Thus, performance the shift of of the a catalyst reduction [38 ,peak39], which to a lower would temperature be reflected would by the reducibility of the catalysts. Thus, the shift of the reduction peak to a lower temperature would indicate indicate the presence of more active oxygen species on catalysts Co-CO3-3T and Co-CO3-5T. It should the presence of more active oxygen species on catalysts Co-CO -3T and Co-CO -5T. It should be pointed be pointed out that a small peak of CO2 production appeared3 at 235 °C for sample3 Co-CO3-1T, which out that a small peak of CO production appeared at 235 C for sample Co-CO -1T, which could be could be attributed to the reduction2 of surface NO3−, as◦ confirmed by the aforementioned3 FT-IR attributedanalysis. to the reduction of surface NO3−, as confirmed by the aforementioned FT-IR analysis.

3000 Co-CO3-1T Co-CO -2T 2500 3 Co-CO3-3T

Co-CO3-5T 2000

1500

1000 concentration (ppm, a.u.) (ppm, concentration concentration (ppm, a.u.) (ppm, concentration 2 2 500 CO CO

0 100 200 300 400 500 600

Temperature (°C)

Figure 6. The e evolutionvolution of CO2 concentrationconcentration during the CO CO-TPR-TPR tests of the catalysts.

2.3. Cat Catalyticalytic Activity Propane oxidation was chosen as the model reaction for short short-chain-chain alkanes. The light light-o-offﬀ curves of thethe third third cooling cooling run run over over the catalysts the catalysts are plotted are inplotted Figure 7ina. TheFigure T 50 7anda. The T90 valuesT50 and (temperatures T90 values at(temperatures conversions ofat 50%conversions and 90%, of respectively) 50% and 90%, in respe propanectively) oxidation in propane are summarized oxidation are in Table summarized2. Catalyst in Co-COTable 2.3 -3TCatalyst exhibited Co-CO the3- best3T exhibited performance the best for propaneperformance oxidation for propane with T 50oxidationand T90 withof 201 T50 and and 236 T90◦ ofC, respectively,201 and 236 which°C, respectively, could be attributed which could to its be highest attributed reducibility. to its highest This catalyst reducibility. was followed This catalyst by catalyst was Co-COfollowed3-5T, by withcatalyst similar Co- activityCO3-5T, inwith the similar high conversion activity in range the high but aconversion little worse range performance but a little when worse the reactionperformance temperature when the was reaction lower thantemperature 230 ◦C, probably was lower due than to the 230 small °C, SSAprobably compared due to with the Co-COsmall 3SSA-3T. Oncompared the other with hand, Co- catalystCO3-3T. Co-COOn the3 other-2T with hand, lower catalyst reducibility Co-CO3 presented-2T with lower lower reducibility catalytic performance presented (thelower T90 catalyticvalue was performance 22 ◦C higher (the than T90 value that of was catalyst 22 °C Co-COhigher3 than-3T). that Finally, of catalyst catalyst Co Co-CO-CO3-33T).-1T showedFinally, nearlycatalyst no Co catalytic-CO3-1T activity showed in nearly the given no catalytic temperature, activity which in the evidenced given temperature, the strong inhibiting which evidenced eﬀect of excessthe strong residual inhibiting sodium effect [31 ].of excess residual sodium [31]. The CO and propene concentrations at the reactor exit were in-situ recorded during the whole reaction process, as shown in Figure 7b,c to get a better understanding of the selectivity of the samples. It was found that CO was not produced with any of the catalysts tested, while traces of propene were detected in the temperature range of 100–200 °C for all catalysts except Co-CO3-1T (inactive), suggesting propene was an intermediate product of propane oxidation over the Co3O4

Catalysts 2020, 10, x FOR PEER REVIEW 7 of 15

Catalystscatalysts.2020 No, 10, obvious 867 difference in propene production was observed, demonstrating the superior7 of 15 selectivity of Co3O4 catalysts for propane oxidation.

100 Co-CO3-1T 14 Co-CO3-1T Co-CO3-2T Co-CO3-2T (%) (%) 2 2 12 80 Co-CO3-3T 12 Co-CO3-3T

Co-CO3-5T Co-CO3-5T 10 60 8

40 6

4 20 CO concentration (ppm) concentration CO CO concentration (ppm) concentration CO 2 Propane conversion to CO to conversion Propane Propane conversion to CO to conversion Propane 0 0 100 150 200 250 300 350 100 150 200 250 300 350 (a) Temperature (°C) (b) Temperature (oC)

14 Co-CO3-1T

Co-CO3-2T 12 Co-CO3-3T Co-CO -5T 10 Co-CO3-5T

6 concentration (ppm) concentration concentration (ppm) concentration 6 6 4 H H 3 3 C C 2

0 100 150 200 250 300 350 o (c) Temperature (oC)

2 FigureFigure 7.7. ((aa)) PropanePropane conversionconversion toto COCO2,(, (bb)) COCO concentration,concentration, andand ((cc)) propenepropene concentrationconcentration asas aa functionfunction ofof the the reaction reaction temperature temperature in in the the oxidation oxidation of of propane propane over over the the catalysts. catalysts.

TableTable 2.2.Catalytic Catalytic performanceperformance andand kinetickinetic analysisanalysis ofof thethe catalystscatalysts forfor propanepropane oxidation. oxidation.

Catalyst T ( C) T ( C) r 108 (mol g 1 s 1) TOF 109 (mol m 2 s 1) E (kJ mol 1) lnA 50T◦50 90T90◦ − − − − a Ea (kJ− Catalyst r × 108 (mol g−1 s−1) TOF× × 109 (mol m−2 s−1) lnA Co-CO3-1T (°C)// (°C) 0.3 0.1 mol//−1) Co-CO3-2T 211 258 6.6 2.4 80 4.8 Co-CO3- Co-CO3-3T 201/ 236/ 9.90.3 3.90.1 76/ 4.1/ Co-CO1T3-5T 206 238 6.9 3.1 92 8.2 Co-CO3- 211 258 6.6 2.4 80 4.8 The2T CO and propene concentrations at the reactor exit were in-situ recorded during the whole Co-CO3- reaction process, as201 shown in236 Figure 7b,c to get9.9 a better understanding3.9 of the selectivity7 of6 the samples.4.1 It was3T found that CO was not produced with any of the catalysts tested, while traces of propene Co-CO3- were detected in2 the06 temperature238 range of6 100–200.9 ◦C for all catalysts3.1 except Co-CO932-1T (inactive),8.2 5T suggesting propene was an intermediate product of propane oxidation over the Co3O4 catalysts. No obviousThe propane diﬀerence oxidation in propene reaction production rates and was TOF observed, were calculated demonstrating at 175 the °C superior, and the selectivity results are of Co O catalysts for propane oxidation. listed3 4 in Table 2. Consequently, catalyst Co-CO3-3T exhibited the largest reaction rate (9.9 × 10−8 mol g−1 sThe−1) and propane the largest oxidation TOF reactionvalue (3.9 rates × 10 and−9 mol TOF m were−2 s−1). calculated Apparent at activation 175 ◦C, and energies the results and areapparent listed in Table2. Consequently, catalyst Co-CO -3T exhibited the largest reaction rate (9.9 10 8 mol g 1 s 1) pre-exponential factors were obtained 3from the Arrhenius plots, as shown in ×Figure− 8a, and− −the and the largest TOF value (3.9 10 9 mol m 2 s 1). Apparent activation energies and apparent results are also displayed in Table× 2.− The pre-−exponential− factors followed the same trend as the Ea pre-exponential factors were obtained from the Arrhenius plots, as shown in Figure8a, and the results did. Catalyst Co-CO3-3T presented the lowest values of Ea (76 kJ mol−1), which was consistent with its arebest also catalytic displayed performance in Table for2. Thepropane pre-exponential oxidation. factors followed the same trend as the E a did. 1 Catalyst Co-CO3-3T presented the lowest values of Ea (76 kJ mol− ), which was consistent with its best catalytic performance for propane oxidation.

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Co-CO3-2T Co-CO3-1T -17.0 -17.5 Co-CO -2T-3T Co-COCo-CO3-1T-2T -17.0 3 33 Co-CO -5T -17.5 Co-CO -3T Co-CO3-3T Co-CO33-2T -17.5 Co-CO -5T Co-CO3-5T -18.0 Co-CO33-3T -17.5 Co-CO -5T -18.0 3 -18.0

lnr -18.5 lnr -18.0 lnr -18.5 lnr -18.5 lnr -18.5 -19.0 -19.0 -19.0 -19.0 -19.5 -19.5 -19.5 0.27 0.28 0.29 0.30 0.31 -19.5 0.235 0.240 0.245 0.250 0.255 0.260 0.265 (a) 0.27 0.28 0.29 0.30 0.31 (b) 0.235 0.240 0.245 0.250 0.255 0.260 0.265 1000/RT (b) 1000/RT (a) 1000/RT (b) 1000/RT Figure 8. Arrhenius plots of lnr vs. 1000/RT for (a) propane oxidation and (b) toluene oxidation. Figure 8. ArrheniusArrhenius plots of lnr vs. 1000/RT 1000/RT for ( a)) propane propane oxidation and ( b) toluene oxidation. Propane oxidation without oxygen was carried out to examine the mobility of the oxygen species Propane oxidation without oxygen was carried out to examine the mobility of the oxygen species in thePropane catalysts. oxidation Figure 9 without shows the oxygen evolution was carried of CO2 out concentration to examine atthe the mobility reactor of exit the during oxygen the species tests. in the catalysts. Figure9 shows the evolution of CO 2 concentration at the reactor exit during the tests. inOne the could catalysts. see that Figure propane 9 shows could the evolutionbe oxidized of intoCO2 COconcentration2 without the at theinvolvement reactor exit of during gaseous the O tests.2 but One could see that propane could be oxidized into CO2 without the involvement of gaseous O2 but Onewith couldthe help see of that oxygen propane species could in bethe oxidized catalysts. into The CO CO2 2without production the sinvolvement over catalysts of Cogaseous-CO3- 3TO2 andbut with the help of oxygen species in the catalysts. The CO productions over catalysts Co-CO -3T and withCo-CO the3- 5helpT were of oxygenalmost thespecies same in and the werecatalysts. much The higher CO22 thanproductions that over over catalyst catalysts Co-CO Co-CO3-2T, 33wher-3T andeas Co-CO -5T were almost the same and were much higher than that over catalyst Co-CO -2T, whereas Co-COnearly 3no-5T CO were2 was almost produced the same with and catalyst were muchCo-CO higher3-1T, which than that agreed over well catalyst with Co-CO the aforementioned3-2T, whereas nearly no CO was produced with catalyst Co-CO -1T, which agreed well with the aforementioned nearlycatalytic no activity CO2 was trend. produced Moreover, with the catalyst surface Co-CO lattice3-1T, oxygen which rather agreed than well the withbulk latticethe aforementioned oxygen in the catalytic activity trend. Moreover, the surface lattice oxygen rather than the bulk lattice oxygen in catalyticcatalysts activityshould trend. be responsible Moreover, thefor surfacethe CO lattice2 evolution oxygen considering rather than thethe bulk relatively lattice oxygenlow reaction in the the catalysts should be responsible for the CO evolution considering the relatively low reaction catalyststemperature. should The be higher responsible CO2 production for the CO over2 2evolution catalysts considering Co-CO3-3T the and relatively Co-CO3 -5Tlow couldreaction be temperature. The higher CO production over catalysts Co-CO -3T and Co-CO -5T could be attributed temperature.attributed to theThe better higher mobility CO2 2 productionof the surface over lattice catalysts oxygen Co-CO 3on them,3-3T which and3 inCo-CO turn 3contributed-5T could beto attributedtotheir the excellent better to mobility the performance better of mobility the surface in propane of latticethe surface oxidation. oxygen lattice on them,Overall, oxygen which theon inthem,mobility turn which contributed of lattice in turn tooxygen contributed their excellentand the to performancetheirreducibility excellent of in performancethe propane samples oxidation. playedin propane Overall,a remarkable oxidation. the mobility role Overall, in ofthe lattice the catalytic mobility oxygen performance andof lattice the reducibility oxygenof the catalystsand of the reducibilitysamplestested. played of the a remarkable samples played role in a the remarkable catalytic performance role in the catalytic of the catalysts performance tested. of the catalysts tested. 70 Co-CO -1T 350 70 3 Co-CO -2T 350 Co-CO3-1T-2T 60 Co-CO -3T Co-CO3-2T-3T 300 60 Co-CO -5T Co-CO3-3T 300 50 Co-CO3-5T 50 250 ) 250 40 癈 40 200 30 200 30 150 20

concentration (ppm) concentration 20 concentration (ppm) concentration 150 Temperature (°C) Temperature (°C) 2 2 20 concentration(ppm)

100 ( Temperature 2 CO CO 10 100

CO 10 50 0 50 0 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Time (min) Time (min) Figure 9. Evolution of CO 2 concentrationconcentration in in propane propane oxidation without oxygen over the catalysts. Figure 9. Evolution of CO2 concentration in propane oxidation without oxygen over the catalysts. Toluene waswas selectedselected asas thethe typicaltypical aromaticaromatic hydrocarbonhydrocarbon forfor catalyticcatalytic activityactivity tests.tests. Figure 10 showsToluene the the light light-o was-off ﬀselected curvescurves oas off the the the catalysts typical catalysts aromatic in in the the third thirdhydr coolingocarbon cooling run. for run. The catalytic The values values activity of T50 of and Ttests.50 Tand90 Figureare T 90listedare 10 showslistedin Table inthe Table3. light-off Different3. Di curvesﬀ erentfrom of fromthat the observedcatalysts that observed inin the the inthird propane the cooling propane oxidation, run. oxidation, The the values catalytic the of catalytic T50 performance and performanceT90 are listed was wasinranked Table ranked as 3. follows: Different as follows: Co from-CO Co-CO3 -that2T 3>-2T observedCo>-COCo-CO3-3T in 3> -3Tth Coe >-propaneCOCo-CO3-5T >3 oxidation,-5T Co->COCo-CO3-1T. the 3The-1T. catalytic most The mostactiv performance activee catalyst, catalyst, wasCo- rankedCo-COCO3-2T,3 as-2T, exhibited follows: exhibited excellentCo-CO excellent3-2T performance > performance Co-CO3-3T for > fortoluene Co-CO toluene oxidation3-5T oxidation > Co-CO with with3-1T. T50 TandThe50 and Tmost90 values T90 activevalues of catalyst, 232 of and 232 andCo-248 CO248°C, 3respectively,◦-2T,C, respectively, exhibited although excellent although it performa possessed it possessednce worse for worse toluene reducibility reducibility oxidation than than with catalysts catalysts T50 and Co Co-CO T-CO90 values33-3T-3T and and of 232Co Co-CO- andCO33 -248-5T.5T. °C,This respectively, fact would indicate although that it possessedthe reducibility worse was reducibility not the key than factor catalysts determining Co-CO3 the-3T catalyticand Co-CO activity3-5T. This fact would indicate that the reducibility was not the key factor determining the catalytic activity

Catalysts 2020, 10, 867 9 of 15

Catalysts 2020, 10, x FOR PEER REVIEW 9 of 15 This fact would indicate that the reducibility was not the key factor determining the catalytic activity ofof didifferentﬀerent catalysts catalysts in in the the toluene toluene oxidation. oxidation. Interestingly, Interestingly, catalyst catalyst Co-CO Co3-1T,-CO with3-1T, nearly with nonearly activity no inactivity the given in the temperature given temperature range for range propane for oxidation,propane oxidation, yielded a yielded toluene a conversion toluene conversion of 68% at 350of 68%◦C, suggestingat 350 °C, suggesting that the inhibiting that the einhibitingﬀect of residual effect of sodium residual in Cosodium3O4 was in Co less3O severe4 was less on toluenesevere on oxidation toluene thanoxidation on propane than on oxidation. propane oxidation. The largest The SSA largest and SSA highly and defective highly defective structure structure of catalyst of catalyst Co-CO3 -2T,Co- whichCO3-2T, were which related were to related more active to more sites active and moresites and surface more adsorbed surface oxygenadsorbed species oxygen [40 species], were [40], supposed were tosupposed be responsible to be responsible for its high for activity its high in tolueneactivity oxidation.in toluene oxidation.

100 Co-CO3-1T Co-CO -2T (%) (%) 3 2 2 80 Co-CO3-3T

Co-CO3-5T 60

20 Toluene conversion to CO to conversion Toluene Toluene conversion to CO to conversion Toluene 0 150 200 250 300 350

Temperature (°C) Figure 10.10. Light-oLight-offﬀ curves ofof thethe catalystscatalysts inin thethe oxidationoxidation ofof toluene.toluene.

Table 3. Catalytic performance and kinetic analysis of the catalysts for toluene oxidation. The toluene oxidation reaction rates and TOF calculated at 213 °C for all catalysts are listed in 8 1 1 9 2 1 1 TableCatalyst 3. Catalyst T50 Co( C)-CO3- T2T90 (withC) ther best10 (mol performance g s ) TOF in toluene10 (mol oxidation m s ) showedE (kJ molthe largest) lnAvalue ◦ ◦ × − − × − − a − −8 −1 −1 −9 −2 −1 ofCo-CO reaction3-1T rate (10.0 309 × 10 mol/ g s ) and 0.3the largest TOF value 0.1(3.7 × 10 mol m s 151). The Arrhenius 18.0 plotsCo-CO were3-2T drawn 232 for all catalysts, 248 as shown 10.0 in Figure 8b. The 3.7 apparent activation 160 energies 22.8 and Co-CO -3T 246 264 3.0 1.2 189 29.1 apparent3 pre-exponential factors obtained from the slopes and intercepts of the lines are also listed Co-CO3-5T 255 276 0.5 0.3 198 29.9 in Table 3. It could be observed that, apart from catalyst Co-CO3-1T, catalyst Co-CO3-2T presented the lowest values of Ea (160 kJ mol−1), agreeing well with its performance in toluene oxidation. The toluene oxidation reaction rates and TOF calculated at 213 ◦C for all catalysts are listed Catalyst Co-CO3-1T displayed the lowest Ea but the excess residual sodium on the surface limited its in Table3. Catalyst Co-CO -2T with the best performance in toluene oxidation showed the largest activity towards toluene oxidation.3 value of reaction rate (10.0 10 8 mol g 1 s 1) and the largest TOF value (3.7 10 9 mol m 2 s 1). × − − − × − − − The Arrhenius plots were drawn for all catalysts, as shown in Figure8b. The apparent activation Table 3. Catalytic performance and kinetic analysis of the catalysts for toluene oxidation. energies and apparent pre-exponential factors obtained from the slopes and intercepts of the lines are also listed in TableT350. It couldT90 be observed that, apart from catalyst Co-CO -1T, catalystEa (kJ Co-CO -2T Catalyst r × 108 (mol g−1 s−1) TOF × 109 (mol m−2 s3−1) lnA3 1 −1 presented the lowest(°C) values(°C) of Ea (160 kJ mol− ), agreeing well with its performancemol ) in toluene oxidation.Co-CO3-1T Catalyst 309 Co-CO 3-1T/ displayed the0.3 lowest Ea but the excess0.1 residual sodium151 on the1 surface8.0 limitedCo-CO its3-2T activity 232 towards 2 toluene48 oxidation.10.0 3.7 160 22.8 Co-CO3-3T 246 264 3.0 1.2 189 29.1 2.4.Co Stability-CO3-5T Test 255 276 0.5 0.3 198 29.9 Three consecutive heating-cooling catalytic cycles were conducted to evaluate the cycling stability of2.4. the Stability most active Test catalysts for propane and toluene oxidation. Figure S4 shows the three-cycle activity curvesThree over consecutive the most active heating catalysts,-cooling namely catalytic Co-CO cycles3-3T forwere propane conducted oxidation to evaluate and Co-CO the 3cycling-2T for toluenestability oxidation.of the most It active can be catalysts seen that for the propane three curves and toluene were almost oxidation. the same Figure for S4 both shows propane the three and- toluenecycle activity oxidation, curves suggesting over the most the good active cycle-stability catalysts, namely of these Co- catalysts.CO3-3T for propane oxidation and Co- CO3-On2T for the toluene other hand, oxidation. long-term It can durability be seen th testsat the over three the curves most active were catalystsalmost the for same propane for both and toluenepropane oxidation and toluene were oxidation, also carried suggesting out (Figure the good 11). Regardingcycle-stability the of propane these catalysts. oxidation, the test was performedOn the as other follows: hand, the long reactor-term was durability heated from tests room over temperaturethe most active to 350 catalysts◦C and for held propane at the latter and toluene oxidation were also carried out (Figure 11). Regarding the propane oxidation, the test was performed as follows: the reactor was heated from room temperature to 350 °C and held at the latter temperature for 1 h, then the temperature was decreased and controlled at 223 °C for 13.5 h and 193

Catalysts 2020, 10, 867 10 of 15

Catalysts 2020, 10, x FOR PEER REVIEW 10 of 15 temperature for 1 h, then the temperature was decreased and controlled at 223 ◦C for 13.5 h and 193°C for◦C another for another 13.5 13.5h. It h.was It wasfound found that thatthe conversion the conversion remained remained quite quite stable stable at both at bothhigh highand andlow lowconversion conversion stages, stages, demonstrating demonstrating the the good good long-term long-term stability stability of of catalyst catalyst Co-CO Co-CO33-3T-3T for for propane oxidation. Regarding the toluene oxidation, the test was conducted by following the same procedure but with the temperature set at 245 ◦°CC for 24 h and 223 ◦°CC for 24 h, respectively. It was observed that the activity couldcould bebe keptkept atat the the high high conversion conversion stage, stage, whereas whereas there there was was an an obvious obvious loss loss of of activity activity at theat the low low conversion conversion stage. stage. Considering Considering the the hysteresis hysteresis phenomenon phenomenon between between thethe heatingheating andand cooling processes, wherewhere thethe hydrocarbonhydrocarbon was was completely completely converted converted into into CO CO2 in2 in the the heating heating run, run, as as shown shown in Figurein Figure S3, S3, the the origin origin of of the the activity activity loss loss at at the the low low conversion conversion stage stage could could be be related related to to the the formation formation of theof the carbonaceous carbonaceous species species on theon the surface surface of the of the catalyst catalyst preventing preventing the oxidationthe oxidation of toluene, of toluene, which which was alsowas observedalso observed by other by other researchers researchers [23,41 [23,41].].

100 100 T = 223 oC T = 245 oC 90 90 T = 193 oC T = 223 oC (%) (%)

2 80 2 80 70 70 60 60 50 50 40 40 30 30 20 20

Propane conversion to CO 10 CO to conversion Toluene 10 0 0 0 3 6 9 121518212427 0 10203040 (a) Time (h) (b) Time (h) Figure 11. The influence influence of time on stream ( a) propane and ( b) toluene oxidation over the most active catalysts (cooling ramp). 2.5. Discussion 2.5. Discussion In this study, it was proved that both the physicochemical properties of the synthesized Co-CO3-xT In this study, it was proved that both the physicochemical properties of the synthesized Co-CO3- catalysts and their catalytic activities for propane and toluene oxidation were influenced by the residual xT catalysts and their catalytic activities for propane and toluene oxidation were influenced by the sodium in the catalysts. The sodium could be inserted into the Co3O4 spinel lattice, inducing defective residual sodium in the catalysts. The sodium could be inserted into the Co3O4 spinel lattice, inducing structures and leading to a smaller crystallite size and larger specific surface area of the Co O catalysts. defective structures and leading to a smaller crystallite size and larger specific surface3 area4 of the However, the residual sodium could meanwhile have a negative influence on the reducibility and Co3O4 catalysts. However, the residual sodium could meanwhile have a negative influence on the oxygen mobility of the catalysts. As a result, the residual sodium significantly affected the catalytic reducibility and oxygen mobility of the catalysts. As a result, the residual sodium significantly performance of the Co3O4 catalysts. affected the catalytic performance of the Co3O4 catalysts. For propane oxidation, catalyst Co-CO3-3T presented the highest activity, with T90 of 236 ◦C For propane oxidation, catalyst Co-CO3-3T presented1 the1 highest activity, with T90 of 236 °C (weight hourly space velocity (WHSV) = 40,000 mL g− h− ). However, catalyst Co-CO3-2T with (weight hourly space velocity (WHSV) = 40,000 mL g−1 h−1). However, catalyst Co-CO3-2T with a little a little more residual sodium performed much worse (T90 = 258 ◦C), and catalyst Co-CO3-1T with more residual sodium performed much worse (T90 = 258 °C), and catalyst Co-CO3-1T with high high sodium content displayed nearly no activity, which indicated that the residual sodium could sodium content displayed nearly no activity, which indicated that the residual sodium could be be destructive to the catalytic activity of the Co3O4 catalysts for propane oxidation. Furthermore, destructive to the catalytic activity of the Co3O4 catalysts for propane oxidation. Furthermore, the the relatively worse activity of Co-CO3-5T than that of Co-CO3-3T suggested that a proper number relatively worse activity of Co-CO3-5T than that of Co-CO3-3T suggested that a proper number of of washing steps of the precipitate during the catalyst preparation was important to maintain good washing steps of the precipitate during the catalyst preparation was important to maintain good textural properties as well as considerable catalytic activity of the Co3O4 catalysts. The better activity textural properties as well as considerable catalytic activity of the Co3O4 catalysts. The better activity of catalyst Co-CO3-3T was determined by its better reducibility and higher oxygen mobility, which of catalyst Co-CO3-3T was determined by its better reducibility and higher oxygen mobility, which favored the formation of active oxygen species that play decisive a role in the catalytic reaction. favored the formation of active oxygen species that play decisive a role in the catalytic reaction. For toluene oxidation, the most active catalyst was Co-CO3-2T (T90 = 248 ◦C at a WHSV of For toluene1 oxidation,1 the most active catalyst was Co-CO3-2T (T90 = 248 °C at a WHSV of 40,000 40,000 mL g− h− ), followed by catalyst Co-CO3-3T (T90 of 264 ◦C). Even catalyst Co-CO3-1T, mL g−1 h−1), followed by catalyst Co-CO3-3T (T90 of 264 °C). Even catalyst Co-CO3-1T, with significant with significant residual sodium, could display certain activity to toluene oxidation. The above results residual sodium, could display certain activity to toluene oxidation. The above results showed that showed that toluene oxidation was less sensitive to residual sodium poisoning than propane oxidation toluene oxidation was less sensitive to residual sodium poisoning than propane oxidation over the over the Co3O4 catalysts. However, excessive residual sodium still played a damaging role in the Co3O4 catalysts. However, excessive residual sodium still played a damaging role in the catalytic oxidation of toluene over the Co3O4 catalysts. A small amount of residual sodium in the bulk of catalyst Co-CO3-2T helped produce a defective structure and large surface area, which were related

Catalysts 2020, 10, 867 11 of 15

catalytic oxidation of toluene over the Co3O4 catalysts. A small amount of residual sodium in the bulk of catalyst Co-CO3-2T helped produce a defective structure and large surface area, which were related to more accessible active sites and more reactive surface adsorbed oxygen species that could facilitate the adsorption and activation of reactants, and thus accelerate the oxidation of toluene.

3. Experimental

3.1. Catalyst Preparation

A series of Co3O4 catalysts were prepared via precipitation using sodium carbonate as the precipitant. Typically, 20 mmol cobalt nitrate hexahydrate (Co(NO ) 6H O, Sigma-Aldrich, St. Louis, 3 2· 2 MO, USA) and 22 mmol sodium carbonate (Na2CO3, Sigma-Aldrich) were dissolved in 100 mL water, respectively. Then, the solution of precipitant was added into the solution of Co(NO ) 6H O 3 2· 2 under continuous stirring and the mixed solution was kept stirring for 1 h at room temperature. After centrifugation, the precipitates were washed with deionized water 1, 2, 3, and 5 times (200 mL each time) in order to obtain samples with various contents of residual sodium. The pH of the liquor would be close to neutral after 3 times of washing. All the precipitates were dried in an oven at 80 ◦C overnight. After that, the dried powders were calcined under static air in a muﬄe furnace at 200 ◦C for 2 h, and then, 500 ◦C for 2 h, respectively. The obtained Co3O4 catalysts were denoted as Co-CO3-xT, where x represents the numbers of washing.

3.2. Catalyst Characterization

TG-DSC analysis of the catalyst precursors (80 ◦C dried overnight) was carried out in ﬂowing 1 1 air (50 mL min− ) at a heating rate of 10 ◦C min− from 50 ◦C to 550 ◦C with a mass of 3–7 mg on a SETARAM Setsys Evolution 12 calorimeter (SETARAM, Caluire, France). An empty 70-mL aluminum pan was used as a reference. ICP-OES was conducted on a HORIBA Jobin Yvon Activa instrument (HORIBA, Paris, France) to determine the catalyst composition. The catalysts were dissolved in a mixture solution of H2SO4 and HNO3 and heated at 250 ◦C before the measurement. XRD patterns of the catalysts were recorded using a Bruker D5005 diffractometer (Bruker, Karlsruhe, Germany) with a Cu Kα radiation (λ = 0.154184 nm) to characterize the structural properties of the catalysts. Samples were scanned in 2θ range from 10◦ to 80◦ with a step size of 0.02◦ and a scanning speed of 2 s per step. Phase identification was conducted by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) database cards. The crystallite size of Co3O4 was calculated using the Scherrer equation based on the six most intense peaks of (111), (220), (311), (400), (511), and (440). Raman spectra were recorded using a HORIBA Jobin Yvon Raman instrument (HORIBA, Paris, France) with an excitation wavelength of 514 nm, 20 s exposure time, 5 accumulation, 50 objective, 1 1 × and 1800 grating lines per mm. The catalysts were scanned from 150 cm− to 800 cm− . FT-IR tests were carried out on a PerkinElmer FT-IR C92712 spectrometer (PekinElmer, Norcross, GA, USA) to get information on the spinel structure of the catalysts. The spectrum was recorded in the 1 1 range of 400–4000 cm− with an instrument resolution of 1 cm− . N -sorption testing of the catalysts was conducted at 196 C on a Micromeritics TRISTAR II 2 − ◦ apparatus (Micromeritics, Norcross, GA, USA). Each catalyst was degassed at 300 ◦C for 3 h before the measurement. The speciﬁc surface area (SSA) of the catalysts was obtained according to the BET method. The total pore volume (Vpore) and the pore size distribution were calculated by the Barrett-Joyner-Halenda (BJH) method. CO-TPR tests was conducted in a conventional flow system equipped with a thermal conductivity detector (SRA, Lyon, France). The catalysts were pre-treated at 300 ◦C for 1 h with a He flow. After cooling 1 down to room temperature, a CO/He flow containing 3000 ppm CO at a flow rate of 100 mL min− was 1 introduced and the catalyst was heated from 50 ◦C to 700 ◦C with a heating rate of 5 ◦C min− . Catalysts 2020, 10, 867 12 of 15

3.3. Activity Test Catalytic oxidation reactions were performed in a U-shaped quartz ﬁxed-bed reactor (220 mm in length and 4 mm in internal diameter) at atmospheric pressure. Also, 150 mg of catalyst, mixed with an amount of silicon carbide (SiC) to get a ﬁxed-bed height of 6 mm, were used for the catalytic reaction. For propane oxidation, the feed gases containing 1000 ppm propane (0.1 vol.% propane + 21 vol.% 1 O2 + 79 vol.% He) were passed through the reactor with a flow rate of 100 mL min− , giving a weight 1 1 hourly space velocity (WHSV) of 40,000 mL g− h− . The reactor was firstly heated to 100 ◦C with a 1 heating rate of 5 ◦C min− and kept at this temperature for 0.5 h to get stabilization. Then, the temperature 1 was increased to 350 ◦C in a step of 2 ◦C min− and held for 1 h. After that, the temperature was decreased to 100 ◦C in the same step as that in the temperature-rise process. For each sample, three consecutive heating-cooling catalytic cycles were carried out. The reactants and products were analyzed online using a gas chromatograph (SRA, Lyon, France) equipped with two thermal conductivity detectors, one for the detection of O2 and CO, and another for the detection of CO2,H2O, C3H6, and C3H8. The conversion of C3H8 was calculated using the following equation:

[CO ] [CO ] ( ) 2 out 2 in XC3H8 % = − 100 (1) 3[C3H8] ∗ where [CO2]out, [CO2]in and [C3H8] represent the outlet and inlet CO2 concentrations, and the initial propane concentration, respectively. For propane oxidation without oxygen, catalysts were pre-treated at 300 ◦C for 1 h with an He flow and then cooled down to room temperature in the same flow. The gas line was switched to a 1 C3H8/He flow (400 mL min− ) containing 1000 ppm C3H8. A heating-cooling run was performed in 1 the temperature range from 30 ◦C to 350 ◦C with a ramp rate of 2 ◦C min− and a 1 h plateau was set up at the highest temperature. In toluene oxidation, the reactant gases containing 1000 ppm toluene, generated by passing an air flow through a pure toluene-containing U-shape tube which was placed in a 5 ◦C thermostatic bath, 1 1 1 were fed into the reactor with a flow of 100 mL min− , corresponding to a WHSV of 40,000 mL g− h− . 1 The reactor was first heated to 150 ◦C with a heating rate of 5 ◦C min− and kept for 0.5 h in order 1 to stabilize the system. Then, the temperature was increased to 350 ◦C in a step of 2 ◦C min− and held for 1 h. Next, the reactor was cooled down to 150 ◦C with the same step. For each sample, three consecutive heating-cooling catalytic cycles were also conducted. CO and CO2 concentrations in the products were in-situ recorded by a Rosemount X-STREAM Gas Infrared Analyzer (Emerson Electric, Ferguson, MO, USA). The conversion of toluene was calculated as follows:

[CO ] [CO ] ( ) 2 out 2 in XC7H8 % = − 100 (2) 7[C7H8] ∗ where [C7H8] represents the initial toluene concentration.

3.4. Kinetic Measurement The kinetic data were measured in a fixed-bed reactor at atmospheric pressure. The linear velocity of the gas in the reactor and the particle size of the catalysts were appropriate to exclude the influence 1 1 of diffusion. The reaction rate, r (mol g− s− ), was calculated using the following equation [42]:

r = X V/M (3) ∗ 1 where X and V are the conversion and the ﬂow rate (mol s− ) of propane or toluene, respectively, M is the weight of the catalyst (g). Catalysts 2020, 10, 867 13 of 15

2 1 The turnover frequency, TOF (mol m− s− ), was calculated as follows [42]:

TOF = X V/S (4) ∗ where S is the surface area (m2) of the catalyst. The apparent activation energy, Ea, was obtained based on the Arrhenius formula.

Ea ln r = − + lnA (5) RT

1 1 where R is the ideal gas constant (8.314 J mol− K− ), T is the reaction temperature (K), and A is the apparent pre-exponential factor. Ea can be inferred from the slope of the linear plot of ln r versus 1/RT. The conversion of propane or toluene to CO2 considered in this analysis was always lower than 10%.

4. Conclusions

A series of Co3O4 catalysts with different contents of residual sodium was prepared using a precipitation method using sodium carbonate as the precipitant and then evaluated for the total oxidation of propane and toluene, respectively. The residual sodium influenced both the physicochemical properties and the catalytic performance of the Co3O4 catalysts. Residual sodium could be partially inserted into the spinel lattice, inducing distortions and resulting in smaller crystallite size and larger specific surface area of the Co3O4 catalysts. Furthermore, the reducibility of the catalysts was inhibited by the presence of residual sodium. For propane oxidation, trace residual sodium could poison the Co3O4 catalysts. The best performance was achieved on Co-CO3-3T, which was traced to the better reducibility and higher oxygen mobility of Co-CO3-3T. In terms of toluene oxidation over the Co3O4 catalysts, the negative effect of residual sodium was lower than that in propane oxidation. Co-CO3-2T was the most active catalyst. The defective structure and larger specific surface area, as well as more surface adsorbed oxygen species on the catalyst, were responsible for the excellent performance. Long-term stability tests showed that the activity of catalyst Co-CO3-3T was stable for propane oxidation, while catalyst Co-CO3-2T could keep its activity at a high conversion level in toluene oxidation.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/8/867/s1, Figure S1: FT-IR spectra of the catalyst precursors, Figure S2: Evolution of CO concentration during the CO-TPR test of the catalysts, Figure S3: Toluene conversion to CO2 as a function of temperature over catalyst Co-CO3-2T in a heating-cooling run, Figure S4: Three-cycle activity curves over (a) Co-CO3-3T for propane oxidation and (b) Co-CO3-2T for toluene oxidation. Author Contributions: G.C. and W.Z. designed the experiments; G.C. performed the experiments; G.C. and W.Z. analyzed the data; G.C. prepared the manuscript and J.L.V., A.G.-F. and Y.G. revised and corrected the manuscript, supervised the Ph.D. students, and managed the research programs. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the National Key Research and Development Program of China (2016YFC0204300), the National Natural Science Foundation of China (21577035), the commission of Science and Technology of Shanghai Municipality (15DZ1205305) and Auvergne Rhone Alpes Region (project PAI 2019 LS 203067). Acknowledgments: This work was financially supported by the University Claude Bernard Lyon 1, the CNRS, and the Auvergne Rhone Alpes Region (project PAI 2019 LS 203067). The China Scholarship Council are also acknowledged for the grants of G.C. and W.Z. Conflicts of Interest: The authors declare no conflict of interest.

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