钴基复合氧化物氧化降解甲苯的结构与催化性能

Structure of the Cobalt-Based Oxide and Its Catalytic

Performance in Oxidation of Toluene*

HU Xiuzhen,JIANG Chongwen,ZHU Jundong,LONG Jie,SONG Quzhi

【摘 要】Abstract：Cobalt-based oxides A-Co (A=Cu,Mn,Ce) and pure cobalt oxide prepared by using sol-gel method have been tested for toluene oxidation.It is found that all binary cobalt-based oxides showed higher catalytic performance in the oxidation of toluene than pure cobalt oxide.The most efficient catalyst was Ce-Co with t10 and t90 (the temperature required for achieving 10% and 90% conversion of toluene,respectively) values of 221 and 238 ℃,respectively.In addition,the stability of the Ce-Co oxide was evaluated at 99% of toluene conversion at 240 ℃ for 50 hours.The characterization techniques of X-ray diffractometry,nitrogen adsorption-desorption,scanning electron microscope,H2 temperature programmed reduction and X-ray photoelectron spectroscopy were used to explain the improved catalytic performance.The results indicated that Ce4+/Ce3+ and Co3+/Co2+ redox couples in the Ce-Co catalyst facilitated the mobility of oxygen and improved the reducibility of surface oxygen accounting for the synergetic effect in the oxidation of toluene.【期刊名称】吉首大学学报（自然科学版）【年(卷),期】2024(039)003【总页数】9

【关键词】Key words：catalytic oxidation;Co-based oxides;sol-gel;VOC;toluene

1 Introduction

Volatile organic compounds (VOCs),emitted from industrial process and biogenic

systems,are a great threat to the human health and the atmosphere[1-4].The strategies for elimination and treatment have attracted increasingly attention of many researchers[5-6].Catalytic oxidation of VOCs into harmless CO2 and H2O is considered as one of the most environment-friendly and cost-effective strategies in the past few decades[7-9].Supported noble metals (Pt,Pd,and Au catalysts[10-13]) are well-known as their high activities for deep oxidation of VOCs.Thus,they have been widely used in industrial processes for abatement of exhausts.However,the high cost of noble metals has increased the interest in the development cheaper transition metal oxide catalysts with comparable activity of noble metal catalyst.Recently,Co3O4 has received a high degree of interest

because of significantly higher catalytic performance in oxidation of VOCs[14-16],[17]11 447.T Garcia et al[18] found Ordered Co3O4 oxides performed well in the deep oxidation of a series representative VOCs.They demonstrated that the existence of a high

concentration of Co2+ ion on the surface explained the higher intrinsic activity.HU F Y et al[19] proposed that the excellent catalytic performances of Co3O4 were associated with large surface area and high surface oxygen concentration.Jiang S J et al[20]conducted the oxidation of toluene over a series of Co3O4 catalysts and found that Co3O4 /CNTs (Carbon nano tubes) exhibited the best catalytic activity compared with cobalt oxides on other supported material (Beta Zeolite,ZSM-5,SBA-15),with a complete toluene conversion temperature at 257 ℃.They confirmed that the morphology or crystal plane of Co3O4 could remarkably alter their catalytic performance.However,the combination of cobalt oxide with other different oxides often affects the textural,morphological,redox and catalytic activity.The synergistic effects have been found in different composite oxides such as cobalt,copper,manganese and chromium catalysts on which their catalytic activities are significantly promoted[21],[22]1 655,[23-24].Higher bulk oxygen mobility (through Mars-Van Krevelen mechanism) and formation of highly active oxygen species activated by the oxygen vacancies are widely recognized as the factors influencing the catalytic activity of Co3O4 and Co3O4-CeO2 binary oxides for VOCs catalytic oxidation[25]3 094.Although numerous studies point out the successful use of cobalt in the oxidation VOCs,how the structure of the cobalt-based oxide is correlated with its catalytic the oxidation of VOCs is not clear.It is noted that the physical chemical properties and activity of catalysts are directly associated to their preparation method.Sol-gel method was preferred due to the advantages of obtaining mixed oxides with a good control of stoichiometry and nano size.In the present work,mixed oxides of Ce-Co,Mn-Co,and Cu-Co were synthesized through sol-gel method.The catalytic activity of these catalysts was evaluated through the oxidation of toluene,which is a very common VOCs in

petrochemical and chemical industries.The attention focused on the maintenance of the catalyst stability was more than 50 hours.The prepared catalysts were characterized by using X-ray diffractometry (XRD),nitrogen adsorption-desorption,scanning electron microscope (SEM),H2 temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS).An attempt to elucidate the structural and surface composition with the catalytic activity will be studied.

2 Experimental

2.1 Preparation of Catalysts

Mixed-oxide catalysts were prepared by a citric acid sol-gel method.Briefly,nitrates with different cations were used as precursors (Cu(NO3)2·3H2O,50%(W.%) of Mn(NO3)2·4H2O,Ce(NO3)3·6H2O,Co(NO3)2·6H2O) in stoichiometric ratio of

A∶Co=1∶1,which A is Mn,Ce and Cu.In a typical synthesis,6.04 g Cu(NO3)2·3H2O and 7.28 g Co(NO3)2·6H2O solution were added into 50 mL deionized water to obtain a mixed solution (1.0 mol/L metal ion concentration).Then,10.51 g citric acid was added into the nitrate solution.The obtained solution was stirred at 80 ℃ for 4 hours to form a gel.This gel was then dried at 110 ℃ for 10 hours and calcined at 500 ℃ (the heating rate of 5 ℃/min) for 3 hours to ensure the removal of carbonaceous residues and the formation of the mixed oxides labeled as Ce-Co.In addition,the single oxide of Co,the binary mixture of Cu-Co and Mn-Co were prepared by the same procedure.2.2 Catalytic Performance Tests

Catalytic performance evaluation experiments were performed in a continuous flow quartz fixed-bed reactor (L=52 cm,Di=0.8 cm) at atmospheric pressure.About 500 mg of catalysts (40～60 mesh) was packed at the isothermal zone of the reactor.The reaction temperature was continuously monitored by a thermocouple tied on the reactor and positioned in the middle of the catalyst bed.5.0×10-3 toluene with a total flow of 155 mL/min was generated by passing a N2 flow through a bottle containing pure toluene chilled in an ethyl alcohol isothermal bath at 5 ℃.Reactive mixture was introduced over the catalysts with heating at a ramp rate of 5 ℃/min from room temperature to 300 ℃.The

concentration of toluene was analyzed via a Shimadzu 2010 gas chromatograph equipped with a flame ionization detector (FID).All experimental runs were repeated three times under steady state conditions and the average conversion of toluene was employed.The complete conversion value of toluene (ηtoluene%) was calculated by the following equation:

where [toluene]in and [toluene]out are the inlet and outlet concentrations of toluene,respectively[26].2.3 Characterization of Catalysts

XRD patterns of the catalysts were recorded on a RigakuMiniFlex 600 X-ray power

diffractometer equipped with a Cu Kα radiation (λ=0.154 056 nm,40 kV).The XRD data was then generally collected in the 2θ range of 10°～80° with a step size of 0.01°.The BET surface area and pore size distributions of all samples were obtained with Nitrogen adsorption-desorption method at -196 ℃ through a Micromeritics ASAP2460

instrument.Before the measurement,samples were outgassed at 200 ℃ for 3 hours under primary vacuum to remove water and other atmospheric contaminants.The specific surface area (SSA) of each catalyst was determined according to the Brunaure-Emmett-Teller (BET) methods.The porous volume and the pore size distribution were obtained via the Barrett-Joyner-Halenda (BJH) methods.Temperature-programmed reduction (TPR) measurements were carried out with Auto Chem.II 2920 equipment (Micromeritics,USA),which the samples (200 mg) were packed into a quartz tube and heated in a 5 vol.% H2/Ar reducing gas mixture at 30 mL/min from room temperature to 900 ℃ by a heating rate of 10 ℃/min,and the detector signal was recorded continuously.The SEM images were taken by a Hitachi 1 080 apparatus.The working voltage was 15 kV.The sample powders were mounted on a double-side adhesive tape and observed at different magnification.XPS measurements were carried out on a 300 W Thermo Fisher Scientific apparatus with an Al Kα line radiation source.Each powder sample was pressed into the double-faced adhesive tape and degassed to remove the volatile contaminants.It was then transferred to the analyzing chamber for XPS analysis.Binding energies (BE) values were calibrated relative to the C 1s peak at 284.8 eV.

3 Results and Discussions

3.1 Catalytic Activities

Fig. 1 shows the catalytic performances of the catalysts in combustion of toluene within the temperature range 140 to 300 ℃.We found that the complete conversion temperature of toluene was lower than 300 ℃ for all the Co based catalysts,which showed better performance with respect to previous reports[27-28].It is worth noting that the binary mixing oxides showed higher activities than pure Co3O4 catalyst,suggesting that there exists a certain kind of synergistic effect between Co and other metal oxides.This synergistic effect can significantly improve the catalytic activities for the oxidation of toluene.However,Ce-Co oxide are the most active catalyst in combustion of toluene with t10 and t90 values of 221 and 238 ℃ from Table 1,respectively.Consequently,the efficiency decreases in the order:Ce-Co>Mn-Co>Cu-Co>Co3O4.In addition,the stability of the Ce-Co oxide was evaluated at 99% of toluene conversion at 240 ℃ for 50 hours in Fig. 2.The conversion of toluene oxidation still maintained at about 99% after 50 hours.Therefore,the Ce-Co mixed oxide catalyst exhibits not only excellent catalytic performance but also good stability.

3.2 XRD Characterization

The crystal structure of each type of catalyst was obtained by comparison with the standard powder diffraction files (PDF) from the International Centre for Diffraction Data (ICDD).The XRD patterns of all the catalysts are shown in Fig. 3.In the XRD pattern of pure cobalt oxides,a series of intensive and sharp diffraction peaks were observed which may ascribed to the cobalt oxide phase (PDF 42-1467).For Ce-Co oxide,the main crystal phases are visible at 2θ=28.6°,33.1°,47.6°,56.4°,which are ascribed to the (111),(200),(220),(311) crystal planes of CeO2 (PDF 43-1002),respectively.Weak diffraction peak of,(311),(440),(511),(440) Co3O4 (PDF 43-1003) was observed at

2θ=36.8°,45°,59°,65.3°,respectively.The typical diffraction peaks of spinel structure broaden and the intensity also decrease.In the Mn-Co oxides XRD pattern,a typical diffraction peak around 18.3°,29.4°,33.0°,36.5°,44.8°,59.1°,60.8° can be contributed to the

(111),(202),(113),(311),(400),(511),(404) crystal plane of spinel structure (Co,Mn)(Co,Mn)2O4 (PDF 18-0408),respectively.Diffraction peak of (111),(311),(440) Co3O4 (PDF 43-1003) was observed at 2θ=19.0°,36.8°,65.3°,respectively.In terms of Cu-Co oxides,the appearance of intense peaks at 2θ=35.5°,38.7° revealed the presence of tenorite (111) CuO (PDF 48-1548),the peaks in 2θ=19.0°,31.3°,36.8°,44.7°,55.6° are ascribed to the presence of (111),(220),(311),(400),(422) Co3O4 phase (PDF 43-1003),respectively.In addition to CuO and Co3O4 phase,a mixed spinel oxide Cu0.76Co2.24O4 (PDF 36-1189) in Cu-Co sample was detected.The characteristic peaks of this phase at 2θ=59.3°,65.1° ascribe to (511),(440) crystal plane.The obtained values in detail are listed in Table 2.The crystallite size of Ce-Co oxide is 6.3 nm,which is the smallest among the prepared catalysts,the mean crystallite sizes were further estimated according to the Debye-Scherer equation.More active sites over the particles on the Ce-Co oxide seemed generating from the small size of the crystallite[2]112.L F Liotta et al[27]and M M Schubert et al[29]also proposed that oxygen vacancies on the surface of the Co3O4 played an important and favorable role in accelerating the adsorption and dissociation of oxygen molecules resulting in the formation of highly active electrophilic O- species.Although the active phase in the oxidation of toluene may involve Co3O4,the interaction between the CeO2 and Co3O4 phase facile would enhance the catalytic activity in the oxidation of toluene since both CeO2 and Co3O4 possess strong redox ability.3.3 N2 Adsorption Desorption Isotherms

BET surface areas,pore size and pore volume are presented in the Table 2.With respect to Co oxide,Mn-Co and Cu-Co,Ce-Co mixed oxide shows the highest surface area value (31 m2/g).The corresponding average pore size of Ce-Co,Mn-Co,Cu-Co mixed oxide and Co oxide are 6.9,15.9,24.7,18.7 nm,respectively.The Ce-Co oxide with the highest surface area