Synthesis Of Nanosized Cobalt Oxide For The Total Oxidation Of Propane

Volatile organic compounds(VOCs)emitted from industries and vehicle exhausts can induce a series of environmental problems,including photochemical smog,broken ozonosphere,and atmospheric haze.Moreover,many VOCs are highly toxic and harmful for human health.Catalytic oxidation is an effective technique to eliminate low concentrations of VOCs.It is widely used in industry due to its advantages such as operability and complete treatment.The performance of catalyst plays a key role in catalytic oxidation.Co3O4 has been extensively studied as a promising catalyst for VOCs oxidation,because of its low cost and high activity in comparison with those of noble metals.The aim of this study is to explore the new synthesis of nanosized Co3O4,and develop highly active cobalt oxide catalyst for the oxidation of propane.The main results and conclusions of this study are as follows:(1)Nanosized cobalt oxides were synthesized by ammonia-etching of cobalt hydroxide and subsequent calcination.The effects of ammonia etching on the structures and catalytic properties of cobalt oxide were investigated.The results suggested that the cobalt oxide is composed of cubic Co3O4 having particle sizes in the range of 10-20 nm;smaller than the analogue prepared through the conventional precipitation method(16-24 nm).Ammonia etching improved the reducibility of cobalt oxide,and facilitated the generation of surface oxygen vacancies and the production of surface-adsorbed active oxygen species.The activity of Co3O4 in propane oxidation was greatly enhanced by ammonia etching,mainly due to the smaller particle size and better redox properties.The facile ammonia-etching synthesis method has potential for large-scale and low-cost production of cobalt oxide catalyst.(2)Nanosized Co3O4 catalyst was prepared through a dispersion-precipitation method involving a reaction between wet cobalt hydroxide and acetic acid,which forms a colloidal dispersion,and subsequent dilution,which destabilizes and precipitates the colloidal particles.The catalyst had a particle size of 5-15 nm and a specific surface area of 82 m2/g.Compared with the analogue prepared by conventional alkali-induced precipitation method,the nanosized catalyst was more reducible and contained a larger amount of active surface oxygen species,as revealed by experiments using temperature-programmed reduction in hydrogen and temperature-programmed desorption of oxygen.The oxygen species could contribute to the observed higher activity in the catalytic oxidation of carbon monoxide and propane.In addition,a catalytic kinetics study revealed that the apparent activation energies for carbon monoxide and propane oxidation over the catalyst were 34.0 and 49.5 kJ/mol,respectively,much lower than those over the analogue(54.7 and 71.1 kJ/mol,respectively).Furthermore,a long-term test(76 h)showed that the nanosized catalyst is highly stable.(3)A simple,one-step strategy for synthesizing silica-supported cobalt catalyst was successfully developed.In this strategy,Co(III)ammonia complex was first prepared and then adsorbed onto porous silica,followed by calcination in air.Adsoption kinetics study revealed that cobalt adsorption on silica proceed via chemisorption with the formation of inner-sphere surface complexes.XRD,IR,Roman,and XPS characterizations revealed that cobalt in the catalyst exists mainly in the form of Co3O4.The obtained cobalt oxide is homogeneously distributed on the silica as indicated by EDS mapping and TEM images.On the premise of good dispersion,cobalt loading as high as 181 mg/g can be achieved by repeated adsorption of Co(III)ammonia complex.Cobalt particle sizes can be tailored in the range of 4.4-8.0 nm by varying cobalt loading.In addition,the cobalt particle size and dispersion can be well maintained even calcined at 650 ?.The as-prepared catalyst showed good catalytic performance and stability toward total oxidation of propane,100%conversion can be achieved at 290 ?,and the initial conversion can be retained after 45 h on-stream reaction.The high activity could be attributed to its good low-temperature reducibility.Catalytic kinetic analysis indicated that propane oxidation over the silica-supported cobalt oxide catalyst proceed through a Langmuir-Hinshelwood mechanism,with an activation energy value of 101,7 kJ/mol.