Study On The Oxygen Vacancy Regulation Of Co-based Catalysts And Its Influence Mechanism On Propane Oxidation And CO₂ Hydrogenation To Methanol

The rapid development of industrialization brings convenience but also causes a series of environmental issues,such as haze,photochemical smoke and greenhouse effects.Since the implementation of"Resolutely Pollution Prevention and Control Battle",the air quality has improved significantly,but regional complex pollution is still prominent.Volatile organic compounds（VOCs）widely exist in transportation,petrochemical,paint printing and other fields.VOCs not only directly pollute the environment,but also form ozone,atmospheric fine particulate matter(PM_2.5)and photochemical smog.On the other hand,the increase of atmospheric carbon dioxide（CO₂）and other greenhouse gas concentration will lead to climate change,China has also put forward the carbon peak and carbon neutrality goals:CO₂emissions strive to reach the peak before 2030,strive to achieve carbon neutrality before 2060.Therefore,it is very important to promote pollution reduction and carbon reduction.Among many VOCs control technologies,catalytic oxidation can efficiently convert VOCs with complex pollution characteristics into non-toxic CO₂and H₂O at relatively low temperatures,which is favored,but it also brings the problem of increasing carbon emissions.If the low concentration of CO₂from the oxidation of VOCs and industrial terminal emissions is separated,purified and concentrated,it can be hydrogenated with hydrogen to achieve efficient conversion of CO₂.Among them,hydrogenation of CO₂to methanol is an important way,because methanol can be used as both primary fuel and raw material to synthesize high-value chemicals.The degradation of VOCs and hydrogenation of CO₂to methanol are in urgent need of developing efficient and stable non-noble metal catalysts.Co based oxides have valence of+2 and+3,because Co has d orbital not filled with electrons,showing good good reducibility,so it is widely used in the field of VOCs catalytic oxidation.While metal Co has a strong H₂dissociation ability,and composite In₂O₃can obtain higher CO₂conversion and methanol selectivity.Hence,Co-based catalysts can be applied to the catalytic reaction of VOCs oxidation and CO₂hydrogenation to methanol.Moreover,the oxygen species mobility of Co-based oxides and the adsorption and activation ability of Co-In₂O₃catalysts for CO₂can be enhanced by oxygen vacancy regulation,thereby improving their catalytic activity in VOCs oxidation and CO₂hydrogenation to methanol.The aim of this study was to increase the ratio of Co²⁺/Co³⁺on the surface of Co based oxide catalysts by metal-organic framework precursors and the complexation of oxygen-containing functional groups in bamboo powder（BP）,and thus increase the oxygen vacancy content of the catalysts;By changing the crystal phase of In₂O₃,the crystal state of metal Co and the molar ratio of Co:In,the Co-In₂O₃interaction and the oxygen vacancy content are regulated.Meanwhile,the structure-activity relationship was revealed by means of catalytic activity evaluation and a series of characterization methods.The main research contents are as follows:（1）Co₃O₄microrods assembled by nanoparticles were prepared by hydrothermal synthesis of Co-BTC precursor followed by calcination,Co₃O₄nanoparticles directly calcined by cobalt nitrate were used as a comparison.Compared with the initial propane conversion,the propane conversion of the comparative sample Co₃O₄decreased by 65%in thermal stability test due to particle agglomeration and surface carbon species deposition.Whereas,Co₃O₄microrods rod can confine Co₃O₄nanoparticles to the microrods and inhibit the migration and agglomeration of Co₃O₄nanoparticles.At the same time,the high surface Co²⁺/Co³⁺ratio induces oxygen vacancy,which can improve oxygen mobility and rapidly oxidize propane,thus having good catalytic performance and stability.In situ DRIFTs results demonstrate that propane is dehydrogenated to propylene and then oxidized to propenol,acrylic acid and CO₂.（2）La Co O₃perovskite was prepared by sol-gel method at low temperature（550 ^oC）using bamboo powder（BP）as complexing agent.In the absence of BP,the separate phases of La₂O₃and Co₃O₄were obtained.As for the precursor with BP,COO-groups can chelate La³⁺and Co²⁺.Moreover,chelation and biological reduction of organic carbon resulted in more Co²⁺in La Co O₃,leading to the generation of abundant oxygen vacancies that contribute to the total oxidation of propane.Alkali metals（Na⁺and K⁺）derived from BP that negatively affect propane catalytic performance can be removed by treatment with an acid solution（p H=1）.Therefore,La Co O₃biosynthesis mediated by BP-1（pretreatment of bamboo powder at p H=1）showes the best propane catalytic activity,thermal stability and high resistance to 5 vol%CO₂and 5 vol%H₂O.（3）In₂O₃/Co₃O₄catalysts were synthesized by impregnation method,and In₂O₃of different crystal phase can be controlled via altering the amount of citric acid during the impregnation process.In the catalyst prepared without citric acid,the crystal phase of In₂O₃is hexagonal（h-In₂O₃）,and cubic In₂O₃（c-In₂O₃）appears with the addition of citric acid.When the molar amount of citric acid reaches 30%of indium,pure c-In₂O₃supported on Co₃O₄is obtained.In₂O₃/Co₃O₄catalysts undergo reconstruction,In₂O₃and Co₃O₄are converted into metal Co and inactive Co₃In C_0.75,and h-In₂O₃is also easily transformed into c-In₂O₃.Under reaction stabilization,except for h-In₂O₃/Co₃O₄catalyst,a small amount of h-In₂O₃was retained,and other catalysts are made of c-In₂O₃,metal Co and Co₃In C_0.75compositions.H₂-TPR and XPS characterization results show that Co₃O₄has stronger interaction with c-In₂O₃,and the electrons of c-In₂O₃are more easily transferred to Co₃O₄,and the low temperature reduction ability of In₂O₃is improved more significantly.After the reaction,the dissociated H atoms on metal Co diffuse more easily to In₂O₃to boost oxygen vacancies formation,thus showing a higher CO₂conversion and methanol space-time yield.（4）BP assisted synthesis of Co₃O₄-In₂O₃catalysts were applied for efficient CO₂hydrogenation to methanol.Under reaction stabilization,Co₃O₄in BP-modified catalyst has a stronger interaction with In₂O₃,and will be transformed into amorphous Co⁰and inactive Co₃In C_0.75（the weight fraction of the catalyst is 69 wt%）.In contrast,without assistance of BP,crystalline Co⁰is formed and the mass content of Co₃In C_0.75also reaches 61 wt%.Density function theory calculation gives the information that H₂molecules are more easily adsorbed and dissociated on amorphous Co⁰compared to crystalline Co⁰,which further facilitates the oxygen vacancy formation of In₂O₃.On the other hand,in situ DRIFTs confirms that methanol synthesis over the Co₃O₄-In₂O₃catalyst follows the dual-site model.That is,active Co⁰and oxygen vacancies are both important,in which the former is associated with H₂dissociation while the latter is closely relatively to CO₂activation.As a result,at the sacrification of some decreased Co⁰but with higher reactive amorphous Co⁰,BP-1-mediated biosynthesis of Co₃O₄-In₂O₃attains a better methanol space-time yield of 13.39 mmol g_cat^-1h^-1at 300 ^oC and 4 MPa.Moreover,the above catalyst shows excellent stability with almost no deactivation in a continuous run of 100 h.（5）On the basis of the previous chapter,the optimal catalyst was selected and its Co:In mole ratio was changed to balance the ability of the catalyst to activate CO₂and H₂.The results show that Co₃O₄is transformed into Co O and Co after reduction and reaction,with highest CO₂conversion rate and methane selectivity.Whereas,Co₃O₄-In₂O₃catalyst showed low CH₄selectivity and high methanol selectivity under experimental conditions.In₂O₃and Co species are consumed in the reaction process due to the inevitable formation of inactive Co₃In C_0.75.Therefore,when the catalyst has a low In₂O₃content with high Co:In molar ratio,resulting insufficient CO₂activation ability and easy decomposition of intermediate products,especially when the Co:In mole ratio is 9:1.When the molar ratio of Co:In reaches 7:3,the catalyst shows the best methanol synthesis performance,with CO₂conversion and methanol selectivity reaching 9.8%and52.5%,respectively.When the molar ratio is further increased,due to the low content of metal Co after the reaction,the hydrogen dissociation ability is insufficient,and the activated CO₂and its intermediate species cannot be further hydrogenated,resulting in a decrease in the methanol yield.At the same time,the physicochemical properties of the optimal catalyst were characterized at different times during the reaction induction period.The results showed that Co₃O₄was reduced to metal Co,and the dissociated H atoms diffused to In₂O₃,which not only participated in the hydrogenation of CO₂,but also facilitate the formation of oxygen vacancy of In₂O₃,accompanied with increasing the yield of methanol.