Exploring The Structure-reactivity Relationship Of SnO₂-based Solid Solution Catalysts For The Combustion Of Volatile Organic Compounds （VOCs） With An XRD Extrapolation Method

With the rapid development of global industrialization,VOCs emission amount is increasing day by day,which can cause serious harm to the ecological environment and human health.Therefore,effective elimination of VOCs pollution has become a matter of great concern.Catalytic combustion technology is considered to be one of the most effective methods to eliminate VOCs pollution.How to prepare cost-effective and efficient VOCs combustion catalysts has become a current research hotspot.In this thesis,a series of SnO₂-based solid solution catalysts have been prepared for the combustion of VOCs,and their catalytic performance has been tested by the combustion of some typical VOCs.The structure–reactivity relationship was systematically investigated by an XRD extrapolation method and some other characterization techniques,with the expectation to provide scientific guidance for the design and preparation of efficient VOCs abatement catalysts.The main research contents are summarized here:Part 1:To elucidate the valence state effect of doping cations,Li⁺,Mg²⁺,Cr³⁺,Zr⁴⁺and Nb⁵⁺with similar radii to Sn⁴⁺（CN=6）have been chosen to dope tetragonal SnO₂ by a sol-gel method.The prepared catalysts have been used for the catalytic oxidation of CO,toluene and propane.XRD and Raman characterization results show that Cr³⁺,Zr⁴⁺and Nb⁵⁺can be successfully incorporated into the SnO₂ lattice to replace part of Sn⁴⁺,resulting in a solid solution structure,which forms more surface defects and promotes the generation of more surface-active oxygen species.However,due to the large deviation of the valence state from Sn⁴⁺,Li⁺and Mg²⁺cannot enter the lattice of SnO₂,but mainly exist on the surface of the catalysts in the form of nitrate,thus inhibiting the generation of surface-active oxygen species.The active oxygen species and acidic sites on the catalysts have been characterized by H₂-TPR,O₂-TPD,Toluene-TPD,NH₃-TPD,EPR,XPS,in situ DRIFTS,and correlated with the reaction activity.The results show that the surface-active oxygen sites play a more important role than surface acid sites in the process of CO oxidation.The Nb⁵⁺-modified solid solution catalyst has the best performance for CO oxidation due to the formation of the largest amount of active oxygen species.In contrast,the surface acidic sites can effectively adsorb and activate the reactant molecules,which play a more important role than the surface-active oxygen species in the combustion of propane and toluene.The Cr³⁺modified solid solution catalysts have the best activity for VOCs combustion due to the formation of the largest number of surface acidic sites.Part 2:Based on the results of part 1,a series of Sn-Cr catalysts with varied Sn/Cr mole ratios have been prepared by a co-precipitation method to optimize their VOCs combustion performance.The lattice capacity of Cr³⁺in SnO₂ was accurately measured by an XRD extrapolation method to be 0.210 g Cr₂O₃/g SnO₂,being equal to a Sn/Cr mole ratio of 70/30,which indicates that Cr³⁺can replace maximally 30%Sn⁴⁺ions in the SnO₂ lattice to form a pure solid solution structure.Whereas,the excess Cr³⁺ions will release out to form Cr₂O₃,which gradually crystallizes as its content increases.The characterization results show that the numbers of surface-active oxygen species and acidic sites of the catalysts rise with the increase of Cr³⁺content and reach a maximum near the lattice capacity when the Cr content is lower than the lattice capacity.Therefore,the catalysts with Cr content close to the lattice capacity（SnCr6-4 and SnCr7-3）present the best toluene combustion performance with a distinct lattice capacity threshold effect.The reaction intermediates and possible mechanisms of toluene combustion have been investigated with in situ DRIFTS.It was found that the toluene molecules were first adsorbed and activated on the surface acidic sites,which interacted in turn with the surface-active oxygen species to form the intermediates such as benzyl alcohol（C₆H₅-CH₂OH）,benzoate（C₆H₅COO^-）and cyclic anhydride etc.,then produced the final products H₂O and CO₂.The synergistic effect of surface-active oxygen species and acidic sites determines the reaction performance.The kinetic data have revealed that the reaction order for toluene concentration is higher than for oxygen concentration,suggesting that the adsorption and activation of toluene is the rate-determining step of toluene combustion.Part 3:Based on the results of the above two parts,in this part,by using the lattice capacity of Nb in SnO₂（molar ratio Nb/Sn=25/75）as the total doping content,a series of catalysts for toluene,propane and propene catalytic combustion were prepared by simultaneous doping of different amounts of Nb and Cr ions into SnO₂matrix.The XRD,Raman,TEM,and Mapping results have confirmed that all the Nb and Cr ions are doped into the lattice of SnO₂,forming Sn_7.5Nb_yCr_2.5-yO_x solid solutions.H₂-TPR,Toluene-TPD,NH₃-TPD,EPR,XPS,and in situ DRIFTS results have demonstrated that the contents of active oxygen species and acidic sites on the catalysts can be effectively improved by changing the Nb/Cr molar ratio to prepare VOCs combustion catalysts with excellent performance.The Sn_7.5Nb_0.1Cr_2.4O_x with an Nb/Cr ratio of 0.10/2.40 has abundance active oxygen species and acidic sites,making it exhibit the best performance in the combustion of toluene,propane and propene,on which the complete oxidation temperature is 305 ^oC,260 ^oC and 310 ^oC,respectively.In addition,the water and sulfur poisoning resistance of the Sn_7.5Nb_0.10Cr_2.40O_x catalyst is significantly improved in comparison with that of the undoped Sn-Cr solid solution.As a result,it has the potential to be applied in real industrial processes.