Controllable Preparation Of SnO₂-Based Catalysts And Their Application In Catalytic Combustion Performance Of Lean Methane

Natural gas,as a clean energy source mainly composed of methane,is gradually receiving widespread attention due to its high calorific value（50-55 MJ/kg）and abundant reserves.However,a large amount of low-concentration methane（<5%）released from natural gas vehicle exhaust into the air exacerbates the global greenhouse effect,causing harm to the environment.Researchers have demonstrated the feasibility of activating methane with catalysts at low temperatures.Methane catalytic combustion is a commonly used method for eliminating low-concentration methane.Currently,most methane combustion catalysts are based on noble metals（such as Pd,Pt,and Au）or non-noble metal oxides（such as Co₃O₄,Mn O₂,and Fe₂O₃）.In this study,firstly,around the Pd/SnO₂model catalyst,the influence of metal-carrier interactions on the activity of methane catalytic combustion was systematically investigated by regulating the carrier morphology and crystal surface structure as well as the dispersed form of the noble metal Pd.Secondly,composite metal oxide catalysts based on SnO₂ were synthesized,and the mechanism of regulating the electronic structure of SnO₂ by doping with hetero-elements（Ce,Cr,Mo）and their influence on the performance of methane catalytic combustion were systematically studied.In this article,different synthetic methods were used to successfully control the morphology of SnO₂ carriers from nanoparticles to two-dimensional nanosheets（SnO₂-NPs（particle-like）,SnO₂-NRs（rod-like）,SnO₂-NSs（sheet-like））.Then,3wt.%Pd was loaded onto the surface of the SnO₂carrier by impregnation（Pd/SnO₂-NPs,Pd/SnO₂-NRs,Pd/SnO₂-NSs）,among which the Pd/SnO₂-NRs catalyst exhibited the best catalytic activity and high-temperature sintering resistance.Experimental analysis found that the interaction between the bivalent Pd component(Pd²⁺)and the carrier could effectively activate the oxygen species at the Pd/SnO₂ interface,increase the concentration of interface oxygen defects,and be conducive to the activation of the C-H bond in the methane component.Through the synergistic effect between the Pd O_x active center and the SnO₂-NRs carrier,the effective improvement of the catalytic activity and stability of Pd/SnO₂-NRs catalyst for methane catalytic combustion reaction was ultimately achieved.Pd/SnO₂-NPs and Pd/SnO₂-NSs catalysts have fewer active oxygen species and lower Pd²⁺content,resulting in lower catalytic activity.In order to further explore the possibility of reducing the Pd loading in the catalyst,1wt.%of Pd was loaded onto the surface of SnO₂-NPs by impregnation to form a（1%Pd/SnO₂）catalyst.The catalytic activity and characterization results were compared with those of the 3wt.%Pd/SnO₂-NPs（3%Pd/SnO₂）catalyst,and the results showed that reducing the Pd loading did not improve the TOF value at low temperature,although it increased the dispersion degree of Pd on the carrier surface,indicating that reducing the size of Pd particles did not improve the catalytic activity.This article synthesized Ce-SnO₂,Cr-SnO₂,and Mo-SnO₂three kinds of composite metal oxide catalysts by co-precipitation method,and evaluated their activities.The results showed that Ce-SnO₂catalyst had the best catalytic performance for methane combustion.Characterization methods such as X-ray diffraction（XRD）and high-resolution transmission electron microscopy（HRTEM）proved that Ce,Cr,and Mo were uniformly doped into the SnO₂ carrier,forming a solid solution structure.BET characterization results and SEM images showed that the specific surface area of Ce-SnO₂,Cr-SnO₂,and Mo-SnO₂ catalysts was significantly larger than that of SnO₂.H₂-TPR characterization results showed that the lattice oxygen in the Ce-SnO₂ catalyst had the highest reduction activity,and O₂-TPD characterization results showed that the Ce-SnO₂catalyst had the highest content of active species.Furthermore,the valence state analysis results showed that the introduction of Ce³⁺/Ce⁴⁺components in the Ce-SnO₂ catalyst effectively regulated the valence state composition(Sn²⁺/Sn⁴⁺ratio)of SnO₂,affected the oxygen vacancy concentration at the material interface,and promoted methane catalytic combustion reaction.