Preparation Of Tin Dioxide-based Porous Ceramics And Its Thermal Catalytic Performance For Methane Conversion

Methane,as a potent greenhouse gas,is widely present in human activities and daily life.Especially with the increasing use of natural gas vehicles（NGVs）,a large amount of low-concentration methane（below 5%）is emitted into the air through incomplete combustion in vehicle exhaust,posing a significant threat to the human living environment.Catalytic combustion can completely convert methane at relatively low temperatures.Monolithic porous ceramic catalysts exhibit excellent chemical stability and reusability.The porous structure can enhance the contact efficiency between the catalyst and methane,reduce the diffusion resistance of methane gas to the active sites of the catalyst,and effectively treat methane emissions.Therefore,in this study,an integrated monolithic tin dioxide（SnO₂）-based porous ceramic catalyst with functional structure was prepared using a salt template method.Various characterization techniques were employed to analyze the structure,morphology,and physical properties of the porous ceramic,and the influence of morphology and structure on the catalytic performance of the porous ceramic catalyst loaded with Pd O was investigated in detail.The catalytic mechanism of the SnO₂-based porous ceramic catalyst was explored.The specific research contents are as follows:（1）Commercial SnO₂ powder was used as the raw material to prepare pure SnO₂ ceramics by spark plasma sintering（SPS）at sintering temperatures ranging from 600°C to 900°C.When the sintering temperature was 600°C,there was no sintering of SnO₂ grains,resulting in a density of75.34%.Sintering at temperatures above 800°C led to complete sintering of SnO₂,which was not suitable for the preparation of porous SnO₂ ceramics.When the sintering temperature was 700°C,partial sintering of the grains occurred,and the compressive strength reached 29.72 MPa.Therefore,the optimal sintering temperature for preparing porous SnO₂ ceramics was 700°C.Based on the sintering temperature of 700°C,the salt templating method was employed to prepare SnO₂ ceramics with different densities.Combining X-ray diffraction（XRD）,energy-dispersive spectroscopy mapping（EDS-Mapping）,and Ag⁺probe detection,the feasibility of salt templating for pore formation was demonstrated.Results from solid density measurements and universal testing machine showed that as the Na Cl content increased,the density and compressive strength of the samples gradually decreased.Among them,the porous SnO₂ ceramic with 20 wt.%Na Cl exhibited a density of 68.26%and a compressive strength of 4.57 MPa.After loading Pd O using the impregnation method,the catalyst exhibited optimal catalytic performance,achieving the catalytic temperature(T₉₀)required for 90%methane conversion at 545°C.Results from three cycles of thermal stability testing confirmed the catalyst’s good thermal stability and reusability.Continuous conversion testing for over 750 minutes demonstrated its excellent catalytic stability.In the exploration of catalyst performance differences,scanning electron microscopy（SEM）revealed that Na Cl enriched the ceramic’s pore structure.Inductively coupled plasma optical emission spectroscopy（ICP-OES）testing showed that the loading of Pd on the porous SnO₂ ceramic catalyst with 20 wt.%Na Cl was 0.30 wt.%,which was 0.13 wt.%higher compared to the 10 wt.%Na Cl sample.This indicated that the porous structure provided more adsorption sites for the loading of Pd.X-ray photoelectron spectroscopy（XPS）data demonstrated that with an increase in Pd loading,the content of oxygen vacancies and active sites also increased,enhancing the catalyst’s methane catalytic activity.（2）In order to further enhance the catalytic activity for methane,under the conditions of a sintering temperature of 700°C and 20 wt.%Na Cl content,the optimal impregnation concentration of the noble metal solution was explored and tested to be 0.01 M.Based on this,SnO₂ porous ceramic catalysts with different graphite contents were obtained by adding varying mass fractions of graphite.Transmission electron microscopy（TEM）and inductively coupled plasma optical emission spectroscopy（ICP-OES）results indicated that the addition of graphite increased the loading amount and uniformity of Pd.Among them,the Pd loading on the SnO₂ porous ceramic catalyst with 10 wt.%graphite was 0.17 wt.%,exhibiting the best catalytic activity with T₉₀ at 427°C.It consistently showed high catalytic activity in three cycles of thermal stability testing,with the catalytic temperature remaining almost constant,demonstrating excellent thermal stability and reusability of the catalyst.The continuous methane conversion testing for over 900 minutes demonstrated good long-term stability of the catalyst.Combining TEM and XPS results,the role of graphite in promoting the growth of Pd nanoparticles and the interaction with the support was analyzed.Pd might be embedded in the SnO₂ lattice in the form of Pd-O-Sn bonds,altering the properties of the SnO₂ lattice and facilitating the generation of oxygen defects,thus enhancing the catalyst’s activity.In exploring the reasons for catalyst stability,graphite as a reducing agent could reduce more Pd^δ+（2<δ≤4）to highly active Pd²⁺species.This allowed the catalyst to maintain efficient catalytic performance during subsequent temperature cycles.Finally,using a self-built electric heating device for experiments,it was found that the SnO₂ porous ceramic with 10 wt.%graphite could reach 332.30°C in just 3 minutes under a DC power of 13.92 W.This indicates that the electric heating conversion path has broad application prospects and advantages.