Synthesis Of Zeolite Confined Pd Catalysts And Catalytic Performance For Combustion Of Light Alkanes

Noble metal nanoparticles(NPs)catalysts exhibit desired activities in various catalytic reactions.However,their application in real-world was still limited,due to suffer from deactivation resulting from severe growth and sintering of noble metal NPs at high reaction temperature.At present,encapsulation of active metal NPs within microporous zeolites are received widespread attention,due to the active metal NPs can be confined and stabilized within zeolite frameworks to prevent them aggregation and growth.Unfortunately,the conventional zeolites with sole microporous nanochannels and the pore size is below 2 nm are limited molecule diffusion and metal site accessibility,and unfavorable to the mass-transfer of the reactants and products.Herein,the hierarchical zeolite confined Pd-based catalysts are prepared by a one pot in situ dry-gel conversion method.In addition,we are study the catalytic combustion performance and reaction mechanism for light alkanes over"Pd-based@Hierarchical zeolite" catalysts.The result shows that mass-transfer effect of reactants and products was improved.Meanwhile,the thermal stability,hydrothermal stability and recycling performance of the catalysts are improved.The paper obtained the following main results:(1)The single crystalline zeolite silicalite-1(S-1)with intra-mesopores encapsulating Pd NPs(Pd@IM-S-1)was synthesized by a facile in situ mesoporogen-free one pot strategy.The HRTEM results show that abundant intra-mesopores are present in Pd@IM-S-1 sample and the mean size of the observed intra-mesopores is 23.9 nm.Using methane and propane as model reactions to explore its catalytic oxidation performance,the results show that the Pd@IM-S-1 catalyst exhibits superior methane and propane combustion performance compared to the Pd@SiO2 and Pd/S-1 catalysts.The excellent catalytic oxidation performance of Pd@IM-S-1 may be due to the introduction of mesopores resulting in expose more active sites,which is improved the mass-transfer of reactants and products.And the presence of the Pd-PdO interfaces makes the catalyst possess higher active sites.More importantly,the as-synthesized Pd@IM-S-1 showed high thermal stability,water resistance and recyclability due to the confinement effect of zeolite shell.The mechanism of propane combustion over Pd@IM-S-1 was also studied in detail by in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS).The results show that the propane oxidation over Pd@IM-S-1 follows the Mars-van-Krevelen mechanism and lattice oxygen from Pd-PdO interfaces participates in the redox cycle reaction.(2)A simple one-pot two-step dry gel conversion method was used to synthesize a hierarchical zeolite confined Pd-CeO2 nanowires adsorption/catalysis bifunctional catalyst(Pd-CeO2NW@S-1).The HRTEM and AC-HAADF-STEM results show that the Pd-CeO2NW@S-1 catalyst retains the complete Pd-CeO2 nanowire structure and contain abundant mesopores.Furthermore,the C3H8-TPD results show that the Pd-CeO2NW@S-1 catalyst can adsorb and enrich propane under low temperature,which is beneficial to the propane oxidation.The Pd-CePO2NW@S-1 catalyst shows excellent propane combustion performance(T90=296?,the reaction rate is 1 × 10-2 mol gPd-1s-1 at 325?).This may be attributed to the following three points.(a)The high dispersion of Pd species.(b)The introduction of mesopores makes it expose more active sites,while improving the mass-transfer effect of reactants and products.(c)Due to the strong interaction between the Pd-CeO2 interfaces and S-1 shell leading to a stronger adsorption force of propane,thereby improving its catalytic activity.Additionally,it is still found that the Pd-CeO2NW@S-1 catalyst has excellent thermal stability,hydrothermal stability and recyclability,which is also due to the confinement effect of zeolite shell.In situ DRIFTS results analysis shows that the degradation mechanism of propane on the Pd-CeO2NW@S-1 catalyst still follows the MvK mechanism,in which lattice oxygen species from Pd-PdO and Pd-CeO2 interfaces participate in the entire redox cycle reaction.