Effect Of Microstructure/Component Regulation Over Palladium-Alumina Catalysts On The Performance Towards Methane Oxidation

It is an effective way to abate the low-concentration methane emissions from natural gas vehicles by catalytic combustion of methane.Noble metal-based catalysts,notably the palladium-based ones,generally exhibit remarkable low-temperature activity.However,in practical applications,the active palladium species are susceptible to sintering or decomposition,meanwhile they could be poisoned by water vapor or sulfur species in the reaction atmosphere,resulting in the deactivation of catalysts.Herein,tailored alumina supported palladium catalysts with different microstructures and surface properties were constructed to realize a stable,sulfur-resistant and efficient methane combustion.Moreover,the catalytic performance,determining factors and the reaction mechanism were studied.The main research contents and findings are presented as follows.Research contents:（1）As for the simple component Pd/Al₂O₃ catalysts,alumina supports with uniform pore structure were prepared through a specific sol-gel method by using small molecular steroid compound-deoxycholic acid（DCA）as the template,and polyvinylpyrrolidone（PVP）as the co-template.The impact of regulating the molar ratio of templates on the pore structure of alumina supports was investigated,and the facile route of tailoring the pore size of alumina was established.The variation in the structure,surface properties and catalytic performance of catalysts prepared with above alumina via incipient wetness impregnation were disclosed.Coupled with the study on the reaction kinetics and DFT calculations,key factors determining the stability of catalysts with different pore size were revealed.（2）Novel alumina supported palladium catalysts were constructed via a designed one-step method,where the palladium precursors were introduced into the sol-gel system containing aluminum precursors,DCA and PVP.The physicochemical properties of anchored palladium species in as-prepared catalysts were explored,then the catalytic performance under steam-containing conditions and at high temperatures were evaluated as well.（3）Based on the above-mentioned studies,designated metal（Mg,La,Zr）or non-metal（Si,P）dopants with different electronic structures were elaborately incorporated into the Pd/Al₂O₃ catalyst under optimized synthesis conditions,and the resulted variations in microstructure and surface properties of catalysts were analyzed.By taking advantage of the in-situ technology and other characterizations,the adsorption,desorption,migration and transformation behavior of sulfur species on catalysts were quantitatively and systematically explored to identify the key factor affecting the sulfur resistance.Research findings:（1）A designed sol-gel method for controllable synthesis of alumina with different pore size（diameter of 5-12 nm）and uniform pore structure was developed.Pd O particles in catalysts prepared with alumina of 5-7 nm pore size were quite small,which could be retained after calcining at high temperatures,indicating that they were confined in small pores.However,during the heating and cooling light-off tests,above catalysts underwent deactivation at high temperatures in the cooling stage（800-500°C）due to the decomposition of small Pd O particles.Although Pd O particles tended to aggregate to a certain extent in catalysts prepared with larger pore size（8-12 nm）alumina,they did not suffer from significant sintering,demonstrating that the proper pore structure was beneficial to the mass and heat transfer during methane combustion to alleviate the sintering.Moreover,the complete conversion of methane was well-maintained at high temperatures during the heating-cooling light-off tests,proving the enhanced thermal stability of larger Pd O particles,which could be explained by the formation of a larger amount of Pd-O-Al bonds according to the DFT calculations.（2）In the novel Pd-Al₂O₃ catalyst constructed by the one-step method,palladium species were anchored on the surface of alumina through the Pd-O-Al bond.Therefore,the small and uniformly distributed Pd O particles over catalysts could be retained even after calcining at high temperatures or going through high-temperature reactions.After methane reduction pretreatment,the Pd-Al₂O₃ catalyst showed a considerably higher activity with a complete methane conversion at 355°C,along with an excellent water resistance at low temperatures.Different from the above findings on Pd/Al₂O₃ catalysts prepared via incipient wetness impregnation method,the strong Pd-O-Al bond in novel Pd-Al₂O₃catalysts prepared through one-step method endowed the anchored small Pd O particles with a high stability,thus they could effectively resist the decomposition or sintering at high temperatures,leading to a superior thermal stability.（3）A proper content（5 wt.%）of dopants was incorporated into Pd/Al₂O₃ catalysts,and no obvious variation in the crystal phase structure of catalysts was observed.However,the surface acidity/basicity of catalysts was effectively tuned.Compared to the Pd/Al₂O₃catalyst,the amount of surface acidic sites over catalysts doped with Si or P obviously increased to 248μmol NH₃/g STP,while the amount of surface basic sites over catalysts doped with Mg or La drastically increased to 143μmol CO₂/g STP.The doping of Zr resulted in a relatively small increment in the number of both surface acidic sites and basic sites over the catalyst（132μmol/g NH₃ STP and 85μmol CO₂/g STP,respectively）.（4）During methane combustion under SO₂ containing conditions,the amount of adsorbed SO₂ on the catalyst was positively correlated with the number of surface basic sites,which was ca.0.59 mmol/g catalyst over those doped with Mg or La.Whereas the increasing surface acidic sites inhibited the adsorption,leading to a lower SO₂ adsorption amount of 0.20-0.31 mmol/g catalyst over those doped with Si or P.The adsorption amount of SO₂ on Pd/Al₂O₃ and Zr-doped catalysts was moderate（0.45-0.50 mmol/g catalyst）.The strong adsorption of sulfur species on the Mg or La doped support inhibited the sulfation of the active Pd O phase,but the formed sulfates were highly stable,which remained in the catalyst（0.36-0.39 mmol/g catalyst）after regeneration,resulting in an inferior catalytic performance towards methane combustion.The weak adsorption of SO₂ on supports caused the sulfation of Pd O and deactivation of Si or P doped catalysts.Although the sulfur content in the regenerated Si or P doped catalysts was quite low（0.11-0.14 mmol/g catalyst）,the oxidation of methane was also suppressed due to the sintering of palladium species or the presence of Pd SO₄.For the Zr-doped catalyst,the formed Zr（SO₄）₂ inhibited the sulfation of active Pd O and the formation of surface/bulk aluminum sulfates,simultaneously.Additionally,Zr（SO₄）₂ species were liable to decompose during the regeneration process（the remained sulfur content is 0.27 mmol/g catalyst）.Meanwhile,the Zr dopant led to a high oxygen mobility,which promoted the regeneration of active Pd O phase,giving rise to an excellent sulfur resistance and regeneration performance.