Studies On Regulation Of Catalyst Active Sites And Reaction Mechanism Towards Hydrogen Purification

Proton exchange membrane fuel cells（PEMFCs）with the use of hydrogen as feed stream,takes the advantages of small-size,high energy conversion efficiency,high-safety and fast start-up speed,which show potential applications in many fields such as fixed power station,portable electronic equipments and electric vehicles.Industrial hydrogen sources nowadays are mainly produced through hydrocarbon reforming or partial oxidation,and the obtained H₂ usually contains 1-10%CO.The residual CO would poison the Pt anode of PEMFCs,resulting in a significant decline in battery life,which is an important bottleneck for the large-scale promotion of PEMFCs.Water-gas shift reaction（WGSR）and preferential oxidation of CO（CO-PROX）are promising solutions to remove residual CO from the feed stream to avoid Pt electrode poisoning.However,there are some challenging tasks:Firstly,fundamental understanding of intrinsic site of catalyst,structure evolution and reaction mechanism under reaction conditions is still controversial,which limits the rational design of high-efficiency catalysts towards hydrogen purification.Secondly,under the reaction conditions,the active components of the catalyst are easy to reconstitute,migrate or aggregate,resulting in the catalyst deactivation.Therefore,the stability of hydrogen purification catalyst needs to be improved,so as to provide guarantee for practical application.In view of the key scientific problems and challenges above,in this dissertation,rational design and preparation of three kinds of heterogenous catalysts were carried out,and their catalytic performance towards WGSR and CO-PROX were studied.Firstly,we regulated the preferentially exposed facets and the oxygen defect concentration of supports for the Pt/CeO₂ catalysts;moreover,the influence of geometric and electronic structure of surface/interfacial sites on the activation adsorption of reactants,as well as the catalytic mechanism were emphatically revealed.Secondly,based on structural topological transformation of layered double hydroxides（LDHs）,we synthesized Pt-Fe（OH）_x composite catalyst and spinel-like oxide catalyst,whose surface/interface structure were tuned systematically.In addition,their catalytic behavior towards PROX reaction was studied and the structure-property correlation was revealed.The main research contents are as follows:1.Highly stable Pt/CeO₂ catalyst with interfacial embedding structure towards WGSRFor the WGSR reaction,the strong metal-support interaction（SMSI）of catalysts plays an important role in improving the catalytic activity and stability,which is an important scientific issue in this field.Herein,using Pt/CeO₂ as the model catalyst,we reported an embedding structure at the interface of Pt clusters and CeO₂（110）,where Pt clusters（～1.6 nm）were partially embedded in the lattice of CeO₂ with 3～4 atomic layers.In contrast,this phenomenon was not found at the interface of Pt and CeO₂（100）.This unique geometry triggers the electronic effect,where electron transfer occurs from Pt clusters to CeO₂（110）support with the formation of Pt^δ+-O_v-Ce³⁺interfacial structure.In situ experimental studies and theoretical calculations show that the Pt^δ+-O_v-Ce³⁺interfacial sites serve as the intrinsic active center of WGSR,which imposes an appropriate strength of CO adsorption and greatly reduces the energy barrier of H₂O dissociation.This is the reason for the excellent catalytic activity of Pt/CeO₂（110）with a high reaction rate of 15.76 mol _CO g _Pt^-1 h^-1 and a TOF value of 2.19 s^-1 at 250°C.Moreover,this interface embedding structure plays a crucial role in stabilizing Pt nanoclusters:the Pt/CeO₂（110）catalyst exhibits a good stability（120 h）under reaction conditions.This study displays direct evidence for the correlation between catalyst microstructure and performance,and provides new insights into the metal-support strong interaction for structure sensitive reactions.2.Studies on the CO preferential oxidation catalyzed by interfacial sites of Pt-Fe（OH）_xA series of highly dispersed Pt-Fe（OH）_x catalysts consisting of platinum-Fe（OH）_x clusters（～2 nm）were prepared by reduction of Pt Fe Mg Al-LDHs precursor under reaction conditions.The optimal catalyst Pt-5Fe/Mg Al O_x,with abundant Pt^δ+–（OH）_x–Fe³⁺interfacial sites,shows excellent catalytic performance towards CO-PROX:CO can be completely removed from the H₂-rich flow at 25°C with a high space velocity of 130,000m L g_cat^-1 h^-1.An extremely wide operation temperature window（25-225°C）is achieved.Notably,the Pt-5Fe/Mg Al O_x catalyst gives a mass specific activity of8.55 mol _CO g _Pt^-1 h^-1 at 20°C,which is higher than other reported Pt-based catalysts.In situ experiments and theoretical calculations reveal that the OH group at the Pt^δ+–（OH）_x–Fe³⁺interfacial site is easily bound to the linearly adsorbed CO at the nearby Pt^δ+site,followed by the formation of*COOH intermediate（0.31 e V）and then its decomposition to CO₂（-0.26 e V）.At the same time,the generated coordination unsaturated Fe²⁺site can activate O₂without energy barrier.In this part,the Pt cluster-transition metal hydroxide composite catalyst is designed and prepared,which has potential application in CO-PROX reaction.3.Fine regulation of local structure of spinel-like oxide catalysts and their catalytic performance towards CO-PROX reactionA series of spinel-like Mn Co O_x catalyst samples were prepared via structural topological transformation of Co₅Mn₁-LDHs precursor,among which the Mn Co O_x-300 catalyst（calcined at 300°C）with rich octahedron-distorted lattice oxygen exhibited an excellent catalytic behavior for CO-PROX reaction.A complete CO removal is achieved in H₂-rich atmosphere at 80°C and a high space velocity(80,000 h^-1).The Mn Co O_x-300 catalyst has a wide reaction temperature window of 80-200°C,which matches well with the operation temperature window of PEMFCs.The performance of Mn Co O_x-300overmatches to the reported non-noble metal catalysts.In addition,stability tests at 80°C demonstrate a satisfactory result:neither activity nor selectivity decreases significantly within 60 h.In situ Raman,in situ TPSR and in situ XAFS reveal that the Co_oct³⁺-O^2--Mn_oct⁴⁺ structure acts as the intrinsic active site in CO-PROX,and an Mv K reaction mechanism is confirmed.In situ XAFS,¹⁸O isotope labeling experiments and DRIFTS experiments verify that the decrease of CO selectivity at high temperatures is due to the competitive oxidation of H₂against CO at the same active site.As the reaction temperature increases from room temperature to 200°C,the intermediate changes from carbonate to HCO₃*and HCOO*.This work provides a new idea for the structure design and performance strengthening of CO-PROX catalyst,which displays fundamental significance and application prospects.