Synthesis And Preferential Co Oxidation Performance Of Composites Containing Hydroxyl-rich Mg-based Support

With the progress of society,the application of clean energy has become an effective way to solve the increased environmental issues.Hydrogen,one of the most ideal clean energy sources,has important applications in many fields,especially as a fuel for proton exchange membrane fuel cells?PEMFCs?.The hydrogen source is mainly obtained by hydrocarbon reforming with a water-gas conversion reaction.Generally,such H₂ production contains about 1%CO,which can poison Pt electrodes in fuel cells.At present,preferential CO oxidation?PROX?is the most promising method for hydrogen purification.However,realizing the CO conversion??99%?in a wide temperature operating window still faces huge challenges.Therefore,it is necessary to synthesize highly efficient and economical catalysts.Currently,PROX catalysts mainly include two types:Cu-Ce based catalysts and noble metal catalysts.Noble metal catalysts have attracted much attention due to their high catalytic activity.Among many noble metal catalysts,Pt based catalysts exhibit good CO adsorption capacity,but their CO temperature operation windows are still narrow and their catalytic activity at low temperature range is poor.Obtaining effective support is the key to solve the problems that Pt based catalysts is facing because catalyst support brings out the capability of the supported metal as a catalytically active center.On the other hand,H₂O/OH,one of the most abundant molecules on earth,plays an important role in many chemical and heterogeneous catalytic reactions.In the PROX reaction,the presence of H₂O/OH could enhance the catalytic activity,but its role is not well clear.Therefore,designing new support,uncovering the role of H₂O/OH and further confirming the catalytic active sites are highly essential for the improvement of PROX performance.In this dissertation,Pt supported on Mg-based rich hydroxyl supports were prepared.Detail investigation shows that the rich surface hydroxyls on the supports enhance the interaction between Pt-based metal and supports,improve the dispersion of Pt metal,and increase catalytic active sites.The obtained PROX catalysts exhibit a high CO conversion,excellent stability and wide temperature window.The main results are as follows:1.Kinetic control synthesis of hydroxyl-rich Mg?OH?₂ nanosheets as the support of PROX:Hydrothermal synthesis was used to control the anisotropic growth of hexagonal Mg?OH?₂ nanosheets by changing the reaction temperature and reaction time.Benefited from the Ostwald ripening process,the thickness of Mg?OH?₂ was tuned from 7.6 nm to 24.0 nm,while the diameter increased from 18.2 nm to 30.2 nm.The grain growth kinetics for the thickness was well described in terms of an equation,D⁵=7.6⁵+6.9×10⁸exp?28.14/RT?.After the deposition of Pt nanoparticles?5 wt%?on the Mg?OH?₂ nanosheets,Mg?OH?₂ with a thickness of 7.6 nm showed the best PROX performance,with a wide temperature window of 140²00 ^oC.This was mainly due to the large specific surface area of Mg?OH?₂,which was more likely to produce surfactant hydroxyl groups,and to form a strong chemical interaction with Pt.This work may provide a new design idea for the controllable synthesis of hydroxyl hydride support for the catalysts of proton exchange membrane fuel cell.2.Hydroxylated MgO supported Pt-Ni nanoalloys for preferential CO oxidation:Different mass ratios of Pt-Ni nanoalloys were supported MgO by introducing surface hydroxylation.The sample with Pt-Ni nanoalloys?mass ratio of Pt to Ni=5:0.5?supported on hydroxylated MgO nanosheets exhibit the best CO-PROX catalytic performance owning the widest temperature window?i.e.,100-200 ^oC?and an excellent durability?i.e.,retention at 180 ^oC for 50 h?.This optimum catalytic performance is attributed to the synergetic effects between Pt,Ni species and hydroxyls that stabilized surface/interface Ni-Pt-?OH?x active sites.In more detail,traces of Ni are conducive to the downshift of Pt d-band center that enables the control of CO adsorption to some extent and does not reduce the O₂ activation ability of Pt.The hydroxyls bonded on the interface could anchor Pt-Ni nanoalloys and participate in the preferential CO reaction to form the*HCO₃ intermediates.The findings may provide important hints about the surface/interface engineering of active sites for many nanoscale alloys-oxide composites.3.Mg induced surface hydroxylation of NiO nanocrystalline for preferential CO oxidation:A series of Pt/Ni_1-xMg_xO catalysts with a low Pt loading of 0.5 wt%were prepared by the coprecipitation and annealing processes followed by a wet impregnation.Mg²⁺was successfully doped into the NiO lattice to form a nanoscale solid solution,and hydroxylation with different concentration was generated on the surface of the solid solution.The analysis results showed that these surface hydroxyl groups could effectively stabilize the Pt²⁺active site under the condition of preferential CO oxidation?CO-PROX?.The formation of CO₂ proceeds through bicarbonate intermediate species with the help of hydroxyl groups.Among all the studied samples,Pt/Ni_0.5Mg_0.5O exhibits the widest temperature operating window?100-240 ^oC?and excellent stability?200 ^oC,50 h?.The results reported here could inspire one to find more new hydroxyl-enriched supports that improve the noble metal utilization efficiency for enhancing the catalyst performance in CO-PROX and other relevant catalytic fields.4.Atomically dispersed Pt catalyst derived from the hydroxyl scissorhands of spinel Mg-based oxides:Atomically dispersed Pt?Pt SAs?supported on the surface of hydroxyl functionalized spinel Mg-based oxides were prepared by a combination of hydrothermal and wet impregnation route.Owing to the oxygen site parameter of u>0.375,MgCo₂O₄ could provide surface Mg²⁺tetrahedral sites to incorporate Pt that were partially oxidized to high valence states.More importantly,Pt aggregation could be effectively avoided with the help of the cutting effect from hydroxyl.The advantage of MgCo₂O₄ structure and the scissorhand role of hydroxyls enable Pt to be strongly anchored on support surface via Pt-O interfacial bonds,which improves the PROX performance of Pt SAs/MgCo₂O₄ catalyst with Pt content as low as 0.23 wt%.Pt SAs/MgCo₂O₄ catalyst achieved a 100%CO conversions at 80 ^oC and exhibited a wide temperature operation windows of 120 ^oC?from 80 to 200 ^oC?,superior to other catalysts ever reported in the application of proton-exchange-membrane fuel cells.Morever,hydroxyl group participated in the formation of formate intermediate species?*COOH?in the catalytic process.These intermediate species can be rapidly decomposed into CO₂.The findings reported in this work would open the door to obtain other atomic active sites of metals via hydroxyl cutting on the magnesium containing supports for many important fields.