Structural Design And Property Of Manganese Oxide Composite Nanomaterials

Energy and environment are the basic conditions for the survival and development of human beings.With the rapid development of economy,the exploitation and utilization of fossil fuels increase sharply,which brings about severe environmental pollution problems.Modern chemical,petroleum,energy and pharmaceutical industries as well as the field of environmental protection.Catalytic processes account for more than 80%of all chemical processes.Therefore,catalytic science and technology play a key role in national economy,energy and environment.In the process of catalytic industry,the development and design of catalysts are the main factors for the development of catalytic science.Transition metal oxides?TMOs?are special because they relatively cheap,abundant and can form different cationic oxidation states.TMOs-based catalysts play an important role in the field of heterogeneous catalysis due to their special electronic structures and their easily adjustable catalytic properties.Manganese oxides?MnOx?is a typical member in the family of TMOs,which is widely used in environmental and energy fields.Many types of synthetic techniques have been developed in the last decade to obtain MnO₂ with precise phase and tunnels to control the material properties.However,it is very difficult to obtain the precise phase content of MnO₂ materials because of the complexity of phase transformation process.Moreover,MnO_x-based catalysts have been intensively in environmental and energy,such as selective reduction of NO_x?ORR?,and oxygen reduction reaction,etc.However,major issues still remain unresolved,such poor resistance of H₂O and SO₂ in SCR field etc.There for my research was focused on the structural design of MnO_x-based composite nanomaterials to overcome those drawbacks.?1?In the first part,I present an approach for the precisely controlled phase transformation of MnO₂ in order to synthesise different compositions of??-MnO₂materials,by adding a trace amount of Zn?acac?₂ as the phase transformation-inducing agent in a hydrothermal reaction.The single-atomic dispersion of Zn might reduce thebarrier of phase transformation of?-MnO₂ to?-MnO₂.The ratio of Zn species present in the single-atomic dispersions and nanoclusters might dominate the generation of?-MnO₂ and?-MnO₂.The results of oxygen reduction reactions indicate that the MnO₂ materials have potential applications as promising catalysts in electrochemical catalysis.?2?Low-temperature oxidation of carbon monoxide?CO?is one of the most studied reactions because of its scientific and industrial applications.In the second part,CO catalytic reaction was used as a probe reaction to study the catalytic performance of Pt/??-MnO_x,Pt/MnO_x.TEM and EDS results indicated that Pt nanoparticles could be selective loaded to the surface of?-MnO₂ in the mixed??-MnO₂ materials.The DFT calculation revealed that?-MnO₂ shows a lower adsorption energy to Pt nanoparticles compared to?-MnO₂.Furthermore,the best low-temperature CO catalytic performance was observed over Pt/??-MnO_x compared to Pt/?-MnO_x and Pt/MnO_x.In order to reduce the complexity of the catalytic system,Pt/MnO_x catalyst was studied.I discovered that the CO performance could be enhanced by?-MnO₂ support materials.Nearly complete CO conversion was obtained over the Pt/MnO_x catalyst below 100 ? under our catalytic conditions.TOF?turnover frequency?of the Pt/MnO_x was five and nine times as Pt/MnO_x-COM?COM,commercial?and Pt/Al₂O₃ at 150 ?,respectively.X–ray diffraction?XRD?Rietveld refinement of fresh and spent catalysts,CO-temperature–programmed desorption?CO-TPD?and diffuse reflectance infrared Fourier transform?DRIFT?combined results allowed us to conclude that O₂ molecules were exhibited strong adsorption and activation ability on the surface of Pt/MnO_x compared to Pt/MnO_x-COM.Moreover,by combining the result of Rietveld of XRD,Raman,CO-TPD and O₂-TPD?inlet CO and inlet CO/O₂ mixture gas as adsorbates,respectively?,we inferred that the interfacial lattice oxygen in the Pt/MnO_x might be the active oxygen species for the low-temperature CO oxidation.?3?In the part ?,a simply approach was proposed to improve the H₂O and SO₂resistance ability over MnO₂ catalysts for low temperature SCR,by modifying with PTFE.Based on XRD,FT-IR,and SEM results,I found that MnO₂ nanowires were tightly packed together with PTFE.CA analysis showed that a more hydrophobic surface was obtained with the introduction of PTFE.Furthermore,PTFE modifying enhanced H₂O and SO₂ resistance,lowering the probability of surface active sites consumed by H₂O and SO₂.?4?.In the part ?,Ce-MnO₂ catalyst was prepared via ion exchange using Ce³⁺as exchange ion and birnessite-type MnO₂ as precursor.Ce-MnO₂ possess the best low-temperature NH₃-SCR activity and excellent resistence to H₂O poisoning compared with K-MnO₂ and Ce_0.38Mn_0.62O₂-CP catalysts.Pyridine-IR result of K-MnO₂-spent catalyst indicated that the Br?nsted and Lewis were almost disappeared due to the poisoning of K.XRD,and XPS of Ce-MnO₂ and Ce_0.38Mn_0.62O₂-CP catalysts suggented that Ce-MnO₂ have more vacancies and Mn on its surface.H₂-TPR and H₂ comsuption indicated that Ce-MnO₂ have a better reduction ability compared to Ce_0.38Mn_0.62O₂-CP.And NO-TPD,and NH₃-TPD results suggest that more types of NO and NH₃ species were exited on the surface of Ce-MnO₂.Pyridine-IR result indicated that the content of Br?nsted on the surface of Ce-MnO₂ was two times toward Ce_0.38Mn_0.62O₂-CP.Thus,Ce-MnO₂ shows a betweer low-temperature NH₃-SCR activity.Furthermore,the Ce-MnO₂ shows a good resistance of H₂O due to the week adsorption of H₂O on the surface of Ce-MnO₂.This study could provide an easy and facile way to prepare efficient and stable NH₃-SCR materials in the high H₂O content NH₃-SCR field.?5?In the last part,Pt/MnO was prepared as the electrocatalyst for the ORR in alkaline media via deposition-precipitation and H₂-reduction approaches.The results revealed that the Mn⁴⁺/Mn³⁺pair and Pt species could act as active components for the ORR in alkaline media.The O-intermediate species limiting the rate of the ORR could be easily dissociated because of the oxygen affinity of the surface of the Pt-based catalysts.Therefore,the Pt mass activities over the Pt/MnO(3.7 A mg_Pt^-1 at 0.7 V)catalysts was 18.5 times as that of a commercial 20%Pt/C catalyst.Moreover,the Pt atoms with different coordination number might control the ORR pathway in alkaline media.Thus,Pt/MnO_x and Pt/MnO showed n values of 3.6 and 4.0,respectively.Moreover,the Pt/MnO catalyst exhibited a good ORR stability after 500 cycles as compared to the Pt/MnO_x catalyst in 0.1 M KOH.This work not only present an approach for increasing the utilization efficiency?UE?of Pt for electrocatalysts,but also provided a new perspective to paving the way for improving the UE of Pt in the commercial electrocatalysts.