Palladium Nanocatalysts And Their Supporting Materials For Electrochemical Reactions

The increase in emissions of environmental pollution caused by the excessive use of fossil fuels,as well as the growing global demand for energy,have led to the need to investigate new alternative sources to produce electric energy in a more environmentally friendly way.One of the most promising technologies that will be used in the near future,are the proton exchange membrane fuel cells?PEMFC?that produce electrical energy from the oxidation of H₂,in the presence of an electrocatalyst?Pt,Pd?,with a theoretical effectiveness greater than 80%and a zero emission of pollutants.However,the stability of commonly used Pt/C and Pd/C catalysts is limited due to the aggregation,coalescence,local Ostwald ripening and detachment of metal nanoparticles from the carbon support that lose a number of active sites.For this reason,it is crucial to design more efficient,economically accessible,and stable electrocatalysts.For this purpose,the synthesis and characterization of new catalysts are required to carry out the necessary reactions in the fuel cells.In view of this situation,this doctoral thesis describes the exploring of the electrochemical activity and stability of new supported nano-particulate electrocatalysts for the oxygen reduction and hydrogen evolution reactions,based on ternary noble metal alloy and metal/semiconductor composites prepared by two synthesis methods?precursors reduction in aqueous solutions in convection oven as well as under hydrothermal and solvothermal conditions?,using different noble metal salts as precursors and several types of metal oxide semiconductor TiO₂ to form electrocatalysts working in acidic and alkaline electrolytes.Employed catalysts were comprehensively assessed electrochemically and physio-chemically using techniques such as:voltammetry,rotating disk electrode technique,TEM,sophisticated spectroscopic techniques?XPS and STEM-EELS?among other methods.The thesis investigates the structure-property relationships through regarding the physicochemical aspects of hybrid nanostructures formation mechanisms and metal-support electronic interaction.We introduce the novel straightforward route to synthesize and the porous ternary Pt-Pd-Ag alloy nanoflowers;the method allows tuning the morphology and particle size.The suggested formation mechanism can contribute to optimization of synthesis parameters of lower cost electrocatalysts.The nanoflowers are superior oxygen reduction reaction electrocatalyst compared to commercial Pt/C and Pd/C in acidic medium in both activity and long-term durability.The improvement in properties are ascribed to the porous structure,3D open framework and beneficial multiple interaction in the multimetallic system.Furthermore,we propose the use of support effect to promote the electrocatalytic reactions without light illumination and without additional support modification by doping or defect generation.Consequently,the increased mass and specific activities and stability?after undergoing up to30 000 cycles of a potential cycling accelerated durability test?resulted from the electron transfer from semiconductor support to metal catalyst.The order and direction of electron transfer were proved by XPS and EELS and are consistent with ORR and HER improvements relative to the same catalysts without TiO₂ supports.Notably,most of the obtained hybrid electrocatalysts remarkably outperformed commercial Pt/C and Pd/C catalysts.Moreover,based on the work functions of metal catalysts and various TiO₂ supports,we propose a mechanism of metal/semiconductor heterojunction and electron transfer for each type of Pd catalyst and TiO₂support.Both model and experiments display that rutile TiO₂ transfer more electron to Pd than anatase,the ascending trend of electron transfer in Pd/faceted anatase TiO₂ is in agreement with an increase of{001}-facet exposure percentage in TiO₂.The order of electron transfer in faceted Pd/anatase TiO₂ system is octahedra>cube>icosahedra,consistent with the peak shifts of EELS and XPS,and ORR improvement.The order of electron transfer was also theoretically confirmed by density functional theory?DFT?computational simulations.We also optimized the experimental conditions of Pd/TiO₂ composite preparation through the novel and promising approaches enabling to establish good interfacial contact between Pd and TiO₂ exhibiting the electron transfer from support to catalyst,which is beneficial for ORR and HER.Thus,we provided solid proof that the electron distribution in Pd/TiO₂system can be modulated and the electron transfer can be manipulated through the tuning of work function and Fermi level by selecting the exposed facet of Pd and TiO₂,leading to enhanced ORR and HER activity and stability.Overall,the results presented on this thesis showed some important promoting effects for the regarded reactions both in acidic and alkaline media and allow us to have a better understanding of the different effects performed by titanium dioxide supports on the activity and stability of palladium-based electrocatalysts.