Active Sites Engineering And Kinetics Study Of Carbon Supported Ru And Pt Catalysts For Hydrogen Generation

The catalytic reaction kinetics has emerged as the basis for the design of industrial catalysts.The current catalytic reaction kinetics mainly focus on the derivation,calculation and measurement of reaction rate constant,while neglecting the underlying relationship with the number and electronic properties of catalytic active site.In this study,exemplified by ammonia borane hydrolysis over noble metal Ru catalysts,the effects of particle size were firstly investigated over a combination of multi-faceted kinetics analysis,advanced structural characterization,and model calculations,where the main active sites were discriminated as the Ru edge site.The heteroatom Cl and Co were used to manipulate the active site environment of Ru,and the negatively geometric effect induced by Cl and the positively electronic effect induced by Co were explored by multiple techniques.Further,the acid-leaching of Co from bimetallic catalysts creates abundant edge-like Ru active sites with higher catalytic activity.Moreover,the carbon-supported Pt catalysts with easily measured electronic properties were chosen as the model catalyst to study the electronic effects by tailoring carbon surface chemistry.Excluding the particle size effects,the descriptor of catalyst activity was identified based on the linear relationship between the Pt electronic structure and kinetic parameters.Subsequently,incorporating the transition state theory and Temkin equation,we derive a new kinetics model involving the structural properties of the catalyst,which can guide the rational design of metal catalyst for this reaction.The main results are summarized as follows:(1)Ru active site identification.The size effect of Ru catalyzed ammonia borane hydrolysis was explored to identify the dominant active sites.By preparing differently sized Ru catalysts,it was found that the catalytic activity exhibited a typical volcano-shaped curve as Ru particle size,compared with the almost unchanged activation energy for these catalysts.The geometric characterization suggested the morphology of truncated hexagonal bipyramid for these supported Ru particles,which involved four types of surface Ru atoms.Further multi-faceted kinetics analysis suggested that there mainly existed one type of active site for this reaction.Hence,the combination of the catalytic active site and the number of the four types of Ru atoms suggested the edge site as the main active site for this reaction,which was further confirmed by kinetic isotopic analysis and DFT calculations.(2)Ru active site engineering.A strategy of tailoring the micro-environment of the Ru active site by introducing Cl and Co heteroatoms was developed.It was demonstrated that the Co could transfer electrons to Ru to increase its surface electron density and lower its binding energy.On the contrary,the strong adsorption of Cl over Ru particle surface could strongly block the active site by steric hindrance,thus inhibiting the activation of water.Therefore,the acid-leaching of Co from bimetallic catalysts creates abundant edge-like Ru active sites with much lower H2O activation barrier,as indicated by the kinetics switch from a Cl coverage-limited regime to a reactant activation regime,thus yielding a significant 4.1-fold increase in hydrogen generation activity of 751.0 molH2·molRu-1·min-1.Further kinetic experiments proved that the catalyst had suitable binding strength with reactants and abundant active sites,thus exhibited excellent activity.(3)Pt electronic structure engineering.Pt with easily detected electronic properties was chosen as active metal.The electrophilic properties of different oxygen-containing functional groups(OCGs)on the surface of the CNTs were investigated by DFT calculation.The type and concentrations of these OCGs on the oxidized CNTs were fine-tuned by heat treatment under inert atmosphere at elevated temperatures to tame the electronic properties of Pt,Excluding the effects of the textural properties,surface defects,polarity and metal particle size,the electronic properties have been identified as the main factor in determining the catalytic activity.A quantified relationship between the carbon surface chemistry and Pt electronic properties was established.It was found that the ratio of electron-withdrawing group to electron-donating group reached the highest at the heat treatment of 400? which the corresponding Pt/CNT-400 catalyst exhibited a 15-fold increase in catalytic activity of 811.3 molH2·molPt-1·min-1.Moreover,this catalyst also demonstrated durable activity after several times usage,which emerged as a highly efficient catalyst for hydrogen production.(4)Nano-kinetics model developing.The regulation mechanism of Pt electronic structure on kinetic parameters was investigated by tailoring the surface OCGs and isolating the effects of textural properties,surface defects,polarity and metal particle size,which demonstrated a strong kinetic compensation effect.It was demonstrated that both the activation entropy and activation enthalpy exhibited linear dependencies on Pt binding energy,and thus Pt binding energy is unprecedentedly identified as an experimentally measurable descriptor of Pt active site.Further incorporating it with the number of Pt active sites,the nano-kinetics modeling is proposed for the first time,which can individually quantify the contributions of the nanoscopic electronic and geometric properties,and precisely predict the catalytic performance.Our results could demonstrate a potentially ground-breaking methodology to design and manipulate metal-carbon catalysts with the desirable properties.