Kinetics Study On The Regulation Of Pt-C Nanocomposite Catalyst For Hydrogen Generation

Carbonaceous material,due to its unique physic-chemical properties,has aroused a considerable of interests in real applications.In the last decades,various carbon materials with different nanostructures and properties are synthesized and applicable to catalytic processes as catalyst support,which endow the catalysts with a unique,effective electron transfer system toward unexpected activity,selectivity and stability patterns.Herein,exemplified with metal-carbon catalyzed hydrolytic dehydrogenation of ammonia borane for hydrogen generation,we focuses mainly on fundamentally understanding the kinetics of metal-carbon catalytic reactions.This aims to elucidate the quantitative relationships between carbon surface chemistry,supported metal nanoparticles properties and the apparent reaction kinetics,further establishing the regulation law of catalytic activity and durability based on the proposed volcanic curve.The insights revealed here could guide the rational design of stable metal-carbon complex catalysts with maximum mass-specific reactivity.The main results are summarized as follows:(1)Active site.An approach to identify the active site of metal catalyst based on kinetics analysis and model calculations is developed.Kinetic studies of differently sized Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane show that the hydrogen generation almost follows a zero-order reaction with respect to ammonia borane in the initial period,and Ea is not very sensitive to Pt particle size.The derivation of relationship between the overall hydrogen generation barrier and the barrier of each active site suggests that only one type of active site is dominant for the reaction.A combination of the normalized TOF based on the number of active site atoms as a function of Pt particle size with model calculations reveals that ca.1.8 nm sized Pt/CNT catalyst is optimum with the highest reactivity arising from the largest amount of active site(i.e.,Pt(111))atoms and optimal electronic properties.(2)Reaction and deactivation mechanism.The nature of this catalytic system has been proposed on the basis of thermodynamic analysis,multiple characterizations,and kinetic(isotopic)analyses.A combination of thermodynamic analysis and FTIR measurement reveals the main reaction as NH3BH3+4H2O?NH4B(OH)4+3H.Isotopic experiments indicate the O-H bond cleavage being in the rate-determining step,which NH3BH2*+H2O*?NH3BH2OH*+H*is discriminated as the rate-determining step.On the other hand,the origin of the support-dependent durability is elucidated.Strongly adsorbed B-containing species and the change in Pt particle size and shape are mainly found to cause catalyst deactivation.(3)Volcano curve.Partial substitution of the Pt catalysts by over 11 times less expensive Ru is studied to obtain highly efficient and cost-effective Pt-based catalysts.It is observed that the Pt-Ru bimetallic catalysts,especially Pt0.5Ru0.5/CNT,deliver higher hydrogen generation activity and durability than the Pt/CNT and Ru/CNT catalysts,indicating a remarkable Pt-Ru synergy.The underlying nature of this synergy is elucidated by combining kinetic and isotopic analyses with multiple techniques.The appropriate activation energy and entropy of activation for the reaction are mainly responsible for the highly active Pt0.5Ru0.5/CNT catalyst.Meanwhile,a volcano-type relationship exists between entropy of activation and hydrogen generation activity,where Pt locates at the right side of the top.This indicates that higher Pt binding energy in principle favors the hydrogen generation.(4)Surface chemistry.A strategy to tune the surface cemistry properties of carbon to tailor the electronic properties of supported metal particle in terms of its catalytic kinetics is proposed.Electron-deficient surface defects instead of electron-rich oxygen containing groups on CNT are found to be mainly responsible for the increased Pt binding energy,where it is a facile and effective method to introduce a number of defects on CNT by acid oxidation and subsequently high temperature treatments.Such defect-rich CNT support immobilized Pt nanocatalysts are highly active.Moreover,the surface defect on CNT could also inhibit the adsorption of boron-containing species and stabilizes the Pt nanoparticles to resist the agglomeration during the reaction,leading to high durability.Therefore,the defect-rich CNT appears to be a promising support material to prepare highly active and durable Pt catalysts.(5)Interface properties.The unresolved details of catalyst reduction methods(i.e.,in situ reduction with AB and ex situ reduction with H2)on the hydrogen generation activity and durability are addressed by combining kinetic and isotopic analyses with multiple techniques.Compared with the Pt/CNT-in situ catalyst,the Pt/CNT catalyst exhibits a unique surface(e.g.,fewer Pt-O bonds)and electronic properties(e.g.,higher Pt binding energy),which give rise to lower activation energy and a stronger ability to activate water and thus produce the higher activity.The differences in the number of Pt-O bonds and the adsorption or even encapsulation of Pt nanoparticles by the B-containing species are primarily responsible for the different catalyst durability.Moreover,the formation of Pt-O bonds within the interface between Pt nanoparticles and f-CNF is found to inhibit the catalytic activity.