Interstitial Boron-Regulated Electronic Structure Of Noble Metal Catalysts For Performance Optimization

Noble metals play an important role in organic synthesis,carbon-carbon coupling,hydrogenation,electrocatalysis and other heterogeneous catalytic reactions.The addition of light metal or transition metal element to lattice of noble metals can effectively regulate their electronic structure and catalysis performance.The peculiarities of light metal element such as B rely on the followings:(i)the light metal atom with relatively smaller atomic size dissolved easily into the lattice interstitial of parent metal,while transition metal atom is more inclined to replace the parent metal atom.(ii)Different from metal-metal alloys with d-d orbital hybridization,alloy with light metal element is usually by orbital hybridization between their s,p orbitals and d orbitals of parent metal atoms.These peculiarities of light metal atom gives us the opportunity to optimize the catalysis perforamnce of noble metal via geometric and electronic effect.In this paper,we realize the precise synthesis of noble metal B interstitial compound with a fixed atomic ratio by solid phase replacement strategy,and the electrocatalytic hydrogen evolution reaction as the research model reaction.The relationship between the bonding mode of boron atoms,boron concentration and boron order in the noble metal crystal lattice and the corresponding catalytic performance of noble metal-B interstitial compound expounded theoretically and experimentally.Besides,the crystal phase and B(s,p)-noble metal(d)orbital hybridization on the influence on the catalysis performance of noble metal-B interstitial compound are also researched.The main contents of this paper are as follows:1.The parer present a joint theoretical and experimental study that reveals the importance of ordering of interstitial boron atoms in Pd-B alloy's near-surface electronic structure and catalytic performance.We theoretically calculated the regulation effect of the distribution and concentration of interstitial B atoms on the d-band electronic structure(such as d-band width and d-band center)of Pd parent and the catalytic properties of hydrogen evolution on the surface.The theoretical results demonstrate that the sub-surface location of interstitial boron atoms with a Pd:B atomic ratio of 2:1 is necessary to ensure an appropriate degree of interatomic s,p-d orbital hybridization,and thereby an optimal surface electronic structure for the hydrogen evolution reaction(HER).We adopt the solid phase synthesis strategy to prepare this highly crystalline intermetallic Pd₂B,and it can realize the desired Pd-B alloy structure.Due to its optimal electronic structure and ordered arrangement of interstitial boron atoms,the intermetallic Pd₂B exhibits Pt-like catalytic activity for HER and has excellent catalytic stability for over seven days.2.We selectively synthesized Pd₆B,Pd₅B₂,and Pd₂B via the solid-phase reaction between Na₂Pd Cl₄and Mg B₂based on the adjustment of experimental parameters,including:(i)the molar ratio of Na₂Pd Cl₄and Mg B₂;(ii)the reaction time;and(iii)the reaction temperature.The regulation of their crystal phases may construct new design principle for Pd-based catalysis materials.The electrocatalytic HER results identify that catalysis performance follows the increased order of Pd,Pd₆B,Pd₅B₂and Pd₂B as the increase of B concentration in Pd lattice interstitial site.We combined DFT theoretical calculation and experiment result demonstrated the reason of enhanced catalysis performance originated form the following several aspects.The strong orbital hybridization between Pd(d)–B(s,p)contributed to the Pd₂B exhibits significantly descend d-band center compared to Pd,Pd₆B,Pd₅B₂,suggesting much weaker binding interaction between the absorbed molecules and catalyst,and then enables absorbed H^*easily activated and dissociated.The bond length of Pd-B decreases in the sequence Pd<Pd₆B<Pd₅B₂<Pd₂B,which is followed by an increase in the B concentration in lattice interstitial site of Pd host lattice.The shorter Pd-B bond length in Pd₂B results in the optimized interaction of Pd-B bond,promoting the stability of Pd atom during catalysis process.Accordingly,a relation was proposed wherein the catalysis performance of Pd-B interstitial compound is co-determined by the B concentration in Pd host lattice,as well as of degree of B(s,p)-Pd(d)orbital hybridization.3.The paper developed the solid phase synthesis method,and synthesized intermetallic rhodium boride(Rh B)comprising an asymmetrically strained hcp Rh sublattice.We investigated the three possible factors(lattice expansion,asymmetric strain,and boron-relevant electronic effect)for the formation of unconventional hcp-Rh sublattice in Rh B through theoretical calculations.The DFT calculation and crystal structure analysis results demonstrated that the covalent interaction of interstitial boron atoms is found to be the main contributor to the generation of asymmetric strains and the stabilization of the hcp Rh sublattice.The electrocatalytic HER results exhibit that the Rh B generates 10 m A/cm²current density at an overpotential of 15 m V,which is similar to that in present of Pt as the catalyst.The absolute value of?G_H*of Rh B is close to that of Pt,which proves that Rh B has excellent HER performance in theory.The further theoretical calculation results demonstrate that the Rh(d)–B(s,p)orbital hybridization induces an optimized electronic structure,which contributes to Rh B as an excellent hydrogen-evolving eletrocatalyst with Pt-like activity.