Structure Regulation Of Pt Catalysts On Nitrogen-doped Carbons And Their Catalytic Oxidation Performance

Carbon nanomaterials have wide applications in catalysis field due to its peculiar structure and excellent physicochemical properties. Nitrogen dopants endow carbon materials interesting electronic characteristics and surface chemistry, making nitrogen-doped carbon nanomaterials a promising candidate as metal-free catalysts or supports for metals. However, the exact mechanism underlying the interaction between N-doped carbons and metallic nanoparticles（NPs） has not been fully understood yet. As one of the most important fundamental issues, it is still under debate which type of nitrogen functionality is responsible for anchoring metallic NPs and the nature of MSI is unclear yet. Therefore, it’s significant to explore the influence of interaction between N-doped support and metal NPs on the electronic structure of metal NPs and catalytic performance. In this work, N-doped carbon nanotubes（NCNT） were used as support to load Pt. Various methods were employed to tune the electronic and geometric structure of Pt catalysts, including tuning the support, changing the loading method and introducing promoter. We spare no efforts to evaluate the catalytic performance of the Pt catalyst and understand the relationship between the structure of metal catalysts and the intrinsic activity of certain reaction. The main contents are as follows:（1） Effects of specific nitrogen functionality and oxidative functionalization of the surfaces of NCNT on the interaction between Pt NPs and NCNT in ethylene glycol（EG） reduction method have been systematically investigated. Their catalytic consequences were studied using aerobic oxidation of glycerol and electro-oxidation of CO as probing reactions. The results suggested that nitrogen dopant obviously enhanced the dispersion of Pt NPs. Strong interaction between Pt and NCNT was observed. Graphitic nitrogen preferentially interacted with Pt NPs, evidenced by strong electron transfer from graphitic nitrogen as electron donor to metallic Pt NPs. The oxygen-containing groups introduced by oxidation of NCNT may reduce the donor-acceptor interaction due to the electronegativity of oxygen. Superior catalytic activity was achieved over Pt/NCNT in the oxidation of glycerol and electro-oxidation of CO, compared with conventional CNT as support. Moreover, for both aerobic oxidation of glycerol and electro-oxidation of CO, it was observed that the good intrinsic activity depended strongly on the electron enrichment of Pt NPs.（2） The influence of Pt catalysts preparation process, including EG reduction, sodium borohydride（NaBH₄） reduction and ex-situ H2 reduction method, on the Pt NPs-NCNT interaction and electronic structure of Pt catalysts have been systematically investigated. Their catalytic consequences were studied in the electrooxidation of glycerol, formic acid and CO, together with ammonia-borane hydrolysis to establish method-structure-activity correlation. The results suggest that Pt4f7/2（0） of Pt catalysts by ex-situ H2 reduction have the highest Pt4f7/2（0） binding energy, due to Pt NPs interacting with defects or pyridinic nitrogen, regardless of the influence of Pt size effect. The catalytic performance was strongly related to the electronic structure of Pt NPs, evidenced by the strong correlation between Pt4f7/2（0） binding energy values and activity. The electron enrichment Pt NPs had better intrinsic activity in electro-oxidation of glycerol and formic acid. The onset potential was lower in glycerol and CO electrooxidation over electron enrichment Pt NPs. Contrarily, the electron deficient Pt NPs had better intrinsic activity in the ammonia borane hydrolysis reaction.（3） The catalytic performance and structural properties of Pt-Bi catalysts were systematically studied to understand the promoting role of Bi for Pt, based on which a simple and feasible production of 1, 3-dihydroxyacetone（DHA） from glycerol may be reached. The results suggest that the selective oxidation of glycerol to DHA catalyzed by Pt supported on NCNT can be significantly promoted in the presence of Bi or Sb in reaction solution. The Bi-promoted Pt/NCNT underwent dynamic surface reconstruction through leaching and adsorption of Bi adatoms, due to the formation of glyceric acid. It has been ascertained that Bi preferentially deposits on the step sites of Pt NPs, and then blocks the terrace sites. The improved DHA selectivity was major attributed to the geometrical effect and complex effect among Bi promoter and Pt NPs with the reactant.（4） A wide spectrum of N-doped carbon materials were reacted with electron probe molecule 7’7’8’8-tetracyanoquinodimethane（TCNQ） and tetrathiafulvalene（TTF） to explore the electron transfer property of N-doped carbon materials as electron donor. The electron would transfer from the highest occupied molecular orbital（HOMO） of N-doped carbon materials to the lowest unoccupied molecular orbital（LUMO） of TCNQ. The strength of the interaction was strongly correlated with the atomic ratio of pyridinic nitrogen to graphitic nitrogen, displaying an inverse volcano. The interaction was weak when the ratio was at a critical point between 0.5 0.6. It can be used to easily assess the nitrogen site distribution over carbon materials. The DFT calculation results suggested that doped nitrogen was beneficial for the electron transfer from graphene to TCNQ. The electron transfer behavior was comprehensively influenced by the graphitic nitrogen and pyridinic nitrogen. Compared to pure graphitic nitrogen doped graphene, the electron of graphitic nitrogen would transfer to the defects caused by pyridinic nitrogen. Hence, the electron transfer was dominated by the pyridinic nitrogen when the ratio was higher than the critical point. Conversely, the electron transfer was dominated by the graphitic nitrogen when the ratio was lower than the critical point.