| Optimizing the energy structure and achieving sustainable development of clean energy is one of the important strategies in China.Supported metal catalysts occupy an important place in all kinds of energy catalysis,and have been attracted extensive attention due to their excellent performance.However,sintering has always been a difficult issue and challenge in the preparation and application of supported metal catalysts.Sintering will greatly change the nano-structure and particle size of the catalytic material.It may lead to a decline in active surface area and performance,and greatly reduce the lifetime and efficiency of catalysis.Therefore,inhibiting the sintering of nanometals is one of the challenges in obtaining long-term stable catalytic systems.This thesis focuses the sintering issues about synthesis and application of carbonsupported catalysts.Starting from the mechanism of sintering,two strategies of supressing metal sintering are developed and further used for high-temperature catalytic reactions as well as the high-temperature synthesis of intermetallic compounds(IMC)catalysts.The relevant research contents are as follows:Chapter Ⅰ introduces the theoretical basis and characterization means of sintering,summarizes the current positive strategies for sintering,and provides a deep understanding of the sintering and prevention strategies for further research.Chapter Ⅱ mainly illustrates the high-temperature anti-sintering of nanocluster catalysts based on the strong interaction between metal and sulfur-doped carbon(S-C)supports.This chapter demonstrates that S-C can efficiently stabilize～1 nm metal nanoclusters(Pt,Ru,Rh,Os,and Ir)against thermal sintering at high temperatures up to 700℃.In-situ X-ray photoelectron spectroscopy(XPS)and X-ray absorption fine structure(XAFS)demonstrate that the enhanced adhesion strength at the metal/sulfurdoped carbon(S-C)interface,arising from the strong and thermally stable metal-sulfur bonding,greatly suppresses the metal nanocluster sintering kinetics by retarding both metal atom diffusion(OR path)and nanoparticle migration(PMC path).The prepared Pt/S-C also showed good stability in high-temperature propane dehydrogenation(PDH).DFT calculation indicates that the charge compensation transfer from S-C to Pt due to strong Pt-S interactions.And the increased electron density of Pt nanoclusters can weaken the bond between Pt and electron-rich group of C=C of propylene on account of electrostatic repulsion,which would enhance the C3H6 desorption and accordingly avoid the catalyst deactivation caused by the deep cracking of C3H8 with enhanced PDH selectivity.On the basis of Chapter Ⅱ,Chapter Ⅲ further systematically explores a special high temperature strong metal support interaction(SMSI)state between Pt and S-C supports,and the reasons for high temperature resistance and sintering.It is found that a special high-temperature SMSI between Pt and S-C supports,evidenced by electron transfer between metal and support,suppressed H2/CO adsorption,and carbon support encapsulation,which results in an excellent sintering resistance of the Pt/S-C catalysts up to 1100℃.The strong chemical interaction between Pt and sulfur atoms doped in carbon matrix dramatically inhibits the Pt overgrowth at moderate temperatures of 300～700℃,along with the subsequent formation of SMSI encapsulation structures on Pt nanoparticles at high temperatures of 900～1100℃.The discovery of high temperature SMSI between Pt and S-C can be extended to other metal and nanostructured carbon carriers to achieve sintering resistant catalysts for various thermal and electrocatalytic applications under harsh conditions.Chapter Ⅳ mainly focuses on the control of particle spatial distribution;the metal sintering is slowed down by optimizing the particle spatial distribution.A significant critical particle distance exist in metal anti-sintering,in which the metal sintering can be significantly suppressed even up to 900℃.By in situ aberration-corrected highangle annular dark-field scanning transmission electron(HAADF-STEM),we identify that particle coalescence sintering can be suppressed by enlarging the particle distance to over the critical value.At the shorter distance catalysts,Pt nanoparticles are located in close proximity and,therefore,are more susceptible to sinter via migration and coalescence.With the sintering proceeding,the nanoparticle coalescence would lead to the decrease of particle density and the enlarged particle distance,which,in turn,slowed down the sintering by coalescence.On the other hand,the wide particle size distribution caused by coalescence could intensify differences in chemical potentials of Pt particles and thus aggravate the Ostwald ripening.By constructing the sintering theoretical model,the critical particle distance is determined by the strength of SMSI,and the sintering critical loading prediction for a given support can be realized.The utility of critical distance concept was investigated under realistic conditions for propane dehydrogenation and the longer distance catalysts showed a good high temperature stability.By virtue of the SMSI strategy described in Chapter Ⅲ,Chapter Ⅴ focus on the high temperature synthesis of intermetallic compounds,and 10 kinds of small sizes(-4nm)Rh-based IMC materials were successfully synthesized.The high temperature annealing treatment of disordered alloy nanoparticles is necessary for the formation of IMC phase,but high temperature annealing inevitably leads to severe sintering of metal nanoparticles,therefore the synthesis of small size IMC becomes challenging.Through the SMSI strategy,successful inhibition of particle sintering under promoting orderly phase formation.The prepared small-size RhSb-IMC facilitates the selective hydrogenation of catalytic functionalized nitroaromatic due to its unique geometric and electronic structure.This SMSI synthesis strategy can extend to the manufacture of other constituent and small-sized polymetallic IMC nanoparticles,providing new approaches to valuable catalysts for a variety of polyphase and electrochemical catalytic processes.Based on the "secure particle distance" strategy proposed in Chapter Ⅳ,ChapterⅥ proposes a more economical and scalable method for preparing 18 kinds of small sizes Pt-based IMC materials on a wide range of commercial carbon black by the critical particle distance strategy with the superior anti-sintering properties.Thanks to the combination of high specific activity in the ordered IMC structure and high active surface area resulting from small particle size,the intermetallic catalyst libraries exhibit outstanding electrocatalytic performance in practical PEMFCs,including high mass activities of 1.3～1.8 AmgPt-1,H2-air fuel cell peak power densities of 1.2～1.4 Wcm-2,high rated power densities of>1 Wcm-2 with an efficient Pt utilization of～0.07 gkW1,as well as excellent stability after 30 K square wave accelerated stress test,all of which surpass the US Department of Energy(DOE)2025 targets.The success of the critical particle diatance strategy will also stimulate future work in the synthesis of other multiordered inter-metal nanoparticle catalysts through high temperature annealing. |
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