Improved methane oxidation activity of P-doped γ-Al2O3 supported palladium catalysts by tailoring the oxygen mobility and electronic properties

Micro-mesoporous P-doped γ-Al₂O₃ with cluster morphology was obtained via an efficient ultrasound-assisted sol-gel process and taken as carrier to construct palladium catalysts for methane oxidation. It was revealed that the structure and properties of catalysts were significantly influenced by the phosphorus precursors with diverse valence and acidity. Dissimilar reducibility of surface hydroxyl and oxygen species is observed in the catalysts derived from different phosphorus sources, indicating the difference in the oxygen mobility and the capacity of the catalysts to convert intermediate CO. The behavior of charge-transfer transition and d-d transition, the transfer ability of electrons from palladium particles into the antibonding 2π* orbitals of CO, together with the surface acidity and electronic density of palladium species was likewise tailored, which demonstrated the metal-support interaction could be tuned, making palladium species behave with diverse status and electronic structures. The optimized properties cooperatively provided an enhancement in catalytic performance of P-containing catalysts.     

Structure and oxygen storage capacity of Pd/Pr/CeOe-ZrO2 catalyst： effects of impregnated praseodymia

Praseodymium （Pr） was impregnated to CeO2-ZrO2 solid solution by an impregnation method. The as-obtained Pr modi- fied CeO2-ZrO2 was impregnated with 1 wt.% Pd to prepare the catalysts. The structure and reducibility of the fresh and hydrother- really aged catalysts were characterized by X-ray diffraction （XRD）, Raman, X-ray photoelectron spectroscopy （XPS）, CO chemi- sorption and H2 temperature-programmed reduction （H2-TPR）. The oxygen storage capacity （OSC） was evaluated with CO serving as probe gas. Effects of impregnated Pr on the structure and oxygen storage capacity of catalysts were investigated. The results showed that the aged Pr-impregnated samples had much higher OSC and better reducibility than the unmodified ones. The scheme of structural evolutions of the catalysts with and without Pr was also established. Partial of the impregnated Pr diffused into the bulk of CeO2-ZrO2 during ageing, which inhibited the sintering, and increased the amount of oxygen vacancies in CeO2-ZrO2 support. Furthermore, those impregnated Pr species which covered on the surface of the support obstructed the strong metal-support interaction between Pd and Ce so as to reduce the encapsulation of Pd as well as the back spill-over of the oxygen during the catalytic process.     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

Precious metal-support interaction in automotive exhaust catalysts

Precious metal-support interaction plays an important role in thermal stability and catalytic performance of the automotive exhaust catalysts. The support is not only a cartier for active compotmds in catalysts but also can improve the dispersion of precious metals and suppress the sintering of precious metals at high temperature; meanwhile, noble metals can also enhance the redox performance and oxygen storage capacity of support. The mechanism of metal-support interactions mainly includes electronic interaction, formation of alloy and inward diffusion of metal into the support or covered by support. The form and degree of precious metal-sup- port interaction depend on many factors, including the content of precious metal, the species of support and metal, and preparation methods. The research results about strong metal-support interaction （SMSI） gave a theory support for developing a kind of new cata- lyst with excellent performance. This paper reviewed the interaction phenomenon and mechanism of precious metals （Pt, Pd, Rh） and support such as A1203, CeO2, and CeO2-based oxides in automotive exhaust catalysts. The factors that affect SMSI and the catalysts developed by SMSI were also discussed.     

有机两相法制备巯基羧酸修饰的小尺寸贵金属纳米粒子

以生物相容性较好的巯基羧酸分子为保护剂、NaBH4为还原剂,采用有机两相法制备金属纳米粒子,实现了对纳米粒子的表面功能化。该方法制备的纳米材料粒径小(5 nm),单分散性好。讨论了巯基羧酸与金属前驱体之间的投料比、还原剂NaBH4的量以及巯基羧酸分子链长等因素对纳米粒子形貌的影响。除Au外,该方法还适用巯基羧酸对Ag、Pt和Pd等多种纳米粒子的表面修饰。     

La、Ba的添加对Pd/Y2O3-ZrO2催化氧化CH4性能的影响

以浸渍法制备了1.0%Pd/Y2O3-ZrO2催化剂,考察了Pd负载过程中La和Ba的添加对Pd/Y-ZrO2催化氧化CH4性能的影响,用BET、XRD、CO脉冲、TEM和H2-TPR等方法对所制备的催化剂进行表征。结果表明,La和Ba的添加降低了Pd/Y2O3-ZrO2催化剂氧化CH4的活性。催化剂中活性金属的分散度及体相PdO的还原性影响催化剂对CH4的氧化活性,La、Ba的添加降低了Pd/Y2O3-ZrO2催化剂中活性金属Pd的分散性,使Pd在高温老化后更容易团聚,同时增强了PdO与载体的相互作用,使PdO不容易被还原。