Preparation, characterization, and reactivity of Pt/SDBC catalysts for the hydrogen-water isotopic exchange reaction

Pt/SDBC catalysts, which are used for the hydrogen-water isotopic exchange reaction, were prepared. TGA experiments showed that the treatment temperature of Pt/SDBC catalysts in inert gas is limited to 400 °C and the maximum allowable heat treatment temperature in oxygen is 200 °C. From nitrogen adsorption and hydrogen chemisorption measurements, it was shown that the dispersion of platinum particles depended on the physical properties, i.e., specific surface area and pore structure of SDBC. It was found that the heat treatment could not impact the structure of SDBC and the oxygen treatment at 150 °C improved the platinum dispersion. It was shown by XPS analysis that the oxygen treatment of impregnated Pt/SDBC increased the fraction of platinum metal state and platinum dispersion. As the supported platinum area increases, the catalytic activity of Pt/SDBC for the hydrogen-water vapor isotopic exchange reaction increases. It indicates that the hydrogen chemisorption measurement can be used to estimate the catalytic activities of Pt/SDBC catalysts. It was not observed that the particle size of supported platinum affected the specific reaction rate at 60 °C. It implies that this reaction is structure insensitive.     

Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores?

Limited availability of platinum is a potential threat to fuel cell commercialization. Since the 1970s, alternative catalysts to the electrochemical reduction of oxygen have been obtained from heat treatment at T > 600 degrees C of carbon with a non-noble metal and a source of nitrogen atoms. However, the process by which the heat treatment activates these materials remains an open question. Here, we report that the activation process of carbon black and iron acetate heat-treated in NH(3) comprises three consecutive steps: (i) incorporation of nitrogen atoms in the carbon, (ii) micropore formation through reaction between carbon and ammonia, and (iii) completion of active sites in the micropores by reaction of iron with ammonia. Step (ii) is the slowest. Moreover, the microporous surface per mass of catalyst controls the macroscopic activity when enough nitrogen atoms are incorporated in the structure of the carbon support. These facts should help in determining the structure of the active sites and in identifying methods to increase the site density of such catalysts.     

Analysis of the Micropore Structure of Various Microporous Materials from Nitrogen Adsorption at 77 K

The micropore structure of four microporous materials (two zeolites, ZSM-5 and Y-82; an activated carbon and an alumina pillared clay) and their binary physical mixtures (50-50 wt%) have been examined by nitrogen adsorption at 77 K. Various micropore sizes have been considered from the stages on the micropore filling mechanism in the microporous materials. The application of the Dubinin-Astakhov (DA) equation to characterize and obtain the adsorption potential distributions of the microporous materials is presented.     

Effect of pressure on the production of hydrogen and nanofilamentous carbon by the catalytic pyrolysis of methane on Ni-containing catalysts

An experimental study of the production of hydrogen and nanofilamentous carbon (NFC) by methane pyrolysis on Ni- and Ni + Cu-containing catalysts at 675°C and pressures of up to 5 atm is reported. As the pressure and the copper content of the catalyst are increased, the stable service life of the catalyst and the hydrogen and NFC yield per unit weight of the catalyst, as well as the specific surface area and micropore volume of the resulting NFC, increase markedly.     

酸性和碱性溶液中金属氮碳材料氧还原和氢析出反应的理论研究

单原子催化剂（SAC）由于其低成本和在各种电催化反应中潜在的高催化活性而被认为是铂族金属的有前景的替代材料,但仍然缺乏对不同金属氮碳材料催化剂之间活性差异的原子机理的理解。在此,通过实验和理论研究相结合,研究了非贵金属氮碳材料（Me-N-C,Me = Fe和Co）作为模型催化剂,以探索在普遍的pH值下氧还原反应（ORR）和氢析出反应（HER）的催化活性以及相对应的反应机理。原子理论模拟表明,Fe-N-C具有比Co-N-C高的ORR活性,这是因为其速率决定步骤的反应势垒较低,而HER的活性趋势却相反。我们的模拟结果与实验观察结果一致。     

Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials

We report the synthesis of zeolite-like carbon materials that exhibit well-resolved powder XRD patterns and very high surface area. The zeolite-like carbons are prepared via chemical vapor deposition (CVD) at 800 or 850 degrees C using zeolite beta as solid template and acetonitrile as carbon precursor. The zeolite-like structural ordering of the carbon materials is indicated by powder XRD patterns with at least two well-resolved diffraction peaks and TEM images that reveal well-ordered micropore channels. The carbons possess surface area of up to 3200 m2/g and pore volume of up to 2.41 cm3/g. A significant proportion of the porosity in the carbons (up to 76% and 56% for surface area and pore volume, respectively) is from micropores. Both TEM and nitrogen sorption data indicate that porosity is dominated by pores of size 0.6-0.8 nm. The carbon materials exhibit enhanced (and reversible) hydrogen storage capacity, with measured uptake of up to 6.9 wt % and estimated maximum of 8.33 wt % at -196 degrees C and 20 bar. At 1 bar, hydrogen uptake capacity as high as 2.6 wt % is achieved. Isosteric heat of adsorption of 8.2 kJ/mol indicates a favorable interaction between hydrogen and the surface of the carbons. The hydrogen uptake capacity observed for the zeolite-like carbon materials is among the highest ever reported for carbon (activated carbon, mesoporous carbon, CNTs) or any other (MOFs, zeolites) porous material.     

Thin-film rotating disk electrode as a tool for comparing the activity of catalysts in the hydrogen oxidation reaction

A thin-film disperse rotating disk electrode is used to study the hydrogen oxidation reaction on platinum catalysts E-TEK with different purification degrees and on disperse palladium catalysts obtained from colloid solutions of organometallic complex precursors with subsequent thermal decomposition in an inert atmosphere or in hydrogen at diverse temperatures. Kinetic currents of the hydrogen oxidation reaction on these catalysts are determined at a potential of 0.025 V. The obtained values of currents may be utilized for performing a comparative estimation of the activity of various catalysts and the degree of their purification from the precursors or accidental impurities.     

Effect of ion exchange by an organic cation on platinum immobilization on clays

Clay-supported platinum catalysts were prepared by ion exchange. The platinum precursor has been reduced either in the presence or in the absence of cinchonidinium cations introduced in the interlamellar space of the cationic clay. The samples were characterized by XRD and TEM. Reduction by NaBH₄ in the absence of cinchonidinium cations and the subsequent introduction of this latter ensured the formation of monodispersed, small platinum crystallites immobilized mostly between the clay sheets. On the other hand, reduction in the presence of the organic cations resulted in migration of the platinum particles on the external surface of the clay.     

Oxidation of a platinum foil with nitrogen dioxide

The oxidation of a polycrystalline platinum foil by nitrogen dioxide at an NO₂ pressures of 10^–6–10^–4 mbar and a temperature of 525 K has been investigated by X-ray photoelectron spectroscopy (XPS). Under these conditions, the platinum oxides PtO and Pt₃O₄ form on the foil surface. A comparison between the data obtained in this study and the data on the oxidation of platinum nanoparticles suggests a hypothesis as to the causes of the size effect in the oxidation of NO over platinum catalysts.     

钬对铂重整催化剂改性作用的研究

采用脉冲微反应技术研究了一系列Pt-Ho催化剂催化甲基环戊烷及环己烷的重整反应,应用Faraday磁天平研究了催化剂的磁性,用氢氧滴定法测定了催化剂上铂的分散度与平均粒径.结果表明,钬能改善活性组分铂的分散度,降低铂的平均粒径,从而增加铂催化剂的活性中心数,提高了铂重整催化剂的活性,进而增强了铂催化剂的抗硫稳定性.Pt-Ho催化剂的活性与磁性有对应关系.     

Preparation and hydrogen storage properties of zeolite-templated carbon materials nanocast via chemical vapor deposition: effect of the zeolite template and nitrogen doping

Carbon materials have been prepared using zeolite 13X or zeolite Y as template and acetonitrile or ethylene as carbon source via chemical vapor deposition (CVD) at 550-1000 degrees C. Materials obtained from acetonitrile at 750-850 degrees C (zeolite 13X) or 750-900 degrees C (zeolite Y) have high surface area (1170-1920 m(2)/g), high pore volume (0.75-1.4 cm(3) g(-1)), and exhibit some structural ordering replicated from the zeolite templates. Templating with zeolite Y generally results in materials with higher surface area. High CVD temperature (> or =900 degrees C) results in low surface area materials that have significant proportions of graphitic carbon and no zeolite-type structural ordering. The nitrogen content of the samples derived from acetonitrile varies between 5 and 8 wt %. When ethylene is used as a carbon precursor, high surface area (800-1300 m(2)/g) materials are only obtained at lower CVD temperature (550-750 degrees C). The ethylene-derived carbons retain some zeolite-type pore channel ordering but also exhibit significant levels of graphitization even at low CVD temperature. In general, the carbon materials retain the particle morphology of the zeolite templates, with solid-core particles obtained at 750-850 degrees C while hollow shells are generated at higher CVD temperature (> or =900 degrees C). We observed hydrogen uptake of up to 4.5 wt % and 45 g H(2)/L (volumetric density) at -196 degrees C and 20 bar for the carbon materials. The hydrogen uptake was found to be dependent on surface area and was therefore influenced by the choice of zeolite template and carbon source. Zeolite Y-templated N-doped carbons had the highest hydrogen uptake capacity. Gravimetric and volumetric methods gave similar uptake capacity at 1 bar (i.e., 1.6 and 2.0 wt % for zeolite 13X and Y-templated N-doped carbons, respectively). Our findings show that zeolite-templated carbons are attractive for hydrogen storage and highlight the potential benefits of functionalization (nitrogen-doping).     

纳米贵金属插入的粘土用于催化选择性加氢反应

 The use of clay minerals in the synthesis of nanosized noble metal particles to give increased catalytic activity was investigated. Nanosized platinum and ruthenium catalysts intercalated in clay (montmorillonite/hectorite) were synthesised and their catalytic activity was evaluated for the selective hydrogenation of three different α,β-unsaturated aldehydes, namely, crotonaldehyde, cinnamaldehyde, and citral, in a gas phase microreactor. The metal nano-sol was prepared by the chemical reduction of its precursor by the micellar technique in the presence of cetyl trimethyl ammonium bromide (CTAB), and the micelle stabilized metal particles were intercalated in the clay mineral by ion exchange. TEM micrographs of the catalyst particles showed that the metal particles were in the nanometre range. The average diameters of the particles were 1–25 nm. The effects of temperature, metal loading, and hydrogen flow rate on the catalytic activity and selectivity for α,β- unsaturated alcohol were studied. The results were correlated with the structural properties of the catalysts. The products formed in each reaction over the different catalysts showed that the catalysts were very active for hydrogenation, and the selectivity for the desired product, namely, α,β-unsaturated alcohol, was good. The metal catalysts intercalated in montmorillonite showed better selectivity than that in hectorite because of its higher acidity. Increased selectivity for α,β-unsaturated alcohol was observed with increased flow rate of hydrogen.     

前驱体中N含量对FeN/C催化剂氧还原活性的影响研究

采用热解法制备FeN/C催化剂,考察催化剂前驱体中氮含量对其氧还原活性的影响. 使用X射线衍射、比表面积和孔径分布测试、透射电子显微镜以及热重分析等方法对催化剂的结构、形貌及催化剂前驱体的热性质等进行表征,使用线性扫描伏安法对催化剂的氧还原活性进行测试. 结果表明,以1,10-菲啰啉为氮源,FeCl₃为铁源,Black Pearl 2000为载体,催化剂前驱体中1,10-菲啰啉含量为20wt%,Fe含量为1wt %时,热处理制备所得催化剂粒子分布均匀,比表面积为824.48 m ²·g ^-1,平均孔隙为10.58 nm,表面的氮元素含量为0.31wt%;并具有最好的氧还原催化活性.催化剂前驱体中氮源含量在热解过程中导致催化剂的比表面积、孔径结构及表面氮元素含量的变化是影响催化剂活性的关键因素.     

Studies on platinum-promoted sulfated zirconia alumina: effects of pretreatment environment and carrier gas on n-butane isomerization and benzene alkylation activities

A series of platinum-promoted sulfated zirconia alumina catalysts (SZA) with different amounts of platinum (0.5, 1, and 2 wt%) were synthesized. Two other catalysts were prepared by mechanically mixing different proportions of the Al-promoted sulfated zirconia with Pt/Al(2)O(3). The 650 degrees C calcined catalysts were characterized by N(2) adsorption/desorption (BET), TPR, and TPD analysis. Butane isomerization activity of the catalysts was studied at 270 degrees C, varying the pretreatment environment and carrier gases. Though the textural properties of the catalysts did not change significantly with platinum loading, the maximum surface area of 116 m(2)/g was exhibited by the catalyst with 1 wt% Pt loading. Under the studied reaction conditions, the air-pretreated catalysts (sulfated zirconia alumina (SZA) and platinated SZA) showed higher n-butane conversion than the N(2)-pretreated catalyst. However, nitrogen was a better carrier gas than H(2), CO(2) or air, and CO(2) and air deactivated the catalyst very fast. Unlike the platinated SZA catalysts, the mechanically mixed catalysts showed an induction phenomenon. A redox mechanism is suggested for butane isomerization over these catalysts. The catalyst SZA was also found to be active for alkylation of benzene with isopropanol, which gave 93% selectivity toward cumene.     

Effect of Preparation Conditions on the Stability of Pt/Al2O3 Catalysts in Methane Combustion

Alumina supported platinum catalysts were prepared by sol-gel method using aluminum-sec-butoxide, platinum acetylacetonate and sec-butanol. Various gelation process were performed either by the help of a gelation agent (water or acetic acid) or by a slow condensation without hydrolysis source. The textural and the structural study of the catalysts using the nitrogen physical adsorption and the hydrogen chemisorption showed that the BET surface area and the metal dispersion varied when the hydrolysis and the gelation processes were modified. The catalytic activities for methane combustion performed on the fresh and aged catalysts were correlated to the metal dispersion values. In addition the catalytic activity loss under reaction mixture seemed to be caused by the metal dispersion decrease. When the catalysts were aged under the reaction mixture at 600°C for 72 h, a BET surface area decrease and a metallic surface area loss were observed. The resistance to sintering observed to be dependant on the hydrolysis and the gelation processes was in favor of the catalyst prepared by acetic acid.     

Pt@MOF-177: synthesis, room-temperature hydrogen storage and oxidation catalysis

The gas-phase loading of [Zn(4)O(btb)(2)](8) (MOF-177; H(3)btb=1,3,5-benzenetribenzoic acid) with the volatile platinum precursor [Me(3)PtCp'] (Cp'=methylcyclopentadienyl) was confirmed by solid state (13)C magic angle spinning (MAS)-NMR spectroscopy. Subsequent reduction of the inclusion compound [Me(3)PtCp'](4)@MOF-177 by hydrogen at 100 bar and 100 degrees C for 24 h was carried out and gave rise to the formation of platinum nanoparticles in a size regime of 2-5 nm embedded in the unchanged MOF-177 host lattice as confirmed by transmission electron microscopy (TEM) micrographs and powder X-ray diffraction (PXRD). The room-temperature hydrogen adsorption of Pt@MOF-177 has been followed in a gravimetric fashion (magnetic suspension balance) and shows almost 2.5 wt % in the first cycle, but is decreased down to 0.5 wt % in consecutive cycles. The catalytic activity of Pt@MOF-177 towards the solvent- and base-free room temperature oxidation of alcohols in air has been tested and shows Pt@MOF-177 to be an efficient catalyst in the oxidation of alcohols.     

Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion

Mixtures of nanosized platinum and palladium particles have been prepared by reduction of salt-containing microemulsion droplets using hydrazine as the reducing agent. To avoid possible negative effects of the presence of sulfur compounds during the preparation the microemulsion was made using the sulfur-free nonionic polyoxyethylene 4 lauryl ether surfactant. Transmission electron microscopy showed that the as-prepared mixtures contained crystalline platinum particles of fairly homogeneous size (20 to 40 nm) with adsorbed amorphous palladium particles 2 to 5 nm in size. Catalyst samples were prepared by depositing the nanoparticles on a gamma-Al(2)O(3) support followed by heating in air at 600 degrees C. Alloyed particles of platinum and palladium with sizes ranging from 5 to 80 nm were obtained during the heating. The majority of the particles had the fcc structure and their compositional range was dependent upon the Pt:Pd molar ratio of the microemulsion. A catalyst prepared from a microemulsion with a 20:80 Pt:Pd molar ratio showed the highest catalytic activity for CO oxidation, while pure platinum and palladium catalysts showed higher sulfur resistance. These results differ from the performance of conventional wet-impregnated catalysts, where a 50:50 Pt:Pd molar ratio resulted in the highest catalytic activity as well as the highest sulfur resistance.     

Fractal Characterization of Wide Pore Range Catalysts: Application to Pd-Ag/SiO(2) Xerogels

Fractal analysis has been applied to characterize the structure of Pd-Ag/SiO(2) catalysts dried under vacuum (150 degrees C and 12 hPa) with different concentrations of Pd-Ag. Nitrogen adsorption-desorption, mercury porosimetry, and small-angle X-ray scattering measurements have been used. The different approaches to fractal analysis and their conditions of applicability are briefly described. Results are discussed in relation to those derived previously from classical interpretations. This analysis shows that Pd-Ag/SiO(2) xerogels exhibit a very open self-similar pore structure analogous to the structure of supercritically dried aerogels and that the micropore texture can be tailored by varying the silver content. Copyright 2000 Academic Press.     

高活性和耐久性非铂氧还原催化剂的研究进展（英文）

质子交换膜燃料电池(PEMFCs)阴极氧还原反应(ORR)动力学迟缓,需要消耗大量的贵金属催化剂,这限制了其商业化应用。目前,原子级分散的M-N-C (M=Fe,Co,Mn等)催化剂受到人们青睐,有望替代铂催化剂。在过去的几十年里,M-N-C催化剂取得了很大的进步,具有优异的ORR活性,而且燃料电池初始性能有希望接近传统的Pt/C催化剂。然而,这些高活性的Fe-N-C催化剂在燃料电池实际工作条件下的稳定性比较差。这篇综述总结了在高效氧还原M-N-C催化剂方面的最近进展,主要概述了作者课题组在限域策略和自旋调控方面的贡献。此外,我们还总结了几种提高活性的有效方法以及近期的关于揭示M-N-C催化剂的降解机制的认识,如金属浸出、碳腐蚀、质子化和微孔淹没都会造成催化剂降解。为改善M-N-C催化剂的寿命,我们概括了文献中的缓解策略,包括控制催化剂中S1/S2位点、使用非铁基催化剂、增强金属氮键、改善碳载体的耐腐蚀性和使用质子缓冲液等。最后,提出了目前原子级分散的M-N-C催化剂的存在的挑战和可能的解决方案。     

Adsorbed States of Hydrogen on Platinum: A New Perspective

The interaction of hydrogen with platinum is enormously important in many areas of catalysis. The most significant of these are in polymer electrolyte membrane fuel cells (PEMFC), in which carbon-supported platinum is used to dissociate hydrogen gas at the anode. The nature of adsorbed hydrogen on platinum has been studied for many years on single-crystal surfaces, on high-surface area-platinum metal (Raney platinum and platinum black), and on supported catalysts. Many forms of vibrational spectroscopy have played a key role in these studies, however, there is still no clear consensus as to the assignment of the spectra. In this work, ab initio molecular dynamics (AIMD) and lattice dynamics were used to study a 1.1 nm nanoparticle, Pt₄₄H₈₀. The results were compared to new inelastic neutron scattering spectra of hydrogen on platinum black and of a carbon-supported platinum fuel cell catalyst and an assignment scheme that rationalises all previous data is proposed.