H2 production through steam reforming of ethanol over Pt/ZrO2, Pt/CeO2 and Pt/CeZrO2 catalysts

The effect of the support nature and metal dispersion on the performance of Pt catalysts during steam reforming of ethanol was studied. H₂ and CO production was facilitated over Pt/CeO₂ and Pt/CeZrO₂, whereas the acetaldehyde and ethene formation was favored on Pt/ZrO₂. According to the reaction mechanism, determined by temperature-programmed desorption (TPD) and Diffuse Reflectance Infrared Spectroscopy (DRIFTS) analysis, some reaction pathways are favored depending on the support nature, which can explain the differences observed on the resulting product distribution.     

Self-rectifying and forming-free resistive switching behaviors in Pt/La2Ti2O7/Pt structure

In this work, we report the resistive switching behavior of an amorphous La₂Ti₂O₇ (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process.     

Conversion of methylcyclopentane (MCP) on Pt/MoO2, Ir/MoO2 and Pt–Ir/MoO2 catalysts

The effect of the metal and reaction temperature was investigated in the conversion of MCP with hydrogen at atmospheric pressure. The highly dispersed 0.5 wt.% Pt/MoO₂, 0.5 wt.% Ir/MoO₂ and 0.25 wt.%–0.25 wt.% Pt–It/MoO₂ metal catalysts were prepared by incipient wetness impregnation or co-impregnation methods. The most active catalyst in the conversion of MCP was Pt/MoO₂ and the most selective to MCP ring opening was Ir/MoO₂. At low temperature, Ir/MoO₂ opened the MCP ring at the secondary–secondary position. High temperature promoted ring opening at the secondary–tertiary positions, which was attributed to the adlineation sites. At low temperatures, Pt/MoO₂ and Pt–Ir/MoO₂ promoted only the ring enlargement reaction while Ir/MoO₂ promoted both ring opening and ring enlargement. Ring enlargement of MCP to cyclohexane and benzene was catalysed by electron deficient adduct sites, while ring opening to 2-meythylpentane (2-MP), 3-methylpentane (3-MP) and n-hexane (n-H) was catalysed by metallic sites. At high temperatures, MCP broken into C₁–C₅ fragments and deactivation of the catalysts was observed. The Ir/MoO₂ showed the highest selectivity for cracking. The differences in selectivity were attributed to the presence of adsorbed agostic species, where the electronic environment of Ir and Pt are different.     

Preparation and Characterization of Pt/C and Pt/TiO2 Electrocatalysts by Liquid Phase Photodeposition

The preparation of carbon and titanium dioxide supported Pt catalysts through a photochemical and photocatalytic routes were investigated. The catalysts were prepared by irradiation with UV-light (365 nm) at room temperature using H₂PtCl₆ and C₁₀H₁₄O₄Pt (Pt(acac)₂) as platinum precursors. The kinetic studies revealed that H₂PtCl₆ produced metallic platinum faster than Pt(acac)₂ and also showed that the amount of platinum deposited on TiO₂ was higher than on carbon. The samples were characterized by X-ray diffraction, SEM/EDS and cyclic voltammetry. X-ray diffraction permitted to identify the crystallographic (111) and (200) planes from platinum metal on the catalysts synthesized, the intensity of peaks depends of the amount of platinum deposited. SEM/EDS test confirmed what it was found by the kinetics studies. The electrocatalytic activity was compared with a commercial Pt E-Tek catalyst (10 wt%). The electrochemical results showed that Pt/C-AA catalyst synthesized by liquid phase photo-deposition method has stability in acid media and high distribution of the actives sites on the electrode surfaces. It could be considered as a candidate for electro-catalyst for polymer electrolyte fuel cell. The Pt/TiO₂ catalysts did not present electrochemical activity.     

不同Pt前体制备Pt/CeO2催化剂对其结构及性能的影响

Pt与载体间的相互作用会影响到本征Pt纳米粒子的催化活性,不同Pt前体制备Pt/CeO₂催化剂会使其表现出完全不同的催化性能。分别采用金属胶体粒子原位沉积法、浸渍法以及浸渍还原的方式制备了Pt/CeO₂催化剂,通过X 射线衍射、程序升温还原、X射线光电子能谱以及高分辨透射电镜对催化剂进行表征,在CO氧化以及甲苯燃烧反应中评价催化剂活性。结果表明,胶体粒子原位沉积法制备Pt/CeO₂催化剂,能够将优先合成好的Pt纳米粒子直接以金属态Pt⁰的形式负载到载体表面,且保证其高度均匀分散,丰富的表面Pt⁰很好地充当了CO、甲苯反应时的活化位点,催化剂表现出优异的性能;浸渍还原法中,Pt纳米粒子之间会发生团聚现象,同时部分Pt又以Pt²⁺的形式与CeO₂之间形成了Pt-O-Ce相互作用,载体表面暴露Pt⁰含量的下降是催化剂表现出较弱活性的主要原因;浸渍法中,以Pt离子对Pt进行负载,Pt完全以Pt²⁺的形式参与到Pt-O-Ce键成键中,表面Pt⁰缺失,催化剂表现出明显的失活现象。Pt/CeO₂催化剂中,起主要活性作用的是金属态Pt⁰,胶体粒子原位沉积法能够实现Pt⁰的直接负载,对于提高Pt基催化剂中Pt的利用率,降低Pt资源消耗都具有重要意义。     

Sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al₂O₃ and Pt/SrO/Al₂O₃ NO _x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO _x storage sites are regenerated when H₂ is used in combination with CO₂ compared to H₂ only.     

CO Spillover and Oxidation on Pt/TiO2

The adsorption of CO has been measured on a 2.5 wt% Pt/TiO₂ catalyst using TPD. A somewhat surprising observation is that (i) CO₂ is produced, even though oxygen is not dosed into the system, (ii) repeated experiments result in the same amount of CO₂ desorption. The results appear to be due to a combination of factors–(i) is due to spillover of CO from the Pt to the TiO₂ support, while (ii) is due to the diffusion of Ti³⁺ into the bulk of the TiO₂ crystallite, which effectively removes the surface non-stoichiometry which might otherwise be expected.     

Transient studies of carbon dioxide reforming of methane over Pt/ZrO2 and Pt/Al2O3

The mechanism of the CO₂ reforming of methane reaction over the Pt/ZrO₂ catalyst was investigated using a temporal analysis of products (TAP) reactor system. For comparative purposes, the reaction pathway using a Pt/Al₂O₃ catalyst was also examined. A reaction sequence is suggested for both catalysts. Over both catalysts, methane decomposition takes place over platinum. The main difference between the two catalysts concerns the carbon dioxide dissociation. Over Pt/Al₂O₃ this step is assisted by hydrogen. Over Pt/ZrO₂ this step takes place over the zirconia support and involves surface vacancies. Moreover, large pools of formate and carbonate species are present on the zirconia. Transient studies conducted to determine the origin of carbon species accumulated during CO₂ reforming revealed that more than 99% of the carbon was derived from the methane molecule over both catalysts. Over the Pt/ZrO₂ catalyst, only a single very reactive carbon species was detected, while over the Pt/Al₂O₃ a second less active species was also formed.     

Reduction of NO by H2 and CO on Pt/TiO2/SiO2

Different catalysts based on platinum and a silica, titania or mixed titania/silica support were studied in NO reduction reactions by CO and H₂ in the temperature range of 25–400C. The mixed oxide catalysts showed considerably lower onset temperatures in NO/CO reactions but this coincided with a maximum in N₂O formation. In NO/H₂ reactions all titania containing catalysts produced more N₂O than silica supported platinum at low temperatures but were more selective to N₂ at high temperatures.     

Ethylene adsorption on Pt/Au/SiO2 catalysts

Ethylene adsorption on a Pt/Au/SiO₂ catalyst (2 wt% Pt; Au/Pt atomic ratio of 10) was studied using adsorption microcalorimetry and FTIR spectroscopy. Ethylene adsorption at 300 K on Pt/Au/SiO₂ produced π‐bonded, di‐σ‐bonded, and ethylidyne species with an initial heat of 140 kJ/mol, compared to a heat of 157 kJ/mol for Pt/SiO₂ on which only ethylidyne species formed. At 203 and 263 K, ethylene adsorbed on Pt as well as on Au surface atoms for the Pt/Au/SiO₂ catalyst. Quantum chemical, DFT calculations indicate that Au exerts a significantly smaller electronic effect on Pt than does addition of Sn to Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

TAP study of NOx storage and reduction on Pt/Al2O3 and Pt/Ba/Al2O3

A systematic mechanistic study of NO storage and reduction over Pt/Al₂O₃ and Pt/BaO/Al₂O₃ is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H₂ pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N₂ on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al₂O₃ is an order of magnitude higher than on Pt/Al₂O₃ showing participation of Ba in the storage even in the absence of gas phase O₂. Either oxygen spillover or transient NO oxidation to NO₂ is postulated as the first steps for NO storage on Pt/BaO/Al₂O₃. The storage on Pt/Ba/Al₂O₃ commences as soon as Pt–O species are formed. Post-storage H₂ reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H₂ pump-probe experiments. NO conversion to N₂ by decomposition is sustained on clean Pt using excess H₂ pump-probe feeds. With excess NO pump-probe feeds NO is converted to N₂ and N₂O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N₂ production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H₂ feed ratio.     

Support Effects in Nerol Hydrogenation over Pt/SiO<Subscript>2</Subscript>, Pt/H-Y and Pt/H-MCM-41 Catalysts

Chemoselective hydrogenation of nerol was investigated over Pt/SiO₂, Pt/H-Y and Pt/H-MCM-41 catalysts. The initial total reaction rates decreased in following order: Pt/H-Y > Pt/SiO₂ > Pt/H-MCM-41. Nerol hydrogenation was found to be an apparent structure sensitive reaction. The selectivities to citronellol at 30% conversion of nerol were 65%, 55% and 25% over Pt/SiO₂, Pt/H-MCM-41 and Pt/H-Y, respectively.     

Tuning product selectivity of CO2 hydrogenation by OH groups on Pt/γ-AlOOH and Pt/γ-Al2O3 catalysts

Herein, we explore how OH groups on Pt/γ-AlOOH and Pt/γ-Al₂O₃ catalysts affect CO₂ hydrogenation with H₂ at temperatures from 250°C to 400°C. OH groups are abundant on γ-AlOOH, but rare at Pt-(γ-AlOOH) interface which is the most favorable site for CO₂ conversion on Pt/γ-AlOOH. This makes CO₂ hydrogenation on Pt/γ-AlOOH form CO weakly bonding to γ-AlOOH, which prefers to desorption from Pt/γ-AlOOH rather than further conversion, thus enhancing CO production on Pt/γ-AlOOH. Different from Pt/γ-AlOOH, OH groups are abundant at Pt-(γ-Al₂O₃) interface which is the most favorable site for CO₂ conversion on Pt/γ-Al₂O₃. This promotes CO₂ hydrogenation on Pt/γ-Al₂O₃ to form CO strongly bonding to Pt, which prefers to further hydrogenation to CH₄, and thereby increases CH₄ selectivity on Pt/γ-Al₂O₃. Therefore, the OH groups at metal-support interface are crucial factor influencing product distribution, and must be considered seriously when fabricating catalysts.     

Hydrogen spillover phenomena on Pt/ZrO2

A Pt/ZrO₂ catalyst has been investigated by temperature-programmed reduction and temperature-programmed desorption of hydrogen. Hydrogen spills over from Pt onto the ZrO₂ surface at about 550°C. One part of spillover hydrogen is consumed by a partial reduction of zirconia. The other part is adsorbed on the surface and is desorbed at about 650°C. This desorption is a reversible one, i.e. it can be followed by a renewed uptake of spillover hydrogen. No connection between dehydroxylable OH groups and spillover hydrogen adsorption has been observed. The adsorption sites for the reversibly bound spillover hydrogen were possibly formed during the reducing hydrogen treatment.     

Effect of hydrogen treatments on ZrO2 and Pt/ZrO2 catalysts

ZrO₂ and Pt/ZrO₂ catalysts have been investigated by TPR, hydrogen chemisorption, TPDH and in the conversion ofn-hexane. At high temperature, ZrO₂ takes up hydrogen. High temperature hydrogen treatment is a precondition of the catalytic activity in then-hexane conversion. Possibly, catalytically active acid sites are formed by this hydrogen treatment. The high temperature hydrogen treatment induces a strong Pt-ZrO₂ interaction.     

Detection of oxygen ion drift in Pt/Al2O3/TiO2/Pt RRAM using interface-free single-layer graphene electrodes

Resistive switching random access memory (RRAM) with oxygen ion drift under electric (E)-field has been intensively studied. However, the findings are insufficient because redox reaction by oxygen ion drift occurs beneath the top electrode, and it is difficult to analyze with a nondestructive method. Therefore, an effective method to circumvent this difficulty is suggested in this study with a Pt/Al₂O₃/TiO₂/Pt device using a single layer graphene (SLG) top electrode. Based on results from spectroscopic analyses, the SLG serves as not only an interface free electrode, but also as a highly effective indicator for proving O⁻ ion drift motion in response to the E-field in RRAM. The origin of asymmetric resistive switching is due to a redox reaction at the interface by oxygen ion drift. The endurance and operation-current distribution are significantly improved with increased thickness of the Al₂O₃ insertion layer, which provides carrier tunneling barrier height. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) is greater than one order of magnitude in a log scale within 1800 cycles. This result demonstrates that control of a localized charge tunneling barrier is a key factor for reliable resistive switching of the scaled-down RRAM.     

Porous Pt/SiO2 catalytic membranes prepared using mesitylene solvated Pt atoms as a source of Pt particles

The preparation of silica catalytic membrane reactors as well as their characterization are discussed. The deposition of platinum by the Pt mesitylene co-condensate method has proved to provide high Pt loading with good catalyst dispersion, also on SiO₂ support. The catalytic activity of the silica membrane has been assessed using the hydrogenation of toluene as model reaction.     

Comparative study of structural properties and NOx storage-reduction behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3

Differences in the NO_x storage-reduction (NSR) behavior of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO₂ exhibits inferior NO_x storage and, particularly, reduction (regeneration) activity compared to the Al₂O₃ supported catalyst. The incomplete reduction of the stored NO_x-species in Pt/Ba/CeO₂ seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO₂ catalyst to reducing H₂ atmosphere in the temperature range 300–500 °C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500 °C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO₂ onto the surface of Pt particles.     

Catalytic Performance of Pt/ZrO2 and Pt/Ce-ZrO2 Catalysts on CO2 Reforming of CH4 Coupled with Steam Reforming or Under High Pressure

CO₂ reforming of methane was performed on Pt/ZrO₂ and Pt/Ce-ZrO₂ catalysts at 1073K under different reactions conditions: (i) atmospheric pressure and CH₄:CO₂ ratio of 1:1 and 2:1; (ii) in the presence of water and CH₄:CO₂ ratio of 2:1; (iii) under pressure (105 and 190 psig) and CH₄:CO₂ ratio of 2:1. The Pt supported on ceria-promoted ZrO₂ catalyst was more stable than the Pt/ZrO₂ catalyst under all reaction conditions. We ascribe this higher stability to the higher density of oxygen vacancies on the promoted support, which favors the cleaning mechanism of the metal particle. The increase of either the CH₄:CO₂ ratio or total pressure causes a decrease in activity for both catalysts, because under either case the rate of methane decomposition becomes higher than the rate of oxygen transfer. The Pt/Ce-ZrO₂ catalyst was always more stable than the Pt/ZrO₂ catalyst, demonstrating the important role of the support on this reaction.     

Pt/TiO2介孔膜的制备及其光催化性能研究

以F127为介孔模板剂,钛酸异丙酯为前驱体,溶胶-凝胶法在载玻片上制备了TiO2介孔膜,用光还原法在TiO2膜表面沉积贵金属Pt,用SEM、XRD、XPS等对TiO2膜进行了表征。结果表明：TiO2膜具有介孔结构,为锐钛矿晶型,沉积在TiO2表面的Pt主要以单质形式存在。光催化结果表明介孔TiO2膜具有较高的光催化活性,表面沉积贵金属Pt后,光催化活性大大提高。