Nickel impregnated Pt/H-β and Pt/H-mordenite catalysts for hydroisomerization of n-hexane

Nickel impregnated Pt/H-β and Pt/H-MOR catalysts with different Ni content were prepared and subjected to hydroisomerization of n-hexane in the presence of flowing H₂ gas. The states of Pt and Ni were identified by ESCA. The particle size measured by TEM shows that average particle size increases with increasing Ni loading. The acidity of the catalysts was measured by TPD of ammonia. The catalytic activity of Ni containing and Ni free Pt/H-β and Pt/H-MOR catalysts was compared and found that addition of Ni up to a threshold value (0.3 wt% for β and 0.1 wt% for MOR) increases the n-hexane conversion and dimethyl butanes selectivity due to better metal-acid synergism and decreases the amount of cracked products. When the Ni amount exceeds the threshold values the conversion decreases and cracked products increase. Further the Ni impregnation of Pt containing acidic supports increases the sustainability of the catalysts and was found to favor the protonated cyclopropane (PCP) intermediate mechanism in n-hexane isomerization. β zeolite was found to be a better potential support than mordenite and the isomerized product mixture shows better octane number.     

H2 and H2/CO oxidation mechanism on Pt/C, Ru/C and Pt–Ru/C electrocatalysts

The oxidation kinetics of H₂ and H₂ + 100 ppm CO were investigated on Pt, Ru and Pt–Ru electrocatalysts supported on a high-surface area carbon powder. The atomic ratios of Pt to Ru were 3, 1 and 0.33. XRD, TEM, EDS and XPS were used to characterize the electrocatalysts. When alloyed with ruthenium, a decrease in mean particle size and a modification of the platinum electronic structure were identified. Impedance measurements in H₂SO₄, at open circuit potential, indicated different mechanisms for hydrogen oxidation on Pt/C (Tafel–Volmer path) and Pt–Ru/C (Heyrowsky–Volmer path). These mechanisms also occur in the presence of CO. Best performances, both in H₂ and H₂ + CO, were achieved by the catalyst with the ratio Pt/Ru = 1. This is due to a compromise between the number of free sites and the presence of adsorbed water on the catalyst. For CO tolerance, an intrinsic mechanism not involving CO electroxidation was proposed. This mechanism derives from changes in the electronic structure of platinum when alloyed with ruthenium.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Simultaneous Removal of NO x and Soot Over Pt–Ba/Al2O3 and Pt–K/Al2O3 DPNR Catalysts

The effect of NO _x storage on the soot combustion activity when alkaline- and alkaline/earth-containing model DPNR catalysts are used is investigated in this work. The influence of different experimental conditions (NO concentration, temperature, and particulate loading) is addressed and discussed in relation to the NO _x storage efficiency and soot oxidation activity as well.     

Pt/Co和Pt/Ni双金属纳米溶胶制备及催化制氢

采用化学共还原法制备聚乙烯吡咯烷酮(PVP)稳定的Pt/Co和Pt/Ni双金属纳米溶胶,采用UV-Vis、TEM等对所合成的Pt/Co和Pt/Ni双金属纳米溶胶进行表征,研究了化学组成对双金属纳米溶胶催化剂催化NaBH4水解制氢的影响. 结果表明,所制双金属纳米溶胶的平均粒径约为2.0 nm,双金属纳米溶胶的催化能力高于单金属Pt, Co, Ni纳米溶胶,Pt/Co和Pt/Ni双金属纳米颗粒优异的催化性能可归因于电荷转移效应,Co或者Ni原子与Pt原子之间发生的电荷转移效应使得Pt原子带负电而Co或者Ni原子带正电,荷电的Pt和Co、Ni原子成为催化反应的活性中心,促进了催化反应的进行.     

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

Electrocatalytic oxidation of methanol and formic acid on dispersed electrodes: Pt, Pt–Sn and Pt/M(upd) in poly(2-hydroxy-3-aminophenazine)

Platinum particles dispersed in a poly(2-hydroxy-3-aminophenazine) film (pHAPh/Pt) provide a better catalyst than smooth Pt for the electrooxidation of methanol and formic acid in perchloric acid aqueous solutions. The catalytic activity of the Pt particles is further enhanced when Sn is codeposited in the polymer film. In the case of formic acid oxidation, the activity of Pt nanoparticles is influenced by adatoms of Tl, Pb and Bi deposited undepotential conditions. The upd-modified Pt particles are much more active than bare Pt particles. The morphology and identity of the metallic dispersion were examined by transmission electron microscopy.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

NOx Adsorption Study over Pt–Ba/Alumina Catalysts: FT-IR and Reactivity Study

The NO _x storage process over Ba/Al₂O₃ and Pt–Ba/Al₂O₃ NSR catalysts has been analyzed in this study by performing experiments at 350 °C with NO₂ and NO/O₂ mixtures using different complementary techniques (Transient Response Method, in situ FT–IR and DRIFT spectroscopies). The collected data suggest that over the Pt–Ba/Al₂O₃ catalyst the NO _x storage process from NO/O₂ mixtures occurs forming at first nitrite species, which progressively evolve to nitrates. In addition, a parallel nitrate formation via disproportionation of NO₂ (formed upon NO oxidation) cannot be excluded.     

Facile synthesis of two-dimensional graphene/SnO₂ /Pt ternary hybrid nanomaterials and their catalytic properties

In this paper, we reported a simple, aqueous-phase route to the synthesis of two-dimensional graphene/SnO(2) composite nanosheets (GSCN) hybrid nanostructures consisting of 5 nm Pt nanoparticles supported on the both sides of GSCN. Functional two-dimensional GSCN were obtained through the reduction of graphene oxide (GO) using SnCl(2) in the presence of polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA). The main advantages of this preparation are that the reduction of GO, the formation of SnO(2) and the functionalization of GSCN were achieved simultaneously through one-pot reaction. GSCN/Pt ternary hybrid nanomaterials were generated by in situ reduction of negatively charged PtCl(6)(2-) precursors adsorbed on the positively charged surface of GSCN through electrostatic attraction. The as-synthesized GSCN/Pt ternary hybrid nanomaterials exhibited high cycle stabilization during the catalytic reduction of p-nitrophenol into p-aminophenol by NaBH(4). Additionally, our approach is expected to extend to other hybrid nanomaterials. We believe that the obtained GSCN/Pt ternary hybrid nanomaterials have great potential for applications in other field, such as electrochemical energy storage, sensors, and so on.     

Supported Pt and Pt–Ru catalysts prepared by potentiostatic electrodeposition for methanol electrooxidation

Methanol electrooxidation was investigated on Pt–Ru electrocatalysts supported on glassy carbon. The catalysts were prepared by electrodeposition from solutions containing chloroplatinic acid and ruthenium chloride. Bulk composition analysis of the Pt–Ru catalyst was performed using an X-ray detector for energy dispersive spectroscopy analysis (EDX). Three different compositions were analyzed in the range 0–20 at.% Ru content. Tafel plots for the oxidation of methanol in solutions containing 0.1–2 M CH₃OH, and in the temperature range 23–50 °C showed a reasonably well-defined linear region. The slope of the Tafel plots was found to depend on the ruthenium composition. The lower slope was determined for the Pt catalyst, varying between 100 and 120 mV dec⁻¹. The values calculated for the alloys were higher, ranging from 120 to 140 mV dec⁻¹. The reaction order for methanol varies from 0.5 to 0.8, increasing with the ruthenium content. The activation energy calculated from Arrhenius plots was found to change with the catalyst composition, showing a lower value around 30 kJ mol⁻¹ for the alloys, and a higher value, of 58.8 kJ mol⁻¹, for platinum. The effect of ruthenium content is explained by the bifunctional reaction mechanism.     

Macroporous Monolithic Pt/γ-Al2O3 and K–Pt/γ-Al2O3 Catalysts Used for Preferential Oxidation of CO

Macro-porous monolithic γ-Al₂O₃ was prepared by using macro-porous polystyrene monolith foam as the template and alumina sol as the precursor. Platinum and potassium were loaded on the support by impregnation method. TG, XRD, N₂ adsorption–desorption, SEM, TEM, and TPR techniques were used for catalysts characterization, and the catalytic performance of macro-porous monolithic Pt/γ-Al₂O₃ and K–Pt/γ-Al₂O₃ catalysts were tested in hydrogen-rich stream for CO preferential oxidation (CO-PROX). SEM images show that the macropores in the macro-porous monolithic γ-Al₂O₃ are interconnected with the pore size in the range of 10 to 50 μm, and the monoliths possess hierarchical macro-meso(micro)-porous structure. The macro-porous monolithic catalysts, although they are less active intrinsically than the particle ones, exhibit higher CO conversion and higher O₂ to CO oxidation selectivity than particle catalysts at high reaction temperatures, which is proposed to be owing to its hierarchical macro-meso(micro) -porous structure. Adding potassium lead to marked improvement of the catalytic performance, owing to intrinsic activity and platinum dispersion increase resulted from K-doping. CO in hydrogen-rich gases can be removed to 10 ppm over monolithic K–Pt/γ-Al₂O₃ by CO-PROX.     

High energy ball-milled Pt and Pt–Ru catalysts for polymer electrolyte fuel cells and their tolerance to CO

High energy ball milling, an industrially amenable technique, has been used to produce CO tolerant unsupported Pt–Ru based catalysts for the oxidation of hydrogen in polymer electrolyte fuel cells. Nanocrystalline Pt_0.5–Ru_0.5 alloys are easily obtained by ball-milling but their performances as anode catalysts are poor because nanocrystals composing the material aggregate during milling into larger particles. The result is a low specific area material. Improved specific areas were obtained by milling together Pt, Ru and a metal leacheable after the milling step. The best results were obtained by milling Pt, Ru, and Al in a 1:1:8 atomic ratio. After leaching Al, this catalyst (Pt_0.5–Ru_0.5 (Al₄)) displays a specific area of 38 m²g^–1. Pt_0.5–Ru_0.5 (Al₄) is a composite catalyst. It consists of two components: (i) small crystallites (4 nm) of a Pt–Al solid solution (1–3 Al wt%) of low Ru content, and (ii) larger Ru crystallites. It shows hydrogen oxidation performance and CO tolerance equivalent to those of Pt_0.5–Ru_0.5 Black from Johnson Matthey, the commercial catalyst which was found to be the most CO tolerant one in this study.     

Influence of Cesium in Pt/NaCsβ on the Physico-Chemical and Catalytic Properties of the Pt Clusters in the Aromatization ofn-Hexane

Various Pt/NaCsβ catalysts with decreasing Na/Cs ratios and a Pt/KL sample taken as a reference catalyst for aromatization ofn-hexane, have been prepared by exchange of NaCsβ and KL zeolites in a Pt tetraammine solution then calcination and reduction. The increase of the Cs content in the NaCsβ zeolitic support results in a decrease of the cyclohexane adsorption and Pt exchange capacities. The Pt/Csβ and Pt/KL samples show similar behaviors which strongly differ from those of the Na-containing Pt/NaCsβ samples in terms of (i) reducibility of the Pt ions after calcination, (ii) shape of the FTIR spectra of CO adsorbed on the Pt clusters after reduction, and (iii) catalytic activity of these clusters in the conversion ofn-hexane. In particular, the selectivity to aromatization is much higher on the Pt/Csβ and Pt/KL catalysts than on the Na-containing Pt/NaCsβ ones. The reasons for the specific behaviors of the Pt/Csβ and Pt/KL samples are discussed.     

Total Oxidation of Methane Over Sulfur Poisoning Resistant Pt/ZrO2 Catalyst: Effect of Pt2+–Pt4+ and Pt2+–Zr4+ Dipoles at Metal-Support Interface

Catalysis Letters - We present a Pt/ZrO2 catalyst that can operate in the harsh conditions of methane oxidation without being deactivated by SO2. XPS analysis of 1%Pt/ZrO2 catalyst revealed the...     

Propane dehydrogenation activity of Pt and Pt–Sn catalysts supported on magnesium aluminate: influence of steam and hydrogen

Propane dehydrogenation was carried out in hydrogen and steam as reaction media on Pt/MgAl₂O₄ and Pt–Sn/ MgAl₂O₄ catalysts. A wide range of Pt and Pt–Sn concentrations was explored. Monometallic Pt catalysts were completely poisoned by steam. Concerning bimetallic Pt–Sn catalysts, tin played an important role related to the activation of platinum particles when the reaction was carried out in steam. On the other hand, tin inhibited cracking reactions leading to an increase of catalysts stability. Activation energy in hydrogen was the same for monometallic and bimetallic catalysts: 22 kcal/mol; while for the reaction in steam, values ranging from 10 to 15 kcal/mol were obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

电感耦合等离子发射光谱法测定Pt/Al_2O_3催化剂中的Pt

用电感耦合等离子发射光谱法测定了Al2O3基催化剂中的Pt,考察了基体及所用试剂对Pt测定的影响,发现Al2O3基体、硫酸等对Pt测定均有负干扰,应在标准溶液与基体匹配的情况下进行测定。该方法的检出限为Pt0.10mg/L;样品的加标回收率96%以上。     

Imaging and chemical probing on the atomic scale: reconstruction and dynamics of the systems O2/Rh and NO2–H2/Pt

This paper presents two case studies of adsorbate-induced surface reconstruction, on the one hand, and dynamical reaction imaging along with local chemical probing, on the other hand. The first one deals with the oxygen-induced reshaping of 3D Rh crystals. Field ion microscopy (FIM) was applied to image in real-space the change from a nearly hemispherical shape in the absence of oxygen toward a polyhedral one in the presence of oxygen. Shape transformation occurs at temperatures of 380–550 K and is associated with the appearance of facets with {111} and {001} orientation. The only high-index planes present in the polyhedral form are of {137} symmetry. (1×2) and (1×3) missing-row reconstructions appear in the {113} and {011} planes. The polyhedral form has also been imaged under in situ conditions of the oxygen–hydrogen reaction on Rh at 505 K. The second case study deals with kinetic non-linearities occurring in the NO₂ reaction with hydrogen on the surface of a 3D Pt crystal reconstructed to a top- and edge-truncated pyramid. The reaction was found to ignite in the {012} corner planes of the crystal. One-dimensional wavefronts were subsequently observed to move along the 211 zone lines. These studies were performed by video-FIM and could be correlated with a local chemical analysis by time-of-flight mass spectrometry of ionised species. The mass spectrum provided information on water product (H₂O⁺ and H₃O⁺) and NO intermediate formation. Strong fluctuations in the NO ₂ ⁺ current indicated the occurrence of NO₂ surface diffusion. These species are most likely responsible for the field ion image formation.     

Stability of Pt–Co/C and Pt–Pd/C based oxygen reduction reaction electrocatalysts prepared at a low temperature by a combined impregnation and seeding process in PEM fuel cells

The stability of Pt–Co/C and Pt–Pd/C electrocatalysts relative to that of a commercial Pt/C catalyst was measured in terms of the loss of the electrochemical surface area (ESA). The electrocatalytic activity was investigated in an acidic solution (0.3 M H₂SO₄) and in a single PEM fuel cell under H₂/O₂ conditions. In the acidic solution, the ESA of the catalyst decreased as the number of repeated potential cycles increased, which is likely to be due to dissolution of the different metals contained within the catalyst structure. In the fuel cell environment, the deterioration of the cell performance increased as the number of repeated potential cycles increased. Thus, the loss of cell performance may be related to the loss of the ESA. In addition, the loss of the catalyst’s ESA affected the cell performance at low-, medium-, and high- current densities, indicating a loss of either the activation potential or an ohmic loss. Among the three electrocatalysts evaluated, the Pt–Co/C based one exhibited the highest electrocatalytic activity in both the acidic solution and in the fuel cell environment.