Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studied on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂, and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst, For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane, and its behavior was explained by a change in the reaction mechanism.     

Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studio on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂ and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst. For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane and its behaviour was explained by a change in the reaction mechanism.     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

Reactions of methylcyclopentane on Rh–Pt catalyst prepared by underpotential deposition of Rh on Pt/SiO2

Rh was deposited on to a well-characterized 3.1% Pt/SiO₂ (InCat-1) parent catalyst by underpotential deposition method to obtain a model Rh–Pt bimetallic catalyst. TEM and EDS was used to determine its mean particle size and bulk composition: the particles of ca. 3 nm contained ca. 60% Pt and 40% Rh. The Rh–Pt catalyst was tested in methylcyclopentane (MCP) reaction between 513 K and 603 K and 60–480 Torr H₂ pressure (with 10 Torr MCP). The parent Pt/SiO₂ as well as a 5% Rh/SiO₂ catalyst were also studied for comparison. Four subsequent treatments with O₂ and H₂ up to T = 673 K were applied on the bimetallic catalyst before the catalytic runs. The overall activity showed positive hydrogen order on all samples, bimetallic Rh–Pt resulting in the lowest TOF values. Ring opening and hydrogenolysis products, as well as unsaturated hydrocarbons were formed from MCP. The selectivity of ring opening products and fragments over Rh–Pt catalyst was between the values observed on Pt and Rh, while the selectivity towards benzene formation was highest on the bimetallic sample, especially at higher temperatures. “Selective” ring opening occurred on all samples, resulting mostly in 2 and 3-methylpentane and less hexane. Different pretreatments with H₂ and O₂ affected slightly the dispersion values and the catalytic behavior of Rh–Pt sample. The selectivities of the Rh–Pt catalyst being between the values observed for Pt/SiO₂ and Rh/SiO₂ indicates that the sample studied represented a real bimetallic catalyst, resembling both components and exhibiting at the same time, new properties in addition to those, characteristic of Pt or Rh. Dedicated to Konrad Hayek.     

Pt催化丙烷脱氢过程中结焦反应的粒径效应与Sn的作用

用乙二醇还原法制备了Pt颗粒平均粒径分别为2.0、4.6、12.1 nm的Pt/Al₂O₃催化剂,同时用浸渍法制备了PtSn/Al₂O₃双金属催化剂,并考察了各催化剂在丙烷脱氢过程中的结焦行为。分别用H₂化学吸附、透射电镜、热重分析、元素分析、红外光谱、拉曼光谱等手段对催化剂进行了表征。表征结果显示,催化剂金属上的结焦速率与Pt金属颗粒粒径密切相关。具有较小Pt颗粒的催化剂金属上的结焦速率明显大于具有较大Pt颗粒的催化剂。具有较小Pt颗粒的催化剂上生成的焦含有较少的氢,其石墨化程度也较高。本研究中PtSn/Al₂O₃催化剂金属上的结焦速率高于Pt/Al₂O₃催化剂,并且在双金属上生成的焦具有更高的石墨化程度。结合Pt/Al₂O₃催化剂上的结焦机理,对高性能丙烷脱氢催化剂提出了新的概念设计。     

Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NO x Storage–Reduction Performance

The effect of Pt location in Pt/Ba/CeO₂ catalysts for NO _x storage–reduction (NSR) was analyzed. The Pt location on BaCO₃ or CeO₂ support was controlled by changing the angle (φ) between the two flame sprays producing these two components. As-prepared flame-made catalysts contain PtO _x which must be reduced during the fuel rich phase to become active for NO _x storage and reduction of NO _x . For Pt on BaCO₃ this process was significantly faster than for Pt on CeO₂. The increased reduction ability of Pt on Ba is reflected in the light off temperatures: for Pt on CeO₂ temperatures around 330 °C were needed to combust 20% of C₃H₆ in air while for Pt on BaCO₃ only 250 °C were required for the same conversion. The ability to control the location of Pt or other noble metals is, therefore, essential to optimize the catalysts for a given Pt/Ba/CeO₂ weight ratio. The best performance was observed when most of the Pt constituent was located near Ba-containing sites.     

Influence of formic acid on electrochemical properties of high‐porosity Pt/TiN nanoparticle aggregates

Platinum‐deposited titanium nitride (Pt/TiN) nanoparticle aggregates with high porosities were successfully prepared via a self‐assembly‐assisted spray pyrolysis method. The addition of formic acid (HCOOH) had a significant influence on the process, promoting the simultaneous formation of metallic Pt and reduction on the surface of the TiN support material. Complete reduction of the Pt/TiN nanoparticle aggregates improved the catalytic activity. The electrochemical surface area (ECSA) of Pt/TiN with HCOOH (Pt/TiN_w/HCOOH) was 87.15 m²/g‐Pt, which was higher than that of Pt/TiN without HCOOH (Pt/TiN_w/o‐HCOOH). The catalytic durability of Pt/TiN_w/HCOOH was twice that of Pt/TiN_w/o‐HCOOH. An effective strategy for obtaining carbon‐free catalysts with high activities and durabilities was identified. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2753–2760, 2013     

Effect of Pt Dispersion on the Catalytic Activity of Supported Pt Catalysts for Diesel Hydrocarbon Oxidation

The catalytic activity of Pt/Al₂O₃ for the total oxidation of hydrocarbon mixture of n-decane and 1-methylnaphthalene was strongly dependent on the Pt dispersion. The maximum hydrocarbon oxidation activity was achieved for Pt/Al₂O₃ with Pt dispersion (D_Pt) of 0.39. On the other hand, the activity of Pt/ZrO₂ monotonously decreased with decreasing Pt dispersion from 1.00 to 0.09. In situ FT-IR spectroscopy suggested that the presence of plural Pt species in metallic state with slight different electric state, which well catalyze the formation of acrylate species as an intermediate, is responsible for the high catalytic activity of Pt/Al₂O₃ with D_Pt = 0.39.     

Improving the Stability and Ethanol Electro‐Oxidation Activity of Pt Catalysts by Selectively Anchoring Pt Particles on Carbon‐Nanotubes‐Supported‐SnO2

To improve the stability and activity of Pt catalysts for ethanol electro‐oxidation, Pt nanoparticles were selectively deposited on carbon‐nanotubes (CNTs)‐supported‐SnO₂ to prepare Pt/SnO₂/CNTs and Pt/CNTs was prepared by impregnation method for reference study. X‐ray diffraction (XRD) was used to confirm the crystalline structures of Pt/SnO₂/CNTs and Pt/CNTs. The stabilities of Pt/SnO₂/CNTs and Pt/CNTs were compared by analyzing the Pt size increase amplitude using transmission electron microscopy (TEM) images recorded before and after cyclic voltammetry (CV) sweeping. The results showed that the Pt size increase amplitude is evidently smaller for Pt/SnO₂/CNTs, indicating the higher stability of Pt/SnO₂/CNTs. Although both catalysts exhibit degradation of electrochemical active surface area (EAS) after CV sweeping, the EAS degradation for the former is lower, further confirming the higher stability of Pt/SnO₂/CNTs. CV and potentiostatic current–time curves were recorded for ethanol electro‐oxidation on both catalysts before and after CV sweeping and the results showed that the mass specific activity of Pt/CNTs increases more than that of Pt/SnO₂/CNTs, indicating that Pt/CNTs experiences more severe evolution and is less stable. The calculated area specific activity of Pt/SnO₂/CNTs is larger than that of Pt/CNTs, indicating SnO₂ can co‐catalyze Pt due to plenty of interfaces between SnO₂ and Pt.     

Irreversible adsorption of Sn adatoms on basal planes of Pt single crystal and its impact on electrooxidation of ethanol

The behaviours of irreversible adsorption (IRA) of Sn adatoms on Pt(1 0 0), Pt(1 1 1) and Pt(1 1 0) electrodes were characterized using cyclic voltammetry. It has revealed that Sn can adsorb irreversibly on Pt(1 0 0) and Pt(1 1 1), while not significantly on Pt(1 1 0) electrode. Quantitative analysis of the relationship between 1 − θ_H and θ_Sn suggests that Sn adatoms may adsorb preferably on hollow sites of Pt(1 1 1) (threefold) and Pt(1 0 0) (fourfold) planes, which is in accordance respectively with the values 0.31 and 0.21 of coverage of IRA Sn adatoms in saturation adsorption determined on these electrodes. The IRA Sn adatoms on different basal planes of Pt single crystal yield different impact on the electrocatalytic oxidation of ethanol. It has revealed that the IRA Sn adatoms on Pt(1 0 0) electrode have declined the activity for ethanol oxidation, while IRA Sn adatoms on Pt(1 1 1) have enhanced remarkably the electrocatalytic activity with Sn coverage θ_Sn between 0.09 and 0.18. The oxidation peak potential E_p and the current density j_p of ethanol oxidation on Pt(1 1 1)/Sn were varied with θ_Sn, and the highest j_p (1258 μA cm⁻²) as well as the lowest E_p (0.20 V) were measured simultaneously at θ_Sn around 0.14. In comparison with the data obtained on a bare Pt(1 1 1), the E_p was shifted negatively by 65 mV and the j_p has been enhanced to about 1.7 times on the Pt(1 1 1)/Sn (θ_Sn = 0.14), which is ascribed to hydroxyl species adsorption at relatively low potentials on Pt(1 1 1)/Sn surfaces. The current study is of importance in revealing the fundamental aspects of modification of the basal planes of Pt single crystal using Sn adatoms, and the impact of this modification on electrocatalytic activity towards ethanol oxidation.     

Coverage and Substrate Effects on the Structural Change of FeOx Nanostructures Supported on Pt

Structural changes of FeO_x nanostructures supported on Pt(111) and Pt foil with response to oxidation and reduction treatments in O₂ and H₂ atmospheres upto 1.0 bar have been investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. We show that submonolayer O–Fe bilayer (FeO) structure on Pt(111) can be transformed to O–Fe–O trilayer (FeO₂) upon oxidation in 5.0 × 10^?6 mbar O₂, while the FeO to FeO₂ transformation happens over the full FeO film only with the O₂ partial pressure above 1.0 × 10^?3 mbar. Reduction of the submonolayer FeO₂ structure back to the FeO structure occurs when exposed to 1.0 mbar H₂ at room temperature (RT). In contrast, the full FeO₂ structure can be kept even under 1.0 bar H₂ exposure condition. The FeO_x coverage and FeO_x/Pt boundary play a critical role in the redox behavior of the supported FeO_x nanostructures. Furthermore, we show that the FeO_x nanostructures supported on Pt foil can be oxidized in a similar way as those on the Pt(111) surface. However, the Pt foil supported FeO₂ nanostructures can be more deeply reduced to the state close to metallic Fe in 1.0 mbar H₂ at RT. The close-packed Pt(111) surface exhibits a stronger confinement effect on the FeO overlayer than the open polycrystalline Pt surface.     

Effect of electrolyte environment and Pt crystallite size on hydrogen adsorption—V

The role of Pt crystallite surface morphology on hydrogen adsorption isotherms in H₂SO₄ and alkaline electrolytes was examined by a potentiodynamic sweep technique. By varying the crystallite size (40–280Å) of highly dispersed Pt electrocatalysts, the relative concentrations of edges, vertices and crystallite faces which contribute to the surface morphology are changed. The potentiodynamic i-V profiles for adsorbed hydrogen oxidation on highly dispersed Pt electrocatalysts in 0.05 and 1 M H₂SO₄ showed similar changes with Pt crystallite size. Only two states of adsorbed hydrogen on highly dispersed Pt were observed in 0.05 M H₂SO₄, compared to four states reported by Angerstein-Kozlowska et al on smooth polycrystalline Pt electrodes.In 1 M NaOH and 35 wt % KOH, less than a monolayer of adsorbed hydrogen was present on highly dispersed Pt electrocatalysts at the reversible hydrogen potential. Two states of chemisorbed hydrogen were observed at 23–91°, while at low temperature (?47°) in 35 wt % KOH, an additional adsorbed hydrogen species was evident in the potentiodynamic i-V curves. A Pt crystallite size effect on the adsorption of hydrogen on highly dispersed Pt in alkaline electrolytes was not deduced.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Electrochemical Reduction of Oxygen on Carbon Supported Pt and Pt/Ru Fuel Cell Electrodes in Alkaline Solutions

A study of O₂ reduction in 1 M NaOH solution at gas diffusion electrodes made from carbon supported Pt and Pt/Ru catalysts is reported. Two Tafel regions were observed for both the Pt and Pt/Ru electrodes. Although the same mechanism was suggested for oxygen reduction on both Pt and Pt/Ru catalysts, the O₂ reduction activity was lower on Ru. Electrochemical Impedance Spectroscopy (EIS) analysis was carried out at different potentials and showed the significant contribution of diffusion on the reaction process and kinetics. The effect of methanol on O₂ reduction was investigated in solutions containing various concentrations of methanol. The electrode performance deteriorated with increasing methanol concentration because of a mixed cathode potential. The methanol tolerance, i. e., the methanol concentration which polarises the O₂ reduction reaction for O₂ reduction, at the Pt/C electrode with a Pt loading of 1.2 mg cm^–2 is 0.2 M methanol in 1 M NaOH.     

In situ d electron density of Pt particles on supports by XANES

The dependency of d electron density of Pt in Pt/SiO₂ catalysts on the particle size was investigated by means of in situ X-ray absorption near-edge structure (in situ XANES) spectroscopy. The d electron density of Pt particles was measured under vacuum, H₂ and ethene, to gain information about ethene hydrogenation on Pt/SiO₂. The intensities of the white lines at L_III and L_II edges in XANES spectra, which are regarded to reflect the unoccupied density of state, varied with the change of particle size under both vacuum and reaction gas atmospheres. The interaction between Pt particle and adsorbates was weak with small particles below 1.5 nm. A new peak induced by Pt-H bonding in the XANES spectra under H₂ was observed for the samples with Pt particle size 1.5 nm. This is related to the change of the turnover frequency and activation energy for ethene hydrogenation by Pt particle size.     

Quantitative analysis of hydrogen adsorbed on Pt particles on SiO2 in the presence of coadsorbed CO by means of L3-edge X-ray absorption near-edge structure spectroscopy

X-ray absorption near-edge structure (XANES) spectra at the Pt L₃-edge for Pt particles supported on SiO₂ under CO adsorption and CO+H₂ coadsorption were recorded to analyze the amount of adsorbed hydrogen in the coadsorbed state on the Pt particles. Adsorbed CO on the Pt particles revealed a new peak at 6 eV above the Pt L₃-edge in the difference spectra before and after CO adsorption in the coverage range 0.10-0.51. Subsequent adsorption of hydrogen at various coverages on the CO-preadsorbed Pt particles broadened and shifted the peak to the higher energy side. The peak was deconvoluted to two components due to adsorbed hydrogen and CO by a linear least-squares fitting technique. It was found that the fitting coefficient with respect to adsorbed hydrogen was proportional to the amount of adsorbed hydrogen. The XANES difference spectra provide a quantitative analysis method for adsorbed hydrogen on supported Pt particles in the presence of coadsorbates like CO. This revised version was published online in July 2006 with corrections to the Cover Date.     

The microwave-assisted ionic liquid nanocomposite synthesis: platinum nanoparticles on graphene and the application on hydrogenation of styrene

The microwave-assisted nanocomposite synthesis of metal nanoparticles on graphene or graphite oxide was introduced in this research. With microwave assistance, the Pt nanoparticles on graphene/graphite oxide were successfully produced in the ionic liquid of 2-hydroxyethanaminium formate [HOCH₂CH₂NH₃][HCO₂]. On graphene/graphite oxide, the sizes of Pt nanoparticles were about 5 to 30 nm from transmitted electron microscopy (TEM) results. The crystalline Pt structures were examined by X-ray diffraction (XRD). Since hydrogenation of styrene is one of the important well-known chemical reactions, herein, we demonstrated then the catalytic hydrogenation capability of the Pt nanoparticles on graphene/graphite oxide for the nanocomposite to compare with that of the commercial catalysts (Pt/C and Pd/C, 10 wt.% metal catalysts on activated carbon from Strem chemicals, Inc.). The conversions with the Pt nanoparticles on graphene are >99% from styrene to ethyl benzene at 100°C and under 140 psi H₂ atmosphere. However, ethyl cyclohexane could be found as a side product at 100°C and under 1,520 psi H₂ atmosphere utilizing the same nanocomposite catalyst.     

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.     

Phase and Texture Evolution in Chemically Derived PZT Thin Films on Pt Substrates

The crystallization of lead zirconate titanate (PZT) thin films was evaluated on two different platinum‐coated Si substrates. One substrate consisted of a Pt coating on a Ti adhesion layer, whereas the other consisted of a Pt coating on a TiO₂ adhesion layer. The Pt deposited on TiO₂ exhibited a higher degree of preferred orientation than the Pt deposited on Ti (as measured by the Full Width at Half Maximum of the 111 peak about the sample normal). PZT thin films with a nominal Zr/Ti ratio of 52/48 were deposited on the substrates using the inverted mixing order (IMO) route. Phase and texture evolution of the thin films were monitored during crystallization using in situ X‐ray diffraction at a synchrotron source. The intensity of the Pt₃Pb phase indicated that deposition on a highly oriented Pt/TiO₂ substrate resulted in less diffusion of Pb into the substrate relative to films deposited on Pt/Ti. There was also no evidence of the pyrochlore phase influencing texture evolution. The results suggest that PZT nucleates directly on Pt, which explains the observation of a more highly oriented 111 texture of PZT on the Pt/TiO₂ substrate than on the Pt/Ti substrate.     

Influence of Pyridine‐Polybenzimidazole Film Thickness of Carbon Nanotube Supported Platinum on Fuel Cell Applications

Pyridine‐polybenzimidazole (PyPBI) films of different thickness (∼1.0–2.4 nm) are wrapped on the surfaces of multi‐walled carbon nanotubes (CNTs). To prepare Pt on PyPBI/CNT (Pt‐PyPBI/CNT) catalysts, Pt⁴⁺ ions are immobilized on these PyPBI wrapped CNTs (PyPBI/CNTs) via Lewis acid‐base coordination between Pt⁴⁺ and :N‐ of imidazole groups, followed by reducing Pt⁴⁺ to Pt nanoparticles. The influence of PyPBI film thickness on the Pt particle size, loading and electrochemical surface area, respectively, of Pt‐PyPBI/CNTs is investigated. Fuel cell performances of the PBI/H₃PO₄ based membrane electrode assemblies (MEAs) prepared from these Pt‐PyPBI/CNT catalysts are also evaluated at 160 °C with unhumidified H₂/O₂ gases. Among the catalysts, the Pt‐PyPBI/CNT catalyst with a PyPBI film thickness of ∼1.6 nm (which is around half of the Pt particle size), a Pt loading of ∼44 wt.%, and a Pt particle size of ∼3.3 nm exhibits the best fuel cell performance.