Coking resistance of Pt–Sn alloys probed by acetylene chemisorption

Acetylene (C₂H₂) is a reactive molecule with a low C : H stoichiometry that can be used to evaluate aspects of the resistance of metal-based catalysts to the formation of carbonaceous residue (coking). Herein we summarize our results for C₂H₂ chemisorption and thermal reaction on four well-defined, ordered surface alloys of Pt–Sn prepared by Sn vapor deposition on Pt(100) and Pt(111) single crystals under UHV conditions. While chemisorption of C₂H₂ under UHV conditions on Pt is completely irreversible, i.e., thermal decomposition leads to complete conversion of the chemisorbed monolayer into surface carbon, alloying with Sn strongly reduces the amount of carbon thus formed. In addition, the temperature for complete dehydrogenation of the carbonaceous residue formed from acetylene decomposition (polymerization) is increased by up to 100 K, from 760 to 860 K. Both of these phenomena are consistent with observations of increased lifetimes and decreased coking for technical Pt–Sn bimetallic catalysts compared to Pt catalysts used for hydrocarbon conversion reactions.     

Aqueous Phase Glycerol Reforming by PtMo Bimetallic Nano-Particle Catalyst: Product Selectivity and Structural Characterization

A carbon supported PtMo aqueous phase reforming catalyst for producing hydrogen from glycerol was characterized by analysis of the reaction products and pathway, TEM, XPS and XAS spectroscopy. Operando X-ray absorption spectroscopy (XAS) indicates the catalyst consists of bimetallic nano-particles with a Pt rich core and a Mo rich surface. XAS of adsorbed CO indicates that approximately 25% of the surface atoms are Pt. X-ray photoelectron spectroscopy indicates that there is unreduced and partially reduced Mo oxide (MoO₃ and MoO₂), and Pt-rich PtMo bimetallic nano-particles. The average size measured by transmission electron microscopy of the fresh PtMo nano-particles is about 2?nm, which increases in size to 5?nm after 30?days of glycerol reforming at 31?bar and 503?K. The catalyst structure differs from the most energetically stable structure predicted by density functional theory (DFT) calculations for metallic Pt and Mo atoms. However, DFT indicates that for nano-particles composed of metallic Pt and Mo oxide, the Mo oxide is at the particle surface. Subsequent reduction would lead to the experimentally observed structure. The aqueous phase reforming reaction products and intermediates are consistent with both C?CC and C?COH bond cleavage to generate H₂/CO₂ or the side product CH₄. While the H₂ selectivity at low conversion is about 75%, cleavage of C?COH bonds leads to liquid products with saturated carbon atoms. At high conversions (to gas), these will produced additional CH₄ reducing the H₂ yield and selectivity.     

In situ hydrogen generation from cycloalkanes using a Pt/CNF catalyst

Pt catalyst supported on carbon nanofibers (CNFs) has been prepared via ion-exchange and it was characterized by XRD, TEM, N₂ physisorption and CO chemisorption. The Pt/CNF catalyst has a small Pt crystallite size in the range of 2–3 nm. This catalyst has been tested in the dehydrogenation of decalin, which is a cycloalkane proposed in the literature as H₂ storage media for vehicles and portable devices. The objective is finding a Pt catalyst suitable for in situ generation of H₂ from chemical storage in decalin. The results revealed that Pt supported on CNF outperforms a Pt catalyst supported on micro–mesoporous activated carbon. Finally, we propose a reactor configuration aiming at the intensification of H₂ production in continuous.     

Structure-Sensitivity of Propylene Hydrogenation over Cluster-Derived Bimetallic Pt-Au Catalysts

A series of Pt and Pt-Au catalysts supported on TiO₂ has been studied using C₃H₆ hydrogenation as a probe reaction to determine the composition of the active catalytic surface. The catalysts were characterized by H₂ chemisorption and TEM analysis to determine concentrations of surface Pt sites for TOF calculations and metal particle size distributions, respectively. Similar TOF values for C₃H₈ formation (approximately 30 sec⁻¹) were observed for a monometallic Pt/TiO₂ and a bimetallic Pt–Au/TiO₂ sample prepared by impregnation from individual salt precursors. In contrast, the TOF for C₃H₈ formation over a Pt₂Au₄/TiO₂ sample prepared from an organometallic Pt₂Au₄ cluster precursor was decreased to 0.07 sec⁻¹, suggesting strong structure sensitivity for the hydrogenation reaction over this catalyst. Characterization results indicate that Pt on the surface of the Pt₂Au₄/TiO₂ catalyst is heavily diluted by Au atoms. In combination with the kinetic results, this suggests that the highly diluted surface ensembles of Pt are too small to effectively catalyze C₃H₆ hydrogenation, although electronic effects induced by the presence of Au adjacent to Pt sites can not be excluded.     

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt-carbon hybrid nanostructures

Three carbon aerogels with different meso-macropore networks were used as supports for Pt catalysts using [Pt(NH₃)₄]Cl₂ as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H₂ chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8-20 wt.% before pre-treatment showed the formation of self-assembled Pt-carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.     

Direct conversion of cellulose into polyols or H2 over Pt/Na(H)-ZSM-5

The direct conversion of cellulose into polyols such as ethylene glycol and propylene glycol was examined over Pt catalysts supported on H-ZSM-5 with different SiO₂/Al₂O₃ molar ratios. The Pt dispersion, determined by CO chemisorption and transmission electron microscopy (TEM), as well as the surface acid concentration measured by the temperature-programmed desorption of ammonia (NH₃-TPD), increased with decreasing SiO₂/Al₂O₃ molar ratio for Pt/H-ZSM-5. The total yield of the polyols, i.e., sorbitol, manitol, ethylene glycol and propylene glycol, generally increased with increasing Pt dispersion in Pt/H-ZSM-5. The one-pot aqueous-phase reforming of cellulose into H₂ was also examined over the same catalysts. The Pt catalyst supported on H-ZSM-5 with a moderate SiO₂/Al₂O₃ molar ratio and a large external surface area showed the highest H₂ production rate. The Pt dispersion, surface acidity, external surface area and surface hydrophilicity appear to affect the catalytic activity for this reaction.     

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.     

Kinetics of acetone hydrogenation over Pt/Al2O3 catalysts

The hydrogenation of acetone to isopropanol has been studied in the vapour phase over Pt/Al₂O₃ catalysts. The rate law obtained at a total pressure of 1 atm and temperatures between 303 and 363 K is of the form V=kP⁰_aP^1/2_H exp (-44 kJ mol^?1 RT^?1). The kinetic results are consistent with a Langmuir-Hinshelwood hydrogenation mechanism involving a half hydrogenated species and a non-competitive chemisorption of acetone and hydrogen on the platinum surface. The specific activity (calculated per platinum surface atom) has been found to be scarcely dependent on the platinum particle size. It is suggested that the chemisorption sites are made of a very small ensemble of platinum atoms.     

Some Insights into the Preparation of Pt/γ-Al2O3 Catalysts for the Enantioselective Hydrogenation of α-Ketoesters

Pt/γ-Al₂O₃ catalysts were prepared by two different impregnation methods and characterized by XRD, TEM, and CO chemisorption. The Pt particle sizes ranged in 2.4–23.3 nm for these 5.0 wt% Pt/γ-Al₂O₃ catalysts. The catalysts were also characterized by FT-IR spectroscopy using CO as a probe molecule before and after the chiral modification with cinchonidine. Two IR bands (2078 and 2060 cm^-1) due to CO linearly adsorbed on the Pt/γ-Al₂O₃ catalyst, calcined at 500 °C before reduction in sodium formate solution were observed, whereas only one IR band at ～2070 cm^-1 was observed for other catalysts. A red shift of the IR band was observed after chiral modification of all the catalysts, except the one with the largest Pt particle size and lowest Pt dispersion. The catalytic performance of the cinchonidine-modified Pt/γ-Al₂O₃ catalysts was tested for the enantioselective hydrogenations of ethyl pyruvate and ethyl 2-oxo-4-phenylbutyrate (EOPB). A 95% ee value was obtained for the ethyl pyruvate hydrogenation and about 83% ee was achieved for the enantioselective hydrogenation of EOPB under the optimized preparation and reaction conditions. It is deduced that the interaction of Pt with γ-Al₂O₃ is a crucial factor for obtaining high activity and that the adsorption abilities (adsorption of reactant, solvent and chiral modifier molecules) of the catalyst surface affect the catalytic performance significantly.     

Support effect on the low-temperature hydrogenation of benzene over PtCo bimetallic and the corresponding monometallic catalysts

PtCo bimetallic and Co, Pt monometallic catalysts supported on γ-Al₂O₃, SiO₂, TiO₂ and activated carbon (AC) were prepared and evaluated for the hydrogenation of benzene at relatively low temperatures (343 K) and atmospheric pressure. Results from flow reactor studies showed that supports strongly affected the catalytic properties of different bimetallic catalysts. AC supported PtCo bimetallic catalysts exhibited significantly better performance than the other bimetallic catalysts, and all the bimetallic catalysts possessed higher activity than the corresponding monometallic catalysts. Results from CO chemisorption and H₂-temperature-programmed reduction (H₂-TPR) studies suggested that different catalysts possessed different properties in chemisorption capacity and reduction behavior, and AC supported PtCo catalysts possessed significantly higher CO chemisorption capacity compared to the other catalysts. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) analysis provided additional information regarding the formation of Pt–Co bimetallic bonds and metallic particle size distribution in the PtCo bimetallic catalysts on different supports.     

Supra‐monolayer coverages on small metal clusters and their effects on H2 chemisorption particle size estimates

H₂ chemisorption measurements are used to estimate the size of supported metal particles, often using a hydrogen‐to‐surface‐metal stoichiometry of unity. This technique is most useful for small particles whose sizes are difficult to estimate through electron microscopy or X‐ray diffraction. Undercoordinated metal atoms at the edges and corners of particles, however, make up large fractions of small metal clusters, and can accommodate multiple hydrogen atoms leading to coverages which exceed 1 ML (supra‐monolayer). Density functional theory was used to calculate hydrogen adsorption energies on Pt and Ir particles (38–586 atoms, 0.8–2.4 nm) at high coverages (≤3.63 ML). Calculated differential binding energies confirm that Pt and Ir (111) single‐crystal surfaces saturate at 1 ML; however, Pt and Ir clusters saturate at supra‐monolayer coverages as large as 2.9 ML. Correlations between particle size and saturation coverage are provided that improve particle size estimates from H₂ chemisorption for Pt‐group metals. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3109–3120, 2018     

Well-dispersed surfactant-stabilized Pt/C nanocatalysts for fuel cell application: Dispersion control and surfactant removal

A simple surfactant-stabilized method was investigated for the preparation of well-dispersed platinum nanoparticles supported on carbon black (Pt/C), using 3-(N, N-dimethyldodecylammonio) propanesulfonate (SB12) as the stabilizer. First, TEM analysis demonstrated that Pt dispersion can be improved by the increase of molar ratio of SB12 to Pt precursor. Moreover, pH environment plays a crucial role in Pt dispersion, and the optimal dispersion with an average Pt particle size of 2.2 nm was obtained under neutral or slightly alkaline environment. Pt dispersion mechanism was shown to involve the electrosteric stabilization of Pt nanoparticles by the zwitterionic surfactant SB12, which is highly pH-dependent. At pH ≥ 7, a stable electrosteric repulsion exists between the Pt particles covered by SB12, where the positively charged part is adsorbed on the particle surface and the negatively charged part (SO₃⁻) and the bulky alkyl chain (C₁₂H₂₅) are pointed away from particles. At pH < 7, protons H⁺ directly interact with Pt particles or SO₃⁻ groups of SB12, resulting in the destruction of the electrosteric stabilization and the following agglomeration of Pt nanoparticles. Furthermore, XPS and cyclic voltammetry showed that the surfactant on Pt particles can be efficiently removed by ethanol wash without any destruction on the dispersion and particle size of Pt, when compared to heat treatment and centrifugation. Electrochemical measurements showed that the ethanol-washed pH-controlled Pt/C catalyst has higher electrochemical surface area and catalytic performance than the commercial one.     

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 Preparation Conditions of Pt/C Catalysts on Oxygen Electrode Performance in Proton Exchange Membrane Fuel Cells

Supported Pt/C catalyst with 3.2 nm platinum crystallites was prepared by the impregnation—reduction method. The various preparation conditions, such as the reaction temperature, the concentration of precursor H₂PtCl₆ solution and the different reducing agents, and the relationship between the preparation conditions and the catalyst performance were studied. The carbon black support after heat treatment in N₂ showed improved platinum dispersion. The particle size and the degree of dispersion of Pt on the carbon black support were observed by transmission electron microscopy (TEM). The crystal phase composition of Pt in the catalyst was determined by X-ray diffraction (XRD). The surface characteristics of the carbon black support and the Pt/C catalyst were studied by X-ray photoelectron spectroscopy (XPS). The electrochemical characteristics of the Pt/C catalysts were evaluated from current—voltage curves of the membrane electrode assembly (MEA) in a proton exchange membrane fuel cell.     

Tunability of Propane Conversion over Alumina Supported Pt and Rh Catalysts

Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C₃H₈:O₂:He = 1:2.25:9) over the temperature range 450–650 °C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 °C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of <10 to >500 nm, and support surface areas in the range of 20–240 m²/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H₂ and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, ≲10–15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C₃H₈, indicative of a direct oxidation pathway in which gas-phase CO and H₂ are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.     

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.     

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Abstract  

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H₂-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C₅+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst.     

Influence of the Pt-particle size on the performance of carbon supported catalysts used in the hydrogenation of citral

The influence of the Pt-particle size in the selective citral hydrogenation is analysed. For that, a Pt/Carbon catalyst has been prepared by impregnation of a carbon xerogel and pre-treated in different conditions to obtain particle sizes varying from 3 to 10 nm. The Pt particle size has been deeply characterized by HRTEM, XRD, XPS and H₂ chemisorption and related with the catalytic performance in each case. Both activity and selectivity to the valuable unsaturated alcohols are favoured for catalysts with increasing Pt-particle size up to around 8 nm, decreasing for larger particles. In this sense, the interaction and transformation of geranial (E-citral) is also favoured regarding neral (Z-citral).     

Synthesis of highly dispersed Pt/C electrocatalysts in ethylene glycol using acetate stabilizer for methanol electrooxidation

Pt/C electrocatalysts were prepared from a solution of H₂PtCl₆ in ethylene glycol in the presence of XC-72 carbon by adding a small amount of sodium acetate as stabilizer. Repeated TEM images showed that the platinum nanoparticles were small and uniform in size and highly dispersed on XC-72 carbon supports when a small amount of sodium acetate solution was added to the synthesis solution. The Pt/C electrocatalysts exhibited very high electrocatalytic activity for liquid methanol oxidation. The effects of adding acetate on Pt particle size and size distribution are discussed. It is demonstrated that acetate can be used as a good stabilizer for preparing Pt/C catalyst with fine and uniform Pt particles.     

Interaction of H2,n-butane and cis-2-butene with Pt and Pt In in NaY: a microcalorimetric study

Pt NaY and Pt In NaY adsorption properties have been compared by using probes like H₂, C₄H₁₀, 2-C₄H₈ and microcalorimetry. It appears that the addition of In to Pt does not change the initial heat of H₂ chemisorption but decreases the initial heat of butane and butene chemisorption.