Size and Shape Dependence on Pt Nanoparticles for the Methylcyclopentane/Hydrogen Ring Opening/Ring Enlargement Reaction

Abstract  

Monodisperse Pt nanoparticles (NPs) with well-controlled sizes in the range between 1.5 and 10.8 nm, and shapes of octahedron, cube, truncated octahedron and spheres (~6 nm) were synthesized employing the polyol reduction strategy with polyvinylpyrrolidone (PVP) as the capping agent. We characterized the as-synthesized Pt nanoparticles using transmission electron microscopy (TEM), high resolution TEM, sum frequency generation vibrational spectroscopy (SFGVS) using ethylene/H₂ reaction as the surface probe, and the catalytic ethylene/H₂ reaction by means of measuring surface concentration of Pt. The nanoparticles were supported in mesoporous silica (SBA-15 or MCF-17), and their catalytic reactivity was evaluated for the methylcyclopentane (MCP)/H₂ ring opening/ring enlargement reaction using 10 torr MCP and 50 torr H₂ at temperatures between 160 and 300 °C. We found a strong correlation between the particle shape and the catalytic activity and product distribution for the MCP/H₂ reaction on Pt. At temperatures below 240 °C, 6.3 nm Pt octahedra yielded hexane, 6.2 nm Pt truncated octahedra and 5.2 nm Pt spheres produced 2-methylpentane. In contrast, 6.8 nm Pt cubes led to the formation of cracking products (i.e. C₁–C₅) under similar conditions. We also detected a weak size dependence of the catalytic activity and selectivity for the MCP/H₂ reaction on Pt. 1.5 nm Pt particles produced 2-methylpentane for the whole temperature range studied and the larger Pt NPs produced mainly benzene at temperatures above 240 °C.     

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.     

Geometric and promoting effects of methylcyclopentane conversion over alkaline Pt/L zeolites

The hydrogenolysis of methylcyclopentane (MCP) has been used as a test reaction to analyze geometric and promoting effects in Pt catalysts supported on alkaline L zeolites. It has been observed that the production of 3-methylpentane (3MP) over these catalysts is significantly higher than over other Pt catalysts. Based on the micro-geometry of the system, we propose that collimating (geometric) effects play an important role in hydrocarbon reactions on these catalysts. On the other hand, the increased activity exhibited by the KL zeolite-supported catalysts relative to SiO₂-supported catalysts might indicate that promoting effects are also operative.     

The hydrogenolysis of methylcyclopentane on platinum model catalysts: Particle size effect due to a reaction occurring at the phase boundary metal-support

The hydrogenolysis of methylcyclopentane at 520 K on platinum model catalysts was studied and the mechanism of this reaction investigated. Catalysts of low metal dispersion (<0.15) were prepared by high-vacuum evaporation of platinum onto thin films of amorphous alumina. After annealing at 770 K in air and hydrogen treatment at 520 K the mean size of the Pt particles was about 10 nm as determined by electron microscopy. On these catalysts the reaction products are mainly 2-methylpentane and 3-methylpentane and only 9–14% n-hexane is formed, as is to be expected for large Pt particles. On top of these catalysts a thin layer of Al₂O₃ (0.3 nm mean thickness) was then deposited in order to reduce the surface area of platinum in relation to the phase boundary platinum/alumina. Thereafter a decrease of the catalytic activity, as well as a shift in the product distribution, was observed. The n-hexane content was significantly enhanced (up to 22%) and the ratio of the three products was comparable to that usually obtained with Pt catalysts of higher dispersion. This result supports a reaction model which consists of two parallel reactions (i) occurring on the platinum surface and producing mainly 2-methylpentane and 3-methylpentane and (ii) occurring on the phase boundary platinum/support and producing additional n-hexane.     

Selective Ring-Opening of Methylcyclopentane Over Titania-Supported Monometallic (Pt, Ir) and Bimetallic (Pt–Ir) Catalysts

An investigation of the selective ring-opening of methylcyclopentane (MCP) was conducted for the first time on Pt/TiO₂, Ir/TiO₂ and Pt?CIr/TiO₂ catalysts with low amounts of noble metals (0.5?wt%) over a temperature range of 180?C400?°C under hydrogen at atmospheric pressure. The catalysts were prepared by impregnation or co-impregnation and characterized by different physico-chemical techniques, including SEM, XRD, H₂-TPR, N₂-sorption, TEM and elemental analysis. The metallic particles were highly dispersed on the TiO₂ support at isodispersion of ~1?nm. The particles exhibited icosahedral Mackay structures limited by (111) planes. The catalytic results show that the activity in the MCP was strongly influenced by the intrinsic nature of the metal and by the temperature. The most active catalyst was Ir/TiO₂. The order of the reactivity as a function of the temperature and total conversion was Ir/TiO₂ 180?°C (???=?2?%)?>?Pt?CIr/TiO₂ 220?°C (???=?27.8?%)?>?Pt/TiO₂ 260?°C (???=?9.9?%). Under these conditions, all of the catalysts exhibited the ability to open the ring of MCP with an atom economy, without unwanted products of cracking and ring-enlargement reactions. The synergy between Pt?CIr bimetallic particles was assessed by the total conversion of MCP, whereas the ring-opening results indicated that the reaction took place on Ir sites. These results suggest that the bimetallic catalyst contained separate entities of two metals. Increased reaction temperatures led to reduced reaction selectivity with respect to ring-opening of MCP versus the cracking side reaction.     

Influence of Particle Size on Reaction Selectivity in Cyclohexene Hydrogenation and Dehydrogenation over Silica-Supported Monodisperse Pt Particles

The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C₆H₁₀, 200–600 Torr H₂, and 273–650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H₂ (H₂: C₆H₁₀ ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (<423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (>573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.     

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.     

Homogeneous Rh-Sn alkoxide coatings on silica surfaces: A novel route for preparation of bimetallic Rh-Sn catalysts

The reaction of [{(COD)Rh}₂Sn(OEt)₆], where COD = 1,5-cyclooctadiene and Et = ethyl, with silanol groups on silica surfaces is shown to lead to near-monolayer coverage of the silica by the Rh-Sn organometallic compound. Heating the supported compound at 498 K yields a catalyst that is active for benzene hydrogenation at room temperature. When the catalyst is reduced in H₂ at 823 K, the benzene hydrogenation activity increases with a simultaneous drop in the activity for n-butane hydrogenolysis. High temperature reduction leads to formation of Rh-Sn alloy particles with an average particle diameter of 2.5 nm. These particles are stable towards oxidation-reduction cycles involving oxidation at 773 K in 15% O₂. When normalized to the benzene hydrogenation activity, the n-butane hydrogenolysis activity of the bimetallic catalyst is suppressed by over 3 orders of magnitude when compared to a monometallic Rh catalyst.     

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.     

Exploring formation pathways of aromatic compounds in laboratory-based model flames of aliphatic fuels

This presentation summarizes our recent experimental and flame modeling studies focusing on understanding of the formation of small aromatic species, which potentially grow to polycyclic aromatic hydrocarbons (PAHs) and soot. In particular, we study premixed flames, which are stabilized on a flat-flame burner under a reduced pressure of ≈15–30 torr, to unravel the important chemical pathways to aromatics formation in flames fueled by small C₃–C₆ hydrocarbons. Flames of allene, propyne, 1,3-butadiene, cyclopentene, and C₆H₁₂ isomers 1-hexene, cyclohexane, 3,3-dimethyl-1-butene, and methylcyclopentane are analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry. Isomer-specific experimental data and detailed modeling results reveal the dominant fuel-destruction pathways and the influence of different fuel structures on the formation of aromatic compounds and their commonly considered precursors. As a specific aspect, the role of resonance-stabilized free radical reactions is addressed for this large number of similar flames of structurally different fuels. While propargyl and allyl radicals dominate aromatics formation in most flames, contributions from reactions involving other resonance-stabilized radicals like i-C₄H₅ and C₅H₅ are revealed in flames of 1,3-butadiene, 3,3-dimethyl-1-butene, and methylcyclopentane. Dehydrogenation processes of the fuel are found to be important benzene formation steps in the cyclohexane flame and are likely to also contribute in methylcyclopentane flames.     

Hydrogenolysis of nitrosodimethyl amine in gas phase over Au/γ-Al2O3 nanocatalyst

Nitrosodimethyl amine (NDMA), as a carcinogenic byproduct in production of unsymmetrical dimethyl hydrazine (UDMH) in space industries, should be decomposed in the vapor phase. A suitable method for this purpose is selective catalytic hydrogenolysis of NDMA over Au/γ-Al₂O₃ nanocatalyst. We synthesized and characterized the Au/γ-Al₂O₃ nanocatalyst by homogeneous deposition-precipitation (HDP)/DP-urea method. Activity of the catalyst was influenced by nanosized Au particles, Au loading and the bed temperature. The optimum parameters for the catalyst were: Au particles <5 nm, Au loading at 1.5 wt% and bed temperature of 35–45 °C. The reaction was strongly sensitive to the Au particle size. The reaction occurred over the catalyst to produce dimethyl amine (DMA) and nitroxyl in a selective manner. The kinetics of NDMA hydrogenolysis over the nanocatalyst was studied in an integral fixed bed reactor. There existed a consistency with the Langmuir-Hinshelwood mechanism involving dissociative adsorption of H₂ and NDMA.     

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.     

Benzene hydroisomerization over Pt/MOR/Al2O3 catalysts

Benzene hydroisomerization is among the promising processes converting benzene into methylcyclopentane (MCP), which is an environmentally friendlier, octane boosting component of motor fuels. Benzene hydroisomerization into MCP over the Pt/MOR/Al₂O₃ (MOR = mordenite) catalytic system is reported here. The dependence of the yield of the target product on the acidic properties of the support and platinum precursor ([Pt(NH₃)₄]Cl₂ or H₂PtCl₆) have been investigated in order to optimize the catalyst composition. The acidic properties of the surface have been altered by introducing 30–95 wt % alumina into the support. Catalytic activity has been measured in the hydroisomerization of cyclohexane and a benzene (20 wt %) + n-heptane (80 wt %) mixture in a flow reactor at 250–350°C, 1.5 MPa, H₂: CH = 3: 1, a cyclohexane LHSV of 6 h^?1, a mixed feedstock LHSV of 2 h^?1, a catalyst bed volume of 2 cm³, and catalyst pellet sizes of 0.25–0.75 mm. The most efficient catalyst for cyclohexane and n-heptane isomerization and benzene hydroisomerization is the platinum-containing catalyst (0.3 wt % Pt) whose support consists of 30 wt % MOR and 70 wt % Al2O3. The highest yield of the target products of isomerization in the presence of this catalyst is attained in the temperature range from 280 to 310°C, which is thermodynamically favorable for MCP formation from benzene. This indicates that this catalyst is promising for the hydroisomerization of benzene-containing gasoline fractions. Use of H₂PtCl₆, a readily available chemical, as the platinum precursor is favorable for commercialization of the catalyst and ensures price attractiveness in its industrial-scale manufacturing.     

Carbon gasification (II) Pt-catalyzed hydrogenation of carbon

The hydrogenolysis up to 90% carbon conversion of graphitized carbon black on platinumcatalyst in a flow reactor, at 873–1073K under 101 KPa hydrogen pressure, has shown that the reaction rates (g carbon reacted (initial surface area)^-1 (time){-1}) reached the maximum in the half way of 90% conversion. The change of Pt particle size over this range of the conversion was also measured by H₂ chemisorption, X-ray line broadening and TEM. Pt particle size calculated from H₂ chemisorption was too large due to the decrease of H₂ uptake on the Pt surface. In the early stage of conversion, Pt particle sizes measured by XRD and TEM did not change significantly. The mechanism of Pt catalyzed carbon gasification is discussed in the light of these observation.     

Reactions of n‐hexane on Pt–Sn/Al2O3 and removal of retained hydrocarbons by hydrogenation

n‐hexane was reacted on two Pt–Sn/Al₂O₃ catalysts, one prepared by coimpregnation (T) the other by using a bimetallic PtSn complex precursor (N). Both catalysts produced isomers, methylcyclopentane fragments and benzene, the aromatic selectivity being higher on catalyst N. The hydrocarbon entities remaining after reaction were removed by hydrogen treatment. They contained methane, C₂–C₅ fragments (not observed on Pt, thus unique for PtSn) and benzene. The possible state and composition of chemisorbed carbonaceous entities during reactions are discussed. More hydrocarbons, including slightly more methane could be hydrogenated off from catalyst N of lower dispersion. The higher overall activity of catalyst N in the presence of more than one C per surface Pt points to its higher coke tolerance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface composition and possible rearrangement of disperse Pt and Rh catalysts: does the presence of carbon and oxygen contribute to different catalytic properties?

The surface composition of Rh and Pt blacks (as determined by XPS) shows carbon and oxygen impurities in the untreated state. Oxygen on Pt is present as adsorbed O as well as OH/H₂O groups and oxidized carbon. Rh was partly oxidized to Rh₂O₃, in agreement with UPS showing hardly any Fermi‐edge intensity in untreated Rh as opposed to untreated Pt. High Fermi‐edge intensities indicated a predominant metallic surface after an in situ treatment with H₂ at 483 K, increasing the purity (XPS) to ∼90%. This treatment reduced Rh to metal and removed its C impurity. Pt, in turn, retained much carbon after H₂ treatment, present mainly as graphitic carbon. A minor amount of CO was also detected, some of the O 1s peak belonging to it. The two metals were tested in methylcyclopentane reactions. Considering the necessity of carbon for nondegradative reactions and oxygen enhancing fragmentation, a correlation is suggested between the typical impurities of Pt and Rh and their respective catalytic propensities: the high fragmentation activity of Rh and the predominant nondegradative reactions to C₆ “ring opening products” on Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

Catalytic behaviour of polycrystalline Pt3Ti in relation to strong metal-support interaction phenomenon

Skeletal isomerization reactions, of hexanes (2-methylpentane and methylcyclopentane) have been carried out in a flow reactor on a polycrystalline intermetallic Pt₃Ti compound which has been powdered and treated under several oxidative and reductive conditions. The results are compared to those obtained with (i) TiO₂ supported Pt catalysts in or not in the Strong Metal-Support Interaction (SMSI) state, (ii) a bulk Pt₃Ti sample studied under static catalytic conditions in an U.H.V. apparatus, and (iii) bulk Pt surfaces. The powdered Pt₃Ti compound shows a very unstable behaviour under hydrogen. Differences in behaviour between the bulk and the decomposed powdered Pt₃Ti samples are observed, showing the important part played by Pt₃Ti in SMSI phenomena. A model is then proposed.     

Direct Aerobic Oxidation of Secondary Alcohols Catalysed by Pt(0) Nanoparticles Stabilized by PV2Mo10O40 5− Polyoxmetalate

Pt(0) nanoparticles stabilized by the H₅PV₂Mo₁₀O₄₀ polyoxometalate were prepared in water by a sequence of redox reactions and supported on α-alumina. Characterization by TEM showed an average particle size of 2.6 nm with an approximate size dispersion of about ±25%. EDS analysis confirmed the amounts of metal and polyoxometalate as indicated from the reaction stoichiometry. These nanoparticles were further used as catalysts for the oxidation of secondary alcohols with molecular oxygen to the corresponding carbonyl products. The oxidation of secondary alcohols was further improved by the use of Pt nanoparticles stabilized by Rh_1.7PV₂Mo₁₀O₄₀.     

Kinetics of ethylene oxidation on plane Pt/SiO2 catalysts in the viscous pressure regime: evidence of support activity

C₂H₄ oxidation on plane Pt/SiO₂ model catalysts with various Pt loadings was studied atT = 373–473 K and in the pressure ranges 10^–6–10² Torr C₂H₄ and 0.3–1500 Torr 02 (1 Torr = 133.3 Pa). Mass spectrometry combined with spatially resolved gas sampling enabled kinetic data to be collected far into the viscous pressure regime. Reaction orders and activation energies were similar to those of a macroscopic Pt surface. However, under fuel-lean conditions the global reaction rate decreases faster than the decrease in metal area. On the other hand, the global rate wasindependent of Pt loading and metal surface area in fuel-rich gas mixtures. This is interpreted in terms of a spillover effect.