Reforming of C6 Hydrocarbons Over Model Pt Nanoparticle Catalysts

Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C₆ hydrocarbons in mixtures with 5:1 excess of H₂: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50?Torr methylcyclopentane in 250?Torr H₂ at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C₆ isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0?nm Pt, which also yielded relatively less benzene.     

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.     

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.     

Highly durable platelet carbon nanofiber-supported platinum catalysts for the oxygen reduction reaction

We report the preparation and characterization of highly durable platinum catalysts supported on platelet-structure carbon nanofibers (Pt/p-CNFs) for the oxygen reduction reaction. The p-CNFs were prepared by liquid phase carbonization of polyvinyl chloride using a porous anodic alumina template at 600 °C; their degree of graphitization was increased by the subsequent heat treatment at higher temperatures of up to 1400 °C. The platinum nanoparticles with ∼3 nm diameter were deposited more uniformly on the p-CNFs compared with those on the commercial Ketjen black (KB). The catalytic activity and durability of the Pt/p-CNFs for the oxygen reduction reaction (ORR) in H₂SO₄ solution were improved by increasing the heat-treatment temperature of p-CNFs. The durability of the Pt/p-CNFs was much higher than that of Pt/KB; in particular, a loss of less than 10% was observed in the ORR activity of Pt/p-CNF heat-treated at 1400 °C after potential cycling from 0.5 to 1.5 V vs. RHE for 200 cycles in an argon-saturated H₂SO₄ aqueous solution.     

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.     

Graphite-supported ultra-small copper nanoparticles – Preparation,characterization and catalysis applications

A convenient preparation of graphite-supported ultra-small copper nanoparticles (Cu NPs) has been developed. Ultra-small Cu NPs, generated by reduction of Cu(OAc)₂ with H₂, were well dispersed onto the graphite support and possess a very narrow distribution in size, ranging from 1.6 to 2.6 nm. The catalytic activity of these ultra-small graphite-supported Cu NPs was evaluated for the Meerwein arylation of pyrroles and the multicomponent synthesis of 1,2,3-triazoles via click reaction.     

Selective Nanocatalysis of Organic Transformation by Metals: Concepts, Model Systems, and Instruments

Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimer-supported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)––inorganic shell (SiO₂) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures (>400 °C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneous-homogeneous, heterogeneous-enzyme and homogeneous-enzyme catalyst systems.     

NO/CH4 reaction over nanodispersed Pt particles

The catalytic behavior of the cubic (≈70%) Pt nanoparticles supported on alumina, with an average diameter of 132 nm, was investigated for NO/CH₄ reaction. It was observed that the formation of reaction products (N₂O, CO and NH₃) is related to the size as well to the shape (facet) of the Pt nanoparticles.

     

Vanadium pentoxide: Synthesis and characterization of nanorod and nanoparticle V2O5 using CTAB micelle solution

Using a surfactant-mediated method (surfactant based on cetyltrimethyl ammonium bromide, CTAB) V₂O₅ nanorod and nanoparticles have been successfully prepared. Morphologies of V₂O₅ nanostructures can be controlled by applying different precursors and by varying reaction conditions within the CTAB soft template. With ammonium metavanadate and sulfuric acid as precursors, nanoparticles are synthesized in the size range of 45–160 nm. Precursors of vanadyl sulfate hydrate and sodium hydroxide yield vanadium pentoxide nanorods with diameters of 30–90 nm and lengths of 260–600 nm. The resulting products are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), variable pressure scanning electron microscopy (VPSEM) and X-ray photoelectron spectroscopy (XPS). Temperature programmed reduction (TPR) is included to test catalytic performance. The results show that V₂O₅ nanoparticles and nanorods achieve better catalytic performance compared to bulk V₂O₅, i.e. lower onset temperature, workability at lower temperatures, and higher H₂ consumption (μmol/g).     

Effect of heat treatment conditions on the growth of MgAl2O4 nanoparticles obtained by sol-gel method

MgAl₂O₄ nanoparticles (NPs) were prepared by sol–gel method using aluminium nitrate, magnesium nitrate and citric acid as starting materials, phenolic formaldehyde resin and carbon black as additives. Growth of MgAl₂O₄ NPs in different heat treatment conditions (temperature, atmosphere, carbon additives and in Al₂O₃-C system) was investigated. MgAl₂O₄ NPs were formed at 600 °C in air atmosphere with serious agglomeration of nanoparticles having diameter of approximate 30 nm. The size of MgAl₂O₄ NPs increased greatly from 40 to 50 nm to several hundreds of nanometres as the temperature was raised from 800 °C to 1400 °C. Partial sintering of NPs was observed upon heating at temperatures higher than 1200 °C in air. In reducing atmosphere, the size of MgAl₂O₄ NPs (about 30–50 nm) changed slightly with increasing temperature. This was attributed to the dispersion of carbon inclusions in the MgAl₂O₄ grain boundaries, inducing a steric hindrance effect and inhibiting the growth of particles. MgAl₂O₄ NPs (30–50 nm) in the Al₂O₃-C system were in-situ formed at high temperatures with the use of dried precursor gels. MgAl₂O₄ NPs can contribute to improving the thermal shock resistance of Al₂O₃-C materials.     

Preparation,characterization and performance of Pd/SiO2 catalyst for benzene catalytic hydrogenation

Palladium nanoparticles supported on silica were prepared by hydrazine reduction in aqueous medium at room temperature. They were characterized by XRD, TEM, EDX, H₂-adsorption, and H₂-TPD. The catalytic properties were evaluated in the gas-phase hydrogenation of benzene in the temperature range of 75–250 °C. Metal particles with a size range of 4.0–25.8 nm were obtained. The metal surface area and hydrogen storage increase with decreasing metal particle size. The H₂-TPD profiles exhibited a main peak appeared at 540 °C with two shoulders at lower (445 °C) and higher (605 °C) temperatures. These peaks were ascribed to strongly adsorbed hydrogen on the surface catalyst. The catalytic activity of the catalysts strongly depends on the metal loading. It increases with decreasing Pd loading. This is ascribed to metal surface area, which increases with decreasing Pd content.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.     

Simultaneous sulfonation and reduction of graphene oxide as highly efficient supports for metal nanocatalysts

The sulfonated reduced graphene oxide (S-rGO) as supports and size-controlled Pt nanoparticles (NPs) for proton exchange membrane fuel cell (PEMFC) catalysts was investigated. The S-rGO was fabricated by a lyophilization-assisted method from a liquid mixture of GO and (NH₄)₂SO₄ with a subsequent thermal treatment in inert gas. Sulfonic acid groups were grafted on GO and a reduction of GO was achieved simultaneously. Transmission electron microscope (TEM) results showed a uniform deposition of Pt NPs on S-rGO (Pt/S-rGO) with a narrow particle size distribution ranging from 2 to 5 nm in diameter. A higher catalytic activity of this novel Pt/S-rGO catalyst was revealed in comparison with that of Pt/GO, Pt/rGO and conventional Pt/C catalysts by cyclic voltammetry and oxygen reduction reaction measurements due to an enhanced triphase boundary. Significantly, the Pt/S-rGO catalyst also presented an excellent electrochemical stability. This new catalyst thus holds a great potential application in PEMFCs in terms of enhanced activity and durability.     

High selectivity and response H2 sensors based on ZnO@ZIF-71@Ag nanorod arrays

Hydrogen (H₂) is widely used in industrial and medical, however its flammable and explosive nature requires economical and effective monitoring to ensure safety. In this work, ZnO@ZIF-71@Ag nanorod arrays were synthesized to provide an effective adsorption response to H₂ through size effect and high catalytic activity by immobilizing Ag nanoparticles (NPs) into the pores of ZIF-71. The results of the gas sensitivity tests showed that the nanorod arrays were significantly more selective towards H₂. Moreover, the response of ZnO@ZIF-71@Ag to 50 ppm H₂ was 11 times higher than that of ZnO@ZIF-71 at a lower operating temperature (150°C). The size of the Ag NPs was demonstrated to be below 10 nm by TEM characterization, suggesting that Ag in the form of quantum dots (QDs) to bring an unignorable catalytic effect for breaking hydrogen molecule (H₂) into highly active atoms ([H]). In addition, the result of Density Function Theory (DFT) calculation revealed that the adsorption energy of Ag-catalyzed [H] (−8.255 eV) was much higher than that of H₂ (−4.222 eV) on ZnO (100), which results in elevated charge transfer to promote hydrogen sensing performance of ZnO@ZIF-71@Ag. In this study, a novel hydrogen sensor based on pore sieving and catalytic sensing mechanisms was obtained, which provides a new reference for the development of hydrogen sensors.     

Single-step polyol synthesis of alloy Pt<Subscript>7</Subscript>Sn<Subscript>3</Subscript> versus bi-phase Pt/SnO<Subscript>x</Subscript> nano-catalysts of controlled size for ethanol electro-oxidation

Disordered alloy and bi-phase PtSn nanoparticles of nominal Pt:Sn ratio of 70:30 atomic % with controlled size and narrow size distribution were synthesized using a single-step polyol method. By adjusting the solution pH it was possible to obtain Pt₇Sn₃ nanoparticles of various sizes from 2.8 to 6.5 nm. We found that the presence of NaOH in the synthesis solution not only influenced the nanoparticle size, but as it was revealed by XRD, it apparently also dictated the degree of Pt and Sn alloying. Three catalysts prepared at lower NaOH concentrations (C_NaOH < 0.15 M) showed disordered alloy structure of the nominal composition, while the other three catalysts synthesized at higher NaOH concentrations (C_NaOH > 0.15 M) consisted of bi-phase nanoparticles comprising a crystalline phase close to that of pure Pt together with an amorphous Sn phase. These observations are plausibly due to the phase separation and formation of monometallic Pt and amorphous SnO_x phases. A proposed reaction mechanism of Pt₇Sn₃ nanoparticle formation is presented to explain these observations along with the catalytic activities measured for the six synthesized carbon-supported Pt₇Sn₃ catalysts. The highest catalytic activity towards ethanol electro-oxidation was found for the carbon-supported bi-phase catalyst that formed the largest Pt (6.5 nm) nanoparticles and SnO_x phase. The second best catalyst was a disordered alloy Pt₇Sn₃ catalyst with the second largest nanoparticle size (5 nm), while catalysts of smaller size (4.5–4.6 nm) but different structure (disordered alloy vs. bi-phase) showed similar catalytic performance inferior to that of the 5 nm disordered alloy Pt₇Sn₃ catalyst. This work demonstrated the importance of producing bi-metallic PtSn catalysts with large Pt surfaces in order to efficiently electro-oxidize ethanol.     

Synthesis of hybrid Pt/TiO2 (anatase)/MWCNTs nanomaterials by a combined sol–gel and polyol process

In the present study, hybrid Pt/TiO₂/MWCNTs nanomaterials are prepared successfully by a combined sol–gel and polyol process. The as-prepared nanomaterials are characterized by X-ray diffraction, high resolution transmission electron microscopy, and thermogravimetry analysis. In addition, its catalytic performance by converting CO into CO₂ is also evaluated. Experimental results show that the hybrid Pt/TiO₂/MWCNTs nanomaterials exhibit a mixture of anatase TiO₂ and Pt phases. Multi-wall carbon nanotubes serve as an excellent supporting material where anatase TiO₂ nanoparticles are decorated with well-distributed Pt nanoparticles. Excellent catalytic performance can be revealed for the hybrid Pt/TiO₂/MWCNTs nanomaterials. When compared with its Pt/TiO₂ counterparts where ～ 100% CO conversion occurred at 150 °C, almost 100% conversion of CO into CO₂ can be observed at a temperature ranged from 30 °C to 100 °C.     

Ambient temperature CO oxidation over gold nanoparticles (14 nm) supported on Mg(OH)2 nanosheets

Au/Mg(OH)₂ catalysts with two different sizes (4 and 14 nm) of Au nanoparticles (NPs) have been prepared by depositing pre-fabricated Au NPs onto Mg(OH)₂ nanosheets (NSs). It was found that 14 nm Au NPs supported on Mg(OH)₂ exhibited unexpected activity for CO oxidation at ambient temperature that could be comparable with those of Au NPs of 4 nm. The Mg(OH)₂ support was suggested to be responsible for the observed catalytic activity of larger gold supported catalysts.     

Pt-rich shell coated Ni nanoparticles as catalysts for methanol electro-oxidation in alkaline media

The Pt-rich shell coated Ni nanoparticles in size of 8.9–12.1 nm were synthesized by chemical deposition via successive reduction of NiCl₂ and H₂PtCl₆, respectively, in an ethylene glycol solution and characterized by TEM, XPS, ICP–AES, and XRD techniques. The electrochemical evaluation showed that as-prepared core–shell structural nanoparticles, as a catalyst for methanol electro-oxidation in alkaline media, not only exhibited better catalytic activity and resistance to carbonaceous intermediate poison than pure solid Pt nanoparticles but also decreased wastage of expensive Pt.     

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.     

Effects of pore structure in nitrogen functionalized mesoporous carbon on oxygen reduction reaction activity of platinum nanoparticles

Nitrogen-doped mesoporous carbons (NMCs) with ordered to disordered pore structures were fabricated on SBA-15 modified with different concentrations of tetraethyl orthosilicate using pyrrole as a carbon source. The carbonization temperature of NMCs was maintained at 800 °C so that the amount and type of nitrogen functionalities were constant. Pt nanoparticles (NPs) were deposited onto NMCs using a modified polyol process. N₂ adsorption isotherms, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize NMCs and Pt NPs. A significant shift in binding energy was found in Pt NPs deposited on NMC with disordered pore structure compared to Pt NPs deposited on NMC with ordered pore structure. Pt NPs deposited on NMC with the disordered pore structure had the highest intrinsic oxygen reduction reaction (ORR) activity among the Pt/NMC catalysts. It showed that the interaction between Pt NPs and NMCs could be modulated for enhancement of the ORR activity of Pt NPs by changing the pore structure of NMCs.