Combination of flame synthesis and high-throughput experimentation: The preparation of alumina-supported noble metal particles and their application in the partial oxidation of methane

Mono and multi-noble metal particles on Al₂O₃ were prepared in one step by flame spray pyrolysis (FSP) of the corresponding noble metal precursors dissolved in methanol and acetic acid (v/v 1:1) or xylene. The noble metal loading of the catalysts was close to the theoretical composition as determined by WD-XRF and LA-ICP-MS. The preparation method was combined with high-throughput testing using an experimental setup consisting of eight parallel fixed-bed reactors. Samples containing 0.1–5 wt% noble metals (Ru, Rh, Pt, Pd) on Al₂O₃ were tested in the catalytic partial oxidation of methane. The ignition of the reaction towards carbon monoxide and hydrogen depended on the loading and the noble metal constituents. The selectivity of these noble metal catalysts towards CO and H₂ was similar under the conditions used (methane: oxygen ratio 2:1, temperature from 300 to 500 °C) and exceeded significantly those of gold and silver containing catalysts.Selected catalysts were further analysed using XPS, BET, STEM-EDXS and XANES/EXAFS. The catalysts exhibited generally a specific surface area of more than 100 m²/g, and were made up of ca. 10 nm alumina particles on which the smaller noble metal particles (1–2 nm, partially oxidized state) were discernible. XPS investigation revealed an enrichment of noble metals on the alumina surface of all samples. The question of alloy formation was addressed by STEM-EDXS and EXAFS analysis. In some cases, particularly for Pt–Pd and Pt–Rh, alloying close to the bulk alloys was found, in contrast to Pt–Ru being only partially alloyed. In situ X-ray absorption spectroscopy on selected samples was used to gain insight into the oxidation state during ignition and extinction of the catalytic partial oxidation of methane to hydrogen and carbon monoxide.     

Anodic catalysts for polymer electrolyte fuel cells: the catalytic activity of Pt/C,Ru/C and Pt-Ru/C in oxidation of CO by O2

The catalytic activity for oxidation of CO by O₂ was investigated on commercial Pt/C, Pt-Ru/C (Pt/Ru atomic ratio = 20, 3, 1, 1/3) and Ru/C. All samples contained 20 wt.% metal. Assuming equal surface and bulk composition, the number of surface Pt and Ru atoms was calculated from the average size of the supported metal particle as determined by TEM. On Pt-Ru/C alloys, the turnover frequency per Ru atom, N_Ru/molecules s⁻¹ Ru-atom⁻¹, was independent of chemical composition. This finding suggests that the active site in these alloys is Ru. In the temperature range 300–400 K, the turnover frequency per active metal atom was 50–300 times higher on Pt-Ru/C than on Pt/C. The turnover frequency was 400 times higher on Ru/C than on Pt/C at 313 K and 90 times higher at 353 K. Addition of water vapor to the reactant mixture left the catalytic activity of Ru/C unchanged but slightly increased the activity of Pt/C. On both catalysts the activation energy and reaction orders were nearly the same as in dry atmosphere. Conversely, the addition of water markedly decreased the activation energy for Pt-Ru(1 : 1)/C alloy (from 19 to 11 kcal mol⁻¹). These findings suggest that fuel cells equipped with Pt-Ru/C anodes perform better than cells with Pt/C anodes. They do so because Ru effectively oxidizes the carbon monoxide present as an impurity in the H₂-reformed fuel.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Hydroxylation of benzene with oxygen and hydrogen over catalysts containing Group VIII metals and heteropoly compounds

The vapor phase hydroxylation of benzene to phenol with a mixture of oxygen and hydrogen over silica supported bi-component catalysts containing Group VIII metals (M) and heteropoly compounds (HPC) was investigated. The productivity of the catalysts was ascertained for various metal and HPC combinations and a range of reaction conditions. The Pt–PMo₁₂/SiO₂ and Pd–PMo₁₂/SiO₂ catalysts of optimal composition provide up to 380 mol phenol/g-atom Pt or Pd/h. The observed catalysis appears to be associated with an interface between metal particles and those of the heteropoly compound, as illustrated by an HREM image of a Pt–PMo₁₂/SiO₂ sample.     

Stabilization and dispersion of PtRu and Pt nanoparticles on multiwalled carbon nanotubes using phosphomolybdic acid,and the use of the resulting materials in a direct methanol fuel cell

PtRu and Pt nanoparticles were deposited on the surface of multiwalled carbon nanotubes (MWCNTs) with the assistance of phosphomolybdic acid (PMo) by a one-pot hydrothermal reduction strategy. Transmission electron microscopy shows a high-density PtRu (or Pt) nanoparticles uniformly dispersed on the surface of the MWCNTs with an average diameter of 1.8 nm for PtRu nanoparticles and 2.4 nm for Pt nanoparticles. Moreover, the as-prepared PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts are highly electroactive for the electrochemical oxidation of methanol. Cyclic voltammograms show a high electrochemical surface area (ESA) and a large current density for methanol oxidation at the modified electrode by PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts. Electrochemical impedance spectroscopy reveals a high CO tolerance for PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts in the electrochemical catalysis of methanol oxidation. For comparison, PtRu/MWCNT and Pt/MWCNT electrocatalysts were prepared in control experiments without PMo. The results demonstrate that PtRu and Pt nanoparticles deposited on MWCNTs in the presence of PMo were superior to those on MWCNTs without PMo in several respects including: (1) a smaller size and a higher dispersion; (2) a higher ESA; (3) a larger current density for methanol oxidation; (4) a higher tolerance for CO poisoning.     

One-step preparation of Pt–M@FP-MWNT catalysts (M = Ru,Ni, Co,Sn, and Au) by γ-ray irradiation and their catalytic efficiency for CO and MeOH

Pt–M@FP-MWNT catalysts (M = Ru, Ni, Co, Sn, and Au) were prepared by one-step γ-ray irradiation. Two different types of functional polymers (FP), such as poly(vinylphenyl boronic acid) (PVPBAc) and poly(vinylpyrorridone) (PVP), were used as anchoring agents, when Pt–M nanoparticles were deposited on the multi-walled carbon nanotube (MWNT) using γ-ray irradiation in aqueous solution at room temperature. The obtained Pt–M@FP-MWNT catalysts were then characterized by XRD, TEM, and elemental analysis. The catalytic efficiency of the Pt–M@FP-MWNT catalysts was examined for CO stripping and MeOH oxidation for use in a direct methanol fuel cell (DMFC). The catalytic efficiency of the Pt–M@FP-MWNT catalyst for MeOH oxidation follows this order: Pt–Sn@FP-MWNT > Pt–Co@FP-MWNT > Pt–Ru@FP-MWNT > Pt–Au@FP-MWNT > Pt–Ni@FP-MWNT catalysts. The CO adsorption capacity of the Pt–M@FP-MWNT catalyst for CO stripping is as follows: Pt–Ru@FP-MWNT > Pt–Sn@FP-MWNT > Pt–Au@FP-MWNT > Pt–Co@FP-MWNT > Pt–Ni@FP-MWNT catalyst.     

Platinum–tin bimetallic nanoparticles for methanol tolerant oxygen-reduction activity

Carbon-supported Pt–Sn/C bimetallic nanoparticle electrocatalysts were prepared by the simple reduction of the metal precursors using ethylene glycol. The catalysts heat-treated under argon atmosphere to improve alloying of platinum with tin. As-prepared Pt–Sn bimetallic nanoparticles exhibit a single-phase fcc structure of Pt and heat-treatment leading to fcc Pt₇₅Sn₂₅ phase and hexagonal alloy structure of the Pt₅₀Sn₅₀ phase. Transmission electron microscopy image of the as-prepared Pt–Sn/C catalyst reveals a mean particle diameter of ca. 5.8 nm with a relatively narrow size distribution and the particle size increased to ca. 20 nm when heat-treated at 500 °C due to agglomeration. The electrocatalytic activity of oxygen reduction assessed using rotating ring disk electrode technique (hydrodynamic voltammetry) indicated the order of electrocatalytic activity to be: Pt–Sn/C (as-prepared) > Pt–Sn/C (250 °C) > Pt–Sn/C (500 °C) > Pt–Sn/C (600 °C) > Pt–Sn/C (800 °C). Kinetic analysis reveals that the oxygen reduction reaction on Pt–Sn/C catalysts follows a four-electron process leading to water. Moreover, the Pt–Sn/C catalyst exhibited much higher methanol tolerance during the oxygen reduction reaction than the Pt/C catalyst, assessing that the present Pt–Sn/C bimetallic catalyst may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell.     

Effect of transition metals (Ni,Sn and Mo) in Pt5Ru4M alloy ternary electrocatalyst on methanol electro-oxidation

Pure Pt, PtRu and Pt₅Ru₄M (M = Ni, Sn and Mo) electrocatalysts were prepared using a NaBH₄ reduction method. The alloy formation and particle size of the electrocatalysts were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The formation of crystalline Pt was confirmed regardless of the addition of Ru and transition metals. The average particle size was found to be about 2.5–3.5 nm.The electrochemical properties of the electrocatalysts were analyzed by methanol electro-oxidation and CO stripping in the half cell. The mass activity and specific activity were obtained through these experiments. Methanol electro-oxidation and the specific activity of the PtRuNi electrocatalyst were much higher than that of PtRu electrocatalyst. The specific activity of methanol electro-oxidation based on EAS for the PtRuSn and PtRuMo electrocatalysts was higher than that of the PtRu, although their mass activity of methanol electro-oxidation was lower.     

Ultra-fine Pt nanoparticles supported on 3D porous N-doped graphene aerogel as a promising electro-catalyst for methanol electrooxidation

Three dimensional porous nitrogen-doped graphene aerogel (3D-NGA) was successfully fabricated via a combined hydrothermal self-assembly, thermal treatment and template-removing process. The as-synthesized Pt/3D-NGA catalysts exhibit an interconnected 3D porous structure, high N-doped level and uniform dispersion of Pt NPs. In studying the electrocatalytic performance of samples towards methanol electrooxidation, we found that Pt/3D-NGA hold a high electrochemical active surface area (ECSA) of 90.7 m² g^− 1 and better catalytic activity as well as stability compared to Pt/G and Pt/3D-GA catalysts. Our studies provide a simple approach to synthesize 3D metal or metal oxide/graphene-based composites, holding great potential for fuel cell applications.     

Transition metal loaded silicon carbide-derived carbons with enhanced catalytic properties

Carbide-derived carbons (CDC) with incorporated transition metal nanoparticles (～2.5 nm) were prepared using a microemulsion approach. Time-consuming post synthesis functionalization of the carbon support material can thus be avoided and nanoparticle sizes can be controlled by changing the microemulsion composition. This synthesis strategy is a technique for the preparation of highly porous carbon materials with a catalytically active component. In particular we investigated the integration of ruthenium, palladium, and platinum in a concentration ranging from 4.45 to 12 wt.%. It was found that the transition metal has a considerable influence on sorption properties of resulting nanoparticle-CDC composite materials. Depending on the used metal salt additive the surface area and the pore volume ranges from 1480 m²/g and 1.25 cm³/g for Pt to 2480 m²/g and 2.0 cm³/g for Ru doped carbons. Moreover, members of this material class show impressive properties as heterogeneous catalysts. The liquid phase oxidation of tetralin and the partial oxidation of methane were studied, and electrochemical applications were also investigated. Primarily Pt doped CDCs are highly active in the oxygen reduction reaction, which is of great importance in present day fuel cell research.     

Thermal performance of vertically-aligned multi-walled carbon nanotube array grown on platinum film

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135 cm² s⁻¹ and the thermal conductivity increases from 1.8 to 3.1 W m⁻¹ K⁻¹ as temperature increases from 243.2 to 453.2 K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.     

Facile preparation and unique H2 adsorption behavior of three-dimensional novel Pt–Ru hollow sphere assemblies

Three-dimensional long range ordered hollow Pt–Ru sphere assemblies were prepared using a sacrificial three-dimensionally ordered macroporous (3DOM) carbon template. Metallic salts, such as a mixture of RuCl₃ with H₂PtCl₆ were infiltrated into the carbon template, and a reduced Pt–Ru phase was produced on the surface of the 3DOM carbon template by a borohydride reduction reaction. The sacrificial template was then burnt off in air at 650 °C. The diameter of the hollow Pt–Ru spheres could be tailored using a different pore size 3DOM carbon template. Assemblies with an outer diameter of 550 nm showed high BET surface area of 584.3 m²/g. In addition, a high hydrogen adsorption stoichiometry (>0.5 H/M) was obtained on the Pt–Ru sphere assemblies, which indicated that most of the metal atoms on the surface were exposed.     

Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems

This paper introduces and discusses the latest research on the use of H₂ generated via the NaBH₄ hydrolysis reaction for proton exchange membrane fuel cells (PEMFCs). To realize the NaBH₄–PEMFC system, many hydrolysis catalysts such as Ru/anion-exchange resins, Pt/LiCoO₂, Co powder/Ni foam, PtRu/LiCoO₂ and Ru/carbon have been proposed. Through these efforts, the hydrolysis reaction conversion approached 100%. In addition, the average H₂ generation rate based on most of the reports generally ranged from 0.1 to 2.8 H₂ l min^− 1 g^− 1 (catalyst), which produced a level of PEMFC performance equivalent to 0.1–0.3 kW g^− 1 (catalyst). However, it was also reported that the H₂ generation rate was 28 H₂ l min^− 1 g^− 1 (catalyst) with the catalyst of Pt/carbon (acetylene black).Considering these reports and the advantageous features of NaBH₄ hydrolysis, the NaBH₄–PEMFC system seems to be technologically feasible and would constitute an alternative system of supplying H₂ in fuel cells.However, some challenges remain, such as the deactivation of the catalyst, the treatment of the by-products, and the proper control of the reaction rate. In addition, if the price of NaBH₄ were to be further reduced, this system could become the most powerful competitor in portable application fields of PEMFC.     

XPS and EXAFS study of supported PtSn catalysts obtained by surface organometallic chemistry on metals: Application to the isobutane dehydrogenation

In this work, well defined alumina and silica supported Pt and PtSn catalysts were prepared by surface organometallic reactions and were characterized by TEM, XPS and EXAFS. These catalysts were tested in the catalytic dehydrogenation of isobutane. XPS results show that tin is found under the form of Sn(0) and Sn(II,IV), being the percentage of Sn(0) lower for alumina supported than for silica supported catalysts. Tin modified platinum catalysts, always show a decrease of approximately 1 eV in the BE of Pt, what would be indicative of an electron charge transfer from tin to platinum. When the concentration of Sn(0) is high enough, in our case Sn(0)/Pt ∼ 0.3, EXAFS experiments demonstrated the existence of a PtSn alloy diluting metallic Pt atoms, for both PtSn/γ-Al₂O₃ and PtSn/SiO₂. This PtSn alloy seems to be not active in the dehydrogenation reaction; however, it is very important for selectivity and stability, inhibiting cracking and coke formation reactions. The ensemble of our catalytic, XPS and EXAFS results, show that bimetallic PtSn/γ-Al₂O₃ catalysts, prepared via SOMC/M techniques, can be submitted to several sequential reaction–regeneration cycles, recovering the same level of initial activity each time and that the nature of the catalytic surface remains practically without modifications.     

Electrodeposited Re-promoted Ni foams as a catalyst for the dry reforming of methane

The dry reforming of methane (DRM) utilizes carbon dioxide (CO₂) as the oxidizing agent in order to produce synthesis gas. Catalyst deactivation via coking, oxidation, and sintering has stymied the industrialization of catalysts for the DRM. Here, we utilized electrodeposition followed by de-alloying in order to synthesize metal alloy foams (5 m²/g). Through this process we have created the first electrodeposited DRM catalyst capable of converting more than 10,000 mL/g 1 h at near-equilibrium conversion. Rhenium promotion was observed over the entire temperature range studied (700–800 °C), with the most dramatic enhancement at 700 °C. After 50 h of reaction, no significant accumulation of carbonaceous deposits were detected, making electrodeposited structures a viable candidate for stable methane conversion catalysts.     

Electro-oxidation of ethanol and ethylene glycol on carbon-supported nano-Pt and -PtRu catalyst in acid solution

Present paper reports kinetics of electro-oxidation of ethanol (EtOH) and ethylene glycol (EG) onto Pt and PtRu nanocatalysts of different compositions in the temperature range of 298–318 K. These catalysts have been characterized by SEM, EDX, XRD, CV and amperometry. It has been observed that apparent activation energies for oxidation of EtOH and EG pass through a minimum at about 15–20 at.% of Ru in the PtRu alloy catalysts. Anodic peak current vs. composition curve also shows a maximum around this composition.The results have been explained by a geometric model, which proposes requirement of an ensemble of three Pt atoms with an adjacent Ru atom onto PtRu surface for an efficient electro-oxidation of EtOH or EG. This is further supported from statistical data analysis of probability of occurrence of such ensembles onto PtRu alloy surface.Present results also suggest that electro-oxidation of EG onto nano-PtRu catalyst surfaces follows a different path from that of EtOH at alloy composition less than 15 at.% of Ru.     

Surface modification of Ni catalysts with trace Pt for oxidative steam reforming of methane

Hysteresis of catalytic performance with respect to temperature increasing and decreasing in oxidative steam reforming of methane (CH₄/H₂O/O₂/Ar = 40/30/20/10) over the monometallic Ni catalysts disappeared by the modification with Pt, and the additive effect of Pt by the sequential impregnation method (Pt/Ni) was much more significant than that by the co-impregnation method (Pt + Ni) in terms of catalytic performance and catalyst bed temperature profile. Characterization results by means of TEM, TPR, EXAFS, and FTIR suggest that the Pt atoms on the Pt/Ni catalysts were located more preferably on the surface to form a PtNi alloy than those on the Pt + Ni catalysts. The modification of Ni with Pt suppressed the oxidation of Ni species near the bed inlet in the oxidative steam reforming of methane at 1123 K, although the species on the monometallic Ni catalysts were oxidized under similar conditions. This can be due to the decreased oxidation rate of the species and the increased reduction rate caused by the surface modification of Ni with Pt. Consequently, the PtNi species can be maintained in the metallic state near the bed inlet, and the species can be the active site for the reforming reaction as well as the combustion reaction, which this leads to a lower bed temperature and smaller temperature gradient than those seen for the monometallic Ni catalysts.     

Microporous metal–organic frameworks constructed by Cu(I)/Zn(II), Tpt and 1,4-benzenedicarboxylate

Two novel metal–organic frameworks, [Cu(tpt)(bdc)_1/2]_n · nH₂O (1) and [Zn(tpt)(bdc)_1/2I]_n (2) (tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine, H₂bdc = 1,4-benzenedicarboxylic acid), have been prepared by hydrothermal reactions. In complex 1, Cu(I) center is in a trigonal coordination environment with bidentate tpt coordinating Cu(I) atoms to form 1D zigzag chains, and bdc ligand links the zigzag chains to form a 2D layered structure. In complex 2, Zn(II) center is in a trigonal–pyramidal environment with bidentate tpt coordinating Zn(II) atoms to form zigzag chains, and bdc links the zigzag chains to form metal–organic framework which contains interesting hexagonal nano-channels.     

Methanation of carbon dioxide over mesoporous Ni–Fe–Ru–Al2O3 xerogel catalysts: Effect of ruthenium content

Mesoporous nickel (35 wt%)–iron (5 wt%)–ruthenium (x wt%)–alumina xerogel (denoted as 35Ni5FexRuAX) catalysts with different ruthenium contents (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a single-step sol–gel method for use in the methane production from CO₂ and H₂. Conversion of CO₂, yield for CH₄, metal surface area, and the amount of desorbed carbon dioxide of the catalysts showed volcano-shaped trends with respect to ruthenium content. Experimental results revealed that metal surface area and the amount of desorbed carbon dioxide of 35Ni5FexRu catalysts were well correlated with conversion of CO₂ and yield for CH₄.     

Direct dehydrogenation of n-butane over Pt/Sn/M/γ-Al2O3 catalysts: Effect of third metal (M) addition

A series of Pt/Sn/M/γ-Al₂O₃ catalysts with different third metal (M = Zn, In, Y, Bi, and Ga) were prepared by a sequential impregnation method for use in the direct dehydrogenation of n-butane to n-butene and 1,3-butadiene. In the direct dehydrogenation of n-butane, Pt/Sn/Zn/γ-Al₂O₃ catalyst showed the best catalytic performance. Catalytic performance decreased in the order of Pt/Sn/Zn/γ-Al₂O₃ > Pt/Sn/In/γ-Al₂O₃ > Pt/Sn/γ-Al₂O₃ > Pt/Sn/Y/γ-Al₂O₃ > Pt/Sn/Bi/γ-Al₂O₃ > Pt/Sn/Ga/γ-Al₂O₃. The catalytic performance increased with increasing metal–support interaction and Pt surface area of the catalyst.