Pd‐W Alloy Electrocatalysts and Their Catalytic Property for Oxygen Reduction

In order to design Pt‐free efficient cathode catalyst and promote the commercialization of fuel cells, different atomic ratio of carbon‐supported Pd‐W alloy catalysts were developed for oxygen reduction reaction (ORR). X‐ray diffraction (XRD) results show the Pd‐W alloys have the similar lattice characteristics to pure Pd. Transmission electron microscopy (TEM) and energy‐dispersive X‐ray spectroscopy (EDS) results show that the Pd‐W alloys disperse on the surface of carbon support uniformly. The results of the electrochemical tests show that the Pd₁₉W/C has two‐fold mass activity over Pd/C, which is hopeful for the application as low‐cost cathode catalyst.     

Highly Durable Platinum based Cathode Electrocatalysts for PEMFC Application using Oxygen and Nitrogen Functional Groups Attached Nanocarbon Supports

The combined effect of oxygen and nitrogen functional groups on highly crystalline carbon supports like multiwalled carbon nanotubes (MWCNT) and MWCNT‐few layer graphene hybrid structures (MWCNT+FLG) have been investigated towards oxygen reduction reaction (ORR) performance and carbon corrosion durability in polymer electrolyte membrane fuel cell (PEMFC) applications. The pristine carbon supports were modified with oxygen and nitrogen functionalities by treating with concentrated mineral acids and subsequent nitrogen plasma treatment assisted with R.F. magnetron sputtering. Pt nanoparticles were dispersed over these chemically modified carbon supports by polyol reduction method. The physicochemical properties of as synthesized electrocatalysts were studied by different techniques such as XRD, TEM, FTIR, Raman and XPS. Electrochemical properties were investigated by cyclic voltammetry and linear sweep voltammetry in 0.1M HClO₄ medium. Compared to commercial Pt/C catalysts, durability show ∼30 % enhancement for the as prepared electrocatalysts due to the presence of large amount of pyrrolic nitrogen and highly oriented graphitic nature of the catalyst supports. The ORR performance were comparable with Pt/C (TEC10E30E) in terms of MSA, 259, 270, 252 A g⁻¹ for Pt/C, Pt/N‐f‐MWCNT, Pt/N‐f‐(MWCNT+FLG) respectively.     

Novel Graphitised Carbonaceous Materials for Use as a Highly Corrosion‐Tolerant Catalyst Support in Polymer Electrolyte Fuel Cells

Cathode catalysts for polymer electrolyte fuel cells (PEFCs) are prepared by depositing Pt nanoparticles on carbon nanospheres (CNSs) and graphitised carbon nanospheres (GCNSs), and their corrosion‐tolerance and electrocatalytic activities for the oxygen reduction reaction are evaluated. Transmission electron micrographs show that the deposited Pt nanoparticles are well dispersed on CNSs. In Pt/GCNS, Pt nanoparticles accumulate selectively along the edges of GCNSs' polygonal surfaces. Electrochemical measurements with a rotating‐ring disk electrode in an O₂‐saturated H₂SO₄ solution show that Pt/GCNS and Pt/CNS produce less H₂O₂ during oxygen reduction, compared to that obtained with a Pt catalyst on carbon black (CB). Thermogravimetric analysis reveals that GCNSs show greater combustion‐tolerance than CNSs and CB. Furthermore, GCNSs show excellent electrochemical corrosion‐tolerance in a H₂SO₄ solution. These results indicate that GCNSs are superior for use as carbon supports, and can serve as cathode catalysts in PEFCs even under oxidative conditions.     

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

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

Chloride free Pt‐ and PtRu‐ Nanoparticles Stabilised by “Armand's Ligand” as Precursors for Fuel Cell Catalysts

Chloride residues on the surface of fuel cell catalysts are known to decrease the catalytic activity, especially for O₂ reduction. Using Armand's ligand, which contains the chloride free DCTA anion, for the colloidal stabilisation of nanoscopic Pt and PtRu catalysts precursors (< 2 nm size) leads to PEMFC and DMFC catalysts with improved activity compared to commercial E‐TEK catalysts as evidenced by both methanol oxidation and CO‐stripping voltametric studies.     

Nanosized Composite Pt‐Ru Catalysts for Production of Modern Modified Fuels

A Pt‐Ru/2 % Ce/(θ+α)‐Al₂O₃ nanosized catalyst was developed for selective catalytic oxidation of CH₄ to synthesis gas. The process was carried out entirely with the formation of synthesis gas at high selectivity by H₂ and CO with H₂:CO = 2.0 ratio only at Pt:Ru = 2:1 or 1:1 atomic ratio and short contact time on Pt‐, Ru‐, and Pt‐Ru low‐percentage catalysts. Samples, which were reduced by H₂ at high temperature, presented a mixture of Pt‐, Ru‐, and Pt‐Ru nanosized particles, its alloy in the mixed catalysts. The correlation between experimental results and data of physicochemical research was established. The activity together with physicochemical properties and quantum chemical calculations for the developed low‐percentage Pt‐Ru catalysts was investigated.     

The Promoting Effect of Europium on PtSn/C Catalyst for Ethanol Oxidation

Well‐dispersed PtSnEu/C and PtSn/C catalysts were prepared by the impregnation–reduction method using formic acid as a reductant and characterised by X‐ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersion X‐ray spectroscopy (EDX) and X‐ray photoelectron spectroscopy (XPS). The synthesised catalysts with different atomic ratios of Pt/Sn/Eu have the Pt face centered cubic (fcc) structure and their particle sizes are 3–4 nm. The PtSnEu/C catalyst is composed of many Pt (0), SnO₂, Eu(OH)₃, a small amount of Pt(II) and partly alloyed PtSn, but no metallic Eu. The electrochemical measurements indicate that in comparison with Pt₃Sn₁/C catalyst, the Pt₃Sn₁Eu₁/C catalyst for ethanol oxidation has more negative onset potential, smaller apparent activation energy and lower electrochemical impedance so that it exhibits very high catalytic activity. Its peak current density increases by 135% and 40%, compared with Pt₃Sn₁/C and Pt₁Ru₁/C (JM) catalysts, respectively. This is because the Eu(OH)₃ formed by adding Eu to PtSn/C catalyst can provide the OH group which is in favour of the removal of adsorbed intermediates and ethanol oxidation.     

SbOx‐promoted pt nanoparticles supported on CNTs as catalysts for base‐free oxidation of glycerol to dihydroxyacetone

Understanding of selective base‐free oxidation of glycerol to dihydroxyacetone (DHA) over Pt‐based catalysts is of paramount scientific and industrial importance. In this work, a comparative study between differently sized SbO_x‐promoted and unpromoted Pt/CNTs catalysts is carried out to decouple the promoter effects from the metal size effects. The introduction of SbO_x appears to enhance both the glycerol oxidation activity and the DHA selectivity, and the largely sized promoted Pt/CNTs catalysts afford a relatively high DHA yield and less C–C bond cleavage. X‐ray photoelectron spectroscopy measurements reveal that the Sb species are mainly in the form of SbO_x, and the differently sized promoted catalysts show similar metal binding energies. Furthermore, theoretical studies on the promotional effects of SbO_x are carried out by DFT calculations. It is found that the presence of the promoter on the catalyst surface favors the preferential activation of the secondary hydroxyl group. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3979–3987, 2018     

Unprecedented Reductive Esterification of Carboxylic Acids under Hydrogen by Reusable Heterogeneous Platinum Catalysts

Supported metal catalysts have been tested for an unprecedented reductive dimerization of carboxylic acids to esters under 8 bar hydrogen in solvent‐free conditions. Among various metal‐loaded tin oxide catalysts, platinum‐loaded tin dioxide (Pt/SnO₂) shows the highest ester yield for the reaction of dodecanoic acid. Among Pt catalysts on various supports, Lewis acidic oxides, especially SnO₂, show high activity. The most active catalyst, 5 wt% Pt/SnO₂ reduced at 100 °C, is effective for the reductive esterification of various carboxylic acids, and the catalyst is reusable for nine cycles, demonstrating the first successful example for the title reaction. Infrared (IR) studies of a model compound (formic acid) on some metal oxides indicate a strong Lewis acid‐base interaction between SnO₂ and the carbonyl oxygen. For Pt/SnO₂ catalysts with different Pt particle sizes, the activity increases with decreasing size of Pt metal. A cooperative catalysis of the Pt metal nanoparticles and the Sn⁴⁺ Lewis acid sites is proposed.

     

Durable Transition‐Metal‐Carbide‐Supported Pt–Ru Anodes for Direct Methanol Fuel Cells

Molybdenum carbide (MoC) and tungsten carbide (WC) are synthesized by direct carbonization method. Pt–Ru catalysts supported on MoC, WC, and Vulcan XC‐72R are prepared, and characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy in conjunction with electrochemistry. Electrochemical activities for the catalysts towards methanol electro‐oxidation are studied by cyclic voltammetry. All the electro‐catalysts are subjected to accelerated durability test (ADT). The electrochemical activity of carbide‐supported electro‐catalysts towards methanol electro‐oxidation is found to be higher than carbon‐supported catalysts before and after ADT. The study suggests that Pt–Ru/MoC and Pt–Ru/WC catalysts are more durable than Pt–Ru/C. Direct methanol fuel cells (DMFCs) with Pt–Ru/MoC and Pt–Ru/WC anodes also exhibit higher performance than the DMFC with Pt–Ru/C anode.     

Preparation of Pt-Co nanocatalysts on carbon nanotube electrodes for direct methanol fuel cells

The electrochemical activities of three types of Pt-Co/CNT catalysts, prepared from different Co depositions, in methanol oxidation have been investigated. X-ray diffraction reveals that these Pt-Co/CNT catalysts possess not only different crystalline sizes but also different levels of atomic distribution. The use of strong reducing agent (NaBH₄) enables the formation of a cobalt layer over the Pt surface, inducing bimetallic Pt-Co particles, whereas direct thermal reduction enables the formation of Pt-Co nanoalloy with a high degree of alloying. It has been shown that the normalized active surface coverage increases the alloying degree of Pt-Co catalysts, indicating the importance of atomic distribution. Cyclic voltammetric measurement also reveals that the Pt-Co/CNT catalyst with a good alloying degree exhibits a better electrochemical activity, high CO tolerance, and long-term durability (> 100 cycles).     

Noncovalent hybrid of CoMn2O4 spinel nanocrystals and poly (diallyldimethylammonium chloride) functionalized carbon nanotubes as efficient electrocatalysts for oxygen reduction reaction

We have grown CoMn₂O₄ spinel nanocrystals on poly (diallyldimethylammonium chloride) functionalized carbon nanotubes (PDDA-CNTs) by noncovalent functionalization and solvothermal techniques. PDDA plays an important role in homogeneously increasing the surface density of available functional groups, which can provide active sites for decoration of CoMn₂O₄ on CNTs. In addition, PDDA preserves the intrinsic properties of CNTs, increases the active sites of catalysts, and enhances the durability of the catalysts. Here, CoMn₂O₄ nanocrystals were uniformly deposited on PDDA-CNTs with loading amounts from 36% to 83%. The as-prepared CoMn₂O₄/PDDA-CNT catalyst showed high current densities for the oxygen reduction reaction (ORR) in alkaline and neutral conditions, which outperformed the Co₃O₄/PDDA-CNT and Pt/C catalysts at medium overpotential, mainly through a 4e reduction pathway. The obtained CoMn₂O₄/PDDA-CNT hybrid exhibited excellent activity and durability when subjected to an oxygen evolution reaction. These results indicate that the CoMn₂O₄/PDDA-CNT hybrid represents a promising alternative to Pt for ORR electrocatalysis, and this non-precious bifunctional electrocatalyst provides a corrosion resistant and protective cathode layer to fuel cells. The excellent activity and stability of the hybrid materials demonstrate the potential of noncovalent coupling inorganic/carbon composites as novel catalytic systems for lithium–air batteries and chlor-alkali production.     

Durability of Pt/C and Pt/MC‐PEDOT Catalysts under Simulated Start‐Stop Cycles in Polymer Electrolyte Fuel Cells

A composite of mesoporous carbon (MC) with poly(3,4‐ethylenedioxythiophene) (PEDOT) is studied as catalyst support for platinum nanoparticles. The durability of commercial Pt/carbon and Pt/MC‐PEDOT as cathode catalyst is investigated by invoking air‐fuel boundary at the anode side so as to foster carbon corrosion at the cathode side of a polymer electrolyte fuel cell (PEFC). Pt/MC‐PEDOT shows higher resistance to carbon corrosion in relation to Pt/C. Electrochemical techniques such as cyclic voltammetry (CV) and impedance measurements are used to evaluate the extent of degradation in the catalyst layer. It is surmised that the resistance of MC‐PEDOT as catalyst support toward electrochemical oxidation makes Pt/MC‐PEDOT a suitable and stable cathode catalyst for PEFCs.     

Synthesis of Ultrafine Size Platinum Nanoparticles on Defective Graphene with Enhanced Performance Towards Methanol Electro‐Oxidation

Graphene nanosheets (GS) were formed by the thermal‐expansion method. Large micropores about 1–2 nm were produced, which might provide abundant anchor sites for fixing catalyst. Platinum nanoparticles (NPs) supported on exfoliated GS (Pt/GS) were synthesized through an improved impregnation approach and mixture gas (5% H₂ in N₂) reduction. SEM and TEM images indicated the simple and clean method can effectively synthesize Pt with uniform dispersion and small size (below 3 nm) on the 2D specific and stratiform GS. The different amounts of Pt loaded on carbon carriers have been investigated respectively to evaluate the preferable electrocatalyst. Experimental results showed that Pt/GS of 20 wt.% initiated CO oxidation at the lowest onset potential in comparison with the commercial Pt/C (JM), indicating a higher CO tolerance of Pt/GS catalysts. In addition, Pt/GS of 20 wt.% exhibited enhanced electrocatalytic activity and high durability towards methanol oxidation. The high performance is exclusively attributed to synergistic effects of exfoliated GS and ultrafine size Pt NPs. Combining a melt‐diffusion strategy with the effective reduction of Pt precursors by the hydrogen gas, this present method is easy to scale up and possesses a significant potential for synthesizing anode electro‐catalyst of direct methanol fuel cells.     

A new synthesis of a highly dispersed and CO tolerant PtSn/C electrocatalyst for low-temperature fuel cell; its electrocatalytic activity and long-term durability

An effective method is developed for preparing highly dispersed and nano-sized PtSn/C electrocatalysts synthesized by borohydride reduction and subsequent hydrothermal treatment. From the XRD patterns, the Pt(2 2 0) peak of the PtSn/C catalysts shift slightly to lower 2θ values with increasing Sn content, compared with that of the Pt/C catalyst, suggesting the alloy formation. Based on the HR-TEM, the PtSn nanoparticles show average particle sizes of approximately 2.3 nm on the carbon surface, which is consistent with XRD data. The XPS result shows that the slight shift in the bulk metallic Pt(0) to higher binding energies is attributed to a significant contribution from the metal-support interaction and the nano-size effect. The methanol and CO oxidations on the PtSn/C catalysts occur at lower potentials as compared to the commercial Pt/C catalyst. This result suggests that Sn has the ability to promote the oxidation of adsorbed CO at lower potentials. In the single-cell and accelerated durability tests, the 3Pt1Sn/C catalyst shows higher performance under a pure H₂ and CO-containing H₂ gases and better durability under a 0.5 M H₂SO₄ solution than the commercial Pt/C catalyst, due to the coexistence of PtSn alloys and Sn oxides.     

Investigations on precious metal/alumina interactions on three-way automotive emission control catalysts,BMA catalysts and model samples

Combined XPS and SIMS measurements, partly supplemented by XRD investigations, were utilized to detect precious metal/support interactions on catalysts containing various precious metals. The Klemm-Bronger reaction between Pt and α-Al₂O₃ on methane/ammonia catalysts was used as a standard reference. Evidence was found for different stages of Pt/Al interactions and changing electronic properties of Pt on the surfaces of these Pt/α-Al₂O₃ catalysts by means of XPS and SIMS, and in the bulk material by XRD. The SIMS investigations on Pt and Rh on γ-Al₂O₃ showed that, even at temperatures around 500 °C, Rh/Al interactions can appear to a small extent in the topmost atomic layers, whereas the Pt/γ-Al₂O₃ specimens did not show any measurable effects. After on-road operation, enhanced SIMS cluster-ion signals as well as anomalous XPS signal contributions were measured on the surfaces of used three-way automotive emission control catalysts. These signals did not appear for the fresh catalysts. By analogy with the related results reported on the methane/ammonia catalysts, the ion signals were used as qualitative surface probes indicating the presence of precious metal/support interactions, especially between Rh and Al.     

Facile synthesis of Pd@Pt octahedra supported on carbon for electrocatalytic applications

Due to the scarcity of Pt, it is highly desirable to construct core‐shell structures with ultrathin Pt shells while maintaining their high electrocatalytic activities. However, it is necessary to preferentially synthesize a core with a specific structure before further formation of core‐shell catalysts with specific morphologies. This prerequisite greatly increases the complexity of the synthesis process. This article describes a synthetic method of core‐shell Pd@Pt octahedra catalysts from Pd nanocubes, truncated nanocubes or truncated octahedra. The formation of octahedral core‐shell structures involves two key factors: (1) the oxidative etching process of Pd atoms at the corner sites; (2) the different reduction rates between Pt and Pd precursors. This mechanism can be extended to synthesize carbon‐supported sub‐8 nm Pd@Pt octahedra from commercial Pd/C catalysts. The derived carbon‐supported Pd@Pt octahedra catalysts performed comparable activity and durability for methanol oxidation reaction with state‐of‐art PtRu/C catalysts. This synthetic method provides an innovative path for large‐scale production of well‐controlled catalysts. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2528–2534, 2017     

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     

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

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

Development of platinum-free catalyst and catalyst with low platinum content for cathodic oxygen reduction in acidic electrolytes

Studies are presented of the kinetics and mechanism of oxygen electroreduction on CoPd catalysts synthesized on XC 72 carbon black. As shown both in model conditions and in tests with the cathodes of hydrogen–oxygen fuel cells with proton conducting electrolyte, the CoPd/C system features higher activity as compared to Co/C. It is found by means of structural analysis that CoPd alloy is formed in the course of the catalyst synthesis. This provides the higher catalytic activity of the binary systems. CoPd/C catalyst is also more stable in respect to corrosion than Pd on carbon black. Measurements on a rotating ring–disc electrode show that the CoPd/C system provides preferential oxygen reduction to water in the practically important range of potentials (E > 0.7 V). The similarity of the kinetic parameters of the oxygen reduction reaction on CoPd/C and Pt/C catalysts points to a similar reaction mechanism. The slow step of the reaction is the addition of the first electron to the adsorbed and previously protonated O₂ molecule. Studies of the most active catalyst in the fuel cell cathodes are performed. Binary PtCo catalysts (metal atomic ratio of 1 : 1) with low platinum content (7.3 wt.%) modified by phosphorus or sulfur are developed and studied. It is demonstrated that the specific activity of the PtCoS/C (Pt : S = 1 : 1) catalytic system for the O₂ reduction reaction exceeds that of a commercial Pt/C catalyst (E-TEK). The tolerance of the catalyst modified with sulfur is at least six times higher than that of Pt/C (E-TEK).