Activity Of Pt Catalysts Research Articles

Understanding enhancing mechanism of Pr6O11 and Pr(OH)3 in methanol electrooxidation

The presence of oxygen vacancies and hydroxyl groups are both favorable for the methanol electrooxidation on Pt-based catalysts. Understanding and differentiating the enhancing mechanism between oxygen vacancies and hydroxyl groups for high activity of Pt catalysts in methanol oxidation reaction (MOR) is essential but still challenging. Herein, we developed two kinds of co-catalyst for Pt/CNTs, Pr6O11 is rich in oxygen vacancies but contains substantially no hydroxyl groups, while Pr(OH)3 possesses abundant hydroxyl groups without oxygen vacancies. After a sequence of designed experiments, it can be found that both oxygen vacancies and hydroxyl groups can improve the performance of Pt/CNTs electrocatalysts, but the enhancing mechanism and improving degree of oxygen vacancies and hydroxyl groups for the MOR are different. Since the oxygen vacancies are more conducive to increasing the intrinsic activity of the Pt catalyst, and the hydroxyl groups play a decisive role in dehydrogenation and deprotonation of methanol, the co-catalysts with both oxygen vacancies and hydroxyl groups mixed with Pt/CNTs have higher catalytic performance. Therefore, hydroxyl-rich Pr6O11·xH2O was prepared and used as MOR electrocatalyst after mixed with Pt/CNTs. Benefiting from the synergistic effect of oxygen vacancies and hydroxyl groups, the Pr6O11·xH2O/Pt/CNTs shows a high peak current density of 741 mA/mg, which is three times higher than that of Pt/CNTs. These new discoveries serve as a promising strategy for the rational design of MOR catalysts with low cost and high activity.

Water–Gas Shift Activity of Pt Catalysts Prepared by Different Methods

Platinum supported on ceria and zirconia was prepared through different preparation methods: Coprecipitation (CP), spray drying (SD), and flame spray pyrolysis (FSP). The catalysts were characterized by XRD, TPR, N2 adsorption, and H2 chemisorption, and the water–gas shift activity in the range 190–310 °C and initial stability at 300–310 °C were tested. Although the spray-dried Pt/CeO2/ZrO2 catalyst shows the highest initial activity, it deactivates rapidly at 300 °C and levels out at similar activity as the coprecipitated Pt/CeO2 and Pt/CeO2/ZrO2 within a few hours. Flame spray pyrolysis appears to be a promising preparation method concerning the stability of catalysts, although the initial activity is rather poor. High activity is related to high Pt dispersion, low reduction temperature, and small support particles. The support particle size is also much affected by the preparation method.

(Invited) Enhancement of Oxidation of Dimethyl Ether at Nanostructured-Metal-Oxide-Supported Noble Metal Catalytic Nanoparticles: Comparison to Behavior of Other Simple Organic Fuels

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, in electrochemical energy conversion systems. In addition to ethanol (biofuel), that can be ideally oxidized to carbon dioxide thus delivering twelve electrons, recent important systems include dimethyl ether as well. But realistically the respective reaction is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials.Platinum has been recognized as the most active catalytic metal towards oxidation of ethanol at low and moderate temperatures. But Pt anodes are readily poisoned by the strongly adsorbed intermediates, namely by CO-type species, requiring fairly high overpotentials for their removal. To enhance activity of Pt catalysts towards methanol and ethanol oxidation, additional metals including ruthenium, tin, molybdenum, tungsten or rhodium are usually introduced as the alloying component. More recently it has been demonstrated that catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol has been significantly enhanced through interfacial modification with ultra-thin monolayer-type films of metal oxo species of tungsten, titanium or zirconium.We pursue a concept of utilization of zirconia nad mixed metal (e.g. zirconium/tungsten or titanium/tungsten) oxide matrices for supporting and activating noble metal nanoparticles (e.g. PtRu) during electrooxidation of dimethyl ether relative to methanol, formic acid and ethanol. Among important issues is incorporation of additional metal (e.g. Rh or Pt) nanostructures capable of weakening, or even breaking, the C-C and C-O bonds in the ethanol and dimethyl ether molecules. On the other hand, rhodium itself is not directly electrocatalytic toward oxidation of ethanol. The oxides and noble metal nanoparticles have been deposited in a controlled manner using the layer-by-layer method. Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. In addition, formation of “nanoreactors” where ethanol and dimethyl ether are partitioned (at Rh, Pt or Pd) to methanolic residues and further oxidized at PtRu cannot be excluded.

Atomic Carbon Layers Supported Pt Nanoparticles for Minimized CO Poisoning and Maximized Methanol Oxidation.

Maximizing activity of Pt catalysts toward methanol oxidation reaction (MOR) together with minimized poisoning of adsorbed CO during MOR still remains a big challenge. In the present work, uniform and well-distributed Pt nanoparticles (NPs) grown on an atomic carbon layer, that is in situ formed by means of dry-etching of silicon carbide nanoparticles (SiC NPs) with CCl4 gas, are explored as potential catalysts for MOR. Significantly, as-synthesized catalysts exhibit remarkably higher MOR catalytic activity (e.g., 647.63 mA mg-1 at a peak potential of 0.85 V vs RHE) and much improved anti-CO poisoning ability than the commercial Pt/C catalysts, Pt/carbon nanotubes, and Pt/graphene catalysts. Moreover, the amount of expensive Pt is a few times lower than that of the commercial and reported catalyst systems. As confirmed from density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) measurements, such high performance is due to reduced adsorption energy of CO on the Pt NPs and an increased amount of adsorbed energy OH species that remove adsorbed CO fast and efficiently. Therefore, these catalysts can be utilized for the development of large-scale and industry-orientated direct methanol fuel cells.

Electrochemical analysis of the porphyrazine-induced enhancement of ORR activity of Pt catalysts for the development of porphyrazine-adsorbed Pt catalysts

The enhancement of the oxygen reduction reaction (ORR) activity of Pt catalysts by porphyrazines (tetraazaporphyrin and phthalocyanine) is analyzed electrochemically. While tetra(t-butyl)tetraazaporphyrin significantly enhances the ORR activity, the enhancement by dilithium phthalocyanine is limited; this molecule increases the ORR at high potentials but decreases it at low potentials. The cyclic voltammograms (CVs) under an Ar atmosphere and a Tafel analysis reveal that the phthalocyanine promotes the ORR by destabilization of OHad, an intermediate of the ORR, at high potentials. However, this positive effect of the phthalocyanine is counteracted by its inhibitory effects at low potentials. Thus, the tetra(t-butyl)tetraazaporphyrin, which does not have a significant inhibitory effect, would be a better enhancer. The method for preparing tetra(t-butyl)tetraazaporphyrin-adsorbed Pt catalysts was investigated. This molecule is not only mainly adsorbed on a carbon black support, but also adsorbed on the surface of Pt nanoparticles. Tetra(t-butyl)tetraazaporphyrin-adsorbed carbon-supported Pt nanoparticles are prepared by an equilibrium adsorption method and an evaporation-to-dryness method. The optimized preparation method is discussed. Using the evaporation-to-dryness method, we also examined the effects of several porphyrins on the ORR of Pt catalysts.

Influences of N species in N-doped carbon carriers on the catalytic performance of supported Pt

Nitrogen-doped carbon carriers could drastically enhance the stability and activity of Pt catalysts in many chemical reactions. However, for the specific active N species chiefly responsible for the Pt performance amelioration, there still is no clear understanding up to now. Herein, density functional theory calculations and the experimental relevancy investigations between the different N species and the intrinsic activity of supported Pt towards a probe reaction of hydrogen evolution reaction were combined to identify the active N species from the pyridinic N, amine N, pyrrolic N, graphitic N and N-oxide in N-doped carbon nanotubes. Simulations demonstrate that five N species all can strengthen the interaction of Pt with carbon nanotubes on different degree, however, only graphitic N could drastically alter the charge and spin densities of Pt atoms and reduce the energy gap between the highest occupied molecular orbitals and the lowest unoccupied ones, revealing that the graphitic N bears the main responsibility for the catalytic performance enhancement of supported Pt especially in both Pt single atom- and cluster-catalyzed reactions with electron-deficiency reactants. Meanwhile, a positive correlation of the intrinsic activity of Pt with the graphitic N was clearly observed from the model reaction of hydrogen evolution reactions with H+ as the electrophilic reactant, further verifying the dominating active structure of graphitic N in N-doped carbon nanotubes. This finding provides a new avenue towards the targeted design of highly efficient Pt-based catalytic materials in Pt-catalyzed chemical reactions by tuning the graphitic nitrogen distribution in nitrogen-doped carbon carriers.

(Invited) Nanostructured-Metal-Oxide Functionalized-Supports for Noble Metal Catalytic Nanoparticles: Enhancement of Electrooxidation of Simple Organic Fuels

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, in electrochemical energy conversion systems. In addition to ethanol (biofuel), that can be ideally oxidized to carbon dioxide thus delivering twelve electrons, recent important systems include dimethyl ether as well. But realistically the respective reaction is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials. Platinum has been recognized as the most active catalytic metal towards oxidation of ethanol at low and moderate temperatures. But Pt anodes are readily poisoned by the strongly adsorbed intermediates, namely by CO-type species, requiring fairly high overpotentials for their removal. To enhance activity of Pt catalysts towards methanol and ethanol oxidation, additional metals including ruthenium, tin, molybdenum, tungsten or rhodium are usually introduced as the alloying component. More recently it has been demonstrated that catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol has been significantly enhanced through interfacial modification with ultra-thin monolayer-type films of metal oxo species of tungsten, titanium or zirconium. We pursue a concept of utilization of mixed metal (e.g. zirconium/tungsten or titanium/tungsten) oxide matrices for supporting and activating noble metal nanoparticles (e.g. PtRu) during electrooxidation of methanol and ethanol. Among important issues is incorporation of Rh nanostructures capable of weakening, or even breaking, the C-C bond in the ethanol molecules. On the other hand, rhodium itself is not directly electrocatalytic toward oxidation of ethanol. The oxides and noble metal nanoparticles have been deposited in a controlled manner using the layer-by-layer method. Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. In addition, formation of “nanoreactors” where ethanol is partitioned (at Rh) to methanolic residues further oxidized at PtRu cannot be excluded.

Rational design of porous structures via molecular layer deposition as an effective stabilizer for enhancing Pt ORR performance

Pt-based catalysts are widely applied in fuel cells as key components of both anode and cathode. However, commercial Pt/C catalysts with Pt nanoparticles dispersed on carbon support exhibit poor durability due to the Ostwald ripening effect, detachment and agglomeration of Pt nanoparticles during fuel cell operation. Herein, we demonstrate for the first time the application of molecular layer deposition (MLD) to stabilize Pt catalysts. By atomic layer deposition (ALD) of Pt particles on MLD-derived interlayer, the Pt catalysts show significantly enhanced oxygen reduction reaction (ORR) activity and durability compared to that without the MLD interlayer. The MLD-derived surfaces with enriched pores are helpful for anchoring the Pt nanoparticles and to avoid agglomeration or detachment from the surface sites. In addition, X-ray adsorption spectroscopy (XAS) results show that the deposition of Pt on the MLD-derived interlayer caused the electron transfer from Pt to substrates, which results in a decrease in the number of electrons in the d orbital of Pt. In addition, the extended X-ray absorption fine structure (EXAFS) result indicates that PtPt bond distance is shortened by the deposition on the MLD-derived NCNT, which leads to the enhanced activity of Pt catalysts. This work provided a novel route for stabilizing Pt catalysts through MLD technique.

Searching for efficient defect types in carbon nanofibers to promote supported Pt catalytic activity for dehydrogenation reaction

• Three CNFs with different defective degrees are synthesized and identified. • N-doping of CNFs is implemented using in-situ synthesis and post treatment methods. • Thermal treatment of N-doped CNFs is adopted for generating edge-like defects. • Pt catalysts supported over various CNFs display different catalytic activity in dehydrogenation. • Edges and edge-like defects in CNFs are proven to be the most efficient active sites in catalysis. Three kinds of carbon nanofibers (CNFs) with different microstructures: platelet CNFs (CNFs-P), fishbone CNFs (CNFs-F) and tubular CNFs (CNFs-T) were synthesized as Pt catalyst supports for decalin dehydrogenation to release hydrogen. Different types of defects including doped nitrogen and edge-like sites were purposefully introduced into three pristine CNFs by employing N-doping and thermal treatment methods. The defective properties of the CNFs were identified by combining the intensity ratio and full width at half maximum of D and G bands derived from Raman spectra of the CNFs. It was revealed together with XPS analysis of Pt 4f 7/2 binding energy that the electronic property of Pt nanoparticles is very sensitive to the surface defect density of CNF supports. Further catalytic performance evaluation showed that Pt supported on the CNFs-P which are featured of the largest quantity of exposed edge sites can provide the most proper electronic property of the catalyst due to suitable metal-support interaction, thereby affording the highest catalytic activity. Moreover, N-doping by both in-situ synthesis and post-treatment manner can favor the Pt catalysts on the tubular and fishbone CNFs to increase their activity, but do harm to the activity of the Pt/CNFs-P. Thermal treatment of N-doped CNFs can generate new edge-like defects by removing N-species, thus beneficial to the activity of Pt catalysts on either tubular or fishbone CNFs. Therefore, the edge sites and edge-like defects on the surface of CNFs are proven to be the best ones, which can promote the catalytic activity of the supported Pt nanoparticles for the dehydrogenation reaction efficiently.

Electrocatalytic Property of Pt Atomic Layers on Pd Nanocrystals for Ethanol Oxidation

Platinum is actually the irreplaceable catalyst for proton-exchange-membrane fuel cells (PEMFCs) and direct-alcohol fuel cells (DAFCs). Due to the high price and rare reserve, further improving the activity while decreasing the loading of Pt catalyst in membrane-electrolyte-assembly (MEA) of fuel cells is crucial. Single atomic Pt layer even single Pt atom catalysts are the most promising and challenging strategy. In this study, catalysts of Pt atomic layers from sub-monolayer to multilayers on Pd tetrahexahedral nanocrystals (THH Pd@Ptn NCs) with high-index faceted and {111}-faceted Pd NCs were prepared through Pt galvanic displacement of UPD Cu. The results demonstrate that the catalytic activity for ethanol electrooxidation is very low on Pt monolayer (THH Pd@Pt1 NCs), while the highest activity is measured on Pt 2-layers (THH Pd@Pt2 NCs) amongst the THH Pd@Ptn NCs (n<1, =1-5) studied. The catalytic activity of THH Pd@Pt2 NCs is even higher than that of pure THH Pt NCs. Through in-situ CO-probe FTIR spectroscopic study, it has revealed that both the interaction of Pt layer with Pd nanoparticle (the strain and ligand effects) and the structure of surface atom arrangement play important roles in manipulating the activity of Pt catalyst. The study has thrown insight into design and preparation of high-active and ultra-low loading Pt catalysts for fuel cell applications. Acknowledgement: This work was supported by Natural Science Foundation of China (21573183, 21621091).

Coupled s-p-d Exchange in Facet-Controlled Pd3Pb Tripods Enhances Oxygen Reduction Catalysis

Summary Efficient oxygen reduction reaction (ORR) catalysts are key for the development of high-performance fuel cells. Palladium (Pd) is a promising catalyst system for ORR given its potential to replace platinum (Pt); however, it usually exhibits lower activity than Pt. Here, we report a class of ordered Pd 3 Pb tripods (TPs) with predominantly {110} facets and show that they achieve extremely high ORR performance in alkaline medium. In contrast to the knowledge that the excellent ORR activity of Pt catalyst is caused by its partially filled d orbital, our first-principle calculations suggest that the strong charge exchange between Pd-4 d and Pb-( sp ) orbitals on the Pd 3 Pb TPs {110} facet results in a Pd-Pb local bonding unit with an orbital configuration similar to that of Pt. Consequently, Pd 3 Pb TPs exhibit much higher ORR activities than commercial Pt/C and commercial Pd/C. Pd 3 Pb TPs are endurable and sustain over 20,000 potential cycles with negligible structural and compositional changes.

A First-Principles Study of O2 Dissociation on Platinum Modified Titanium Carbide: A Possible Efficient Catalyst for the Oxygen Reduction Reaction

The adsorption and dissociation of O2 on the Pt modified TiC(001) surfaces with different Pt coverages of 1/4, 1/2, 3/4, and 1 ML are comparatively investigated using ab initio density functional theory calculations. The geometric and electronic structures are analyzed in detail. The strong interaction of Pt atoms with the TiC(001) is beneficial to improving the stability and activity of Pt catalyst. Compared with Pt(111), the MLPt/TiC(001) (3 × 3) has a positive impact on promoting the scission of the O–O bond (leading to a dissociation barrier comparable to that on Pt(111)) and weakening the adsorption of atomic O (the dissociation product of O2), which shed meaningful light on the important role of TiC(001) as support to improve the efficiency of Pt for oxygen reduction reaction.

Boosting Pt oxygen reduction reaction activity by tuning the tin oxide support

The influence of the dopant element on the physicochemical properties of the SnO2 support and on the oxygen reduction reaction (ORR) activity of the supported Pt nanoparticles was systematically studied. The dopant element and concentration significantly influence the SnO2 crystal structure and the electrical conductivity, as well as the ORR activity in 0.1M HClO4 of the supported Pt catalysts. ORR activities significantly exceeding that of Pt deposited on carbon were observed for Pt nanoparticles supported on Nb- and W-doped SnO2. No linear correlation between the support conductivity and the ORR activity was observed, suggesting that other factors, such as (electronic) metal–support interactions, could play a role in the ORR activity of Pt catalysts supported on metal oxides.

CO-Terminated Pt/Au Codeposition on Titania Nanotube Arrays (TNAs)

Forming platinum monolayers on less-expensive materials is an ingenious way to enhance the mass activities for oxygen reduction reaction (ORR) and decrease the amount of this scarce and precious metal in fuel cell technology. Previously, we observed low activities of Pt catalysts deposited directly on titania nanotube arrays (TNAs). Here, we propose a gold interlayer between Pt and TiO2. The synthesis is attempted by the codeposition of Pt and Au from CO-saturated solution. The activity of the Pt/Au codeposited catalyst is higher than in the case when Pt is deposited alone. This result suggests that Au is a promising way to enhance ORR activity. The activity of this catalyst, however, is still much lower than that of the Pt monolayer on bulk gold. This is explained by the negative effects of CO, disrupting the chance of monolayer formation by preadsorption to the TNA substrate.

Operando and multimodal studies of speciation and activity of Pt catalysts during the hydrogenation of ethylene

Operando and multimodal studies of speciation and activity of Pt catalysts during the hydrogenation of ethylene

Modification of Catalytic Performance of Supported Pt Catalysts by Coverage with Silica Layers

Supported Pt catalysts were covered with hydrophilic or hydrophobic silica layers. Both silica-coated Pt catalysts showed high tolerance to high-temperature sintering of Pt metal particles. The coverage with hydrophobic silica enhanced the activity of Pt catalysts for alkene hydrogenation in hydrophilic solvents.

Promoting Influence of Activated Carbon used in Carbon Paste Electrode on Platinum Nanoparticles Efficiency in Methanol Electrooxidation

Direct methanol fuel cell (DMFC) have the advantages of low emissions of carbon dioxide, high power density and high energy density, so they have a great potential to replace fossil fuels. It is well known that Pt with high activity is an ideal catalyst for methanol oxidation in an acidic medium. However, it is an expensive material, and cost of the DMFC containing Pt is prohibitively high and it becomes poisoned by COads which is formed during the methanol oxidation. In this work, for the first time, the preparation of activated carbon(YBC) doped carbon paste electrode (YBCPE) and carbon paste electrode (CPE) were carried out for comparison. The electrochemical properties of YBCPE and CPE were investigated by electrochemical impedance spectroscopy. The electrodes of Pt/YBCPE and Pt/CPE were prepared by electrodeposition of platinum on YBCPE and CPE, respectively, at a constant potential (−0.1V vs. Ag|AgCl(sat)) in 0.5M H2SO4 containing 3mM H2PtCl6. The behavior of electrodeposition of platinum on YBCPE and CPE was studied through the application of theoretical models, and the morphology of Pt/YBCPE and Pt/CPE were characterized by SEM. The electrochemical activities of these catalysts towards methanol electrooxidation were examined by cyclic voltammetry. The influence of YBC mass percentage in YBCPE on both the electrode activity and long-term stability of electrodes in methanol electrooxidation was studied. It was shown that the addition of activated carbon improved the electrocatalytic activity of Pt catalysts on corresponding YBCPE toward methanol electrooxidation.

Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts.

High‐Performance Low‐Platinum Electrode for Proton Exchange Membrane Fuel Cells: Pulse Electrodeposition of Pt on Pd/C Nanofiber Mat

AbstractA novel electrode (E−P electrode) with a nanofiber structure and Pd/C@dendritic Pt catalysts is prepared by using electrospinning and pulse electrodeposition (PED) techniques. The maximum power density of the E−P electrode is 1.43‐fold larger than that of the conventional electrode at the same cathode Pt loadings of 0.1 mg cm−1. Owing to the in situ deposition of dendritic Pt on the surface of Pd in the Pd/C nanofiber mat, almost all Pt catalysts are accessible for oxygen. The electronic tuning between Pd and Pt enhances the oxygen reduction reaction activity of Pt catalysts. The large Pt surface area of the E−P electrode mitigates the oxygen‐transport resistance in comparison with that of the conventional electrode. After the accelerated degradation test for 10000 cyclic voltammetry cycles, the maximum power density of the E−P electrode only decreases by 12 %. The long‐term stability of the E−P electrode is ascribed to the Pd/C@dendritic Pt catalysts and nanofiber structure.

The influence of deposited potential on the ORR activity of Pt catalysts on glassy carbon electrode

Herein, we have demonstrated how different deposited potential scan affect the ORR activity of Pt catalysts on glassy carbon electrode (Pt/GC).