PtPd Alloy Research Articles

The effect of Pt/Pd ratio on the oxidation activity and resistance to sulfur poisoning for Pt-Pd/BEA diesel oxidation catalysts with high siliceous content

This study investigates the effect of the Pt/Pd ratio on the oxidation activity and sulfur poisoning/regeneration of diesel oxidation catalysts (DOC) using beta zeolites with high siliceous content as support. Formation of Pt-Pd alloy leads to contraction of the cell lattice of Pt in the bimetallic catalysts, improving not only the sintering resistance of Pt but also retaining a high fraction of Pd in Pd2+ form. Moreover, the Pt-Pd alloy also improves the oxidation resistance of the particles, which enhances the activity of the catalysts for CO and C3H6 oxidation. Bimetallic catalysts also favor NO reduction at a lower temperature than the monometallic Pt although they showed lower values for the absolute conversion of NO due to a decrease in the total number of the Pt active sites. In addition, the bimetallic catalysts significantly improved the sulfur resistance as compared to the monometallic Pd catalyst. Moreover, the bimetallic catalysts could easily recover their activity for NO and C3H6 oxidation by thermal treatment either in lean conditions or in H2. The reduction with H2 was necessary to recover completely the activity of the CO and C3H8 oxidation.

Surface Structure Engineering of PtPd Nanoparticles for Boosting Ammonia Oxidation Electrocatalysis.

Achieving high catalytic ammonia oxidation reaction (AOR) performance of Pt-based catalysts is of paramount significance for the development of direct ammonia fuel cells (DAFCs). However, the high energy barrier of dehydrogenation of *NH2 to *NH and easy deactivation by *N on the Pt surface make the AOR show sluggish kinetics. Here, we have put forward an alloying and surface modulation tactic to optimize Pt catalysts. Several spherical PtM (M = Co, Ni, Cu, and Pd) binary nanoparticles were controllably loaded on reduced graphene oxide (rGO). Among others, spherical PtPd nanoparticles displayed the most efficient catalytic activity. Further surface engineering of PtPd nanoparticles with a cubic-dominant structure has resulted in dramatic AOR activity improvements. The optimized (100)Pt85Pd15/rGO exhibited a low onset potential (0.467 V vs reversible hydrogen electrode (RHE)) and high peak mass activity (164.9 A g-1), much better than commercial Pt/C. Nevertheless, a short-term stability test along with morphology, structure, and composition characterizations indicate that the leaching of Pd atoms from PtPd alloy nanoparticles, their structure transformations, and the possible poisoning effects by the N-containing intermediates could result in the catalyst's activity loss during the AOR electrocatalysis. A temperature-dependent electrochemical test confirmed a reduced activation energy (∼12 kJ mol-1 decrease) of cubic-dominant PtPd compared to Pt/C. Density functional theory calculations further demonstrated that Pd atoms in Pt decrease the reaction energy barrier of electrochemical dehydrogenation of *NH2 to *NH, resulting in an excellent catalytic activity for the AOR.

O3 titration for synthesis of Pt-Pd core-shell structural catalyst and its catalytic performance on C-H bond activation

A novelty catalyst synthesis method is introduced in this investigation, which employs O3 pulse to induce a segregation of Pd-Pt species to synthesize Pt-Pd core shell structural catalyst. In Pt-Pd alloy, O3 releases highly active oxygen (O*) species (O3→O2 + O·), which combine with Pd species or Pt species selectively. This combination induces a relative migration between Pd and Pt atoms, forming Pt-core@PdO-shell structural catalyst, which meets the thermodynamic stability of Pt-Pd alloy. Core shell structural catalyst displays a better catalytic performance for C-H bond activation. Experimental results indicate core-shell structural catalyst produces more highly active oxygen species than alloy catalyst does. These highly active oxygen species activate C-H bond in alkanes, contributing to a higher catalytic reactivity. In addition, the interaction between Pt-core and PdO-shell lowers the binding energy of ionic Pd species, which facilitates the release of reactive oxygen species.

Electrocatalytic performance impact of various bimetallic Pt-Pd alloy atomic ratio in robust ternary nanocomposite electrocatalyst toward boosting of methanol electrooxidation reaction

The low-loading of precious platinum (Pt) metal electrocatalysts development by alloying with less expensive of earth-abundant transition metals exhibiting high electrocatalytic activity and durability performance towards methanol oxidation reaction (MOR) for new generation sustainability of direct methanol fuel cell (DMFC) application has aroused increasing consideration. In this typical study, the as-prepared ternary nanocomposite electrocatalysts with controllable composition of the bimetallic Pt-Pd alloy nanoparticles (NPs) in acidic media were synthesized through a facile one-step hydrothermal-assisted formic acid reduction reaction. The main study on the critical impact of the bimetallic Ptx-Pdy alloy atomic ratio (x-y = 1:1, 2:3, 3:7, 1:4, 1:9) in the as-prepared ternary RGO/bimetallic Ptx-Pdy alloy/0.90CeO2 nanocomposite electrocatalyst upon its suitability as anode electrocatalyst towards the electrocatalytic performance of MOR was thoroughly evaluated at constant operating conditions. The results clearly demonstrated that the compositions of the bimetallic Pt-Pd alloy NPs can be easily adjusted by varying the Ptx-Pdy alloy atomic ratio that can contribute to the significant impact on the electrocatalytic activity of MOR in DMFC. Upon increase in the composition of Pd from the bimetallic Pt-Pd alloy atomic ratio of 1:1 to 2:3 led to increased electrocatalytic activity, long-term stability, durability cycles and charge transfer resistance with respect to the MOR. However, it was continuously decrease with further increase of the Pd proportion in the Pt-Pd alloy NPs atomic ratio of 3:7 to 1:9. The maximum peak current density of the MOR (39.83 mA cm-2) was obtained in the present research work for the as-synthesized ternary nanocomposite electrocatalyst with the bimetallic Pt-Pd alloy atomic ratio of 2:3. The as-synthesized ternary nanocomposite electrocatalyst with low noble Pt content through the alloying strategy could promotes practically employed as anode electrocatalyst under acidic media in DMFC application.

Pd(1 1 1)/SnO2(1 0 1) heterostructure on porous N-carbon material as enhanced catalyst for formic acid electrooxidation

High specific surface area carbon materials act as a critical factor of fuel cell catalyst carriers. In this study, the PtxPdy/SnO2/GC nanocatalysts were successfully synthesized at different molar ratios of Pt and Pd to conduct the formic acid oxidation. In addition, the N-doped porous carbon carrier (GC) was prepared with graphene oxide and gelatin through the solvated volatilization/nanometer SiO2 template/KOH activation, and the prepared carrier GC took up a large specific surface area (SBET = 1104.5 m2/g). The fine-tuning of the atomic ratio of Pt and Pd was demonstrated to further guide the growth of ultra-small nanocrystals that achieved an average size of 2.52 nm and displayed uniform distributions on the prepared GC carrier. Compared with PtPd/GC, PtSnO2/GC and Pt/GC catalysts, Pt1Pd1/SnO2/GC took up a larger electrochemical activity area of 528.93 cm2/mg, and the positive sweep peak current density reached 2258.45 mA/mgPt at 0.75 V. As confirmed by the DFT analysis, the heterojunction structure formed between the Pt-Pd (111) and SnO2 (101) surfaces would contribute to the electron transport. The optimal electrochemical performance was reasonably attributed to the synergistic effect exerted by PtPd alloy and SnO2, as well as N-doped porous carbon carrier.

Pt-Based Single Atom Electrocatalysts By Atomic Layer Deposition for Fuel Cell Related Catalytic Reactions

Platinum is a key component of the catalysts in a proton-exchange membrane fuel cell (PEMFC), which exhibited good oxygen reduction reaction (ORR) activity. Notwithstanding the superior properties of Pt catalysts, the low abundance of Pt, and subsequent high cost have created a major barrier for the large-scale commercialization of Pt catalysts in PEMFCs. One effective route for reducing the cost of Pt is to increase the atom utilization efficiency by reducing the particle size to clusters or even single atoms catalysts.[1] Herein, we will demonstrate our recent approaches about the preparation of Pt-based single atom catalysts by atomic layer deposition method (ALD). Nearly 100% Pt single atom catalysts were successfully deposited on N-doped carbon nanotubes. Pt-Pd single atom alloys were prepared through ALD method and applied in electrochemical oxygen reduction for the first time. We also successfully prepared successfully prepared Pt-Ru dimers on NCNTs through ALD process. All these different types of single atom catalysts exhibited superior activity in ORR and HER reactions. In conclusion, we demonstrate a series types of Pt single atoms, including high-quality Pt single atom, Pt-Pd single atom alloy and Pt-Ru dimer structures. The results show that the formation of single atom structure can effectively improve the electrochemical performance. It is also worth to mention that we have first introduced ALD technique in the single atoms. It is believed that our study opens up a new avenue of developing new types of Pt-based catalysts for fuel cells and brings new understanding about catalytic performances of single atoms.[1] L. Zhang, X. Sun et al, Pt-Based Electrocatalysts with High Atom Utilization Efficiency: From Nanostructures to Single Atoms Energy Environ. Sci. 2019, 12, 492-517.[2] L. Zhang, X. Sun et al, Atomic Layer Deposited Pt-Ru Dual-Metal Dimers and Identifying the Active Sites of Dimer for High Hydrogen Evolution Reaction. Nature Commun. 2019, 10, 4936.[3] L. Zhang, X. Sun et al, Pt/Pd Single Atom Alloys as Highly Active Electrochemical Catalysts and the Origin of Excellent Activity. ACS Catalysis, 2019, 9, 9350.[4] L. Zhang, X. Sun et al, New Insight of Pyrrole-Like Nitrogen for Boosting Hydrogen Evolution Activity and Stability of Pt Single Atoms. Small 2020, in press.

4-Propylphenol Hydrogenation over Pt-Pd Bimetallic Catalyst in Aqueous Ethanol Solution without External Hydrogen

Ring-hydrogenation of 4-propylphenol (4-PP) to 4-propylcyclohexanone, cis- and trans-4-propylcyclohexanols proceeded over graphite-supported palladium catalysts (Pd/G) in aqueous ethanol solution at 573 K without using external hydrogen gas. Compared to Pd/G, graphite-supported platinum (Pt/G) catalysts were active only for hydrogen production and not for the ring-hydrogenation under the same reaction conditions. We have found that the addition of platinum to palladium enhanced the cis- and trans-4-propylcyclohexanols yields. The optimum molar ratio of platinum to palladium in the Pt-Pd/G catalysts was found to be 1:2. The formation of Pt-Pd alloy sites was studied by TEM and EXAFS analyses.

PtPd‐decorated Multibranched Au@Ag Nanocrystal Heterostructures for Enhanced Hydrogen Evolution Reaction Performance in Visible to Near Infrared Light

AbstractPtPd‐decorated multibranched Au@Ag nanocrystals (NCs) were synthesized to form heterostructures with enhanced electrochemical performance for water splitting in the visible/near infrared (vis‐NIR) light region. Three kinds of heterostructures including PtPd‐covered, PtPd‐edged, and PtPd‐tipped Multi‐branched Au@Ag NCs were fabricated by controlling the growth process of PtPd alloy nanoparticles on Au@Ag NCs. PtPd‐covered Multi‐branched Au@Ag nanocrystal exhibited the best photo‐assisted electrocatalytic activity. The initial overpotential under vis‐NIR light is 33 mV with a Tatel slope of 101 mV/dec while the overpotential is 150 mV at 10 mA cm−2. The enhanced catalyst activity is ascribed to samples with heterogeneous structures and changed morphology and composition. In addition, localized surface Plasmon resonance (LSPR) effect from the surface of samples resulted in the effective separation of electron‐hole pairs (photoelectric effect). Finally the photothermal effect generated from the absorbance of Au nanostructures in near‐infrared light increased the temperature of the catalyst. The synergy of structure, photoelectricity, and photothermal effect endows the catalyst with excellent photo‐assisted electrocatalytic activity.

Rational Design of Pt-Pd-Ni Trimetallic Nanocatalysts for Room-Temperature Benzaldehyde and Styrene Hydrogenation.

Nanostructures of the multimetallic catalysts offer great scope for fine tuning of heterogeneous catalysis, but clear understanding of the surface chemistry and structures is important to enhance their selectivity and efficiency. Focussing on a typical Pt-Pd-Ni trimetallic system, we comparatively examined the Ni/C, Pt/Ni/C, Pd/Ni/C and Pt-Pd/Ni/C catalysts synthesized by impregnation and galvanic replacement reaction. To clarify surface chemical/structural effect, the Pt-Pd/Ni/C catalyst was thermally treated at X=200, 400 or 600 °C in a H2 reducing atmosphere, respectively termed as Pt-Pd/Ni/C-X. The as-prepared catalysts were characterized complementarily by XRD, XPS, TEM, HRTEM, HS-LEIS and STEM-EDS elemental mapping and line-scanning. All the catalysts were comparatively evaluated for benzaldehyde and styrene hydrogenation. It is shown that the "PtPd alloy nanoclusters on Ni nanoparticles" (PtPd/Ni) and the synergistic effect of the trimetallic Pt-Pd-Ni, lead to much improved catalytic performance, compared with the mono- or bi- metallic counterparts. However, with the increase of the treatment temperature of the Pt-Pd/Ni/C, the catalytic performance was gradually degraded, which was likely due to that the favourable nanostructure of fine "PtPd/Ni" was gradually transformed to relatively large "PtPdNi alloy on Ni" (PtPdNi/Ni) particles, thus decreasing the number of noble metal (Pt and Pd) active sites on the surface of the catalyst. The optimum trimetallic structure is thus the as synthesised Pt-Pd/Ni/C. This work provides a novel strategy for the design and development of highly efficient and low-cost multimetallic catalysts, e. g. for hydrogenation reactions.

Mechanism investigation of PtPd decorated Zn0.5Cd0.5S nanorods with efficient photocatalytic hydrogen production combining with kinetics and thermodynamics

Different components of PtPd bimetallic cocatalysts modified Zn0.5Cd0.5S nanorods have already been designed and prepared in this study. The obtained hybrid photocatalysts were tested and characterized by XPS, ICP-OES and UV-Vis spectra, TEM and EDX tools. Such characterizations can prove the formation of PtPd bimetallic alloy particles in hybrid catalysts. Under visible light illumination, an outstanding hydrogen producing rate of 9.689 mmol·g−1·h−1 and a high AQY efficiency up to 10.43% at 420 nm are achieved in this work. In addition, thermodynamics (DFT calculations) and kinetics (Photoluminescence emission, photocurrent responses, electrochemical impedance spectroscopy and surface photovoltage spectra) investigations illustrate that PtPd bimetallic alloy has similar catalytic thermodynamic properties to Pt, which can greatly boost the charge separation and speed up the charge transfer, and decrease the activation energy of H2 generation. Notably, the calculation data suggests that Pt is thermodynamically favorable, while PtPd alloy is kinetically beneficial to H2 production, which can be ascribed to the higher activity of PtPd/Zn0.5Cd0.5S than Pt/Zn0.5Cd0.5S. This work can propose a fresh perspective for preparing high efficiency hybrid photocatalysts.

Octahedral Growth of PtPd Nanocrystals

PtPd nanoparticles are among the most widely studied nanoscale systems, mainly because of their applications as catalysts in chemical reactions. In this work, a combined experimental-theoretical study is presented about the dependence of growth shape of PtPd alloy nanocrystals on their composition. The particles are grown in the gas phase and characterized by STEM-HRTEM. PtPd nanoalloys present a bimodal size distribution. The size of the larger population can be tuned between 3.8 ± 0.4 and 14.1 ± 2.0 nm by controlling the deposition parameters. A strong dependence of the particle shape on the composition is found: Pd-rich nanocrystals present more rounded shapes whereas Pt-rich ones exhibit sharp tips. Molecular dynamics simulations and excess energy calculations show that the growth structures are out of equilibrium. The growth simulations are able to follow the growth shape evolution and growth pathways at the atomic level, reproducing the structures in good agreement with the experimental results. Finally the optical absorption properties are calculated for PtPd nanoalloys of the same shapes and sizes grown in our experiments.

Efficient Schottky Junction Construction in Metal-Organic Frameworks for Boosting H2 Production Activity.

Manipulation of the co‐catalyst plays a vital role in charge separation and reactant activation to enhance the activity of metal‐organic framework‐based photocatalysts. However, clarifying and controlling co‐catalyst related charge transfer process and parameters are still challenging. Herein, three parameters are proposed, V transfer (the electron transfer rate from MOF to co‐catalyst), D transfer (the electron transfer distance from MOF to co‐catalyst), and V consume (the electron consume rate from co‐catalyst to the reactant), related to Pt on UiO‐66‐NH2 in a photocatalytic process. These parameters can be controlled by rational manipulation of the co‐catalyst via three steps: i) Compositional design by partial substitution of Pt with Pd to form PtPd alloy, ii) location control by encapsulating the PtPd alloy into UiO‐66‐NH2 crystals, and iii) facet selection by exposing the encapsulated PtPd alloy (100) facets. As revealed by ultrafast transient absorption spectroscopy and first‐principles simulations, the new Schottky junction (PtPd (100)@UiO‐66‐NH2) with higher V transfer and V consume exhibits enhanced electron‐hole separation and H2O activation than the traditional Pt/UiO‐66‐NH2 junction, thereby leading to a significant enhancement in the photoactivity.

Effects of Hydrothermal Aging on CO and NO Oxidation Activity over Monometallic and Bimetallic Pt-Pd Catalysts

By combining scanning transmission electron microscopy, CO chemisorption, and energy dispersive X-ray spectroscopy with CO and NO oxidation light-off measurements we investigated deactivation phenomena of Pt/Al2O3, Pd/Al2O3, and Pt-Pd/Al2O3 model diesel oxidation catalysts during stepwise hydrothermal aging. Aging induces significant particle sintering that results in a decline of the catalytic activity for all catalyst formulations. While the initial aging step caused the most pronounced deactivation and sintering due to Ostwald ripening, the deactivation rates decline during further aging and the catalyst stabilizes at a low level of activity. Most importantly, we observed pronounced morphological changes for the bimetallic catalyst sample: hydrothermal aging at 750 °C causes a stepwise transformation of the Pt-Pd alloy via core-shell structures into inhomogeneous agglomerates of palladium and platinum. Our study shines a light on the aging behavior of noble metal catalysts under industrially relevant conditions and particularly underscores the highly complex transformation of bimetallic Pt-Pd diesel oxidation catalysts during hydrothermal treatment.

The role of Pd–Pt Interactions in the Oxidation and Sulfur Resistance of Bimetallic Pd–Pt/γ-Al2O3Diesel Oxidation Catalysts

Diesel oxidation catalysts (DOC) were investigated for oxidation activity, NO conversion stability, and sulfur poisoning/regeneration on Pd/Al2O3, Pt/Al2O3, and Pd-Pt/Al2O3 catalysts. The Pd/Al2O3 catalyst was more active for CO and hydrocarbon (C3H6 and C3H8) oxidation, while the Pt/Al2O3 catalyst efficiently oxidized NO. The formation of a Pd-Pt alloy in the Pd-Pt/Al2O3 catalyst maintained Pd in a more reduced phase, resulting in the superior activity of this catalyst for the oxidation of CO, C3H6, and NO in comparison with its monometallic counterparts. The Pd-Pt alloy not only provided more low-temperature activity but also retained the stability of NO oxidation. The Pd-Pt alloy also favored the spillover of SO2 to the alumina support, resulting in significantly higher adsorption capacity of the Pd-Pt/Al2O3 catalyst, extensively prolonging its lifetime. However, the stable sulfates on Pd-Pt/Al2O3 made it difficult to completely regenerate the catalyst. The bimetallic sample showed higher activity for CO, C3H8, and C3H6 after sulfur poisoning and regeneration.

Advanced ternary RGO/bimetallic Pt-Pd alloy/CeO2 nanocomposite electrocatalyst by one-step hydrothermal-assisted formic acid reduction reaction for methanol electrooxidation

A facile, rapid, green, and novel clean synthesis of advanced RGO/bimetallic Pt-Pd alloy/CeO2 nanocomposite electrocatalyst with the enhanced electrocatalytic performance of methanol oxidation reaction (MOR) has been successfully carried out through a one-step hydrothermal-assisted formic acid reduction reaction without applying any template or surfactant. The as-prepared electrocatalysts were extensively characterized by X-ray Powder Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray Spectroscopy (EDX), High-Resolution Transmission Electron Microscopy (HRTEM), and Field-Emission Scanning Electron Microscopy (FESEM) to confirm the formation, deposition and homogenous distribution of Pt-Pd alloy and CeO2 nanoparticles (NPs) on the surface of RGO. Meanwhile, the electrocatalytic activity and the long-term stability performance of the as-prepared electrocatalysts towards MOR were employed by cyclic voltammogram and chronoamperometry, respectively. Noticeably, the as-prepared RGO/bimetallic Pt-Pd alloy/CeO2 nanocomposite electrocatalyst presented very outstanding electrocatalytic performance with higher maximum forward peak current density (69.82 mA cm−2) than those of RGO/monometallic Pt/CeO2 (23.81 mA cm−2) and RGO/monometallic Pd/CeO2 (1.39 mA cm−2) toward MOR in acidic medium denoting to the large surface area and excellent conductivity of RGO, homogenous distribution of Pt-Pd alloy electrocatalyst as well as a synergistic effect between Pt-Pd alloy NPs, RGO, and CeO2 NPs. Moreover, the RGO/bimetallic Pt-Pd alloy/CeO2 electrocatalyst also possesses excellent stability and exceptional poisoning tolerance through the advantages of utilizing Pt-Pd alloy NPs and the synergistic effect of CeO2 NPs. Therefore, this study may open a new facile route with the convenient experimental procedure, clean, reasonable cost, easy to handle, no time consuming, and easy to scale-up for the large quantity production of an advanced anode electrocatalyst for direct methanol fuel cell application.

Highly Selective and Efficient Electrocatalytic Semihydrogenation of Diphenylacetylene to Z-Stilbene Using Pt-Pd Alloy Cathode Catalyst

The electrical energy can be provided from renewable energy source such as solar and wind, thus making the electrocatalytic hydrogenation process is expected one of important technology to reduce carbon dioxide emission in organic synthesis industry. The selective hydrogenation of alkynes to Z-alkenes is highly important for the construction of numerous valuable compounds, such as bioactive molecules, natural products, flavors, and industrial materials. Although a number of methods for the stereoselective synthesis of Z-alkenes have been developed so far, the Lindlar catalytic semihydrogeneation of alkynes is a long-standing synthetic transformation and still remains the favored system. HoweveTo develop more environmentally benign and efficient semihydrogenation systems, we have demonstrated electrocatalytic hydrogenation of diphenylacetylene (DPA) in a proton exchange membrane (PEM) reactor. 1 In our system, the oxidation of water was employed as an anodic process (Figure 1 (a)).Especially, in this work, we have investigated systematically influences of catalyst materials such as Pt, Pd, and PtPd alloy for a PEM reactor on the product selectivity for Z-stilbene and partial current density for Z-stilbene formation in the hydrogenation of diphenylacetylene. As a result, PtPd alloy with a composition of Pt1Pd99 was found to be a most suitable catalyst material in order to achieve both high partial current density for Z-stilbene formation and high Z-stilbene selectivity (Figures 1 (b) and (c)). As shown in Figure 1 (b), it was also found that the partial current density for Z-stilbene is much improved by the addition of a small amount of Pt which has the high hydrogenation activity. Moreover, the synergy of Pt electrocatalysis for the formation of adsorbed hydrogen species (Had) (H+ + e− → Had) and Pd catalysis for hydrogenation of DPA (2Had + DPA → Z-stilbene) might be required in order to achieve the high stereoselective formation of Z-stilbene. Acknowledgement This work was support by JST CREST Grant No. JPMJCR18R1, Japan. The authors gratefully acknowledge for ISHIFUKU Metal Industry corporation the generous gift for cathode catalysts. References 1) A. Fukazawa, J. Minoshima, K. Tanaka, Y. Hashimoto, Y. Kobori, Y. Sato, M. Atobe, ACS Sustain. Chem. Eng. 2019, 7, 11050–11055. Figure 1

Selective Dissolution of Palladium and Consequent Enrichment of Platinum in Pt-Pd Alloys

Introduction Polymer electrolyte fuel cells (PEFCs) are expected to be widespread as a clean energy source, and platinum (Pt) alloys are used as a cathode catalyst for low-temperature operation. Pt-palladium (Pd) alloy catalysts show higher oxygen reduction reaction activity than Pt catalysts.1 Furthermore, Pd exhibits a higher corrosion resistance than less noble metals such as cobalt, nickel, and cupper under PEFCs operating conditions. Thus, the durability of Pt-Pd alloy catalysts is expected to be greater than that of the other alloys. In this study, dissolution behavior of Pt-Pd alloys under potential cycling simulating PEFCs operating conditions was investigated. Experimental Single-phase and polycrystalline Pt-50at.% Pd (Pt-50Pd) and Pt-75Pd were prepared by arc-melting. A total of 1,000 cycles of potential cycling test (CV) was conducted with the potential range of 0.05-1.4 V vs. SHE, simulating start-up/shut-down cycles. The test solution (0.5 M H2SO4) was sampled and replaced every 100 cycles, and the amount of dissolved Pd and Pt from the alloys was quantified using an inductively coupled plasma-mass spectrometer (ICP-MS). After the CV, the outermost surface of the alloys was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Results and Discussion The CV of Pt-50Pd consists of hydrogen (H) adsorption/desorption peaks, oxide formation/reduction peaks. These features are similar to the CV features of pure Pt, indicating that the Pt-enriched layer formed on the alloy surface after the 1,000th cycles of CV. On the other hand, the peak positions of the oxide formation/reduction of Pt-75Pd are in good agreement with those of pure Pt. However, H absorption peak was observed in the CV of Pt-75Pd. Thus, Pd atoms were partially exposed to the surface after the CV.The amount of dissolved Pt from Pt-50Pd and Pt-75Pd was almost similar that from pure Pt. In contrast, the amount of dissolved Pd from Pt-50Pd and Pt-75Pd was about 80-90% smaller than that from pure Pd. When the amount of dissolved Pt was compared with that of Pd of Pt-50Pd and Pt-75Pd during the last 100 cycles during CV, the ratio of dissolved Pt to Pd was 1:1.7 and 1:4.6, respectively. Hence, Pd selectively dissolved from Pt-Pd alloy, which confirmed that the Pt-enriched layer formation on the surfaces. The outermost surface analysis by TOF-SIMS also supported the Pt-enrichment. The dissolution rate of Cu and Pd from Pt-75Cu and Pt-75Pd during 100 cycles of CV were 3300 and 3.6 μg cm- 2, respectively, indicating that the selective dissolution rate of Pd is extremely small when the Pd concentration in the alloy is above parting limit.2 Based on these experimental results, the structures of Pt-enriched layer formed on Pt-50Pd and Pt-75Pd as a result of the selective dissolution of Pd might be different. References J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R Adzic, Angew. Chem. Int. Ed., 44(14) 2132 (2005)M. Artymowicz, J. Erlebacher, R.C. Newman, Philos. Mag., 89 1663 (2009).

(Invited) Time-Resolved Dissolution Analysis of Precious Metals Using a Solution Flow Cell Combined with ICP-MS

Introduction Platinum (Pt), palladium (Pd), and Pt-Pd alloys exhibit excellent catalytic activity for oxygen reduction reaction;1 thus, they are expected to be used as cathode catalyst in polymer electrolyte fuel cell (PEFC). However, one of the problems is their dissolution under PEFC operating condition. A measurement system, an inductively coupled plasma-mass spectrometer (ICP-MS) combined with solution flow cell, is proposed to detect dissolved ions, and time and potential-resolved measurement for metals could be possible under potential cycling using this system.2 Even though the trivial dissolution of Pt and Pd lead to a performance loss of PEFC, improvement of detection sensitivity is required for the system. In this study, we firstly developed a new system, ICP-MS combined with the commercially available flow cell, to improve detection sensitivity. Then the time-resolved measurement of dissolution rates of Pt and Pd was conducted with high detection sensitivity. Experimental Cross-flow cell (BAS Inc.) was employed for the experiments. Pt or Pd as a working electrode and glassy carbon as a counter electrode were embedded in the cell together. Screw type Ag/AgCl electrode (RE-3VT, BAS Inc.) was used as a reference electrode. A silicon gasket was used to form the flow circuit whose shape is elliptical with a 100 μm thickness, and a test solution (0.5 M H2SO4) flew in the circuit at a rate of 110 μL min-1. The outlet of the cell directly connected ICP-MS (7700x, Agilent Technology), and time-resolved measurements of Pt and Pd were conducted under potentiostatic polarization and potential cycling. Detection limits of Pt and Pd using this system were confirmed to be 0.13, 0.39 pg cm‒2 s‒1, which was lower than those using the previous system. 2 Results and Discussion Firstly, Pt or Pd was polarized to 1.4 V (vs. standard hydrogen electrode) to confirm the system operation. Pt or Pd dissolved immediately after polarization as the following reactions:Pt → Pt2+ + 2e- ,Pd → Pd2+ + 2e- .After polarization, Pt or Pd dissolution detected by ICP-MS after around 20 s. Thus, we should consider this delay time in the time-resolved measurements, and the potential step from open circuit potential to 1.4 V was applied to WE for the calibration of delay time before every experiment. Pt and Pd were subjected to potential cycling between 0.05 – 1.4 V at 10 mV s-1, onset potentials for Pt and Pd dissolution were around 1.0 V and 0.73 V in the anodic scan, respectively. These values were more negative than those decided by the previous system.2 Thus we successfully detected a trivial amount of dissolved Pt and Pd using the developed system. Acknowledgments The authors acknowledge Open Facility Center, Materials Analysis Division, Tokyo Institute of Technology, for assistance with the ICP-MS analysis. References Song, Z. Liang, K. Nagamori, H. Igarashi, M. B. Vukmirovic, R. R. Adzic and K. Sasaki, ACS Catal., 10 4290 (2020).A. Topalov, I. Katsounaros, M. Auinger, S. Cherevko, J. C. Meier, S. O. Klemm and K. J. Mayrhofer, Angew. Chem. Int. Ed. Engl., 51 12613 (2012).

Hetero-shaped coral-like catalysts through metal-support interaction between nitrogen-doped graphene quantum dots and PtPd alloy for oxygen reduction reaction

The development of efficient and durable electrocatalysts is essential for the practical application of fuel cells. Metal-support interaction (MSI) is an important concept in heterogeneous catalysis, which can construct the structure of catalyst and improve the reaction activity and stability of catalyst. Based on this, a simple and efficient method is innovatively proposed to synthesize coral-like special structure catalyst by utilizing the crush capacity of high energy γ-rays and the structure-induced effect of MSI between PtPd alloy and nitrogen-doped graphene quantum dots. The electrocatalyst demonstrates superior long-term durability due to the structure of catalyst is not easy to collapse under suitable MSI. Meanwhile, the unique coral-like structure provides a larger contact area, abundant mass transfer channels, and full exposure of active sites, which are advantageous to improve electrocatalytic activity. The oxygen reduction reaction performance of optimal catalyst (NGQDs140-PtPd) was far superior to that of the commercial 20% Pt/C in alkaline medium, including onset potential (959 mV), half-wave potential (860 mV) and stability (the residual current density was 86.5% after long-term stability test). Therefore, coral-like shaped catalysts are expected to become a member of high-efficiency oxygen reduction catalysts in practical applications.

Enhancement of Ammonia Gas Sensing Properties of GaAs-Based Schottky Diodes Using Ammonium Sulfide Surface Passivation

In this work, enhancement of gas sensing characteristics of GaAs-based Schottky diode sensors passivated by ammonium sulfide are studied and demonstrated. The GaAs surface is passivated by saturated ammonium sulfide solution to remove its native oxide and reduction of surface state densities. The measurements indicate a significant improvement in electrical characteristics of Pt-Pd(alloy)/GaAs diode where the Schottky barrier height increases from 0.688 to 0.758eV after surface passivation. The Schottky diode with passivated GaAs surface exhibits a response of 63% upon exposure to 600ppm ammonia gas at a working temperature of 150°C, which is about 2.5 times higher than that of the non-passivated sensor. This noteworthy improvement is attributed to the decrement of metal-semiconductor interface states, preventing the Fermi level pinning. Furthermore, the response (recovery) times of 29 (138)s is obtained upon exposure to 600ppm ammonia at 150°C, showing a value of around 2 times lower than that for the non-passivated sensor.