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Impact of IrRu oxygen evolution reaction catalysts on Pt nanostructured thin films under start-up/shutdown cycling
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Crystal growth of incommensurate members of 2H-hexagonal perovskite related oxides: Ba{sub 4}M{sub z}Pt{sub 3−z}O{sub 9} (M=Co, Ni, Cu, Zn, Mg, Pt)
Millimeter sized crystals of six oxides of approximate composition Ba{sub 4}M{sub z}Pt{sub 3-z}O{sub 9} (M=Co, Ni, Cu, Zn, Mg, Pt) were grown from molten K{sub 2}CO{sub 3} fluxes and found to crystallize in a 2H hexagonal perovskite-related structure type. The compositions of these incommensurate structures, which belong to the A{sub 3n+3m}A′{sub n}B{sub 3m+n}O{sub 9m+6n} family of 2H hexagonal perovskite related oxides, were characterized by X-ray diffraction, energy dispersive spectroscopy, and magnetic susceptibility measurements. The specific synthetic considerations, crystal growth conditions, and magnetic susceptibility measurements are discussed. - Graphical abstract: SEM image and average commensurate unit cell of Ba{sub 4}Pt{sub 3}O{sub 9.} - Highlights: • Single crystals of the series Ba{sub 4}A′{sub z}Pt{sub 3-z}O{sub 9} were grown via a molten carbonate flux. • Ba{sub 4}Pt{sub 3}O{sub 9} and all substitutional variants are incommensurate, composite structures. • All compounds have an approximate stoichiometry of Ba{sub 4}A′Pt{sub 2}O{sub 9.}.
Temperature-Dependent Evolution of the Oxidation States of Cobalt and Platinum in Co 1– x Pt x Clusters under H 2 and CO + H 2 Atmospheres
Co1-xPtx clusters of 2.9-nm size with a range of atomically precise Pt/Co atomic ratios (x = 0, 0.25, 0.5, 0.75, 1) were synthesized using the mass-selected low-energy cluster beam deposition (LECBD) technique and soft-landed onto an amorphous alumina thin film prepared by atomic layer deposition (ALD). Utilizing ex situ X-ray photoemission spectroscopy (XPS), the oxidation state of the as-made clusters supported on Al2O3 was determined after both a 1-h-long exposure to air and aging for several weeks while exposed to air. Next, the aged duster samples were characterized by grazing-incidence X-ray absorption spectroscopy (GIXAS) and then pretreated with diluted hydrogen and further exposed to the mixture of diluted CO and H-2 up to 225 degrees C at atmospheric pressure, and the temperature-dependent evolutions of the particle size/shape and the oxidation states of the individual metal components within the dusters were monitored using in situ grazing-incidence small-angle X-ray scattering and X-ray absorption spectroscopy (GISAXS/GIXAS). The changes in the oxidation states of Co and Pt exhibited a nonlinear dependence on the Pt/Co atomic ratio of the dusters. For example, a low Pt/Co ratio (x <= 0.5) facilitates the formation of Co(OH)(2), whereas a high Pt/Co ratio (x = 0.75) stabilizes the Co3O4 composition instead through the formation of a Co-Pt core-shell structure where the platinum shell inhibits the reduction of cobalt in the core of the Co1-xPtx alloy dusters. The obtained results indicate methods for optimizing the composition and structure of binary alloy clusters for catalysis.
Co-Pt core-shell nanostructured catalyst prepared by selective chemical vapor pulse deposition of Pt on Co as a cathode in polymer electrolyte fuel cells
A new type of PtCo/C catalyst for use as a cathode in polymer electrolyte fuel cells was prepared by selective chemical vapor pulse deposition (CVPD) of Pt on the surface of Co. The activity of the prepared catalyst for oxygen reduction was higher than that of a catalyst prepared by sequential impregnation (IMP) with the two metallic components. This catalytic activity difference occurs because the former catalyst has smaller Pt crystallites that produce stronger Pt-Co interactions and have a larger Pt surface area. Consequently, the CVPD catalyst has a great number of Co particles that are in close contact with the added Pt. The Pt surface was also electronically modified by interactions with Co, which were stronger in the CVPD catalyst than in the IMP catalyst, as indicated by X-ray diffraction, X-ray photoemission spectroscopy, and cyclic voltammetry measurements of the catalysts.
First-principles study of nitric oxide oxidation on Pt(111) versus Pt overlayer on 3d transition metals
Catalytic oxidation of NO to NO{sub 2} is a significant research interest for improving the quality of air through exhaust gas purification systems. In this paper, the authors studied this reaction on pure Pt and Pt overlayer on 3d transition metals using kinetic Monte Carlo simulations coupled with density functional theory based first principles calculations. The authors found that on the Pt(111) surface, NO oxidation proceeds via the Eley–Rideal mechanism, with O{sub 2} dissociative adsorption as the rate-determining step. The oxidation path via the Langmuir–Hinshelwood mechanism is very slow and does not significantly contribute to the overall reaction. However, in the Pt overlayer systems, the oxidation of NO on the surface is more thermodynamically and kinetically favorable compared to pure Pt. These findings are attributed to the weaker binding of O and NO on the Pt overlayer systems and the binding configuration of NO{sub 2} that promotes easier N-O bond formation. These results present insights for designing affordable and efficient catalysts for NO oxidation.
Pt Catalysts Modified with Manganese Oxide through Atomic Layer Deposition for CO Oxidation: Establishing a Coating Layer above or below Pt
The effect of atomic layer deposition (ALD) of manganese oxide on CO oxidation over Pt/Al2O3 is investigated. Two types of catalysts with a thin MnOx layer coated on or under the Pt nanoparticles (NPs) are prepared, and a pristine Pt/Al2O3 catalyst is used as a reference sample. The addition of the ALD layers improves the catalyst durability and both catalysts with MnOx exhibit greater stability after thermal aging at 1073-K. Besides, MnOx promotes CO oxidation through active Pt-MnOx interfacial sites, showing a reaction rate up to 35-fold faster than that of unmodified Pt. Interestingly, the CO adsorption ability of the Pt catalysts with MnOx overcoats is greatly suppressed after the high-temperature calcination; however, the reaction rate of CO oxidation is still excellent. This phenomenon is because the uniformly covered MnOx with two atomic layers diminishes the exposed Pt surface, but the reaction near the Pt-MnOx interfacial site still occurs. Our results guide the effective use of ALD, which is actively applied to heterogeneous catalysis, and provide a novel direction to control metal-assisted interactions.
Enhanced high temperature performance of MgAl2O4-supported Pt-BaO lean NOx trap catalysts
The structural and chemical characteristics of Pt/BaO lean-NO{sub x} trap (LNT) catalysts supported on {gamma}-Al{sub 2}O{sub 3} and MgAl{sub 2}O{sub 4} are compared in this study. The Pt-BaO/MgAl{sub 2}O{sub 4} sample shows relatively low NO{sub x} uptake at temperatures below 300 C, and the temperature of maximum NO{sub x} uptake (T{sub max}) is shifted to 350 C in comparison to that of Pt-BaO/Al{sub 2}O{sub 3} (T{sub max} {approx}250 C). More importantly, the NO{sub x} uptake over the MgAl{sub 2}O{sub 4}-supported catalyst at 350 C is twice that of the alumina-based one. The shift toward the higher temperature NO{sub x} uptake is explained by the larger interfacial area between Pt and BaO, due to smaller Pt clusters as evidenced by TEM and Pt L3 EXAFS. In situ TR-XRD results demonstrate that the formation of a BaAl{sub 2}O{sub 4} phase in the BaO/MgAl{sub 2}O{sub 4} LNT catalyst occurs at a temperature about 100 C higher than on BaO/Al{sub 2}O{sub 3}, which may also represent a beneficial attribute of the BaO/MgAl{sub 2}O{sub 4} LNT with respect to catalyst stability.
Systematic study on the discharge product of Pt-based lithium oxygen batteries
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Structure and Desorption Kinetics of Acetonitrile Thin Films on Pt(111) and on Graphene on Pt(111)
Acetonitrile thin films were prepared on Pt(111) and on graphene on Pt(111) using molecular beam techniques. Temperature programed desorption (TPD) experiments of acetonitrile on Pt(111) displayed first-order kinetics for monolayer desorption and zero-order kinetics for multilayer desorption. We used reflection adsorption infrared spectroscopy (RAIRS) to characterize the orientation of the monolayer on Pt(111). Acetonitrile molecules have been shown to adopt a bridging structure where the C=N bond breaks to form two single bonds to underlying platinum atoms. When Tdeposition = 120 K our RAIRS spectra are consistent with this bridging configuration. However, when Tdeposition was lowered to 60 K molecules retained the C=N bond and adopted a vertical orientation with the nitrogen closest to the platinum. On graphene, the TPD experiments show that both the monolayer and multilayer desorb with zero-order kinetics. Our RAIRS spectra suggest that the monolayer consists of acetonitrile molecules oriented with their molecular axes parallel to the graphene substrate. This orientation did not change when Tdeposition was varied from 60 to 120 K. Our results provide a more complete picture of how acetonitrile molecules interact with Pt(111) and with graphene surfaces.
Pt skin on Pd–Co–Zn/C ternary nanoparticles with enhanced Pt efficiency toward ORR
Exploring highly active, stable and relatively low-cost nanomaterials for the oxygen reduction reaction (ORR) is of vital importance for the commercialization of proton exchange membrane fuel cells (PEMFCs). A highly active, durable, carbon supported, and monolayer Pt coated Pd–Co–Zn nanoparticle is synthesized via a simple impregnation–reduction method, followed by spontaneous displacement of Pt. By tuning the atomic ratios, we obtain the composition–activity volcano curve for the Pd–Co–Zn nanoparticles and determined that Pd : Co : Zn = 8 : 1 : 1 is the optimal composition. Compared with pure Pd/C, the Pd 8 CoZn/C nanoparticles show a substantial enhancement in both the catalytic activity and the durability toward the ORR. Moreover, the durability and activity are further enhanced by forming a Pt skin on Pd 8 CoZn/C nanocatalysts. Interestingly, after 10 000 potential cycles in N 2 -saturated 0.1 M HClO 4 solution, Pd 8 CoZn@Pt/C shows improved mass activity (2.62 A mg -1 Pt) and specific activity (4.76 A m -2 total), which are about 1.4 and 4.4 times higher than the initial values, and 37.4 and 5.5 times higher than those of Pt/C catalysts, respectively. After accelerated stability testing in O 2 -saturated 0.1 M HClO 4 solution for 30 000 potential cycles, the half-wave potential negatively shifts about 6 mV. Our results show that the Pt skin plays an important role in enhancing the activity as well as preventing degradation.
Well-defined Pt(0) heterogeneous hydrosilylation catalysts supported by a surface bound phosphenium
Single atom, low valent transition metals are important for heterogeneous catalysis but are challenging to generate and stabilize in a well-defined manner. Herein, we explored the functionalization of silica with well-defined N-heterocyclic phosphenium (NHP) ions to heterogenize low-valent metals. The surface electro-statically bound [NHP] + coordinate to Pt(0) precursors resulting in well-defined, chemisorbed [(NHP)Pt(0)L n ] + sites. The resulting materials catalyze the hydrosilylation of alkynes and exhibit activities and selectivities that rival the current industry standard homogeneous catalysts. The catalysts leach Pt limiting their recyclability; however, recycling studies support that the high regioselectivities arise from heterogeneous sites and Pt particles do not form on the surface. Here we suspect that this phosphenium-based immobilization strategy will result in stable, tunable, low valent heterogeneous transition metal catalysts in a wider array of catalytic reactions.
Double-Layer Kagome Metals Pt 3 Tl 2 and Pt 3 In 2
The connectivity and inherent frustration of the kagome lattice can produce interesting electronic structures and behaviors in compounds containing this structural motif. Here we report the properties of Pt 3 X 2 (X = In and Tl) that adopt a double-layer kagome net structure related to that of the topologically nontrivial high-temperature ferromagnet Fe 3 Sn 2 and the density wave hosting compound V 3 Sb 2 . We examined the structural and physical properties of single crystal Pt 3 Tl 2 and polycrystalline Pt 3 In 2 using X-ray and neutron diffraction, magnetic susceptibility, heat capacity, and electrical transport measurements, along with density functional theory calculations of the electronic structure. Our calculations show that Fermi levels lie in pseudogaps in the densities of states with several bands contributing to transport, and this is consistent with our Hall effect, magnetic susceptibility, and heat capacity measurements. Although electronic dispersions, characteristic of simple kagome nets with nearest-neighbor hopping, are not clearly seen, likely due to the extended nature of the Pt 5d states, we do observe moderately large and non-saturating magnetoresistance values and quantum oscillations in the magnetoresistance and magnetization associated with the kagome nets of Pt.
Kinetic and mechanistic study of bimetallic Pt-Pd/Al2O3 catalysts for CO and C3H6 oxidation
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Origin of Multiple Peaks in the Potentiodynamic Oxidation of CO Adlayers on Pt and Ru-Modified Pt Electrodes
The research of the electrooxidation mechanism of CO ad on Pt based catalysts is very important for designing more effective CO-tolerant electrocatalysts for fuel cells. We have studied the origin of multiple peaks in the cyclic voltammograms of CO stripping from polycrystalline Pt and Ru modified polycrystalline Pt (Pt/Ru) surfaces in both acidic and alkaline media by differential electrochemical mass spectrometry (DEMS), DFT calculations, and kinetic Monte Carlo (KMC) simulations. A novel CO ad electrooxidation kinetic model on heterogeneous Pt and Pt/Ru catalysts is proposed to account for the multiple peaks experimentally observed. In this model, OH species prefer to adsorb at low-coordination sites or Ru sites and, thus, suppress CO repopulation from high-coordination sites onto these sites. Therefore, CO ad oxidation occurs on different facets or regions, leading to multiplicity of CO stripping peaks. This work provides a new insight into the CO electrooxidation mechanism and kinetics on heterogeneous catalysts.
Predicted Impacts of Pt and Ionomer Distributions on Low-Pt-Loaded PEMFC Performance
Low-cost, high performance proton exchange membrane fuel cells (PEMFCs) have been difficult to develop due to limited understanding of coupled processes in the cathode catalyst layer (CCL). Low-Pt-loaded PEMFCs suffer losses beyond those predicted solely due to reduced catalyst area. Although consensus links these losses to thin ionomer films in the CCL, a precise mechanistic explanation remains elusive. In this publication, we present a physically based PEMFC model with novel structure-property relationships for thin-film Nafion, validated against PEMFC data with low Pt loading. Results suggest that flooding exacerbates kinetic limitations in low-loaded PEMFCs, shifting the Faradaic current distribution. As current density increases, protons travel further into the CCL, resulting in higher Ohmic overpotentials. We also present a parametric study of CCL design parameters. We find that graded Pt and ionomer loadings reduce Ohmic losses and flooding, but individually do not provide significant improvements. However, a dual-graded CCL (i.e., graded Pt and ionomer) is predicted to significantly improve the maximum power density and limiting current compared to uniformly loaded CCLs. This work highlights the importance of accurate transport parameters for thin-film Nafion and provides a pathway to low-cost PEMFCs via precise control of CCL microstructures.