An XPS and STM Study of Oxidized Platinum Particles Formed by the Interaction between Pt/HOPG with NO<Subscript>2</Subscript>

X-ray photoelectron spectroscopy (XPS) (with AlK_α and AgL_α radiations) and scanning tunneling microscopy (STM) were used to study the interaction of two model samples prepared by vacuum evaporation of platinum on highly oriented pyrolytic graphite (HOPG) with NO₂ at room temperature. According to STM data, platinum evaporation on the graphite surface produced particles of a flattened shape. In the Pt/HOPGS1 sample with a lower concentration of platinum, the average diameter of particles d and the height-to-diameter ratio h/d were 2.8 nm and 0.29, respectively. In the Pt/HOPG-S2 sample with a higher concentration of platinum, the average values of d and h/d were 5.1 nm and 0.32. When the samples interacted with NO₂ (P ≈ 3 × 10^–6 mbar), the particles of metallic platinum completely converted to the particles of PtO_2· Upon oxidation, the shape of larger platinum particles in the Pt/HOPG-S2 sample did not change, although for the dispersed particles in the Pt/HOPG-S1 samples under these conditions, the h/d ratio increases. The reduction of oxide to metal particles on heating the Pt/HOPG-S1 sample in vacuum at 460°С is accompanied by an increase in the size of particles. Their shape became more round compared to the initial one. It was found that X-ray radiation affects the state of platinum in the oxidized sample by reducing the surface layer of PtO₂ to PtO.     

X-ray photoelectron study of the interaction of H<Subscript>2</Subscript> and H<Subscript>2</Subscript>+O<Subscript>2</Subscript> mixtures on the Pt/MoO<Subscript>3</Subscript> model catalyst

The effects of H₂ and H₂ + O₂ gas mixtures of varying composition on the state of the surface of the Pt/MoO₃ model catalyst prepared by vacuum deposition of platinum on oxidized molybdenum foil were investigated by X-ray photoelectron spectroscopy (XPS) at room temperature and a pressure of 5–150 Torr. For samples with a large Pt/Mo ratio, the XP spectrum of large platinum particles showed that the effect of hydrogen-containing mixtures on the catalyst was accompanied by the reduction of molybdenum oxide. This effect results from the activation of molecular hydrogen due to the dissociation on platinum particles and subsequent spill-over of hydrogen atoms on the support. The effect was not observed at low platinum contents in the model catalyst (i.e., for small Pt particles). It is assumed for the catalyst that the loss of its hydrogen-activating ability is a consequence of the formation of platinum hydride. Possible participation of platinum hydride as intermediate in hydrogen oxidation to H₂O₂ is discussed.     

Oxidation of methanol on Pt(Mo) electrodes obtained using galvanic displacement method

The electrocatalytic Pt-Mo system was obtained by formation of platinum particles on the Mo surface under its contact with PtC₆²⁻ (PtCl₄²⁻) under the open circuit conditions. Cyclic voltammograms of the obtained Pt(Mo) electrodes feature well pronounced peaks of hydrogen adsorption and desorption on Pt particles. Nonuniform platinum distribution across the electrode surface was found. Pt(Mo) electrodes showed a higher specific activity in the reaction of methanol oxidation in the potential range of 0.35–0.45 V (RHE) as compared to Pt/Pt.     

Size effect in the oxidation of platinum nanoparticles on graphite with nitrogen dioxide: An XPS and STM study

The interaction of NO₂ with model catalysts prepared by platinum evaporation onto the surface of highly oriented pyrolytic graphite has been investigated at room temperature and a pressure of 3 × 10^?6 Torr by X-ray photoelectron spectroscopy and scanning tunneling microscopy. In the catalyst containing only small (<2.5 nm) platinum particles, these particles oxidize to PtO and PtO₂. The action of NO₂ on the graphite support and on the graphite-supported Pt catalyst causes graphite oxidation. The oxygen concentration in the model catalyst is higher than on the support. This is supposed to be due to the spillover of oxygen atoms from platinum particles to graphite.     

The effect of electrochemically treated glassy carbon on the activity of supported Pt catalyst in methanol oxidation

Effect of electrochemical oxidation of glassy carbon on deposition of platinum particles and electrocatalytic activity of platinum supported on oxidized glassy carbon (Pt/GC_OX) were studied for methanol oxidation in H₂SO₄ solution. Platinum was potentiostatically deposited from H₂SO₄ + H₂PtCl₆ solution. Glassy carbon was anodically polarised in 0.5 M H₂SO₄ at 2.25 V vs. saturated calomel electrode (SCE) during 35 s. Electrochemical treatment of GC support, affecting not significantly the real Pt surface area, leads to a better distribution of platinum on the substrate and has remarkable effect on the activity. The activity of the Pt/GC_OX electrode for methanol oxidation is larger than polycrystalline Pt and for more than one order of magnitude larger than Pt/GC electrode. This increase in activity indicates the pronounced role of organic residues of GC support on the properties of Pt particles deposited on glassy carbon.     

Distinctive features of supported catalysts prepared from platinum carbonyl clusters

The formation of Pt/γ-Al₂O₃ and Pt/C catalysts from platinum carbonyl clusters H₂[Pt₃(CO)₆]_n (n = 2, 5) is studied. The strength of interaction between clusters (strong Lewis bases) and the support and the state of platinum in catalysts are governed by the acceptor strength of the support. The formation of a stable platinum compound with a surface of γ-Al₂O₃ (strong Lewis acid) is shown for a Pt/γ-Al₂O₃ catalyst by the method of radial distribution functions. In a Pt/C catalyst containing the same amount of Pt supported on a carbon material known to be a weaker acceptor, metallic platinum is formed along with surface-bonded platinum. Proceeding from the existence of the active phase of catalysts in the form of a surface platinum complex and platinum crystallites, the properties of catalysts are discussed in the complete oxidation of methane and the dehydrogenation of cyclohexane, as well as the high dispersity of platinum and its thermal stability     

New process for loading highly active platinum on carbon black surface for application in polymer electrolyte fuel cell

The deposition of platinum on various carbon blacks was carried out by forming active functional groups on the surface of the carbon support, and exchanging these active groups with different platinum complexes. Using H₂PtCl₆ solution, an impregnation rather than an exchange takes place. However, using divalent platinum complexes [Pt(NH₃)₄]²⁺, a fast exchange takes place which leads to extremely small platinum particles highly dispersed on the surface of carbon black. A comparison of the catalytic activities of platinum supported on various carbon blacks was also carried out. The performances of Pefc (Polymer Electrolyte Fuel Cell) based on the process of the ion exchange are reported.     

Interface-reactions of Pt/TiO2: Comparative electrical, XPS-, and AES-depth profile investigations

Summary Pt/TiO₂-interfaces show Schottky-barrier or ohmic behavior depending on the diffusion of Pt into TiO₂ subsurface layers. Diffusion effects are negligible for TiO₂ rutile surface structures in the absence of oxygen at T1070 K. This leads to Schottky-barrier electrical behavior. If bulk diffusion of Pt is pronounced we observe ohmic behavior. This is the case for all TiO₂ samples with nonstoichiometric Magneli-subsurface phases and for TiO₂ rutile surface phases at T1070 K in the presence of oxygen. The latter is explained by reduced activation barriers of surface platinum during the first step of in-diffusion if platinum is present as an ion (Pt⁴⁺ or Pt²⁺) at the surface. Ions are formed as intermediates during the formation and decomposition of platinum oxide at the three-phase-boundary O₂/Pt/TiO₂.     

The influence of plasma chemical treatment of a platinum catalyst on its activity in the dehydrogenation of cyclohexane

The influence of plasma chemical treatments on the catalytic activity of 0.64 wt % Pt/SiO₂ and 1.0 wt % Pt/SiO₂ platinum catalysts in the dehydrogenation of cyclohexane was studied. The state of the surface of the catalysts was examined using X-ray photoelectron spectroscopy. Temperature hysteresis caused by the formation of active carbon was observed in flow experiments. It was shown that the reaction on the initial catalysts occurred on neutral and positively charged Pt particles, and that the active centers contained carbon. After catalyst treatment with a high-frequency plasma in H₂, its activity increased by many times because of the formation of a large number of low-activity centers on positively charged platinum particles also containing carbon. Glow discharge plasma in Ar sharply decreased catalytic activity, and the reaction then predominantly occurred on centers localized on neutral Pt particles, whereas centers on positive Pt particles were blocked. The state of the substrate (silica gel) did not change under the action of plasmas of both kinds.     

EuroPt-1 catalyst: Radial distribution of electron density X-ray diffraction and EXAFS studies

The supported Pt/SiO₂ (EuroPt-1) catalyst has been studied by the radial distribution of electron density (RDED) and EXAFS techniques. The starting sample of the catalyst was stored in air, not subjected to any further treatment, and contained metal platinum Pt⁰ and platinum oxide PtO in a ～1:2 ratio. An analysis of the EXAFS data was based on three possible structural models of platinum particles. Model 1 suggested that there was one Pt-Pt short contact, which was the same in the bulk of the particles and on the surface. Model 2 considered two different Pt-Pt distances for the particle volume and surface. For model 3, we additionally assumed that the corresponding Debye-Waller factors differed. For the oxidized sample, model 2 was most reliable, and the Pt-Pt distance between the surface atoms was shortened by ～0.14 Å. For the reduced samples, the structural data obtained are consistent with model 3.     

Dispersion and states of platinum ions in BEA-zeolite pores: effect of the framework basicity

Pt/Cs-BEA materials prepared by a classical ion-exchange procedure using two Cs-BEA supports with different Cs loadings, and a reference acidic Pt/H-BEA, have been studied to investigate the effect of the framework basicity (evaluated by FT-IR of adsorbed CO₂) on the state of platinum species after the initial steps (introduction of Pt complex by ion-exchange and subsequent calcination) of the preparation procedure. DR-UV data revealed that the framework basicity affects the structure of the Pt²⁺ complexes introduced as countercations in the zeolite by ion exchange. FT-IR spectra of adsorbed CO indicated that zeolite basicity rules the fate of platinum species in the subsequent calcination. Hence, in Pt/H-BEA essentially well dispersed Pt^δ+ (4≥δ≥1) are present, while PtO _x particles progressively prevail as the basic character of the zeolite increases.     

Surface and structural properties of pt/ceo2catalyst under preferential co oxidation in hydrogen (PROX)

Summary Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO₂with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO₂earlier [16]. The optimum temperature region is T￡373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H₂/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO_1.695structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO₂.</o:p>     

Electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto glassy carbon and into Nafion® films

A comparative investigation of electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto a glassy carbon surface (Pt/GC) and within a thin Nafion® film formed on a GC electrode (Pt/Nf/GC) is described. As test reaction the methanol oxidation in sulfuric acid solutions is used. Dependences of the steady-state specific reaction rates upon potential and methanol concentration were established, as well as those of the platinum surface coverage with methanol chemisorption products upon concentration. It was shown that at higher platinum loadings (above 60 μg cm⁻²) the specific activities of Pt/GC and Pt/Nf/GC are nearly the same and close to that of smooth platinum. At such loadings the surface coverage of the platinum deposit surface with organic particles does not differ from that of smooth platinum. At very low platinum loadings in the polymeric matrix (10–30 μg cm⁻²) a considerable decrease in the adsorption of strongly chemisorbed methanol particles is observed. These deposits are characterized by a low specific activity, which may be caused by the decrease of the platinum particle’s size, leading to a decrease in the amount of weakly bound methanol particles participating in the limiting reaction step.     

The use of galvanic displacement in synthesizing Pt(Cu) catalysts with the core-shell structure

The formation of a Pt(Cu) bimetallic catalyst on the carbon support by galvanic displacement of copper electrodeposits with platinum (PtCl ₆ ^2? as the displacing agent) is systematically studied. Composition, structure, and electrocatalytic properties of samples corresponding to different stages of copper displacement are analyzed. For substantially long displacement times, the formation of stable Pt(Cu)_st particles with the atomic ratio Pt: Cu ≈ 7: 3 is observed. The Pt(Cu)_st/C electrodes are shown to be close to the Pt/C electrode as regards the adsorption of hydrogen and copper atoms and the specific activity in methanol oxidation (with 0.5 M H₂SO₄ as the supporting electrolyte). Such electrocatalytic behavior of Pt(Cu)_st particles makes it possible to infer the formation of the “core(Pt, Cu)-shell(Pt)” structure, as confirmed by the XPS data.     

X-ray photoelectron spectroscopic study of the interaction of supported metal catalysts with NO<Subscript>x</Subscript>

The reactions of the platinum and rhodium model catalysts applied to aluminum oxide with NO_x (10 Torr NO + 10 Torr O₂) were studied by X-ray photoelectron spectroscopy. The reaction conducted at room temperature formed on the surface of the oxide support the NO _3,s ^? nitrate ions characterized by the N1s line at 407.4 eV and O1s line at 533.1 eV and the NO _2,s ^? nitrite ions characterized by the N1s line with a binding energy of 404.7 eV. At the same time, the Pt4f and Rh3d lines of the supported platinum particles are shifted toward higher binding energies by 0.5–1.0 eV and 0.7–1.2 eV, respectively. It is assumed that the binding energies increase due to changes in the chemical state of the platinum metal in which oxygen is dissolved. The reaction of NO_x with Pt/Al₂O₃ at 200°C forms platinum oxide defined by the Pt4f _7/2 line with a binding energy of 72.3 eV.     

Platinum colloid supported on graphite: X-ray photoelectron spectroscopy study

Two states of platinum, Pt⁰ and Pt^+δ, with the binding energy of the Pt(4f_7/2) line at 71.4±0.1 and 72.2±0.3 eV, are revealed by XPS in colloidal platinum deposited onto graphite. The latter state is assigned to surface Pt atoms partially oxidized due to the interaction with water and/or oxygen-terminated sulfonate group in an organic shell. The colloid deposition is found to be temperature dependent with the apparent activation energy of ∼40 kJ/mol.     

Dependence of the adsorption and catalytic properties of a copper-platinum catalyst on the structure of metal particles and the composition of the catalyst surface

The kinetics of H₂ desorption from the surface of a copper-platinum catalyst deposited on silica gel ([1 wt % Pt + 0.15 wt % Cu]/SiO₂) and the kinetics of C₆H₁₂ dehydrogenation were studied. The effects of copper introduction in a platinum catalyst on the structural characteristics of platinum particles, the composition of their surface, and the effects of plasmochemical treatments on these parameters were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The metal-H atom bond energies (E _Pt-H) and the catalytic activity were found to increase in the presence of Cu. This was explained by the formation of new hydrogen adsorption centers (due to the Cu^+δ adatoms) and catalytic centers composed of Cu^+δ adatoms and carbon atoms. The mean diameter of Pt particles (D) increased twofold. The microstresses (ɛ) in the particles increased after the catalyst was treated with glow discharge plasma in Ar and O₂ and with high-frequency plasma in H₂ (HF-H₂). The observed changes in the bond energy E _Pt-H and kinetic parameters were explained by the increase in microstresses in Pt particles.     

Oxidation of formic acid on platinum electrodeposited on polished and oxidized glassy carbon

Formic acid oxidation at platinum electrochemically deposited on polished (GC/Pt) and oxidized glassy carbon (GC_ox/Pt) was examined with the objective of studying the effect of electrochemical treatment of the support on deposition of platinum and on the activity of Pt catalyst. The electrodes were characterised by STM and XPS techniques. The oxidative treatment of the support leads to deposition of smaller Pt particles in comparison with the one on the polished substrate. The XPS spectra indicated the increased fraction of functional (acidic) groups on the treated support as well as the higher fraction of oxygen containing species on Pt catalyst deposited on oxidised referring to Pt deposited on polished substrate.The activity of GC_ox/Pt electrode is increased by the factor of 2–4 for formic acid oxidation compared to the activity of GC/Pt electrode. This result is explained by the oxidative removal of CO_ad species leading to enhanced amount of Pt free sites available for direct formic acid oxidation to CO₂.     

Synthesis of nanostructured electrocatalysts based on magnetron ion sputtering

Nanostructured platinum catalysts for electrochemical systems with proton-exchange membranes (PEMs) have been synthesized by magnetron ion sputtering on a carbon support. The design of the powder support stirrer has been optimized to ensure uniform surface coverage with platinum metal nanoparticles. The deposition parameters (discharge power, deposition time, and bias voltage) that make it possible to obtain electrocatalysts with a large specific surface area (up to 44 m²/g) have been determined. The resulting catalysts have been studied by transmission electron microscopy and X-ray diffraction. The samples with platinum particles 3 to 4 nm in size uniformly distributed over the carbon surface and forming a single phase exhibit the greatest efficiency. The electrodes based on the synthesized electrocatalysts have been tested in a liquid electrolyte and as a component of a fuel cell and PEM water electrolyzer. The voltage across the fuel cell with the synthesized Pt/C electrocatalyst (44 m²/g) at a current density of 1 A/cm² is as high as 0.55 V, which corresponds to a specific power of 550 mW/cm². Qualitative correlations between the parameters of the synthesized catalysts and the deposition conditions have been established.     

Effect of metal loading and carbon support microstructure on the dispersion of Pt/C catalysts prepared viaadsorption of chloroplatinic acid

Summary Adsorption of<span lang=EN-US style='mso-ansi-language:EN-US'>H₂PtCl₆onto a carbon support followed by reduction of the adsorbed platinum species with H₂at 250°C leads to<span lang=EN-GB style='mso-ansi-language:EN-GB'>Pt/C catalysts, which show universal volcano-like dependence of platinum dispersion on the metal loading in the range from 0.01 to 0.55mmol (Pt)/m²(S_BET)<span lang=EN-GB style='mso-ansi-language:EN-GB'>when highly disordered carbons, namely, active carbons and carbon blacks, are used as supports. The maximal dispersion D(CO/Pt) = 0.8 is attained at<span lang=EN-US style='mso-ansi-language:EN-US'>0.18mmol (Pt)/m² (S_BET). With<span lang=EN-GB style='mso-ansi-language:EN-GB'>other factors being equal, the dispersion of platinum supported on carbons with a more regular crystal structure, especially Sibunit-type supports, proves to be the highest and independent of the metal loading.The differences between the two groups of carbon supports are explained by the differences in the state of the adsorbed platinum precursors.