Platinum Samples Research Articles

Application of hot stage micro-FT-i.r. to the study of organic functional group changes during pyrolysis

A modified Leitz hot stage fitted to a Spectra-Tech microscope and interfaced with a Nicolet 20-SXC Fourier transform infrared (FT-i.r.) analyser was used to collect infrared (i.r.) spectra from a series of heat treated coal or kerogen samples. Hot stage modifications included installation of an i.r. transparent ZnSe cover disc and routing of a microthermocouple to the interior of a 50 μl platinum sample pan. Spectra were collected in the reflected light mode prior to and then during or after heat treatment. This provided a means to monitor organic functional group changes or depletion resulting from heat treatment. Obstacles encountered and their solutions are discussed.

Calorimetric measurements of the thermal relaxation in nanocrystalline platinum

The enthalpy release during the heating of nanocrystalline platinum samples has been measured and at least two different contributions to the excess Gibbs free energy have been identified by controlling the grain size. About one half of the excess enthalpy is released during a first relaxation process without any measurable variation of grain size followed by a second relaxation which can be attributed to grain growth. The isothermal heat release of the grain growth is analyzed to calculate the excess grain boundary enthalpy.

Characterization of zeolite-supported Pt-Cu bimetallic catalyst by xenon-129 NMR and EXAFS

Samples of platinum supported on NaY zeolite (Pt/NaY), Cu/NaY, and bimetallic PtCu/NaY have been prepared by using a scheme that consists of ion exchange of the metalammine cations, activation in flowing O 2, and subsequent reduction in H 2. These samples have been characterized by 129Xe NMR and extended X-ray absorption fine structure (EXAFS). The chemical shift in 129Xe NMR of the xenon adsorbed on PtCu/NaY has been found to be significantly lower than that on Pt/NaY with the same Pt content under the same experimental conditions, and therefore bimetallic PtCu clusters are formed in supercages of NaY zeolite in both the cases of a sequential clustering method and a simultaneous ion exchange preparation method. The peak intensities at Pt-Pt coordination distance in the Fourier transforms of EXAFS spectra for Pt/NaY and PtCu/NaY are very similar, which indicates that the reduction of Cu precursor has resulted in no appreciable changes in the PtPt coordination structure of the Pt cluster. It thus appears, from these results, that during the reduction of Cu precursor the bimetallic PtCu clusters are formed with a model of a Cu covering on Pt clusters in the supercage of NaY zeolite.

Electrochemical monitoring of the thermal reordering of platinum single-crystal surfaces after metallographic polishing from the early stage to the equilibrium surfaces

In this paper the voltammetry of hydrogen adsorption-desorption on platinum is presented as an alternative standard laboratory method to monitor a complex surface reorganization process such as that occurring during the thermal reordering of the thick perturbed layer resulting from the polishing of oriented platinum samples. By using an electrochemically perturbed Pt (111) surface as a test surface sensitive to atomic reorganization, a thermal decontamination procedure has been developed which allows the cleaning of a disordered platinum surface with minimum change in its initial crystalline structure. This decontamination procedure has been applied successfully to newly polished Pt (111), (100), (110) and (775) surfaces. After decontamination, no preferential site distribution relative to their crystalline direction is detected as resulting from polishing. The modification of the clean perturbed surface during annealing to its final equilibrium state is described, giving evidence for the disappearance of the surface defects. The significant diminution of the mean total electric charge during the reordering process is related to the elimination of the microroughness present on the newly polished surfaces. The effect of a cooling atmosphere, either air or a H 2-Ar stream, after the flame-cleaning procedure was also studied with the well-ordered samples. Major differences for the two types of cooling were observed for Pt (100), giving rise to two types of voltammograms, while for Pt (111) no difference was detected. The differences are interpreted in terms of surfaces with different degrees of two-dimensional long-range order. On the other hand, it is demonstrated that the rearrangement of randomly distributed (110) sites in steps with long-range order along the step rows makes the corresponding hydrogen adsorption state narrower.

Effect of the nature of promoting chromium-containing compounds on the isomerization and dehydrocyclization activities of Pt-Cr/Al 2O 3 catalysts

Preparation of Pt-Cr/Al 2O 3 catalysts is achieved by treatment of a monometallic alumina-supported platinum sample (0.16 wt.-% platinum) with CrO 2Cl 2 vapour or by impregnation of this sample with aqueous solutions of various chromium-containing compounds such as (CrO 3) x, K 2Cr 2O 7 and (NH 4) 2Cr 2O 7 or their mixtures. The change in acidity of the support is estimated by measuring thermal desorption of ammonia. The activity of the obtained samples in isomerization and dehydrocyclization is studied in model reactions with n-pentane, methylcyclopentane and n-hexane. It is established that modification of Pt/Al 2O 3 catalysts by chromium ions directly influences the support acidity, depending on the nature of the compounds used, and exercises an indirect effect on the state of the metallic platinum centers. The electronic state of the active sites changes simultaneously due to the presence of electron-exchange effects between platinum and chromium. As a result of these combined effects, the rate of the reaction and selectivity of the catalysts change.

Formation, reactivity and catalytic activity-selectivity of Pt(CL x)(CONH) y species prepared by interaction of H 2PtCl 6 with nylon supports

Samples of platinum supported on nylon-66 have been prepared by the adsorption of hexachloroplatinic acid from aqueous solution and characterised. After reduction these catalysts were selective in the hydrogenation of benzene (373 – 493 K, H 2:hydrocarbon = 18) to cyclohexene rather than cyclohexane, while in the hydrogenation of cyclohexene (373 – 453 K, H 2: hydrocarbon = 214) the samples were active without simultaneous disproportionation. Such selectivity was greatest for Pt/nylon with highest Pt loading. Apparent turnover numbers for both reactions on Pt were low (possibly reflecting the difficulty of assessing the active surface areas of these catalysts). Activation energies for both reactions suggest the importance of reactant product diffusion into the porous support. Since the active species ( i.e. -NH⋯ PtCl x⋯CO-, where x remains uncertain)are thought to be coordinatively bonded to support amide groups, it is an important finding that Pt in a positive oxidation state is active in these reactions.

Transmission electron microscopy of scanning tunneling tips

The resolution of an scanning tunneling microscope (STM) when used for topographic measurement is mainly limited by the shape of the tip. We here report on the shape and structure of tips observed with high resolution electron microscopy before and after the use in a STM. Tungsten tips made by electropolishing and Pt–Ir tips made by electropolishing, cutting, and ion milling are studied. During electropolishing, the tungsten tips easily get covered with a rather thick, unwanted contamination and/or oxide layer. We have studied such tips as well as clean tips with transmission electron microscopy before and after they were used in a STM working in ambient air. We find that heavily oxide-covered tips often suffer severe bending on a platinum sample, probably because the oxide layer is deformed (broken) before tunneling can occur, or flattening in the STM. On the contrary, oxide free tips normally do not bend, and tunneling condition with a stable tunnel current is easily established.

Pt clusters in BaKL zeolite: Characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

Platinum supported on BaKL zeolite was characterized by Transmission Electron Microscopy (TEM), hydrogen chemisorption, and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results of all three techniques indicate the presence of highly dispersed platinum in the zeolite pores. There is no evidence of platinum outside the zeolite pores. The EXAFS data determine a Pt-Pt coordination number of 3.7, suggesting that the average platinum cluster in the zeolite consists of 5 or 6 atoms, consistent with the TEM and chemisorption data. The EXAFS data also provide evidence of the platinum-zeolite interface, indicated by Pt-O contributions at 2.14 and 2.70 A, and a Pt-Ba contribution at 3.8 A. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.

Scanning tunneling microscopy of metal particles on graphite

Supported metal catalysts are typically 10 to 100Å diameter particles on inert supports such as Al2O3 or graphite. The shape of catalytic particles determines the number and type of active sites on which a reactant can adsorb. The particle morphology can be studied by TEM which gives atomic resolution, but little information about the surface topography, or by SEM which yields topographic information but only at resolutions of >20Å. However, Scanning Tunneling Microscopy (STM), a surface analysis technique developed in 1981, is a very sensitive surface probe with vertical resolution of >0.1Å. In this work, catalytic systems of small platinum and rhodium particles supported on graphite substrates have been studied using STM to determine the surface morphology of the particles.The supported particle samples were prepared by first evaporating approximately 20Å of metal film on an Highly Oriented Pyrolitic Graphite (HOPG) substrate. A typical platinum sample was then placed in a tube furnace and annealed in hydrogen at 850°C for 4 hours; a typical rhodium sample was annealed in hydrogen at 750°C for 2 hours.

Effect of chlorine in the formation of PtRe alloys in [formula omitted] catalysts

The role of chlorine and the thermal pretreatments of the samples in PtRe alloying during the reduction of Pt Al 2O 3 and PtRe Al 2O 3 systems have been examined by using the reaction of n-butane hydrogenolysis with the aid of temperature-programmed reduction (TPR) experiments. The presence of chlorine modifies only slightly the TPR profiles, and rhenium reduction is always catalyzed by the presence of platinum. This effect depends strongly on the hydration degree of the support, while it is almost independent of the chlorine content of the sample. However, chlorine greatly decreases the catalytic activity of both Pt and PtRe catalysts in n-butane hydrogenolysis. Selectivity to methane (measured as C 3 C 1 ratio) is close to 1 for the platinum samples, while in the PtRe systems it remains close to 0.5 for the chlorine-free catalyst after all the pretreatments, but changes from 0.5 to 1.0 when the chlorinated catalyst is predried at increasing temperatures, indicating differences in the degree of PtRe alloying. Chlorination/dechlorination experiments confirm the role of chlorine on the above results and point to the existence of oxychlorinated Pt and/or Re species that may strongly interact with the progressively dehydrated Al 2O 3 support, leading to higher metallic dispersions and lower PtRe alloying in the chlorinated catalysts after hydrogen reduction.

Formation of Pt particles in Y-type zeolites: The influence of coexchanged metal cations

Samples of platinum, supported on NaY, HNaY, and M NaY (M = Ca 2+ , La 3+ , or Fe 2+ ) , have been studied by temperature-programmed reduction (TPR), desorption (TPD), and extended X-ray absorption fine structure (EXAFS) to investigate the mechanism of the formation of Pt particles and the influence of H + and added metal cations on it. Size and location of Pt particles in a zeolite matrix were found to be strongly dependent on the distribution of Pt ions between the supercages and the sodalite cages, which is controlled by the calcination temperature ( T c) prior to the reduction. At low T c (e.g., 360 °C), the majority of the Pt 2+ ions are in the supercages; after reduction small particles (5–10 Å) are formed which are also located in the supercages. This is also true for Pt Y with or without coexchanged multivalent cations. The Pt 0O 2− distance is 2.65 Å as derived from the k 1-weighted EXAFS functions. At medium T c (e.g., 450 °C), the Pt 2+ ions are distributed between supercages and sodalite cages. In this case, the Pt 2+ ions in the supercages are reduced first to form small particles which become nucleation sites for Pt 0 atoms leaving the sodalite cages above 400 °C. Eventually, these Pt particles can become larger than the supercages. At high T c (e.g., 550 °C), most Pt 2+ ions migrate to sodalite cages and require a high reduction temperature ( T r). The reduced Pt 0 atoms then migrate from sodalite cages to supercages, and in the absence of nucleation sites they finally coagulate on the external surface of the zeolite crystals to form large Pt particles. However, the presence of coexchanged multivalent cations, e.g., Fe 2+, which can effectively block sodalite cages and hexagonal prisms, thus forcing Pt 2+ ions to stay in supercages, can prevent the formation of large particles on the external surface of zeolites even at high T c. The evolution of hydrogen at T max = 450 °C, caused by oxidative reaction of Pt 0 atoms with hydroxyl groups during TPD, as indicated by FTIR, can be used to determine the presence of Pt in sodalite cages.

Surface characterization by energy distribution measurements of secondary electrons and of ion‐induced electrons

AbstractInstruments for surface microanalysis (e.g. scanning electron or ion microprobes, emission electron or ion microscopes) use the current of emitted secondary electrons or of emitted ion‐induced electrons for imaging of the analysed surface. These currents, integrating over all energies of the emitted low energy electrons, are, however, not well suited to surface analytical purposes. On the contrary, the energy distribution of these electrons is extremely surface‐sensitive with respect to shape, size, width, most probable energy, and cut‐off energy. The energy distribution measurements were performed with a cylindrical miror analyser and converted into N(E), if necessary. Presented are energy spectra of electrons released by electrons and argon ions of some contaminated and sputter cleaned metals, the change of the secondary electron energy distribution from oxidized aluminium to clean aluminium, and the change of the cut‐off energy due to work function change of oxidized aluminium, and of a silver layer on a platinum sample. The energy distribution of the secondary electrons often shows detailed structures, probably due to low‐energy Auger electrons, and is broader than the energy distribution of ion‐induced electrons of the same object point.

An X-Ray Photoelectron Spectroscopic Study of the Free Surface Segregation of Impurities in Platinum and Palladium

X-ray photoelectron spectroscopy has been used to observe the free surface segregation of impurities in samples of platinum and palladium. Heating in vacuo to 600°C promoted the surface segregation of silver, gold, and carbon in platinum and the surface segregation of silver and sulfur in palladium. All species segregated in their elemental form. The core level binding energies of the silver and gold segregants were lower than those measured for the corresponding pure metal states.

Simultaneous determination of desorption parameters and barrier heights for release of previously absorbed tracer amounts of cesium and potassium from a platinum sample

Neutral and ionic desorption of cesium and potassium from a polycrystalline platinum surface was investigated with a field reversal apparatus in the temperature interval 1600–1900 K under UHV conditions. The desorbing atoms and ions were released from previously absorbed tracer amounts of alkali from a molecular beam. The Arrhenius desorption parameters (k = Aexp(− E/KBT)) found are E+ = 1.80 eV. E0 = 3.27eV, A+ = 4.9 × 1011s−1 and A0 = s−1 for cesium and E+ = 2.47 eV, E0 = 3.65 eV, A+ = 3.2 × 1013s−1 and A0 = 5.8 × 1013s−1 forpotassium. The subscripts + and 0 indicate ionic and neutral desorption respectively. A rate limiting step in the diffusion out to the surface was observed. The rate limiting barriers found were Eout = 6.68 eV for cesium and Eout = 6.87 eV for potassium.

Simultaneous determination of desorption parameters and barrier heights for release of previously absorbed tracer amounts of cesium and potassium from a platinum sample

Neutral and ionic desorption of cesium and potassium from a polycrystalline platinum surface was investigated with a field reversal apparatus in the temperature interval 1600–1900 K under UHV conditions. The desorbing atoms and ions were released from previously absorbed tracer amounts of alkali from a molecular beam. The Arrhenius desorption parameters ( k = A exp(− E/ K B T)) found are E + = 1.80 eV. E 0 = 3.27 eV, A + = 4.9 × 10 11 s −1 and A 0 = s −1 for cesium and E + = 2.47 eV, E 0 = 3.65 eV, A + = 3.2 × 10 13 s −1 and A 0 = 5.8 × 10 13 s −1 forpotassium. The subscripts + and 0 indicate ionic and neutral desorption respectively. A rate limiting step in the diffusion out to the surface was observed. The rate limiting barriers found were E out = 6.68 eV for cesium and E out = 6.87 eV for potassium.

Investigation of thermal degradation of polystyrene with the aid of thermal analysis

When the DTA of polystyrene is carried out in air in a platinum sample holder, an anomalously high endothermic effect is observed. This effect was found to be related to gas-phase catalytic styrene oxidation occurring on the surface of both operating and reference platinum sample holders. Methodological recommendations are given concerning the DTA procedure for organic substances, making it possible to avoid this type of anomaly.

Observation of LEED threshold effects on polycrystalline platinum

We report the observation of LEED threshold interference structure on polycrystalline samples of platinum. These structures, observed at low energy by high-resolution electron reflection measurements, are interpreted in terms of a recently developed theoretical framework. Our analysis shows how observation and detection of threshold effects is possible even on polycrystalline samples under certain conditions. These conditions are described with emphasis on the use of threshold effects for studying polycrystalline samples.

Volatilisation studies of cadmium compounds by the combined quartz furnace and flame atomic absorption method: effects of magnesium chloride and ascorbic acid additives

A flame atomic absorption spectrometer was coupled to a modified thermoanalytical quartz furnace for the element-specific detection of evolved decomposition products. Heating programmes with a constant heating rate (0.8 °C s–1) and with an exponentially decreasing heating rate (typically 50–5 °C s–1) were applied in the range 25–950 °C. Sample holders made of platinum, electrographite and solid pyrolytic graphite were used in furnace atmospheres of variable oxygen concentration. Decomposition and vaporisation of Cd(NO3)2, CdCl2, MgCl2, ascorbic acid and mixtures of these compounds were studied. The radiation scatter of the aggregates formed from the ascorbic acid by decomposition was detected in a flow cuvette. Because of the extremely long life of the atomic cadmium vapour, cold vapour detection of this species was also feasible in the combined system. This was used to supplement the flame atomic absorption measurements in elucidating vaporisation mechanisms. Samples in the form of aqueous solutions and occasionally as solids were applied to the furnace. It was found that the hydrolysis of CdCl2 and MgCl2 takes place to a lesser extent on increasing the sample mass and the heating rate and by using pyrolytic graphite and platinum sample holders. The enhanced rate of hydrolysis on the surface of porous graphite is explained by an adsorption effect (adsorption hydrolysis). The fraction of a hydrolysing chloride salt evolved in chloride form is also dependent on the pre-heating conditions. In the presence of ascorbic acid, Cd(l), CdO(s) and MgO(s) are formed from both chloride and nitrate salts of these elements in the first period of heating. On the surface of the pyrolytic sample holder, the formation of Cd(l) is enhanced in the presence of ascorbic acid and thus complete evaporation of cadmium can be achieved at low temperature (320–460 °C). The vaporisation of CdO(s) takes place by thermal dissociation from both platinum and graphite surfaces at a temperature (ranging from 580 to 800 °C) that depends on the effective oxygen concentration close to the sample. The decomposition products of the nitrate anion activate the graphite surface, which enhances the vaporisation rate of CdO(s) from such a surface.

Catalytic hydrogenation of cyclohexene: VI. Turnover rate of the gas-phase reaction on unsupported platinum powders

A platinum powder, suitable for conventional studies of adsorption and catalysis, was prepared by washing a potassium-contaminated commercial platinum black in nitric acid. The acid-washed powder was potassium free, as shown by Auger electron spectroscopy. Surface areas were measured by the BET method with dinitrogen and argon. On the acid-washed platinum powder, the turnover rate for hydrogenation of cyclohexene under standard conditions was practically the same as on supported samples of platinum and a large clean single crystal, as determined by others. On that evidence, it is concluded that the surface of the acid-washed powder can be considered a clean surface for studies of adsorption and catalysis.

Theory of spin-polarized photoemission from nonmagnetic metals: Platinum.

We develop a fully relativistic theory of the process of photoemission from paramagnetic metals and illustrate it by explicit calculations of the photocurrent and the spin polarization of electrons photoemitted from a single-crystal platinum sample. Our results are in good agreement with the available experimental data and indicate that energy-, angle-, and spin-resolved photoemission spectroscopy is a powerful new probe of the electronic structure even in the case of nonmagnetic metals.