Small Platinum Particles Research Articles

Effect of Particle Size on the Adsorption of O and S Atoms on Pt: A Density-Functional Theory Study

In order to evaluate the effect of size on the adsorption energies on small Pt particles, we have performed a series of DFT-GGA calculations on Pt clusters of varying sizes, between 3 and 25 atoms, and a Pt(111) slab, with and without O and S adsorbates. We have found a significant variation in the energy of adsorption as a function of cluster size. In particular, energies of adsorption on the Pt10 cluster showed the largest deviation from those on the slab surface. Structural and energetic properties compare excellently to published experimental data. The Pt4 clusters with and without adsorbates were found to have reduced symmetry due to the Jahn−Teller effect. A simple rule between the cohesive energy and the size of small close-packed platinum particles is derived and confirmed by our first-principle calculations, the extrapolation of which excellently matches both experimental and computational bulk cohesive energies.

00/00702 Characterization of recycle solvent and solvent hydrogenation in 150 ton/day pilot plant for NEDOL process

In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/γ-Al2O3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/γ-Al2O3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H2S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H2S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H2S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt–S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.

Testing and characterisation of Pt/ASA for deep HDS reactions

In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/γ-Al 2O 3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/γ-Al 2O 3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H 2S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H 2S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H 2S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt–S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.

Imaging faces of shadowed magnetite (Fe(3)O(4)) crystals from magnetotactic bacteria with energy-filtering transmission electron microscopy.

We used energy-filtering transmission electron microscopy to image magnetite crystals isolated from uncultured magnetotactic bacteria. These magnetite crystals were shadowed in high vacuum with platinum at 45 degrees. The shadowed crystals were observed in a Zeiss (Thornwood, NY) CEM902 transmission electron microscope. Imaging shadowed crystals with inelastically scattered electrons provided information of the decoration pattern of small platinum particles over crystal surfaces, and thus information on surface characteristics of crystals. Results were comparable to those obtained from scanning electron microscopy using a field emitter gun. Electron energy loss spectra of the crystals as well as of the supporting film were recorded to evaluate variations of image contrast with energy losses. Results indicated that the contrast is attenuated with inelastic imaging and that the effect of contrast tuning caused a contrast inversion at a given point between 100 and 150 eV. We believe this approach can be useful for studying multilayered materials by transmission electron microscopy.

Imaging faces of shadowed magnetite (Fe3O4) crystals from magnetotactic bacteria with energy‐filtering transmission electron microscopy

We used energy-filtering transmission electron microscopy to image magnetite crystals isolated from uncultured magnetotactic bacteria. These magnetite crystals were shadowed in high vacuum with platinum at 45°. The shadowed crystals were observed in a Zeiss (Thornwood, NY) CEM902 transmission electron microscope. Imaging shadowed crystals with inelastically scattered electrons provided information of the decoration pattern of small platinum particles over crystal surfaces, and thus information on surface characteristics of crystals. Results were comparable to those obtained from scanning electron microscopy using a field emitter gun. Electron energy loss spectra of the crystals as well as of the supporting film were recorded to evaluate variations of image contrast with energy losses. Results indicated that the contrast is attenuated with inelastic imaging and that the effect of contrast tuning caused a contrast inversion at a given point between 100 and 150 eV. We believe this approach can be useful for studying multilayered materials by transmission electron microscopy. Microsc. Res. Tech. 46:319–324, 1999. © 1999 Wiley-Liss, Inc.

Outstanding Performances of Magnesia-Supported Platinum–Tin Catalysts for Citral Selective Hydrogenation

Magnesia-supported Sn–Pt catalysts were prepared by reaction between preformed platinum metal particles and Sn(n-C4H9)4. HRTEM and H2-chemisorption data confirm the synthesis of very small tin-decorated platinum particles (∼1.0 nm mean diameter). Preliminary studies on catalytic hydrogenation of citral in a trickle-bed reactor reveal that 97% selectivity to unsaturated alcohols at 100% conversion can be easily achieved. The electronic ligand effect produced by the basic support is supposed to be fundamental to enhance the selectivity to unsaturated alcohols and to stabilise the bimetallic particles.

The Effect of CO Adsorption at Room Temperature on the Structure of Supported Pt Particles

To improve the understanding of the applicability of CO-FTIR spectroscopy for probing the electronic properties of catalysts, the effect of CO adsorption on the geometry of small metal particles in Pt/LTL and Pt/SiO2 catalysts with varying support acidities was determined by comparison of X-ray absorption spectra before and after CO adsorption. At room temperature, the platinum particles (first shell coordination number N > 5) supported on SiO2 were stable under CO atmosphere. By contrast, the smaller platinum particles (N < 5) in zeolite LTL reconstructed with the formation of very small platinum-CO aggregates upon admission of CO at room temperature. For both Pt/SiO2 and Pt/LTL the linear-to-bridge ratio (L/B) of the CO infrared bands is a function of the support acidity/alkalinity. In the case of Pt/SiO 2, the L/B ratio directly reflects the electronic properties of the catalytically active metal, since the metal particles do not reconstruct. The results show that the structure of the platinum particles in LTL zeolite, which participate in catalytic reactions is not the same as the Pt-CO aggregate analyzed by FTIR. Nevertheless, both the catalytic activity and L/B ratio are a function of support acidity.

Contribution of shape resonance and Pt–H EXAFS in the Pt L2,3 X-ray absorption edges of supported Pt particles: Application and consequences for catalyst characterization

The electronic and geometric effects induced by hydrogen chemisorption on small platinum particles supported on high surface-area saponite clay and zeolite LTL were studied by near edge X-ray absorption fine structure (XAFS) spectroscopy. A new subtraction procedure was developed to separate the electronic from geometric effects. A significant Pt–H extended X-ray absorption fine structure (EXAFS) scattering (structural effect) was found for energy values between 0 and 20 eV. In addition, the Pt–H antibonding state (electronic effect) was found to produce a shape-resonance and was isolated from the near edge of the L3 X-ray absorption spectrum. Moreover, for Pt/LTL the shape and energy of the shape-resonance was found to strongly depend on the acidity/alkalinity of the support material, implying a direct influence of the support on the electronic properties of the platinum particles. The results of the study of the resonance state and the Pt–H EXAFS scattering demonstrate the potential of these techniques for characterization of hydrogen chemisorption, metal-promoter, and metal-support effects in catalysis research.

Electrochemical and microscopic characterisation of platinum-coated perfluorosulfonic acid (Nafion 117) materials†

Platinum-coated Nafion 117 structures were characterised using electrochemical measurements of platinum surface area and a number of microscopy techniques. The morphology and composition of the platinum deposits were related to their preparation conditions in terms of platinum salt concentration, electrolyte flow and the surface roughness of Nafion 117. Platinum surface areas achieved were higher than the values predicted for ideal spherical platinum particles of average diameter. This is due to a fine microstructure, which realises much smaller platinum particles than average (down to about 50 nm) allied to the geometry produced by their clustering to form nodules (about 0.1–1.5 µm diameter micro-nodules and about 3–5 µm macro-nodules). Adherent platinum deposits with high surface areas were promoted by using roughened Nafion 117 membranes and enhancing the mass transport of chloroplatinate and tetraborohydrate ions by magnetic stirring of the electrolyte. A flow cell produced much more reproducible platinum-coated Nafion 117 structures. At best, platinum surface areas of 30–50 m2 g–1 Pt were achieved at platinum penetration depths of 5–30 µm into the membrane surface.

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 H 2PtCl 6 solution, an impregnation rather than an exchange takes place. However, using divalent platinum complexes [Pt(NH 3) 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.

Spectroelectrochemical Study of the Role Played by Carbon Functionality in Fuel Cell Electrodes

X‐ray absorption spectroscopy was used to identify specific types of nitrogen and sulfur‐based carbon functionality present in the carbon black supports of fuel cell anodes and cathodes. The effects of these functional groups on the electrocatalytic performance of small platinum particles, dispersed on the carbon, during methanol oxidation and oxygen reduction were assessed. Electrodes functionalized with nitrogen had enhanced catalytic activities toward oxygen reduction and methanol oxidation relative to untreated electrodes. Although electrodes with sulfur functionality had higher oxygen reduction activities than untreated carbons, the activity of these electrodes toward methanol oxidation was found to be lower than electrodes manufactured from untreated carbon. It was found that carbon supports functionalized with both nitrogen and sulfur initiated the formation of Pt particles smaller in size than those observed on untreated carbon supports.

Preparation of Highly Dispersed Platinum Particles in HZSM-5 Zeolite: A Study of the Pretreatment Process of [Pt(NH3)4]2+

The pretreatment of [Pt(NH3)4]2+, ion exchanged into zeolite HZSM-5, was studied using temperature-programmed techniques and diffuse-reflectance UV/VIS spectroscopy. The pretreatment is an important part of the preparation of bifunctional Pt/HZSM-5 catalysts, which are active in the hydroisomerization of alkanes. Pretreatment in a helium atmosphere leads to autoreduction and the conversion of part of the NH3ligands into H2and N2. Pretreatment in an oxygen-containing atmosphere leads to the oxidation of the NH3ligands into N2, N2O, and H2O. Temperature-programmed pretreatment with oxygen gives rise to three reaction-rate maxima if the sample is not dried, and the conclusion is that [Pt(NH3)3(H2O)]2+is formed as an intermediate in the first step. If temperature-programmed pretreatment with oxygen is carried out on a previously dried sample, oxidation of the ligands is a two-step process: three of the NH3ligands are oxidized in one rapid step and the last ligand is oxidized at a higher temperature. Careful pretreatment in an oxygen-containing atmosphere of [Pt(NH3)4]HZSM-5 leads to the formation of the pale green PtHZSM-5 in which platinum is present as Pt2+coordinated to the zeolite framework. Small platinum particles are formed by subsequent reduction of these platinum ions in flowing hydrogen.

The Origin of Sulfur Tolerance in Supported Platinum Catalysts: The Relationship between Structural and Catalytic Properties in Acidic and Alkaline Pt/LTL

The reactivity, structure, and sulfur tolerance is compared for platinum supported on acidic and alkaline LTL zeolite. In the absence of sulfur, EXAFS spectroscopy indicates that small metallic platinum particles of approximately 6 to 14 atoms/cluster are present. The TOF for neopentane hydrogenolysis and isomerization is ca 100 times higher on the acidic LTL due to the metal–support interaction. Saturation of the platinum by H2S, results in the formation of surface Pt–S bonds with a bond length of 2.33 Å. Comparison of the EXAFS results of the sulfur poisoned catalysts, indicates that the S to Pt ratio is lower for platinum on the acidic zeolite. Catalytically, the initial activity (per gram) of both catalysts is greatly reduced after sulfur poisoning due to the loss of exposed platinum, however, the initial neopentane TOFs are nearly unchanged. Because of its higher TOF, the catalytic activity (per gram) of sulfur poisoned, acidic Pt/LTL is comparable to that of nonsulfur poisoned alkaline Pt/LTL. In both sulfur poisoned catalysts the neopentane isomerization selectivity increases compared to the sulfur free catalyst. Although the initial TOFs of the nonsulfur poisoned and sulfur poisoned catalysts are the same, there is a rapid loss in activity due to coke formation in the sulfur poisoned, alkaline LTL, while rate of deactivation by coke in the sulfided, acidic LTL is much lower. The increased sulfur tolerance of acidic supported noble metal catalysts appears to result primarily from the higher intrinsic TOF. In addition, because of its resistance to coke deactivation, the acidic supported sulfur tolerant catalyst is able to maintain stable catalytic activity.

ChemInform Abstract: MCM‐41 as a Support for Small Platinum Particles: A Catalyst for Low‐ Temperature Carbon Monoxide Oxidation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

CO-induced decomposition of small Pt particles in K-LTL zeolite

Reduced Pt/K-LTL has been studied with XAFS spectroscopy before and after CO admission at room temperature. The results of the EXAFS data-analysis show that after reduction very small platinum metal particles are present consisting of five to six atoms. CO admission at room temperature leads to complete decomposition of the platinum metal particles and the formation of a platinum carbonyl cluster most probably stabilised by the zeolite walls. Modelling shows that the platinum carbonyl cluster just fits inside the pores of the zeolite-LTL. Analysis of the white line intensities of platinum LII and LIII X-ray absorption edges reveals that after reduction and in the presence of chemisorbed hydrogen the platinum metal particles in Pt/K-LTL have 0.12% more d-band vacancies than bulk platinum metal. This value increases to 0.34% after CO admission implyingπ-backdonation from platinum to CO, a positive charge on Pt in the newly formed cluster or a combination of both. The results of this study have a large impact on the interpretation of existing literature FTIR CO data obtained on Pt/K-LTL. The results question the validity of CO chemisorption carried out to determine the dispersion of very small platinum particles.

Investigation of methane oxidation on Pt/Al 2O 3 catalysts under transient reaction conditions

The oxidation of methane on alumina-supported platinum catalysts has been investigated for various gas mixtures as a function of the state of oxidation or reduction of the platinum surface. It has been shown that the activity for methane oxidation goes through a sharp maximum a few seconds after the introduction of the reactants to either a pre-reduced or a pre-oxidised catalyst. The results are inter-preted in terms of a Langmuir-Hinshelwood mechanism in which the optimum rate of reaction corresponds to a platinum surface partially covered by both oxygen- and carbon-containing species. The position of the maximum in activity is very reproducible for pre-reduced catalysts but varies in an imprecise manner for pre-oxidised catalysts. These effects are thought to be related to morphological changes in the reduced and subsequently re-oxidised small platinum particles. It appears that even after re-oxidation at 300°C these platinum particles have a strong ‘memory’ of previous reduction treatments.

MCM-41 as a support for small platinum particles: a catalyst for low-temperature carbon monoxide oxidation

MCM-41 is loaded with platinum via different pathways, resulting in highly active oxidation catalysts.

An in situ FTIR study of the electrochemical oxidation of methanol at small platinum particles

It is shown for the first time that it is possible to employ electrochemical in situ Fourier transform infrared (FTIR) spectroscopy to study the electro-oxidation of methanol at small platinum particles. A number of differences with respect to the behaviour at a bulk metal electrode are observed; these observations demonstrate beyond doubt the very significant difference between bulk and particle electrode mechanisms for the oxidation of methanol, and reveal the dangers of extrapolating from the former to the latter.

Benzene hydrogenation on platinum and iridium catalysts. Variation of the toxicity of sulfur with the nature of the support

Benzene hydrogenation was studied at 323 K on fresh and sulfurized catalysts. The activity of small platinum and iridium particles was sensitive to the electronic properties of the metal, which, in turn, depended on the acidity of the support. The toxicity of sulfur, which varies with both the nature of the metal and the support, can be explained by the combination of geometric (negative) and electronic (negative or positive) effects.

Intermetallic Pt-Cr Clusters in Zeolites as Models of Bimetallic Aromatization and Reforming Catalysts: I. Characterization of Oxidation States, Dispersion, and Local Structure

A variety of both structural (EXAFS, XANES, XRD, TEM) and surface-sensitive (XPS, LEISS) techniques have been applied to study the formation, dispersion, composition, and local structure of zeolite-hosted platinum-chromium metal alloys. Both XPS and XANES indicated a two-step reduction of Cr(VI) to Cr(III) and Cr(0) states, while the analysis of EXAFS data demonstrates the formation of platinum-chromium alloy particles after reduction at 823 K. Chromium stabilises small (8-10 Å) platinum particles at lower temperatures (623 K) and when alloyed the particle size increased to 12-20 Å. The Cr/Pt ratio in the alloy is low (<20% chromium), and is independent of the overall Cr/Pt ratio in the ZSM-5, possibly due to space limitations inside the zeolite voids. TEM suggests the formation of some flattened, raft-like metal particles at the external surface of the zeolite. Based on LEISS measurements of surface segregation in a Pt 70Cr 30 alloy film and thermodynamic considerations, a model of the catalyst particles is proposed in which the outermost monolayer consists entirely of platinum and the second shell is enriched in the additive metal, chromium.