Small Pd Particles Research Articles

Ostwald ripening of Pd particles on the (101̄2̄) surface of sapphire

Morphological changes of small Pd particles on the (101̄2̄) surface of α-Al 2O 3 single crystal were investigated by Auger electron spectroscopy in the temperature range 833–923 K. It was shown that practically there was no decrease of the effective thickness during heat treatments under 6×10 −9 mbar. Supposing that the observed change in the Auger intensities is due to the surface Ostwald ripening, it was established that the process was controlled by surface diffusion. It was shown that in this case, taking into account the dissolution of small particles, the normalised Auger intensity is inversely proportional to the normalised average radius. The surface mass transport diffusion coefficients were determined and they were in the range 10 −15–10 −17 m 2 s −1 in the investigated temperature interval. Their temperature dependence can be given as D s ′= 6 +486 −5.93 ×10 −5 exp{[−(203±30) kJ mol −1]/RT} m 2 s −1 .

Nucleation of Metals on Aluminum by Laser Irradiation and the Effect on Ni‐P Electroless Deposition

The mechanism of direct Ni‐P electroless deposition on aluminum by pulsed yttrium aluminum garnet (YAG) laser irradiation in solution was investigated by X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) with energy‐dispersive X‐ray (EDX) analysis. Aluminum specimens covered with porous anodic oxide films were irradiated with a pulsed YAG laser in solutions containing , , , or ions, and then localized Ni‐P electroless plating was attempted at the laser irradiated area in solution. It was found that laser irradiation in solutions containing , , or ions causes the deposition of small metallic particles of Pd, Cu, or Ni at the laser‐irradiated area on the aluminum surface which was exposed to the solution after removal of anodic oxide film. The deposition of the metals is due to a redox reaction between the aluminum substrate and the , , or ions. During laser irradiation in solution, Ni‐P and Ni were deposited by redox reactions between and as well as between and Al. Palladium and nickel (Ni‐P) particles deposited during the laser irradiation acted as catalytic centers for the subsequent Ni‐P electroless plating, while Cu particles did not promote the electroless plating. © 1999 The Electrochemical Society. All rights reserved.

Pd L3-Edge XANES Spectra of Supported Pd Particles Induced by the Adsorption and the Absorption of Hydrogen

Abstract We measured Pd L3-edge X-ray absorption near-edge structure (XANES) spectra for small Pd particles dispersed on inorganic oxide supports (SiO2 and Al2O3) and the spectra changes induced by the adsorption and the absorption of hydrogen. When the XANES spectra were measured under the atmosphere of H2, a new peak appeared at about 8 eV above the inflection point of the absorption edge. The peak position was invariant but its intensity decreased with the decrease of Pd particle size. This peak is caused by hydrogen absorption in the Pd particles. Hydrogen at the subsurface region little affected the Pd L3-edge XANES spectra. A new peak also appeared for the Pd sample accompanied with hydrogen adsorbed on the surface. The peak energy relative to the edge was independent of the Pd particle size and the peak intensity increased with the amount of adsorbed hydrogen. The XANES spectra can quantify the adsorbed and absorbed hydrogen atoms. We presented a model of Pd particles composed of surface, subsurface and bulk atoms.

NO Reduction by CH4in the Presence of O2over Pd-H-ZSM-5

An investigation of the interaction of NO and NO2with Pd-H-ZSM-5, as well as the reduction of NO by CH4, has been conducted using mass spectrometry andin situinfrared spectroscopy. Prior to reaction most of the Pd in Pd-H-ZSM-5 (Pd/Al=0.048) is present as Pd2+cations. NO reduction by CH4in the absence of O2results in the progressive reduction of Pd2+cations above 610 K and the formation of small Pd particles. Reduction of Pd2+cations is significantly suppressed when O2is added to the feed of NO and CH4.In situinfrared spectroscopy reveals the presence of NO+and NO as the principal adsorbed species. NO+is present as a charge-compensating cation (e.g., Z−NO+) and is believed to be formed via the reaction 2 Z−H++2 NO+1/2 O2=2 Z−NO++H2O. NO+does not react with CH4at temperatures up to 773 K. Adsorbed NO reacts with CH4above 650 K and CN species are observed as intermediates. The latter species react with both NO, O2, and presumably NO2. Based on the accumulated data, a mechanism is proposed to explain the reduction of NO by CH4both in the presence and absence of O2.

In Situ Oxidation and Reduction of Small Pd Particles on Silica

Abstract Palladium based catalyst have technological applications in catalytic converters in automobile industry for the combustion of NOx and CO gasses. Pd can also adsorb up to 900 times its volume H2 and is useful for purifying H2. Electron holography studies with model Pd on silica catalysts [1] show that under some circumstances, the Pd particles may form voids. The reason for the formation of these voids and the conditions under which they are formed are not well understood. We have done in situ and ex situ studies on oxidation and reduction of similar model Pd/SiO2 to develop a better understanding of the conditions under which voids are formed. The catalyst samples were prepared by impregnating silica with acetyl acetoneate (acac) and oxidizing in air and then reducing in H2 at 300°C [1]. High resolution images (Fig. 1) of Pd particles show a central feature with low contrast attributed to the presence of voids.

Observation of a low-energy adsorbate core-level satellite for CO bonded to palladium: Coordination-dependent effects

A strong low-energy shake-up satellite for CO adsorbed on Pd is observed. The occurrence of the satellite is established for the CO/1 ML Pd/Mo(110) system at a coverage where CO adsorbs exclusively on-top. Comparisons with CO adsorbed on Pd single-crystal surfaces and small supported Pd particles indicate that the strongly increased satellite intensity is due to the decreased CO-Pd interaction strength for on-top adsorbed CO. This can be used to get further insight into the structure and bonding properties of the adsorbate system. Since a low-energy shake-up feature may be misinterpreted as a chemically shifted component, the conclusion is that great care has to be taken in the evaluation of adsorbate core-level spectra for systems with large variations in adsorption strength depending on the adsorbate sites. Large variations in the CO site distribution may furthermore occur depending on the nature of the Pd substrate: Adsorption of CO on 1 ML Pd/Mo(110) leads to an overlayer dominated by an on-top species and, likewise, the CO overlayer formed on small Pd particles after large doses has a large fraction of on-top bonded species. This is in strong contrast to Pd single-crystal surfaces, where CO adsorbed in more highly coordinated sites is abundant.

A Study of the Dynamics of Pd Oxidation and PdO Reduction by H2and CH4

Palladium is one of the most active catalysts for the catalytic combustion of methane. Since Pd is oxidized during methane combustion to PdO and PdO is required for high activity, it is of interest to understand the dynamics of Pd oxidation and the structure of the oxide formed as well as the dynamics of PdO reduction by H2and CH4. In the present study isothermal and temperature-programmed oxidation and reduction were used to probe the dynamics of the oxidation and reduction of zirconia-supported Pd. During uptake in oxygen, a monolayer of oxide is generated immediately, and upon further oxidation, the oxide forms a shell around a core of metal with thicknesses that increase with the oxidation temperature. The initial oxide is amorphous and subsequently transforms to crystalline PdO. The dynamics of Pd oxidation suggest that oxidation follows the Cabrera–Mott theory. Reduction of PdO by H2occurs in a shellwise manner, consistent with a shrinking core mechanism, while reduction in CH4occurs via an autocatalytic, nucleation mechanism. In the latter case, small particles of Pd must first be formed on which CH4can dissociate. The fragments (H and CHx(x=3–1)) diffuse to the metal–oxide boundary where reduction of the oxide occurs. Consistent with this picture, the kinetics of PdO reduction are first order in Pd initially, but then they become zero order in Pd.

Influence of Alumina Surface Structure on Growth and Adsorption Properties of Pd Particles

The partial CO dissociation observed on small Pd particles deposited on γ-alumina and only molecular adsorption of CO on Pd/(0001) α-alumina model catalysts show that the surface structure of alumina can influence the reactivity of supported clusters. To understand the effect of metal–support interaction, Pd particles were deposited on alumina substrates of different surface structure [(0001), (1-102)]. The surface of alumina substrates was investigated by means of reflection high energy electron diffraction (BREED). The growth of Pd particles, their structure and orientation were observed by RHEED. In order to compare the catalytic properties of supported Pd particles, the study of CO and O adsorption was performed by temperature-programmed desorption (TPD) using molecular beam techniques.

Palladium Supported on Alumina Catalysts Prepared by MOCVD and Impregnation Methods

AbstractConventional MOCVD method has been explored to prepare Pd supported catalysts. Pd and PdO phases were found on the surface of the support. Small Pd particles about 1 to 3 nm and dispersions up to 19%were obtained by MOCVD. TPR results indicated that several surface Pd compounds are reduced. At temperatures below 25°C, PdO, the main compound, is completely converted to metallic Pd which forms hydrides. At higher temperatures, between 500 to 800°C, the reduction peaks could be attributed to Pd-support interactions and a strong support dehydroxylation. All catalysts were inactive in benzene hydrogenation and a significant conversion was only detected a temperatures above 100 °C. This was explained by the reduced accessibility of Pd sites imposed by the carbon contamination and by the Pd-Al2O3interactions.

INFLUENCE OF SURFACE STRUCTURE ON THE MECHANISM OF CO ADSORPTION AND CATALYTIC OXIDATION ON PALLADIUM

The adsorption and oxidation of carbon monoxide on small alumina-supported Pd particles have been studied by temperature-programmed desorption (TPD) using molecular beam techniques. The results of size effect studies show clearly different surface properties of bulk metal and supported clusters. Surprising effect of partical CO dissociation was observed on small Pd particles deposited on γ-alumina, prepared by thermal oxidation of aluminum, and not on Pd/α-alumina model catalysts. The activation energy of CO desorption, Ed, was determined as a function of particle size. The Ed decreased with decreasing particle size. The CO and oxygen sticking probability measurements indicated the CO and oxygen diffusion over the support. It was shown that the oxygen adsorption capacity and the reactivity of alumina-supported catalysts were higher than those of Pd(111).

Oxygen desorption from α-Al 2O 3 (0001) supported Rh, Pt and Pd particles

Temperature-programmed desorption (TPD) was used to study the interaction of oxygen with both large (∼ 10 nm) and small (<2 nm) Rh, Pt and Pd particles supported on α-Al 2O 3(0001). The results of this study indicate that O 2 desorption from large particles of Rh, Pt and Pd is similar to that from low-index planes of the respective metal single crystals. Additionally, O 2 desorption from small Rh particles is similar to that from the close-packed planes of bulk Rh. In contrast, O 2 desorption from small particles of Pt and Pd occurs at a significantly higher temperature than that from the respective, low-index single crystal surfaces despite a much higher saturation oxygen coverage on the small particles. Possible explanations for the differences in O 2 desorption as a function of particle size for the various metals is discussed.

Study of CO interaction with alumina-supported Pd particles

Anomalous effects such as the enhanced sticking probability of CO and O 2 and the partial CO dissociation were measured on small Pd particles deposited on γ-alumina. Recently, a dependence of the sticking coefficient on the substrate has also been reported on Pd (0001) α-alumina but no dissociation has been observed in this case. We present a detail study of CO adsorption/desorption and catalytic reaction properties for a series of alumina substrates. Particularly, we used α-alumina of surface orientations (0001) and (11̄02). Before the Pd deposition, the substrates were exposed to annealing at different temperatures in vacuum and in the air. The adsorption and reaction parameters of Pd particles change with crystallographical structure and stoichiometry of the substrate surface, essentially due to the effect of reactants diffusion over the substrate.

Microstructure of [formula omitted] catalyst: Effect of high temperature reduction in hydrogen

The effect of high temperature reduction (HTR) in hydrogen (up to 1180 K) on the microstructure of 9 wt.-% PdCeO2 catalyst was studied by HRTEM and XRD methods. Reduction of the catalyst at or above 973 K caused severe recrystallization of CeO2 and Pd with simultaneous strong interaction between the two components appearing as three phenomena: epitaxial growth of small Pd particles on CeO2 (most frequently with [111]Pd∥[111]CeO2); decoration of large Pd particles with ordered CeO2 overlayer and expansion of the lattice parameter of Pd (by 2.1%). The origin of the Pd lattice expansion is discussed and diffusion of Ce species into the Pd lattice seems to be the most probable one. HTR caused also phase transformations in the ceria support. At 973 K and 1100 K, whole CeO2 was transformed into oxygen deficient CeOx phase exhibiting the same or similar structure but with expanded lattice parameter (by 2.8%). At 1180 K most ceria was transformed into hexagonal A-Ce23. The CeOx phase appeared to be stable in hydrogen and in vacuum at room temperature, but upon exposure to air at room temperature it rapidly reoxidised to CeO2. Ce2O3 also reoxidised to CeO2 but much slower. Another consequence of HTR at or above 773 K was formation of pits in CeO2 crystallites, mainly on (112)-type crystal faces. The pits (1–10 nm) exhibited well defined walls parallel to CeO2 lattice fringes and they could possibly constitute nucleation sites for strongly bonded, epitaxial oriented Pd particles.

Methanol synthesis on palladium nanoparticles: Size effect

The turnover number (TON) of Pd/SiO2 in methanol synthesis increases by one order of magnitude with the growth of Pd particle size from 10 to 30 A. Further particle size increase to 60 A does not alter the TON. The fraction of bridged adsorbed CO decreases with the growth of Pd dispersion. Low TON of Pd nanoparticles is caused by the decrease of CO chemisorption strength on small electron-deficient Pd particles.

Influence of substrate structure on activity of alumina supported Pd particles: CO adsorption and oxidation

Recently the unexpected effects of partial CO dissociation were reported for small Pd particles deposited on γ-alumina, prepared by thermal oxidation of aluminium whilst this behaviour was not observed on Pd α- alumina model catalysts. In this study we compared the CO adsorption and oxidation properties of Pd particles deposited on α-alumina substrates of different surface stoichiometry with those of Pd on γ-alumina. It was shown that the oxygen adsorption capacity as well as the reactivity of α-alumina supported catalysts was lower than those of γ-alumina supported catalysts. The CO and oxygen sticking probability measurements indicated the CO and O diffusion over the support that in the case of CO decreased with T. The temperature dependent CO oxidation rate was explained in terms of its limitation by the CO diffusion process which is less important in the case of the aluminium rich alumina surface.

Low-temperature CO oxidation on iron oxide supported palladium

For small Pd particles on hematite support, specific catalytic activity in the reaction of low-temperature CO oxidation was found to be greatly enhanced indicating strong synergism. Possible reasons for this pnenomena such as CO and oxygen spillover and stabilization of the oxidized Pd forms due to support effect are considered.

Nanosecond time-resolved degenerate four-wave mixing measurements of small Pd particles: Thermal phase grating analysis

Reflection by the thermal phase grating formed in methanol containing small Pd metal particles has been measured by using a degenerate four-wave mixing optics equipped with a nanosecond Nd:YAG laser and a streak camera. Although the integrated intensity of the reflection depended on the cubic power of the laser intensity, the temporal profile of the reflection in a pulse duration showed the delayed response, the theoretical profile of which has been formulated by considering the formation of thermal phase grating. The theoretical calculation based on heat conduction equation was in good agreement to the experimental results with respect to the shape of the profile, the intensity, and the polarization dependence.

Formation mechanism of a crystalline Ag50Pd50solid solution by impact ball–materials interaction

The formation of the crystalline Ag50Pd50 solid solution during mechanical alloying under shock interaction of Ag and Pd metallic powders has been studied using electron microscopy and X-ray diffraction. The process involved for impact interaction occurs via the incorporation of the small Pd particles into the Ag matrix in which, under the action of mechanical strain, the mutual diffusion of Ag and Pd leads to the formation of two solid solutions, one rich in Ag and the other rich in Pd. Further grinding allows the homogenization of the two Ag and Pd solid solutions whose composition changes to finally reach the initial Ag50Pd50 composition. The defects as dislocations generated by the impact conditions may be responsible for the reaction mechanism.

Polymer-Stabilized Pd Sols: Kinetics of Sol Formation and Stabilization Mechanism

The effects of different water-soluble polymers such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and poly-2-vinylpyridine (P2VP) on the mechanism of sol formation and stabilization of aqueous nanometer-sized Pd sols have been studied using TEM and UV/VIS photospectroscopy. In the case of UV/VIS spectroscopy an (empirical) relation between the absorbance at high wavelength and the particle size is found. A weakly adsorbing polymer (PVA) does not affect the sol formation. On the other hand, strongly adsorbing polymers like PVP and P2VP retard the growth of the particles. Two other parameters that affect the particle size are the reducing agent (H3PO2) concentration and the oxygen content of the solution. The effect of the polymer concentration and the molecular weight of the polymer on the stability and sedimentation of the Pd sol indicates that the weakly adsorbing PVA does not adapt its conformation during adsorption onto the very small Pd particles. Therefore, adsorption of Pd particles on a PVA coil is probably a more realistic model for this system. On the other hand, strongly adsorbing polymers like PVP and P2VP will adapt their conformation, resulting in a kind of "beads on a string" structure. The sol formation/stabilization mechanism of P2VP sols is complex compared to that of PVP-stabilized sols, because of the polyelectrolyte character of P2VP.

The influence of particle size on CO oxidation on Pd/alumina model catalyst

The CO oxidation on Pd(111) and small Pd particles deposited on an alumina support has been studied by means of the thermal molecular beam technique using the transient method at temperatures between 300 and 500 K. The surface is predosed with a known coverage of oxygen and then exposed to the beam of carbon monoxide at a given temperature until the reaction is complete. The temperature and coverage dependent CO oxidation data were obtained for alumina supported Pd particles as well as for a Pd(111) single crystal. The activation energy of the Langmuir-Hinshelwood reaction was calculated for all samples. It was found that the small particles were more reactive. The activation energy E of CO oxidation on 2.5 nm particles is 19 kJ/mol whilst it is 45 kJ/mol for a Pd(111) single crystal. The long-time CO2 production during the transient CO oxidation on small Pd particles, compared to the Pd(111), is explained by the diffusion of oxygen atoms into the subsurface region of Pd particles.