Palladium Sites Research Articles

Adsorbate migration on PdAg(111)

Scanning tunneling microscopy (STM) was used to study the adsorption of oxygen on the PdAg(111) surface. Oxygen appears as dark holes of ca 40pm depth, whereas palladium atoms appear as bright spots. By comparing consecutive STM images, oxygen was found to occupy only palladium sites: the oxygen was found to travel exclusively to and from the palladium atoms. The surface concentration of oxygen was low under all the experimental conditions used, as indicated by STM as well as Auger electron spectroscopy.

Catalytic oxidation of volatile organic compounds 1. Oxidation of xylene over a 0.2 wt% Pd/HFAU(17) catalyst

The transformation of o-xylene in low concentration (1700 ppmv) in air over 0.2 wt% Pd/HFAU catalyst with framework and total Si/Al ratios of 17 was investigated in a flow reactor at temperatures between 150°C and 320°C. At short time-on-stream (TOS), whatever the temperature, no xylene appears at the reactor outlet. Below 250°C, this total conversion is due only to the retention of heavy reaction products inside the zeolite pores (“coke”), whereas above this temperature it is due partly to this retention, partly to the oxidation of xylene into CO 2. At higher temperatures, the conversion of xylene into CO 2 increases with TOS. At 290°C, it becomes complete for TOS>30 min. The amount of coke passes through a maximum at a reaction temperature of 230°C. The composition of coke at TOS=60 min was determined at various temperatures. At 150°C, coke consists mainly of (i) aromatic hydrocarbons (65 wt%): methyldiphenylmethane, methylfluorene, methylanthracene, and phenanthrene compounds, and of (ii) oxygenated aromatic compounds, mainly with hydroxy groups. At 200°C, oxygenated compounds become predominant whereas above 250°C coke is mainly constituted by very polyaromatic compounds resulting from the transformation of oxygenated compounds. Mechanisms involving successively protonic and palladium sites (bifunctional catalysis) are proposed to explain the formation of the main coke components.

Preparation and Crystal Structure of the Equiatomic Rare Earth Palladium Silicides NdPdSi, SmPdSi,α-GdPdSi, andα-TbPdSi

The title compounds were prepared by arc-melting of the elemental components. Whereas NdPdSi and SmPdSi are already present after the arc-melting,α-GdPdSi andα-TbPdSi are formed only during the annealing at 800°C. The four compounds crystallize with the recently reportedα-YbAuGe type structure, which was refined forα-GdPdSi:Pnma,a=2108.0(4) pm,b=433.9(1) pm,c=745.6(1) pm,Z=12,R=0.026 for 1447 structure factors and 62 variable parameters. The lanthanoid atoms are situated between two-dimensionally infinite nets of condensed, puckered hexagons formed by alternating palladium and silicon atoms, with Pd–Si distances varying between 251 and 262 pm. In the third dimension these nets are linked via weak Pd–Pd (300 pm), Pd–Si (283 pm), and Si–Si bonds (261 pm). The refinements of the occupancy parameters suggested that ca. 2% of the palladium sites are occupied by silicon atoms and vice versa. The structural relationships between the two modifications of GdPdSi and the structures of AlB2, KHg2, TiNiSi,α-UFeGe, EuAuGe, and EuAuSn are discussed.

STM study of the (111) and (100) surfaces of PdAg

The (111) and (100) surfaces of the Pd 67Ag 33 alloy have been imaged with atomic resolution by scanning tunneling microscopy. On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720–920 K) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency towards formation of isolated palladium sites. On the (100) surface, the palladium concentration in the first layer is extremely low and the system has to be forced into a non-equilibrium state to find palladium atoms in the first monolayer. Chemical contrast here amounts to a 60 pm apparent height difference.

Resonant L3-M 4,5N 4,5 Auger electron spectroscopy as a probe for the local electronic structure inPd xAg 1− x alloys

The Pd L 3- M 4,5 N 4,5 Auger spectra from the pure metal and severalPd x Ag 1− x alloys were studied for sub-threshold excitation energies utilising the resonant line narrowing. Upon alloying, a characteristic line broadening with decreasing palladium concentration is observed. The measured energy shifts in the Auger parameter are in agreement with an increase in the local d-charge density at the palladium sites in thePd x Ag 1−x alloys.

A kinetic study of methanol decomposition catalyzed over plate-type palladium catalyst

A kinetic study of the catalytic methanol decomposition to carbon monoxide and hydrogen has been carried out in the pressure range of methanol up to 8 atm at 200 and 250°C over a palladium catalyst supported on an oxidized aluminum plate. The reaction pathway can be proposed as (i) dissociative adsorption of methanol to methoxyl groups and hydrogen adsorbed on palladium sites, (ii) decomposition of the methoxyl groups to carbon monoxide and hydrogen adsorbed, and (iii) desorption of the surface carbon monoxide and hydrogen species. It is suggested that the second step is rate-determining and the surface hydrogen species enhance the decomposition of the methoxyl groups.

Physico-chemical characteristics and CO oxidation studies over Pd/(Mn 2O 3 + SnO 2) catalyst

The mixed oxide catalyst (Mn 2O 3 + SnO 2) prepared by the coprecipitation method has been impregnated with Pd metal and it's catalytic behaviour for CO oxidation reaction has been investigated. In the coprecipitated material, Mn 2O 3 and SnO 2 were found to crystallise at 875 K and 1175 K, respectively, which are significantly higher than the crystallisation temperatures of individual oxides prepared under similar conditions. Results of catalytic oxidation of CO, carried out using the pulse method for the mixed oxide system and the individual oxides, suggest significant synergistic effects between these two oxides. The impregnation of palladium metal facilitated CO oxidation and the catalyst Pd/(Mn 2O 3 + SnO 2) was found to be quite effective for CO oxidation even at room temperature. Further, the CO disproportionation has been observed on palladium sites in the temperature range 350 to 400 K for the individual oxide systems.

“Uphill” hydrogen diffusion effects and hydrogen diffusion coefficients in palladium

Information concerning measurements of hydrogen diffusion coefficients over α- and β-phase ranges of hydrogen content in the palladium/hydrogen system is summarized. The utilization of palladium as a hydrogen purification membrane has constituted a background for observations of “uphill” hydrogen diffusion effects. Observations of “uphill” features have been recorded for membranes in forms of both tubes and sheets. Experimental procedures have been developed to introduce hydrogen into outer (external) surfaces and to monitor hydrogen permeation through the walls by changes of either pressures of hydrogen gas within the tubular membranes or of electrode potentials at inner (opposite) surfaces. For clear observations of the “uphill” effects, both surfaces of the membranes have been coated with palladium black. Following discharges of hydrogen at the outer surfaces of membranes initially containing hydrogen, the changes of hydrogen pressure within the tubes or equivalent pressures calculated from electrode potentials, showed initial periods of decreases of the hydrogen contents at opposite surfaces. This permeation behaviour has been held accountable to expansions by hydrogen of interstitial palladium sites, that have initiated strain gradients extending from outer to inner surfaces, which then induced Gorsky effect operations. A discussion is presented concerning influences of these strain gradient considerations on forms of dependence of values of hydrogen diffusion coefficients D H, on overall hydrogen contents in palladium membranes.

FT-IR characterization of alkali-doped Pd catalysts for the selective hydrogenation of phenol to cyclohexanone

A detailed FT-IR investigation of CO, phenol and cyclohexanone adsorbed on alkali-doped Pd catalysts has been carried out in order to get information on the influence of the support and of the addition of promoters (alkali or alkaline earth metals) on the catalytic performance of Pd-supported catalysts in the selective hydrogenation of phenol. The obtained results are consistent with a mechanism that involves the reaction of phenol, adsorbed on the support as a phenolate species, located nearby Pd sites on which hydrogen is activated. The basicity of the support plays an important role in the adsorption of the phenol. Promoters have been found to modify the electronic properties of the palladium sites.

The Arsenides LnPd 3As 2 ( Ln = La-Nd, Sm, Gd) and Structure Refinement of CePd 2- xAs 2 with the ThCr 2Si 2 Structure

The title compounds were prepared in well-crystallized form by annealing the corresponding binary arsenides in a NaCl/KCl flux. The compounds Ln Pd 3As 2 crystallize with a new monoclinic structure type, which was determined from single-crystal X-ray data of GdPd 3As 2: C2/ m, a = 1656.3(6) pm, b = 404.6(2) pm, c = 993.7(4) pm, β = 107.85(2)°, Z = 6, R = 0.025 for 1728 structure factors and 58 variable parameters. These arsenides belong to a large structural family with a metal to metalloid ratio of 2:1. Somewhat unusual features in the structure of GdPd 3As 2 are the (distorted) octahedral coordination of one gadolinium site and the square-planar coordination of arsenic atoms around two palladium sites. All of these, however, are also observed for the corresponding atoms in the previously reported, closely related structure of Th 5Fe 19P 12. CePd 2- x As 2 has the tetragonal ThCr 2Si 2 structure ( a = 425.1(2)pm, c = 1026.1(6)pm, R = 0.023 for 244 F values and 11 variables) with an As-As distance of 247.1(1) pm. The refinement of the occupancy parameter of the palladium position resulted in a value of 87.9(2)% corresponding to the formula CePd 1.758(4)As 2. It is argued that the formation of these defects reduces antibonding (destabilizing) Pd-Pd interactions.

Feasibility of heavy-boson superconductivity

Deuterons are bosons and as such, under proper conditions, will Bose condense. A configuration which permits this phenomenon is described in which deuterons are weakly bound at interstitial sites of the metal palladium. It is argued that at a moderate value of the deuterium-to-palladium concentration ratio, a magnetically driven current in a single-crystal loop of this material will become “class B” superconducting at the critical temperature, T c, where “class B” denotes heavy-boson charge flow. Two distinct regimes are examined in which the deuteron kinetic energy, E, is large or small compared to the activation energy, E a. In the domain E< E a, stemming from the London criterion for Bose-condensation and the tunneling matrix element, an expression for T c is obtained which, for deuteron propagation in palladium, is noted to be infinitesimally small, rendering the proposed phenomenon unfeasible. In the domain E > E a, first-order estimates of the nearly-free-particle model, together with the London criterion, give the critical value T c≈9.1 K. It is shown that the condition E > E a may be established for sufficiently short rise time of an imposed magnetic field. A calculation is included which indicates that at these low temperatures, phonon effects in the present model are minimal. Metallurgical limitations on obtaining a single-crystal loop of palladium, with the fcc structure continuously maintained, imply the condition ϱ≳1.75 × 10 3 b, where ϱ is the radius of curvature of the palladium current loop and b is its diameter.

Synthesis of mixed-metal sulfido clusters with a cuboidal molybdenum-palladium sulfide (Mo3PdS4) core which coordinate alkene to the unique palladium site surrounded by sulfido ligands

Synthesis of mixed-metal sulfido clusters with a cuboidal molybdenum-palladium sulfide (Mo3PdS4) core which coordinate alkene to the unique palladium site surrounded by sulfido ligands

Hydrogen and oxygen at metal/oxide interfaces

Using mostly electrochemical methods the chemical potential, diffusion coefficient, partial molar volume and the resistivity increment of hydrogen were determined at room temperature in internally oxidized PdAl, PdMg, PdZn and PdZr alloys. In all samples hydrogen is trapped at the formed metal/oxide interfaces, where irreversible trapping (with respect to anodic polarization) can be distinguished from reversible one. In the cases of Pd Al 2 O 3 and Pd MgO phase boundaries the oxide precipitates were characterized by electron microscopy with regard to their size and their orientation and, therefore, the coverage of the phase boundaries with irreversibly bound hydrogen could be calculated as 1.5·10 15 H/cm 2 or 7·10 14 H/cm 2, respectively, which is about one monolayer of the densest packed oxygen planes of the corresponding oxides. This equivalence and the irreversible nature of the bonding indicates the formation of OH bonds at the phase boundaries. During irreversible trapping a larger volume change is observed as compared to dissolution of hydrogen in regular octahedral sites of palladium. The measured partial molar volumes correspond to an increase of the phase boundary “thickness” of 0.3 to 0.9 Å between Pd and oxide. The irreversible component of hydrogen trapping disapears after annealing in the vapor of the alloying addition (Al, Mg, …) or in vacuum and appears again after a second treatment in air. As an explanation of this effect we propose that annealing in metal vapor or in vacuum, respectively, changes the composition at the phase boundary. At high oxygen activities oxygen segregates to the phase boundaries. In comparison to the oxygen in the oxide the segregated one is less strongly bound and, therefore, can be desorbed at low O-activities. Furthermore the missing bonds to the stable oxide allow the formation of OH bonds.

Ab initio computations of one and two hydrogen or deuterium atoms in the palladium tetrahedral site

We report ab initio cluster computations of deuterium atoms in a tetrahedral palladium site, performed at the UHF level with extended basis sets. An interstitial deuteron is found to be energetically favored over an interstitial deuterium atom. Computations with one interstitial deuterium atom or with one deuteron reveal an increase in electron density near the deuteron. Not only valence electrons but also inner core electrons of the palladium atoms are present in the vicinity of the deuteron. A potential energy curve is calculated with two deuterium atoms approaching each other, with one deuterium fixed at the center of a tetrahedral site. With regard to the environment provided by molecular deuterium, our results show that the tetrahedral site does not favor a closer deuteron encounter. No metastable dimer geometry is found. The two deuterons repel each other despite the screening. These computations represent more than 250 CPU h of a new-generation biprocessor vector computer.

Oxidation of carbon monoxide over palladium on zirconia prepared from amorphous PdZr alloy I. Bulk structural, morphological, and catalytic properties of catalyst

Catalysts highly active for the oxidation of carbon monoxide have been prepared by oxidation of amorphous and crystalline Pd 1Zr 2 metal alloys. The oxidation was carried out in situ; i.e., the fresh metal alloys were exposed to CO-oxidation conditions at temperatures of 473–553 K. Under these conditions, the initially almost inactive metal alloys underwent a series of solid state reactions and were finally transformed into stable and highly active microporous catalysts. Bulk structural characterization using X-ray diffraction and electron microscopy revealed that the final catalysts were made up of small intergrown and poorly crystalline domains of different palladium and zirconia phases. Palladium was present in three forms in the active catalysts, as a solid solution of palladium and oxygen, as metallic palladium, and as PdO. Zirconia existed in monoclinic and tetragonal forms. Comparative kinetic measurements carried out in a differential fixed bed reactor in the temperature range 300–500 K indicated that the activity of the palladium species in the catalysts derived from the metal alloys was more than an order of magnitude higher than the activity of the palladium species in a palladium on zirconia catalyst prepared by impregnation. The comparison of the activities was based on turnover frequencies calculated on the basis of accessible palladium sites determined by CO chemisorption. The higher activity of the catalysts derived from the metal alloys is attributed to their unique structural and chemical properties leading to extremely large interfacial areas and possibly to enhanced oxygen transfer via the solid-solid interphase.

Penetration of hydrogen into subsurface sites of silica-supported palladium during temperature-programmed desorption

Penetration of hydrogen into subsurface sites of silica-supported palladium during temperature-programmed desorption

ChemInform Abstract: Preparation and Characterization of a Resin-Supported Palladium Catalyst.

The process of the ligand release and the reduction of [Pd(NH3)4]-form sulfonic resin has been studied by chemical analysis, X-ray photoelectron spectroscopy and adsorption techniques. The removal of ammonia ligands remarkably increased with hydrogen treatment above 353 K, and was complete at around 393 K. It is suggested that the released ammonia partly transfers to –SO3− groups to form –SO3NH4 groups as the reduction proceeds. The dispersion of the metal particles prepared by hydrogen reduction ranged from 0.11 to 0.31 depending on the loading concentration of the palladium cation. The metal particles are considered to be entrapped in the pores of the resin. The formation of cationic palladium sites by the low temperature reduction is also suggested.

Preparation and Characterization of a Resin-Supported Palladium Catalyst

Abstract The process of the ligand release and the reduction of [Pd(NH3)4]-form sulfonic resin has been studied by chemical analysis, X-ray photoelectron spectroscopy and adsorption techniques. The removal of ammonia ligands remarkably increased with hydrogen treatment above 353 K, and was complete at around 393 K. It is suggested that the released ammonia partly transfers to –SO3− groups to form –SO3NH4 groups as the reduction proceeds. The dispersion of the metal particles prepared by hydrogen reduction ranged from 0.11 to 0.31 depending on the loading concentration of the palladium cation. The metal particles are considered to be entrapped in the pores of the resin. The formation of cationic palladium sites by the low temperature reduction is also suggested.

The Measurement of the Dispersion of Pd/C Catalysts

AbstractA 10 w/o Pd/C catalyst from Johnson Matthey has been examined by three different techniques: chemisorption, line broadening of X‐ray diffraction, and TEM (transmission electron microscopy) to investigate the size of the palladium crystallites. The dispersion of this catalyst was determined to be 20%. Among these techniques, the X‐ray diffraction was found to be the most convenient method. Through this method, the dispersion of Pd was found to decrease on catalyzing hydroxylamine reactions at 330 K. The deactivation of Pd/C for these reactions was correlated to the sintering of palladium. H2 chemisorption at 373 K was found to be a good way to accurately measure the number of active palladium sites in the Pd/C catalysts.

Skeletal rearrangement of hydrocarbons on the rare earth mixed valence intermetallic compound CePd 3

The skeletal rearrangements (isomerization, hydrogenolysis, dehydrocyclization) of methylcyclopentane, 2-methylpentane, and 3-methylhexane have been studied on the intermetallic compound CePd 3 and compared with results obtained on classical supported palladium catalysts prepared by impregnation. CePd 3 exhibited a very low activity; the reaction mechanisms involved were quite different from what is known of palladium catalysts and they were attributed instead to cerium atoms present in an oxidized form. However, an activation with air at room temperature or at 120 °C dramatically increased the catalytic activity and the reactions involved were characteristic of palladium atoms, with a surprising selectivity toward aromatization and 1–5 ring closure. This enhancement was interpreted as due to a complete rearrangement of the surface, yielding active palladium sites on the surface. This phenomenon was related to the specific properties of cerium atoms which can undergo fast fluctuations of valence as revealed by photoelectron spectroscopy.