Size effects in the CO-oxidation over supported Pd particles

Low pressure CO oxidation was studied over small supported Pd particles by means of static secondary ion mass spectrometry (SSIMS). The SSIMS spectra of adsorbed layers on Pd particles with diameters below ～7nm contained, besides other species, Pd _n X⁺ ions (X=C, O, CO;n=1–3). In experiments with Pd films, Pd _n C⁺ species were much less abundant or even absent from the mass spectra. We conclude that CO decomposed in appreciable amounts only on Pd particles. No significant influence of the support material (mica, MgO,α-Al₂O₃) was seen and posthumous TEM analysis revealed well defined particle morphologies with largely (111) and (100) facets. At total pressures below 10^?4 Pa, the relative carbidic intensities Pd _n C⁺/Pd _n ⁺ , which were taken as proportional to the carbon surface concentrations, first increased with increasing temperatures and then decreased after they passed a maximum at T=400–440 K. The smaller the particle size, the more intense were the maxima. For T ?440 K, carbon was removed from the surface via reaction with oxygen. Strong hysteresis effects in the C_ad and CO_ad concentrations occurred during temperature variation measurements. The Pd _n C⁺/Pd _n ⁺ ratios displayed maxima during heating but not during cooling. While the Pd _n CO⁺/Pd _n ⁺ ratios were always high at 300 K and always low at T>550 K, they were more intense in the medium temperature range when subjecting the samples to heating. These hysteresis effects are discussed in terms of asymmetric adsorption and reaction behaviour of CO/O₂ mixtures, involving inhibition of CO₂ formation by adsorbed carbon and carbon monoxide.     

Density functional study on structures, stabilities, and electronic properties of size-selected Pd n Si q (n = 1–7 and q = 0, +1, −1) clusters

The density functional theory (DFT) calculations within the framework of generalized gradient approximation have been employed to systematically investigate the geometrical structures, stabilities, and electronic properties of Pd _n Si ^q (n = 1–7 and q = 0, +1, ?1) clusters and compared them with the pure ${\text{Pd}}_{n + 1}^{q}$ (n = 1–7 and q = 0, +1, ?1) clusters for illustrating the effect of doping Si atom into palladium nanoclusters. The most stable configurations adopt a three-dimensional structure for both pure and Si-doped palladium clusters at n = 3–7. As a result of doping, the Pd _n Si clusters adopt different geometries as compared to that of Pd _n+1. A careful analysis of the binding energies per atom, fragmentation energies, second-order difference of energies, and HOMO–LUMO energy gaps as a function of cluster size shows that the clusters ${\text{Pd}}_{4}^{ + }$ , ${\text{Pd}}_{4}$ , ${\text{Pd}}_{8}^{ - }$ , ${\text{Pd}}_{5} {\text{Si}}^{0, + , - }$ , and ${\text{Pd}}_{7} {\text{Si}}^{0, + , - }$ possess relatively higher stability. There is enhancement in the stabilities of palladium frameworks due to doping with an impurity atom. In addition, the charge transfer has been analyzed to understand the effect of doped atom and compared further.     

Adsorption-Induced Segregation as a Method for the Target-Oriented Modification of the Surface of a Bimetallic Pd–Ag Catalyst

The effect of palladium segregation was studied which resulted from the effect of CO and O₂ on the surface structure and catalytic characteristics of the Pd–Ag₂/Al₂O₃ catalyst. The IR-spectroscopic study of adsorbed CO showed that Pd₁ centers isolated from each other by silver atoms predominated on the surface of reduced Pd–Ag₂/Al₂O₃, as evidenced by the almost complete absence of absorption bands typical for the multicentred CO adsorption. In the course of catalyst treatment with CO and O₂, the intensity of absorption bands characteristic of the multicenter CO adsorption considerably increased due to the transformation of a portion of monatomic Pd₁ centers into multiatomic Pd_n ones as a result of the surface segregation of Pd. In this case, a substantial increase in the catalyst activity in the liquid-phase hydrogenation of diphenylacetylene was observed. It was established that, after treatment with CO, the catalyst selectivity for the formation of a target olefin (stilbene) remained almost constant, whereas the treatment with O₂ led to a decrease in the selectivity because of more considerable surface modification.     

Effects of precipitants and surfactants on catalytic activity of Pd/CeO<Subscript>2</Subscript> for CO oxidation

A series of precipitants and commercial surfactants (soft templates) were employed to synthesize mesoporous/nano CeO₂ by a hydrothermal method. As-prepared CeO₂ was impregnated with palladium and employed for low-temperature catalytic oxidation of CO. It was found that both soft templates and precipitants had significant effects on the morphology, particle size, crystallinity, and porous structure of the CeO₂, having a significant effect on the surface palladium abundance, molar ratios of surface species, and catalytic activity of the final impregnated Pd/CeO₂. Using ammonia as precipitant could facilitate increased surface palladium abundance and surface molar ratios of PdO/Pd _SMSI , Ce³⁺/(Ce³⁺ + Ce⁴⁺), and O_surface/O_lattice. The catalytic activity of the final Pd/CeO₂ catalysts could be enhanced as well. The optimal P123-assisted ammonia-precipitated Pd/CeO₂ catalyst exhibited over 99% catalytic conversion of CO at 50 °C.     

Synthesis and structure of polynuclear carbonylphosphine clusters of palladium

Reductive condensation of Pd(OAc)₂ in dioxane in the presence of CO and PR₃ (R = Et, Buⁿ) with addition of CF₃COOH leads to the formation of decanuclear Pd₁₀(μ₃-CO)₄(μ₂-CO)₈(PBuⁿ₃)₆ (I) and Pd₁₀(CO)₁₄(PBuⁿ₃) (II) at Pd(OAc)₂:PR₃ molar ratios of 1:4–1:10 and 1:1.5–1:2.5, respectively. The use of CH₃COOH instead of CF₃COOH results in tetranuclear clusters Pd₄(CO)₅(PR₃)₄ (III) and Pd₄(μ₂-CO)₆(PBuⁿ₃) (IV). I ? III and III → IV transformations occur in organic media. The structures of I (space group P2₁/n, Z = λMo, 12125 independent reflections, R = 0.047) and IV (Pz:3, Z = λMo, 3254 reflections, R = 0.098) were established by X-Ray diffractions analysis. Cluster I is a 10-vertex Pd₁₀ polyhedron, an octahedron with four unsymmetrically centered non-adjacent faces. The average PdPd distances in the octahedron are 2.825 Å, in the eight short Pd_oct.Pd_cap. bonds with the “equatorial” Pd atoms of the inner octahedron, bridged by the μ₂-CO ligands, are 2.709 Å, and in the four elongated (without bridging CO groups) bonds with the apical Pd atoms of the octahedron are 3.300–3.422 Å. The PBuⁿ₃ ligands are coordinated to the apical Pd atoms and the capping atoms (PdP 2.291–2.324 Å). Cluster IV is tetrahedral, with the CO ligands symmetrically bridged; PdPd 2.778–2.817; PdP 2.232–2.291; PdC 2.06 Å (average).     

Two isomers of Pd2(S2NC7H4)4

The dichloromethane solvates of the isomers tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁴N:S;κ⁴S:N‐dipalladium(II)(Pd—Pd), (I), and tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁶N:S;κ²S:N‐dipalladium(II)(Pd—Pd), (II), both [Pd₂(C₇H₄NS₂)₄]·CH₂Cl₂, have been synthesized in the presence of (o‐isopropylphenyl)diphenylphosphane and (o‐methylphenyl)diphenylphosphane. Both isomers form a lantern‐type structure, where isomer (I) displays a regular and symmetric coordination and isomer (II) an asymmetric and distorted structure. In (I), sitting on an centre of inversion, two 1,3‐benzothiazole‐2‐thiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the other two benzothiazolethiolate units are bonded to the same Pd atoms by, respectively, a Pd—S and a Pd—N bond. In (II), three benzothiazolethiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the fourth benzothiazolethiolate unit is bonded to the same Pd atoms by, respectively, a Pd—S bond and a Pd—N bond.     

Influence of CeO2 addition on the physicochemical and catalytic properties of Pd/TiO2 catalysts in CO oxidation

The influence of CeO₂ addition on the formation of the microstructure, electronic state, and catalytic properties of Pd/TiO₂ supported catalysts in CO oxidation were investigated. It was shown that, when Pd is supported on titanium dioxide modified with cerium dioxide, annealing at 500°C results in the formation of Pd/(CeO₂-TiO₂) catalysts with a nanocrystalline structure composed of incoherently intergrown fine anatase crystals and interblock boundaries in which palladium and cerium are stabilized. The higher catalytic activity of Pd/(CeO₂-TiO₂) catalysts compared to Pd/TiO₂ catalysts is explained by the smaller size of Pd particles and the higher proportion of palladium in the Pd^δ+ state.     

Investigation of the state of palladium in the Pd/SO4/ZrO2 system by diffuse-reflectance IR spectroscopy

The Pd/ZrO₂ and Pd/SO₄/ZrO₂ systems were investigated by diffuse-reflectance IR spectroscopy using CO as the probe molecule. For the Pd/ZrO₂ system, the behavior of the metal is characteristic of the weak metal-support interaction. Intense bands attibuted to the bridging CO species indicate the formation of large metal particles in the reduced systems. Modification of the ZrO₂ support with SO₄ ^2? anions leads to an increase in the metal—support interaction and makes the metal more resistant to reduction. On the surface promoted by SO₄ ^2? anions, metal particles with a positive charge (Pd⁺ and Pd^δ+) were observed. The smaller the size of the metal clusters and the higher degree of oxidation of sulfur, the stronger the influence of acidic protons and surface sulfur compounds on the metal.     

Low-temperature oxidation of carbon monoxide over (Mn1 − x M x )O2 (M = Co, Pd) catalysts

(Mn_{1 ? x} M _x )O₂ (M = Co, Pd) materials synthesized under hydrothermal conditions and dried at 80°C have been characterized by X-ray diffraction, diffuse reflectance spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, and adsorption and have been tested in CO oxidation under CO + O₂ TPR conditions and under isothermal conditions at room temperature in the absence and presence of water vapor. The synthesized materials have the tunnel structure of cryptomelane irrespective of the promoter nature and content. Their specific surface area is 110–120 m²/g. MnO₂ is morphologically uniform, and the introduction of cobalt or palladium into this oxide disrupts its uniformity and causes the formation of more or less crystallized aggregates varying in size. The (Mn,Pd)O₂ composition contains Pd metal, which is in contact with the MnO₂-based oxide phase. The average size of the palladium particles is no larger than 12 nm. The initial activity of the materials in CO oxidation, which was estimated in terms of the 10% CO conversion temperature, increases in the following order: MnO₂ (100°C) < (Mn,Co)O₂ (98°C) < (Mn,Co,Pd)O₂ (23°C) < (Mn,Pd)O₂ (?12°C). The high activity of (Mn,Pd)O₂ is due to its surface containing palladium in two states, namely, oxidized palladium (interaction phase) palladium metal (clusters). The latter are mainly dispersed in the MnO₂ matrix. This catalyst is effective in CO oxidation even at room temperature when there is no water vapor in the reaction mixture, but it is inactive in the presence of water vapor. Water vapor causes partial reduction of Mn⁴⁺ ions and an increase in the proportion of palladium metal clusters.     

Sorption of aqueous palladium onto synthetic hydroxyapatite

The sorption of Pd(II) on hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) has been studied at 25 °C as a function of pH, in 0.01 M NaClO₄, and 0.01 and 0.025 M Ca(ClO₄)₂ aqueous background electrolytes and Pd(II) concentration (9.3 to 47 ??M), trying to minimize some types of reactions, such as solid dissolution of and metal precipitation. The radiotracer palladium, ¹⁰⁹Pd, obtained by neutron irradiation, has been used to calculate the palladium??s distribution coefficients K _d between aqueous and solid phase. A mathematical treatment of results has been made by ion-exchange theory in order to interpret palladium sorption onto treated solid. For this, we take into account the existence of active sites at the hydroxyapatite surface, and the aqueous solution chemistry of palladium as well as the effect of phosphate anions from solid dissolution. The results can be explained as evidence of sorption of the species PdOH⁺, and of a mixed hydroxo complex of Pd²⁺ like (XCaO^?)?CPdOH⁺·nH₂O fixed onto {??Ca?COH} surface sites of the hydroxyapatite.     

Coordination Chemistry of Cyclopropenylidene‐Stabilized Phosphenium Cations: Synthesis and Reactivity of Pd and Pt Complexes

A straightforward synthesis of cyclopropenylidene‐stabilized phosphenium cations 1 a – g through the reaction of [(iPr₂N)₂C₃⁺Cl]BF₄ with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L₂](BF₄)₂ and electrochemical methods. The cyclopropenium ring induces a phosphite‐type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a – g still act as two‐electron donor ligands, forming adducts with Pd^II and Pt^II precursors. Conversely, in the presence of Pd⁰ species, an oxidative insertion of the Pd atom into the C_carbene–phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of Pt^II complexes was tested; all of the cationic phosphines accelerated the prototype 6‐endo‐dig cyclization of 2‐ethynyl‐1,1′‐biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A.     

Self‐Assembly Film of Zeolite Y Nanocrystals Loading Palladium on an Au Electrode for Electrochemical Applications

Nano‐scale zeolite Y crystals were synthesized, and palladium nanoparticles were prepared in the supercage of the zeolite by “ship‐in‐a‐bottle” approach. A novel method to fabricate zeolite‐modified electrode (ZME) loading Pd nanoparticles was developed, in which the zeolite Y loading Pd²⁺ ions was self‐assembled on (3‐mercaptopropyl) trimethoxysilane‐attached Au surface to form the stable and density packed multilayers (SAM‐ZME). The structures of zeolite Y and the SAM‐ZME were investigated by using TEM, XRD and SEM techniques. Pd²⁺ ions in the SAM‐ZME were converted into Pd nanoparticles (Pd_n⁰) by two steps consisting of the electrochemical reduction as well as the succeeding admission and release of CO. The redox couple [Fe(CN)₆]^3?/4? was used to probe the electron‐transfer barrier properties during self‐assembling process. Moreover, the special properties of the SAM‐ZME loading Pd_n⁰ were studied by using cyclic voltammetry and CO‐probe in situ FTIR spectroscopy. The results illustrated that Pd_n⁰ in the SAM‐ZME exhibits higher electrocatalytic activity for oxidation of adsorbed CO than that of ZME prepared in our previous study by zeolite coating method. The present study is of importance in design and preparation of SAM‐ZME, which poccesseses excellent properties for the immobilization of electrocatalysts or biomolecules.     

Synthesis and structure of the new heteronuclear palladium-mercury cluster

Interaction of the tetra-nuclear phosphinecarbonyl complex of zero-valent palladium, Pd₄(CO)₅(PEt₃)₄ (I), with a four-fold excess of 8-(α-bromomercuryethyl)quinoline gives a new neutral heteronuclear cluster Pd₄Hg₂Br₂(CO)₄(PEt₃)₄ (IV). The structure of (IV) was determined by an X-ray analysis. The molecule of (IV) contains a “butterfly” Pd₄(μ₂-CO)₄(PEt₃)₄ moiety, whose triangular “wings” are capped by Hg atoms bonded also to bromo-ligands. The metal Hg₂Pd₄ polyhedron consists of two heteroatomic Pd₃Hg tetrahedra with a common PdPd edge. The IR- and ³¹P-NMR spectra of (IV) were also studied.     

Novel mixed-ligand palladium complexes [Pd2(acac)3NO3] and [Pd(acac)NO3]n involving O,O- and γ-C-bonded acetylacetonate linkers

The reaction of the palladium nitrate trans-[Pd(NO₃)₂(H₂O)₂] with acetylacetone affords mononuclear [Pd(acac)₂] (acac = acetylacetonate), mixed-ligand binuclear [Pd₂(acac)₃NO₃] (1) and polynuclear [Pd(acac)NO₃]_n (2) complexes depending on the Pd:acetylacetone ratio in the reaction mixture. The binuclear 1 and insoluble polynuclear 2 were isolated and studied by single-crystal X-ray diffraction (1) and solid-state ¹³C MAS NMR (1 and 2). It was found that in both compounds the Pd ions are linked together through bridging acetylacetonate ligands where one metal atom is connected to the usual O,O-donor sites, whereas the other metal atom forms a bond with the γ-carbon center. Based on a topological quantum-chemical method, the Pd-γ-C bond was classified as a strained dative bond.     

Palladium clusters: Stoichiometric and catalytic reactions

The mechanistic aspects of organic reactions catalyzed with palladium clusters and stoichiometric reactions of carbonyl and carbene clusters are discussed. Palladium carbonyl carboxylates Pd₄(CO)₄ (OCOR)₄(RMe, CMe₃, Ph, CF₃, CCl₃) undergo thermolysis above 110–130°C, giving rise to CO₂, CO and diacyls. In solutions of aromatic compounds the insertion of carbon dioxide into the aromatic CH bond or activated CH bond of alkylaromatic compounds was observed in the course of the thermolysis. The decomposition of palladium carbene carboxylate cluster Pd₄(Ph₂C)₄(OAc)₄ at 80°C has been found to involve inner sphere carbene oxidation during which an oxygen atom is transferred from the carboxylate group to the carbene ligand. Analogously, the reaction of [Pd(OAc)₂PPh₃]₂ with formic acid, a reaction involving intermediate cluster formation, includes the transfer of an oxygen atom from the formate droup to the P atom of a phosphorus containing ligand, supposedly a diphenylphosphido bridging group. Positional and geometric α-alkene isomerization in aqueous PdCl²⁻₂ solution has been found to be catalyzed by palladium (I) complexes of type Pd₂Cl²⁻₄. Colloidal clusters containing more than 500 palladium atoms in the metal core, which are soluble in polar organic solvents, have been found to catalyze the oxidative reactions of alkenes, toluene, alcohols and formic acid. Alcohols bearing at least one hydrogen atom in α-position undergo dehydration under mild conditions in non-acidic solution containing a Pd, Mo octanuclear anionic cluster [Pd₄Mo₄(CO)₁₂Cp]²⁻. The reaction of benzyl alcohol gives rise to trans-stilbene. All these catalytic reactions can be rationalized within a scheme including the oxidative addition of the substrates across cluster metal-metal bond as a key step.     

Reactivity of Pd3(dppm)3(CO)n+ and Pd3(dppm)3(CO)(RCCR)n+ (n?=?0, +1, +2) Towards F?. Evidence of Reactive Intermediates and X-Ray Structure of [Pd3(dppm)3(MeO2CC≡CCO2Me)(F)]PF6

Abstract  

The reactivity of the trinuclear palladium cluster [Pd₃(dppm)₃(CO)] ⁿ⁺ (dppm = bis(diphenylphosphinomethane); n = 2, 1) towards F⁻ was investigated by electrochemical and spectroscopic methods. The reaction depends on the charge of the cluster. The chemical reduction of the cluster dication is observed in the presence of F⁻ generating the paramagnetic monocationic cluster. Spin-trapping experiments with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) provided evidence for the radical F^• as an intermediate. In a similar manner to the dication, the monocationic cluster [Pd₃(dppm)₃(CO)]⁺ is also reduced, but in a slower process, by the F⁻ ion to produce [Pd₃(dppm)₃(CO)]⁰. Additionally, the alkyne cluster adducts [Pd₃(dppm)₃(CO)(RCCR)] ⁿ⁺ (n = 2, 1; R = CO₂Me) are also reactive towards F⁻. Particularly, the dication adduct leads to a metastable fluoride adduct [Pd₃(dppm)₃(CO)(RCCR)(F)]⁺. The electroreductive behavior of this adduct involves electron-transfer steps and F⁻ exchange equilibriums, for which digital simulation enables the extraction of the thermodynamic parameters (standard potentials and equilibrium constants). Concurrently, the monocation adduct [Pd₃(dppm)₃(CO)(RCCR)]⁺ with F⁻, leads to a disproponation generating 0.5 equiv. of [Pd₃(dppm)₃(CO)(RCCR)(F)]⁺ and 0.5 equiv. of [Pd₃(dppm)₃(CO)(RCCR)]⁰. The former slowly evolves to [Pd₃(dppm)₃(RCCR)(F)]⁺, which was described by X-ray diffraction method.     

Activation of the C-O bond on the surface of palladium: An <Emphasis Type="Italic">In situ</Emphasis> study by X-ray photoelectron spectroscopy and sum frequency generation

An in situ study of the adsorption of CO on atomically smooth and defect Pd(111) surfaces was performed over wide ranges of temperatures (200–400 K) and pressures (10^–6-1 mbar) by X-ray photoelectron spectroscopy and sum frequency generation. Both of the techniques indicated that CO was adsorbed as three-fold hollow, bridging, and terminal species to form well-known ordered structures on the surface. In the course of the in situ experiments, no signs of CO dissociation or of the formation of carbonyl structures (Pd(CO)_n, n > 1) were detected. The mechanism of C-O bond activation in the course of methanol decomposition on the surface of palladium was considered. It was found that the adsorption of methanol on the surface of palladium essentially depends on pressure. Along with the well-known reaction path of methanol dehydrogenation to CO and hydrogen, a slow process of methanol decomposition with C-O bond cleavage was observed at elevated pressures. In this case, the formation of carbon deposits finally resulted in the carbonization and complete deactivation of the surface. A mechanism for C-O bond activation on the surface of palladium was proposed; the geometry of adsorption complexes plays an important role in this mechanism.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 288–301.Original Russian Text Copyright © 2005 by Kaichev, Bukhtiyarov, Rupprechter, Freund.     

Preparation of the Pd/C catalysts: A molecular-level study of active site formation

This review summarizes the results of molecular-level studies on the mechanism of Pd/C catalyst formation from the PdCl₂ precursor. Two processes occur in acidic media during the contact of H₂PdCl₄ with carbon: (a) adsorption of palladium chloride to form surface complexes and (b) redox interaction between PdCl₂ and carbon with the formation of palladium metal particles. The ratio between these adsorbed palladium species depends on the conditions of adsorption and especially on the size of carbon support grains and the oxidative atmosphere. The observations are explained by the fact that carbon support exhibits electrochemical and ligand properties. X-ray diffraction, X-ray scattering, XPS, and high-resolution electron microscopy revealed that the nanostructure of carbon materials, in particular the extent of their three-dimensional ordering, is crucial for the ligand properties. The presence of two forms, metallic and ionic, of sorbed palladium determines the bimodal size distribution of the metal. After the reduction of ionic species, metal particles are “blocked” with support. The nature of the ionic forms of palladium (mostly (PdCl₂)_n) clusters chemically and epitaxially bound to the carbon surface suggests the mechanisms of the bimodal distribution of the supported metal particles on the surface and the methods for the control of the ratio between “blocked,” low-dispersed, and highly-dispersed particles in the catalyst. One of these methods is the use of palladium polynuclear hydroxo complexes (PHCs) with low oxidation potentials as starting compounds for catalysts preparation. The data on the PHC structure in a solution and its change upon the adsorption of PHC on the surface of the carbon material obtained by the¹⁷O,²³Na,¹³³Cs, and³⁵Cl NMR techniques are discussed. PHCs are shown to be a clew of the [Pd(OH)₂]_n polymeric filament, whose fractions are bound with alkali metal ions. When PHC is adsorbed on the surface of the carbon support and then dried, palladium oxide is formed from which highly dispersed metal particles are formed during reduction. The nature of alkali metal ions in PHC affects the activity of the Pd/C catalyst. An important role of the ligand, electrochemical, and lyophilic properties of carbon material during the formation of the species of the active catalyst component is discussed.     

Laser ablation doping of poly crystalline tin dioxide films with palladium

Laser ablation of palladium was studied and velocity (energy) distributions of palladium ions evaporated by an Kr-F laser in a vacuum were obtained. The optimum values of energy fluence (fluence rate) of laser radiation for doping tin dioxide films, at which neither multiply charged Pd^N+ ions nor ionized clusters Pd _N ⁺ , occur in a plasma, were determined. From time-of-flight probe measurement data, Pd⁺ implantation depths in SnO₂ films were calculated, which qualitatively agree with the results obtained by secondary neutral mass spectrometry. Electric conductivity measurements on the obtained films in a gas phase showed that introduction of palladium into polycrystalline SnO₂ films by laser ablation significantly enhanced their gas sensitivity to hydrogen.     

Hydrodechlorination of 4-Chlorophenol on Pd-Fe Catalysts on Mesoporous ZrO2SiO2 Support

A mesoporous support based on silica and zirconia (ZS) was used to prepare monometallic 1 wt% Pd/ZS, 10 wt% Fe/ZS, and bimetallic FePd/ZS catalysts. The catalysts were characterized by TPR-H₂, XRD, SEM-EDS, TEM, AAS, and DRIFT spectroscopy of adsorbed CO after H₂ reduction in situ and tested in hydrodechlorination of environmental pollutant 4-chlorophelol in aqueous solution at 30 °C. The bimetallic catalyst demonstrated an excellent activity, selectivity to phenol and stability in 10 consecutive runs. FePd/ZS has exceptional reducibility due to the high dispersion of palladium and strong interaction between FeOx and palladium, confirmed by TPR-H₂, DRIFT spectroscopy, XRD, and TEM. Its reduction occurs during short-time treatment with hydrogen in an aqueous solution at RT. The Pd/ZS was more resistant to reduction but can be activated by aqueous phenol solution and H₂. The study by DRIFT spectroscopy of CO adsorbed on Pd/ZS reduced in harsh (H₂, 330 °C), medium (H₂, 200 °C) and mild conditions (H₂ + aqueous solution of phenol) helped to identify the reasons of the reducing action of phenol solution. It was found that phenol provided fast transformation of Pd⁺ to Pd⁰. Pd/ZS also can serve as an active and stable catalyst for 4-PhCl transformation to phenol after proper reduction.