Key Role of a-Top CO on Terrace Sites of Metallic Pd Clusters for CO Oxidation

Pd-based catalysts are the most widely used for CO oxidation because of their outstanding catalytic activity and thermal stability. However, fundamental understanding of the detailed catalytic processes occurring on Pd-based catalysts under realistic conditions is still lacking. In this study, we investigated CO oxidation on metallic Pd clusters supported on Al₂O₃ and SiO₂. High-angle annular dark-field scanning transmission electron microscopy revealed the formation of similar-sized Pd clusters on Al₂O₃ and SiO₂. In contrast, CO chemisorption analysis indicated a gradual change in the dispersion of Pd (from 0.79 to 0.2) on Pd/Al₂O₃ and a marginal change in the dispersion (from 0.4 to 0.24) on Pd/SiO₂ as the Pd loading increased from 0.27 to 5.5 wt %; these changes were attributed to differences in the metal-support interactions. Diffuse reflectance infrared Fourier-transform spectroscopy revealed that fewer a-top CO species were present in Pd supported on Al₂O₃ than those in Pd supported on SiO₂, which is related to the morphological differences in the metallic Pd clusters on these two supports. Despite the different dispersion profiles and surface characteristics of Pd, O₂ titration demonstrated that linearly bound CO (with an infrared signal at 2090 cm⁻¹) reacted first with oxygen in the case of CO-saturated Pd on Al₂O₃ and SiO₂, which suggests that a-top CO on the terrace site plays an important role in CO oxidation. The experimental observations were corroborated by periodic density functional calculations, which confirmed that CO oxidation on the (111) terrace sites is most plausible, both kinetically and thermodynamically, compared to that on the edge or corner sites. This study will deepen the fundamental understanding of the effect of Pd clusters on CO oxidation under reaction conditions.     

Palladium particles growth on various aluminas

Pd was deposited from a filament onto anα-Al₂O₃ single-crystal. The Pd 3dP ^5/2 binding energy change versus deposition time was used as a calibration of the Pd clusters size. The experiment was then reproduced onγ-Al₂O₃. The Pd 3d ^5/2 binding energy showed that the clusters growth mechanism was now different. Thermal treatments produced a dispersion of the metal or a clusters size increase depending on the temperature or the nature of the support. Oxygen exposure dispersed Pd in every case. A mechanism is proposed to interpret this phenomenon.     

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)₂ and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd₃P_0.8 → Pd₅P₂ → PdP₂. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.     

Factors determining the chemoselectivity of phosphorus-modified palladium catalysts in the hydrogenation of chloronitrobenzenes

The precursor nature effect on the state of the Pd–P surface layer in palladium catalysts and on their properties in the liquid-phase hydrogenation of chloronitrobenzenes under mild conditions has been investigated. A general feature of the Pd–P-containing nanoparticles obtained from different precursors and white phosphorus at P/Pd = 0.3 (PdCl₂ precursor) and 0.7 (Pd(acac)₂ precursor) is that their surface contains palladium in phosphide form (BE(Pd3d _5/2) = 336.2 eV and BE(Р2р) = 128.9 eV) and Pd(0) clusters (BE(Pd3d_5/2) = 335.7 eV). Factors having an effect on the chemoselectivity of the palladium catalysts in chloronitrobenzenes hydrogenation are considered, including the formation of small palladium clusters responsible for hydrogenation under mild conditions.     

l‐Arginine‐Triggered Self‐Assembly of CeO2 Nanosheaths on Palladium Nanoparticles in Water

Pd@CeO₂ core–shell nanostructures with a tunable Pd core size, shape, and nanostructure as well as a tunable CeO₂ sheath thickness were obtained by a biomolecule‐assisted method. The synthetic process is simple and green, as it involves only the heating of a mixture of Ce(NO₃)₃, l ‐arginine, and preformed Pd seeds in water without additives. Importantly, the synthesis is free of thiol groups and halide ions, thus providing a possible solution to the problem of secondary pollution by Pd nanoparticles in the sheath‐coating process. The Pd/CeO₂ nanostructures can be composited well with γ‐Al₂O₃ to create a heterogeneous catalyst. In subsequent tests of catalytic NO reduction by CO, Pd@CeO₂/Al₂O₃ samples based on Pd cubes (6, 10, and 18 nm), Pd octahedra (6 nm), and Pd cuboctahedra (9 nm) as well as a simply loaded Pd cube (6 nm)–CeO₂/Al₂O₃ sample were used as catalysts to investigate the effects of the Pd core size and shape and the hybrid nanostructure on the catalytic performance.     

Photochemistry of 1,2-diphenyl-2,2-dimethyl-propanone-1 in micellar solutions. Cage effects,isotope effects and magnetic field effects.

The cage reaction resulting from photolysis of PhCOC (CH₃)₂Ph and PhCOC(CD₃)₂Ph in micellar solution is shown to be subject to substantial magnetic field and magnetic isotope effects.     

The vibrational spectra of complexes with planar monothio-oxamides—I. The Ni(II), Pd(II), Pt(II), Cu(II) and Zn(II) complexes of N,N′-dimethylmonothio-oxamide

The new complexes M(LH)₂ (M = Pd,Pt), ML(M = Pd,Cu) and ML · H₂O (M = Ni,Zn), where LH₂ = N,N′-dimethylmonothio-oxamide, have been prepared. The complexes were characterized by metal analyses, thermal methods and spectral (i.r., Raman, u.v.—vis.) studies. The vibrational analyses of the complexes are given using NH/ND, CH₃/CD₃ and metal isotopic substitutions. The Ni(II), Pd(II), Pt(II) and Cu(II) compounds are square planar. The monoanion LH⁻ shows a chelated bidentate S,O-coordination, while the doubly deprotonated L²⁻ acts as a bridging S,N/N,O-tetradentate ligand giving polymeric structures.     

Density functional study of AgnPd and AgnPdH clusters

Small Ag_nPd (n = 5) clusters and their hydrides Ag_nPdH (n = 5) have been studied by density functional theory calculations. For bare clusters, the structures in which the Pd atom has a maximum number of neighboring Ag atoms tend to be energetically favorable. Hydrogen prefers binding to Ag? Pd bridge site of Ag_nPd clusters except for Ag₅Pd. The binding energy has a strong odd–even oscillation. The electron transfers are from Ag atoms to Pd in bare clusters and are from metal clusters to H in cluster hydrides. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ligand-Protected Ultrasmall Pd Nanoclusters Supported on Metal Oxide Surfaces for CO Oxidation: Does the Ligand Activate or Passivate the Pd Nanocatalyst?

Herein, we report on the synthesis of ultrasmall Pd nanoclusters (∼2 nm) protected by L-cysteine [HOCOCH(NH₂)CH₂SH] ligands (Pd_n(L-Cys)_m) and supported on the surfaces of CeO₂, TiO₂, Fe₃O₄, and ZnO nanoparticles for CO catalytic oxidation. The Pd_n(L-Cys)_m nanoclusters supported on the reducible metal oxides CeO₂, TiO₂ and Fe₃O₄ exhibit a remarkable catalytic activity towards CO oxidation, significantly higher than the reported Pd nanoparticle catalysts. The high catalytic activity of the ligand-protected clusters Pd_n(L-Cys)_m is observed on the three reducible oxides where 100 % CO conversion occurs at 93–110 °C. The high activity is attributed to the ligand-protected Pd nanoclusters where the L-cysteine ligands aid in achieving monodispersity of the Pd clusters by limiting the cluster size to the active sub-2-nm region and decreasing the tendency of the clusters for agglomeration. In the case of the ceria support, a complete removal of the L-cysteine ligands results in connected agglomerated Pd clusters which are less reactive than the ligand-protected clusters. However, for the TiO₂ and Fe₃O₄ supports, complete removal of the ligands from the Pd_n(L-Cys)_m clusters leads to a slight decrease in activity where the T_100% CO conversion occurs at 99 °C and 107 °C, respectively. The high porosity of the TiO₂ and Fe₃O₄ supports appears to aid in efficient encapsulation of the bare Pd_n nanoclusters within the mesoporous pores of the support.     

Pd Loaded NiCo Hydroxides for Biomass Electrooxidation: Understanding the Synergistic Effect of Proton Deintercalation and Adsorption Kinetics

The key issue in the 5-hydroxymethylfurfural oxidation reaction (HMFOR) is to understand the synergistic mechanism involving the protons deintercalation of catalyst and the adsorption of the substrate. In this study, a Pd/NiCo catalyst was fabricated by modifying Pd clusters onto a Co-doped Ni(OH)₂ support, in which the introduction of Co induced lattice distortion and optimized the energy band structure of Ni sites, while the Pd clusters with an average size of 1.96 nm exhibited electronic interactions with NiCo support, resulting in electron transfer from Pd to Ni sites. The resulting Pd/NiCo exhibited low onset potential of 1.32 V and achieved a current density of 50 mA/cm² at only 1.38 V. Compared to unmodified Ni(OH)₂, the Pd/NiCo achieved an 8.3-fold increase in peak current density. DFT calculations and in situ XAFS revealed that the Co sites affected the conformation and band structure of neighboring Ni sites through CoO₆ octahedral distortion, reducing the proton deintercalation potential of Pd/NiCo and promoting the production of Ni³⁺−O active species accordingly. The involvement of Pd decreased the electronic transfer impedance, and thereby accelerated Ni³⁺−O formation. Moreover, the Pd clusters enhanced the adsorption of HMF through orbital hybridization, kinetically promoting the contact and reaction of HMF with Ni³⁺−O.     

The vibrational spectra of complexes with planar monothio-oxamides—II. Complexes of N(s)-methylmonothio-oxamide

The new complexes Pd(SH₂)₂ · H₂O, M(SH) · H₂O (M = Ni, Pd, Pt) and M(SH) · 0.5H₂O (M = Cu, Zn), where SH₃ = N_(s)-methylmonothio-oxamide, have been prepared. The complexes were characterized by metal analyses, thermal methods and i.r., Raman and u.v./vis spectroscopic studies. The vibrational analyses of the complexes are given using NH/ND, CH₃/CD₃ and metal isotopicsubstitutions. The Ni(II), Pd(II), Pt(II) and Cu(II) compounds are square planar. The vibrational spectra show that in Pd(SH₂)₂ · H₂O the coordination occurs via the sulphur atom of the deprotonated thioamide group and the oxygen atom of the neutral primary amide group, in a bidentate chelated fashion. The doubly deprotonated SH²⁻ ion behaves as a bridging bis-bidentate ligand giving polymeric structures.     

Interpretation of infrared intensities of propane and deutero-propanes by electrooptical parameters

The interpretation of IR intensities of CH₃CH₂CH₃, CD₃CH₂CD₃ and CH₃CD₂CH₃ in terms of bond parameters (electrooptical parameters, e.o.p. s) is discussed. Sets of e.o.p. s derived from a previous study on methanes and ethanes are used as starting points to derive more complete sets of e.o.p. s which fit the intensities of propane very well. The intensities from ethanes have been used as a constraint on the e.o.p. s of CH₃ groups. The results are discussed and compared with values from quantum mechanics for the derivatives of the molecular dipole moment. Since the parameters derived in this work predict satisfactorily the IR intensities of polyethylene, they should form a good starting set for the interpretation of the intensities of n-paraffins.     

Structural characterization of catalytically active metal clusters in polymer membranes

Metal clusters were generated and stabilized in pore free, mechanically stable poly(amide imide) (PAI) polymer membranes in high dispersion and high concentration of typically 15wt-%. These membranes have been successfully tested for catalytic applications. Pure Pd-loaded and bimetallic Pd/Ag, Pd/Cu, Pd/Co, Pd/Pb PAI films were investigated by means of x-ray absorption spectroscopy, small and wide angle x-ray scattering and transmission electron microscopy to characterize the structure of the metal clusters in the protective polymer. The measurements consistently show a homogeneous distribution of metallic nanoclusters of 10–30 Å size within the membranes. Also, a smaller amount of larger aggregates up to 300 Å is observed in most of the films. The precise cluster size distribution critically depends on the solvents used as well as on other preparation parameters such as the stirring time of the metal precursor/polymer solution. In case of Pd/Ag and Pd/Pb bimetallic films no clear evidence for the formation of bimetallic clusters in the membrane, i.e. alloying of both metal components, is found. In Pd/Cu and Pd/Co membranes, chlorine from CuCl₂ and CoCl₂ precursors reacts with Pd which may influence the Pd catalytic behavior. Reduction and oxidation of the metal nanoclusters is quantitatively studied by means of x-ray absorption spectroscopy. Membrane properties are discussed with respect to catalytic applications.     

Synthesis,properties, and activity of nanosized palladium catalysts modified with elemental phosphorus for hydrogenation

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing various palladium phosphides (PdP₂, Pd5P₂, and Pd₆P) and Pd(0) clusters. The main stages of the formation of palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd₆P phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.     

Nanosized hydrogenation catalyst based on palladium bisacetylacetonate and phosphine: Formation,the origin of activity,and properties

The nature and catalytic properties of a hydrogenation catalyst based on Pd(acac)₂ and PH₃ are considered. As demonstrated by a variety of physicochemical methods (IR and UV spectroscopy, ³¹P and ¹H NMR, electron microscopy, and X-ray powder diffraction), nanoparticles consisting of various palladium phosphides (Pd₆P, Pd_4.8P, and Pd₅P₂) and Pd(0) clusters form under the action of dihydrogen during catalyst preparation. The promoting effect of phosphine at low PH₃: Pd(acac)₂ ratios is mainly due to the ability of phosphine to increase the extent of dispersion of the catalyst.     

Studies of oxidative addition-reductive elimination reactions,and the crystal structure of the palladium(IV) complex Me2(p-BrC6H4CH2)Pd(phen)Br

Selectivity in reductive elimination of ethane and RMe has been observed for benzyl and phenacyl complexes Me₂RPd(L₂)Br (L₂ = bipy, phen), with product ratios dependent upon R and L₂, and cationic intermediates detected by ¹H NMR spectroscopy for oxidative addition of CD₃I and phenacyl bromides to Me₂Pd(L₂). The crystal structure of fac-Me₂(p-BrC₆H₄CH₂)Pd(phen)Br has been determined.     

The Structure of Pd2(CH≡CC6H5)2(C5H7O2)3(BF3)2BF4

IR and NMR data showed that the ionic complex Pd₂(CHCC₆H₅)₂(C₅H₇O₂)₃(BF₃)₂BF₄ isolated in the reaction Pd(Acac)₂ + PA + 5BF₃OEt₂ (Acac is C₅H₇O₂, PA is phenylacetylene) is an adduct of two complexes, namely, (Acac)PdBF₄ and [(PA)₂Pd(C³-Acac · BF₃)]⁺(Acac · BF₃)^– (coordinatively unsaturated). On dissolution in deuteroacetone or deuteromethanol, the [(Acac)PdF₂BF₂Pd(C³-Acac · BF₃)(PA)₂]⁺(Acac · BF₃)^– adduct decomposed to Pd(Acac)₂, 2BF₃ · L (L = (CD₃)₂CO, CD₃OD) and the [L(PA)₂Pd(C³-Acac]⁺BF₄ ^– complex.     

Density functional theory study of p‐chloroaniline adsorption on Pd surfaces and clusters

The adsorption mode of aromatic molecules on transition metal surfaces plays a key role in their catalytic transformation. In this study, by means of density functional theory calculations, we systematically investigate the adsorption of p‐chloroaniline on a series of Pd surfaces, including stepped surfaces, flat surfaces, and clusters. The adsorption energies of p‐chloroaniline on these substrates [Pd(221), Pd(211), Pd(111), Pd(100), Pd₁₃‐icosahedral, Pd₁₃‐cubo‐octahedron, Pd₅₅] are ?1.90, ?2.13, ?1.70, ?2.11, ?2.53, ?2.65, ?2.23 eV, respectively. Benzene ring is adsorpted on catalyst rather than amine group in p‐chloroaniline molecular. A very good linear relationship is further found between the adsorption energies of p‐chloroaniline and the d‐band center of both Pd surfaces and clusters. The lower of d‐band center of Pd models, the stronger adsorption of p‐chloroaniline on catalysts. In addition, the frontier molecular orbital and density of states analysis explain the adsorption energy sequence: cluster Pd₁₃ > stepped Pd(221) surface > flat Pd(111) surface. © 2014 Wiley Periodicals, Inc.     

Catalytic activity of bimetal-containing Co,Pd systems in the oxidation of carbon monoxide

The catalytic activity of low-percentage Co,Pd systems on ZSM-5, ERI, SiO₂, and Al₂O₃ supports in the oxidation of CO was studied. The activity of bimetal-containing catalysts was shown to depend on the nature of the catalyst and the amount and ratio of their active components. According to the results of thermoprogrammed reduction with H₂ (H₂ TPR) and X-ray photoelectron spectroscopy (XPS) data, the metals are distributed as isolated cations or Co^δ+-O-Pd^δ+ clusters with cobalt and palladium cations surrounded by off-lattice oxygen in Co,Pd systems. The 0.8% Co,0.5% Pd-ZSM-5 bimetal catalysts were found to be more active due to the presence of clusters.     

Crossed Molecular Beam Study of H+CH4 and H+CD4 Reactions: Vibrationally Excited CH3/CD3 Product Channels

We study the H+CH₄/CD₄→H₂/HD+CH₃/CD₃ reactions using the time sliced velocity map ion imaging technique. Ion images of the CH₃/CD₃ products were measured by the (2+1) resonance enhanced multi-photon ionization (REMPI) detection method. Besides the CH₃/CD₃ products in the ground state, ion images of the vibrationally excited CH₃/CD₃ products were also observed at two collision energies of 0.72 and 1.06 eV. It is shown that the angular distribution of the products CH₃/CD₃ in vibrationally excited states gradually vary from backward scattering to sideways scattering as the collision energy increases. Compared to the CH₃/CD₃ products in the ground state, the CH₃/CD₃ products in vibrationally excited states tend to be more sideways scattered, indicating that larger impact parameters play a more important role in the vibrationally excited product channels.