The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.     

The adsorption of acetic acid on clean and oxygen-covered Au/Pd(100) alloy surfaces

The adsorption of acetic acid is studied on clean and oxygen-covered Au/Pd(100) alloys as a function of gold content by temperature-programmed desorption and reflection–absorption infrared spectroscopy. Au/Pd(100) forms ordered alloys such that, for gold coverages above ~ 0.5 monolayers, only isolated palladium atoms surrounded by gold nearest neighbors are present. Predominantly molecular acetic acid forms on Au/Pd(100) alloy surfaces for gold coverages greater than ~ 0.56 ML, and desorbs with an activation energy of ~ 59 kJ/mol. Heating this surface also forms some η¹-acetate species which decompose to form CO and hydrogen. On alloy surfaces with palladium–palladium bridge sites, η¹-acetate species initially form, but rapidly convert into η²-species. They thermally decompose to form CO and hydrogen, with a small portion rehydrogenating to form acetic acid between 280 and 321 K depending on gold coverage. The presence of oxygen on both Pd(100) and Au/Pd(100) alloys facilitates acetate dehydrogenation so that only η²-acetate species form on these surfaces. The presence of oxygen also serves to stabilize the acetate species.     

基于密度泛函理论研究掺杂Pd石墨烯吸附O2及CO

基于第一性原理的密度泛函理论研究了单个O₂和CO气体分子吸附于本征石墨烯和掺杂钯(Pd)的石墨烯的体系, 通过石墨烯掺Pd前后气体分子的吸附能、电荷转移及能带和态密度的计算, 发现掺Pd后气体分子吸附能和电荷转移显著增大, 这是由于Pd的掺杂, 在本征石墨烯能带中引入了杂质能级, 增强了石墨烯和吸附气体分子间的相互作用; 氧化性气体O₂和还原性气体CO吸附对石墨烯体系能带结构和态密度的影响明显不同, 本征石墨烯吸附O₂后, 费米能级附近态密度变大, 掺Pd后在一定程度变小; 吸附还原性的CO后, 石墨烯费米能级附近态密度几乎没有改变, 表明掺杂Pd不会影响石墨烯对CO的气体灵敏度, 但由于CO对石墨烯的吸附能增大, 可以提高石墨烯对还原性气体的气敏响应速度.     

Absolute coverages for the various surface phases of CO and O adsorbed on Pd(110)

The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the ¹²C(d, p)¹³C and ¹⁶O(d, p₁)¹⁷O¹ reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 10¹⁴ CO molecules cm⁻²) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML.     

Rietveld refinement and activity of CO oxidation over Pd/Ce0.8Zr0.2O2 catalyst prepared via a surfactant-assisted route

A surfactant-assisted preparation method was successfully used to synthesize the Ce_0.8Zr_0.2O₂ nanophases that had homogeneous chemical composition, enhanced thermal stability and improved catalytic activity. Structural refinement by the Rietveld method confirmed that many cationic lattice defects were formed in the ceria-zirconia crystals due to incorporation of the cationic surfactant in the network of the solid. Zirconium dissolving into cubic structure of ceria improved its thermal stability but led to structure distortion. FT-IR in situ characterization showed that CO chemisorption on the palladium loaded ceria-zirconia catalyst formed linear and multibonded CO–metal complexes. It was also evidenced that CO₂ was produced in the CO chemisorption procedure from the pathways of CO disproportionation and CO reacting with lattice oxygen of the catalyst. Owing to high dispersion of the loaded metal palladium and defective structure of the ceria-zirconia support, the Pd/Ce_0.8Zr_0.2O₂ catalyst calcined at a temperature range between 873 and 1173 K not only exhibited a more stable catalytic activity for CO oxidation but also performed a lower light-off temperature at cool start below 373 K in comparison with Pd/CeO₂.     

Elucidation of the nature of active oxygen in the reaction of low-temperature oxidation of CO on single crystal surfaces platinum and palladium

The temperature-programmed reaction (TPR) method, high-resolution electron energy loss spectroscopy (HREELS), and molecular beam method were used to elucidate the role surface reconstruction, subsurface oxygen (O_subs), and CO_ads concentration play in the low-temperature oxidation of CO on the Pt(100), Pt(410), Pd(111), and Pd(110) surfaces. The possibility of the formation of so-called hot oxygen atoms, which arise at the surface at the instant of dissociation of O_2ads molecules and can react with CO_ads at low temperatures (～150 K) to form CO₂, was examined. It was revealed that, when present in high concentration, CO_ads initiates the phase transition of the Pt(100)-(hex) reconstructed surface into the (1 × 1) non-reconstructed one and blocks fourfold hollow sites of oxygen adsorption (Pt₄-O_ads), thereby initiating the formation of weakly bound oxygen (Pt₂-O_ads), active in CO oxidation. For the Pt(410), Pd(111), and Pd(110) surfaces, the reactivity of O_ads with respect to CO was demonstrated to be dependent on the surface coverage of CO_ads. The ¹⁸O_ads isotope label was used to determine the nature of active oxygen reacting with CO at ～150–200 K. It was examined why a CO_ads layer produces a strong effect on the reactivity of atomic oxygen. The experimental results were confirmed by theoretical calculations based on the minimization of the Gibbs energy of the adsorption layer. According to these calculations, the CO_ads layer causes a decrease in the apparent activation energy E _act of the reaction due to changes in the type of coordination and in the energy of binding of O_ads atoms to the surface.     

Precious Metals: A Convenient Recycle Procedure for Laboratory Rejects with Spectroscopic Characterisation of the Products

The recycling process of metals is also a good opportunity to get in touch with the economic and environmental issues involving the routine in the chemistry laboratory. This strategy has been introduced regularly in the laboratory in order to reduce the waste material storage. the recovery process focusing on the precious metals, gold and palladium, gives as a result the phosphine derivatives of these metals, compounds that are extremely useful in different areas: catalysis, organometallic syntheses, and in medicine (the gold derivatives are used in the treatment of arthritis and neoplasias). AuPPh₃Cl and Pd(PPh₃)₂Cl₂ were prepared from rejects stored in the laboratory using a new synthetic route and were characterised by ₃₁P {₁H}, UV/visible spectra and elemental analysis.     

Isomers of OCS and their reaction with H2O on singlet potential energy surface

The isomers of the carbonyl sulfide (OCS) molecule are investigated in detail at CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level of theory. One cyclic isomer was identified along with three different linear minima of the OCS molecule. Three interconversion transition states were also located between cyclic and linear forms of OCS. Among these four isomers, the singlet potential energy surface (PES) for the molecule–molecule reaction between the three most energetically favoured isomers of OCS and H₂O has been explored theoretically at the CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level. This singlet PES comprises of three paths. Path 1 is the reaction of linear OCS molecule with water producing the major product P1 (CO₂?+?H₂S), minor product P2 (S?+?HCOOH) and two isomers via 14 minima and 15 transition states. The Path 2 is an isomerization process in which cyclic isomer of OCS reacts with water molecule via another initial barrierless aduct producing five isomers of the OCS–H₂O system through five interconversion transition states. The reaction of linear COS isomer with water is shown in Path 3. This path produces the radicals SH and COOH from another COS–H₂O complex via a transition state. Among these three products, the product P1 is energetically most favoured. The overall exothermicity of the product channels for the formation of major product P1 on PES is calculated to be about 10.60?kcal/mol possessing initial high entrance barriers of 45.48 and 55.47?kcal/mol in two possible pathways. As the process is favoured thermodynamically but not kinetically, the reaction is expected to be very slow.     

DFT study of CO adsorption on neutral and charged Pd<Subscript>n</Subscript>(n = 1–7) clusters

Density functional calculations have been performed to investigate CO adsorption on neutral, cationic and anionic Pd_n (n=1–7) clusters. From the results, it is observed that the binding of CO molecule to neutral and cationic palladium clusters takes place via 1-, 2- and 3-fold coordination. On the other hand, only terminal adsorption of CO molecule is possible in anionic clusters barring bridging adsorption in Pd₇ ^- cluster.     

Confirmation of Complex Formation Between Ethylene and Mercury Halides by Matrix-Isolation Infrared Spectroscopy

Since DeKock¹ first prepared the Ni(CO)_n (n = 1,2,3 and 4) series in argon matrices, a number of unstable and transient coordination compounds have been synthesized by reacting metal vapor directly with the ligand in inert gas matrices at low temperatures. These include metals such as Pt, Pd and Ni, and ligands such as CO, N₂′ NO, O₂ and PF₃. Van Leirsburg and DeKock² prepared the MX₂-L type complexes by reacting metal halide vapor such as NiF₂ and NiCl₂ directly with ligands such as CO, NO and N₂ in argon matrices. Thus far, no reports are available on the synthesis of metal halid-ethylene complexes in inert gas matrices. In this letter, we wish to report the confirmation of such complex formation by matrix-isolation infrared spectroscopy.     

The reaction of carbon monoxide with palladium supported on cerium oxide thin films

In this study we probe the reaction of carbon monoxide with Pd nanoparticles supported on cerium oxide thin films. With the use of soft X-ray photoelectron spectroscopy (sXPS), and temperature programmed desorption (TPD) the surface intermediates and pathways leading to reaction products of CO on Pd supported on ceria were investigated. When Pd is supported on the stoichiometric CeO₂ surface (Ce⁺⁴) only the molecular adsorption of CO on Pd is visible (286.4 eV). All of the CO desorbs below 520 K, however a small amount of O exchange between the CO and the ceria was indicated through the acquisition of labeled ¹⁸O from the substrate in the desorbed CO. The Pd nanoparticles are activated on partially reduced CeO_x to promote the dissociation of <10% of the CO as indicated by a C-Pd species (284.4 eV) in sXPS. The C recombines with O from the ceria and desorbs between 600 and 700 K. The majority of the CO does not dissociate, however, and the degree of dissociation does not increase with the degree of ceria reduction. This result is in contrast with Rh nanoparticles supported on ceria where the degree of dissociation increased with the degree of ceria reduction and nearly total dissociation was obtained when the ceria was highly reduced.     

A theoretical study on the reaction mechanism and kinetics of allyl alcohol (CH2 = CHCH2OH) with ozone (O3) in the atmosphere

Volatile organic compounds (VOCs) play a major role in the physical and chemical process of the tropospheric chemical reactions in both polluted and remote environments. A theoretical work has been presented on the VOC of allyl alcohol with O₃ molecule is investigated using density functional theory methods. The reaction profile is initiated through the cycloaddition of ozone which leads to the formation of primary ozonide with minimal relative energy barrier of 1.31 kcal/mol which decomposes to form carbonyl molecule and carbonyl oxide. Carbonyl oxide, i.e. criegee intermediates reacts with various atmospheric species to produce more hazardous and toxic end products to the environment. The condensed form of Fukui function was calculated to predict reactive sites of the primary and secondary reaction profile. The rate coefficient using CVT with SCT over the temperature range of 258–358K is analysed and also to study the atmospheric effects of allyl alcohol in the atmosphere. The predicted rate coefficient for the favourable reaction pathway of k_p1 found to be 1.190 ×10^?15 cm³/molecule/sec and comparable with the experimental result at 298 K. The atmospheric lifetime of allyl alcohol was found to be around 10 hours in addition to that global warming potentials are compared with the CO₂.     

Spectral and luminescent properties of mono- and binuclear cyclometalated complexes of Pd(II) and Pt(II) based on 4,6-diphenylpyrimidine with ethylenediamine and orthophenanthroline

The [M(N_^∧N)(Hdphpm)]ClO₄ and [(M(N_^∧N))₂(μ-dphpm)](ClO₄)₂ complexes (M = Pd(II), Pt(II); (N_^∧N) is ethylenediamine (En) and orthophenanthroline (Phen); Hdphpm^? and dphpm^2? are the mono- and bisdeprotonated forms of 4,6-diphenylpyrimidine) are obtained and characterized by ¹H NMR spectroscopy and electronic absorption and emission spectroscopy. The magnetic nonequivalence of protons of (N_^∧N) ligands is explained by a difference in the trans-effect of the carbanion and pyrimidine parts of the cyclometalated ligand. The long-wavelength absorption bands and the vibrationally structured luminescence bands of ethylenediamine complexes are attributed to optical transitions in the {M(Hdphpm)} and {M₂(μ-dphpm)} metal-complex fragments. The complexes with orthophenanthroline exhibit two low-energy optical transitions involving π* orbitals localized on the cyclometalated and chelating ligands; the difference in their energies depends on the metal and is much larger for Pt(II) than for Pd(II). It is found that the replacement of Pd(II) by Pt(II) in the [(M(phen))₂(μ-dphpm)]²⁺ complexes changes the direction of the photoexcitation energy degradation due to the energy transfer between the {M₂(μ-dphpm)} bridging fragment and peripheral phenanthroline ligands.     

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd_0.75Cu_0.25(111) single crystal surface with surface composition of about Pd_0.7Cu_0.3. The main objective was to make a comparison with the previously studied Pd_0.67Ag_0.33(111) (surface composition Pd_0.1Ag_0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10⁻⁹ mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.     

Synthesis of carbon supported palladium nanoparticles catalyst using a facile homogeneous precipitation-reduction reaction method for formic acid electrooxidation

A highly dispersed and ultrafine carbon supported Pd nanoparticles (Pd/C) catalyst is synthesized by a facile homogeneous precipitation-reduction reaction method. Under the appropriate pH conditions, [PdCl₄]²⁻ species in PdCl₂ solution are slowly transformed into the insoluble palladium oxide hydrate (PdO·H₂O) precipitation by heat treatment due to a slow hydrolysis reaction, which results in the generation of carbon supported PdO·H₂O nanoparticles (PdO·H₂O/C) sample with the high dispersion and small particle size. Consequently, a highly dispersed and ultrafine Pd/C catalyst can be synthesized by PdO·H₂O → Pd⁰ in situ reduction reaction path in the presence of NaBH₄. As a result, the resulting Pd/C catalyst possesses a significantly electrocatalytic performance for formic acid electrooxidation, which is attributed to the uniformly sized and highly dispersed nanostructure.     

Computational studies of the halooxyhalocarbenes XOCX (X = F,Cl)

Ab initio calculations at the B3LYP, MP2, MP4 and CCSD(T) levels of theory were performed to predict the stability of the halooxyhalocarbenes, XOCX (X = F, Cl). The calculations indicate that the nonlinear FOCF molecule is stable with an energy 16 kJ mol^?1 below the energy of possible reacting fragments F₂ and CO. However, a nonlinear equilibrium structure for ClOCCl was located, but it was found to be about 192 kJ mol^?1 higher in energy than the energy of Cl₂ and CO. The charge distribution in these molecules was analysed using the atoms in molecules (AIM) method.     

Bromine-adsorption behavior of a one-dimensional bromine-bridged mixed-valence palladium (II), (IV) complex, [Pd(chxn)2][PdBr2(chxn)2]Br4

The bromine-adsorption behavior of [Pd(chxn)₂][PdBr₂(chxn)₂]Br₄ (chxn: 1R,2R-cyclohexanediamine) has been investigated by elemental analysis, X-ray powder diffraction (XRD), ¹H NMR and ESR. From XRD, the reaction of the palladium complex with bromine was revealed to be reversible depending on the partial pressure of bromine around the complex. Elemental analysis suggested the adsorption of two Br₂ molecules per the formula unit. Temperature dependence of the ESR intensity showed the existence of Curie and thermally activated spins in both compounds with and without bromine-adsorption. The spin density, as well as the fraction of Curie spins, decreased after bromine-adsorption suggesting the oxidation of paramagnetic Pd(III) into Pd(IV). ¹H NMR spin-lattice relaxation of these compounds is determined by the coupling with the thermally activated spins as well as with the Curie spins.     

Structures,electronic properties and reaction paths from Fe(CO)5 molecule to small Fe clusters

The geometries, electrical characters and reaction paths from Fe(CO)₅ molecule to small Fe clusters were investigated by using all-electron density functional theory. The results show that in the decomposition process of pentacarbonyl-iron, Fe(CO)₅ molecule prefers to remove a carbon monoxide and adsorb another Fe(CO)₅ molecule to produce nonacarbonyldiiron Fe₂(CO)₉ then Fe₂(CO)₉ gradually removes carbon monoxide to produce small Fe clusters. As It can be seen from the highest occupied molecule orbital–lowest unoccupied molecule orbital gap curves, the Fe(CO)_n=3,?and?5 and Fe₂(CO)_n=3,?7?and?9 intermediates have higher chemical stability than their neighbors. The local magnetic moment of the carbon monoxide is aligning anti-ferromagnetic. The effect of external magnetic field to the initial decomposition products of Fe(CO)₅ can be ignored.     

Palladium-vacancy pairs in p-type and n-type germanium

Time differential perturbed angular correlation spectroscopy was performed for n-type and p-type germanium samples that had been recoil-implanted with the probe ¹⁰⁰Pd/¹⁰⁰Rh. Two different measurements with the detectors aligned along <100> and <110> crystallographic directions of germanium confirm that the palladium probe pairs with the nearest neighbour vacancy and not a dopant. The pairs interact with a coupling constant of υ _Q = 10.7(2) MHz and this agrees with what has been observed in undoped Ge. This further suggests that the palladium pairs with a vacancy in germanium, doped or un-doped. It was also observed that there are more Pd-V pairs in Ga-doped than there are in Sb-doped Ge. DFT calculations of the binding energies of the Pd-defect complexes support the experimental observations.     

Theoretical study of the coadsorption of CO and O2 on doped cationic gold clusters MAun + (M = Ti, Fe, Au; n = 1, 6, 7)

We present a study based on first-principles calculations of the adsorption of CO on selected equilibrium configurations of MAu_nO₂ ⁺ (M = Ti, Fe; n = 1, 6, 7) complexes resulting from the adsorption of O₂ on doped cationic gold clusters MAu_n ⁺. Empirical rules for the formation of CO₂-MAu_nO⁺ complexes are outlined. The desorption energy of CO₂ is calculated. The adsorption of a second CO molecule on the residual MAu_nO⁺ complex leads in some cases to the formation of CO₂-MAu_n ⁺. The desorption of a second CO₂ molecule brings back to the initial doped gold cluster MAu_n ⁺.