Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Palladium”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed2006

Palladium monolayer and palladium alloy electrocatalysts for oxygen reduction.

We investigated the oxygen-reduction reaction (ORR) on Pd monolayers on various surfaces and on Pd alloys to obtain a substitute for Pt and to elucidate the origin of their activity. The activity of Pd monolayers supported on Ru(0001), Rh(111), Ir(111), Pt(111), and Au(111) increased in the following order: Pd/Ru(0001) < Pd/Ir(111) < Pd/Rh(111) < Pd/Au(111) < Pd/Pt(111). Their activity was correlated with their d-band centers, which were calculated using density functional theory (DFT). We found a volcano-type dependence of activity on the energy of the d-band center of Pd monolayers, with Pd/Pt(111) at the top of the curve. The activity of the non-Pt Pd2Co/C alloy electrocatalyst nanoparticles that we synthesized was comparable to that of commercial Pt-containing catalysts. The kinetics of the ORR on this electrocatalyst predominantly involves a four-electron step reduction with the first electron transfer being the rate-determining step. The downshift of the d-band center of the Pd "skin", which constitutes the alloy surface due to the strong surface segregation of Pd at elevated temperatures, determined its high ORR activity. Additionally, it showed very high methanol tolerance, retaining very high catalytic activity for the ORR at high concentrations of methanol. Provided its stability is satisfactory, this catalyst might possibly replace Pt in fuel-cell cathodes, especially those of direct methanol oxidation fuel cells (DMFCs).

Alloys↗
PubMed2005

(Di-2-pyridylamine-kappa2N,N')(naphthalene-2,3-diolato-kappa2O,O')palladium(II) monohydrate and (di-2-pyridylamine-kappa2N,N')(3-oxidonaphthalene-2-carboxylato-kappa2O,O')palladium(II).

In the title complexes, [Pd(C10H6O2)(C10H9N3)].H2O, (I), and [Pd(C11H6O3)(C10H9N3)], (II), the Pd(II) centers have a distorted cis-square-planar geometry. In (I), the Pd(II) atom is coordinated to two N atoms of the di-2-pyridylamine (DPA) ligand and two O atoms of the naphthalene-2,3-diolate (ND) dianion. In (II), the Pd(II) atom is coordinated to two N atoms of the DPA ligand, one carboxylate O atom and one oxide O atom from the 3-oxidonaphthalene-2-carboxylate (NC) ligand. The dihedral angle between the planes of the two pyridine rings of DPA in (I) is 16.20 (12) degrees and that in (II) is 29.45 (10) degrees . In (I), the molecules are linked by N-H...O and O-H...O hydrogen bonds to generate centrosymmetric dimers. In (II), molecules are linked by N-H...O and C-H...O hydrogen bonds to generate spirals.

Amines↗
PubMed2000

A New Highly Efficient Catalyst System for the Coupling of Nonactivated and Deactivated Aryl Chlorides with Arylboronic Acids Palladium-Catalyzed Reactions for Fine Chemical Synthesis, Part 17. The authors thank C. Fuhrmann for the excellent support in the laboratory, Dipl.-Chem. W. Mägerlein for ligand samples of 2-(dicyclohexylphosphanyl)toluene and -anisole as well as DMC(2) for generous donations of palladium compounds. Part 16: F. Vollmüller, W. Mägerlein, S. Klein, J. Krause, M. Beller, Adv. Synth. Catal., in press.

Explore the source record for details and available documents.

Journal Article↗
PubMed1997

Complex Formation and Ligand Substitution Reactions of (2-Picolylamine)palladium(II) with Various Biologically Relevant Ligands. Characterization of (2-Picolylamine)(1,1-cyclobutanedicarboxylato)palladium(II).

The influence of a 2-picolylamine (pic) ligand on the reactivity of Pd(II) complexes was investigated by detailed equilibrium and kinetic studies. Reactions of [Pd(pic)(H(2)O)(2)](2+) with chloride, 1,1-cyclobutanedicarboxylic acid (CBDCAH(2)), inosine (ino), and inosine 5'-monophosphate (5'-IMP) were studied. A significantly higher reactivity for the first reaction step involving the displacement of one coordinated solvent molecule on [Pd(pic)(H(2)O)(2)](2+) was observed for the nucleoside inosine (k(10)( degrees )(C) = 25 400 +/- 200 M(-)(1) s(-)(1)) than for the nucleotide 5'-IMP (k(10)( degrees )(C) = 7100 +/- 300 M(-)(1) s(-)(1)) and for CBDCAH(-) (k(25)( degrees )(C) = 5380 +/- 70 M(-)(1) s(-)(1); DeltaH() = 54 +/- 2 kJ mol(-)(1); DeltaS() = 10 +/- 4 J K(-)(1) mol(-)(1); DeltaV() = -0.2 +/- 0.7 cm(3) mol(-)(1)). The results are compared and discussed in reference to data reported for closely related systems in the literature. The molecular structure of [Pd(pic)(CBDCA)] in solution and in the solid state was resolved. [Pd(pic)(CBDCA)].2H(2)O crystallizes in the space group P2(1)/c (monoclinic, a = 5.659(5) Å, b = 18.320(5) Å, c = 14.027(5) Å, beta = 97.748(5) degrees, V = 1440.94(14) Å(3), Z = 4).

Journal Article↗
PubMed2000

Synthesis and structural characterization of configurational isomers of tungsten-palladium complexes with bridging diphenyl(dithioalkoxycarbonyl)phosphine as a ligand and phosphine transfer from tungsten to palladium.

Treatment of the complex [W(CO)5[PPh2(CS2Me)]] (2) with [Pd(PPh3)4] (1) affords binuclear complexes such as anti-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2Me)PPh2]W(CO)5] (3), syn-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2Me)PPh2]W(CO)5] (4), and trans-[W(CO)4(PPh3)2] (5). In 3 and 4, respectively, the W and Pd atoms are in anti and syn configurations with respect to the P-CS2 bond of the diphenyl(dithiomethoxycarbonyl)phosphine ligand, PPh2(CS2Me). Complex 3 undergoes extensive rearrangement in CHCl3 at room temperature by transfer of a PPh3 ligand from Pd to W, eliminating [W(CO)5(PPh3)] (7), while the PPh2CS2Me ligand transfers from W to Pd to give [[(Ph3P)Pd[mu-eta 1,eta 2-(CS2Me)PPh2]]2] (6). In complex 6, the [Pd(PPh3)] fragments are held together by two bridging PPh2(CS2Me) ligands. Each PPh2(CS2Me) ligand is pi-bonded to one Pd atom through the C=S linkage and sigma-bonded to the other Pd through the phosphorus atom, resulting in a six-membered ring. Treatment of Pd(PPh3)4 with [W(CO)5[PPh2[CS2(CH2)nCN]]] (n = 1, 8a; n = 2, 8b) in CH2Cl2 affords syn-[(Ph3P)2Pd[mu-eta 1,eta 2-[CS2(CH2)nCN]PPh2]W(CO)5] (n = 1, 9a; n = 2, 9b). Similar configurational products syn-[(Ph3P)2Pd[mu-eta 1,eta 2-(CS2R)PPh2]W(CO)5] (R = C2H5, C3H5, C2H4OH, C3H6CN, 11a-d) are synthesized by the reaction of Pd(PPh3)4 with [W(CO)5[PPh2(CS2R)]] (R = C2H5, C3H5, C2H4OH, C3H6CN, 10a-d). Although complexes 11a-d have the same configuration as 9a,b, the SR group is oriented away from Pd in the former and near Pd in the latter. In these complexes, the diphenyl(dithioalkoxycarbonyl)phosphine ligand is bound to the two metals through the C=S pi-bonding and to phosphorus through the sigma-bonding. All of the complexes are identified by spectroscopic methods, and the structures of complexes 3, 6, 9a, and 11d are determined by single-crystal X-ray diffraction. Complexes 3, 9, and 11d crystallize in the triclinic space group P1 with Z = 2, whereas 6 belongs to the monoclinic space group P2/c with Z = 4. The cell dimensions are as follows: for 3, a = 10.920(3) A, b = 14.707(5) A, c = 16.654(5) A, alpha = 99.98(3) degrees, beta = 93.75(3) degrees, gamma = 99.44(3) degrees; for 6, a = 15.106(3) A, b = 9.848(3) A, c = 20.528(4) A, beta = 104.85(2) degrees; for 9a, a = 11.125(3) A, b = 14.089(4) A, c = 17.947(7) A, alpha = 80.13(3) degrees, beta = 80.39(3) degrees, gamma = 89.76(2) degrees; for 11d, a = 11.692(3) A, b = 13.602(9) A, c = 18.471(10) A, alpha = 81.29(5) degrees, beta = 80.88(3) degrees, gamma = 88.82(1) degrees.

Journal Article↗
PubMed2006

Palladium-catalyzed hydrogenation: detection of palladium hydrides. A joint study using para-hydrogen-enhanced NMR spectroscopy and density functional theory.

Pd(PEt3)2(OTf)2, acting as an in situ source of Pd(PEt3)2, reacts with an alkyne and hydrogen via phosphine loss to form the detectable hydride-containing species Pd(PEt3)2(H)(CHPhCH2Ph), cis- and trans-Pd(PEt3)2(H)(CPh=CHPh), and Pd2(PEt3)3(H)(CHPhCH2Ph)2+, which map onto the reaction scheme predicted by density functional theory.

Journal Article↗
PubMed2001

Palladium(II)-catalyzed oxidation of aldehydes and ketones. 1. Carbonylation of ketones with carbon monoxide catalyzed by palladium(II) chloride in methanol.

Unsubstituted or alkyl-substituted cyclic ketones react with PdCl2 in methanol under a CO atmosphere to give mainly acyclic diesters along with some acyclic chloro-substituted monoesters. The monosubstituted cyclic ketones, 2-hydroxy- and 2-methoxycyclohexanone, do not give ring cleavage but rather produce 2-(carbomethoxy)cyclohex-2-en-1-one. 13CO labeling experiments indicate one CO is inserted in forming the diester product so the second ester group must arise from the original ketone group. Two mechanisms are possible for the diester reaction. One involves initial Pd(II)-CO2CH3 insertion across the double bond of the enol form of the ketone while the second involves initial addition of Pd(II)-OCH3 followed by CO insertion into the new Pd(II)-carbon bond formed. Pd(II) elimination and acid-catalyzed ring cleavage produce the second methyl ester group in both routes. The chloro-substituted monoester is formed by initial Pd(II)-Cl insertion across the double bond followed by the acid-catalyzed ring cleavage. The 2-(carbomethoxy)cyclohex-2-en-1-one must result from elimination of water or methanol from the alpha-ketoester product formed by the initial methoxycarbonylation of the enol form of the ketone. As expected, the acyclic ketone, 2-decanone, formed methyl acetate and a mixture of methyl nonanoate and 1-chlorooctane as products.

Journal Article↗
PubMed2001

Palladium(II)-catalyzed asymmetric cyclization of (Z)-4'-acetoxy-2'-butenyl 2-alkynoates. Role of nitrogen-containing ligands in palladium(II)-mediated reactions.

Pd(OAc)(2) combined with nitrogen-containing ligands (e.g., 2,2'-bipyridine) catalyzed the cyclization of (Z)-4'-acetoxy-2'-butenyl 2-alkynoates (1) in acetic acid to afford the alpha-(Z)-acetoxyalkylidene-beta-vinyl-gamma-butyrolactones (2) with high efficiency and high stereoselectivity. The nitrogen-containing ligands, like halides, served to favor beta-heteroatom elimination over beta-hydride elimination in Pd(II)-mediated reactions. The generality of this ligand effect was probed in both stoichiometric and catalytic reactions. With these results in hand, the catalytic asymmetric protocol was achieved with high enantioselectivity (up to 92% ee) when pymox (pyridyl monooxazoline) or bisoxazoline was used. The absolute configuration of the products and the synthetic utility of this asymmetric transformation were established through the convenient synthesis of (3S)-(+)-A-factor.

Journal Article↗
PubMed2002

Transmetalation of palladium enolate and its application in palladium-catalyzed homocoupling of alkynes: a room-temperature, highly efficient route to make diynes.

A novel pathway for the homocoupling reaction has been achieved using a similar protocol as the cross-coupling reaction. Ethyl bromoacetate is chosen to initiate the coupling reaction through oxidative addition to a Pd(0) species, and an PdBr(enolate) intermediate is formed. This intermediate can undergo double transmetalation with an alkynyl copper reagent, and reductive elimination produces a variety of diynes in high yields.

Journal Article↗
PubMed2005

Temperature dependence of the kinetics for the complete oxidation of methane on palladium and palladium oxide.

The kinetics for the complete combustion of methane was studied on a Pd foil in the regions where the oxide and then the metal were the bulk stable phases. The use of a model catalyst allowed the kinetics to be studied at higher temperatures than are possible on supported catalysts since heat and mass transport limitations could be avoided for this nonporous model catalyst. For all reaction conditions, CH4 and O2 reaction orders remained the same at about 0.7 and 0, respectively. With PdO as the stable phase, the water reaction order increased from -1 to 0 and the apparent activation energy (Ea) decreased from 125 to 30 kJ mol(-1) as the reaction temperature increased from 600 to 880 K. We propose that as the temperature is increased water desorbs from the sites responsible for combustion and as a result water inhibition and Ea decrease. To investigate the rate of reaction on Pd versus PdO, the rates were measured around the Pd-PdO transition temperature. The turnover rate decreased from 3.0 s(-1) to 0.3 s(-1) at the transition temperature (907 K with 1.5 Torr O2 and 0.30 Torr CH4) when PdO decomposed to Pd metal, showing that PdO was more active than Pd metal for methane oxidation at this temperature. The reaction orders for Pd metal in the range of 933-1003 K were 0.7, 0, and 0 for methane, water, and O2, respectively, with an apparent activation energy of 125 kJ mol(-1). Thus, the turnover rate and Ea changes suggest that the reaction mechanism for methane oxidation on Pd is different from the one on PdO.

Journal Article↗
PubMed2006

Chemical state study of palladium powder and ceria-supported palladium during low-temperature CO oxidation.

The chemical state of Pd at the surfaces of two sizes of Pd powders and ceria-supported Pd during low-temperature CO oxidation has been studied using X-ray photoelectron spectroscopy (XPS). During oxidation in O(2), metallic Pd powder is converted into PdO, and the thickness of the PdO layer increases with increasing reaction temperature. A similar Pd oxidation process occurs during the catalytic CO oxidation reaction, but the extent of the Pd oxidation is less due to the presence of CO which is a reducing agent. The reaction rate data indicate that the larger size Pd powder is about three times more active than the smaller size Pd powder on a surface area basis at 160 degrees C. Catalytic CO oxidation data obtained from 10 wt % Pd supported on nanocrystalline ceria powder indicate that there is a strong chemical interaction between the ceria and the supported Pd. The Pd is present as PdO on the fresh catalyst. During reaction, small amounts of Pd metal and PdO(2) are formed at 50 degrees C while less Pd metal and only a small amount of PdO(2) are present after running the reaction at 110 degrees C. At 50 degrees C, the catalytic activity decays rapidly due to accumulation of carbonate or bicarbonate species on the surface. This decay does not occur at 110 degrees C due to decomposition of the bicarbonate species.

Journal Article↗