Oxidation State Of Palladium Research Articles

Low-temperature deactivation and oxidation state of Pd/γ-Al 2O 3 catalysts for total oxidation of n-hexane

Five weight percentage of Pd/γ-Al 2O 3 catalysts were used for the oxidation of n-hexane. The activities of catalysts were greatly influenced by pre-treatments. The hydrogen-treated catalysts mainly with metallic palladium showed higher oxidation activities than the air-treated catalysts mostly with palladium oxide, even if the palladium oxide was known to be the active phase for high-temperature combustion of methane. The XPS and XRD analyses suggested that the oxidation state of palladium did not change during oxidation reaction at lower temperature of 180 °C. The reduced catalysts whose initial activities at lower temperatures were higher than the oxidized catalysts showed a gradual deactivation. This kind of low-temperature deactivation was attributed to the deposit of carbonaceous intermediates, which were evidenced through temperature-programmed analyses such as TGA and TPO.

A test reaction to assess the presence of Brönsted and the softness/hardness of Lewis acid sites in palladium supported catalysts

Cyclic ethylene acetal of α-bromopropiophenone gives rise to three distinctive products, each one being specific to Bronsted, hard Lewis and soft Lewis sites, respectively. In the presence of Bronsted sites, hydrolysis of cyclic acetal forms the corresponding α-bromophenone. Hard or soft Lewis sites promote selectively a rearrangement of the phenyl or ethylenedioxy group to form 2-phenylpropionate ester or dioxine, respectively. This test reaction has been used to assess the influence of the support on the catalytic activity of palladium in zeolites. Three series of supports including alkali-exchanged faujasite, Beta zeolites and sepiolites have been tested. It has been observed that the nature of the zeolite plays an important role on the palladium selectivity, basic supports increasing the softness of the palladium Lewis sites. The catalytic test correlates well with electrochemical data about the reduction potential of supported palladium, XPS characterization of the oxidation state of palladium supported on the surface and activity in the Suzuki reaction.

Pyrrole-appended derivatives of O-confused oxaporphyrins and their complexes with nickel(II), palladium(II), and silver(III).

Condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde formed, instead of the expected 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin, a pyrrole addition product [(H,pyr)OCPH]H(2); this product can formally be considered as an effect of hydrogenation of 3-(2'-pyrrolyl)-5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin ([(pyr)OCPH]H). The new oxacarbaporphyrinoid presents the (1)H NMR spectroscopy features of an aromatic molecule, including the upfield shift of the inner H21 atom. Insertion of NiCl(2) or PdCl(2) into [(H,pyr)OCPH]H(2) gave two structurally related organometallic complexes, [(pyr)OCP]Ni(II)] and [(pyr)OCP]Pd(II)], in which the metal ions are bound by three pyrrolic nitrogens and the trigonally hybridized C21 atom of the inverted furan. The reaction of [(H,pyr)OCPH]H(2) with silver(I) acetate yields a stable Ag(III) complex [(C(2)H(5)O,pyr)OCP]Ag(III)] substituted at the C3 position by the ethoxy and pyrrole moieties. The macrocyclic frame of [(H,pyr)OCPH]H(2) is conserved. Addition of trifluoroacetic acid to [(C(2)H(5)O,pyr)OCP]Ag(III)] yielded a new aromatic complex [(pyr)OCP]Ag(III)](+). The structures of [(pyr)OCP]Ni(II)] and [(C(2)H(5)O,pyr)OCP]Ag(III)] have been determined by X-ray crystallography. In both molecules the macrocycles are only slightly distorted from planarity and the nickel(II) and silver(III) are located in the NNNC plane. The dihedral angle between the macrocyclic and appended-pyrrole planes of [(pyr)OCP]Ni(II)] reflects the biphenyl-like arrangement with the NH group pointing out toward the adjacent phenyl ring on the C5 position. Tetrahedral geometry around the C3 atom was detected for [(C(2)H(5)O,pyr)OCP]Ag(III)]. The Ni[bond]C and Ag[bond]C bond lengths are similar to other nickel(II) or silver(III) carbaporphyrinoids where the trigonal carbon atom coordinates the metal ion. The trend detected in the (13)C chemical shifts for the appended-pyrrole resonances has been rationalized by the extent of effective conjugation between the macrocycle and the appended pyrrole moiety controlled by the hybridization of the C3 atom and the metal ion oxidation state. The dianionic or trianionic macrocyclic core of the pyrrole-appended derivatives is favored to match the oxidation state of nickel(II), palladium(II), or silver(III), respectively.

Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/γ-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pre-treatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/γ-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.

1,2-Oxopalladation versus pi-allyl palladium route. A regioconvergent approach to a key intermediate for cyclopentanoids synthesis. New insights into the Pd(II)-catalyzed lactonization reaction.

Regioconvergent synthesis of the key lactone 1 from an equimolar mixture of the two olefins 4 and 5 was achieved by unique Pd(II) chemistry. The synthetic versatility of lactone 1 has been demonstrated in the synthesis of iridoids and of the endo-Corey lactone 2, which is a key intermediate for the F(2)-isoprostane synthesis. Upon exposure of the sodium salts of 4 and 5 to a catalytic amount of Pd(OAc)(2) under oxygen, in the presence of AcOH, an isomeric lactone 12 was obtained in addition to the title compound 1. The Pd(II) lactonization was optimized by fine-tuning all the factors participating in the catalytic cycle: solvent, oxidant, co-oxidant, and Pd(II) source. The Hosokawa's heterobimetallic couple emerged as the catalyst of choice. With a Cu(II)-Pd(II) couple, the redox process was transferred to copper, and the formal oxidation state of palladium remained constant during the reaction. By virtue of this new methodology, lactone 1 was obtained in a rewarding 60% yield, along with isomeric lactone 12 in 30% yield. A detailed mechanistic study was carried out in order to elucidate the formation of lactones 1 and 12. Lactone 1 was formed from either olefin 8 or olefin 10; on the other hand, lactone 12 was formed exclusively from olefin 10. An intramolecular 1,2-acyloxypalladiation was invoked for the transformation of 8 into 1, whereas the pi-allyl complexes 13 and 11 were involved in the transformation of olefin 10 into 12 and 1, respectively.

Catalytic properties and characterization of Pd supported on hexaaluminate in high temperature combustion

The catalytic activities of palladium-supported Sr 0.8La 0.2MnAl 11O 19 (SLMA) and the effect of phase transformation of palladium on the catalytic activity after high temperature treatments were studied by XRD, TPD, temperature programmed reaction (TPRX) and temperature programmed reoxidation (TPRO). Also the interaction of palladium with Sr 0.8La 0.2MnAl 11O 19 at high temperature was studied by XPS. PdO formed on Sr 0.8La 0.2MnAl 11O 19 was dissociated to metallic Pd after calcination at 1000 °C. TPD and TPRX study revealed that the surface oxygen of PdO cluster was the major source of oxygen to react with CH 4 in the absence of O 2. TPRO after TPRX experiments showed the reoxidation of catalysts was dependent on the calcination temperature. High temperature treatments caused the interaction of palladium and Sr 0.8La 0.2MnAl 11O 19 support resulting in the change of the oxidation state of palladium.

Propane oxidation over Pd/LaFe0.8Co0.2O3 catalyst

Pd/LaFe0.8Co0.2O3 was found to be more active than the Pd/Al2O3 catalyst in propane oxidation. Activity and the oxidation state of palladium were strongly affected by the redox ratio (S) of reactants. Higher propane conversion could be obtained under rich conditions at higher temperatures, contributing to steam reforming. The presence of excess steam would bring about inhibition and oscillation.

The response of tin acetylacetonate and tin dioxide-based gas sensors to hydrogen and alcohol vapours

The aim of this work is to investigate the properties of gas sensors with active layers prepared by pulsed laser deposition (PLD) technology. The active layers were deposited on planar sensor chips with interdigital platinum electrodes. The deposition was carried out from tin dioxide and tin acetylacetonate (SnAcAc) targets by KrF excimer laser. In some cases Pd catalyst was sputtered on the surface of the active layer. The ‘as-deposited’ sensors were submitted to heat treatment. The chemical composition of heat treated active layers was studied by XPS method. This method revealed the oxidation state of palladium and the distribution between organic and inorganic phase during the deposition of SnAcAc. The DC responses of the sensors to a reducing atmosphere containing 1000 ppm of hydrogen, methanol, ethanol, n-propanol and n-butanol were also measured. The maximum sensitivity S (ratio of sensor resistances S= R air/ R gas) achieved 67 for hydrogen, 21 for methanol, 50 for ethanol, 71 for n-propanol and 44 for n-butanol. The temperature of maximum sensitivity ( T max) to distinct gases, the influence of molecular weight of detected gas on T max and the influence of molecular weight on the sensor response speed are also discussed.

Preparation of Alumina Supported Palladium Catalysts by Sol-Gel Method

The chemical reactivity of the aluminium-sec-butoxide (ASB) and the palladium acetylacetonate Pd(acac)2, used as precursors for the preparation of the alumina supported palladium catalysts by sol-gel method was investigated by the spectroscopic study of the precursor mixture during ageing, using FTIR, UV-VIS and 27Al NMR. The obtaind results showed that acetylacetonate ligands were linked to aluminum when the mixture was aged at 40°C. This was proved by the bands observed at 1530 and 1600 cm−1 in the FTIR spectra, the band obtained at 289 nm in the UV-VIS spectra and the 27Al NMR sharp peak at 3 ppm. Furthermore, in order to avoid the Pd(acac)2 reduction to metallic palladium by the SB occurring when the mixture is aged for 3 h, an optimum ageing time should be selected. The precursors modification and the preservation of the palladium oxidation state during ageing could be the required conditions to create a bond between palladium and aluminium during the gelation step. This should be the reason of the thermal stability improvement of the alumina supported palladium catalyst prepared by the sol-gel method.

Acid Strength of Support Materials as a Factor Controlling Oxidation State of Palladium Catalyst for Propane Combustion

The support effect on the low temperature catalytic combustion of propane over palladium catalyst was studied by using a series of metal oxides as support materials: MgO, ZrO2, Al2O3, SiO2, SiO2–ZrO2, SiO2–Al2O3, and SO42−–ZrO2. The catalytic activity varied with the kind of the support materials; a support material with moderate acid strength gave maximum conversion. In order to discuss more details, 5 wt% Pd/ZrO2, Pd/Al2O3, and Pd/SiO2–Al2O3 were subjected to catalytic testing under the various conditions, and the catalysts after the test were characterized by XRD and XPS. The activity changed with the oxygen concentration, and the concentration providing the maximum activity depended on the support material. The maximum activity was obtained at high concentration on Pd/SiO2–Al2O3, at moderate concentration on Pd/Al2O3, and at low concentration on Pd/ZrO2. This sequence corresponded to that of acid strength of the support materials measured by Hammett indicators. The relation between the oxidation state of palladium and its catalytic activity under the various mixtures indicated that the catalytic activity on each catalyst depends on the oxidation state of palladium, and that partially oxidized palladium is effective for propane combustion. Moreover, palladium on acidic support kept the metallic state even under high oxygen concentration, indicating that acidic support gives palladium the resistibility against oxidation. These results suggest one guideline for the design of active palladium catalyst; an acidic support must be chosen under the mixture of the higher oxygen concentration and vice versa.

Effect of Oxidation State of Palladium in Polyaniline Layers on Sensitivity to Hydrogen

It is shown that the ratio of Pd0/Pd2+ in polyaniline (PANI) has a major effect on the response of film to hydrogen. The palladium clusters were incorporated in PANI as the result of spontaneous reduction of PdSO4 by electroactive PANI in the leucoemeraldine or nigraline form, respectively. The two forms of PANI were prepared, by terminating the electrochemical deposition at –0.1 V (PANI–0.1 V) and by terminating it at +0.7 V (PANI0.7 V). A higher amount of Pd is incorporated in the in PANI when the reduction of Pd2+ is initiated from the leucoemeraldine form. The ratio of the amine/imine units correlates well with the changes of the Pd0/Pd2+ratio as shown by the XPS. The changes of the work function were measured by a Kelvin Probe. A substantial decrease of WF was observed when the PANI–0.1 V-Pd composite film was exposed to hydrogen. On the other hand the WF of PANI0.7 V-Pd film increases upon exposure to hydrogen. The change in the electron donor/electron acceptor behavior towards hydrogen is governed by the Pd0/Pd2+ratio in the PANI/Pd matrix.

Oxidation state of palladium as a factor controlling catalytic activity of Pd/SiO 2–Al 2O 3 in propane combustion

The oxidation state of palladium on SiO 2–Al 2O 3 used for propane combustion was examined by XPS and XRD, and the correlation of the catalytic activity with the oxidation state of palladium was systematically studied. The propane conversion over 5 wt% Pd/SiO 2–Al 2O 3 was measured in the range 1.0≤ S≤7.2 ( S is defined as [O 2]/5[C 3H 8] based on stoichiometric ratio). The propane conversion strongly depended on the S value and reached the maximum at S=5.5. The oxidation state of palladium also changed with the S value; palladium particles were more oxidized under the reaction mixture of higher S value. On the sample used for the reaction at S=5.5, both of metallic palladium and palladium oxide were found. It is concluded that partially oxidized palladium which has optimum ratio of metallic palladium to palladium oxide shows the highest catalytic activity in propane combustion.

Nitric Oxide Reduction with Methane over Pd/TiO2Catalysts

Reduction of NO with methane was investigated over Pd/TiO2catalysts using isotopically labeled species. Steady-state as well as unsteady-state isotopic labeling studies have been performed using the switches14N16O→15N18O,16O2→18O2, CH4+15NO+O2→ CH4+14NO+O2, CH4+15NO+O2→CH4+O2,13CH4→NO,13CH4+NO→He,13CH4+NO→NO,13CH4+NO+O2→NO,13CH4→O2, CH4+15NO→CH4+15N18O, and N2O→He, which have given some important clues regarding the mechanistic aspects of the NO–CH4interaction. Based on these studies, it was concluded that N2is formed through direct participation of CH4whereas N2O formation is mainly a result of NO decomposition reaction. It was also observed that at least three types of reactions occur simultaneously on the surface, namely NO+CH4, CH4oxidation and NO decomposition. The extent of these reactions is determined by the oxidation state of palladium on the surface.

Total ethane oxidation over Pd/α-Al 2O 3: the palladium oxidation state under reaction conditions

The total oxidation of ethane was studied over Pd/α-Al 2O 3 catalysts in an oxygen rich atmosphere. After palladium reduction by hydrogen treatment at 573 K it was found that catalysts became more active with time on stream. The increase in activity is a consequence of bulk palladium oxide formation. The extent of palladium oxidation at the steady state was determined by oxygen uptake measurements before and after the reaction at 573 K and by hydrogen titration at 323 K after the reaction. A previous oxidation or a high temperature treatment of the reaction mixture did not affect the level of activity and a time dependent activation process was not observed. The catalytic activity of palladium does not depend on the initial metal dispersion. Moreover, the rate of reaction was found to depend on the total amount of palladium. This result suggests that under reaction conditions all palladium atoms become active for C 2H 6 oxidation.

X‐ray photoelectron spectroscopy studies of fired palladium electrode ink materials

AbstractThe oxidation state of palladium in an electrode ink dispersion, air‐dried on barium titanate dielectric substrates, has been studied using x‐ray photoelectron spectroscopy (XPS) as a function of both firing temperature and firing atmosphere. Several different states have been identified in amounts which vary over the firing temperature range of interest, i.e. palladium metal, a so‐called PdOads species, PdO and palladium in one or more higher oxidation states. No significant differences were observed for the two different contacting atmospheres used during firing, namely air and flowing nitrogen. Owing to overlap of the Pd 3p3/2 and the O 1s spectral envelopes, peak fitting of the oxygen signal was not attempted. Small amounts of residual carbonaceous ash were detected for all the samples studied over the whole range of firing temperatures employed.XPS analysis of a sample of the palladium ink, fired to 1350°C in air and subsequently etched using a fast atom bombardment (FAB) argon source, showed that the sub‐surface palladium was metallic in nature although the original metal surface was composed almost entirely of PdO. Thus, for these palladium inks, when fired to temperatures comparable with those used in multilayer ceramic capacitor (MLCC) manufacture, apart from a thin surface oxide layer the palladium present is in the metallic form, as required for electrode layer conduction.

Studies of the oxidation state and location of palladium species in Al13-pillared montmorillonite

Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) have been used to establish the oxidation state of palladium in Pd(NH3)2+4-doped Al13-pillared Texas montmorillonite clay (Pd/Al13 STx) after various sample treatments. Heating of Pd/Al13 STx in flowing oxygen at 350 and 500 °C produces Pd3+which appears to migrate into the montmorillonite lattice. Both Pd2+ and Pd3+ are strongly bound to the internal and external surfaces of the montmorillonite layers. Some of the Pd2+ species present after heating can be reduced to Pd+ by subjecting the sample to flowing hydrogen at ambient temperature. Pd3+ cations generated during heating are essentially unaffected by this reductive treatment.

Solvent, not palladium oxidation state, is the primary determinant for successful coupling of terminal alkynes with iodo-nucleosides

Coupling of iodo-nucleosides with terminal alkynes such as 3-(acylamino)propynes, whose initial products readily undergo secondary cyclization reactions, can be effected smoothly by the standard catalysis with (Ph3P)2PdCl2/CuI/Et3N in dimethylformamide.

X‐Ray Photoelectron Spectroscopy of Palladium Fluorides

AbstractX‐ray photoelectron spectra of several Palladium fluorine compounds have been investigated. In all these phases Palladium atoms have a more or less distorted octahedral coordination. Satellite Structures generally occur in Pd 3d3/2 and Pd 3d5/2 levels and broad F 1s peaks are observed. Shifts in binding energies can be correlated with Palladium oxidation State and covalency. XPS results clearly confirm the unusual +III oxidation State of Palladium in NaPdF4, Na3PdF6, K3PdF6, and K2NaPdF6.

Palladium compounds with +III oxidation state

The trivalent oxidation state of palladium has been stabilized and unequivocally characterized in several fluoride series. NaPdF 4, whose structure can be derived from the KBrF 4 type, is obtained under high pressure and is quenched back to ambient conditions. Elpasolite compounds of A 2BPdF 6 formulation (A, B = alkali metal) are synthesized via solid state reactions. Between 200 and 500 K every A 2BPdF 6 compound undergoes a tetragonal cubic phase transition. The ESR spectra are characteristic of a doublet ground state. The t 6 2g e 1 g configuration of Pd(+III) is stabilized versus the t 5 2ge 2 g high-spin alternative by an important Jahn-Teller splitting of the 2E g state. In most cases ESR spectra are consistent with tetragonally elongated PdF 6 octahedra; an orthorhombic component occurs for Cs 2KPdF 6 and Cs 2RbPdF 6. ▪ ESR spectra with tetragonal (a-K 2NaPdF 6) and orthorhombic (b-Cs 2KPdF 6) symmetries.

Palladium Compounds with +III Oxidation State

AbstractTrivalent oxidation state of palladium has been unambiguously characterized in elpasolites of A2BPdF6 formulation (A, B = alkali metal). At room temperature K2LiPdF6 is cubic (Fm3m space group, a = 8.154 Å) while all other A2BPdF6 compounds possess a tetragonal F4/mmm symmetry with c/a > 1. Phase transitions in Pd elpasolites have been followed using low‐and hightemperature X‐ray diffraction. Between 200 and 500 K every A2BPdF6 compound undergoes a tetragonal ⇄ cubic phase transition. The ESR signals are characteristic of a doublet ground state; they bring up the unequivocal evidence of a te configuration of Pd(+III). This configuration is stabilized versus the te high ‐spin alternative by an important Jahn‐Teller splitting of the 2Eg‐state. ESR results confirm that the PdF6 octahedra are tetragonally elongated in most cases; an additional orthorhombic component is found for Cs2KPdF6 and Cs2RbPdF6. The occurence of Pd(+III) in a low‐spin state is corroborated by magnetic measurements.