Pd 3d Binding Energy Research Articles

Pt modulates the electronic structure of Pd to improve the performance of Pd-based catalytic combustion catalyst

The electronic modulation between the catalytic active components can improve the catalytic activity and stability of the catalyst. The Pd-based catalysts can easily react with SOX to form stable and inactive sulfates. In this paper, the Pd–Pt-based catalytic combustion catalyst was prepared by replacing part of Pd with a small amount of Pt. The storage tank VOCs catalytic combustion activity and the anti-SO2 poisoning performance of the Pd–Pt-based catalyst and Pd-based catalyst were tested. The Pd 3d binding energy of each Pd-based catalyst was detected by XPS characterization, and the electronic structure changes of Pd active components was analyzed by the change of Pd 3d binding energy. The effect of electrons transfer between Pd and Pt on the improvement of catalytic combustion activity and SO2 poisoning resistance of Pd-based catalysts was analyzed. The results show that the Pt addition can increase the electron cloud density of the Pd active components, and improve the performance of the Pd active components to adsorb and activate oxygen. The reaction of Pd and SOX to form sulfate needs to gain electrons. The increase in the electron cloud density of the Pd active components in Pd–Pt-based catalyst makes it difficult for the Pd active components to adsorb SOX and difficult to react with SOX to form sulfate, thereby preventing the Pd active components from being poisoned and deactivated.

Elucidating Interaction between Palladium and N-Doped Carbon Nanotubes: Effect of Electronic Property on Activity for Nitrobenzene Hydrogenation

Nitrogen dopants of carbon materials remarkably improve the stability and tune the catalytic performance of supported metal nanoparticles. However, it is still controversial how the Pd–N metal–support-interaction (MSI) influences the catalysis. Herein, the density function theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) were combined to rationalize the Pd–N MSI. DFT calculations suggested that Pd adsorbs on N-doped carbon nanotubes (N@CNTs) and donates electrons to pyridinic nitrogen. It was further experimentally proved using XPS through a titration method by gradually increasing Pd content or changing the N content of support by a postheat-treatment. The Pd catalysts display electron-deficiency depending on the intensity of MSI between Pd and pyridinic nitrogen, measured by Pd 3d binding energy. It paves the way to the rational synthesis of Pd catalysts with a tunable electronic state for the targeted catalytic reaction. Using the hydrogenation of nitrobenzene as the probe reaction,...

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.

FeOx Coating on Pd/C Catalyst by Atomic Layer Deposition Enhances the Catalytic Activity in Dehydrogenation of Formic Acid

Hydrogen generation from formic acid (FA) has received significant attention. The challenge is to obtain a highly active catalyst under mild conditions for practical applications. Here atomic layer deposition (ALD) of FeOx was performed to deposit an ultrathin oxide coating layer to a Pd/C catalyst, therein the FeOx coverage was precisely controlled by ALD cycles. Transmission electron microscopy and powder X-ray diffraction measurements suggest that the FeOx coating layer improved the thermal stability of Pd nanoparticles (NPs). X-ray photoelectron spectroscopy measurement showed that deposition of FeOx on the Pd NPs caused a positive shift of Pd3d binding energy. In the FA dehydrogenation reaction, the ultrathin FeOx layer on the Pd/C could considerably improve the catalytic activity, and Pd/C coated with 8 cycles of FeOx showed an optimized activity with turnover frequency being about 2 times higher than the uncoated one. The improved activities were in a volcano-shape as a function of the number of FeOx ALD cycles, indicating the coverage of FeOx is critical for the optimized activity. In summary, simultaneous improvements of activity and thermal stability of Pd/C catalyst by ultra-thin FeOx overlayer suggest to be an effective way to design active catalysts for the FA dehydrogenation reaction.

B-N Codoped Graphene as a Novel Support for Pd Catalyst with Enhanced Catalysis for Ethanol Electrooxidation in Alkaline Medium

B-N-doped graphene (B-N-G) was designed as a novel carrier for Pd nanoparticles (NPs) and it was used as the electrocatalyst for ethanol oxidation in alkaline medium. B and N species were introduced onto the graphene surface by the process of B and N sequentially incorporation. According to XPS results, most of B and N atoms are bonded with C atoms separately, which could increase the spin density and activate the neighboring carbon atoms, and further cause strong interaction between Pd and substrates. So, Pd/B-N-G catalyst presents most metallic Pd content and least Pd2+ content, as well as decreased Pd-3d binding energy. TEM results showed that Pd NPs dispersed most uniformly with an average size of 2.9 nm on Pd/B-N-G. Electrochemical evaluation showed that Pd/B-N-G catalyst exhibited much improved catalytic performance and poisoning tolerance for ethanol oxidation in alkaline medium as compared to those of Pd/B-G, Pd/N-G and Pd/G. B-N-G carrier promoted the high dispersion of Pd NPs due to the coexisting active sites introduced by B-N codoping, which improved the utilization efficiency of Pd. Furthermore, the enhanced electronic effects between Pd particles and the support caused by B-N codoping, may cause synergistic effects and thereby eventually improve the catalytic performance.

Preparation of Pd-Co-based nanocatalysts and their superior applications in formic acid decomposition and methanol oxidation.

Formic acid (FA) and methanol, as convenient hydrogen-containing materials, are most widely used for fuel cells. However, using suitable and low-cost catalysts to further improve their energy performance still is a matter of great significance. Herein, PdCo and PdCo@Pd nanocatalysts (NCs) are successfully prepared by the facile method. Pd 3d binding energy decreases due to the presence of Co. Consequently, PdCo@Pd NCs exhibit high catalytic activity and selectivity toward FA dehydrogenation at room temperature. The gas-generation rate at 30 min is 65.4 L h(-1) g(-1) . PdCo/C has the worst catalytic performance in this reaction, despite the fact that it has a high gas-generation rate in the initial 30 min. Furthermore, both PdCo and PdCo@Pd NCs have enhanced electrocatalytic performance toward methanol oxidation. Their maximum currents are 966 and 1205 mA mg(-1) , respectively, which is much higher than monometallic Pd/C.

Alumina support and Pdn cluster size effects on activity of Pdn for catalytic oxidation of CO

A series of model catalysts were prepared by depositing different size Pd(n) clusters on alumina films grown to variable thickness on a Ta(110) support. Samples were characterized by a combination of X-ray photoelectron spectroscopy, low energy He+ scattering, and temperature-programmed reaction and desorption (TPR/ TPD). For the activity studies, the samples were first exposed to 18O2 at Tox, and then to 13CO at 180 K, where CO sticks to Pd, but not to the alumina support. CO oxidation activity increased with increasing thickness of the alumina support up to approximately 4.5 nm, but was constant for greater thicknesses. Activity increased, with Tox up to 400 K, but then declined for Tox = 500 K. Activity was also found to be non-monotonically dependent on deposited cluster size, with Pd(n) (n < or = 6) being generally more reactive than the larger clusters studied. Activity was only weakly correlated with exposed Pd binding sites, which decreased with increasing cluster size, however, there does appear to be a correlation between activity and electronic structure, as probed via the Pd 3d binding energy. Unlike previous systems we have studied, the activity of small Pd(n) on these alumina films was quite stable, with essentially no changes observed in up to eight successive TPR experiments.

Characterization of palladium oxide catalysts supported on nanoparticle metal oxides for the oxidative coupling of 4-methylpyridine

Palladium catalysts supported on various metal oxides were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate why these catalysts do not show any correlation between the measured Pd surface area and the catalytic activity for the oxidative coupling of 4-methylpyridine to 4,4′-dimethyl-2,2′-bipyridine. The XPS data confirm the classification of n-Al 2O 3(+), n-MgO and p-SiO 2 as non-interacting supports, since the Pd 3d 5/2 binding energy (BE) of palladium on these supports is 336.1 eV, consistent with bulk PdO. In contrast, catalysts supported on p-TiO 2, n-ZnO, n-ZrO 2, n-ZrO 2(CeO 2), and n-CeO 2 have Pd 3d 5/2 BEs ranging from 336.3 to 337.4 eV, which reveal varying degrees of metal-support interactions. Metal support interactions leading to electron deficient Pd 2+ species are likely beneficial for the reaction due to a facilitated C–H insertion step. While both the PdO/p-TiO 2 and PdO/n-TiO 2 catalysts have a Pd 3d BE of 336.3 eV, their differences in activity can be attributed to (1) the PdO/n-TiO 2 catalyst as prepared having a significantly higher number of hydroxyl groups on the surface compared with the PdO/p-TiO 2 catalyst, and (2) the p-TiO 2 support being crystalline with an anatase phase, while the n-TiO 2 support is nearly amorphous. The presence of surface hydroxyl groups before reaction could hinder the first C–H activation step, and an anatase phase of the support can result in more favorable palladium-support interactions compared with an amorphous TiO 2. The XPS data also indicates that while Pd-support interactions are beneficial, very strong interactions, such as in the case of CeO 2, can lead to migration of the support over Pd, which reduces the Pd surface area and explains the lower than expected activity of the PdO/n-CeO 2 catalyst. On some supports in this study leaching may occur during the reaction, but the characterization data indicate that other factors of catalyst deactivation are more important. XRD reveals that the complete reduction of the PdO particles on the surface is very fast compared to the reaction time. This observation explains why reducible supports with mobile oxygen are beneficial in this reaction. These supports can facilitate the reoxidation of palladium due to strong metal-support interactions. Migration of the support over the active palladium species is another deactivation pathway that appears to be more severe than leaching on these catalysts.

Influence of CeO 2 and La 2O 3 on properties of palladium catalysts used for emission control of natural gas vehicles

Pd/YZ-Al 2O 3 (Y and Zr modified Al 2O 3, and hereafter, labelled as Al) catalysts with 4 wt% additive CeO 2 and/or La 2O 3 were prepared and characterized by X-ray photoelectron spectroscopy (XPS), NO-temperature programmed desorption (NO-TPD), N 2-adsorption/desorption (Brunauer-Emmet-Teller BET method), X-ray diffraction (XRD) and CO-chemisorption. Catalytic activities for CH 4, CO and NO conversion were tested in a gas mixture simulated the emissions from natural gas vehicles (NGVs) operated under stoichiometric conditions. The results indicated that all catalysts exhibited excellent catalytic performances for CH 4 and CO oxidation and the promoting effect of CeO 2 or La 2O 3 was significant for NO conversion. XPS results showed that the electron density around Pd was increased by CeO 2 and/or La 2O 3, the binding energy of Pd 3d decreased as the order: Pd/Al > Pd/Ce/Al > Pd/La/Al > Pd/CeLa/Al. The electron-rich Pd showed Rh-like catalytic properties which exhibited good activity for the reduction of NO. NO-TPD results showed that the addition of CeO 2 and/or La 2O 3 increased NO adsorption on surface, and promoted the conversion of NO.

Size-dependent oxidation of Pd n ( n ⩽ 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pd n ( n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O 2 at temperatures ranging from 100 to 500 K. After O 2 exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd 4, which did not oxidize under any conditions. The inertness of Pd 4/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O 2 exposures at 300 K or above, but He + scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.

Performance of Pd catalysts supported on nanocrystalline α-Al 2O 3 and Ni-modified α-Al 2O 3 in selective hydrogenation of acetylene

Nanocrystalline α-Al 2O 3 and Ni-modified α-Al 2O 3 have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl 2O 4 spinel was formed on the Ni-modified α-Al 2O 3 after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al 2O 3 powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al 2O 3 and Ni-modified α-Al 2O 3 in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al 2O 3 supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al 2O 3–sol–gel > Pd/Ni-modified α-Al 2O 3-solvothermal ≈ Pd/α-Al 2O 3–sol–gel > Pd/α-Al 2O 3-solvothermal ≫ Pd/α-Al 2O 3-commerical. As revealed by NH 3 temperature program desorption studies, incorporation of Ni atoms in α-Al 2O 3 resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al 2O 3–sol–gel where only NiAl 2O 4 was formed, suggesting that the electronic properties of Pd may be modified.

Impact of palladium silicide formation on the catalytic properties of Pd/SiO 2 catalysts in liquid-phase semihydrogenation of phenylacetylene

In this study, palladium silicide was formed on the sol–gel derived SiO 2 supported Pd catalysts when they were prepared by ion-exchange method using Pd(NH 3) 4Cl 2 as a palladium precursor. No other palladium phases (PdO or Pd 0) were evident after calcinations at 450 °C for 3 h. The Pd/SiO 2 catalysts with Pd silicide formation were found to exhibit superior performance than commercial SiO 2 supported ones in liquid-phase semihydrogenation of phenylacetylene. From XPS results, the binding energy of Pd 3d of palladium silicide on the Pd/SiO 2 catalyst shifted toward larger binging energy, indicating that Pd is electron deficient. This could probably result in an inhibition of a product styrene on the Pd surface and hence high styrene selectivities were obtained at high phenylacetylene conversions. The formation of Pd silicide, however, did not have much impact on specific activity of the Pd catalysts since the TOFs were quite similar among the various catalysts with or without palladium silicides if their average particle sizes were large enough. The TOFs decreased by an order of magnitude when palladium dispersion was very high and their average particle sizes were smaller than 3–5 nm.

Catalytic methanol decomposition at low temperatures over palladium supported on metal oxides

Palladium catalysts supported on metal oxides have been prepared by a coprecipitation method for the selective decomposition of methanol to CO and H 2. The catalysts whose supports are ZrO 2, Pr 2O 3, and CeO 2 are significantly active at 200–300°C. Turnover frequency determined from the activity of the catalysts containing 15 wt% of Pd at 200°C, increased with an increase in the binding energy of Pd 3d 5/2 for the catalysts, indicating that a positively charged Pd species, most likely Pd +, is preferable for the low-temperature decomposition of methanol. The coprecipitation method is advantageous in the preparation of catalysts containing a large amount of Pd over an impregnation method because smaller Pd particles interacting strongly with the support can be produced by the former technique.

Effect of sodium on the electronic properties of Pd/silica-alumina catalysts

The effect of the alkali ion content on the electronic properties of palladium, supported on natural pumice and on synthetic aluminosilicates, was studied by X-ray photoelectron spectroscopy (XPS) measurements. To this aim a series of catalysts of palladium supported on aluminosilicates with various amounts of sodium and potassium in the bulk structure were prepared and analysed. The Pd 3d 5/2 binding energy shifts of the different catalysts with respect to pure metal Pd 3d 5/2 binding energy were followed as a function of the Na/Pd atomic ratio. For reasons of comparison, a silica supported Pd catalyst whose surface had been doped with sodium was also studied. A correlation of the Pd 3d binding energy with the sodium content was found, regardless on whether sodium was present in the bulk structure or just at the surface of the catalyst. The effect was independent of the catalyst preparation procedure. The analysis of the intensity of the photoelectron peaks of Pd 3d and Si 2p of the lower surface area catalysts, using the Kerkhof-Moulijn model, yielded particle sizes in agreement with values obtained from small angle X-ray diffraction measurements (SAXS). For the large surface area supported catalysts a monolayer type of metal distribution was found. The intensity ratios of the Na 1s and Si 2p photoelectron peaks are in accordance with those estimated from the bulk Na/Si atomic ratios of the synthetic and natural pumice support. Therefore any segregation of the alkali ions on the palladium particle of the corresponding catalysts are ruled out. The electronic effect is thus discussed in terms of the formation of an electron donor composite formed by the metal, the oxygen and the alkali ion.

Electronic structure of palladium deposits on alumina

We have examined palladium deposits on the (0001) face of α-alumina, using X-ray photoemission. The evolutions of the Pd3d binding energy (BE) and intensity were followed for various palladium vapour fluxes. Energy referencing with use of the Al 2p peak confirms the electrostatic origin of the decreasing BE for increasing palladium deposits. Taking the metal Fermi level as a reference shows a general tendency to increasing BE with cluster size, the details of the evolution depending on the actual structure of the deposits. This behaviour is attributed to valence band broadening, as a consequence of the changes in the number of nearest neighbours.

Electronic structure of palladium deposits on alumina

Abstract We have examined palladium deposits on the (0001) face of α-alumina, using X-ray photoemission. The evolutions of the Pd3d binding energy (BE) and intensity were followed for various palladium vapour fluxes. Energy referencing with use of the Al 2p peak confirms the electrostatic origin of the decreasing BE for increasing palladium deposits. Taking the metal Fermi level as a reference shows a general tendency to increasing BE with cluster size, the details of the evolution depending on the actual structure of the deposits. This behaviour is attributed to valence band broadening, as a consequence of the changes in the number of nearest neighbours.

Further characterisation of Pd deposited on an extensively oxidised Zn(001) support

Palladium overlayers have been deposited onto both oxidised Zn(001) surfaces (ZnO x ) and polycrystalline zinc oxide powder discs, and have subsequently been characterised by XPS, AES and SIMS. The growth mode was found to vary between the two surface types. Oxidation of the deposited metal was not observed, however a negative shift in the Pd 3d binding energy with increasing coverage is reported. There is some evidence of zinc-palladium alloying at the interface. Both surfaces were found to be stable at elevated temperatures for short periods of time, with a slow interdiffusion occurring at the interface during longer intervals.

Characterization of the Pd/a-Al2O3 supported cluster system

Palladium deposits supported on amorphous alumina were prepared in ultrahigh vacuum and studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and energy loss spectroscopy (ELS). The growth of clusters with deposition time was followed using TEM. The XPS, AES, and TEM results support a consistent model for the distribution of palladium on the alumina substrate, with the amount of material within the clusters accounting for the majority of the palladium deposited. The combined results of XPS, ELS, and TEM indicate that the deposit growth with deposition time can be tentatively separated into the following size regimes: monolayer islands, ‘‘small’’ clusters and metallic three-dimensional clusters. The size-dependent XPS Pd 3d binding energy shifts are analyzed within the initial-state-final-state framework. Component fitting of the Pd 3d spectra is consistent with assigning one of the components to the layer of interfacial Pd atoms.