Pd CO Research Articles

Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors

The effects of CO and H2S as catalyst inhibitors on the rate of toluene hydrogenation were studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO2 suffered serious negative effects from catalyst inhibitors; however, Pd/TiO2–SiO2 exhibited high CO and H2S tolerance because the acidic support decreased the electron density of the Pd metal particles, which, in turn, decreased the interaction between the Pd surface and CO (or H2S). The TiO2–SiO2-supported Pd catalyst exhibited activity greater than that of the TiO2–SiO2-supported Pt catalyst in the presence of CO; however, it exhibited lower activity in presence of H2S. Catalyst characterization after sulfidation with H2S revealed that Pd particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity in the presence of H2S.

Eggshell membrane templated Y2O3@Pd catalyst for enhanced methanol oxidation and CO tolerance

Macroporous Pd and Y2O3@Pd network catalysts are synthesised using eggshell membrane as a template and explored for the methanol oxidation reaction (MOR) to replace the Pt catalysts and enhance the electrocatalytic activity and CO poisoning tolerance. The scanning electron microscopy observation, porosity, X-ray diffraction patterns and X-ray photoelectron spectra are used to characterise the resultant catalysts. The results reveal that the well crystalline Pd is in its metallic state and Y2O3 has crystalline hexagonal structure, and there is an interaction between Pd and Y2O3 on the interface of Pd and Y2O3. The cyclic voltammetric studies at room temperature indicate an enhanced electrocatalytic activity for MOR in acidic medium. Because of the preferential formation of Y–CO bond over Pd–CO bond, the resultant Y2O3@Pd catalysts remain excellent electrocatalysis toward methanol oxidation and CO poisoning tolerance. The enhanced electrocatalytic activity, excellent stability and CO tolerance, along with lowered cost, have promised the Y2O3@Pd to be a high performance and robust catalyst for methanol oxidation in direct methanol fuel cells.

Ab initio study of CO adsorption on PdGa(1 1 0)

CO adsorption is analyzed using Density Functional Theory (DFT) calculations. Changes in the electronic structure of PdGa(110) surface and CO bond after adsorption are addressed. CO is located only on Pd atop geometry with a tilted configuration (9.13° from the perpendicular to the surface) and no interaction with Ga is detected. The Pd–Pd bond strength decreases 54.2% as the new Pd–CO bond is formed. The C–O bond length change less than 1%, compared to the gas phase value, while its bond overlap population decrease 46.2%. The effect of CO is limited to its first Pd neighbor. Analysis of orbital interaction reveals the Pd–CO bond mainly involves s–s and s–p orbitals with less participation of Pd 4d orbitals. The computed CO vibration frequencies after adsorption shows a red shift from vacuum to ward 2013.85cm−1, which agrees with previous experimental data on PdGa intermetallic.

The effect of CO on intermetallic PdZn/ZnO and Pd2Ga/Ga2O3 methanol steam reforming catalysts: A comparative study

The catalytic properties in methanol steam reforming (MSR) of PdZn/ZnO and Pd2Ga/Ga2O3 were investigated and related to the actual surface composition. MSR selectivities of around 90% were observed on both catalysts. In situ FTIR spectroscopy was utilized to clarify the origin of the CO formed as a by-product. Exposure to CO at room temperature caused a degradation of both intermetallic surfaces, which was more severe on Pd2Ga. Likely, the strong Pd–CO interaction led to enrichment of Pd at the surface, as observed by the CO spectra resembling that of CO on metallic Pd. A limited stability of the PdZn and Pd2Ga surfaces was also observed in methanol/water, most likely due to the CO formed as a by-product in MSR. Therefore, the surface under reaction conditions consists of domains of metallic Pd (or a Pd-rich alloy) in addition to the intermetallic surface. This effect was more pronounced at lower temperatures. A faster in situ regeneration of the intermetallic surface by the hydrogen produced in MSR at elevated temperatures (>500K) is proposed. Degradation by CO and re-reformation of the alloy by H2 likely leads to a certain steady state of the surface. X-ray absorption spectroscopy measurements indicate that the effect of CO apparently goes beyond the surface only.

Catalytic reactivity of face centered cubic PdZnα for the steam reforming of methanol

Addition of Zn to Pd changes its catalytic behavior for steam reforming of methanol. Previous work shows that improved catalytic behavior (high selectivity to CO2) is achieved by the intermetallic, tetragonal L10 phase PdZnβ1, where the Pd:Zn ratio is near 1:1. The Pd–Zn phase diagram shows a number of other phases, but their steady-state reactivity has not been determined due to the difficulty of precisely controlling composition and phase in supported catalysts. Hence, the role of Zn on Pd has generally been studied only on model single crystals where Zn was deposited on Pd(111) with techniques such as TPD and TPR of methanol or CO. The role of small amounts of Zn on the steady-state reactivity of Pd–Zn remains unknown. Therefore, in this work, we have synthesized unsupported powders of phase pure PdZnα, a solid solution of Zn in fcc Pd, using a spray pyrolysis technique. The surface composition and chemical state were studied using Ambient Pressure-XPS (AP-XPS) and were found to match the bulk composition and remain so during methanol steam reforming (MSR) (Ptot=0.25mbar). Unlike the PdZnβ11 phase, we find that PdZnα is 100% selective to CO during methanol steam reforming with TOF at 250°C of 0.12s−1. Steady-state ambient pressure micro-reactor experiments and vacuum TPD of methanol and CO show that the α phase behaves much like Pd, but Zn addition to Pd improves TOF since it weakens the Pd–CO bond, eliminating the poisoning of Pd by CO during MSR over Pd. The measured selectivity for fcc PdZnα therefore confirms that adding small amounts of Zn to Pd is not enough to modify the selectivity during MSR and that the PdZnβ1 tetragonal structure is essential for CO2 formation during MSR.

Enhanced methanol oxidation and CO tolerance using CeO2-added eggshell membrane-templated Pd network electrocatalyst

Macroporous Pd and CeO2-added Pd network catalysts have been synthesized using eggshell membrane (ESM) as a template for enhanced methanol oxidation and CO tolerance. The microstructural characterization revealed a hierarchically ordered macroporous network of Pd reproducing the fibrous structure of ESM for a Pd-only catalyst, and a flower-like CeO2-decorated Pd morphological architecture for the CeO2-added Pd catalyst synthesized by a precipitation method. XRD patterns indicated Pd and CeO2 phases with good crystallinity. The cyclic voltammetry studies showed an enhanced electrocatalytic activity for methanol oxidation in acidic aqueous medium. Because of the preferential formation of Ce–CO bonds over Pd–CO bonds, the incorporation of CeO2 into Pd-based catalysts results in an increased CO tolerance, making it a robust catalyst for methanol oxidation in direct methanol fuel cells.

Highly active Pd-based catalysts with hierarchical pore structure for toluene oxidation: Catalyst property and reaction determining factor

A series of micro/mesoporous hybridized materials with coke-resistant ZSM-5 and three-dimensional large pore KIT-6 as their micro- and mesoporous components were synthesized and investigated for toluene deep oxidation. The characterization results show that most of Al atoms are tetrahedrally coordinated in the framework of the samples, and the ratio of ZSM-5 to KIT-6 can be systematically controlled by varying the Si/Al molar ratio in the synthesis mixture. The acid sites over the supports are favorable for Pd nanoparticle dispersion and metallic Pd oxidation due to their electrophilic characters. All Pd supported composite materials are found to be powerful catalysts for the total oxidation of toluene. Particularly, Pd/ZK-50 possesses the highest catalytic activity with 90% toluene decomposition at 197 °C, this temperature is much lower than that of Pd/KIT-6 ( T 90 = 234 °C). CO 2 desorption capability has positive effects on catalytic activity, while the influence of toluene adsorption property is negligible. The superior activity of the composite catalysts can be ascribed to the cumulative of specific surface area, support acidity, Pd particles dispersion and CO 2 desorption capability.

Formation of Carbon–Sulfur and Carbon–Selenium Bonds by Palladium-Catalyzed Decarboxylative Cross-Couplings of Hindered 2,6-Dialkoxybenzoic Acids

A simple route to diaryl sulfides using a decarboxylative palladium-catalyzed reaction between electron-rich 2,6-dialkoxybenzoic acid derivatives and diaryl disulfides is reported. This coupling proceeds efficiently in the presence of Pd(CF(3)CO(2))(2) and Ag(2)CO(3) in a 65:1 mixture of 1,4-dioxane and tetramethylene sulfoxide (TMSO). We present also the first formation of a carbon-selenium bond via a palladium-catalyzed decarboxylative cross-coupling.

Nanoparticulate Pd Supported Catalysts: Size-Dependent Formation of Pd(I)/Pd(0) and Their Role in CO Elimination

A combination of time-resolved X-ray absorption spectroscopy (XAS), hard X-ray diffraction (HXRD), diffuse reflectance infrared spectroscopy (DRIFTS), and mass spectrometry (MS) reveals a series of size-dependent phenomena at Pd nanoparticles upon CO/(NO+O(2)) cycling conditions. The multitechnique approach and analysis show that such size-dependent phenomena are critical for understanding Pd CO elimination behavior and, particularly, that different Pd(I) and Pd(0) centers act as active species for a size estimated by XAS to be, respectively, below and above ca. 3 nm. The relative catalytic performance of these two noble metal species indicates the intrinsic higher activity of the Pd(I) species.

Enhancement of electrochemical properties on Pd–Cu/C electrocatalysts toward ethanol oxidation by atmosphere induced surface and structural alteration

The preparation and modification of high effective Pd–Cu/C electrocatalysts toward ethanol oxidation reaction (EOR) in alkaline solution are investigated. A simple strategy through atmosphere-induced surface/structural modification is applied to promote their electrochemical performance. The alterations of EOR activities, structures and surface species of the prepared catalysts induced by H2, O2 or CO treatments are analyzed by CV, XRD and XPS techniques, respectively. It is observed that the degree of Pd–Cu alloying, surface Pd oxidation, CO oxidation and EOR activity are all promoted by the CO treatment at 470K and the current density at −0.2V of the as-prepared sample is enhanced about 190% due to the formation of perfect alloy core/PdO shell structure.

Reaction Kinetics and Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PM-IRAS) Investigation of CO Oxidation over Supported Pd−Au Alloy Catalysts

Pd−Au bimetallic model catalysts, synthesized either as thin films on Mo(110) or as nanoparticles on a TiO2 thin film, were used in CO oxidation at both low (1 × 10−7 Torr) and elevated (8−16 Torr) CO pressures. Alloying with Au forms isolated Pd sites that are incapable of dissociating O2. This inability causes reactivity loss at low reactant pressures. Moreover, alloying also electronically modifies surface sites and affects the reaction activation energy. At elevated pressures, Pd preferentially segregates to the surface to form contiguous Pd sites and CO oxidation reactivity is regained. Under stoichiometric conditions and relatively low temperatures, Pd−Au alloys show superior reactivity compared with pure Pd due to more facile CO desorption (less CO inhibition). Under net oxidizing conditions, metallic Pd displays superior reactivity due to its higher capability of dissociating O2. However, pure Pd oxidizes and loses reactivity more readily than do Pd−Au alloys.

Associative versus dissociative binding of CO to 4d transition metal trimers: A density functional study

Density functional calculations were performed to determine equilibrium geometrical structures, transition states and relative energies for M(3) clusters (M = Nb, Mo, Tc, Ru, Rh, Pd, Ag) reacting with CO, leading to proposed reaction pathways. For the Nb(3), Mo(3), and Tc(3) clusters, the lowest energy structure correlates to dissociated CO, with the C and O atoms bound on opposite sides of the metal triangle. For all other trimers, the lowest energy structures maintain the CO moiety. In the case of Pd(3) and Ag(3) the dissociated geometries lie higher in energy than the sum of the separated reactants. In most cases, several multiplicities were found to be similar in energy and for Mo(3)CO and Pd(3)CO singlet-triplet minimum energy crossing points were identified. In the case of Rh(3)CO, minimum energy crossing points for the doublet, quartet, and sextet reaction pathways were determined and compared. The electron densities of pertinent M(3)CO species were investigated using Natural Bond Order calculations. It was found that the effect of the metal trimer on the energy of the pure p-type pi* antibonding orbital of carbon monoxide directly correlates with the occurrence of CO dissociation.

CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC

The performance of H 2/O 2 proton exchange membrane fuel cells (PEMFCs) fed with CO-contaminated hydrogen was investigated for anodes with PdPt/C and PdPtRu/C electrocatalysts. The physicochemical properties of the catalysts were characterized by energy dispersive X-ray (EDX) analyses, X-ray diffraction (XRD) and “in situ” X-ray absorption near edge structure (XANES). Experiments were conducted in electrochemical half and single cells by cyclic voltammetry (CV) and I–V polarization measurements, while DEMS was employed to verify the formation of CO 2 at the PEMFC anode outlet. A quite high performance was achieved for the PEMFC fed with H 2 + 100 ppm CO with the PdPt/C and PdPtRu/C anodes containing 0.4 mg metal cm −2, with the cell presenting potential losses below 200 mV at 1 A cm −2, with respect to the system fed with pure H 2. For the PdPt/C catalysts no CO 2 formation was seen at the PEMFC anode outlet, indicating that the CO tolerance is improved due to the existence of more free surface sites for H 2 electrooxidation, probably due to a lower Pd–CO interaction compared to pure Pd or Pt. For PdPtRu/C the CO tolerance may also have a contribution from the bifunctional mechanism, as shown by the presence of CO 2 in the PEMFC anode outlet.

CO adsorption on Pd atoms deposited on MgO, CaO, SrO and BaO surfaces: density functional calculations

The adsorption properties of CO molecules adsorbed on Pd atoms supported on various sites of MgO, CaO, SrO and BaO surfaces have been studied by means of a density functional cluster model approach. Clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surface, and ions that were the nearest neighbors to the adsorption site were allowed to relax to equilibrium. The metal Pd atoms are stabilized with different binding energies on the examined defect-free and defect-containing surfaces of the oxide supports. We considered oxide anions, neutral and charged anion vacancies located at the regular (001) surfaces. CO is used as a probe molecule to characterize where the Pd atoms are located. This is done by analyzing how the Pd–CO binding energy changes as a function of the substrate site, where the Pd atom is bound, and on the basicity of the oxide support.

Oxidation, Reduction, and Reactivity of Supported Pd Nanoparticles: Mechanism and Microkinetics

We have studied the oxidation and reduction kinetics of Pd nanoparticles on Fe3O4 as well as the CO oxidation kinetics on partially oxidized Pd nanoparticles. The structural properties of the Pd/Fe3O4 model catalyst as well as its adsorption behavior have been studied in detail previously. Here we present the results of fully remote-controlled pulse sequence molecular beam (PSMB) experiments, using CO and O2 molecular beams of variable intensity. It is found that at 500 K and above large quantities of oxygen are incorporated into a Pd interface oxide and, subsequently, into a Pd surface oxide. The Pd oxide coexists with metallic Pd over a broad range of conditions. We identify two reaction regimes as a function of oxygen coverage: fast CO oxidation on the O-precovered metallic Pd and slow CO oxidation involving reduction of the Pd oxide phases. The reaction orders for both reaction regimes are determined, showing a complex flux dependent behavior in the latter case. Experiments at 500 K reveal that there is a slow equilibrium between chemisorbed oxygen on the Pd metal and oxygen incorporated in the Pd oxide phases. The corresponding rate and equilibrium constants are determined, showing that at low to intermediate oxidation levels the equilibrium strongly favors the Pd oxide. As a consequence, there is a continuous depletion of chemisorbed oxygen on the metallic Pd metal at 500 K and above. An analysis of the CO oxidation kinetics on partially oxidized Pd particles in combination with microkinetic modeling suggests that the reduction of the Pd oxide is likely to proceed via two competing reaction channels: The first channel proceeds via decomposition of the oxide and release of oxygen onto the metal, followed by reaction with CO. The second channel is likely to involve a direct reaction with the oxide, possibly via a minority of active sites on the Pd oxide.

Activation of C–H, C–C and C–I bonds by Pd and cis-Pd(CO) 2I 2. Catalyst–substrate adaptation

We have studied the oxidative addition reactions of methane and ethane C–H, ethane C–C and iodomethane C–I bonds to Pd and cis-Pd(CO) 2I 2 at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (DFT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal–ligand coordination complex. For both Pd and cis-Pd(CO) 2I 2, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (S N2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from Pd to cis-Pd(CO) 2I 2, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of Pd for C–I over C–H activation. Obviously, cis-Pd(CO) 2I 2 is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C–X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO) 2I 2 complex, e.g., C–X or X–C along an I–Pd–CO axis, or in between two I–Pd–CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst–substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies Δ E ≠ are decomposed into the activation strain Δ E strain ≠ of and the stabilizing TS interaction Δ E int ≠ between the reactants in the activated complex: Δ E ≠ = Δ E strain ≠ + Δ E int ≠ .

Interaction of CO with PdCu surface alloys supported on Ru(0 0 0 1)

The adsorption and desorption of CO on monolayer PdCu surface alloys on a Ru(0 0 0 1) substrate and, for comparison, on Pd/Ru(0 0 0 1), and Cu/Ru(0 0 0 1) monolayer films, have been studied by temperature programmed desorption and infrared reflection absorption spectroscopy, aiming at a detailed understanding of the different effects controlling the chemical properties of bimetallic surfaces, geometric ensemble effects, electronic ligand effects and strain induced effects. The results show that all of these effects are present in these surfaces, acting in a cooperative, synergetic way. Strain effects lead to a destabilization of the Pd–CO and a stabilization of the Cu–CO interaction as compared to CO adsorption on the respective bulk substrates. Geometric ensemble effects are imposed by the highly disperse distribution of the Cu and Pd atoms, reducing the number of otherwise favorable Pd 3 sites, and electronic ligand effects lead to an increasing stability of the Pd–CO bond with increasing Cu content.

On the binding of carbonyl to a single palladium atom

All-electron CCSD(T), QCISD(T) and MP4(SDQ) calculations including relativistic effects via the use of the IORAmm Hamiltonian have been performed for PdCO. The optimized molecular geometry is in nice agreement with the recently obtained experimental data. The Pd–CO bond dissociation energy is estimated to be 38.8 kcal/mol. The vibrational spectrum of PdCO is calculated and a reassessment of the experimental datum for the frequency of bending mode is suggested.

CO-induced morphology modification in buffer-layer-assisted growth of Pd nanostructures

The effects of chemisorbed CO on the aggregation and coalescence of Pd nanometer-sized clusters during Xe buffer-layer-assisted growth (BLAG) have been studied with transmission electron microscopy. The Pd clusters were either grown on Xe and then exposed to CO or they were grown on solid CO condensed on Xe at 20 K. In the former case, the adsorbed CO presented no barrier to aggregation when clusters came into contact during Xe desorption, and they retained their diffusion-limited cluster–cluster kinetics. It did, however, significantly impede their coalescence, thus producing with branched islands with thinner branches and lower profile than those produced without CO. The critical radius for crossover from compact to branched growth was ∼2 nm for CO-clad Pd compared to ∼4 nm for bare Pd clusters. In the second case when Pd clusters were grown on CO on Xe, their rate of growth during Xe desorption was slower, possibly because of the presence of mixed Pd–CO moieties among them. We also show that BLAG and desorption-assisted coalescence observed with rare gas buffers also occur with CO, with differences that reflect adatom and cluster interactions with the CO.

The effect of specific chloride adsorption on the electrochemical behavior of ultrathin Pd films deposited on Pt( [formula omitted]) in acid solution

The electrochemical behavior of thin Pd films supported on a Pt(1 1 1) electrode is investigated by cyclic voltammetry and in situ Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that in perchloric acid solution underpotential deposition of hydrogen (H upd) and hydroxyl adsorption (OH ad) is in strong competition with the adsorption of Cl − anions, the latter being present as a trace impurity in HClO 4. The interaction of Cl − with Pd is rather strong, controlling the adsorption of H upd and OH ad as well as the kinetic rate of CO oxidation. The microscopic insight (the binding sites) of the adsorbed CO (CO ad) and the rate of CO oxidation (established from CO 2 production) on Pt(1 1 1) modified with a (sub)monolayer of Pd is elucidated by means of Fourier infrared (FTIR) spectroscopy. The appearance of both the characteristic Pt(1 1 1)–CO ad and Pt(1 1 1)–1 ML Pd–CO ad stretching bands on a Pt(1 1 1) surface covered by 0.5 ML Pd confirms previous findings that the Pd atoms agglomerate into islands and that the bare Pt areas and the Pd islands behave according to their own surface chemistry. The systematic increase of the Pd surface coverage results in a gradual change in the catalytic properties of Pt(1 1 1)– xPd electrodes towards CO oxidation, from those characteristic of bare Pt(1 1 1) to those which are characteristic for Pt(1 1 1) covered with 1 ML of Pd.