Pd Ensembles Research Articles

Role of different Pd/Pt ensembles in determining CO chemisorption on Au-based bimetallic alloys: A first-principles study

Using spin-polarized density functional calculations, we investigate the role of different Pd/Pt ensembles in determining CO chemisorption on Au-based bimetallic alloys through a study of the energetics, charge transfer, geometric and electronic structures of CO on various Pd/Pt ensembles (monomer/dimer/trimer/tetramer). We find that the effect of Pd ensembles on the reduction of CO chemisorption energy is much larger than the Pt ensemble case. In particular, small-sized Pd ensembles like monomer show a substantial reduction of CO chemisorption energy compared to the pure Pd (111) surface, while there are no significant size and shape effects of Pt ensembles on CO chemisorption energy. This is related to two factors: (1) the steeper potential energy surface (PES) of CO in Pd (111) than in Pt (111), indicating that the effect of switch of binding site preference on CO chemisorption energy is much larger in Pd ensembles than in Pt ensembles, and (2) down-shift of d-band in Pd ensembles/up-shift of d-band in Pt ensembles as compared to the corresponding pure Pd (111)/Pt (111) surfaces, suggesting more reduced activity of Pd ensembles toward CO adsorption than the Pt ensemble case. We also present the different bonding mechanism of CO on Pd/Pt ensembles by the analysis of orbital resolved density of state.

The origin of active sites for direct synthesis of H2O2 on Pd/TiO2 catalysts: Interfaces of Pd and PdO domains

Direct synthesis of H2O2 from H2 and O2 on Pd/TiO2 catalysts has been systematically studied to gain understanding of the origin of active sites. For the first time, the structure of Pd ensembles has been demonstrated to be finely tuned by the variation of Pd loading without change in the particle size. The structure of Pd/TiO2 catalysts has been thoroughly characterized using multiple techniques such as HRTEM, XRD, in situ DRIFTS, XPS, XANES, and EXAFS. With the variation of Pd loading in the range 1.0–5.0wt%, the size (ca. 2.4nm in diameter) and crystallinity of Pd particles have shown no discernible differences, while the surface configuration of Pd atoms changed significantly. Performance (a selectivity of 61% to H2O2 and a rate in TOF of 630h−1) was optimal for a catalyst of 1.0wt% Pd at 283K and 1atm using a semibatch reactor. We demonstrate that part of the surface Pd atoms can be oxidized (47.6% for 1.0wt% Pd and 35.2% for 5.0wt% Pd) by the adsorbed oxygen with the assistance of TiO2 even at 283K, and form Pd–PdO ensembles. The electronic structure of the surface Pd atoms has proved to change dynamically in different atmospheres. We assume that H2O2 synthesis occurs at the interfaces of the Pd and PdO domains. In addition, the deep insight into the origin of active sites from this paper, as a case study, reminds us that the structure of active sites of support catalysts can be potentially tuned by varying the metal loading.

Adsorption of small hydrocarbons on the three-fold PdGa surfaces: the road to selective hydrogenation.

Intermetallic compounds are a promising class of materials as stable and selective heterogeneous catalysts. Here, the (111) and (-1-1-1) single crystal surfaces of the PdGa intermetallic compound were studied as model catalysts with regard to the selective hydrogenation of acetylene (C2H2) to ethylene (C2H4). The distinct atomic surface structures exhibit isolated active centers of single atomic and three atomic Pd ensembles, respectively. For the two prototypal model catalyst surfaces, the adsorption sites and configurations for hydrogen (H2), acetylene, and ethylene were investigated by combining scanning tunneling microscopy, temperature-programmed desorption, and ab initio modeling. The topmost Pd surface atoms provide the preferred adsorption sites for all studied molecules. The structural difference of the Pd ensembles has a significant influence on the adsorption energy and configuration of C2H2, while the influence of the ensemble structure is weak for C2H4 and H2 adsorption. To approach the question of catalytic performance, we simulated the reaction pathways for the heterogeneous catalytic hydrogenation of acetylene on the two surfaces by means of density functional theory. Due to the geometrical separation of the Pd sites on the surfaces, the steric approach of the reactants (H and C2Hx) was found to be of importance to the energetics of the reaction. The presented study gives a direct comparison of binding properties of catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore of major relevance to the knowledge-based design of highly selective hydrogenation catalysts.

Role of Feed Composition on the Performances of Pd-Based Catalysts for the Direct Synthesis of H2O2

The role of feed composition, in particular the O2 to H2 ratio and the concentration of CO2, has been investigated on two Pd/N-CNT and PdAu/N-CNT catalysts. There is a significant influence on the catalytic behavior, but the effect is complex, and cannot be analyzed in terms of conventional kinetic approaches, due to the presence also of a change in the catalyst characteristics as a function of time on stream (in the fresh samples). Depending on the O2 to H2 feed and catalyst nature, the trend of productivity and selectivity varies differently with the time on stream. A simplified kinetic model has been developed which well accounts for the observed behavior. The model is based on the concepts that (i) the selective sites are associated to small Pd terraces covered by chemisorbed O2 with limited sites for H2 chemisorption, and (ii) during the reaction, due to catalyst modifications, a change of available chemisorbed H2 for unselective parallel formation of H2O and H2O2 hydrogenolysis occurs. The changes during time on stream are related to (i) the removal of PVA capping agent, with an initial increase of available Pd surface area leading also to an increase of the H2 chemisorption sites for unselective parallel conversion and consecutive H2O2 hydrogenolysis, and (ii) for the longer times on stream aggregation of some Pd nanoparticles leading to some decrease in the available Pd surface area. The isolation of Pd ensembles by Au (PdAu/N-CNT) influences this trend of productivity and selectivity to H2O2 as a function of the O2 to H2 ratio in the feed. The change is consistent with the model indicated above.

Efficient and Durable Au Alloyed Pd Single-Atom Catalyst for the Ullmann Reaction of Aryl Chlorides in Water

Ion exchange resin supported Au alloyed Pd single atoms have been explored to serve as an effective and robust catalyst for the Ullmann reaction of aryl halides under mild conditions in aqueous media, in particular for the activation of less reactive aryl chlorides. The catalysts were prepared with an ion exchange-NaBH4 reduction method and submitted to extensive characterizations by HRTEM, XRD, EXAFS, and DRIFTS techniques. XRD patterns demonstrated the formation of Au-Pd alloys. EXAFS and DRIFTS characterization results showed that with an increase of Au/Pd molar ratios, the continuous Pd ensembles on the surface were gradually separated and eventually isolated by Au atoms, confirming that the Au alloyed Pd single-atom catalyst was formed. The catalysts exhibited excellent performance for the Ullmann reaction of aryl chlorides, and the turnover number (TON) increased exponentially with a decrease of the amount of Pd in the catalysts. On the basis of these characterization and catalytic results, the Au alloyed Pd single-atom was proposed as the active site for the reaction. The catalyst exhibited excellent catalytic performance for a broad scope of substrates and could be reused at least 8 times with no change in yield. This Au alloyed Pd single-atom catalyst bridges the gap between homogeneous and heterogeneous catalysis in organic transformations and may open a new vision to develop other efficient single-atom catalysts for green synthesis of fine chemicals.

Chemoselective Hydrogenation of Nitrostyrene to Aminostyrene over Pd- and Rh-Based Intermetallic Compounds

A noble catalytic system based on intermetallic compounds was developed for the chemoselective hydrogenation of p-nitrostyrene (NS) to p-aminostyrene (AS). The main concept of the catalyst design was to construct polar active sites consisting of two types of metals with different electronegativities. We prepared a series of Pd- and Rh-based intermetallics and investigated their catalytic properties in detail in the catalytic transfer hydrogenation of nitrobenzene and NS in the presence of 4-methyl-1-cyclohexene (MC) or methanol as a hydrogen donor. FT-IR studies of adsorbed CO confirmed that the number of Pd ensembles exposed on a surface was greatly decreased by the formation of an intermetallic phase; hence, the particle surface consisted of two metal elements being coadjacent at the atomic level. The product distribution achieved with Pd catalysts was dependent on the electronegativity of the second metal element: more electronegative metals gave higher AS selectivity and lower p-ethylnitrobenzene sele...

Resolving Anisotropic Interfacial Phenomena Using, In Situ, Grazing Transmission Ultra-Small Angle X-Ray Scattering Together with Electrochemical Impedance Spectroscopy

Inherent to many electrochemical systems is phase formation at or near the electrode surface. Resolving these phases, in-situ, is challenging with the presence of the necessary electrolyte between the working and counter electrodes. The high X-ray flux available at the Advanced Photon Source (APS) allows these phases to be resolved, in-situ, using small angle X-ray scattering (SAXS) in a grazing transmission geometry shown below. Using this geometry, dimensions both parallel and perpendicular to the surface can be resolved, in-situ, concurrently with an electrochemical technique such as electrochemical impedance spectroscopy (EIS). In this work we show how combined SAXS and EIS is used to resolve unique nanoparticle-solvent interactions that occur in a deep eutectic solvent. Using Ultra-small angle X-ray scattering (USAXS), together with EIS, we were able to resolve ~ 100 nm sized layers that reside above Pd and Ag nanoparticle ensembles on a glassy carbon surface. This study was also performed on the high-energy pinhole SAXS setup, where the distance between the opposing layers was also resolved. While USAXS/SAXS provides information about the size, shape and structure of these phases, the EIS provides their electrochemical response for a comprehensive understanding of interfacial chemistry. These results show how combined USAXS/SAXS and EIS can be used as complementary techniques to resolve otherwise elusive electroactive phases at the interface of electrochemical systems. This technique is accessible through the general user program at the APS. Some general considerations of the experimental setup are presented, including: signal to noise, geometry and cell capabilities. An overview of beamline capabilities is also presented and consists of USAXS/pinSAXS, USAXS, wide angle X-ray scattering (WAXS), USAXS imaging and the high-energy pinhole SAXS setup. With these capabilities, the 15-ID-D beamline offers the potential for a wide variety of in-situ electrochemical experiments towards an understanding of elusive interfacial phenomena.

The effect of the size of surface Pd island ensembles on electron transfer of adsorbed perchlorate ions on Au(111)

A progressive increase in the size of Pd ensembles on a mica-supported Au(111) single crystal surface can facilitate electron transfer of perchlorate ions at lower anodic potential in CV curves than pure Au(111) due to a strong ligand effect and Pd-Au neighbouring pairs at edge sites render a higher degree of electron transfer.

Insight into active sites of Pd–Au/TiO2 catalysts in hydrogen peroxide synthesis directly from H2 and O2

H2O2 synthesis directly from H2 and O2 and side reactions in this system over a variety of Pd–Au/TiO2 catalysts have been studied systemically. The effects of Pd/Au ratio on H2O2 synthesis and subsequent H2O2 hydrogenation were investigated. The catalysts were characterized using multiple techniques including transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (CO-DRIFTS), X-rays photoelectron spectroscopy (XPS), and temperature-programmed desorption of H2/O2 (H2-/O2-TPD). By detailed analysis of these spectroscopies and the kinetics of this reaction, new insights into various active sites of Pd upon alloying with Au were elaborated. Pd monomer surrounded by Au atoms could be the primary active site for H2O2 formation. In addition, Pd ensembles were assumed to be more active for H2O2 hydrogenation than monomeric Pd sites; the over-oxidation of H2 to H2O likely occurred at the interface between Au and TiO2.

Mechanistic Insights in the Catalytic Synthesis of Vinyl Acetate on Palladium and Gold/Palladium Alloy Surfaces

The reaction pathways for the synthesis of vinyl acetate monomer (VAM) are explored on model palladium and gold–palladium alloy single crystal catalysts by combining experiments carried out in ultrahigh vacuum together with density functional theory calculations and Monte Carlo simulations. Previous work by Goodman has shown that both pure palladium and gold–palladium alloys catalyze VAM formation at high pressures, thereby paving the way for fundamental studies of the pathways for this reaction. The coverages of the reactants and products on the surface were found to play an important role in controlling both the reaction pathways and the selectivity. The high coverages on the catalyst under reaction conditions favor bond-forming reactions while inhibiting bond-breaking reactions. On Pd(111), the reaction is initiated by the coupling of ethylene and surface acetate species to form an acetoxyethyl-palladium intermediate, a bond-forming reaction. The high coverages also act to control the selectivity since VAM is stabilized on the crowded surface. The gold in model Au/Pd(111) and Au/Pd(100) alloys gold preferentially segregates to the surface. In the case of Au/Pd(111) alloys, there is a slightly repulsive interaction between the gold and palladium atoms, resulting in a larger proportion of isolated palladium sites than would be expected if they were randomly distributed, while the longer-range interactions on Au/Pd(100) lead to the formation of ordered surface structures and the existence of isolated palladium sites for gold coverages greater than 0.5 ML. Higher coverages of Au on the Au/Pd(111) and Au/Pd(100) alloys decrease the population of bridging Pd sites and thus increase Pd site isolation. This eliminates the larger Pd ensembles that that lead to the decomposition of VAM and ethylene thus increasing the reaction selectivity and weakens the adsorption of ethylene and acetate which enhances the rate of reaction. Higher coverages of Au, however, also suppress the activation of O2 which decrease the rate of acid deprotonation thus resulting in optimal Au/Pd compositions.

Hydrodechlorination catalysis of Pd-on-Au nanoparticles varies with particle size

Trichloroethene (TCE), a common carcinogen and groundwater contaminant in industrialized nations, can be catalytically degraded by Au nanoparticles partially coated with Pd (“Pd-on-Au NPs”). In this work, we synthesized Pd-on-Au NPs using 3, 7, and 10nm Au NPs with Pd surface coverages between 0–150% and studied how particle size and composition influenced their TCE hydrodechlorination (HDC) activity. We observed volcano-shape dependence on both Au particle size and Pd surface coverage, with 7nm Au NPs with Pd coverages of 60–70% having maximum activity. Using extended X-ray absorption fine-structure spectroscopy, we found a strong correlation between catalytic activity and the presence of 2-D Pd ensembles (as small as 2–3 atoms). Aberration-corrected scanning transmission electron microscopy further confirmed the presence of Pd ensembles. The Pd dispersion and oxidation state generally changed from isolated, metallic Pd atoms to metallic 2-D Pd ensembles of varying sizes, and to partially oxidized 3-D Pd ensembles, as Pd surface coverage increased. These changes occurred at different surface coverages for different Au particle sizes. These findings highlight the importance of controlling particle size and surface coverage in bimetallic catalysts.

Structure and local reactivity of PdAg/Pd(111) surface alloys

Motivated by a recent detailed experimental study [Y. Ma et al., Phys. Chem. Chem. Phys., 2011, 13, 10741], the structure and local reactivity of PdAg/Pd(111) surface alloys were studied using periodic density functional theory calculations. As a probe of the local reactivity, CO adsorption energies were evaluated as a function of concentration and configuration of silver and palladium atoms and the CO coverage and related to the underlying electronic structure. According to the calculations, the formation of PdAg/Pd(111) surface alloys is found to be energetically stable. We find in accordance with the experiment that the adsorption on the surface alloy is dominated by ensemble effects, whereas electronic ligand and strain effects effectively cancel each other out. Furthermore, we elucidate the mechanism of CO adsorption on small Pd ensembles upon higher exposure.

Atomic ensemble effects on formic acid oxidation on PdAu electrode studied by first-principles calculations

Using first-principles methods, we investigated the reaction pathways of the formic acid oxidation on Pd(111) and PdAu(111) surfaces. The dehydrogenation of formic acid can simultaneously occur via C–H and O–H activation on different Pd ensembles. However, on the contiguous ensembles without threefold Pd hollow, the reaction of COOH → CO + OH proceeds with a high activation energy (∼1.00 eV), and the direct pathway is predominant for H production from formic acid. Our results indicates that the proper arrangement of Au and Pd sites can significantly improve electrocatalytic activity of PdAu catalyst for formic acid oxidation attributed to the reduction of poisoning species of COad.

First-principles investigations of O2 dissociation on low-coordinated Pd ensembles over stepped Au surfaces

The adsorption and dissociation of O2 on Pd monomer or trimer incorporated into Au(322) and Au(321) surfaces are systematically investigated by first-principles calculations and nudged-elastic-band simulations. We found that the contiguous low-coordinated Pd ensembles alloyed into step edges of Au surfaces are required for O2 dissociation with an enhanced adsorption energy (∼−1.00 eV). The dissociative barrier of O2 is mainly related to the size of Pd ensembles, and the activation energy is about 1.00 eV on Pd trimers. However, the Pd monomer is less active for the adsorption and dissociation of O2. Additionally, the O spillover from Pd to Au sites only occurs at elevated temperature, and the diffusion processes are highly endothermic. The calculated results indicate that the Pd-modified Au step edge with a contiguous Pd ensemble is the reactive center for supplying atomic oxygen on Pd-doped Au catalysts.

Role of Small Pd Ensembles in Boosting CO Oxidation in AuPd Alloys.

We present a theoretical explanation on how PdAu alloy catalysts can enhance the oxidation of CO molecules based on density functional theory calculations of CO adsorption and oxidation on AuPd/Pd(111) surfaces. Our study suggests that the enhanced activity is largely attributed to the possible existence of "partially-poisoned" Pd ensembles that accommodate fewer CO molecules than Pd atoms. Whereas the oxidation of preadsorbed CO is likely governed by O2 trapping, our study shows that small Pd ensembles such as dimers and compact trimers tend to provide more active sites than larger ensembles; CO adsorbed on a Pd monomer is found to react hardly with O2 to form CO2. In addition, we find the tendency of CO-induced Pd agglomeration, which may in turn facilitate CO oxidation by creating more dimers and compact trimers as compared with the adsorbate-free surface where monomers are likely prevailing.

The effect of CNTs on structures and catalytic properties of AuPd clusters for H2O2 synthesis

The structures and catalytic properties of AuPd clusters supported on carbon nanotubes (CNTs) for H(2)O(2) synthesis have been investigated by means of density functional theory calculations. Firstly, the structures of AuPd clusters are strongly influenced by CNTs, in which the bottom layers are mainly composed of Pd and the top layers are a mix of Au and Pd due to the stronger binding of Pd than Au on CNTs. Especially, it is found that O(2) adsorption on the Pd/CNTs interfacial sites is much weaker than that on the only Pd sites, which is in contrast to transition metal oxide (for example TiO(2), Al(2)O(3), CeO(2)) supported metal clusters. Furthermore, Pd ensembles on the interfacial sites have far superior catalytic properties for H(2)O(2) formation than those away from CNT supports due to the changes in electronic structures caused by the CNTs. Therefore, our study provides a physical insight into the enhanced role of carbon supports in H(2)O(2) synthesis over supported AuPd catalysts.

Pd–Au bimetallic catalysts: understanding alloy effects from planar models and (supported) nanoparticles

Pd-Au bimetallic catalysts often display enhanced catalytic activities and selectivities compared with Pd-alone catalysts. This enhancement is often caused by two alloy effects, i.e., ensemble and ligand effects. The ensemble effect is a dilution of surface Pd by Au. With increasing surface Au coverage, contiguous Pd ensembles disappear and isolated Pd ensembles form. For certain reactions, for example vinyl acetate synthesis, this effect is responsible for reaction rate enhancement via the formation of highly active surface sites, e.g., isolated Pd pairs. The disappearance of contiguous Pd ensembles also switches off side reactions catalyzed by these sites. This explains the selectivity increase of certain reactions, for example direct H(2)O(2) synthesis. The ligand effects are electronic perturbation of Pd by Au. Via direct charge transfer or by affecting bond lengths, the ligand effects cause the Pd d band to be more filled, moving the d-band center away from the Fermi level. Both changes make Pd more "atomic like" therefore binding reactants and products more weakly. For certain reactions, this eliminates a so-called "self-poisoning" effect and enhances activity/selectivity.

Catalytic activity of Pd ensembles incorporated into Au nanocluster for CO oxidation: A first-principles study

We investigated the catalytic activity of Pd atoms incorporated into Au(111) facet through first-principles calculations, and found that the Pd monomer, dimer, and trimer are highly reactive for the reaction of CO + O 2 → CO 2 + O via association mechanism, in which an intermediate state (OOCO) is formed. Significantly, a low energy barrier (0.19–0.32 eV) was found for the formation of OOCO. The atomic oxygen left by CO + O 2 → CO 2 + O reaction can be removed by another CO on Pd-decorated Au cluster via Langmuir–Hinshelwood or Eley–Rideal mechanism. Our studies indicate Pd ensembles incorporated into Au(111) facet markedly improve the catalytic activity of gold nanocluster.

Pd ensemble effects on oxygen hydrogenation in AuPd alloys: A combined density functional theory and Monte Carlo study

Using density functional theory and cluster expansion-based Monte Carlo simulations, we examine the effect of Pd dispersion on the energetics and barriers for the reaction of O 2 with H atoms to form H 2O and H 2O 2 on a AuPd/Pd(1 1 1) alloy surface. Our calculations show that this hydrogenation reaction is considerably affected by the distribution of Pd and Au atoms in the surface layer. In particular, on isolated Pd monomers surrounded by less active Au atoms, the activation barrier to form H 2O 2 is appreciably lowered due to the suppression of O–O bond cleavage. In contrast, the reactivity to H 2O on the Pd dimer is predicted to be enhanced compared to pure Pd. Using Monte Carlo simulations we also predict Pd ensemble populations in the AuPd surface layer as a function of temperature and composition. Due to the favorability of Au–Pd interactions over Pd–Pd, we find that small ensembles, particularly monomers, preferentially exist. This study highlights how theoretical investigation of bimetallic alloys, particularly the surface arrangement of atoms and the influence of ensembles on reaction energetics, can offer insight into the design of catalysts and tailoring of reaction conditions.

Catalytic activity of Pd ensembles over Au(111) surface for CO oxidation: A first-principles study

Employing the first-principles pseudopotential plane-wave methods and nudged-elastic-band simulations, we studied the reaction of CO oxidation on Pd-decorated Au(111) surface. We found that the contiguous Pd ensembles are required for the CO + O(2) reaction. Interestingly, Pd dimer is an active site for the two-step reaction of CO+O(2)→OOCO→CO(2)+O, and a low energy barrier (0.29 eV) is found for the formation of the intermediate metastable state (OOCO) compared to the barrier of 0.69 eV on Pd trimer. Furthermore, the residual atomic O in the CO + O(2) reaction can be removed by another CO on Pd dimer with the barrier of 0.56 eV close to the value of 0.52 eV on Pd monomer via Langmuir-Hinshelwood mechanism. The higher energy barriers (0.96 and 0.64 eV) are also found for the CO + O reaction on Pd trimers. The calculated results indicate Pd dimer is highly reactive for CO oxidation by O(2) via association mechanism on Pd-decorated Au(111) surface.