Structural model of CO dissociation on Pd particles

In order to modelize the CO dissociation observed on palladium particles, by means of Static Secondary Ion Mass Spectrometry, theoretical investigations were performed on small Pd clusters by using a local approximation of the density functional theory. Several types of defects were studied concerning their ability to dissociate the CO molecule.     

Density functional study on the activation of methane over Pd2, PdO,and Pd2O clusters

Simple functional models for elementary steps in the total oxidation of methane over supported palladium catalysts were investigated using density functional theory. Three simple cluster models were proposed, namely, the palladium dimer and PdO diatomic and linear Pd₂O, to probe the mechanism of the methane activation on metallic and oxidized palladium phases, respectively. The strongest adsorption was found on Pd₂, where also the C(SINGLE BOND)H bond became easily activated; however, no stable product of the C(SINGLE BOND)H bond scission was indicated. Similar hydrogen activation took place on Pd₂O and, in addition, adsorbed methyl and OH species formed the most stable system after crossing a moderate energy barrier. The same product was previously found stable also in the case of PdO dimer but the activation barrier was high. On the Pd₂O cluster, the process of energy barrier crossing was accomplished in two steps: easy formation of a free hydrogen moiety and actual oxidation, which made the overall process less demanding energetically. © 1997 John Wiley & Sons, Inc.     

Coulomb blockade of small Pd clusters

Single-electron tunneling through Au substrate-alkanethiol-Pd cluster-tip junctions is investigated with scanning tunneling spectroscopy. The measured I(V) curves reveal several characteristic features of the Coulomb blockade, namely, the presence of a Coulomb gap and a Coulomb staircase. By using the orthodox theory of single-electron tunneling, the capacitances and resistances of the double junction system as well as the fractional charge are extracted from the experimental data.     

Catalytic effect of Pd nanoparticles on electroless copper deposition

This study elucidates the application of Pd nanoparticles as catalysts of electroless copper deposition and their catalytic effect on the deposition kinetics and microstructure in an electroless copper bath. Quartz crystal microgravimetry (QCM) and high-resolution field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that the kinetic changes associated with electroless copper deposition (ECD) comprised two stages—the incubation period and the acceleration period. In the incubation period, small copper particles were deposited. In the acceleration period, the ECD rate increased rapidly and continuously conducting films with large grains were formed. Leaner sweep voltammetry (LSV) and mixed potential theory (MPT) were applied to examine the catalytic powers of the prepared Pd nanoparticles and the related electrochemical kinetics in the ECD bath.     

Pd6簇与H2分子相互作用的密度泛函理论研究

通过相对论有效核势密度泛函理论计算,优化了Pd6(H)2和Pd6(H)4等簇的平衡几何结构,预测了氢分子在Pd6簇表面上的吸附行为与活化解离性质.计算结果表明,单态的Pd6簇可以活化两个氢分子;第一个H2和第二个H2吸附解离过程速率决定步骤的能垒分别是66.4和24.5kJ/mol、在形成的分子氢配合物Pd6(H2)和Pd（H)2H2中,H2主要作为给电子配体.在最稳定的二氢簇合物Pd6(H)2中,H倾向与3个Pd相互作用,形成面位氢的多核成键吸附方式.     

A combined electrochemical and DFT study of the lattice strain effect on the surface reactivity of Pd

We report a combined study of electrochemical experiments and ab initio calculations on tuning the surface reactivity of Pd via a compressive lattice strain achieved by employing nanoparticles of Pd-Cu alloys with a Pd-rich surface.Surface oxygen-containing species were used as the probing molecule for revealing the surface reactivity.Both density functional theory (DFT) calculations and experiments showed linear relationships,with very close slopes,between the adsorption strength of OH_(ads) and the Pd lattice constant.Not only is this work a successful realization of controllable modulation in the surface reactivity,but it also provides valuable information for the rational design of Pd-based catalysts for fuel cell applications.     

Reduced Pd density of states in Pd/SAM/Au junctions: the role of adsorbed hydrogen atoms

Experiments have shown that a Pd monolayer deposited electrochemically on a Au-supported self-assembled monolayer (SAM) of 4-mercaptopyridine (Mpy) exhibits a strongly reduced Pd local density of states (LDOS) at the Fermi energy (E(f)). Understanding the origin of this modified electronic structure is crucial for the use of the sandwich design as a platform for future nanoelectronics. Here we suggest that hydrogen adsorption might be the origin of the modified electronic properties. We performed periodic density functional theory calculation to explore the influence of hydrogen adsorption on the geometric and electronic structure of a Pd/Mpy/Au(111) complex. Dissociative adsorption of H(2) on a Pd monolayer on top of a Mpy SAM is a strongly exothermic process leading to atomic hydrogen atoms preferentially located at the hollow sites. Due to the formation of a strong Pd-H bond the Pd-SAM interaction realized via one-fold N-Pd bonds is substantially weakened. Upon hydrogen adsorption, the Pd LDOS becomes significantly modified exhibiting a drastic reduction of the density of states at E(f). The calculated spectra are in a good agreement with the experiment for a hydrogen coverage corresponding to two monolayers which is still thermodynamically allowed.     

Acetate coverage effect on the reactivity of vinyl acetate synthesis on Pd/Au alloy surfaces

Vinyl acetate (VA) synthesis on Pd/Au(111) and Pd/Au(100) surfaces has been systematically investigated through first-principles density functional theory (DFT) calculations. The DFT results showed that for VA synthesis, the ‘Samanos’ reaction mechanism (i.e., direct coupling of coadsorbed ethylene and acetate species and subsequent β-hydride elimination to form VA) is more favorable than the ‘Moiseev’ mechanism (i.e., ethylene first dehydrogenates to form vinyl species which then couple with the coadsorbed acetate species to form VA). More importantly, it was found the surface coverage of acetate has a significant effect on the reactivity of VA synthesis, and the activation energy of the rate-controlling step on Pd/Au(100) surface is smaller than that on Pd/Au(111) surface (0.88 vs. 0.95 eV), indicating the former is more active than the latter.     

Formation of Pd/Au nanostructures from Pd nanowires via galvanic replacement reaction

Bimetallic nanostructures with non-random metal atoms distribution are very important for various applications. To synthesize such structures via benign wet chemistry approach remains challenging. This paper reports a synthesis of a Au/Pd alloy nanostructure through the galvanic replacement reaction between Pd ultrathin nanowires (2.4 +/- 0.2 nm in width, over 30 nm in length) and AuCl3 in toluene. Both morphological and structural changes were monitored during the reaction up to 10 h. Continuous changes of chemical composition and crystalline structure from Pd nanowires to Pd68Au32 and Pd45Au55 alloys, and to Au nanoparticles were observed. More interestingly, by using combined techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS), UV-vis absorption, and extended X-ray absorption fine structure (EXAFS) spectroscopy, we found the formation of Pd68Au32 non-random alloy with Au-rich core and Pd-rich shell, and random Pd45Au55 alloy with uniformly mixed Pd and Au atom inside the nanoparticles, respectively. Density functional theory (DFT) calculations indicated that alkylamine will strongly stabilize Pd to the surface, resulting in diffusion of Au atoms into the core region to form a non-random alloy. We believe such benign synthetic techniques can also enable the large scale preparation of various types of non-random alloys for several technically important catalysis applications.     

Adsorption of thiols on the Pd(111) surface: a first principles study

Using density functional theory formalism, we have investigated the adsorption behavior of thiols on the Pd(111) surface. Two different thiol molecules, viz. (a) methane thiol and (b) thiophene 2-thiol (TSH), were used as model adsorbates for this purpose. The results revealed that whereas the methane thiol molecule undergoes spontaneous dissociative chemisorption onto the palladium surface, the adsorption of the thiophene 2-thiol molecule does not involve cleavage of the S-H bond, leading to weak interaction energy or physisorption. The variation in the adsorption behavior has been explained based on the difference in the electronic environment of the terminal sulfur atom. The nature of binding at the interface has been analyzed through calculation of the partial density of states of the sulfur atom at the interface.     

甲烷在清洁 Pd(111) 及氧改性的 Pd(111) 表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.     

Influence of the hydroxylation of gamma-Al2O3 surfaces on the stability and diffusion of single Pd atoms: a DFT study

Using recent well-defined models of gamma-Al2O3 surfaces, we study the interaction of single Pd atoms with gamma-Al2O3 surfaces corresponding to realistic pretreatment conditions by means of density functional theory periodic calculations. For relevant hydroxylation states of the surface, we determine potential energy surfaces (PES) that depict the relationship between structure and interaction at the metal-oxide interface. This approach enables the determination of the low-energy diffusion paths of the adsorbed Pd species. We applied classical transition-state theory to derive the temperature-dependent hopping rate of Pd on gamma-Al2O3 surfaces. Our work provides new insight into the chemisorption and diffusion process of single Pd atoms on alumina and show that the binding energy and hopping rate of Pd atoms decrease as the surface OH coverage increases. These results offer new highlights on Pd cluster formation at the initial nucleation steps on gamma-Al2O3 surfaces.     

The structure of furan reaction products on Pd(111)

Previous experimental studies of the interaction of molecular furan, C(4)H(4)O, with Pd(111) have led to the conclusion that partial dissociation leads to two coadsorbed reaction products, CO and a C(3)H(3) species. Using density functional theory (DFT), a range of possible molecular conformation and adsorption sites of the C(3)H(3) species have been explored and the lowest energy structures, and associated C 1s photoelectron core-level binding energy shifts (CLSs), have been determined. Comparison of these CLS values with published experimental measurements allows one possible conformation to be rejected. New simulations of the C 1s scanned-energy mode photoelectron diffraction (PhD) spectra for several of lowest-energy structures found in DFT are compared with the results of an earlier experimental study. The lowest energy structure found in DFT is not consistent with the PhD data, suggesting that energy barriers to achieve the associated conformation cannot be overcome in the dissociation process. Through consideration of the results of both methods, the most probable surface structures are discussed.     

DFT Study of Thiophene Adsorption on the Pd（111） and Pt（111） Surfaces

Thiophene adsorption on the(111) surfaces of Pd and Pt have been investigated by density functional theory.The results indicate that the adsorption at the hollow sites is the most stable.To our interest,the molecular plane of thiophene ring is distorted with C=C bond being elongated to 1.450  and C-C bond being shortened to 1.347 ,and the C-H bonds tilt 13.91～44.05o away from this plane.Furthermore,analysis on population and density of states verified the calculated adsorption geometries.Finally,charge analysis suggests that thiophene molecule is an electron acceptor,reflecting the interaction between the lone pair of sulfur and the d-orbitals of metal.     

Adsorption energy of the As atom on the Pd(111) surface according to the density functional theory data

Density functional theory in the plane wave basis set is used to study As adsorption on the Pd(111) surface in order to determine the action of a palladium chemical modifier used in electrothermal atomic absorption spectrometry (ETAAS). The calculated heat of desorption of the arsenic atom is 435 kJ/mole, which corresponds to the activation energy of arsenic atomization of 439 kJ/mole (in the range of high temperatures T > 1828 K), obtained by ETAAS. Based on the calculated data, the action of the palladium modifier for the determination of As is assumed to be controlled by the process of chemisorption.     

Adsorption and Decarbonylation Reaction of Furfural on the Pd(111) and M/Pd(111)(M=Ni,Cu and Ru) Surfaces

By performing with density functional theory(DFT) method, the detailed adsorption process and the catalytic decarbonylation mechanisms of furfural over Pd(111) and M/Pd(111)(M = Ni, Cu, Ru) surfaces toward furan were clarified. The results of atomic size factor, formation energy and d-band center showed that Ru/Pd(111) surface was the most stable and active. The adsorption energies of furfural on the different surfaces followed the order Ru/Pd(111) Cu/Pd(111) Pd(111) Ni/Pd(111). After analyzing Mulliken atomic charge population and the deformation density, we can find that on Ru/Pd(111) surface, the number of charge transfer was the most and the interaction was the strongest. Therefore, its adsorption energy was the highest. Furthermore, the furfural decarbonylation pathway is more kinetically feasible on bimetallic surface, and the reaction is the most likely to occur on Ru/Pd(111).     

DFT Comparison the Performance of Pd10Sn5 and Pd10Ag5 Electrocatalyst for Reduction of CO2

CO₂ electrocatalysis as a hydrocarbon is a promising means of achieving economical CO₂-mediated hydrogen energy cycling. Hydrocarbons are renewable hydrogen storage materials. The development of reliable metal alloy electrocatalysts is an urgent but challenging task associated with such systems, although there is still a lack of precise reaction mechanism design. In this study, the performance of Pd₁₀Ag₅ alloy nanoparticles (NPs) and Pd₁₀Sn₅ alloy nanoparticles (NPs) on the electrocatalytic reaction of CO₂ was compare. The kinetic and density functional theory (DFT) calculations show that the selectivity of the Pd-based bimetallic catalyst to the C2 product is greater than that of C1, and the stability of Pd₁₀Ag₅ is better and less affected by the reaction environment. However, the catalytic performance of the Pd₁₀Sn₅ electrocatalyst in the liquid phase is the best. The insight obtained from the calculations is used to develop criteria for identifying new and improved catalysts for electrochemical CO₂ reduction.     

Theoretical research on Pd cluster catalysts for the direct dehydrogenation of propane to propylene

In this article, the feasibility of catalytic dehydrogenation of propane by Pd clusters (Pd₇, Pd₆C, Pd₆Si, Pd₆Ge, and Pd₆Sn) was investigated by using density functional theory (DFT). It was found that Pd₆Sn has the strongest electron mobility and the ability to activate C H bonds, and the highest adsorption barrier (−75.16 kcal/mol) with propylene. The first pathway of the Pd₆Sn-catalyzed primary reaction has the lowest decisive step barrier (16.65 kcal/mol), and the second pathway of the secondary reaction has the highest decisive step barrier (62.25 kcal/mol). It was demonstrated that both the catalyst's electron-leaping ability and the ability to activate C H bonds were the key factors affecting the activity, and the adsorption strength of the catalyst to the product was the main factor affecting the selectivity. It was shown that Pd cluster-catalyzed PDH is theoretically feasible and Pd₆Sn is likely to be a potential cluster catalyst for propane dehydrogenation.     

The role of water in the adsorption of oxygenated aromatics on Pt and Pd

Catalytic processing of biomass‐derived oxygenates to valuable chemical products will contribute to a sustainable future. To provide insight into the conversion of processed sugars and lignin monomers, we present density functional theory studies of adsorption of phloroglucinol, a potentially valuable biomass derivative, on Pt(111) and Pd(111) surfaces. A comprehensive study of adsorption geometries and associated energies indicates that the bridge site is the most preferred adsorption site for phloroglucinol, with binding energies in the range of 2–3 eV in the vapor phase. Adsorption of phloroglucinol on these metal surfaces occurs via hybridization between the carbon p_z orbitals and the metal d and d_yz orbitals. With explicit solvent, hydrogen bonds are formed between phloroglucinol and water molecules thereby decreasing binding of phloroglucinol to the metal surfaces relative to the vapor phase by 20–25%. Based on these results, we conclude that solvent effects can significantly impact adsorption of oxygenated aromatic compounds derived from biomass and influence catalytic hydrogenation and hydrodeoxygenation reactions as well. © 2012 Wiley Periodicals, Inc.     

Systematic study of the lowest energy states of Pd,Pd2, and Pd3

A systematic study has been carried out for the determination and characterization of the lowest states of Pd, Pd₂, and Pd₃ using some of the best ab initio tools available at present (conventional and DFT). Full electron ab initio calculations using the HF, MP2, MP3, MP4, and QCI methods were compared with DFT methods using several gradient-corrected functionals as well as the hybrid B3LYP functional that performed very well for the energetics studies of these small clusters. A suitable basis set has been found to perform considerably well with palladium atoms, another of double-ζ quality has been found insufficient to reproduce basic characteristics of the smallest palladium clusters. The results indicate that the ground state for Pd is a singlet. The dimer is a triplet; however, it is very difficult to ascertain due to the closeness between singlet and triplet states (0.9 kcal/mol). The trimer ground state was found to be a triplet with a separation from the lowest singlet of 3.2 kcal/mol. The lowest triplet and singlet of Pd₃ were practically equilateral triangles. © 1997 John Wiley & Sons, Inc.