Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture

Palladium is crucial for industry‐related applications such as heterogeneous catalysis, energy production, and hydrogen technologies. In many processes, atomic H and C species are proposed to be present in the surface/near‐surface area of Pd, thus noticeably affecting its chemical activity. This study provides a detail and unified view on the interactions of the H and C species with Pd nanoparticles (NPs), which is indispensable for insight into their catalytic properties. Density functional calculations of the interplay of C and H atoms at various concentrations and sites on suitable Pd NPs have been performed, accompanied by catalysis‐relevant experiments on oxide‐supported bare and C‐modified Pd NPs. It is shown that on a Pd₇₉ NP a subsurface C atom destabilizes nearby atoms H at low coverage. Our experiments confirm that H atoms bind more weakly on C‐containing Pd NPs than on C‐free NPs. Various factors related to the presence of both H and C atoms on a Pd₇₉ surface, which may influence the penetration of H atoms from the surface into the subsurface area, have been investigated. Carbon atoms facilitate the subsurface penetration of atomic H both thermodynamically and kinetically when the surface is densely covered by H atoms. Moreover, subsurface H atoms are also energetically favored, even in the absence of C atoms, when several facets of the NP are covered by H atoms.     

Mg-Ti-H 体系晶体结构与相稳定性的第一性原理计算

采用基于密度泛函理论的第一性原理赝势平面波方法计算了不同Ti含量的MgxTi(1-x)H2(x=0.25,0.5,0.75,0.875)体系的平衡晶格结构、总能量及稳定性.结果表明:氢原子在晶胞中的位置接近于四面体间隙位置;H―Ti原子间距小于H―Mg原子间距,表明Ti较Mg具有更强的"亲氢性",Ti原子在吸引周围H原子的同时削弱了H―Mg键的强度;随Ti含量的增加,体系的稳定性和脱氢温度均降低,且MgxTi(1-x)H2体系与MgH2相比,稳定性更差,脱氢温度更低,表明Ti可降低MgxTi(1-x)H2体系的分解温度并加速其脱氢动力学过程,表现出脱氢催化活性.     

Structure, stability, and mobility of small Pd clusters on the stoichiometric and defective TiO2 (110) surfaces

We report on the structure and adsorption properties of Pd(n) (n = 1-4) clusters supported on the rutile TiO(2) (110) surfaces with the possible presence of a surface oxygen vacancy or a subsurface Ti-interstitial atom. As predicted by the density functional theory, small Pd clusters prefer to bind to the stoichiometric titania surface or at sites near subsurface Ti-interstitial atoms. The adsorption of Pd clusters changes the electronic structure of the underlying surface. For the surface with an oxygen vacancy, the charge localization and ferromagnetic spin states are found to be largely attenuated owing to the adsorption of Pd clusters. The potential energy surfaces of the Pd monomer on different types of surfaces are also reported. The process of sintering is then simulated via the Metropolis Monte Carlo method. The presence of oxygen vacancy likely leads to the dissociation of Pd clusters. On the stoichiometric surface or surface with Ti-interstitial atom, the Pd monomers tend to sinter into larger clusters, whereas the Pd dimer, trimer, and tetramer appear to be relatively stable below 600 K. This result agrees with the standard sintering model of transition metal clusters and experimental observations.     

Subsurface Carbon: A General Feature of Noble Metals

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble‐metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low‐coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low‐coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity.     

Hydrogen evolution and Cu UPD at stepped gold single crystals modified with Pd

The evolution of hydrogen on Au(332) and Au(665) surfaces modified with Pd was studied by cyclic voltammetry in hydrogen-saturated sulfuric acid. A strong catalytic activity of Pd decorating the steps, and even monoatomic rows, reflected in the exchange current density for the hydrogen evolution reaction, was found. In comparison, the activity of Pd at terrace sites is negligible. This is explained by the previously observed weak adsorption of hydrogen at monoatomic Pd rows according to the Sabatier principle. For Au(665)/Pd electrodes where the Pd steps have been blocked with Cu, the catalytic activity decreases to values in the same order of magnitude of those for Au(665) surfaces modified with more than a full monolayer of Pd. No direct evidence of hydrogen spillover from Pd-covered areas to the Au substrate was found. Cu underpotential deposition measurements also suggest that no alloy formation takes place between the Cu atoms and the underlying Pd film, nor between Pd and the gold substrate. Dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

Structural, energetic, and electronic properties of hydrogenated titanium clusters

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti-Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti-Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed.     

Shell-anchor-core structures for enhanced stability and catalytic oxygen reduction activity

Density functional theory is used to evaluate activity and stability properties of shell-anchor-core structures. The structures consist of a Pt surface monolayer and a composite core having an anchor bilayer where C atoms in the interstitial sites lock 3d metals in their locations, thus avoiding their surface segregation and posterior dissolution. The modified subsurface geometry induces less strain on the top surface, thus exerting a favorable effect on the surface catalytic activity where the adsorption strength of the oxygenated species becomes more moderate: weaker than on pure Pt(111) but stronger than on a Pt monolayer having a 3d metal subsurface. Here we analyze the effect of changing the nature of the 3d metal in the subsurface anchor bilayer, and we also test the use of a Pd monolayer instead of Pt on the surface. It is found that a subsurface constituted by two layers with an approximate composition of M(2)C (M = Fe, Ni, and Co) provides a barrier for the migration of subsurface core metal atoms to the surface. Consequently, an enhanced resistance against dissolution in parallel to improved oxygen reduction activity is expected, as given by the values of adsorption energies of reaction intermediates, delayed onset of water oxidation, and/or low coverage of oxygenated species at surface oxidation potentials.     

Density functional study of AgnPd and AgnPdH clusters

Small Ag_nPd (n = 5) clusters and their hydrides Ag_nPdH (n = 5) have been studied by density functional theory calculations. For bare clusters, the structures in which the Pd atom has a maximum number of neighboring Ag atoms tend to be energetically favorable. Hydrogen prefers binding to Ag? Pd bridge site of Ag_nPd clusters except for Ag₅Pd. The binding energy has a strong odd–even oscillation. The electron transfers are from Ag atoms to Pd in bare clusters and are from metal clusters to H in cluster hydrides. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Kinetic Evidence for the Influence of Subsurface Oxygen on Palladium Surfaces Towards CO Oxidation at High Temperatures

Transient state kinetics of the catalytic oxidation of CO with O₂ on Pd‐surfaces has been measured under isothermal conditions by using a molecular beam approach. Systematic studies were carried out as a function of reaction temperature and CO+O₂ composition. With sufficient kinetic evidence, we have demonstrated the positive influence of subsurface oxygen towards CO‐adsorption and oxidation to CO₂ at high temperatures (600–900 K) on Pd‐surfaces, and the likely electronic nature of the surface changes with oxygen in the subsurface. These studies also provide a direct proof for CO‐adsorption with a significantly reactive sticking coefficient at high temperatures on Pd‐surfaces exhibiting a significant subsurface O‐coverage.     

First-principles study of Ti-catalyzed hydrogen chemisorption on an Al surface: a critical first step for reversible hydrogen storage in NaAlH4

Complex metal hydrides are perhaps the most promising hydrogen storage materials for a gradual transformation to a hydrogen-based economy. We have used a computational approach to aid the ongoing experimental effort to understand the reversible hydrogen storage in Ti-doped NaAlH(4) and propose a plausible first step in the rehydrogenation mechanism. The study provides insight into the catalytic role played by the Ti atoms on an Al surface in the chemisorption of molecular hydrogen and identifies the local arrangement of the Ti atoms responsible for the process. Our results can potentially lead to ways of making other similar metal hydrides reversible.     

Hydrogenation Catalysts Based on Polynuclear Palladium Complexes with Organophosphorus Ligands

A new procedure is proposed for the preparation of hydrogenation catalysts. This procedure includes the synthesis of cyclic tetranuclear palladium complexes with bridging diphenylphosphide ligands followed by a reaction with Pd(CH₃COO)₂ in the presence of hydrogen to form nanosized particles. In the test catalysts, the ensembles of palladium atoms (or palladium hydrides) immobilized on supramolecular structures formed by the association of phosphinidene and phosphide complexes of palladium are responsible for the catalytic activity.     

Activation and Spillover of Hydrogen on Sub‐1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi‐Hydrogenation of Alkynes

The search for efficient nontoxic catalysts able to perform industrial hydrogenations is a topic of interest, with relevance to many catalytic processes. Herein, we describe a mechanistic phenomenon for the activation and spillover of hydrogen for remarkable selectivity in the semi‐hydrogenation of acetylene over sub‐1 nm Pd nanoclusters confined within sodalite (SOD) zeolite (Pd@SOD). Specifically, hydrogen is dissociated on the Pd nanoclusters to form hydrogen species (i.e., hydrogen atoms and hydroxyl groups) that spill over the SOD surfaces. The design and utilization of the small‐pore zeolite SOD (six‐membered rings with 0.28×0.28 nm channels) is crucial as it only allows H₂ diffusion into the channels to reach the encapsulated Pd nanoclusters and thus avoids over‐hydrogenation to form ethane. Pd@SOD exhibits an ethylene selectivity of over 94.5 %, while that of conventional Pd/SOD is approximately 21.5 %.     

Hydrogen in Palladium and Storage Properties of Related Nanomaterials: Size,Shape, Alloying,and Metal-Organic Framework Coating Effects

One of the key issues for an upcoming hydrogen energy-based society is to develop highly efficient hydrogen-storage materials. Among the many hydrogen-storage materials reported, transition-metal hydrides can reversibly absorb and desorb hydrogen, and have thus attracted much interest from fundamental science to applications. In particular, the Pd−H system is a simple and classical metal-hydrogen system, providing a platform suitable for a thorough understanding of ways of controlling the hydrogen-storage properties of materials. By contrast, metal nanoparticles have been recently studied for hydrogen storage because of their unique properties and the degrees of freedom which cannot be observed in bulk, i. e., the size, shape, alloying, and surface coating. In this review, we overview the effects of such degrees of freedom on the hydrogen-storage properties of Pd-related nanomaterials, based on the fundamental science of bulk Pd−H. We shall show that sufficiently understanding the nature of the interaction between hydrogen and host materials enables us to control the hydrogen-storage properties though the electronic-structure control of materials.     

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.     

Stimulating effect of graphite admixture on hydrogen sorption-desorption properties of mechanically activated titanium powder

The effect of graphite admixture on hydrogen sorption-desorption properties of mechanically activated electron microscopy, temperature-programmed reaction, and temperature-programmed desorption techniques. The major effect of graphite addition was found to be the formation of microporous carbon matrix particles containing randomly distributed titanium particles only several nanometers in diameter. This powder architecture enhances hydrogen transport to the titanium surface without hindrances and promotes titanium-hydrogen interaction by modifying the titanium surface and subsurface layers with interstitial carbon atoms.     

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.     

单原子分散的Au/Cu(111)表面合金的表面结构与吸附性质

Au-Cu双金属合金纳米颗粒对包括CO氧化和CO2还原等在内的多个反应有较好的催化活性,然而关于其表面性质的研究却相当匮乏。在此工作中,我们通过对低覆盖度的Au/Cu(111)和Cu/Au(111)双金属薄膜退火,制备出了单原子级分散的Au/Cu(111)和Cu/Au(111)合金化表面,并利用高分辨扫描隧道显微镜(STM)和扫描隧道谱(STS)进一步研究了掺杂原子的电子性质及其对CO吸附行为的影响。研究发现,分散在Cu(111)表面的表层和次表层Au单原子在STM上表现出不同衬度。在-0.5 e V附近,前者表现出相较于Cu(111)明显增强的电子态密度,而后者则明显减弱。吸附实验表明表层Au单原子对CO的吸附能力并没有得到增强,甚至会减弱其周围Cu原子的吸附能力。与Au在Cu(111)表面较好的分散相反,Cu原子倾向于钻入Au(111)的次表层,并且形成多原子聚集体。且Cu原子受Au(111)衬底吸电子作用的影响,其对CO的吸附能力明显减弱。这个研究结果揭示了合金表面的微观结构与性质的关联,为进一步阐明Au-Cu双金属催化剂的表面反应机理提供参考。     

Catalytic effects of subsurface carbon in the chemisorption of hydrogen on a Mg(0001) surface: an ab-initio study

Ab initio density functional theory (DFT) calculations are performed to explore possible catalytic effects on the dissociative chemisorption of hydrogen on a Mg(0001) surface when carbon is incorporated into Mg materials. The computational results imply that a C atom located initially on a Mg(0001) surface can migrate into the subsurface and occupy an fcc interstitial site, with charge transfer to the C atom from neighboring Mg atoms. The effect of subsurface C on the dissociation of H2 on the Mg(0001) surface is found to be relatively marginal: a perfect sublayer of interstitial C is calculated to lower the barrier by 0.16 eV compared with that on a pure Mg(0001) surface. Further calculations reveal, however, that sublayer C may have a significant effect in enhancing the diffusion of atomic hydrogen into the sublayers through fcc channels. This contributes new physical understanding toward rationalizing the experimentally observed improvement in absorption kinetics of H2 when graphite or single walled carbon nanotubes (SWCNT) are introduced into the Mg powder during ball milling.     

Hydrogenation of fullerites in the presence of intermetallic compounds or metals

Fullerene hydrides containing 24–26 H atoms per fullerene molecule were obtained by hydrogenation of solid-phase mixtures of fullerenes with either intermetallic compounds LaNi₅, LaNi_4.65Mn_0.35, CeCo₃ or V and Pd metals with gaseous hydrogen at 1.0–2.5 MPa and 573–673 K. These fullerene hydrides decompose at 800 K with evolution of H₂. Upon subsequent heating to 1000 K, vanadium reacts with fullerene to yield a cubic phase of vanadium carbide. The intermetallic compounds react with fullerene with the formation of a metallic phase of the 3d-metal and destruction of fullerene. Palladium does not react with fullerene. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 679–683, April, 1997.     

氢和硫原子在Pd、Au和Cu及PdAu、PdCu合金(111)表面吸附的密度泛函研究

用密度泛函理论研究了氢和硫原子在金属Pd、Au、Cu以及合金PdM3、Pd2M2 和Pd3M(111)表面的吸附(M=Au, Cu), 得到了覆盖率为0.25时最稳定的吸附位、结合能以及吸附前后表面的驰豫情况. 结果表明, 氢和硫均与Pd形成最稳定的吸附, Cu次之, Au的吸附最弱, 其在三种纯金属(111)表面的最稳定吸附位均为fcc位. 由于PdAu合金具有较大的晶格常数, Pd3Au 合金吸附氢的结合能甚至较纯Pd更大, 除此之外, 氢和硫在PdM合金表面的吸附基本随M组分的增加而减弱, 而最稳定的吸附位随金属种类和组成变化而变化. 根据计算得到的吸附结合能, 发现与PdCu合金相比, PdAu合金在Au含量较低(约25%, 摩尔分数)时, 氢和硫吸附的结合能下降较慢, 而Au含量较高(跃50%)时, 结合能迅速下降, 这表明含金量为25%-50%的PdAu合金有可能在保持相近透氢性能的同时, 比PdCu合金具有更好的抗硫毒性.