Electrocatalysis by ad-atoms: Part IV. Enhancement of the oxidation of formic acid on Pt-Au(subs) and Au-Pt(subs) electrodes by bismuth ad-atoms

The roles of surface and bulk for electrocatalysis have been investigated. Bi ad-atoms enhance a platinum electrode to a great extent for HCOOH oxidation. In order to examine whether bulk platinum atoms are necessary for the electrocatalysis, monolayer or submonolayer amounts of platinum atoms were made to deposit on a gold electrode which is quite inactive for the oxidation. The deposition of a complete monolayer of platinum atoms on a gold electrode makes the electrode as active as a platinum electrode itself. Bi ad-atoms enhance this electrode to the same extent as they do a platinum electrode for the oxidation. Thus surface atoms, Pt and Bi atoms, having no bulk atoms on a quite inactive electrode work effectively for the electrocatalysis, the platinum atoms for the adsorption of the main reactant and the Bi atoms for blocking of the sites against the formation of poison. It is the adsorptive property of the surface that controls the electrocatalytic activity.     

Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pt-transition metal alloys: Pt-skin versus Pt-skeleton surfaces

The surface properties of PtM (M = Co, Ni, Fe) polycrystalline alloys are studied by utilizing Auger electron spectroscopy, low energy ion scattering spectroscopy, and ultraviolet photoemission spectroscopy. For each alloy initial surface characterization was done in an ultrahigh vacuum (UHV) system, and depending on preparation procedure it was possible to form surfaces with two different compositions. Due to surface segregation thermodynamics, annealed alloy surfaces form the outermost Pt-skin surface layer, which consists only platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components. The measured valence band density of state spectra clearly shows the differences in electronic structures between Pt-skin and sputtered surfaces. Well-defined surfaces were hereafter transferred out from UHV and exposed to the acidic (electro)chemical environment. The electrochemical and post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable during and after immersion to an electrolyte. In contrast all sputtered surfaces formed Pt-skeleton outermost layers due to dissolution of transition metal atoms. Therefore, these three different near-surface compositions (Pt-skin, Pt-skeleton, and pure polycrystalline Pt) all having pure-Pt outermost layers are found to have different electronic structures, which originates from different arrangements of subsurface atoms of the alloying component. Modification in Pt electronic properties alters adsorption/catalytic properties of the corresponding bimetallic alloy. The most active systems for the electrochemical oxygen reduction reaction are established to be the Pt-skin near-surface composition, which also have the most shifted metallic d-band center position versus Fermi level.     

Co-adsorption of CO onto a Ag-modified Pt(111)--restructuring of a Ag UPD layer monitored by EC-STM

The effect of co-adsorption of CO on an underpotentially deposited (UPD) silver monolayer on a Pt(111) single crystal electrode in 0.05 M sulfuric acid is investigated for the first time by means of electrochemical scanning tunneling microscopy (EC-STM). Pure electrochemical experiments suggest that the co-adsorption of CO onto Pt single crystal electrodes previously modified by a monolayer of Ag, forces Ag atoms of the first UPD monolayer into a second adlayer. The present EC-STM studies reveal the formation of a large-area Ag network after the co-adsorption of CO. The resulting Ag nanostructures formed on wide Pt(111) terraces are approximately 0.5 nm high and 10 nm wide. The desorption of the newly formed second Ag adlayer, the oxidation of CO and the desorption of Ag atoms from the first adlayer are monitored by EC-STM and simultaneously detected in the corresponding CVs in three different oxidation peaks. EC-STM images recorded afterwards show the unchanged Pt surface. The presence of Ag on the surface leads to a downward shift of the onset of oxygen adsorption on the Pt(111) surface.     

Experimental and theoretical investigation of the stability of Pt-3d-Pt(111) bimetallic surfaces under oxygen environment

The stability of the Pt-3d-Pt(111) (3d = Ti, V, Cr, Mn, Fe, Co, or Ni) bimetallic surface structures in the presence of adsorbed oxygen has been investigated by means of density functional theory (DFT). The dissociative binding energies of oxygen on Pt-3d-Pt(111) (i.e., subsurface 3d monolayer) and 3d-Pt-Pt(111) (i.e., surface 3d monolayer) were calculated. All of the Pt-3d-Pt(111) surfaces were found to have weaker oxygen binding energies than pure Pt(111) whereas all of the 3d-Pt-Pt(111) surfaces were found to have stronger oxygen binding energies than pure Pt(111). The total heat of reaction was calculated for the segregation for 3d metal atoms from Pt-3d-Pt(111) to 3d-Pt-Pt(111) when exposed to a half monolayer of oxygen. All of the Pt-3d-Pt(111) subsurface structures were predicted to be thermodynamically unstable with adsorbed oxygen. In addition, the segregation of subsurface Ni and Co to the surfaces of Pt-Ni-Pt(111) and Pt-Co-Pt(111) was investigated experimentally using Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). AES and HREELS confirmed the trend predicted by DFT modeling and showed that both the Pt-Ni-Pt(111) and Pt-Co-Pt(111) surface structures were unstable in the presence of adsorbed oxygen. The activation barrier of the segregation of surbsurface Ni and Co atoms was determined to be 15 +/- 2 and 7 +/- 1 kcal/mol, respectively. These results are further discussed for their implication in the design and selection of cathode bimetallic electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrode membrane (PEM) fuel cells.     

Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.     

Synthesis and characterization of platinum monolayer oxygen-reduction electrocatalysts with Co–Pd core–shell nanoparticle supports

We synthesized Pt monolayer electrocatalysts for oxygen-reduction using a new method to obtain the supporting core–shell nanoparticles. They consist of a Pt monolayer deposited on carbon-supported Co–Pd core–shell nanoparticles with the diameter of 3–4 nm. The nanoparticles were made using a redox-transmetalation (electroless deposition) method involving the oxidation of Co by Pd cations, yielding a Pd shell around the Co core. The quality of the thus-formed core–shell structure was verified using transmission electron microscopy and X-ray absorption spectroscopy, while cyclic voltammetry was employed to confirm the lack of Co oxidation (dissolution). A Pt monolayer was deposited on the Co–Pd core–shell nanoparticles by the galvanic displacement of a Cu monolayer obtained by underpotential deposition. The total noble metal mass-specific activity of this Pt monolayer electrocatalyst was ca. 3-fold higher than that of commercial Pt/C electrocatalysts.     

单层 FeO 薄膜表面周期性氧缺陷结构的形成

 采用扫描隧道显微镜和 X 射线光电子能谱对含有次表层 Fe 的 Pt 表皮结构 (Pt skin), 即 0.4 ML Fe 的 Pt/Fe/Pt(111) 表面, 在 1.1 × 10?7 kPa 氧气气氛退火过程中的变化进行了研究. 结果表明, 当退火温度为 600 K 时, 氧气在 Pt/Fe/Pt(111) 表面上解离吸附并诱导表面局域结构的重构; 750 K 时次表层 Fe 可以扩散到表面并被氧化; 当升至 850 K 时, 在样品表面形成单层 FeO 结构, 并且 FeO 表面具有周期性的缺陷. 这种缺陷是由于单层 FeO 薄膜的摩尔条纹单胞中 fcc 位上一个或多个氧原子缺失形成的, 其中多原子空位被确定为缺失 6 个氧原子所致. FeO 表面缺陷结构的研究为理解 Fe-Pt 催化剂在氧化气氛中的结构稳定性以及构造表面活性位提供一定的基础.     

Dissolution of oxygen reduction electrocatalysts in an acidic environment: density functional theory study

Density functional theory is employed to determine the reaction thermodynamics of a group of chemical and electrochemical reactions chosen to investigate the dissolution of metal atoms from oxygen reduction reaction catalysts in an acid medium. Once a set of thermodynamically allowed reactions is established, those reactions are selected to investigate the relative stabilities of Pt atoms and of other transition metal atoms (Ir, Pd, Rh, Ni, and Co) toward the dissolution reactions. The dissolution reactions that are found thermodynamically favorable are electrochemical and involve adsorbed oxygenated compounds that are intermediate species of the oxygen reduction reaction. Iridium is found to be the most stable among the various pure metals in comparison to Pt. Most of the metals alloyed with Pt cause a decrease of the Pt stability against dissolution, except for Ni, which does not affect it. On the other hand, the influence of Pt on the stability of the second metal in the alloy follows the same trend as in pure metal catalysts, with Ir being the most stable. When both atoms in a PtM alloy are involved in dissolution reactions, alloyed Ir is also found more stable than Pt in a given dissolution reaction, and the same behavior is found in alloyed Co for most of the reactions studied.     

Pt monolayer electrocatalysts for O2 reduction: PdCo/C substrate-induced activity in alkaline media

We measured the activity of electrocatalysts, comprising Pt monolayers deposited on PdCo/C substrates with several Pd/Co atomic ratios, in the oxygen reduction reaction in alkaline solutions. The PdCo/C substrates have a core-shell structure wherein the Pd atoms are segregated at the particle’s surface. The electrochemical measurements were carried out using an ultrathin film rotating disk-ring electrode. Electrocatalytic activity for the O₂ reduction evaluated from the Tafel plots or mass activities was higher for Pt monolayers on PdCo/C compared to Pt/C for all atomic Pd/Co ratios we used. We ascribed the enhanced activity of these Pt monolayers to a lowering of the bond strength of oxygenated intermediates on Pt atoms facilitated by changes in the 5d-band reactivity of Pt. Density functional theory calculations also revealed a decline in the strength of PtOH adsorption due to electronic interaction between the Pt and Pd atoms. We demonstrated that very active O₂ reduction electrocatalysts can be devised containing only a monolayer Pt and a very small amount of Pd alloyed with Co in the substrate. Dedicated to Professor Oleg Petrii on the occasion of his 70th birthday on August 24, 2007.     

Pt(x)Ru(1-x)/Ru(0001) surface alloys-formation and atom distribution

The formation of PtRu surface alloys by deposition of submonolayer Pt films on a Ru(0001) substrate and subsequent annealing to about 1350 K and the distribution of the Pt atoms in the surface layer were investigated by scanning tunneling microscopy. Quantitative statistical analysis reveals (i) negligible losses of Pt into subsurface regions up to coverages close below 1 monolayer, (ii) a homogeneous distribution of the Pt atoms over the surface, and (iii) the absence of a distinct long-range or short-range order in the surface layer. In addition, the density of specific adsorption ensembles is analyzed as a function of Pt surface content. Possible conclusions on the process for surface alloy formation are discussed. The results are compared with the properties of PtRu bulk alloys and the findings in previous adsorption studies on similar surface alloys (H. Rauscher, T. Hager, T. Diemant, H. Hoster, F. Bautier de Mongeot and R. J. Behm, Surf. Sci., 2007, 601, 4608; T. Diemant, H Rauscher and R. J. Behm, J. Phys. Chem. C, in press).     

Amine-assisted synthesis of concave polyhedral platinum nanocrystals having {411} high-index facets

High-index surfaces of a face-centered cubic metal (e.g., Pd, Pt) have a high density of low-coordinated surface atoms and therefore possess enhanced catalysis activity in comparison with low-index faces. However, because of their high surface energy, the challenge of chemically preparing metal nanocrystals having high-index facets remains. We demonstrate in this work that introducing amines as the surface controller allows concave Pt nanocrystals having {411} high-index facets to be prepared through a facile wet-chemical route. The as-prepared Pt nanocrystals display a unique octapod morphology with {411} facets. The presence of high-index {411} exposed facets endows the concave Pt nanocrystals with excellent electrocatalytic activity in the oxidation of both formic acid and ethanol.     

A study of electronic structures of Pt3M (M=Ti,V,Cr,Fe,Co,Ni) polycrystalline alloys with valence-band photoemission spectroscopy

The surface valence-band densities of states (DOS) of Pt(3)M (M=Ti,V,Cr,Fe,Co,Ni) polycrystalline alloys were investigated with ultraviolet photoemission spectroscopy. Upon annealing the ion-sputter-cleaned alloys at high temperatures, the observed valence-band DOS spectra clearly show the modified electronic structures on the surfaces suggesting the surface segregation of Pt as predicted in thermodynamic models. The measured d-band centers and widths for the annealed alloy surfaces show qualitatively the same trend as predicted by density-functional-theory calculations based on the model of a Pt "skin" on the topmost surface layer and a subsurface layer enriched in the 3d transition metal.     

Surface Buckling and Subsurface Oxygen: Atomistic Insights into the Surface Oxidation of Pt(111)

Platinum is a catalyst of choice in scientific investigations and technological applications, which are both often carried out in the presence of oxygen. Thus, a fundamental understanding of platinum’s (electro)catalytic behavior requires a detailed knowledge of the structure and degree of oxidation of platinum surfaces in operando. ReaxFF reactive force field calculations of the surface energies for structures with up to one monolayer of oxygen on Pt(111) reveal four stable surface phases characterized by pure adsorbate, high‐ and low‐coverage buckled, and subsurface‐oxygen structures, respectively. These structures and temperature programmed desorption (TPD) spectra simulated from them compare favorably with and complement published scanning tunneling microscopy (STM) and TPD experiments. The surface buckling and subsurface oxygen observed here influence the surface oxidation process, and are expected to impact the (electro)catalytic properties of partially oxidized Pt(111) surfaces.     

Carbon on transition metal surfaces

The review surveys the conditions of formation and properties of four forms of surface carbon on transition metals, to wit, adsorbed atoms and clusters, surface carbide and graphite, and their role in the physical and chemical processes on the surface. The first-order phase transition in the adlayer, when graphite islands coexist with carbon gas, are considered. The effect of intercalation, when atoms (Cs, K, Na, Ba, Pt, Si) penetrate spontaneously under the graphite islands physisorbed on the metal, and its mechanism are discussed. An analysis is made of the poisoning of platinum-group metal catalysts in the reaction of dissociation, when graphite islands characterized by extreme adsorption and catalytic passivity form in the adlayer. The method of CsCl dissociation to probe the surface carbon is treated. Attention is drawn to the adsorption of a number of atoms (Cs, K, Ba, Pt) on a graphite monolayer on metals, and the properties of such systems are discussed. The effects observed in coadsorption of CsCl molecules with K, Na, Ba, Tm atoms on a graphite monolayer on metals are covered. By analogy with the bulk carbides, surface carbides of fixed stoichiometry and very strong metal-carbon bonding have been revealed to form on the surface of transition metals (W, Re, Mo). The effect of displacement of surface carbon into the bulk of the metal stimulated by the adsorption of some atoms (Si, S, O) is discussed. The carbon clusters adsorbed on metals are considered. The transport of surface carbon, its desorption and diffusion between the surface and the bulk of the metal with a single- and double-phase adlayer are reviewed.     

Layer Dependence of Graphene for Oxidation Resistance of Cu Surface

We studied the oxidation resistance of graphene-coated Cu surface and its layer dependence by directly growing monolayer graphene with different multilayer structures coexisted, di-minishing the influence induced by residue and transfer technology. It is found that the Cu surface coated with the monolayer graphene demonstrate tremendous difference in oxidation pattern and oxidation rate, compared to that coated with the bilayer graphene, which is considered to be originated from the strain-induced linear oxidation channel in monolayer graphene and the intersection of easily-oxidized directions in each layer of bilayer graphene, respectively. We reveal that the defects on the graphene basal plane but not the boundaries are the main oxidation channel for Cu surface under graphene protection. Our finding indi-cates that compared to putting forth efforts to improve the quality of monolayer graphene by reducing defects, depositing multilayer graphene directly on metal is a simple and effective way to enhance the oxidation resistance of graphene-coated metals.     

Effect of Zn on the adsorption of CO on Pd(111)

Introduction of a second metal can greatly modify the surface reactivity of a host metal. Recently Jeroro and Vohs found that Pd(111) deposited with 0.03-0.06 monolayer of Zn might possess unique activity to methanol steam reforming reaction. To investigate the distribution of the deposited Zn, we examined the adsorption of CO on two types of model systems. In the first model, Zn is in the top-layer of Pd(111) only, while in the second model Zn is placed in the subsurface exclusively. It is found that Zn atoms in the topmost layer show negligible effect on CO adsorption especially at hollow sites, whereas the second layer Zn atoms affect significantly the interaction of CO with the substrate. It is revealed that the negligible influence of the first layer Zn on CO adsorption is due to the offsetting of the ligand effect by the strain effect. On the other hand, the ligand effect dominates the CO adsorption in the second model where the strain effect is insignificant. It is demonstrated that the d-band centers correlate well with the binding energies of the second model, whereas no such good correlation exists for the first model. Our results show that the subsurface plays a more important role and the observed dramatic modification of surface reactivity of Pd(111) deposited with 0.03-0.06 ML Zn is most likely originated from the subsurface Zn atoms, if the coverage is not underestimated and the deposited Zn atoms are distributed uniformly within a layer.     

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.     

Niobium oxide-supported platinum ultra-low amount electrocatalysts for oxygen reduction

We demonstrate a new approach to synthesizing high-activity electrocatalysts for the O(2) reduction reaction with ultra low Pt content. The synthesis involves placing a small amount of Pt, the equivalent of a monolayer, on carbon-supported niobium oxide nanoparticles (NbO(2) or Nb(2)O(5)). Rotating disk electrode measurements show that the Pt/NbO(2)/C electrocatalyst has three times higher Pt mass activity for the O(2) reduction reaction than a commercial Pt/C electrocatalyst. The observed high activity of the Pt deposit is attributed to the reduced OH adsorption caused by lateral repulsion between PtOH and oxide surface species. The new electrocatalyst also exhibits improved stability against Pt dissolution under a potential cycling regime (30,000 cycles from 0.6 V to 1.1 V). These findings demonstrate that niobium-oxide (NbO(2)) nanoparticles can be adequate supports for Pt and facilitate further reducing the noble metal content in electrocatalysts for the oxygen reduction reaction.     

Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrolyte fuel cells studied by EC-XPS

CO tolerance at pure Pt, Pt-Co, and Pt-Ru alloys was investigated by X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-XPS) in order to discover a hint for designing higher performance anode catalysts. After the electrochemical stabilization and/or CO adsorption, these electrodes were immediately transferred to the XPS chamber without exposure to air to avoid contamination of the surfaces. It was revealed that alloying with Co or Ru modified the electronic structures of Pt atoms, resulting in a positive core level (CL) shift of Pt 4f(7/2) which could weaken the Pt-CO interaction. For the Pt-Co alloy electrode, the Pt 4f(7/2) CL shift remained after the electrochemical stabilization despite Co dissolution and formation of a Pt skin layer. Changes in surface core level shifts (DeltaSCLSs) induced by CO adsorption were evaluated and related to the CO adsorption energy. The values of DeltaSCLS at these alloys were smaller than that of pure Pt, indicating that Ru and Co are effective elements to weaken the bond strength of Pt-CO.     

Electrochemical processes on multi-component cathodic catalysts PtM and PtM<Subscript>1</Subscript>M<Subscript>2</Subscript> (M = Co,Ni, Cr): the effect of surface composition on the catalyst stability and its activity in O<Subscript>2</Subscript> reduction

The multi-component nanocatalysts based on platinum-transient metals alloys applied onto dispersed carbon material are considered as the most promising catalysts, which can be substituted for platinum in the fuel cell cathodes. The electrocatalytic activity of platinum in the PtM₁/C and PtM₁M₂/C alloys increases by several times with simultaneous increase in the stability. From the results obtained by structural and electrochemical methods, it is found that the synthesized binary and ternary catalysts are the metal alloys, whose surface is enriched in platinum as a result of surface segregation and subsequent chemical or electrochemical treatment. Under the corrosive attack, the less-noble metal, which has not entered into the alloy, dissolves, and the core-shell structures form. The properties of platinum in the shell differ from its properties in Pt/C due to the ligand effect of the core (metal alloy). As a result, the surface coverage with oxygen chemisorbed from water decreases in the binary and ternary systems. This causes an increase of the catalytic activity in the O₂ reduction reaction due to a decline in the effect of surface blocking against molecular oxygen adsorption and a decrease in the platinum dissolution rate, because the oxidation of platinum by water is the onset of corrosion process. For the catalytic systems studied, the mass activity decreases in the following order: 20% Pt in PtCoCr/C > 7.3% Pt in PtCo/C ≥ 7.3% Pt in PtCr/C and PtNi/C ≥ 40% Pt/C. The application of PtCoCr/C catalyst as the cathode in a low-temperature hydrogen-air fuel cell enabled one to reduce the platinum consumption by one half on retention of its performance.