Redox Replacement Reaction Research Articles

(Invited) Growth and Catalytic Performance of Pd and Pd-Pt Alloy Ultrathin Films on Au By Surface Limited Redox Replacement of H UPD Layers in One-Cell Configuration

In the ongoing effort of solving the energy problem of humanity the design, fabrication and testing of catalysts has been playing a central role for the last couple of decades. Besides the known application of Pd hydrogen gas storage a growing interest has been seen in the use of this metal and its alloys in catalytic reactions with relevance to fuel cells. While Pt is still considered best catalyst for fuel cell applications its high cost and limited supply calls more and more for intensifying the effort of replacing Pt with Pd. Means of growing Pd on conductive materials include a variety of expensive vacuum deposition approaches that need sophisticated equipment and high temperature. An alternative to those is electrodeposition that besides the standard bulk deposition, offers a variety of ultra-thin film growth approaches that could facilitate the deposition of uniform and flat continuous Pd films on polycrystalline Au. Among those, the electrochemical atomic layer deposition of Pd under optimum conditions was first developed for automated flow cell. Our group has been working recently on developing fine-tuned approaches for the growth of epitaxial Pd on Au using always readily available one-cell configuration instead, of sophisticated automated flow cells. The growth uses surface limited redox replacement (SLRR) reaction of underpotentially deposited (UPD) Cu or H sacrificial layers. After a set of iteratively applied deposition cycles, the as-deposited Pd or Pt-Pd films are tested for surface roughness using characteristic HUPD and CuUPD cyclic voltammetry (CV) curves and the efficiency of the deposition technique is measured by stripping charge determination. Also, XPS characterization was used for determining both, the alloy composition in the case of Pt-Pd deposition and the presence / absence of impurities incorporating in the deposit in the case of pure Pd growth. The quasi-2D growth resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in-situ STM. Consequently, the accordingly deposited films at minimum thickness are tested for catalytic activity and durability in formic acid oxidation reactions. In this presentation results of this work will be presented in detail and critically discussed. The emphasis is on the illustration of deposition approach that has been developed to take place in one cell and in the specifically in the case of SLRR of HUPD results in a theoretically contamination free deposits. It is also demonstrated that this approach provides for maximum deposition efficiency engaging also adsorbed H in the replacement scenario. The CV results indicate the deposition of smooth Pd films taking place for up to 30 SLRR cycles for HUPD and CuUPD sacrificial layers, respectively. This is followed by a rapid transition to dendritic growth at higher thickness. The underlying kinetic reasons for such abrupt transition are also considered and critically discussed. Finally, the applicability of the proposed facile SLRR approach has been demonstrated through testing for catalytic activity and durability in formic acid oxidation of ultrathin films of Pd, Pt and Pt-Pd alloys with different composition. The critical analysis of the testing outcome suggested that a 1:2 Pt-Pd alloy exhibits activity and durability that are comparable with those of pure Pt and substantially better to those of pure Pd.

Design and Electrocatalytic Properties of Pt - Pd/Au Nanostructures

The key scientific and technological challenge of the future hydrogen economy is to develop new materials with low-content of Pt and Pt-group noble-metals that exhibit high activity, improved selectivity and stability by controlling their structure and composition. Design of Pt-bi and tri-metallic catalysts of different shapes, sizes and configurations (i.e. core-shell nanoparticles, nanoalloys, nanoclusters) are the most promising approaches to resolve challenges of fuel cells commercialisation, particularly in automotive industry and integrated power system technologies. The extensive studies have been conducted on various Pt-Pd bimetallic structures over the past few decades. Epitaxial ultrathin films of Pd (1-10 ML) on single crystal surfaces of Pt(hkl) and Au(hkl) have been shown to exhibit higher electrocatalytic activity for formic acid oxidation (FAO) than the bulk Pd and bulk Pt 1, 2. Pd and PdAu-alloy nanoparticles covered by electrodeposited Pt monolayer shell have been shown to be excellent catalysts for oxygen reduction reaction with much higher activity and stability than the bulk Pt 3, 4 . In a three component system Pt-nanoclusters (Pt-nc) on Au@Pd (core@shell) particles showed higher activity for FAO dependent on the thickness of Pd shell 5. In this work, we explored the effect of sub-ML Pt - nc on the electrocatalytic activity and stability behaviour of Pd thin films electrodeposited on Au. Uniform distribution of Pt-nc was achieved by reduction of the spontaneously adsorbed [PtCl4]2- complex on the surfaces known as spontaneous deposition6 and by the surface limited redox replacement reaction (SLRR) of adsorbed hydrogen 7. The study was focused on the deposition of sub-ML Pt-nc on epitaxial Pd films on Au of different thicknesses between 2ML and 10 ML. It has been shown that 2ML Pd films are pseudomorphic and adsorbs hydrogen, while for higher thicknesses (3-10 ML) Pd films maintain their epitaxial structure with the underlying Au and feature energetically separated H adsorption and absorption processes 8. The comparison of Pd films of different thickness allowed us to explore the effect of sub ML Pt-nc on H-sorption behaviour and FAO activity. The study showed that presence of Pt-nc contributed to the significant changes in the kinetics of H-sorption and increased the stability of Pd toward dissolution during potential cycling in 0.1 M H2SO4 and in particular during FAO. The FAO on Pd films modified with Pt-nc exhibited significant changes compared to the pure Pd/Au surfaces featuring increased COads poisoning. However, the stability of these surface toward dissolution was significantly better than that of pure Pd/Au. For example, 200 potential cycles of FAO with a scan rate of 50 mV/s on 4 ML Pd/Au resulted in a complete dissolution of Pd film. However, deposition of ~ 0.5 ML of Pt via SLRR of H on the 4 ML Pd/Au improved the surface stability such that the same number of potential cycles resulted in a loss of ~ 1 ML of Pd (equivalent thickness measured by the changes of hydrogen absorption).

The structure-reactivity relationships of using three-dimensionally ordered macroporous LaFe1−xNixO3 perovskites for chemical-looping steam methane reforming

Chemical-looping steam methane reforming (CL-SMR) is a novel technology for syngas and hydrogen production without purification process. A series of three-dimensionally ordered macroporous (3DOM) LaFe1−xNixO3 (x = 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) perovskite-type oxides were synthesized using the polystyrene colloidal crystal templating method. The structural and physico-chemical properties of the obtained oxides were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauere-Emmette-Teller (BET) surface area technologies. The structure-reactivity relationships and effects of Ni-substitution on the improvement of reactivity and resistance to carbon formation were investigated in a thermo-gravimetric analyzer and a fixed-bed reactor. It was found that the as-prepared oxides obtained standard perovskite structures and the well-ordered skeleton was surrounded with uniform close-packed macroporous windows. Ni-substitution improved the ability for oxygen supply but simultaneously enhanced the methane dissociation. While the openness channel and large surface area of 3DOM perovskite allowed low mass-transfer resistances and provided high active sites for reaction. Complicated factors synergistically affected the reactivity and an optimal value for Ni-substitution is confirmed with x = 0.1 by comprehensively considering from the points of reactivity, resistance to carbon formation, as well as hydrogen generation capacity. During the following successive redox reactions, 3DOM LaFe0.9Ni0.1O3 exhibited good regenerability and thermo-stability probably converting 90% of CH4 into syngas in methane reforming stage and generating ∼210–220 ml hydrogen in steam splitting stage.

Hydrogen by chemical looping reforming of ethanol: The effect of promoters on La2-xMxNiO4-λ (M = Ca, Sr and Ce) oxygen carriers

Chemical looping reforming (CLR) of ethanol is a novel method to produce hydrogen. It’s vital to select an appropriate oxygen carrier (OC) with favorable catalytic activity and high stability for successive redox processes. In current research, the intelligent perovskites La2-xMxNiO4-λ (M=Ca, Sr and Ce) were used as OCs and prepared by co-precipitation method. The ‘intelligence’ was embodied in exceptional self-recovery ability of perovskite during oxidation stage of CLR process, which was confirmed by the movement that Ni ions escaped and regained back to original position during oxidation step. The experimental result indicated that the movement effectively restrained the growth of metallic Ni and resulted in high activity of OC during life-time service and aging. Furthermore, the influence of promoters doping on the regeneration characteristics of OCs was investigated by BET, TEM, XRD, ICP-OES, TG and a fixed-bed experiment. From the perspective of catalytic performance, the addition of promoter strongly improved the reducibility of OCs, and the optimal substitution is Ca. The LNO-Ca OC exhibited favorable reducibility of nickel ions, shorter ‘dead time’, higher yield of hydrogen, superior ethanol conversion, stronger anti-carbon ability and excellent reproducibility of perovskite during successive redox reactions.

Deposition of Pt Nanoclusters on Au and Their Catalytic Activity

Electrocatalytic studies of Pt nanoclusters (Pt-nc) distributed on foreign metals have shown to exhibit different and often much higher activity than the bulk Pt. In recent years, 2D Pt sub-ML catalysts designed using surface limited redox replacement reaction (SLRR) of an underpotentially deposited (UPD) monolayer of Cu have been used as model systems in studies of the coverage-dependant behaviour during formic acid, methanol, ethanol and hydrogen oxidation reactions.1-4 The effects of the substrate (ligand effect) as well as the size-dependant strain (geometric effect) have been suggested to play an important role in the Pt-nc activity. However, studies on single layered and multilayer Pt-nc suggests that there is an important correlation between the nanoclusters size (2D and 3D) and coverage (controlling the methods of deposition) that needs to be understood better.5 Formic acid oxidation (FAO) is an ideal reaction to study as a function of size and coverage of Pt-nc as many studies recently reported the low-coverage Pt-nc on Au as the ‘most active’ bimetallic system configurations for this reaction.2, 6-8 Here in this talk we will present a study of FAO activity and stability dependence on the size and coverage of 2D and 3D Pt-nc designed by two different methods: spontaneous deposition and the SLRR via Pb UPD. SD is a method that produces uniformly distributed multi-layered Pt-nc by reduction of the spontaneously adsorbed [PtCl4]2- complex.9A systematic study of Pt-nc coverage and distribution included the time of the Pt-complex adsorption and the number of the applied SD cycles on the evolution of the 3D nature of Pt-nc as well as the distribution i.e. on Au substrate coverage. A particular attention was given to the different surface processes used as analytical tools for the determination of Pt coverage: 1) oxidation/reduction of AuO and PtO surface oxides and 2) adsorption processes such as H-UPD and CO. The discrepancies in the values of Pt coverage obtained by different methods requires better understanding of these processes on Pt-nc/Au. The study has shown that up to 5 SD cycles the Pt-nc size and coverage increases up to ~ 60% of Au substrate coverage. Subsequent cycles results in the growth of exiting Pt-nc height but not in the formation of new clusters. The SLRR deposition of 2D Pt-nc using Pb UPD has been controlled by galvanic replacement of electrodeposited Pb-sub ML layer on Au. The study of isolated Pt-nc of comparable coverage (0.23 ±0.02) created by SD and SLRR methods showed the differences in their FAO behaviour that can be attributed to the clusters size (2D and 3D) and coverage. The analysis showed that SD Pt-nc with smaller diameter but larger variation of height than SLRR Pt-nc exhibit 5 times higher activity for FAO. Stability of SD and SLRR Pt-nc during FAO cycling showed that both Pt-nc structures follow a similar trend of changes that can be correlated with the dissolution of Pt. The SD Pt-nc however, showed higher (longer) stability during FAO that could be attributed to their size and larger percentage of 2ML height clusters compared to the SLRR Pt-nc.

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

We studied the initial stage of a Pt monolayer produced by surface-limited redox replacement (SLRR) using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Different from the widely accepted understanding that a metallic monolayer island is formed, our XAFS showed that the Pt monolayer, initially present on the Au(111) substrate, was mainly in the form of a planar [PtCl4]2- complex with its molecular plane parallel to the Au(111). This result provides a new insight into the mechanism of SLRR.

Synergistic improvements in stability and performance of the double perovskite-type oxides La2−xSrxFeCoO6 for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) is a potential route to efficiently co-produce syngas and hydrogen. Development of oxygen carrier with high activity, good recyclability, strong resistance to carbon deposition and excellence capacity for steam splitting is highly desired for this process. The article investigated a novel and unique structure of double perovskite-type oxides La1−xSrxFeCoO6 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) as oxygen carrier. XRD, XPS and H2-TPR technologies were adopted to characterize the physical and chemical properties of them. Meanwhile, isothermal reactions and cyclic redox reactions were carried out in a fixed-bed reactor to determine the influences of Sr-substitution on the reactivity of La1−xSrxFeCoO6. XRD results confirmed the formation of double perovskite crystal structure for all the samples, while substitution of Sr induced a certain degree of Fe/Co disorder generating oxygen vacancies and/or higher oxidation states of metal cations. Synergistic interactions between surface metal ions, such as Fe4+/Fe5+ with Co3+ which were detected by XPS, strongly enhance the reducibility of oxygen carriers. Three zones including total oxidation of methane by surface oxygen, partial oxidation of methane by lattice oxygen and carbon deposition were divided. Among the six samples with different substitution of Sr, La0.6Sr0.4FeCoO6 exhibited the best oxygen transport ability, thermal stability, as well as capacity for hydrogen generation. A stable CH4 conversion at ∼90% with desired H2/CO ratio at 2.0–2.5 in the methane reduction stage, and an average hydrogen yield at ∼5.9mmol/g oxygen carrier with ∼93.8% of hydrogen concentration in the steam oxidation stage were obtained during twenty successive redox reactions, which made them very attractive for the purpose of chemical looping partial oxidation of carbon fuel in real applications.

Pulse Potential Deposition - an Experimental Protocol for Growth of High Quality Thin Films Via Surface Limited Red-Ox Replacement Reaction

Pulse Current Deposition method for growth of thin films and functional coatings has been in use in academia and industry for quite some time [[1]]. However, with few exemptions [[2]], the application of the pulse potential deposition for growth of metallic coatings was mainly confined to academic studies where the conductivity of the substrate, substrate morphology and size do not present a major challenge. Over the years, different deposition protocols have been developed which leverage the benefit of Underpotentially Deposited Monolayers (UPD ML) as a surfactants, mediators or sacrificial materials in deposition process [[3]]. In these examples, the essential step that results in the desired growth mode is achieved by precise control of the electrode potential at which action of the UPD ML is exploited. One of them is deposition via Surface Limited Redox Replacement (SLRR) reaction [[4],[5]], where the main role of UPD layer is to serve as sacrificial reducing metal for deposition of controlled amount of more noble metal. Both, thermodynamics and kinetics of this deposition process are now reasonably well understood which opens more opportunities for practical applications [[6]]. In this talk we show that the control of deposition process during pulse potential deposition is facile and extendable to large area sample plating. The experimental study focuses on design of simple pulse potential function (on/off) for deposition of noble metal coatings on model substrates such as Au and on industrially relevant substrates such as Cu and Ru. The “on” step of the potential cycle is designed to produce a full UPD ML on the substrate surface. The main growth of the deposit occurs during the “off” step of the potential cycle. In this step, at OCP, the deposition of noble metal occurs via SLRR of UPD ML [5]. We will discuss the important parameters of the potential pulse function such as pulse time, off time, and their relation to the deposit morphology, composition and stress. The terminal effect as well as the effect of solution conductivity and solution design in terms of the mutual ratio of UPD metal and depositing metal ion concentration will be discussed as well. [1] J.C. Puippe and F. Leaman, Theory and Practice of Pulse Plating, AESF, (1986). [2] Modern Electroplating V, editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010). [3] Chapter 27- Applications to Magnetic Recording and Microelectronic Technologies, S.R. Brankovic, N. Vasiljevic, N. Dimitrov, Modern Electroplating V, editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010). [4] S. R. Brankovic , J. X. Wang and R.R. Adzic, Surface Science , 474, L173, (2001). [5] N. Dimitrov, Electrochimica Acta, 209, 599 (2016). [6] Chapter 3: Electrochemical Surface Professes and Opportunities for Material Synthesis, Stanko R. Brankovic and Giovanni Zangari in Electrochemical Engineering Across Scales: From Molecules to Processes, Editors: R. C. Alkire, P. N. Bartlett, J. Lipkowski, Advances in Electrochemical Science and Engineering, vol. 15, Willey-VCH (2015) p. 59-107.

Predicting the Electrochemical Stability of Core-Shell Catalysts from First Principles

The rational design of heterogenous electrocatalysts requires a detailed understanding of the physicochemical properties of catalytic surfaces under applied voltage and in contact with a liquid electrolyte. Significant progress has been made recently in part to refined synthetic approaches that provide an unprecedented level of control over the surface chemistry of catalytic electrodes. One such strategy involves the underpotential deposition of a sacrificial component such as copper followed by a surface limited redox replacement reaction with a nobler component such as platinum. These approaches have enabled the design of cost effective core-shell nanostructures with ultrathin films that often exhibit higher performance and higher durability compared to traditional elemental electrocatalysts. Nevertheless, the efficacy of these electrocatalysts for particular reactions can only be determined after performing detailed experimental studies aimed at measuring the activity, selectivity, and durability of the catalyst under a wide range of experimental conditions. In principle, this process can be accelerated by performing virtual experiments on a computer that can reveal the thermodynamic properties of catalytic electrodes in electrochemical environments. The latter relies heavily on the development of predictive models that are capable of describing the complex nature of the electrode-solution interface in an efficient and effective manner. Recently, we have reported a first principles model of electrodeposition phenomena [Weitzner, S. E. and Dabo, I. npj Computational Materials, in press] that incorporates a polarizable continuum dielectric region and a model ionic countercharge to efficiently describe the liquid electrolyte within density-functional calculations of the interface. Electrification of the quantum–continuum interface enables a direct assessment of the stability of electrodeposited species in electrochemical environments. In this talk, we present recent progress on the modeling of underpotential deposition phenomena and surface limited redox replacement reactions under realistic environmental conditions. We additionally discuss the performance of the interfacial model in describing the stability of bimetallic and trimetallic core-shell nanoparticles for catalysis applications.

Observations of copper deposition on functionalized carbon nanotube films

This research details the spontaneous electroless and electrodeposition of copper onto carbon nanotube (CNT) film from a copper sulfate electrolyte. Inhomogeneous electrodeposition was found to occur on pristine CNT film due to differences in available active nucleation sites and the hydrophobic nature of the film. In order to improve the electrochemical response of CNT films, oxidative pre-treatments such as heat treatment and anodization were investigated. These treatments were shown to increase the amount of oxygen containing defects at the surface of CNT film material. Incorporation of functional groups were shown to enhance the wetting of the aqueous electrolyte and created a highly active surface suitable for homogenous electrodeposit. Cathodic polarization curves showed that the presence of functional groups decreased the required level of polarization for copper deposition. Nevertheless, after a certain level of functionalization the polarization starts to decrease due to increased resistivity of the CNT film. Oxygen grafted CNT films were also shown to exhibit enhanced adsorption and reduction of copper without applied voltage due to redox replacement reactions, which were observed to increase with enhanced levels of functionalization.

Boron nitride supported Ni nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on Ni nanoparticles supported on boron nitride spheres (BNSP) and sheets (BNSH) as catalysts for hydrogen generation via hydrolysis of ammonia borane (NH3BH3, AB). The Ni/BN catalysts were prepared through a redox replacement reaction. The structure, morphology, and chemical composition of the obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma emission spectroscopy (ICP). The characterization results showed that Ni nanoparticles were well dispersed on the BNSP and BNSH supports for the four Ni/BN catalysts prepared (Ni atomic contents of 6.8, 9.0, 9.2, and 12 wt%). The catalytic activity toward the hydrolysis of AB was found to correlate with the loading of the Ni/BN catalysts, which showed great cycle performance. Theoretical calculations revealed that BN sheets significantly influenced the electric character of Ni catalysts and thereby their catalytic properties.

Synthesis and Evaluation of Core-Shell Electrocatalysts for Oxygen Reduction Reaction

The slow kinetics of the oxygen reduction reaction (ORR) at the cathode and high cost of electrocatalysts (commonly Pt and Pt-based alloys) for this reaction are major hurdles that hinder the large scale production of proton exchange membrane fuel cells (PEMFCs).[1] A core-shell structure consisting of an atomically thin layer (up to several atoms thick) of Pt deposited on Pd has been recognized as a promising ORR electrocatalysts to replace bulk Pt-based materials.[2] Ideally, all of the Pt atoms will be fully utilized since they are all on the surface in the case of an atomically thin layer of Pt on a core material. The large scale synthesis of Pd@Pt core-shell catalysts, however, has not been fully realized. This study reported two synthesis methods to prepare gram scale batch of Pd@Pt/C electrocatalysts with the assistance of citric acid. The first is Cu-mediated-Pt-displacement method involving the displacement of an underpotentially deposited (UPD) Cu monolayer by Pt.[3] In this method, the Cu monolayer deposited on Pd then is displaced by Pt via a surface limited redox replacement (SLRR) reaction: Pd@Cu + PtCl4 2- → Pd@Pt + Cu2+ + 4Cl-. Recent studies have shown that the SLRR reaction is rather complicated and poorly-controlled resulting in formation of 3D Pt islands instead of a uniform overlayer, especially in the large batch size synthesis. By adding citric acid in the displacement reaction, the activity of Pd@Pt/C can be enhanced by 2 times compared with that synthesized without citric acid. The second method is direct chemcial reduction of PtCl4 2- with the assistance of citric acid without preformation of a Cu UPD layer. Comparing with other reducing agents, the reducing power of citric acid is neither too strong nor too week and allows the reduction reaction to occur at the room temperature, which significantly simplifies the reaction process. In addition, the strong interaction of citric acid with metals may reduce the Pd dissolution and introduce charge transfer from adsorbed citric acid molecules to Pd surface, which can also be used to reduce Pt cations. Both methods resulted good quality of core-shell catalyts in terms of activity and durability. The Pt activities of Pd@Pt/C preapred by the Cu-mediated-Pt-displacement and chemcial reduction methods were 0.95 and 0.78 A mg-1 at 0.9 V, respectively. Fuel cell performance will be also reported and the role of citric acid will be discussed in the presentation.

A XAFS Investigation of Pt Monolayer on Au(111) Obtained from Displacement Reaction

The urgent need of our modern society for the shift to clean energy from fossil energy resources has driven many research efforts into proton exchange membrane fuel cells (PEMFCs), which emit no greenhouse gases during operation and thus the applications to the automobile is promising. However, several issues concerning the electrocatalysts, including the cost and durability, have greatly inhibited the practical use of PEMFCs. To ultimately increase the surface area of mainly Pt-based fuel cell catalyst, and thus reduce the cost, 2D metal monolayer catalyst has attracted many research interests1. Surface-limited redox replacement reaction (SLRR)2, as a convenient, low cost way to achieve monolayer deposition of noble metal such as Pt, Ru, has attracted extensive attentions and lead to its wide applications3 , 4. However, there were only limited numbers of studies regarding the fundamental mechanism of the replacement reaction. For example, there is still controversy regarding the chemical state of the monolayer Pt obtained from displacement reaction5 , 6. The correct understanding of the stoichiometry and Pt monolayer structure is of fundamental importance to better design the electrocatalysis. In this work, the Pt monolayer on well-defined Au(111) synthesized from SLRR of Cu UPD was studied by in situ PTRF-XAFS (polarization-dependent total reflect fluorescence X-ray absorption fine structure), which is especially powerful in characterizing the surface structure and electronic state of monolayer adsorbate on atomically flat surface even in electrochemical condition7. Measurements were conducted in two polarizations, s- and p-, whose surface orientations are parallel and perpendicular to the polarization direction of the incident X-ray, respectively. The in situ XANES (X-ray absorption near edge structure) spectra taken at 0.3 V (vs. sat. Ag/AgCl) presented a larger white line intensity compared to the spectrum of Pt foil as shown in Fig. 1(a). The Pt L edge intensity is a direct reflection of the level of unfilled d state, i.e, the Pt monolayer was not completely reduced Pt0 even at this potential which was at the overpotential region for Pt deposition from [PtCl6] 2-. Furthermore, by analyzing the EXAFS (extended X-ray absorption fine structure) spectra, we found that the Pt-Cl instead of Pt-Pt oscillation in s-polarization (Fig. 1(b)). This result diverts from the previous understanding that Pt4+ in [PtCl6]2-was reduced and Pt-Cl bond breaks when replacing Cu UPD layer. Further XPS measurement also provided the evidence that the Pt monolayer had the Pt-Cl complex structure and the reduction kinetics was very slow, meaning the Pt complex monolayer is stable on the Au(111) surface. Figure 1. a) Pt L3-edge XANES intensity comparison between the Pt monolayer at E=0.3 V (s- and p-polarization) with standard reference samples with Pt oxidation state 0, +2, and +4 (Pt foil, K2PtCl4 and H2PtCl6·6H2O, respectively). b) Pt L3-edge EXAFS comparison of s- and p- polarization of Pt monolayer at 0.3 V with kweight = 3. Reference: (1) Uosaki, K.; Ye, S.; Naohara, H.; Oda, Y.; Haba, T.; Kondo, T. J. Phys. Chem. B 1997, 101, 7566. (2) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173. (3) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W. P.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131, 17298. (4) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220. (5) Gokcen, D.; Bae, S. E.; Brankovic, S. R. J. Electrochem. Soc. 2010, 157, D582. (6) Cheng, S.; Rettew, R. E.; Sauerbrey, M.; Alamgir, F. M. ACS Appl. Mater. Inter. 2011, 3, 3948. (7) Asakura, K. In Catalysis; Spivey, J. J., Gupta, M., Eds.; RSC Publishing: London, 2012; Vol. 24, p 281. Figure 1

Electroreduction of Nitroaromatic Compounds at Electrochemically Reduced Graphene Oxide Supported Bimetallic Ag@Pd Nanorods Modified Electrodes

AbstractThe design and fabrication of nanostructured electrode with high activity at low cost are crucial elements in studying the toxicity of environmental pollutants. Here, we develop a combined step of generating Electrochemically Reduced Graphene Oxide (ERGO) nanosheets on the surface of the glassy carbon electrode where an effective seed mediated growth followed by a galvanic exchange process were introduced for the direct growth of Ag core @Pd shell nanorods (Ag@Pd NRDs). The resulting electrode possesses a large surface area, interconnected porous networks, uniform distribution of bimetallic Ag@Pd NRDs with extremely thin size of Pd generation and good electrical conductivity, which are highly desirable for the electrocatalytic reduction of nitroaromatic compounds (NACs). In the fabrication step, the shell like Cu at the bimetallic NRDs acts as a sacrificial template for forming a thin layer of Pd at Ag NRDs surface by redox replacement reaction. Thus, the resultant Ag@Pd NRDs on ERGO modified electrode was profoundly tested for the electrochemical sensing of NACs with high sensitivity, selectivity and a very low detection limit of 1.8×10−11 M. Differential Pulse Voltammetry (DPV) was used to study the linear range of 4‐nitroaniline (4‐NA) between 1.0×10−9 M and 1.2×10−8 M. The modified electrode exhibits better reproducibility and long term stability. In addition, the modified electrode out performed well in the real sample analysis containing NACs in the presence of different interfering cations and anions.

The Role of Citric Acid in Perfecting Platinum Monolayer on Palladium Nanoparticles during the Surface Limited Redox Replacement Reaction

Cu-mediated-Pt-displacement method that involves the displacement of an underpotentially deposited (UPD) Cu monolayer by Pt has been extensively studied to prepare core-shell catalysts. It has been found that Pt clusters instead of a uniform Pt monolayer were formed in the gram batch synthesis. With a suitable surfactant, such as citric acid, the Pt shell could be much more uniform. In this study, the role of citric acid in controlling the Cu-Pt displacement reaction kinetics was studied by electrochemical techniques and theoretical approaches. It was found that citric acid strongly adsorbed on Pd, Pt, Cu/Pd, and Pt/Pd surfaces, especially in the double layer region in acid solutions. The strong adsorption of citric acid slowed down the Cu-Pt displacement reaction. The main characteristics of such strong interaction most likely arises from the OH groups in the citric acid molecule according to the molecular dynamics simulation results.

The synthesis of PdPt/carbon paper via surface limited redox replacement reactions for oxygen reduction reaction

Surface-limited redox replacement reactions using the electrochemical atomic layer deposition (EC-ALD) technique were used to synthesize PdPt bimetallic electrocatalysts on carbon paper substrate. Electrocatalysts having different Pd:Pt ratio were electrodeposited by varying the deposition cycles of Pd and Pt. Diverse structural shapes of PdPt electrocatalysts were obtained with scanning electron microscopy (SEM). Higher number of deposition cycle produced more agglomerates and less dense particles on the support surface. The EDX profiles confirmed the presence of Pd and Pt particles on carbon paper. Cyclic voltammograms (CV) and linear scanning voltammetry (LSV) in oxygen showed that the electrodeposited catalysts were active for ORR and some were less affected by the methanol introduced in the vessel.

(Invited) Reaction Kinetics of Metal Deposition Via Slrr of UPD ML Studied By Surface Reflectivity Measurements

The deposition protocol based on Surface Limited Redox Replacement (SLRR) reaction of UPD monolayers [[i]] has gained a lot of attention in recent years [[ii]-,[iii],[iv],[v],[vi],[vii],[viii],[ix]]. The monolayers as well as thin films of arbitrary thickness can be deposited with great fidelity using this approach [9,[x]]. Their nucleation and morphology is found to be a direct function reaction kinetics of SLRR [[xi]]. Understanding of the interplay between the experimental conditions during SLRR on one side and resulting SLRR reaction kinetics on the other is necessary if the true control of the deposit morphology is to be assumed using this deposition protocol. Our work explores a fundamental relation between kinetics of Au deposition via SLRR of Pb UPD ML and experimental parameters of the SLRR process such as concentration of the UPD metal and depositing metal ions in the solution, effect of complexing agents and ions such as citrate, effect of temperature and effect of supporting electrolyte. The reaction solution in our kinetics studies contained both Pb2+ and Au3+ ions in 0.1 M HClO4 as a supporting electrolyte. The Pb coverage as a function of time during SLRR is calculated form the temporal change of the surface reflectivity. The typical results for Pb UPD ML coverage change during Au deposition via SLRR of Pb UPD ML obtained from reflectivity data are shown in Figure 1. Data such as one in Figure 1 are fitted with the model-rate equation of first order in terms of Pb UPD coverage which also takes into account the contribution of parallel SLRR of Pb UPD due to oxygen reduction reaction (ORR). This allowed a more realistic and more reproducible values of the rate constant, k’, for Au deposition process to be obtained. Our results suggest a linear relation between the rate constant k’ and Au3+ concentration. The increase in Pb2+ ion concentration for one order of magnitude increases the value of the fundamental rate constant for Au deposition via SLRR of Pb UPD ML for almost 100%. The presence of citrate ions in the solution lowers the value of rate constant but not significantly. The presented data show that effective manipulation of deposition kinetics can be achieved by proper choice of UPD and depositing metal ions concentration. This should have a strong implication when deposit morphology control is considered. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922.

Highly porous honeycomb manganese oxide@carbon fibers core–shell nanocables for flexible supercapacitors

Core–shell electrodes composed of highly porous honeycomb manganese oxide@carbon fibers (HMO@CFs) are synthesized by a facile hydrothermal approach involving an in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon fibers. In this reaction, the carbon fibers serve as not only a sacrificial reductant, but also the substrate. The porous HMO@CFs core–shell nanocables deliver remarkable electrochemical performance with a high capacitance (295.24Fg−1 at 100mAg−1), high good rate capability, and superior cycling stability (about 96.4% specific capacitance retained after 3000 cycles). The maximum energy density of 22.2Whkg−1 (at a power density of 400Wkg−1) and power density of 12,000Wkg−1 (at an energy density of 10Whkg−1) can be achieved at an operating voltage of 1.6V. The fabrication method is simple, cost-effective, and readily scalable thereby having large commercial potential.

Physico–chemical Modelling of Adlayer Phase Formation via Surface–limited Reactions of Copper in Relation to Sequential Electrodeposition of Multilayered Platinum on Crystalline Gold

Electrochemical physico-chemical models, describing isothermically surface coverage dependency with electrode potential of underpotentially deposited Cu adlayers (CuUPD) as well as successive surface-limited redox replacement (SLRR) reactions between CuUPD and PtCl62–, to form multilayered Pt deposits on crystalline Au, have been explored. Modelling of such phase formation phenomena take into account heterogeneity effects and extent of adatom interactions within the adlayers on the base Au substrate and gradually formed Pt multilayered deposits. Insights on exchanged electrons, phenomenological reaction kinetics and Gibbs free energy variations during various deposition stages involving SLRR reactions are divulged.

Growth of Ultra-Thin PtxPb1-x Electrocatalysts By Surface Limited Redox Replacement and Study of Their Adsorption Properties

The development of inexpensive, active and stable Pt catalysts relevant to fuel cell applications remains a very active area of research. The most promising approaches to improve the performance of Pt catalysts while reducing the total noble metal content include Pt thin-film overlayers1 and Pt-X alloy skin and skeleton structures that comprise a Pt-rich surface supported on a Pt-X alloy2. The confinement of the dimensions to a few atomic layers, coupled with the neighbourhood of Pt to another metal, causes electronic and geometrical effects that substantially alter the catalytic performance and adsorption binding energies. Furthermore, this work is motivated by promising results concerning the catalytic activity of PtxPb1-x alloys for formic acid oxidation3 and the suppression of CO poisoning4. Thus, in this work we have explored the effect of controlling film thickness and Pb content on adsorption and catalytic activity.One of the most successful methods to grow thin film overlayers is known as Surface Limited Redox Replacement (SLRR)5. In this method, a monolayer of underpotentially deposited (UPD) metal is galvanically displaced by Pt. Recently, it was demonstrated that both the UPD step and the redox replacement by more noble ions can be performed in a single electrochemical cell, assisted by Pb UPD6. Recent work has shown the application of Pb UPD to the formation of Pt4Cu overlayers7. However, the application of this method to Pt alloy thin films has not been thoroughly explored.In this work, we explored the SLRR-based deposition of PtxPb1-x alloys in a single solution containing 0.1 M NaClO4, 1 mM Pb(ClO4)2, 1 mM HClO4 and 0.5 mM K2PtCl4. The deposition protocol included (1) Pb UPD on a Au substrate by application of a 1 s pulse at -0.85V vs MSE and (2) a redox replacement reaction under open circuit conditions. The OCP was monitored and interrupted at selected values in the potential region for Pb UPD on Pt. In this way, the Pb content in each layer was controlled. Films of 10 ML in thickness of different composition were grown by repeated deposition cycles.The variation in surface composition with upper limit potential and thickness has been verified by XPS measurements. The coverage of H UPD has been shown to decrease dramatically with Pb content, as shown in Figure 1A. In fact, the estimated Pt active areas obtained from the H UPD charge are much lower than the ones measured by XPS on the same alloys. This observation can possibly be explained by a screening effect of Pb causing an inhibition of H-adsorption. A similar effect has been observed in other Pt bimetallic systems8,9.In this work, we have also examined the CO stripping behaviour on alloys. CO stripping was performed by polarising the electrode at -0.6 V in CO saturated 0.5 M H2SO4solution. Linear sweeps in the CO free solution are presented in Figure 1B.A negative shift of the onset potential for CO stripping and change in peak shape as the Pb content increases can be observed. These changes can be ascribed to an electronic effect from Pb, or a bifunctional effect affecting the onset of OH adsorption10. The charges obtained by integration of the stripping peaks are in quantitative agreement with those obtained by H UPD, indicating the coverage obtained by CO follows the same trend. References 1 R.R. Adzic, J. Zhang et al., Top. Catal. 46, 13 (2007). 2 V. R. Stamenkovic, B. S. Mun et al., J. Am. Chem. Soc. 128, 8813 (2006). 3 S.-M. Hwang, J. E. Bonevich et al., J. Electrochem. Soc. 158, B1019 (2011). 4 N. de-los-Santos-Álvarez, L.R. Alden et al., J. Electroanal. Chem. 626, 14 (2009). 5 S. R. Brankovic, J. X. Wang et al., Surf. Sci. 474, L173 (2001). 6 M. Fayette, Y. Liu et al., Langmuir 27, 5650 (2011). 7 L. Bromberg, M. Fayette et al., Electrocatalysis 4, 24 (2013). 8 Dennis F. van der Vliet, Chao Wang et al., Angew. Chem. Int. Ed. 51, 3139 (2012). 9 H. Schulenburg, J. Durst et al., J. Electroanal. Chem. 642, 52 (2010). 10 H. A. Gasteiger, N. M. Markovic et al., J. Phys. Chem. 98, 617 (1994).