Redox Replacement Reaction Research Articles

Monolayer Catalyst Design Using Slrr of UPD Monolayers – Fundamental Aspects and Interplay of UPD, Reaction Kinetics, and Nucleation Kinetics

The deposition protocol based on surface limited Redox replacement reaction (SLRR) of UPD monolayers [[i]] has gained a lot of attention in recent years. It has been extensively used for preparation of highly active catalysts [[ii],[iii],[iv],[v],[vi],[vii]], and for growth of homo/heteroepitaxial 2D films with arbitrary thickness [[viii],[ix]]. The concept behind this deposition protocol has potential for a wide range of applications, yet very little is known about the governing phenomena determining morphology of deposited catalyst monolayers.In this talk we will review fundamental relations controlling the catalyst monolayer nucleation as a prelude for its morphology design. The analytical model describing the nucleation density as a function on reaction stoichiometry, rate constant, reaction order, and fundamental parameters describing surface transport will be presented and discussed with reflection to experimental data. We will show the practical relevance of SLRR reaction kinetics on deposition flux and nucleation kinetics of metal/catalyst monolayers. The different approaches for manipulation of the reaction kinetics and thermodynamics will be discussed within the frame of experimental design/protocols to control the morphology of deposited catalyst monolayers. The emphasis will be made on some convenient and yet allowed approximation relevant to the real systems.In the second part of the talk the spectroscopic data will be presented emphasizing that catalyst morphology i.e. nanocluster size is an important design parameter directly affecting the monolayer catalyst performance. Examples of Pt/Pd(hkl) and Pt/Au(hkl) will be discussed with in the light of the Pt monolayer morphology. In the last part of the talk we will show the examples of Pb UPD on Pt and Ru modified Au(111) as a prelude to design of the novel 2D catalyst monolayers and nanoclusters configurations. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. [1] SRBrankovic@uh.edu

Electrodeposition of Pd-Based Binary Catalysts on Carbon Paper Via Surface-Limited Redox-Replacement Reactions for Oxygen Reduction Reaction

Direct alcohol fuel cells (DAFCs) continue to extensive attention as potential power sources for portable and stationary applications [1]. The oxygen reduction reaction (ORR) involving the four electron transfer remains a challenge for DAFCs due to its slow kinetics. Research activities worldwide are focussed on improving the performance of ORR catalysts, reducing the amount of Pt required in DAFCs and in addition, developing catalysts tolerant to alcohol (alcohol crossover). Pd has attracted enormous attention as it is more abundant and also cheaper than Pt [2-5].In this presentation, Electrochemical Atomic Layer Deposition (ECALD) was used in the preparation of binary ORR catalysts. The binary nanoclusters were synthesised via surface-limited redox-replacement reaction (SLR3) with Cu as a sacrificial metal. Co-deposition of the two metals on carbon paper was also performed. ECALD is the appropriate fabrication methodology because the resultant catalysts showed good dispersion of the nanoclusters on the carbon paper. This will reduce the amount of Pt and increase the utilization of the catalyst. The prepared binary nanoclusters were evaluated for the ORR.

Nucleation of Pt Monolayers Deposited via Surface Limited Redox Replacement Reaction

The study of Pt nucleation on Au(111) deposited via surface limited redox replacement of underpotentially deposited Cu monolayer is presented. Series of Pt deposition experiments were performed where starting coverage of an underpotentially deposited Cu monolayer varied from 0 to 1. Exsitu Scanning Tunneling Microscopy and statistical image processing are used to characterize the morphology of deposited Pt monolayers. The Pt nucleation density and the average size (area) of Pt nanoclusters is used as the main descriptors of Pt monolayer morphology. Results are discussed within the framework of analytical model developed to describe qualitatively our data. The relevance of our work for design of Pt monolayer catalyst is illustrated studying the CO electrosorption on Pt monolayers with different nucleation densities and average size of Pt nanoclusters.

Facile synthesis of well-ordered manganese oxide nanosheet arrays on carbon cloth for high-performance supercapacitors

Well-ordered manganese oxide (MnO2) nanosheet arrays (NSAs) were grown directly on carbon cloth via a simple in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon cloth without any other oxidant or reductant addition. The morphology of MnO2 NSAs was examined by scanning and transmission electron microscopy and the phase structure of nanosheets (NSs) was analyzed by X-ray diffraction spectroscopy. Based on a series of time-dependent experiments, a possible growth process for this structure was proposed. The MnO2 NSAs supported on carbon cloth were directly used as integrated electrodes for electrochemical capacitors. The ordered MnO2 NSAs yielded high-capacitance performance with a high specific capacitance of 2.16 F cm−2 at a charge and discharge current density of 5 mA cm−2 and 1.01 F cm−2 at 20 mA cm−2 with a cycling ability (61.4% of the initial specific capacitance remains after 3000 cycles). The MnO2 nanosheet arrays with large surface area and high degree of ordering, combined with the flexible carbon cloth substrate can offer great promise for large-scale supercapacitor applications.

Electro-deposition of Pd on Carbon paper and Ni foam via surface limited redox-replacement reaction for oxygen reduction reaction

Pd nanostructured catalysts were electrodeposited by surface-limited redox replacement reactions using the electrochemical atomic layer deposition technique. Carbon paper and Ni foam were used as substrates for the electrodeposition of the metal. Supported nanostructured Pd electrodes were characterized using electrochemical methods and scanning electron microscopy. Carbon paper and Ni foam produced good quality deposits with some agglomeration on Ni foam. The EDX profiles confirmed the presence of Pd particles. Cyclic voltammograms of the electrodeposited Pd on substrates showed features characteristic of polycrystalline Pd electrodes. In the acidic electrolyte a very weak oxygen reduction reaction (ORR) activity was observed on Pd/Carbon paper electrode when compared to Pd/Ni foam electrode. The Pd/Ni foam electrode showed improved ORR activity in alkaline medium.

Hydrogen sorption properties of bare and Rh-modified Pd nanofilms grown via surface limited redox replacement reactions

Pd nanofilms were grown using electrochemical atomic layer deposition (E-ALD) and used as a platform for investigations into changes in hydrogen absorption/desorption kinetics as a function of the coverage of Rh. Surface limited redox replacement (SLRR) reactions were used to form Pd atomic layers. That is, Pd atomic layers were grown on polycrystalline Au by first depositing a sacrificial Cu atomic layer using underpotential deposition (UPD), and then exchanging it for PdCl42− ions at open circuit potential (OCP). That cycle was then repeated 15 times to form one of the Pd nanofilms used in this study. Rh was deposited on the 15 cycle Pd films from a RhCl63− solution at constant potential or using an E-ALD procedure, similar to that employed to form the Pd nanofilms. Cyclic voltammetry (CV) of the Pd films modified with various coverages of Rh were compared with unmodified Pd nanofilms. The resulting CVs indicated that the presence of Rh enhanced the rates of hydrogen absorption and desorption into and out of the underlying Pd nanofilm. Rh overlayers formed at 0V for 60s produced the greatest kinetic enhancement. Two possible explanations for the observed behavior are proposed and discussed.

Crown Jewel-Structured Au/Pd Nanoclusters as Novel Catalysts for Aerobic Glucose Oxidation

Low-coordination Au sites have been proved to play a key role in defining the catalytic activity of Au nanoclusters (NCs). At the present time, it is still of great interest and challenge to design and synthesize catalysts containing the desired amount of low-coordinated Au atoms by a simple, easy, and large-scale method. In this study, PVP-protected 'crown jewel'-structured Au/Pd (CJ-Au/Pd) catalyst containing an abundance of top Au atoms were prepared by redox replacement reaction between Pd NCs and Au3+ ions. The catalytic activity of the CJ-Au/Pd NCs for aerobic glucose oxidation is about 3 times higher than that of the Au/Pd alloy NCs prepared by alcohol reduction method, although all of these NCs possess almost the same particle size.

Dual-signal anodic stripping voltammetric determination of trace arsenic(III) at a glassy carbon electrode modified with internal-electrolysis deposited gold nanoparticles

A glassy carbon electrode (GCE) modified with internal-electrolysis deposited gold nanoparticles (AuNPsied) was applied to sensitively and selectively detect As(III) by anodic stripping linear sweep voltammetry (ASLSV). The AuNPsied/GCE was prepared based on the redox replacement reaction between a supporting-electrolyte-free aqueous HAuCl4 and a copper sheet in saturated KCl separated by a salt bridge. Under optimum conditions (0.5M aqueous H2SO4, 300-s preconcentration at −0.4V), the ASLSV peak current for the As(0)–As(III) oxidation responded linearly to As(III) concentration from 0.02 to 3μM with a limit of detection (LOD) of 0.9nM (0.07μgL−1) (S/N=3), while that for the As(III)–As(V) oxidation was linear with As(III) concentration from 0.02 to 1μM with a LOD of 4nM (0.3μgL−1) (S/N=3). An appropriate high-scan-rate for ASLSV can enhance both the sensitivity and signal-to-noise ratio. This method was applied for analyses of As(III) in real water samples.

The Electrochemical Atomic Layer Deposition of Pt and Pd Nanoparticles on Ni Foam for the Electro-Oxidation of Alcohols

Electrodeposition of Pt and Pd metal by surface limited redox replacement reactions was performed using the electrochemical atomic layer deposition. Carbon paper and Ni foam were used as substrates for metal deposition. Supported Pt and Pd electrodes were characterised using electrochemical methods and scanning electron microscopy (SEM). Cyclic voltammograms of the electrodeposited Pt and Pd on substrates showed features characteristic of polycrystalline Pt and Pd electrodes respectively. Ethanol oxidation in alkaline media was highly favoured on Pd/Ni foam than on Carbon paper while methanol oxidation was favoured more on Carbon paper than on Ni foam. Carbon paper and Ni foam produced good quality deposits and the EDX confirmed the presence of Pt and Pd particles.

Coverage-dependent electro-catalytic activity of Pt sub-monolayer/Au bi-metallic catalyst toward methanol oxidation

The coverage-dependent catalytic properties of a Pt nanofilm formed on a Au substrate were investigated for the electro-oxidation of methanol. The coverage of Pt nanofilm was precisely fabricated by the formation of coverage-controlled under potential deposition of Cu adlayer and followed by the surface limited redox replacement reaction with different Pt complex ions. The STM images exhibit the formation of Pt nano-film on Au substrate with different coverage. It was clearly shown that the activity of Pt nanofilm deposited on Au substrate toward the methanol electro-oxidation was highly sensitive to its surface coverage. Pt–Au bimetallic catalyst was found to become active at the Pt surface coverage near 0.2 and reached its maximum around 0.6. The electro-catalytic activity as well as CO tolerance on Pt–Au bimetallic system was found higher than those on a Pt electrode.

Multilayered Nanoclusters of Platinum and Gold: Insights on Electrodeposition Pathways, Electrocatalysis, Surface and Bulk Compositional Properties

Electrochemical, surface and bulk compositional properties of multilayered nanoclusters of Pt and Au, electrochemically deposited on glassy carbon under conditions involving sequential surface–limited redox–replacement reactions (performed at open–circuit) and voltammetric dealloying of templating adlayers of electrodeposited Cu, have been studied in the direction of electrocatalytic applications. Variations in open–circuit potentials during redox–replacement steps indicated thermodynamically–favored formation of Pt(s) and Au(s). Unique bimetallic interfacial active sites, Pt|Au or Au|Pt, were effectively generated as evidenced by their distinct surface electrochemistry and multicomponent X–ray photoelectron spectral features. The bulk and surface–to–near surface distribution of Pt and Au appeared to be influenced by the stoichiometry of the surface redox–replacement reactions and sequential dealloying processes through which the nanoclusters were synthesized. Interactions between metal centers, carbon and oxygen containing surface functional groups on the glassy carbon appeared to have played a significant role in the overall stabilization and catalytic activity of the nanoclusters. Profound effects were also found on interfacial charge–transfer and adsorptive properties involving carbon monoxide and its subsequent electrooxidation to CO2, as well as on the electrocatalytic activity involving formic acid oxidation reaction, where the Pt–rich (Pt|Au) exhibited the highest activity.

Layer-by-Layer Deposition of Pd on Pt(111) Electrode: an Electron Spectroscopy–Electrochemistry Study

Ultrathin palladium films, at coverages that ranged from one half to eight monolayers, were prepared, one layer at a time, on a well-defined Pt(111) single-crystal electrode surface by surface-limited redox replacement reaction (galvanic exchange) of Cu initially coated via underpotential deposition. Adlayers produced by the galvanic-exchange method were completely free of any residual Cu after the displacement-by-Pd step. Analysis of the current-potential profiles in the hydrogen-adsorption region indicated that at one monolayer coverage, the film was essentially pure terrace, devoid of steps, which did not exhibit hydrogen absorption. At higher coverages, a Stranski–Krastanov growth mode was manifested by the emergence of a voltammetric peak characteristic of hydrogen adsorption–desorption at step sites. Peaks diagnostic of hydrogen adsorption on both terrace and steps persisted even at a Pd-film thickness of eight monolayers; the intensities were considerably diminished, however, which indicated that hydrogen absorption into the now-bulk-like film had become dominant.

Hydrogen Sorption Properties of Bare and Rhodium-capped Palladium Multilayers Grown via Surface Limited Redox Replacement (SLRR) Reactions

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Electrochemical Atomic Layer Deposition (E-ALD) of Pt Nanofilms Using SLRR Cycles

This paper describes platinum nanofilm formation via the electrochemical form of atomic layer deposition (E-ALD), where the E-ALD cycles are based on surface limited redox replacement (SLRR) reaction. SLRR is where an atomic layer (AL) of a reactive (sacrificial) metal is exchanged for a more noble metal. In the present study both Cu and Pb AL were investigated as sacrificial atomic layers for replacement with both Pt(II) and Pt(IV) precursors. 25 E-ALD cycles were used to form Pt nanofilm deposits. Initial deposits contained an order of magnitude more Pt than expected, evidenced by a factor of 7 increase in surface roughness. Overly positive potentials achieved during the exchange promoted excess deposition and surface roughening. It is proposed that anionic Pt precursors adsorbed more strongly at high potentials, making them difficult to rinse from the cell. Those remaining adsorbed Pt anions are then reduced to Pto when the potential was shifted negative for deposition of the sacrificial element. The result was Pto formation at a large overpotential, which, contributed to excessive Pt deposits and roughening. Increased rinsing of the anionic Pt precursors from the cell eliminated the excess Pt deposition and roughening, resulting in the expected layer by layer growth of an ALD process.

Self-assembly of well-ordered whisker-like manganese oxide arrays on carbon fiber paper and its application as electrode material for supercapacitors

Self-assembled well-ordered whisker-like manganese dioxide (MnO2) arrays on carbon fiber paper (MOWAs) were synthesized via a simple in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon fiber paper (CFP) without any other oxidant or reductant addition. The CFP serves as not only a sacrificial reductant and converts aqueous permanganate (MnO4−) to insoluble MnO2 in this reaction, but also a substrate material and guarantees MnO2 deposition on the surface. The electrochemical properties were examined by cyclic voltammograms (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode cell. According to the CV results, the ordered MOWAs yield high-capacitance performance with specific capacitance up to 274.1 F g−1 and excellent long cycle-life property with 95% of its specific capacitance kept after 5000 cycles at the current density of 0.1 A g−1. The high-performance hybrid composites result from a synergistic effect of large surface area and high degree of ordering of the ultrathin layer of MnO2 nanowhisker arrays, combined with the flexible CFP substrate and can offer great promise in large-scale energy storage device applications.

Kinetics of Metal Deposition via Surface-Limited Redox Replacement Reaction

In this work, we examined the kinetics of metal deposition via surface-limited redox replacement reaction (SLRR). The model system was Pt submonolayer deposition on Au(111) via redox replacement of Pb and Cu underpotentially deposited (UPD) monolayers (MLs) on Au(111). The deposition kinetics were studied through the formalism of the comprehensive analytical model developed to fit open circuit potential (OCP) transients from our deposition experiments. Reaction half time, reaction order, and reaction rate constant were extracted and discussed within the frame of their application to the design and control of deposition experiments. Through the scope of the analytical model, different aspects of SLRR (i.e., the nature of the UPD metal monolayer participating in the redox reaction, the transport limitation, and the role of the anions/electrolyte) were investigated in terms of their effects on the reaction kinetics.

Pt Nano-Structures Formed through Surface Limited Redox Replacement Reaction with Cu Adlayer on Au(111) Surfaces

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Stoichiometry of Pt Submonolayer Deposition via Surface-Limited Redox Replacement Reaction

The work exploring the stoichiometry of Pt deposition via surface-limited redox replacement of the underpotentially deposited (UPD) Cu monolayer on Au(111) is presented. The Cu UPD monolayer is formed from solution, whereas the Pt deposition via surface-limited redox replacement reaction is carried out in solution at open-circuit potential. Our results indicate that the Pt submonolayers have two-dimensional morphology and linear dependence of their coverage on the amount (coverage) of the replaced Cu UPD monolayers. Our analysis shows that the oxidation state of Cu during redox replacement reaction is , suggesting that four Cu UPD adatoms are replaced by each deposited Pt adatom. This work stresses the general importance of the anions, determining the stoichiometry of metal deposition reaction via surface-limited redox replacement of the UPD monolayers.

Electrochemical quartz crystal microbalance study on Au-supported Pt adlayers for electrocatalytic oxidation of methanol in alkaline solution

Underpotential deposition (UPD) of Cu on an Au electrode followed by redox replacement reaction (RRR) of CuUPD with a Pt source (H2PtCl6 or K2PtCl4) yielded Au-supported Pt adlayers (for short, Pt(CuUPD-Pt4+)n/Au for H2PtCl6, or Pt(CuUPD-Pt2+)n/Au for K2PtCl4, where n denotes the number of UPD-redox replacement cycles). The electrochemical quartz crystal microbalance (EQCM) technique was used for the first time to quantitatively study the fabricated electrodes and estimate their mass-normalized specific electrocatalytic activity (SECA) for methanol oxidation in alkaline solution. In comparison with Pt(CuUPD-Pt2+)n/Au, Pt(CuUPD-Pt4+)n/Au exhibited a higher electrocatalytic activity, and the maximum SECA was obtained to be as high as 35.7 mA μg−1 at Pt(CuUPD-Pt4+)3/Au. The layer-by-layer architecture of Pt atoms on Au is briefly discussed based on the EQCM-revealed redox replacement efficiency, and the calculated distribution percentages of bare Au sites agree with the experimental results deduced from the charge under the AuO x -reduction peaks. The EQCM is highly recommended as an efficient technique to quantitatively examine various electrode-supported catalyst adlayers, and the highly efficient catalyst adlayers of noble metals are promising in electrocatalysis relevant to biological, energy and environmental sciences and technologies.

E-ALD of Cu Nanofilms on Ru/Ta Wafers Using Surface Limited Redox Replacement

This paper describes the formation of Cu nanofilms on a Ru/Ta-coated wafer using electrochemical atomic layer deposition (E-ALD). The initial steps involved cleaning and oxide removal from the Ru/Ta substrate using ultrahigh vacuum electrochemical system. Auger spectroscopy was used to follow the relative amounts of oxygen, Ru, and Cu on the wafer: as-received, after electrochemical treatment, after ion bombardment, and after Cu deposition. An automated flow cell electrodeposition system was employed to grow Cu nanofilms with up to 200 cycles using surface limited redox replacement (SLRR) reaction. The open-circuit potential was used to follow the exchange of Pb for Cu in the SLRR reaction. Electron probe microanalysis was used to determine the homogeneity of the Cu films. The use of a complexing agent (citrate) greatly improved the homogeneity. Different concentrations of citrate were investigated, containing 2 or 4 mM citrate. Atomic force microscopy images of the Cu films showed the ingress morphology to be the same as the egress when 4 mM citrate was used. A prominent Cu(111) peak was displayed in the X-ray diffraction pattern for 200 cycles of Cu grown on the Ru/Ta-coated wafer.