Surface Limited Redox Replacement Reaction Research Articles

(Invited) Electroless Cu, Pb and Ag Monolayer Deposition – Generality of an Electroless Monolayer Deposition Phenomenon

Recent progress in fuel cell catalysis has been enabled in great part by facile methods for catalyst monolayer synthesis based on surface limited redox replacement (SLRR) of underpotentially deposited (UPD) monolayers. Although this approach has been very effective, it has shown a limited applications in majority of heterogeneous gas phase catalysis problems where substrates are oxide based materials with very limited conductivity. In this talk we point out a new possibility for catalyst monolayer synthesis via SLRR on metal surfaces which are not part of a broadly conductive substrates such as metal nanoparticles imbedded in various oxide or zeolite supports. This is enabled by discovery of an electroless (E-less) Pb, Cu and Ag monolayer deposition phenomenon. Fundamentals of the E-less monolayer deposition are reviewed, and new data illustrating Cu and Ag monolayer deposition are presented. The focus is on emphasizing generality of the E-less monolayer deposition phenomenon, its self-termination and surface selectivity towards substrates at which Cu, Ag and Pb form UPD monolayer. Examples of the thin films and catalyst monolayer deposition via SLRR of E-less Cu, Ag and Pb monolayer are presented under the auspices of a newly developed two-step electroless atomic layer deposition protocol (E-less ALD)1.1) Electroless Pb Monolayer Deposition – Prelude for Further Advances in Catalyst Monolayer Synthesis via Surface Limited Redox Replacement Reaction”, K. Ahmadi, N. Dole, D. Wu, T. Salavati‐Fard, L. C. Grabow, F. C. Robles Hernandez, and S. R. Brankovic, ACS Catalysis, 11, 4650 (2021).

Electroless Pb Monolayer Deposition—Prelude for Further Advances in Catalyst Monolayer Synthesis via Surface Limited Redox Replacement Reaction

We point out a novel opportunity for catalyst monolayer and core–shell structures synthesis via the surface limited redox replacement (SLRR) reaction. It is enabled by discovery of an electroless P...

Electrosynthesis and characterization of PdIr using electrochemical atomic layer deposition for ethanol oxidation in alkaline electrolyte

This work reports Pd and PdIr thin films electrodeposition via surface limited redox replacement reaction using electrochemical atomic layer deposition technique. Cyclic voltammetry (CV) showed Pd and PdIr voltammetric signatures confirming the successful utilization of Cu as sacrificial metal for Pd and Ir ions. The resulting deposits were further characterized using XRD, EDS, XPS, SEM and AFM, confirming their structural morphology and surface roughness. The catalytic activity of the thin films towards the oxidation of ethanol was determined using CV. PdIr showed a more negative onset potential and higher current (−0.423 V; 0.664 mA) than Pd (−0.402 V; 0.494 mA) thin film, indicating a more catalytic behavior. Further, PdIr showed a higher if/ib ratio of 1.094 than Pd (if/ib = 0.94) indicating higher tolerance to poisoning by ethanol intermediates than Pd. Moreover, chronoamperometry (CA) confirmed that the addition of Ir to Pd improves the stability of the PdIr film. The resultant behaviour was also confirmed by EI spectroscopy with smaller Rct = 0.23 (kΩ) for PdIr than that of Pd; Rct = 14.8 (kΩ) when.Subjected at a fixed potential of −0.2 V. The higher catalytic activity of PdIr towards EOR makes it a promising candidate as electrocatalyst for alkaline direct ethanol fuel cell (DEFCs) than Pd.

Strain Effect on Electrolytic Hydrogen Isotope Separation

The effect of strain in Pt and Pd monolayer catalyst is explored for electrolytic hydrogen isotope separation. Positive/negative strain increases/decreases strength of adsorbed hydrogen bond in overpotential region where recombination of hydrogen atoms is the rate determining step in hydrogen evolution reaction. As a result, the increased/decreased hydrogen isotope separation efficiency of Pt and Pd monolayers is expected as compared to bulk Pt and Pd. The positive/negative strain rises/lowers diffusion barrier for adsorbed hydrogen atoms. This effect favors/retards recombination of isotopes with smaller mass. As the surface is stretched/compressed, the neighboring adsorption sites separate/approach each other which affects heavier hydrogen isotopes more/less and results in their lower/higher probability for recombination. All these fundamental consideration indicate that Pt and Pd monolayers with different level of strain should have much different separation factors than corresponding bulk electrodes. To study described effects, Pt and Pd monolayers we synthesized electrochemically on Au(111) and Ru(0001) electrodes each yielding qualitatively different strain levels (Au-positive, Ru-negative). Pd monolayers are synthesized using electrodeposition[1] while Pt monolayers were synthesized using deposition via surface limited redox replacement reaction[2]. The adsorption strength of hydrogen isotopes is studied by infra-red spectroscopy. These results will are input in our classical models for isotope separation[3] and to our DFT calculations. The calculated ratio between the rates of hydrogen and deuterium recombination and separation factors for Pt and Pd monolayers and corresponding bulk electrodes are compared to experimentally measured ones. The following discussion is focused on understanding and quantification of strain effects on separation efficiency of Pt and Pd monolayers. [1] H. –F. Waibel, M. Kleinert, L. A. Kibler, and D. M. Kolb, Electrochim. Acta, 47, 1461 (2002). [2] S. R. Brankovic, J. X. Wang and R.R. Adzic, Surface Science, 474, L173 (2001). [3] B. E. Conway, Proceedings of The Royal Society of London, A. Mathematical and Physical Sciences, 247, 400 (1958).

Electroless Deposition of Pb Monolayer: A New Process and Application to Surface Selective Atomic Layer Deposition.

The present work demonstrates an electroless (e-less) deposition of Pb monolayer on Au and Cu surface whose morphology and properties resemble its underpotentially deposited counterpart. Our results and analysis show that the e-less Pb monolayer deposition is a surface selective, surface controlled, self-terminating process. Results also show that the electroless Pb monolayer deposition is enabling a phenomenon for new deposition method called "electroless atomic layer deposition" (e-less ALD). Here, the e-less Pb monolayer serves as reducing agent and sacrificial material in surface limited redox replacement reaction with noble metal ions such as Pt n+, i.e., Pt deposition. The e-less ALD is highly selective to the metal substrates at which Pb forms the e-less monolayer. The full e-less ALD cycle leads to an overall deposition of a controlled amount of the noble metal. Repetition of the two-step e-less ALD cycle an arbitrary number of times leads to formation of a highly compact, smooth, and conformal noble metal thin film with applications spanning from catalyst synthesis to semiconductor technology. The process is designed for (but not limited to) aqueous solutions that can be easily scaled up to any size and shape of the substrate, deeming its wide applications.

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).

Reaction Stoichiometry and Mechanism of Pt Deposition via Surface Limited Redox Replacement of Copper UPD Layer on Au(111)

The stoichiometry and reaction mechanism of Pt deposition via surface limited redox replacement (SLRR) of Cu underpotential-deposited (UPD) monolayer on Au(111) was studied using in situ polarization dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). We proposed that Pt deposition via SLRR of Cu UPD monolayer leads to formation/deposition of Pt-surface species mainly consisting of Pt(II) chloride with a square planar local structure [PtCl4] parallel to the Au surface (60%) which has a strong interaction of the Pt complex with the Au substrate. The rest (40%) was one-monolayer Pt metal cluster. This result provides a new understanding into the mechanism and stoichiometry of the SLRR reaction, which has a wide application for synthesis of monolayer catalysts.

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.

Electroless UPD Monolayer Deposition - a Prelude to Highly Selective Electroless Atomic Layer Deposition Process

The improved understanding of nucleation kinetics has led to various discoveries in which the thin film growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. Exploiting some of these results has led to invention of several new methods and protocols for electrodeposition where underpotentially deposited (UPD) monolayer is used as a mediator, surfactant or sacrificial template. The one successful example is so called “deposition via surface limited redox replacement (SLRR) of UPD monolayer,” which gained a lot of applications for synthesis of noble metal thin films with different functionalities. This protocol/method represents the combination of the potential controlled step (formation of UPD monolayer) and the electroless step (SLRR of UPD monolayer by more noble metal ions, i.e., galvanic displacement). However, our studies show that there is still a lot of room left for improvements and further simplifications. These should expand application of this deposition protocol in many areas where traditional atomic layer deposition (ALD) process has been used. In this talk we present results demonstrating an electroless ALD process in which a Pb UPD monolayer is used as a reducing agent and sacrificial material in SLRR reaction with noble metal ions such as Pt, Pd, and Ru and Cu. The full deposition cycle involves sequential exposure of the substrate such as Cu and Ru to the solution for electroless Pb UPD monolayer deposition and then to the solution for SLRR reaction and noble metal deposition. This results in an overall deposition of controlled amount of noble metal which is the function of the areal density of deposited Pb monolayer and stoichiometry of the SLRR reaction. Therefore, the process mimics, to a great extent, the standard ALD cycle where adsorption of the metal precursors and surface catalyzed reaction are replaced by electroless Pb UPD monolayer deposition and SLRR reaction. If the two-step SLRR cycle is repeated an arbitrary number of times, a highly compact, smooth, and conformal noble metal thin film can be grown. The deposition process is highly selective to the metal substrates at which Pb forms an UPD monolayer providing an advantage when certain integration requirements are considered. The process is designed for (but not limited to) aqueous solutions with fairly simple and stable chemistry that can be easily scaled up to any size and shape of the substrate surface. Results demonstrating details and underlying phenomena controlling this process will be discussed. In addition, an example of a high quality of Pt, Pd, Cu and Ru films grown on Cu and Ru substrate will be shown, as well as the applications of this process for metallization of structures relevant to semiconductor device fabrication.

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.

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.

Editors' Choice—Reaction Kinetics of Metal Deposition via Surface Limited Redox Replacement of Underpotentially Deposited Monolayer Studied by Surface Reflectivity and Open Circuit Potential Measurements

Presented work studies the relation between kinetics of metal deposition via surface limited redox replacement (SLRR) of underpotentially deposited (UPD) monolayer (ML) and experimental parameters of reaction solution such as meal ions concentrations and supporting electrolyte concentration. The model system is Au deposition on Au(111) via SLRR of Pb UPD ML. The rate constant of the SLRR reaction for different solution designs is determined from temporal change of electrode surface reflectivity and from the open circuit potential transients’ analysis. The obtained results show clearly that reaction kinetics of metal deposition via SLRR of UPD ML is significantly affected by the design of the reaction solution i.e. the UPD metal ion, depositing metal ion, and supporting electrolyte concentrations. The ten-fold change of concentration of either solution parameter produces approximately the same change in the value of the rate constants. The presented results have fundamental importance for the future development and application of the metal deposition via SLRR of UPD ML. They offer a link between the reaction solution design and expected trend in SLRR reaction rate, which transposes to successful control of deposition flux, nucleation density and resulting morphology of the deposit.

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

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.

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.