CO Electro-oxidation Reaction Research Articles

Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

Electrocatalytic Properties of Platinum Catalysts Prepared by Pulse Electrodeposition Method Using SnO2 as an Assisting Reagent

PtSnO 2 /C catalysts with uniformly Pt spherical nanoclusters were synthesized by a novel pulse electrodeposition method using SnO 2 as an assisting reagent. The morphology and crystallinity of the catalysts were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively. Spherical nanoclusters obtained by introducing SnO 2 reagent into supports were more and finer than Pt/C catalyst nanoclusters prepared by the same method without SnO 2 . The electrocatalytic activities of catalysts toward methanol oxidation reaction and oxygen reduction reaction were investigated by cyclic voltammetry measurements. In comparison with Pt/C catalysts, the PtSnO 2 /C catalysts had larger active surfaces and exhibited higher electrocatalytic activity for methanol oxidation and oxygen reduction. The possible reason for the increased activity was analyzed and ascribed to both the promotion of CO electro-oxidation reaction kinetics and oxygen sources provided by SnO 2 . It is implied in the study that the method of mixing cocatalyzing materials with Pt-based catalysts is promising and will be potential in design and fabrication of electro-catalysis.

Promoting the current for methanol electro-oxidation by mixing Pt-based catalysts with CeO 2 nanoparticles

Pt particles were prepared through microwave radiation reduction of chloroplatinic acid in ethylene glycol and then adsorbed on carbon nanotubes (CNTs). The thus prepared catalyst was denoted as CNTs-Pt. CeO 2 nanoparticles were prepared through molten salt method. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to characterize the morphology and structure of CNTs-Pt and CeO 2. Cyclic voltammetry (CV), CO stripping and chronoamperometry methods were used to characterize the electrochemical behaviors of the catalysts. The results showed that CeO 2 nanoparticles and CNTs-Pt catalysts can be mixed homogeneously and the current of methanol oxidation can be greatly increased by the mixed CeO 2 nanoparticles. The reason for the increased activity was analyzed and ascribed to the promotion of CO electro-oxidation reaction kinetics by CeO 2. The method of mixing co-catalyzing materials with Pt-based catalysts is effective and can find wide application in electro-catalysis.

Bifunctional amorphous alloys more tolerant to carbon monoxide

The aim of this work is the study of the electrochemical behaviour of methanol and CO electro-oxidation reaction with amorphous metallic alloys of compositions (NiNb) 99(PtX) 1 (X = Ru, Sn, compositions in at.%), which are obtained by mechanical alloying, that results to in powders used as modified carbon paste electrodes (MCPEs). The presence of tin drastically decreases the reactivity towards methanol electro-oxidation of the alloy, followed by the Ni 59Nb 40Pt 1 and Ni 59Nb 40Pt 0.6Ru 0.4 alloys. Differential electrochemical mass spectrometry (DEMS) studies demonstrate that the Ni 59Nb 40Pt 0.6Ru 0.4 and Ni 59Nb 40Pt 0.6Sn 0.4 electrodes show the best tolerance to CO produced by methanol deprotonation, giving rise to more negative onset potential values for the CO 2 production, although the voltammograms for the stripping of a CO monolayer clearly show that the electrode that possesses a better tolerance to CO adsorbed on the surface of the electrode is Ni 59Nb 40Pt 0.6Sn 0.4, decreasing the onset potential values 281 and 158 mV with regard to the Ni 59Nb 40Pt 1 and Ni 59Nb 40Pt 0.6Ru 0.4, respectively. The catalytic activity of the electrode formed might be attributed to an adsorbed state of CO unique in this alloy.

CO Oxidation on Pt‐Modified Rh(111) Electrodes

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: Anion effects on CO surface mobility

The influence of anion adsorption on the CO electrooxidation reaction on stepped Rh[ n(1 1 1) × (1 1 1)] electrodes was investigated by comparing voltammetric and chronoamperometric data obtained in 0.5 M HClO 4 with previously published results obtained in 0.5 M H 2SO 4. Compared to sulfuric acid media, complete stripping of the CO adlayer requires fewer cycles in perchloric acid and the resulting stripping voltammetry peaks are shifted to considerably lower potentials, attesting to the reduced influence of more weakly adsorbed anions. The absence of a shoulder prior to the main oxidation peak and the higher symmetry of the main peaks implies that the electrooxidation reaction in perchloric acid is not only faster than in sulfuric acid but probably also not diffusion limited, which suggests the mean field approximation as the best mathematical model for the reaction kinetics rather than nucleation and growth. The chronoamperometric transients recorded at various potentials show only a single oxidation peak with a slight tailing at longer times. Only the Rh(1 1 1) transients display tailing, which is Cottrellian in nature. The surface diffusion coefficient of CO deduced from the Cottrell plots is more than four orders of magnitude larger in perchloric than in sulfuric acid (HClO 4: 1 × 10 −12 < D < 2 × 10 −11 cm 2 s −1 vs. H 2SO 4: 1 × 10 −16 < D < 8 × 10 −16 cm 2 s −1), which also suggests a mean field model for HClO 4 rather than nucleation and growth. Apparently, specific anion adsorption on rhodium surfaces not only affects the rate, but also the dynamics of the CO electrooxidation reaction. Thus, by varying the adsorption strength of the anion, we could, in principle, influence the diffusion rate of adsorbates on the surface and, therefore, the reaction dynamics and the overall reaction rate.

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: a chronoamperometric study

The CO electro-oxidation reaction has been studied on Rh[ n(1 1 1) × (1 1 1)]-type electrodes in 0.5 M H 2SO 4 using chronoamperometry. The transients recorded on Rh(1 1 1), Rh(5 5 4), Rh(5 5 3) and Rh(3 3 1) are characterized by a current plateau, visible directly after charging of the double layer, followed by a main oxidation feature, which consists of two peaks, a pre-peak and a main peak. The current density in the plateau region is nearly constant over time and, thus, is of (quasi) zeroth order in CO coverage. A plot of log( j plateau) vs. E final gives a linear relationship with a slope of ca. 45 mV dec −1, suggesting a second electron transfer as the rate determining step. Analogously to platinum, the current density plateau was ascribed to a Langmuir–Hinshelwood type reaction between CO ads and OH ads with no effective freeing of sites for OH adsorption due to relaxation of the CO adlayer. The presence of two peaks, rather than one, in the main oxidation feature can be explained by assuming a low surface mobility of CO and high oxidizability of rhodium surfaces. Indeed, dual step chronoamperometry shows that the mobility of CO on rhodium surfaces in aqueous media is very low. Since rhodium surfaces are known to oxidize readily, the pre-peak and main peak can be ascribed to CO reacting with OH, which adsorbs fast at the steps and more slowly at terrace sites. Since the geometry of the steps is nearly the same on each surface, the pre-peak appears structure insensitive, while the main peak shifts considerably with the step density. Introducing randomly distributed crystalline defects by cycling the electrodes repeatedly up to the surface oxidation region prior to each potential step experiment, results in a negative shift of the main peak, while the position of the pre-peak remains fixed. From the data presented, we conclude that the reaction nucleates at the steps and grows in the direction of the terraces.

CO oxidation on stepped Rh[ n (1 1 1) × (1 1 1)] single crystal electrodes: a voltammetric study

The CO electrooxidation reaction has been studied on Rh[ n(1 1 1) × (1 1 1)]-type electrodes in 0.5 M H 2SO 4 using cyclic voltammetry. Examination of the blank cyclic voltammograms (BCVs) of the electrodes under investigation [Rh(1 1 1), Rh(5 5 4), Rh(5 5 3) and Rh(3 3 1)] shows that a distinction can be made between the electrodes on the basis of their voltammetric profiles. An increase in reversibility of the peaks in the hydrogen/(bi)sulfate region and an increase in the potential at which surface oxidation occurs, has been observed for decreasing step density. Higher reversibility of the peaks in the low-potential region therefore indicates more steps and/or defects on the surface. Cycling the potential to within the oxidation region leads to disordering of the surface, which, unlike results reported previously, can be identified in the BCV. Calculations on the hydrogen/(bi)sulfate adsorption/desorption charge seem to indicate that (bi)sulfate prefers adsorption on the terraces and not on the (1 1 0) steps. The fact that disordering of the surface leads to lower charges in the hydrogen/(bi)sulfate region, supports this notion. Both CO stripping and also bulk CO oxidation experiments showed a pronounced effect of the surface structure on the reaction rate. In general, the activity of the electrodes increases for increasing step density. Also disordering of the surface increases the activity, due to an increasing number of defects. For CO stripping experiments, on well-ordered electrodes a pre-shoulder is observed prior to the main oxidation peak, which can be ascribed to CO oxidation next to or at the steps. The presence of the pre-shoulder as well as a pronounced tailing of the main oxidation peak suggests that diffusion of CO on rhodium may be slow.

High Throughput Experimental and Theoretical Predictive Screening of Materials − A Comparative Study of Search Strategies for New Fuel Cell Anode Catalysts

A comparative study of experimental and theoretical combinatorial and high-throughput screening methods for the development of novel materials is presented. Both methods were applied to the development of new anode fuel cell alloy catalysts with improved CO tolerance. Combinatorial experimental electrocatalysis was performed on a 64-element electrode array. Sputter-deposited ternary thin-film electrocatalysts of composition PtRuM (M = Co,Ni,W) were screened in parallel for their methanol oxidation activity, and their individual geometric and specific chronoamperometric current density were monitored and evaluated against standard PtRu catalysts. Density functional theory calculations of a variety of model ternary PtRuM alloy catalysts yielded detailed adsorption energies and activation barriers. Feeding these thermodynamic and kinetic data into a simple micro kinetic model for the CO electro oxidation reaction, the relative activities of a number of PtRuM ternary alloys were calculated. The experimental a...