Z. Jusys

Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation—: A DEMS study

Abstract The activity of unsupported high surface area PtRu catalysts of different Pt:Ru ratio towards preadsorbed CO and the methanol oxidation reaction (MOR) in 0.5 M sulfuric acid solution has been studied at room temperature using differential electrochemical mass spectrometry (DEMS). Adsorbed CO monolayer stripping DEMS experiments show that (i) the contribution of double-layer charging increases with the Ru content, reaching up to 50% of the total stripping charge at approximately 40 at% Ru; (ii) the onset of COad oxidation for Ru containing catalysts starts at approximately 0.3 VRHE, about 0.15 V more negative compared with Pt; and (iii) both the onset potential and the peak potential for COad-stripping depends on the Ru content, reaching the most negative values at medium Ru contents, for 20–60 at% Ru. COad-stripping was furthermore used to determine the active surface area of the PtRu catalysts. Based on the electron yield of 1.9 electrons per CO2 product molecule COad can be identified as the stable adsorbed product of methanol dehydrogenation on all PtRu catalysts. Potentiodynamic methanol oxidation experiments show a clear effect of the chemical composition of the PtRu catalysts. The onset of CO2 formation occurs most negative, at slightly below 0.3 VRHE, for PtRu catalysts containing about 40–60 at% Ru. In the technologically interesting potential regime of 0.4–0.5 VRHE PtRu catalysts containing small or medium amounts of Ru (15, 42, 46 at%) are most active, while at more positive potentials more Pt rich catalysts containing approximately 15 at% Ru are most active. These activities refer to the inherent chemical activity obtained by normalizing the oxidation current to the active surface area determined by COad-stripping. Without normalization, the Pt rich catalyst (15 at% Ru) would appear as the most active one, underlining the necessity to properly account for variations in the active surface area. For all compositions methylformate formation starts at potentials around 0.5 VRHE, about 0.2 V positive of the onset of CO2 formation, indicating that at low anodic potentials complete oxidation of methanol to CO2 is preferred. The high current efficiency of both the PtRu and the Pt catalysts for the MOR, with electron yields of slightly above six electrons per CO2 product molecule, is attributed to readsorption and complete oxidation of partially oxidized reaction intermediates, which is more facile for electrodes with high catalyst loadings, as used here, than for electrodes with lower loadings or smooth electrodes.

Transport effects in the oxygen reduction reaction on nanostructured, planar glassy carbon supported Pt/GC model electrodes

The role of transport and re-adsorption processes on the oxygen reduction reaction (ORR), and in particular on its selectivity was studied using nanostructured model electrodes consisting of arrays of Pt nanostructures of well-defined size and separation on a planar glassy carbon (GC) substrate. The electrochemical measurements were performed under controlled transport conditions in a double-disk electrode thin-layer flow-cell configuration; the model electrodes were fabricated by colloidal lithography techniques, yielding Pt nanostructures of well defined and controlled size and density (diameter: 140 or 85 nm, height: 20 or 10 nm, separation: from 1-2 to more than 10 diameters). The nanostructured model electrodes were characterized by scanning electron microscopy and electrochemical probing of the active surface area (via the hydrogen adsorption charge). The electrocatalytic measurements revealed a pronounced variation of the hydrogen peroxide yield, which increases by up to two orders of magnitude with increasing separation and decreasing size of the Pt nanostructures. Similar, though less pronounced effects were observed upon varying the electrolyte flow and thus the mass transport characteristics. These effects are discussed in a reaction model which includes (i) direct reduction to H(2)O on the Pt surface and (ii) additional H(2)O(2) formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H(2)O(2) molecules on the Pt surface.

Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: In situ FTIR spectroscopy and on-line DEMS studies

The interaction of colloid-based, carbon supported Pt/C (40 wt%), PtRu/C (45 wt%) and Pt3Sn/C (24 wt%) catalysts with ethanol and their performance for ethanol electrooxidation were investigated in model studies by electrochemical, in situ infrared spectroscopy and on-line differential electrochemical mass spectrometry measurements. The combined application of in situ spectroscopic techniques on realistic catalysts and under realistic reaction (DEMS, IR) and transport conditions (DEMS) yields new insight on mechanistic details of the reaction on these catalysts under the above reaction and transport conditions. Based on these results, the addition of Sn or Ru, though beneficial for the overall activity for ethanol oxidation, does not enhance the activity for C-C bond breaking. Dissociative adsorption of ethanol to form CO2 is more facile on the Pt/C catalyst than on PtRu/C and Pt3Sn/C catalysts within the potential range of technical interests (<0.6 V), but Pt/C is rapidly blocked by an inhibiting CO adlayer. In all cases acetaldehyde and acetic acid are dominant products, CO2 formation contributes less than 2% to the total current. The higher ethanol oxidation current density on the Pt3Sn/C catalyst at these potentials results from higher yields of C2 products, not from an improved complete ethanol oxidation to CO2.

On the CO tolerance of novel colloidal PdAu/carbon electrocatalysts

The synthesis and characterization (physical and electrochemical) of novel PdAu:Vulcan XC 72 electrocatalysts are described. The catalysts were prepared via deposition and activation of preformed bimetallic colloidal precursors. Physical characterization of the catalysts involved high-resolution transmission electron microscopy and powder X-ray diffraction. Electrochemically, the PdAu-catalysts were characterized by means of the thin-film rotating disk method and by differential electrochemical mass spectrometry. H2 ,C O and CO:H2 oxidations on PdAu:Vulcan electrodes with different bulk compositions were used as probe reactions to determine the CO tolerance of this catalyst system and its physical origin. We propose that the CO-tolerance mechanism at low overpotentials for PdAu:Vulcan electrodes is governed by both a lower CO steady-state surface coverage compared to PtRu:Vulcan due to a lower CO adsorption energy and finite CO oxidation rates, which both result in free surface sites for H2 oxidation to take place. At elevated temperatures (60°C) this effect is more pronounced, providing more free active Pd sites for H2 oxidation. CO:H2 oxidation measurements (1000 and 250 ppm CO) showed the superior activity of PdAu compared with a state-of-the-art PtRu catalyst at technically relevant potentials, especially in Pd-rich alloys.

Mesoscopic mass transport effects in electrocatalytic processes

The role of mesoscopic mass transport and re-adsorption effects in electrocatalytic reactions was investigated using the oxygen reduction reaction (ORR) as an example. The electrochemical measurements were performed on structurally well-defined nanostructured model electrodes under controlled transport conditions in a thin-layer flow cell. The electrodes consist of arrays of Pt ultra-microelectrodes (nanodisks) of defined size (diameter approximately 100 nm) separated on a planar glassy carbon (GC) substrate, which were fabricated employing hole-mask colloidal lithography (HCL). The measurements reveal a distinct variation in the ORR selectivity with Pt nanodisk density and with increasing electrolyte flow, showing a pronounced increase of the H2O2 yield, by up to 65%, when increasing the flow rate from 1 to 30 microL s(-1). These results are compared with previous findings and discussed in terms of a reaction model proposed recently (A. Schneider et al., Phys. Chem. Chem. Phys., 2008, 10, 1931), which includes (i) direct reduction to H2O on the Pt surface and (ii) additional H2O2 formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H2O2 molecules on the Pt surface. The potential of model studies on structurally defined catalyst surfaces and under well-defined mass transport conditions in combination with simulations for the description of electrocatalytic reactions is discussed.

Electrooxidation of CO and H2/CO mixtures on a carbon-supported Pt catalyst—a kinetic and mechanistic study by differential electrochemical mass spectrometry

We report the results of a differential electrochemical mass spectrometry study on the electrooxidation of CO and H2/CO mixtures on a high surface area carbon-supported Pt/Vulcan catalyst. The measurements were performed in a thin-layer flow-cell, allowing the quantitative determination of the individual reaction rates under controlled mass transport conditions as a function of the electrode potential, parallel to the Faraday current. COad monolayer oxidation (stripping) experiments in a 0.5 M H2SO4 solution saturated with H2 or with 2% CO in H2 show that (i) the hydrogen oxidation reaction (HOR) in CO-free, H2-saturated solution starts already at potentials right at the onset of CO oxidation (0.15 VRHE), as evidenced by the simultaneous increase in Faraday current, the increase in the CO2 signal, and the decrease in H2 content in the solution, and that (ii) in CO containing solution the onset of the HOR is shifted to more positive potentials due to continuous CO readsorption. In the former case 90% of the mass transport limited current is reached already at a potential where only ∽5% of the saturated CO adlayer is oxidized, in the latter case the limiting HOR current is reached only in the potential regime of the main CO stripping peak. Furthermore, in the second case CO oxidation in the pre-wave regime is enhanced, while the HOR is suppressed in this potential regime compared to CO stripping in CO-free H2-saturated electrolyte. As a result the ignition of the HOR in the H2/CO(2%) saturated electrolyte occurs only together with the main CO oxidation peak. The data are direct proof that the HOR proceeds in an almost closed CO adlayer, most likely in small (fluctuating) holes of the adlayer and has to compete with CO readsorption. Kinetic measurements at constant electrode potential (0.04 V) in H2/CO(2%)-saturated solution show that CO adsorption from an H2/CO mixture follows a precursor-type adsorption kinetics. For the HOR the data show substantial deviations from both a (1 − θCO) and (1 − θCO)2 dependence of the HOR rate on the CO coverage, pointing to a more complex mechanism than a simple site-blocking mechanism.

Cyclic voltammetry and quartz crystal microgravimetry study of autocatalytic copper(II) reduction by cobalt(II) in ethylenediamine solutions

Abstract Cyclic voltammetry (CV) and electrochemical quartz crystal microgravimetry (EQCM) were used to study the reactions in a new type autocatalytic (electroless) copper deposition system using cobalt(II)–ethylenediamine complex as a reducing agent. The rates of Co(II) oxidation and Cu(II) reduction (partial reactions of the autocatalytic process) as well as those of Cu anodic dissolution and Co(III) reduction were measured both in separate Co(II)–En, Cu(II)–En, and in complete electroless plating systems (pH 6–8) as a function of Cu potential (from −0.4 to 0.05 V vs. SHE), in unstirred solution and under flow-through (wall-jet) conditions. The experimental results show that the autocatalytic process results from a coupling of partial electrochemical reactions of Co(II) oxidation and Cu(II) reduction on the Cu surface, some deviations from partial reactions additivity being explained by ligand (En) concentration changes at the Cu surface in the separate solutions of Co(II) and Cu(II) and in the complete system. The Co(II) oxidation rate increases with the solution pH alongside the increase in CoEn 3 2+ complex concentration in the solution. The partial reaction rate increases remarkably at solution flow conditions, this effect being related both to the transport of reacting Cu(II) and Co(II) species to the electrode, and to the removal of Co(III) species from the Cu surface. Chloride ions possibly act as a bridge between reacting complex species and the Cu electrode.

Stability of Nanostructured Pt/Glassy Carbon Electrodes Prepared by Colloidal Lithography

The stability of nanostructured Pt/glassy carbon (GC) model electrodes upon exposure to a realistic electrochemical/electrocatalytic reaction environment (continuous reaction, continuous electrolyt ...

Autocatalytic reduction of Cu(II) to copper by Co(II) studied by electrochemical quartz crystal microgravimetry in a wall jet cell

Abstract Electrochemical Quartz Crystal Microgravimetry (EQCM) studies were carried out in a new type of electroless copper plating solution containing Co(II)-ethylenediamine species as a reducing agent for Cu(II)-ethylenediamine complex. The rates of partial reactions of Cu(II) reduction and Co(II) oxidation were studied as a function of the electrode potential (−0.3 V to 0.05 V vs. SHE) under stopped-flow and wall jet conditions. The increase in the rate of partial reactions and the overall process under wall jet conditions suggests diffusion limitations occur in the transport of reacting species (Cu(II) and Co(II)) to the electrode and/or inhibiting products of reaction (Co(III) and ethylenediamine) to the bulk of the solution. Anodic dissolution of copper starts at 0.0 V.

Complete Quantitative Online Analysis of Methanol Electrooxidation Products via Electron Impact and Electrospray Ionization Mass Spectrometry

We report on a novel approach for complete quantitative online product analysis in electrocatalytic reactions, combining electron impact ionization mass spectrometry (EI-MS) and electrospray ionization mass spectrometry (ESI-MS) for simultaneous detection of both volatile and nonvolatile reaction products. The potential of this method is demonstrated using continuous methanol oxidation in a flow cell. The overall reaction rate was followed via the Faradaic current; CO(2) formation was monitored mass spectrometrically via a membrane inlet system, and formaldehyde and formic acid were detected by ESI-MS after a derivatization-extraction-separation procedure introduced recently (Zhao, W.; Jusys, Z.; Behm, R. J. Anal. Chem.2010, 82, 2472-2479) providing quantitative data on the product distribution. In a more general sense, this approach is applicable for a wide range of reactions at the solid-liquid interface or in liquid phase.