Electrocatalytic oxidation of formic acid and methanol on Pt deposits on Au(111)

This work presents characteristics of Pt deposits on Au(111) obtained by the use of spontaneous deposition and investigated by electrochemical scanning tunneling microscopy (EC-STM). On such prepared and STM characterized Au(111)/Pt surfaces, we studied electrocatalytic oxidation of formic acid and methanol. We show that the first monatomic layer of Pt displays a (square root 3 x square root 3)R30 degrees surface structure, while the second layer is (1 x 1). After prolonged deposition, multilayer Pt deposits are formed selectively on Au(111) surface steps and are 1-20 nm wide and one to five layers thick. On the optimized Au(111)/Pt surface, formic acid oxidation rates are enhanced by a factor of 20 compared to those of pure Pt(111). The (square root 3 x square root 3)R30 degrees-Pt yields very low methanol oxidation rates, but the rates increase significantly with further Pt growth.     

Controlled deposition of Pt on Au nanorods and their catalytic activity towards formic acid oxidation

Platinum submonolayer decorated gold nanorods with controlled coverage were prepared by the addition of Au nanorods into the growth solution of Pt in the presence of NH₂OH · HCl as the growth agent. The properties of Au nanorods decorated by Pt submonolayer were investigated by various techniques including transimission electron microscopy, X-ray diffraction, and electrochemical methods. The Pt decorated Au nanorods on carbon black showed significantly higher activity on formic acid electrooxidation than the conventional Pt/C catalysts. They showed different reaction path of formic acid electrooxidation by suppressing the formation of poisoning intermediate CO.     

The role of surface oxides in formic acid oxidation on Au

Compared to Pt or Pd electrodes, Au is a poor catalyst for the direct anodic oxidation of HCOOH, but the formation of Au surface oxides in acidic solutions is accompanied by a fast oxidation of HCOOH. This fast reaction is not simply a secondary reaction of Au surface oxides since those oxides are kinetically stable in HCOOH solutions. They do oxidize HCOOH only via a slow and purely electrochemical process which occurs on free Au sites and is “driven” by oxide reduction. The fast HCOOH oxidation is due to a highly reactive intermediate which is able either to form stable Au oxides Au_nO_m or to react with HCOOH. Our results are consistent with the model that by the charge transfer step a reactive non-equilibrium {Au…O> species is formed which converts to stable equilibrium oxides Au_nO_m after migration and rearrangement steps. Pre-equilibrium <Au…O> oxidizes HCOOH and this oxidation is of lower order with respect to <Au…O> compared with the formation of Au_nO_m.     

Self-assembly of mixed Pt and Au nanoparticles on PDDA-functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel cells

Pt and Au nanoparticles with controlled Pt?:?Au molar ratios and PtAu nanoparticle loadings were successfully self-assembled onto poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene (PDDA-G) as highly effective electrocatalysts for formic acid oxidation in direct formic acid fuel cells (DFAFCs). The simultaneously assembled Pt and Au nanoparticles on PDDA-G showed superb electrocatalytic activity for HCOOH oxidation, and the current density associated with the preferred dehydrogenation pathway for the direct formation of CO(2) through HCOOH oxidation on a Pt(1)Au(8)/PDDA-G (i.e., a Pt?:?Au ratio of 1?:?8) is 32 times higher than on monometallic Pt/PDDA-G. The main function of the Au in the mixed Pt and Au nanoparticles on PDDA-G is to facilitate the first electron transfer from HCOOH to HCOO(ads) and the effective spillover of HCOO(ads) from Au to Pt nanoparticles, where HCOO(ads) is further oxidized to CO(2). The Pt?:?Au molar ratio and PtAu nanoparticle loading on PDDA-G supports are the two critical factors to achieve excellent electrocatalytic activity of PtAu/PDDA-G catalysts for the HCOOH oxidation reactions.     

Highly efficient submonolayer Pt-decorated Au nano-catalysts for formic acid oxidation

A novel structure of catalyst, submonolayer Pt-decorated Au, has been synthesized with minimal use of Pt and shows markedly improved activity toward formic acid oxidation where it facilitates the direct oxidation of formic acid by suppressing the formation of poisonous species COads via the "ensemble" effect.     

Butylphenyl-functionalized Pt nanoparticles as CO-resistant electrocatalysts for formic acid oxidation

Butylphenyl-functionalized Pt nanoparticles (Pt-BP) with an average core diameter of 2.93 ± 0.49 nm were synthesized by the co-reduction of butylphenyl diazonium salt and H(2)PtCl(4). Cyclic voltammetric studies of the Pt-BP nanoparticles showed a much less pronounced hysteresis between the oxidation currents of formic acid in the forward and reverse scans, as compared to that on naked Pt surfaces. Electrochemical in situ FTIR studies confirmed that no adsorbed CO, a poisoning intermediate, was generated on the Pt-BP nanoparticle surface. These results suggest that functionalization of the Pt nanoparticles by butylphenyl fragments effectively blocked the CO poisoning pathway, most probably through third-body effects, and hence led to an apparent improvement of the electrocatalytic activity in formic acid oxidation.     

Titanium-supported nanoporous bimetallic Pt–Ir electrocatalysts for formic acid oxidation

Novel titanium-supported nanoporous network bimetallic Pt–Ir/Ti electrocatalysts (S1:Pt₅₉Ir₄₁/Ti, S2:Pt₄₄Ir₅₆/Ti, S3:Pt₂₂Ir₇₈/Ti) have been successfully fabricated by the hydrothermal process. The nanoparticles of Pt and Ir were deposited on the titanium substrates in the presence of formaldehyde as a reduction agent. The electrocatalytic activity of these electrocatalysts towards formic acid oxidation in 0.5 M H₂SO₄ + 0.5 M HCOOH solutions was investigated using cyclic voltammograms (CVs), linear sweep voltammograms (LSVs), chrono amperometry and electrochemical impedance spectroscopy (EIS). The CVs of S1, S2 and S3 exhibit two anodic peaks in the forward scan and one anodic peak in the reverse scan which are similar to the pure Pt. Their LSVs show that the three samples present significantly high current densities of formic acid oxidation compared to the Pt electrode. It is observed from the chrono amperometric measurements at potential 600 mV that the sample S2 delivers a steady-state current density that is 545 times larger than that for the pure Pt electrode. EIS analysis shows that the impedances on both the imaginary and real axes of S1, S2 and S3 are much lower than those of the pure Pt. Among the three samples (S1, S2 and S3), S2 exhibits the highest electrocatalytic activity towards the formic acid oxidation.     

Ensemble size estimation in formic acid oxidation on Bi-modified Pt(111)

The physical dimensions of ensemble structures formed by adsorbed Bi on Pt(111) were estimated and correlations were established between ensemble size and oxidation activity. Measurements were made by examining electrochemical scanning tunneling microscopy (EC-STM) images of Bi irreversibly adsorbed on Pt(111). Percentage coverages of Bi, CO, poison, and ensemble were determined by both EC-STM and cyclic voltammetry. As the fractional Bi coverage increased, from 0.03 to 0.21, the ensemble size decreased, from approximately 9 to 1 nm. Poison formation was inhibited at ensemble sizes less than 2 nm and the turnover frequency of formic acid was enhanced by a factor of 100–200. Bi is hypothesized to not only physically produce the ensembles, but may also chemically alter their electronic properties.     

Two-parameter stochastic resonance in a model of electrochemical oxidation of formic acid on Pt

Stochastic resonance (SR) is shown in a two-parameter system, a model of electrochemical oxidation of formic acid on Pt. The driving current and the saturation coverage for carbon monoxide are two control parameters in this model. Modulation of an excitable focal stable state close to a Hopf bifurcation by a weak periodic signal in one parameter and noise in the other parameter is found to give rise to SR. The results indicate that the noise can enlarge a weak periodic signal and lead the system to be ordered. The scenario and novel aspects of SR in this system are discussed.     

Electrochemical-reduced graphene oxide-modified carbon fiber as Pt–Au nanoparticle support and its high efficient electrocatalytic activity for formic acid oxidation

A simple electrochemical approach is developed to prepare reduced graphene oxide (RGO)-wrapped carbon fiber (CF) as a novel support for Pt–Au nanocatalysts. The obtained composite electrodes have been characterized by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDX), thermal gravimetric analysis (TGA), and electrochemical methods. SEM images reveal that the Pt–Au nanoparticles deposited on RGO-wrapped CF (RGO/CF) electrode display smaller particle size and more uniform dispersion than those on the bare CF electrode. Cyclic voltammetry, linear sweep voltammetry, chronoamperometry, chronopotentiometry, Tafel plots, and electrochemical impedance spectroscopy (EIS) analyses demonstrate that the introduced RGO on CF electrode surface is beneficial to the dispersion of Pt–Au nanoparticles, as a consequence, to the enhancement of the electrocatalytic activity and the antipoisoning ability of Pt–Au towards formic acid electrooxidation.     

Localized Pd overgrowth on cubic Pt nanocrystals for enhanced electrocatalytic oxidation of formic acid

Binary Pt/Pd nanoparticles were synthesized by localized overgrowth of Pd on cubic Pt seeds for the investigation of electrocatalytic formic acid oxidation. The binary particles exhibited much less self-poisoning and a lower activation energy relative to Pt nanocubes, consistent with the single crystal study.     

Application of differential electrochemical mass spectrometry to the electrocatalytic oxidation of formic acid at a modified Bi/Pt electrode surface

Ohne Zusammenfassung     

Surface engineering of Pt surfaces with Au and cobalt oxide nanostructures for enhanced formic acid electro-oxidation

This study aims to mitigate the CO poisoning of platinum (Pt) surfaces during formic acid electro-oxidation (FAEO), the essential anodic reaction in the direct formic acid fuel cells (DFAFCs). For this purpose, a glassy carbon (GC) electrode was amended sequentially with Pt (n-Pt), gold (n-Au), and cobalt oxide (n-CoOx) nanostructures. Fascinatingly, the ternary modified n-CoOx/n-Au/n-Pt/GC catalyst (for which n-Pt, n-Au, and n-CoOx were sequentially and respectively assembled onto the GC surface) exhibited a remarkable electrocatalytic enhancement toward FAEO, which surpassed ca. 53 times that of the Pt/GC catalyst. Additionally, it exhibited a much (ca. 18 times) higher stability after 3000 s of continuous electrolysis. The observed enhancement was proven to originate from driving the reaction mechanism principally to the desirable direct dehydrogenation pathway on the expense of the poisoning dehydration path. The impedance and CO stripping measurements confirmed the prevailing of both the electronic and third body effects in the catalytic enhancement.     

Enhanced electrocatalytic activity of Cu-modified,high-index single Pt NPs for formic acid oxidation

A key goal of nanoparticle-based catalysis research is to correlate the structure of nanoparticles (NPs) to their catalytic function. The most common approach for achieving this goal is to synthesize ensembles of NPs, characterize the ensemble, and then evaluate its catalytic properties. This approach is effective, but it excludes the certainty of structural heterogeneity in the NP ensemble. One means of addressing this shortcoming is to carry out analyses on individual NPs. This approach makes it possible to establish direct correlations between structures of single NPs and, in the case reported here, their electrocatalytic properties. Accordingly, we report on enhanced electrocatalytic formic acid oxidation (FAO) activity using individual Cu-modified, high-indexed Pt NPs. The results show that the Cu-modified Pt NPs exhibit significantly higher currents for FAO than the Pt-only analogs. The increased activity is enabled by the Cu submonolayer on the highly stepped Pt surface, which enhances the direct FAO pathway but not the indirect pathway which proceeds via surface-absorbed CO*.

Single-crystal Pt nanoparticles with a diameter of ∼200 nm were electrosynthesized, covered with a single monolayer of Cu, and then fully characterized. The resulting materials exhibit excellent electrocatalytic properties for formic acid oxidation.     

Electrochemical fabrication of multiplicate palladium hierarchical architectures and their electrocatalysis toward oxidation of formic acid

Dendritic, cactoid, splintery flowers-like and spinous flowers-like micro/nano-Pd hierarchical architectures were successfully deposited on the conductive substrates without assistance of any templates. Distinct from other general electrodeposition at a constant potential or cyclic potential, we utilized pulse potentials as deposition and dissolution potential, which were controlled by a simple and convenient electrochemical method—differential pulse amperometry. It was found that the morphologies of these novel micro/nanoparticles could be regulated with different pulse potentials. The resulting nanostructures were characterized by scanning electron microscopy and X-ray diffractometry. The results show that series of Pd micro/nanoparticles were bounded on the different index facets. It means that the growth direction could be effectively controlled by regulating the pulse potentials. Moreover, the as-synthesized Pd micro/nanoparticles also exhibited strikingly difference in catalytic activity toward electrooxidation of formic acid.     

High activity of Pt(4)Mo alloy for the electrochemical oxidation of formic acid

Surface processes on Pt4Mo alloy well-defined by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were studied in acid solution by cyclic voltammetry. It was established that Mo in the alloy is much more resistant toward electrochemical dissolution than pure Mo. During the potential cycling of Pt4Mo surfaces in completely quiescent electrolyte, hydrous Mo-oxide could be generated on Mo sites. Investigation of the formic acid oxidation revealed that this type of Mo-oxide enhances the reaction rate by more than 1 order of magnitude with respect to pure Pt. Surface poisoning by CO(ads) is significantly lower on Pt4Mo alloy than on pure Pt. The effect of hydrous Mo-oxide on the HCOOH oxidation rate was explained through the facilitated removal of the poisoning species and through its possible influence on the intrinsic rate of the direct reaction path.     

EQCM analysis of Bi oxidation mechanism on a Pt electrode

The electro-oxidation behaviours of bismuth (Bi) species adsorbed on a platinum (Pt) anode were investigated using an in situ electrochemical quartz crystal microbalance analysis in the measurement of cyclic voltammetry (CV) and electrochemical impedance spectroscopy under potentiostatic mode. In the CV of Bi modified Pt, there were four distinct features in the current–potential curves during an anodic scan: (i) the adsorption of water molecules in acidic media, (ii) the formation of Bi oxide, (iii) ionization of Bi oxide and (iv) partial desorption of Bi species. During a cathodic scan, the Bi modified Pt surface recovered to its original state via the reduction of Bi oxide and re-deposition of Bi ion. Surface mass data with electrical charge change and impedance measurements of Bi modified Pt supported the electrocatalytic oxidation of bismuth species as the responsible mechanism.     

Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100): rotation and temperature effects

We investigated the oscillatory behavior in the kinetics of formic acid electrooxidation on Pt(100) in 1 mM HClO₄ solution. We studied the effect of different experimental parameters on the oscillatory behavior, viz. defined HCOOH mass-transport to the electrode surface by using the rotating disk electrode technique, the temperature of the supporting electrolyte, and the nature of anions. We suggest that the interdependence of the reaction steps during HCOOH oxidation, the adsorption of anions and the competition for adsorption sites among the reaction partners and intermediates lead to complex non-linear kinetics. It was evident that once the individual reactions in the dual path mechanism reach steady state the oscillations vanish. These conditions can be reached either by enhanced formic acid reaction rates induced by electrode rotation or by increased temperature. Under specific conditions of anion and formic acid concentration, relaxational oscillations can be transformed into mixed-mode oscillations.     

Aerobic oxidation of methanol to formic acid on Au20-: a theoretical study on the reaction mechanism

The aerobic oxidation of methanol to formic acid catalyzed by Au(20)(-) has been investigated quantum chemically using density functional theory with the M06 functional. Possible reaction pathways are examined taking account of full structure relaxation of the Au(20)(-) cluster. The proposed reaction mechanism consists of three elementary steps: (1) formation of formaldehyde from methoxy species activated by a superoxo-like anion on the gold cluster; (2) nucleophilic addition by the hydroxyl group of a hydroperoxyl-like complex to formaldehyde resulting in a hemiacetal intermediate; and (3) formation of formic acid by hydrogen transfer from the hemiacetal intermediate to atomic oxygen attached to the gold cluster. A comparison of the computed energetics of various elementary steps indicates that C-H bond dissociation of the methoxy species leading to formation of formaldehyde is the rate-determining step. A possible reaction pathway involving single-step hydrogen abstraction, a concerted mechanism, is also discussed. The stabilities of reactants, intermediates and transition state structures are governed by the coordination number of the gold atoms, charge distribution, cooperative effect and structural distortion, which are the key parameters for understanding the relationship between the structure of the gold cluster and catalytic activity in the aerobic oxidation of alcohols.     

Hydrogen evolution reaction and formic acid oxidation by decorated nanostructural Pt/Pd on a copper-filled nanoporous stainless steel

In this work, a 304 stainless steel (SS) was anodized to prepare nanoporous SS (NPSS) with an average size of about 75 nm and then filled with copper (Cu/NPSS) using pulsed electrodeposition method. Afterward, a nanostructural Pt and Pd film was deposited by galvanic replacement (GR) on the Cu/NPSS to prepare modified electrode (PtPd/Cu/NPSS) for hydrogen evolution reaction (HER) and formic acid electrooxidation (FAO). The electrocatalytic activity of the modified electrode and its structural characterization have been studied by voltammetric methods, electrochemical impedance spectroscopy (EIS), inductively coupled plasma optical emission spectrometry (ICP-OES), and field emission scanning electron microscopy (FESEM). The results show that the nanostructural Pt₁Pd₁/Cu/NPSS composition, with low Pt loading and suitable stability, has a good electrocatalytic performance toward HER (E_Onset = + 12 mV vs. NHE) and FAO (E_Onset = ?180 mV vs. NHE). For HER observed a high mass activity of noble metals (87.54 mA cm^?2μg _Pd+Pt ^?1 ) in comparison with Pt deposited Cu/NPSS (41.5 mA cm^?2 μg _Pt ^?1 ) at the same applied potential of ? 0.25 V versus NHE. Also, the fabricated electrocatalysts with more electrochemically active surface area in comparison with Pd/Cu/NPSS and Pt/Cu/NPSS revealed more resisting to the poisoning components and good stability for FAO.