HCOOH Electrooxidation Research Articles

Sb Surface Modification of Pd by Mimetic Underpotential Deposition for Formic Acid Oxidation

The newly proposed mimetic underpotential deposition (MUPD) technique was extended to modify Pd surfaces with Sb through immersing a Pd film electrode or dispersing Pd/C powder in a Sb(III)-containing solution blended with ascorbic acid (AA). The introduction of AA shifts down the open circuit potential of Pd substrate available to achieve suitable Sb modification. The electrocatalytic activity and long-term stability towards HCOOH electrooxidation of the Sb modified Pd surfaces (film electrode or powder catalyst) by MUPD is superior than that of unmodified Pd and Sb modified Pd surfaces by conventional UPD method. The enhancement of electrocatalytic performance is due to the third body effect and electronic effect, as well as bi-functional mechanism induced by Sb modification which result in increased resistance against CO poisoning.

The Catalytic Behaviour of Pt Clusters on Au and Pd/Au As a Function of Their Surface Coverage and Density

Pt bimetallic structures such as Pt core-shell layers, alloys and clusters on different substrates have been a focus of intense research because they generally exhibit properties different from bulk Pt. The often improved catalytic activity of these structures makes them highly attractive for fuel cells applications[1]. Our understanding and decoupling of the effects responsible for the enhanced catalytic behaviour, such as bi-functional, electronic and ensemble effects, is incomplete. Numerous studies have been conducted so far with the aim of correlating surface activity with the structure and distribution of Pt on or in the catalyst surface [2-5]. It has been shown that the catalytic activity of Pt nanoparticles decorated with ruthenium clusters (Ru/Pt catalyst) is twice that of the commercial 50 : 50 Ru/Pt alloy catalyst [2]. An enhancement of CO tolerance has been also found on supported Pt/Ru catalyst obtained by spontaneous deposition for a considerably lower Pt loading than in commercial catalyst[3]. The modified Au surface with Pt nanosize clusters [6] coverage as low as 15-25% of the substrate area, exhibit very high activity and incredible selectivity toward dehydrogenation of formic acid (HCOOH). Our interest in this work are 2-dimensional model systems of Pt clusters electrodeposited on Au and Pd/Au. We used different electrochemical protocols to control Pt clusters’ size, i.e. in terms of diameter, height and distribution: i) spontaneous adsorption of [PtCl4]2- complex and its potential controlled reduction e.g. the method often called ‘spontaneous deposition’ (SD) [2, 7] and ii) modified surface limited redox replacement (SLRR) method based on the galvanic replacement or partial underpotentially deposited metal layers. For each method successive application of optimized electrodeposition protocols yielded increased Pt surface coverage ranging from 0.25 to 1 area fraction. The surface coverage of Pt/Au and Pt/Pd/Au as well as the active area were determined electrochemically using H-UPD and CO-adsorption. The Pt clusters structure such the diameter, height and distribution were examined by scanning tunnelling microscopy (STM). The catalytic behaviour of Pt- nanostructures is analysed during HCOOH electro-oxidation and it is illustrated in the figure bellow.

Structurally ordered Pt–Zn/C series nanoparticles as efficient anode catalysts for formic acid electrooxidation

Structurally ordered Pt3Zn and PtZn nanoparticles with ultrasmall particle sizes were obtained and exhibited enhanced catalytic activity toward HCOOH electrooxidation.

Insights into the promotion effect of macrocycle molecule on HCOOH electro-oxidation

Macrocycle molecules were found to be highly promotive to HCOOH electro-oxidation on Pt surface since 2007. But it is still not known how the macrocycle molecules induce the promotion effect. The difficulty is that it’s very hard to obtain the coverage of macrocycle molecules during the reaction. In this paper, the under-potential deposited hydrogen (UPD-H) is used as the probe to measure the coverage of macrocycle molecules on Pt surface. Subsequently, the kinetics of adsorption and the promotion effect of the macrocycle molecule (copper phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid (CuPcTsA)) is systematically studied. We find that the coverage of CuPcTsA increases very fast at the beginning 500s during the adsorption, and saturates after 1000s. Usually, the activity of CuPcTsA modified Pt electrode increases with the increasing coverage of CuPcTsA under a certain CuPcTsA concentration during the range 3–25μM. But this behavior follows different curves for different CuPcTsA concentrations. The selectivity (direct pathway to CO intermediate pathway) also increases with the increasing coverage. Surprisingly, the selectivity from different CuPcTsA concentrations follows a same curve. The order of the reaction selectivity is ∼1 and ∼3 for the CuPcTsA coverage lower and higher than 0.34. The desorption is not a simple reverse process of the adsorption, and has some memory of its initial state, where the desorption starts.

PtBi intermetallic and PtBi intermetallic with the Bi-rich surface supported on porous graphitic carbon towards HCOOH electro-oxidation

Well-dispersed PtBi intermetallic and PtBi intermetallic with Bi-rich surface supported on the porous graphitic carbon have been synthesized by using hydrogen gas to reduce the Pt and Bi precursors. TEM images indicated that the simple and clean method can effectively synthesize those PtBi intermetallics with uniform dispersion and small size (below 3nm) on the porous graphitic carbon. Electrochemical characterization revealed that PtBi intermetallic with Bi-rich surface showed extremely high mass activity and stability towards oxidizing of formic acid. Combining precursors’ melt-diffusion strategy with the following effective reduction by the hydrogen gas, the proposed method is easy to scale up and has promising potential for synthesizing anode electro-catalyst of direct formic acid fuel cells.

Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces.

Pd-Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd-Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd-Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd-Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd-Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd-Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.

3D-nanoarchitectured Pd/Ni catalysts prepared by atomic layer deposition for the electrooxidation of formic acid

Three-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.

Synthesis of highly active SnO2-CNTs supported Pt-on-Au composite catalysts through site-selective electrodeposition for HCOOH electrooxidation

In this paper, site-selective electrodeposition method is developed to prepare Pt-on-Au composite catalysts for HCOOH electrooxidation. In the synthesis, Au nanoparticles are prepared through reduction of NaAuCl4 by Sn(OH)2 supported on carbon nanotubes and after thermal transformation of Sn(OH)2 to SnO2, composite supports (Au/SnO2-CNTs) were obtained. By adjusting the potential of Au/SnO2-CNTs coated glassy carbon electrode, Pt entities are selectively deposited on Au particles to form SnO2-CNTs supported Pt-on-Au composite catalysts. The morphology and composition of the formed composite catalysts are characterized by instrument techniques such as X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The site-selectivity of Pt electrodeposition on Au is confirmed by electrochemical measurements. The composite catalysts are tested to have high Pt mass activity toward HCOOH oxidation reaction, demonstrating that the synthesis method developed in this paper has promising potential for developing high performance composite catalysts. The Pt content effect on the catalytic activity is also tested for catalyst performance optimization.

High activity of PtBi intermetallics supported on mesoporous carbon towards HCOOH electro-oxidation

Well-dispersed PtBi intermetallics supported on the modified mesoporous carbon-46 (PtBi/MPC) has been synthesized by using hydrogen gas to reduce the platinum and bismuth precursors. The results of TEM, XRD, EDX, and XPS exhibited that PtBi nanoparticles are intermetallics. Electrochemical characterization revealed that PtBi/MPC showed extremely high mass activities and stability towards oxidizing of formic acid. These high performances may attribute to synergistic effects of electron and geometry effects of these intermetallics and the specific properties of MPC. With the reported promotional effect of Bi, such Pt-based intermetallics supported on MPC is a promising anode catalyst for direct formic acid fuel cells.

The Role of Co‐Adsorbed CO and OH in the Electrooxidation of Formic Acid on Pt(111)

Synergy with potential: Analysis of relevant mechanistic pathways by density functional theory, reveals the synergistic role of co-adsorbed CO and OH in promoting HCOOH electrooxidation on Pt(111). Kinetic models derived from these studies show the atomistic surface phenomena underlying the experimental CV observation in the potential range between 0.0 and 1.2 V (see picture).

Formic acid electrooxidation over carbon-supported nanoparticles of non-stoichiometric palladium carbide

Catalytic performance of the 5 wt.% Pd/C and 5 wt.% PdC0.14/C catalysts for formic acid electrooxidation was compared under potentiodynamic (0.06–1.0 V vs. RHE, sweep rate 50 mV s−1) and potentiostatic (0.2 and 0.3 V vs. RHE) regimes in 0.5 M HCOOH + 0.1 M H2SO4 solution at 25 °C. The non-stoichiometric palladium carbide catalyst demonstrates superior catalytic performance and stability during HCOOH electrooxidation as compared to the unmodified Pd/C precursor. The observed phenomenon is explained in terms of modification of electronic and adsorption properties of palladium caused by the embedding of carbon atoms in the interstitial cites of the crystalline lattice of Pd nanoparticles.

Effect of Carbon Supports on Electrocatalytic Reactivity of Au–Pd Core–Shell Nanoparticles

The role of particle-substrate interactions on the reactivity of bimetallic nanostructures is investigated in the case of Au–Pd core–shell nanoparticles supported on Vulcan XC-72R (Vulcan). Core–shell nanostructures (CS) featuring 19 nm Au cores and Pd shells with thicknesses between ca. 1 and 10 nm were synthesized by controlled colloidal methods and subsequently incorporated in the carbon support. X-ray diffraction, energy dispersive X-ray analysis, and high resolution transmission electron microscopy confirmed the CS nature of the nanostructures, which remain unaffected upon incorporation onto the carbon matrix. Their electrochemical properties toward CO and HCOOH electro-oxidation were studied, using cyclic voltammetry and chronoamperometry. The results show that the CO stripping potential becomes independent of the average Pd lattice strain in the case of Vulcan supported CS. This behavior is significantly different to the trend observed in CS assemblies at In-doped SnO2 electrodes. Formic acid oxida...

Significantly Enhancing Catalytic Activity of Tetrahexahedral Pt Nanocrystals by Bi Adatom Decoration

Tetrahexahedral Pt nanocrystals (THH Pt NCs) bounded by high-index facets possess a high density of active sites and display therefore a higher catalytic activity in comparison with those enclosed by low-index facets. In the current communication, we report, for the first time, the decoration of THH Pt NC surfaces by using Bi adatoms and have demonstrated that the catalytic activity of the Bi decorated THH Pt NCs toward HCOOH electrooxidation has been drastically enhanced in comparison with bare THH Pt NCs. It has also been revealed that the catalytic activity of Bi decorated THH Pt NCs for all coverages investigated always exhibits a higher catalytic activity that is about double that of Bi decorated Pt nanospheres. The study is of great importance regarding both fundamentals and applications.

Butylphenyl-functionalized palladium nanoparticles as effective catalysts for the electrooxidation of formic acid

Monodisperse butylphenyl-functionalized palladium (Pd-BP, dia. 2.24 nm) nanoparticles were synthesized through co-reduction of butylphenyldiazonium and H(2)PdCl(4) by NaBH(4). Because of this unique surface functionalization and a high specific electrochemical surface area (122 m(2) g(-1)), the Pd-BP nanoparticles exhibited a mass activity ∼4.5 times that of commercial Pd black for HCOOH electrooxidation.

The formation of palladium nanocomposite catalysts on the porous silicon used as anodes of fuel cells

Palladium nanoparticles on the porous silicon were synthesized by radiation-chemical reduction in the solution of reversed micelles. The Pd nanoparticles obtained are electron deficient. The porosity, the type of conductivity, the silicon matrix pore geometry, and precursor parameters influence the size, the shape and the charge state of palladium catalysts. The mechanism of H2 and HCOOH electrooxidation on porous silica in the presence of Pd+/Pd redox pair is proposed.

Pt Flecks on Colloidal Au (Pt∧Au) as Nanostructured Anode Catalysts for Electrooxidation of Formic Acid

Nanostructured Pt-on-Au electrocatalysts (coded as Ptm∧Au, m being the atomic Pt/Au ratio), prepared by Pt deposition on Au colloids in two size ranges (Au-I, 10.0 ± 1.2 nm; Au-II, 3.0 ± 0.6 nm) (Zhao and Xu, Phys. Chem. Chem. Phys. 2006, 8, 5106), were employed for the electrooxidation of formic acid (HCOOH) at concentrations of 0.2−3.2 M by cyclic voltammetry. The HCOOH electrooxidation over a Pt shell fully covering Au-I colloids (Ptm∧Au-I at m > 0.2, Pt dispersion 50%) was 5 times and their mass-specific activity 25 times higher than those of the conventional Pt/C and core@shell structured Pt∧Au-I cat...

Enhanced electrocatalytic oxidation of formic acid by platinum deposition on ruthenium nanoparticle surfaces

PtRu alloy nanoparticles were prepared by the spontaneous adsorption of Pt(acac) 2 on the surface of freshly prepared octanethiolate-protected ruthenium (RuSC8) nanoparticles. UV–vis spectroscopic measurements suggested that the adsorption was facilitated by the strong affinity of the platinum metal center to the ruthenium surface. Scanning tunneling spectroscopic studies showed that the resulting PtRu nanoparticles exhibited enhanced conductance as compared to the original ruthenium nanoparticles, where the adsorbed Pt centers might serve as the effective sites for electron-tunneling across the tip/particle/substrate junction. The nanoparticles were then loaded onto a glassy carbon (GC) electrode and their catalytic activities in the electro-oxidation of formic acid were examined and compared. Remarkably, despite the presence of an organic protecting monolayer on the particle surface, apparent electrocatalytic activity was observed. Voltammetric measurements suggested that a Pt skin layer was formed on the Ru nanoparticle surface, most likely as a result of electro-reduction of the adsorbed Pt(acac) 2. On the basis of the onset potential and current density in the electro-oxidation of HCOOH, the resulting PtRu nanoparticles displayed much higher electrocatalytic activity than the Ru counterparts and commercial PtRu nanoparticle catalysts from BASF. It was also found that the electrocatalytic activity of the PtRu nanoparticles was comparable to that of FePt alloy nanoparticles and PtRu bulk electrocatalysts, despite a minimal loading of Pt in the catalysts. Consistent responses were observed in electrochemical impedance spectroscopic measurements, where the charge-transfer resistance on PtRu/GC was found to be at least an order of magnitude smaller than that on Ru/GC and commercial PtRu catalysts. These measurements suggest that the PtRu thin-layer nanoparticles might be used as a promising candidate for the electrocatalytic oxidation of formic acid.

A versatile electroless approach to controlled modification of Sb on Pt surfaces towards efficient electrocatalysis of formic acid

A new facile approach towards developing superior Pt-based catalysts for HCOOH electrooxidation has been proposed, which is exemplified with a mimetic underpotential deposition (MUPD) of Sb on Pt surfaces to attain a favorable coverage. Suitable Sb modification was achieved simply through immersing a bulk Pt electrode or dispersing Pt/C powders in a Sb(III) solution mixed with ascorbic acid (AA). AA serves as the mild reducing agent to ensure freshly reduced Pt surfaces for Sb modification, as demonstrated by the negatively shifted open circuit potential. The catalytic activity towards HCOOH electrooxidation on the above Sb-modified Pt/C catalyst far exceeds that on commercial Pt–Ru/C or Sb-modified Pt/C through traditional irreversible adsorption. This electroless approach is generally applicable to all types of Pt surfaces, in particular suited for upgrading Pt/C for practical anode catalysts of direct formic acid fuel cells.

Controllable Synthesis of Pd Nanocatalysts for Direct Formic Acid Fuel Cell (DFAFC) Application: From Pd Hollow Nanospheres to Pd Nanoparticles

The controllable synthesis of nanosized carbon-supported Pd catalysts through a surface replacement reaction (SRR) method is reported in this paper. Depending on the synthesis conditions the Pd can be formed on Co nanoparticles surface in hollow nanospheres or nanoparticles structures. Citrate anion acts as a stabilizer for the nanostructures, and protonation of the third carboxyl anion and hence the nanostructure and size of the resulting catalysts are controlled via the pH of the synthesis solution. Pd hollow nanospheres, containing smaller Pd nanoparticles, supported on carbon are formed under the condition of pH 9 reaction solution. Meanwhile, highly dispersed carbon-supported Pd nanoparticles can be formed with higher pH (pH ≥ 10). All catalysts prepared through the SRR method show enhanced activities for the HCOOH electro-oxidation reaction compared to catalysts reduced by NaBH4.

Electrocatalytic properties of Au(111)-Pd quasi-single-crystal film electrodes as probed by ATR-SEIRAS

Electrochemical and electrocatalytic properties of thin films Au(111-25 nm), which are quasi-single-crystal electrodes 25 nm thick made of gold with the (111) preferential orientation, and same electrodes modified with a monolayer (ML) of palladium are studied in 0.1 M solutions of HClO4 and H2SO4 employing voltammetric techniques and surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS). Spectroscopic experiments demonstrate strong adsorption of electrolyte species (H2O, OHads, anions) on the Pd surface. The weak and reversible adsorption of CO on Au(111-25 nm) does not change the interfacial-water structure. Adsorption of CO on the Pd-modified film results in an irreversibly adsorbed CO adlayer stabilized by co-adsorbed isolated water species. Various electrooxidation mechanisms are discussed. Electrochemical and spectroscopic investigations on the adsorption and electrooxidation of HCOOH on bare and 1 ML Pd-Au(111-25 nm) electrodes reveal that electrooxidation proceeds in both cases via a direct or dehydrogenation pathway. This mechanism involves the formation of formate as intermediate, which is detected by in situ ATR-SEIRAS. The reactivity on Pd-modified surfaces is higher than on bare gold. The specifically adsorbed anions (sulfate/bisulfate) and the oxide formation on the substrate surface lower the reactivity for CO and HCOOH on both surfaces.