CO-like Species Research Articles

Platinum–palladium bimetallic nanoparticles on graphitic carbon nitride modified carbon black: A highly electroactive and durable catalyst for electrooxidation of alcohols

A composite catalyst consisting of Pt–Pd bimetallic nanoparticles (NPs) and graphitic carbon nitride (g-C3N4) modified carbon black (CB) is designed and fabricated by a facile two-step approach. The resulting catalyst exhibits unprecedented high catalytic activity and excellent long-term stability for electrooxidation of alcohols (methanol, ethanol, ethylene glycol and glycerol) in alkaline media. The superior electrochemical performance of the composite catalyst is attributed to the specific characteristics of the unique nanostructure and the synergistic effects of individual components, including the complementary roles of Pt (dehydrogenation site) and Pd (removal of CO-like species), the template effect of the planar amino group of g-C3N4 for the dispersed decoration of Pt–Pd NPs, the high specific surface area of CB for the rapid diffusion of electrolyte and removal of the carbonaceous intermediates, as well as the structural stability of the support based on covalent interaction between g-C3N4 and CB for maintaining the durability of the catalytic system.

PdxCu100−xnetworks: an active and durable electrocatalyst for ethanol oxidation in alkaline medium

Rational control of the composition and the nanostructure of catalysts can significantly affect their catalytic efficiency. Here, we demonstrated a one-pot, room temperature and aqueous synthesis of submicrometer-sized PdCu networks as superior catalysts for ethanol oxidation reaction (EOR) in alkaline medium. When correlating the activity of PdCu network catalysts for EOR with Cu content, the PdCu network catalysts show a composition-dependent ‘volcano’ behavior. The ‘volcano’ behavior is rationalized with the Sabatier principle where the reaction rate passes through a maximum when metals adsorb reaction intermediates moderately. The composition-optimized Pd73Cu27 network was intensively compared with a commercial Pd/C catalyst and it showed a superior electrocatalytic performance with a 178% enhancement in Pd-based mass activity, 160% improvement in long-term stability and better tolerance of CO-like species poisoning.

The electrooxidation mechanism of formic acid on platinum and on lead ad-atoms modified platinum studied with the kinetic isotope effect

Kinetics and mechanism of formic acid (FA) oxidation on platinum and upd-lead ad-atoms modified platinum electrodes have been studied using unlabelled and deuterated compounds. Poisoning of the electrode surface by CO-like species was prevented by suppression of dissociative chemisorption of FA due to a fast competitive underpotential deposition of lead ad-atoms on the Pt surface from an acidic solution containing Pb2+ cations. Modification of the Pt electrode with upd lead induced a catalytic effect in the direct electrooxidation of physisorbed FA to CO2. With increasing degree of H/D substitution, the rate of this reaction decreased in the order: HCOOH > DCOOH ≥ HCOOD > DCOOD. HCOOH was oxidized 8.5-times faster on a Pt/Pb electrode than DCOOD. This primary kinetic isotope effect proves that the C–H- and O–H-bonds are simultaneously cleaved in the rate determining step. A secondary kinetic isotope effect was found in the dissociative chemisorption of FA in the hydrogen adsorption–desorption range on a bare Pt electrode after H/D exchange in the C–H bond, wherein the influence of deuterium substitution in the O–H group was negligibly small. Thus the C–H bond cleavage is accompanied by the C–OH and not the O–H bond split in the FA decomposition, producing CO-like species on the Pt surface sites.

Synthesis of a low palladium dosage catalyst and its high performance promoted by Fe2O3 for ethanol electrooxidation

In this paper, a low palladium dosage hybrid catalyst (Pd/Fe2O3) for ethanol electrooxidation promoted by Fe2O3 has been fabricated by a facile solvothermal approach. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses reveal that the prepared face-centered cubic Pd nanoparticles with the average diameter of ca. 5 nm are well dispersed on the surface of the as-prepared dendritic Fe2O3. The resulting Pd/Fe2O3 hybrid catalyst with a total loading of 8.4 wt% Pd shows higher mass activity (450.8 mA mg−1 Pd) than Pd/XC-72 catalyst (359.9 mA mg−1 Pd) and JM commercial 20% Pt/C catalyst (144.0 mA mg−1 Pt) towards ethanol electrooxidation, and better stability as well. Fe2O3 has a promoting effect on oxidizing and removing the adsorbed CO-like intermediate species during electrooxidation of ethanol. These results indicate that due to the low cost and enhanced performance, the Pd/Fe2O3 hybrid catalyst may be a superior candidate for direct ethanol fuel cells (DEFCs).

Fabrication of a novel PtPbBi/C catalyst for ethanol electro-oxidation in alkaline medium

This work presents a general strategy to fabricate a new type of ternary intermetallic PtPbBi/C nano-catalyst. X-ray diffraction, transmission electron microscope and X-ray photoelectron spectroscopy data demonstrate that some Pb atoms are imbedded into Pt lattice to form PtPb alloy particles in the microwave action. The PtPb/C catalyst surface is further modified by Bi via a spontaneous and irreversible chemisorption approach. Ethanol oxidation peak current on PtPbBi/C (217.9mAcm−2) is about 4.1 times higher than that on Pt/C (53.6mAcm−2) in cyclic voltammograms. Tafel plots show that both ethanol dehydrogenation and removal of CO-like species are greatly enhanced on the novel PtPbBi/C catalyst. Constant potential and linear current sweep tests indicate that the addition of Pb and Bi causes the formation of abundant oxygenated species on the catalyst surface at low potential. These oxygenated species can react with poisonous intermediates formed during ethanol oxidation, and help the release of free Pt active sites for further adsorption and electro-oxidation of ethanol molecules.

Palygorskite promoted PtSn/carbon catalysts and their intrinsic catalytic activity for ethanol oxidation

Abstract PtSn nanoparticles deposited on a stable electrocatalytic support containing conventional carbon and palygorskite are prepared using pre-treated palygorskite and are evaluated by ethanol oxidation. The electrocatalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM and XRD show that the pre-treatment has an effective impact on the morphology of palygorskite, but does not destroy its architecture. XPS results imply that introduction of palygorskite can facilitate the formation of Sn oxides on the surface of catalyst. Sn oxides increase the amount of surface oxygen-containing species and in turn facilitate to oxidize CO-like species adsorbed on Pt active sites, which has been proved by CO stripping voltammetry. Cyclic voltammetry (CV) measurements show that the addition of palygorskite into the carbon as support improves the electrocatalytic activity in ethanol oxidation. The developed new composite support is catalytically more active for ethanol electrooxidation than the conventional carbon support and shows good stability, offering great potential for widespread application of palygorskite as support for electrocatalysis.

Catalytic effect of molybdenum oxo-species on reduction of allyl alcohol at a Pt electrode in strongly acidic solutions

Kinetics of the electroreduction of molybdenum(VI) species and allyl alcohol (AA) alone as well as of AA after the addition of Na2MoO4 to perchloric acid solutions (HClO4) on a polycrystalline Pt electrode was characterized under the conditions of cyclic voltammetry (CV). Using the XPS technique evidence was provided that mixed valent Mo(V/VI) layer is formed upon reduction of Mo(VI) moieties present in 1M HClO4 solution, while the cationic Mo(VI) species existing in strongly acidified solutions are reduced successively to relevant Mo(V) and Mo(III) species at E<0.25V. It was ascertained that a relatively fast adsorption of AA precedes its hydrogenation to propene and propane in the absence of Mo(VI) in the electrolyte solution. However, the Pt surface is partially poisoned with CO-like species due to a rupture of the C3-carbon backbone in the potential range of hydrogen adsorption–desorption. A considerable enhancement of the cathodic current corresponding to the AA electroreduction was discovered in the presence of cationic Mo(VI) species in 4M HClO4 solutions. Simultaneously, the rate of electroreduction of this species was effectively increased on the addition of AA in comparison with that in the absence of AA. According to the proposed reaction pathways, AA is reduced via a catalytic chemical reaction with Mo(III) and/or Mo(V) cationic species. The cyclic regeneration of the electroactive Mo(VI) and/or Mo(V) species accounts for the observed catalytic phenomenon. Furthermore, the decomposition of the C3-carbon skeleton of AA molecules and formation of strongly adsorbed CO-like species is eliminated in the presence of Mo(VI). The optimal conditions for monitoring of AA and/or Mo(VI) in electrochemical devices with a Pt electrode are specified.

Tungsten carbide as supports for Pt electrocatalysts with improved CO tolerance in methanol oxidation

One anti-CO-poisoning Pt–WC/C catalyst for methanol electro-oxidation is prepared in this work, through depositing platinum on tungsten carbide support using an intermittent microwave heating (IMH) method. The catalyst presents an improved methanol oxidation performance evidenced by a negative shift in onset potential, and increase of peak current density, compared with a commercial Pt/C one. CO stripping experiments indicate that the adsorbed CO is able to be oxidized and removed from the Pt–WC/C catalyst more easily, attesting the enhanced capability of anti-poisoning to CO-like species. Theoretical calculation further provides evidence that the surface electronic structure in Pt–WC/C and Pt/C catalysts is likely different. WC supports could lead to much stronger negative electronic property, which is beneficial for avoiding CO adsorption on the Pt–WC/C catalyst. In the mean time, the electron donating effect generated by WC supports also promotes the ability to oxidize the adsorbed CO-like species on catalysts. In good agreement with experimental results, the theoretical calculation proves the anti-CO-poisoning nature of the Pt–WC/C catalyst, and well explains the origin of the improvement in the electrochemical catalytic performance for effectively accelerating the oxidation of CO to CO2 in methanol oxidation.

The origin of the high performance of tungsten carbides/carbon nanotubes supported Pt catalysts for methanol electrooxidation

The Pt supported on WC modified MWCNT catalysts (PtWC/MWCNT) were synthesized by the combination of organic colloidal and intermittent microwave heating (IMH) methods for the first. The results proved the better performance of the PtWC/MWCNT catalyst than that of Pt/C for methanol oxidation in terms of the onset potential and peak current density. The synergistic effect between Pt nanoparticles and WC and the structure effect of the MWCNTs could be the reasons to result in the high activity. The CO stripping test provided the evidence that the onset potential shift for methanol oxidation is consistent with the reduction in the overpotential for the CO oxidation on PtWC/MWCNT catalyst. Therefore, the mechanism of the high performance for methanol oxidation on PtWC/MWCNT catalyst is probably the easier oxidation of CO-like species which cause high overpotential for further oxidation of methanol.

Structure and chemical composition of supported Pt–Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnO x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0–3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO 2 nanoparticles, characterized with the nominal 0.264 nm spacing of SnO 2 (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO x catalyst. PtSnO x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.

Electrochemical analysis of high temperature methanol electro-oxidation at Pt-decorated Ru catalysts

Methanol electro-oxidation at Pt-decorated unsupported Ru catalysts having a Pt loading of 0.1 mg cm −2 has been investigated in situ in direct methanol fuel cells at high temperatures. The chemistry and morphology of the decorated catalyst has been studied by X-ray techniques and electron microscopy. An analysis of the stripping behaviour of adsorbed CO-like species has been carried out to obtain information on the electrocatalytic activity of the Pt-decorated Ru anode in relation to carbon supported Pt–Ru (1:1) alloy and bare unsupported Ru catalysts. The promoting effect of decorating Pt particles has been investigated by cyclic voltammetric analysis in the presence of a continuous methanol supply. The decorated catalyst has shown lower intrinsic catalytic activity with respect to the Pt–Ru alloy; yet, the decorating particles promote methanol dehydrogenation and adsorption of methanolic residues on the Ru support allowing the reaction to occur at significant rates even in the presence of small Pt amounts.

A laboratory survey of the thermal desorption of astrophysically relevant molecules

The thermal desorption characteristics of 16 astrophysically relevant species from laboratory analogues of the icy mantles on interstellar dust grains have been surveyed in an extensive set of preliminary temperature programmed desorption experiments. The species can be separated into three categories based on behaviour. Water-like species have a single relevant desorption coincident with water. CO-like species show the volcano desorption and co-desorption of trapped molecules, monolayer desorption from the surface of water ice, and multilayer desorption if initially present in sufficient abundance in an outer layer separated from the water ice. Intermediate species show the two desorptions of trapped molecules, and may show a small monolayer desorption for molecules small enough to have a limited ability to diffuse through the structure of porous amorphous water ice. Methods by which the results obtained under laboratory conditions can be adapted for astrophysical situations are discussed.

Analysis of the high-temperature methanol oxidation behaviour at carbon-supported Pt–Ru catalysts

Methanol oxidation behaviour at three Pt–Ru catalysts varying by the concentration of active phase on the carbon support has been investigated in a wide temperature range (80–130 °C). An increase of the adsorbed methanolic residue stripping charge is observed with the increase of catalyst dispersion. As the temperature is increased, the stripping peak potential shifts more negatively accounting for a lower activation barrier for the reaction. An increase of temperature above 90 °C also produces a strong decrease in the coverage of adsorbed methanolic residues. The fuel cell performance is significantly enhanced by catalysts with intrinsically high catalytic activity, whereas the methanol reaction rate appears to be less influenced by an increase in coverage of active species. Catalysts characterized by a higher degree of alloying and metallic behaviour on the surface appear to be more active towards methanol oxidation. However, the physico-chemical properties of the catalysts have less influence on the anode electrochemical behaviour at high temperature since CO poisoning is alleviated under such conditions. The decrease of CO-like species coverage with temperature and the methanol tolerance characteristics of a Pt/C cathode are also discussed in relation to the crossover drawback of direct methanol fuel cells.

The influence of carbon dioxide on PEM fuel cell anodes

The influence of CO 2 on the performance of PEM fuel cells was investigated by means of fuel cell experiments and cyclic voltammetry. Depending on the composition and microstructure of the fuel cell anode, the effect varies from small to significant. Adsorbed hydrogen plays a dominant role in the formation of CO-like species via the reverse water–gas shift reaction. Platinum sites which are not utilized in the electrochemical oxidation of hydrogen are thought to catalyze this reverse-shift reaction. Alloying with ruthenium suppresses the reverse-shift reaction.

Elucidation of the reaction pathways of allyl alcohol at polycrystalline palladium electrodes

The electrochemical behaviour of allyl alcohol at palladium electrodes was studied by cyclic voltammetry, chronoamperometry and on-line mass spectrometry (DEMS). The latter allows the detection of the volatile products generated during the electroreduction and electrooxidation processes. C 3-hydrocarbons (propene and propane) and acrolein were detected as the bulk products, whereas C 2-hydrocarbons and CO 2 are related to the adsorbed species. The dissociation of the alcohol produces ethine in the 0.20–0.35 V potential range, which reduces to ethane. Adsorbed acrolein and C 2-hydrocarbonated residues seem to be formed in addition to CO-like species. The results were compared with those previously obtained at platinum and gold, as well as with other unsaturated alcohols, namely benzyl and propargyl alcohol.

Electrochemical behaviour of oxocarbons on single crystal platinum electrodes Part V. Tetrahydroxy-p-benzoquinone in 0.5 M sulphuric acid medium

The electrochemical behaviour of tetrahydroxy-p-benzoquinone (THQ) on Pt(111), Pt(100) and Pt(110) surfaces has been studied in sulphuric acid solutions by cyclic voltammetry. The experimental results show that THQ is the oxocarbon having a lesser tendency to form CO-like species after adsorption on platinum surfaces. THQ is directly adsorbed on Pt(111) and Pt(100), although it is not stable and undergoes a slow reaction to CO adspecies on Pt(100) surfaces. Clear evidence of CO adspecies has been found only on Pt(100) electrodes.

The influence of the surface structure on the adsorption of ethene, ethanol and cyclohexene as studied by DEMS

Using differential electrochemical mass spectrometry, the adsorption of ethene, ethanol and cyclohexene on mono- and polycrystalline Pt electrodes was studied. Experiments in H 2 18O showed that a part of the adsorbed ethene molecules contains oxygen. Ethanol mainly dissociates upon adsorption, yielding an adsorbed CO-like species and a CH x fragment which can be cathodically desorbed as methane. From a polycrystalline Pt electrode, adsorbed cyclohexene can be cathodically desorbed as benzene and cyclohexane in the same ratio as adsorbed benzene, and is therefore most likely to be identical to adsorbed benzene; cyclohexene adsorption on Pt(111) and Pt(110) mainly yields an oxygenated species in a similar reaction as that which occurs for a part of the adsorbing ethene.

Microcalorimetric study of reactive surface area on demineralized coal chars

Microcalorimetric studies of oxygen adsorption reveal that the reactive surface area (RSA) of coal chars is composed of a distribution of site types. The nature of the distribution is a function of the starting coal; however, two basic site types dominate for the three representative (bituminous, subbituminous, and lignite) coals studied. Following low-temperature outgassing treatments (773 K) the majority of reactive sites on all samples were such that oxygen adsorption at 300 K led to the formation of CO-like species. In contrast, following high-temperature outgassing (1173 K) both CO-like and CO 2 -like species formed during oxygen adsorption at 300 K. The kinetics of oxygen adsorption, however, were very much a function of the identity of the parent coal and the treatment temperature

A voltammetric study of glyoxylic acid behaviour on platinum single-crystal electrodes in sulphuric acid medium

The electrochemical behaviour of glyoxylic acid has been studied by linear sweep voltammetry in acidic solutions. The oxidation of glyoxylic acid takes place mainly at potentials higher than 1.0 V/RHE, and has been found to be structure sensitive. At lower potentials, the formation of poisoning intermediates due to dissociative adsorption of glyoxylic acid is the predominant process. Two kinds of stable residues can be distinguished: CO-like species, and species which probably maintain the CC bond intact. The formation of these species depends on the applied potential and does not take place at open-circuit conditions. The oxidation of adsorbed CO takes place in the usual potential range for this species. Only a slight positive shift is observed, attributable to specific adsorption of glyoxylate anions which is stronger than that of bisulphate. The C 2 residues require potentials of about 1.0 V/RHE to be stripped oxidatively from the surface

Electrochemical behaviour of squaric acid on single-crystal platinum electrodes with basal orientations in aqueous sulphuric acid medium

The electro-oxidation of squaric acid (SQA) on basal platinum single-crystal surfaces in aqueous 0.5 M H 2SO 4 solutions has been studied. Voltammetric results show that two different processes occur in the overall reaction. In the potential range 0.7–0.8 V vs. reversible hydrogen electrode, the oxidation of strongly bonded species formed after adsorption of SQA at less positive potentials takes place. This process is a structure-sensitive reaction and its characteristics allow us to assume that CO-like species are the stable adsorbed residues. At more positive potentials, the direct oxidation of SQA takes place overlapping with the oxidation of the platinum surface; Pt(110) is the most active electrode for this reaction. Its sensitivity to the surface structure indicates that this reaction involves the interaction of SQA molecules with the first platinum surface oxides. Only a slight inhibition effect on the direct oxidation of SQA due to strongly adsorbed CO is observed on Pt(100) and Pt(110) electrodes.