CO Ads Research Articles

Electrochemical oxidation of small organic molecules on hydrothermal synthesized Pt and PtCo/ordered mesoporous carbon

Ordered mesoporous carbon (OMC) supported nanosized Pt and PtCo alloy electrocatalysts were synthesized by ethylene glycol hydrothermal reduction route and their electrochemical oxidation activity toward several typical small organic molecules (SOMs) was investigated. Structural characterization revealed that Pt and PtCo nanoparticles with mean diameter of 3–4 nm were well dispersed on OMC (BET specific surface of 2220 cm 2/g). Electrochemical measurements confirmed that electrochemical oxidation behaviors of SOMs on synthesized Pt/OMC and PtCo/OMC catalysts were complicated and the carbon chain length of SOMs and the intermediates, especially CO ads oxidation behaviors on Pt active sites are two main factors which affect the SOMs oxidation.

Deactivation of carbon supported palladium catalyst in direct formic acid fuel cell

A new carbon black supported palladium catalyst for direct formic acid fuel cell applications has been prepared and characterized by X-ray diffraction. Bi-modal distribution of Pd crystallite sizes was observed. The average Pd size for crystallites in small size and large size ranges were about 2.7 nm and 11.2 nm, respectively. The initial activity of the catalyst in the oxidation of formic acid tested in a fuel cell was similar to a commercial well dispersed 20 wt.% Pd/Vulcan. The rates of the fuel cell power decay were measured for formic acid of two purities for various current loadings. The results showed that various mechanisms contribute to the decrease of cell power with time. In direct formic acid fuel cell (DFAFC) fed with a very pure HCOOH accumulation of CO 2 gas bubbles in anode catalyst layer is responsible for observed power decay. In DFAFC fed with a pure for analysis (p.a.) grade formic acid the formation of CO ads poison from the formic acid impurities is the main deactivation reason.

Methanation of CO2: Further insight into the mechanism over Rh/γ-Al2O3 catalyst

The methanation of CO2 was performed on Rh/γ-Al2O3 catalyst at temperatures between 50 and 150°C at 2bar of pressure in a pulse reactor. Experiments confirm the formation of methane at low temperature and pressure. The formation of formiate species during the adsorption of CO2 can be excluded. After reaction with CO2, the catalyst is oxidized. Oxidation is not observed in the presence of CO. The CO2 is adsorbed dissociatively, forming CO (ads) and O (ads). Gem-dicarbonyl Rh(CO)2 species are more reactive than the Rh–CO linear species. The type of adsorbed species depends on the Rh oxidation state. The formation of methane by hydrogenation of CO2 and CO is carried out with 100% selectivity. The activation energy for the hydrogenation of CO2 and CO is lower than values presented in the literature which have been obtained at higher temperature. In the presence of CO2 and CO, the reaction of methanation of CO2 seems to be inhibited by CO. When oxygen is added in low amount in the reactant gas feed, a positive effect on methanation is observed. When the amount of oxygen is too high, oxygen has a negative effect. These results are in agreement with thermodynamics equilibrium calculations, except when O2 is present, confirming the importance of kinetic effects in the reaction. These results open new perspectives of application of catalysis, in order to recycle CO2 in the presence of H2.

Self-assembly of HPW on Pt/C nanoparticles with enhanced electrocatalysis activity for fuel cell applications

We report here a novel method to immobilize water soluble tungstophosphoric acid (H 3PW 12O 40, HPW) on Pt/C nanoparticles via the electrostatic interaction between the negatively charged HPW and the positively charged functional groups of chitosan which has been attached to Pt/C nanoparticles to provide positively charged sites for the self-assembly of HPW. The HPW assembled Pt/C catalysts (donated as Pt/C-chitosan-HPW) were characterized by XRD, FTIR, TGA, zeta potential, and X-ray photoelectron spectroscopy (XPS). The results indicate that HPW assembled on chitosan-functionalized Pt/C is very stable and Pt/C-chitosan-HPW catalyst has a higher utilization efficiency as compared to that of pristine Pt/C catalyst. Electrochemical activity of Pt/C-chitosan-HPW catalysts for methanol oxidation and oxygen reduction reaction (ORR) is significantly higher than that of Pt/C catalysts without assembled HPW. The enhanced electrocatalytic activities of HPW assembled Pt/C catalysts are most likely due to the synergistic effect between assembled HPW and Pt/C nanoparticles and the presence of HPW leads to a downward shift in the d-band center of Pt catalyst and facilitates the oxidative removal of CO ads poisoning species for methanol oxidation and desorption of O ads species for ORR on Pt catalysts.

Investigation of grain boundary formation in PtRu/C catalyst obtained in a polyol process with post-treatment

The grain boundary formation in PtRu/C catalyst obtained in a polyol process with post-treatment was investigated by scanning transmission electron microscopy, transmission electron microscopy (TEM) and High resolution TEM. The crystalline structure and surface composition of the PtRu/C catalysts were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The electrochemical activities were evaluated by CO stripping voltammetry and linear sweep voltammetry measurements in combination with in situ IR reflection-absorption spectroscopy. As-prepared isolated spherical nanoparticles on the carbon support started to interconnect after washing procedure, and the interconnection between the particles was greatly promoted by reduction post-treatment at 80 °C; grain boundary formation occurred in the interconnected particles with increasing reduction temperature to 200 °C, and the particles reconstructed severely with further increasing reduction temperature to 400 °C. The defects at the grain boundary served as active sites for methanol electro-oxidation by weakening CO ads adsorption on Pt sites and facilitating OH ads formation, and the PtRu/C catalyst treated in 5% H 2/Ar at 200 °C for 10 h had the greatest catalytic activity for methanol electro-oxidation among the PtRu/C catalysts treated under various atmospheres and temperatures.

Elucidating the Ionomer-Electrified Metal Interface

The competitive adsorption of Nafion functional groups induce complex potential dependencies (Stark tuning) of vibrational modes of CO adsorbed (CO(ads)) on the Pt of operating fuel cell electrodes. Operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion were correlated by density functional theory (DFT) calculated spectra to elucidate Nafion functional group coadsorption responsible for the Stark tuning of CO(ads) on high surface area fuel cell electrodes. The DFT calculations and observed spectra suggest that the side-chain CF3, CF2 groups (i.e., of the backbone and side chain) and the SO3(-) are ordered by the platinum surface. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles, consistent with experimental and calculated spectra, suggest direct adsorption of the CF3 and SO3(-) functional groups on Pt. Such adsorption partially orders the Nafion backbone and/or side-chain CF2 groups relative to the Pt surface. The coadsorption of CF3 is further supported by Mulliken partial charge calculations: The CF3 fluorine atoms have the highest average charge among all types of Nafion fluorine atoms and are second only to the sulfonate oxygen atoms.

Promotion of the Oxidation of Carbon Monoxide at Stepped Platinum Single-Crystal Electrodes in Alkaline Media by Lithium and Beryllium Cations

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.

Enhanced electrochemical activity of Pt nanowire network electrocatalysts for methanol oxidation reaction of fuel cells

In this work, Pt nanowire networks supported on high surface area carbon (Pt NWNs/C) are synthesized as electrocatalysts for direct methanol fuel cells (DMFCs). The electrocatalytic behavior of Pt NWNs/C catalysts for the methanol and adlayer CO oxidation reactions is investigated and the results are compared with the Pt nanoparticles (NPs) supported on carbon (Pt NPs/C). The results indicate that Pt NWNs are characterized by interconnected nanoparticles with large number of grain boundaries, downshifted d-band center and reduced oxophilicity, which results in the enhanced surface mobility of oxygen-containing species such as CO ads and OH ads. The enhanced surface mobility of CO ads and OH ads in turn facilitates the removal of intermediate CO species during the methanol oxidation. The activity of the Pt NWNs/C electrocatalyst for the methanol oxidation reaction and electrooxidation of adsorbed CO is also evaluated by cyclic voltammetry, CO stripping, and kinetic analysis. The results show that Pt NWNs/C catalysts have a significantly higher electrocatalytic activity for the methanol oxidation reaction as compared to Pt NPs/C catalysts. The enhanced electrocatalytic activity of Pt NWNs/C catalysts is mainly due to the existence of large number of the grain boundaries of the interconnected nanoparticles of the unique Pt NWN structure.

Reaction mechanism of CO oxidation on Cu2O(111): A density functional study

The possible reaction mechanisms for CO oxidation on the perfect Cu(2)O(111) surface have been investigated by performing periodic density functional theoretical calculations. We find that Cu(2)O(111) is able to facilitate the CO oxidation with different mechanisms. Four possible mechanisms are explored (denoted as M(ER1), M(ER2), M(LH1), and M(LH2), respectively): M(ER1) is CO((gas))+O(2(ads))→CO(2(gas)); M(ER2) is CO((gas))+O(2(ads))→CO(3(ads))→O((ads))+CO(2(gas)); M(LH1) refers to CO((ads))+O(2(ads))→O((ads))+CO(2(ads)); and M(LH2) refers to CO((ads))+O(2(ads))→OOCO((ads))→O((ads))+CO(2(ads)). Our transition state calculations clearly reveal that M(ER1) and M(LH2) are both viable; but M(ER1) mechanism preferentially operates, in which only a moderate energy barrier (60.22 kJ/mol) needs to be overcome. When CO oxidation takes place along M(ER2) path, it is facile for CO(3) formation, but is difficult for its decomposition, thereby CO(3) species can stably exist on Cu(2)O(111). Of course, the reaction of CO with lattice O of Cu(2)O(111) is also considered. However, the calculated barrier is 600.00 kJ/mol, which is too large to make the path feasible. So, we believe that on Cu(2)O(111), CO reacts with adsorbed O, rather than lattice O, to form CO(2). This is different from the usual Mars-van Krevene mechanism. The present results enrich our understanding of the catalytic oxidation of CO by copper-based and metal-oxide catalysts.

Carbon supported PtRh catalysts for ethanol oxidation in alkaline direct ethanol fuel cell

Owing to the formation of an oxametallacyclic conformation, the C–C bond cleavage is the preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt can increase the CO 2 yield during the ethanol oxidation. However, in acidic media the slow oxidation kinetics of CO ads to CO 2 limits the overall reaction rate. In this work, we prepare carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in alkaline media. Cyclic voltammetry tests demonstrate that the Pt 2Rh/C catalyst exhibits a higher activity for the ethanol oxidation than Pt/C does. Linear sweep voltammetry tests show that the peak current density on Pt 2Rh/C is about 2.4 times of that on Pt/C. The enhanced electro-activity can be ascribed not only to the improved C–C bond cleavage in the presence of Rh, but also to the accelerated oxidation kinetics of CO ads to CO 2 in alkaline media.

Influence of the nature of the precursor salts on the properties of Rh–Ge/TiO 2 catalysts for citral hydrogenation

Four sets of Rh–Ge/TiO 2 bimetallic catalysts were prepared by surface redox reaction (i.e., catalytic reduction method) between hydrogen activated on a Rh parent catalyst and a Ge salt dissolved in aqueous solution. The four sets of catalysts differ depending on the presence or absence of chloride ions in the Rh and Ge precursor salts used (i.e., RhCl 3 vs. Rh(NO 3) 3, GeCl 4 vs. GeO 2). Samples were reduced either at a lower temperature (300 °C) or at a temperature chosen to induce a strong metal–support interaction (SMSI) effect (500 °C). Catalysts were characterized by elemental analysis, transmission electronic microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO and evaluated for their activity for the gas phase dehydrogenation of cyclohexane and selective hydrogenation of citral. Regardless of the nature of the Rh and Ge precursor salts, the catalytic reduction method causes the Ge to be in intimate contact with the Rh particles, favoring the citral hydrogenation toward unsaturated alcohols (UA: nerol and geraniol). For low Ge loadings, the bimetallic effect can be combined with the SMSI effect. It was observed that the UA selectivity is directly correlated to the ratio R ( R = ∑A(CO ads on oxidized Rh ⩾1+ species)/∑A(CO ads on total exposed Rh species)) determined by FTIR. A better UA selectivity is obtained when bimetallic catalysts possess a surface in a predominantly oxidized state, a situation that is enhanced when chlorinated rhodium and germanium precursors are used.

Carbon nanotube supported platinum–palladium nanoparticles for formic acid oxidation

Pt, Pd and Pt x Pd y alloy nanoparticles (Pt 1Pd 1, Pt 1Pd 3, atomic ratio of Pt to Pd is 1:1, 1:3, respectively) supported on carbon nanotube (CNT) with high and uniform dispersion were prepared by a modified ethylene glycol method. Transmission electron microscopy images show that small Pt and Pt x Pd y nanoparticles are homogeneously dispersed on the outer walls of CNT, while Pd nanoparticles have some aggregations and comparatively larger particle size. The average particle sizes of Pt/CNT, Pt 1Pd 1/CNT, Pt 1Pd 3/CNT and Pd/CNT obtained from the Pt/Pd (2 2 0) diffraction peaks in the X-ray diffraction patterns are 2.0, 2.4, 3.1 and 5.4 nm, respectively. With increasing Pd amount of the catalysts, the mass activity of formic acid oxidation reaction (FAOR) on the CNT supported catalysts increases in both cyclic voltammetry (CV) and chronoamperometry (CA) tests, although the particle size gets larger (thus, the relative surface area gets smaller). The CV study indicates a ‘direct oxidation pathway’ of FAOR occurred on the Pd surface, while on the Pt surface, the FAOR goes through ‘CO ads intermediate pathway’. Pd/CNT demonstrates 7 times better FAOR mass activity than Pt/CNT (2.3 mA/mgPd vs. 0.33 mA/mgPt) at an applied potential of 0.27 V ( vs. RHE) in the CA test.

Reaction mechanism of methanol decomposition on Pt‐based model catalysts: A theoretical study

The decomposition mechanisms of methanol on five different Pt surfaces, the flat surface of Pt(111), Pt-defect, Pt-step, Pt(110)(1 x 1), and Pt(110)(2 x 1), have been studied with the DFT-GGA method using the repeated slab model. The adsorption energies under the most stable configuration of the possible species and the activation energy barriers of the possible elementary reactions involved are obtained in this work. Through systematic calculations for the reaction mechanism of methanol decomposition on these surfaces, we found that such a reaction shows the same reaction mechanism on these Pt-based model catalysts, that is, the final products are all H (H(ads)) and CO (CO(ads)) via O-H bond breaking in methanol and C-H bond scission in methoxy. These results are in general agreement with the previous experimental observations.

High efficient electrocatalytic oxidation of formic acid on Pt/polyindoles composite catalysts

Four novel composite catalysts have been developed by the electrodeposition of Pt onto glassy carbon electrode (GCE) modified with polyindoles: polyindole, poly(5-methoxyindole), poly(5-nitroindole) and poly(5-cyanoindole). As-formed composite catalysts are characterized by SEM, XRD and electrochemical analysis. Compared with Pt nanoparticles, respectively, deposited on the bare GCE and on the GCE modified with polypyrrole, the four newly developed composite catalysts exhibit higher catalytic activity towards formic acid electrooxidation by improving selectivity of the reaction via dehydrogenation pathway and thus mostly suppressing the generation of poisonous CO ads species. The enhanced performance is proposed to come from the synergetic effect between Pt and polyindoles and the increase of electrochemical active surface area (EASA) of Pt on polyindoles.

Temperature effect on the electrode kinetics of ethanol electro-oxidation on Sn modified Pt catalyst through voltammetry and impedance spectroscopy

The effects of temperature and ethanol concentration on the activity of electrodeposited PtSn catalysts towards ethanol oxidation had been investigated. Electro-codeposition from a bath of Pt and Sn precursors led to both alloyed and unalloyed PtSn system depending on the deposition conditions. A study of the temperature dependence of ethanol oxidation in the range of 10–70 °C afforded the determination of the activation energies on Pt and PtSn catalyst formulations. The onset potential of ethanol oxidation was lowered by ca. 300 mV for all PtSn catalysts as compared to Pt demonstrating that surface Sn and/or its oxides can provide oxygen species at much lower potentials compared to Pt. The considerable lowering of the activation energy for the unalloyed PtSn catalyst was due to its effectiveness for the coupled processes of dissociative ethanol adsorption and CO ads oxidation. The appearance of an inductive loop at temperatures >30 °C in the Nyquist plots, attributable to CO oxidation, for the unalloyed PtSn catalyst formulation, further demonstrated its ability to mitigate CO poisoning of the electrode surface. The effect of ethanol concentration on the electro-oxidation process was revealed through the determination of apparent reaction order, by the help of voltammetry and impedance studies in the ethanol concentration range of 0.1–1.0 M.

The role of the support in COadsmonolayer electrooxidation on Ptnanoparticles: Pt/WOxvs.Pt/C

The electrocatalytic properties of home-made Pt nanoparticles supported onto WO(x) were determined for the electrooxidation of a CO(ads) monolayer and compared with that of a commercial Pt/C having the same Pt particle size. By combining electrochemical and spectroscopic techniques, we found that Pt/WO(x) nanoparticles exhibit a very high tolerance to CO at low electrode potentials (E = 0.1 V vs. RHE), which was never reported in the literature before. CO adsorption at E = 0.1 V vs. RHE on Pt/WO(x) yields CO(2) production as observed by Fourier-transform infrared spectroscopy (FTIR). When the gas bubbling in solution changes from CO to Ar, the current attenuates and the CO(2) production vanishes. This points towards a limited number of "active sites" and a slow step in the electrocatalytic process. When H(2) is used to purge the electrolyte from CO, a steep and continuous increase of the H(2) electrooxidation current is observed pointing towards continuous liberation of the Pt catalytic sites. The high tolerance to CO of Pt/WO(x) is discussed in terms of strong metal-support interaction (SMSI), which involves formation of a metal-oxide film partially covering the Pt nanoparticles (encapsulation) and creation of W-OH groups upon H(+) insertion at low electrode potentials.

CO tolerance and catalytic activity of Pt/Sn/SnO 2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO 2 nanowires (SnO 2NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO 2NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO 2NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO ads. For a carbon paper electrode loaded with 0.1 mg cm −2 Pt and 0.4 mg cm −2 SnO 2NW, a conversion of 54% SnO 2NW into Sn metal (0.17 mg cm −2) initiates the CO ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO 2NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.

Reaction of Br2 with Adsorbed CO on Pt, Studied by the Surface Interrogation Mode of Scanning Electrochemical Microscopy

Scanning electrochemical microscopy surface interrogation (SI-SECM) in the cyclic voltammetry mode was successfully used to detect and quantify adsorbed CO on a Pt electrode by reaction with electrogenerated Br(2). The two-electrode setup used in this new technique allowed the production of Br(2) on an interrogator tip, which reported a transient positive feedback above a Pt substrate at open circuit as an indication of the reactivity of this halogen with CO((ads)). Br(-) and CO(2) are shown to be the main products of the reaction (in the absence of O(2)), which may involve the formation of bromophosgene as a hydrolyzable intermediate. Under saturation conditions, CO((ads)) was reproducibly quantified at the polycrystalline Pt surface with theta(CO) approximately = 0.5. The reaction is shown to be blocked by the action of pre-adsorbed cyanide, which demonstrates the surface character of the process. The formation of CO(2) as an end product was further tested in a bulk experiment: addition of Pt black to a mixture of Br(2) in 0.5 M H(2)SO(4) through which CO was bubbled gave a precipitate of BaCO(3) in a saturated solution of Ba(OH)(2). The use of SI-SECM allowed access to a reaction that would otherwise be difficult to prove through conventional electrochemistry on a single electrode.

Attraction−Repulsion Mechanism for Carbon Monoxide Adsorption on Platinum and Platinum−Ruthenium Alloys

Cluster and periodic density functional theory (DFT) of carbon monoxide adsorbed atop on Pt (CO ads ) show that ruthenium alloying weakens both the CO ads internal and C-Pt bonds and reduces the CO ads adsorption energy. A new theoretical model based on the π-attraction σ-repulsion is used to explain the above results. This model correlates (1) Mulliken population, (2) density-of-states analysis of the CO ads orbitals, (3) the individual interaction of these orbitals with the metal lattice bands, and (4) their polarizations within the CO ads molecule. In this study, the σ interaction has both attractive and repulsive components via electron donation to the metal bands and Pauli repulsion, respectively. Cluster DFT shows that the overall weakening of the CO ads internal bond upon alloying is due to the dominance of reduced σ donation to the metal (which weakens the CO ads internal bond) over increased π bonding between the carbon and oxygen. However, periodic DFT calculations show that both the σ donation and the CO ads internal π bonding are simultaneously reduced. The C-Pt bond weakening upon alloying is primarily due to increased exchange repulsion between the adsorbate and the substrate. The adsorbing Pt atom sp/d z 2 orbitals population increase upon alloying for both calculations.

The origin of high activity of Pt–Au surfaces in the formic acid oxidation

We have investigated the oxidation of HCOOH on Au-modified Pt (Au@Pt) and Pt-modified Au (Pt@Au) electrodes to establish the role of the ensemble and the electronic effect in the catalysis of this reaction. The surfaces were modified by the electrodeposition of sub-monolayers. Cyclic voltammetry in the supporting electrolyte and CO ads stripping have shown that the electronic Pt–Au interactions are insignificant on Au@Pt, but considerable on Pt@Au. Oxidation of CO ads on Au@Pt commenced at the same potentials as on bare Pt, whereas on Pt@Au the oxidation was delayed for ∼0.2 V. Both types of Pt–Au surfaces were found to be more active toward the oxidation of HCOOH compared to bare Pt, exhibiting the maximum activity at Pt fraction between 0.15 and 0.25 with the enhancement factor of more than two orders of magnitude. On bimetallic surfaces the amount of CO ads formed by HCOOH dehydration was low and decreased with decreasing Pt fraction. It was concluded that the high activity of Pt–Au surfaces is caused by the increased selectivity toward the HCOOH dehydrogenation. Similar behavior of Pt–Au surfaces in the HCOOH oxidation, regardless on the Pt–Au electronic interactions, leads to the conclusion that the ensemble effect plays dominant role in the catalysis of HCOOH oxidation.