“…Clearly, the CO oxidation on nanoparticle Pd is sample size independent. Since a dependence of the formic acid electrooxidation activity upon reduction in the particle size is found (Figure ), we conclude that the activity increase is not due to facilitating the CO oxidation process but rather due to the reaction leading to CO 2 formation directly. , 5 Voltammetric CO stripping with Pd nanoparticles (9 nm) in 0.5 M H 2 SO 4 . The scan rate was v = 5 mV/s.…”
Section: Resultsmentioning
confidence: 76%
“…It is generally accepted that the oxidation of formic acid occurs via the dual path mechanism: , one path involving a direct carbon dioxide formation and another an inhibiting intermediate formation . At low potentials, adsorbed CO produced from formic acid decomposition is the inhibiting intermediate. ,, Therefore, it is important to understand the CO oxidation behavior at the Pd nanoparticles, as it will be referred to below. At higher potentials, formate intermediate is the poisoning species .…”
We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.
“…Clearly, the CO oxidation on nanoparticle Pd is sample size independent. Since a dependence of the formic acid electrooxidation activity upon reduction in the particle size is found (Figure ), we conclude that the activity increase is not due to facilitating the CO oxidation process but rather due to the reaction leading to CO 2 formation directly. , 5 Voltammetric CO stripping with Pd nanoparticles (9 nm) in 0.5 M H 2 SO 4 . The scan rate was v = 5 mV/s.…”
Section: Resultsmentioning
confidence: 76%
“…It is generally accepted that the oxidation of formic acid occurs via the dual path mechanism: , one path involving a direct carbon dioxide formation and another an inhibiting intermediate formation . At low potentials, adsorbed CO produced from formic acid decomposition is the inhibiting intermediate. ,, Therefore, it is important to understand the CO oxidation behavior at the Pd nanoparticles, as it will be referred to below. At higher potentials, formate intermediate is the poisoning species .…”
We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.
“…In the Pt-oxide region, at potentials well positive of PZC, − it is possible that HCOO – behaves just as a spectator anion at the surface. Significantly, Vela et al, studying HCOOH and HCOONa oxidation at Pd in acid and alkaline solutions by in situ FTIR spectroscopy, showed the presence of a predominant surface configuration of HCOO – in alkaline solution with the two O-atoms bound strongly to the Pd surface and the C–H bond normal to the surface. Employing surface enhanced infrared absorption (SEIRA) spectroscopy, a surface-sensitive in situ FTIR technique, to study HCOOH oxidation in acid at Pt nanoparticles, Miki et al show that HCOO ads is most likely to be in a bridging configuration on the Pt-surface with the molecular plane normal to the surface or slightly tilted .…”
The electro-oxidation of formate (HCOO − ) on polycrystalline Pt in highly basic medium (pH = 14) has been investigated by cyclic voltammetry, differential electrochemical mass spectrometry, potential step-linear sweep voltammetry, and adsorbate stripping experiments. Low oxidation currents were observed for HCOO − oxidation indicating that HCOO − is only weakly active under these conditions. The activity of HCOO − is confined to low potentials (0.2 V < E < 0.7 V vs RHE). Our results suggest a dual pathway mechanism for HCOO − oxidation, analogous to HCOOH oxidation in acid. In the case of HCOO − oxidation, the direct and indirect pathway analogues are referred to as the primary and secondary pathways, respectively. Formation of CO ads from HCOO − was observed to be slow. We also find that oxidation of CO ads (the secondary pathway) is strongly coupled to the oxidation of HCOO − through the primary pathway. Moreover, the oxidation of CO ads did not appreciably enhance HCOO − oxidation through the primary pathway, signifying the presence of other inhibitory processes such as OH adsorption. Adsorbate stripping experiments revealed, in addition to CO ads , the presence of a stable adsorbate that can undergo oxidation. We also find that the latter can be converted to CO ads in the H upd region. On the basis of the results obtained, a tentative mechanism of HCOO − oxidation in basic media is proposed.
“…The oxidation of formic acid on Pt involves a dual path mechanism, a dehydrogenation path to the direct formation of CO 2 and a dissociative adsorption to form poisoning CO species (a dehydration path). 34,35 Thus, it is reasonable to assign the rst peak for supported Pd catalysts to the dehydrogenation path while the second peak is related to a dehydration path as specied for the Pt/C catalyst. 5,35 One could note that the onset potential for the second peak is similar to that of CO ad oxidation in CO stripping voltammograms, indicating that the second peak is indeed related to the oxidation of adsorbed CO or COlike intermediates, that has been proved by surface-enhanced infrared absorption spectroscopy.…”
Section: Resultsmentioning
confidence: 99%
“…34,35 Thus, it is reasonable to assign the rst peak for supported Pd catalysts to the dehydrogenation path while the second peak is related to a dehydration path as specied for the Pt/C catalyst. 5,35 One could note that the onset potential for the second peak is similar to that of CO ad oxidation in CO stripping voltammograms, indicating that the second peak is indeed related to the oxidation of adsorbed CO or COlike intermediates, that has been proved by surface-enhanced infrared absorption spectroscopy. 36 The ratio of the rst peak current to the second peak current could give an indication of which is the main reaction path.…”
A novel method to self-assemble PMo 12 on CNTs followed by a thermal treatment under hydrogen was developed for a highly dispersed Mo 2 C/CNTs hybrid and was further used to support Pd towards formic acid oxidation. TEM examinations show that the CNTs are uniformly covered by Mo 2 C particles with a diameter of 1-2 nm. The Pd-Mo 2 C/CNTs catalyst exhibits a more negative onset potential (À0.16 V) than Pd/CNTs (À0.13 V) and Pd/C (À0.12 V), showing the highest intrinsic electrocatalytic activity, while its peak current density (845 mA mg À1 Pd) is $1.6 and 2.3 times higher than that of Pd/CNTs (534 mA mg À1 Pd) and Pd/C (371 mA mg À1 Pd), indicating the highest electroactive surface area. The Pd-Mo 2 C/ CNTs catalyst also demonstrates excellent long-term stability, which is likely to have contributions from the incorporation of Mo 2 C that significantly enhances the dehydrogenation path while lessening the dehydration path, resulting in the poisoning reduction of the Pd catalysts. It is reasonably proposed that the assembled Mo 2 C acts not only as a support to enhance the catalyst uniformity, but also as a cocatalyst to directly promote both electrocatalytic activity and stability. This work provides an efficient method to prepare Mo 2 C/CNTs as a promising support towards oxidation of small organic molecules in fuel cells.
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