Methanol oxidation at platinized copper particles prepared by galvanic replacement

Bimetallic Pt-Cu particles have been prepared by galvanic replacement of Cu precursor nanoparticles, upon the treatment of the latter with a chloro-platinate acidic solution. The resulting particles, typically a few tens of nm large, were supported on high surface area carbon (Vulcan® XC–72R, Cabot) and tested as electrodes. Surface electrochemistry in deaerated acid solutions was similar to that of pure Pt, indicating the existence of a Pt shell (hence the particles are denoted as Pt(Cu)). Pt(Cu)/C supported catalysts exhibit superior carbon monoxide and methanol oxidation activity with respect to their Pt/C analogues when compared on a per electroactive surface area basis, due to the modification of Pt activity by Cu residing in the particle core. However, as a result of large particle size and agglomeration phenomena, Pt(Cu)/C are still inferior to Pt/C when compared on a mass specific activity basis

New type bimetallic catalysts for fuel cell electrocatalytic applications

In this thesis the galvanic replacement method was used in order to prepare practical polymetallic electrocatalysts on a fuel cell technology carbon substrate (Vulcan XC 72R). In more detail, Pt(Cu)/C and Pt(Ni)/C bimetallic catalysts were prepared by chemical and electroless deposition of Cu and Ni on Vulcan XC 72R powder, followed by galvanic replacement of Cu and Ni by Pt. The method was also applied to the semiconductor substrate of TiO₂ powder (Degussa P-25) for the preparation of alternative fuel cell catalysts and photoelectrochemical applications. The Pt(Cu)/TiO₂ bimetallic catalysts were prepared by photocatalytic deposition of Cu on TiO₂ powder, followed by galvanic replacement of Cu by Pt. The prepared catalysts were fully characterized by physicochemical techniques. Τransition Electron Microscopy (TEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) were used. Based on the results, the composition, morphology and structure of prepared catalysts were correlated to their preparation conditions. Cyclic voltammetry and slow scan rate voltammetry were used for the electrochemical characterization of the catalysts towards the methanol (CH₃OH) and carbon monoxide (CO) oxidation reactions. The enhanced effect of Cu and Ni in the catalytic activity of Pt towards electrochemical oxidation of methanol and CO was confirmed. The overall catalytic activity was interpreted based on the modification of active surface area, Pt properties and the efficient use of Pt. The prepared catalysts showed higher inherent catalytic activity when compared with a commercial Pt/C catalyst (20% w/w Pt) but (due to smaller electroactive surface area) comparable or inferior mass specific activity towards the oxidation of methanol.Στην παρούσα διδακτορική διατριβή επιτεύχθηκε η επέκταση της μεθόδου της γαλβανικής αντικατάστασης και η καθιέρωσή της στην παρασκευή πρακτικών πολυμεταλλικών ηλεκτροκαταλυτών σε υπόστρωμα άνθρακα της τεχνολογίας των στοιχείων καύσης (Vulcan XC-72R). Πιο συγκεκριμένα, παρασκευάστηκαν διμεταλλικοί καταλύτες Pt(Cu)/C και Pt(Ni)/C μέσω χημικής και μη ηλεκτρολυτικής απόθεσης Cu και Ni στον άνθρακα Vulcan XC-72R, ακολουθούμενης από γαλβανική αντικατάσταση του Cu και του Ni από Pt. Επίσης, επιτεύχθηκε η εφαρμογή της μεθόδου και στο ημιαγώγιμο υπόστρωμα ΤiO₂ (Degussa P-25) για εναλλακτικούς καταλύτες στοιχείων καύσης και φωτοηλεκτροχημικές εφαρμογές. Παρασκευάστηκαν διμεταλλικοί καταλύτες Pt(Cu)/TiO₂ μέσω φωτοκαταλυτικής απόθεσης Cu σε σκόνη TiO₂ (Degussa P-25), ακολουθούμενης από γαλβανική αντικατάσταση του Cu από Pt. Οι παρασκευασθέντες διμεταλλικοί καταλύτες χαρακτηρίστηκαν πλήρως με χρήση φυσικοχημικών τεχνικών. Χρησιμοποιήθηκαν η ηλεκτρονική μικροσκοπία διέλευσης (Τransition Electron Microscopy, TEM), η περίθλαση ακτίνων Χ (X-ray Diffraction, XRD), η φασματοσκοπία διασποράς ενέργειας (Energy Dispersive Spectroscopy, EDS) και η φασματοσκοπία φωτοηλεκτρονίων ακτίνων Χ (X-ray Photoelectron Spectroscopy, XPS). Με βάση τα αποτελέσματα συσχετίστηκε η σύσταση, η μορφολογία και η δομή των παρασκευασθέντων καταλυτών με τις συνθήκες παρασκευής τους. Χρησιμοποιήθηκαν η κυκλική βολταμμετρία και η βολταμμετρία αργής σάρωσης του δυναμικού για τον ηλεκτροχημικό χαρακτηρισμό των καταλυτών ως προς την αντίδραση οξείδωσης της μεθανόλης (CH₃OH) και του μονοξειδίου του άνθρακα (CO). Επιβεβαιώθηκε το ενισχυτικό αποτέλεσμα των μετάλλων Cu και Ni στην καταλυτική ικανότητα του Pt για την ηλεκτροχημική οξείδωση της μεθανόλης και του μονοξειδίου του άνθρακα. Η συνολική καταλυτική ικανότητα ερμηνεύτηκε βάσει της τροποποίησης της ενεργής επιφάνειας, των ιδιοτήτων και της αποτελεσματικής χρήσης του Pt. Από τη σύγκριση με εμπορικούς καταλύτες προέκυψε ότι οι παρασκευασθέντες καταλύτες έχουν μεγαλύτερη εγγενή καταλυτική ικανότητα από εμπορικό καταλύτη Pt/C (20 % w/w) αλλά (λόγω μικρότερης ηλεκτροενεργής επιφάνειας) συγκρίσιμη ή υποδεέστερη ειδική κατά μάζα ενεργότητα για την οξείδωση της μεθανόλης

The use of bimetallic Au(Cu)-coated microelectrodes for improved detection of cystine

Au shell – (Au-Cu) core coatings on carbon microdisc electrodes (30 m diameter) have been prepared by a two-step technique whereby Cu particles electrodeposited onto carbon supports(first step) had their surface layers replaced by Au (second step). The latter has been achieved by means of spontaneous partial replacement of the non-precious metal deposits of Cu by Au upon their immersion in the chlorolaurate-based solution: 3 Cu/C + 2 AuCl4 2 Au (Cu)/C + 3 Cu2+ + 8 Cl . The Au-Cu coated microelectrodes, u(Cu)/C, were subsequently used for voltammetric determination of cystine and their performance compared to a bulk Au RDE. The voltammetric picture was similar at both electrodes. A peak corresponding to adsorbed cystine oxidation at ca. +1.2 V vs. Ag/AgCl and a plateau at ca. +1.6 V corresponding to cystine oxidation from the bulk solution under mass transfer control, were recorded. The latter permitted an estimate of the apparent number of electrons associated with cystine oxidation which were calculated to be around 5 in both cases. However, the peak current was found to be much higher at the Au-Cu microelectrodes, even after correction for increased roughness was taken into account. This increase is attributed to an electronic effect of Cu on Au which decreases the affinity of the latter for excessive oxide formation and/or poisonous carbonaceous intermediates of cystine oxidation. Both the peak and plateau currents varied linearly with the concentration range of 1 10-5 – 1 10-4 M cystine and, in the former case, with a detection limit (3 ) estimated to be about 2 10-6 M

Electrocatalysts Prepared by Galvanic Replacement

Galvanic replacement is the spontaneous replacement of surface layers of a metal, M, by a more noble metal, Mnoble, when the former is treated with a solution containing the latter in ionic form, according to the general replacement reaction: nM + mMnoblen+ → nMm+ + mMnoble. The reaction is driven by the difference in the equilibrium potential of the two metal/metal ion redox couples and, to avoid parasitic cathodic processes such as oxygen reduction and (in some cases) hydrogen evolution too, both oxygen levels and the pH must be optimized. The resulting bimetallic material can in principle have a Mnoble-rich shell and M-rich core (denoted as Mnoble(M)) leading to a possible decrease in noble metal loading and the modification of its properties by the underlying metal M. This paper reviews a number of bimetallic or ternary electrocatalytic materials prepared by galvanic replacement for fuel cell, electrolysis and electrosynthesis reactions. These include oxygen reduction, methanol, formic acid and ethanol oxidation, hydrogen evolution and oxidation, oxygen evolution, borohydride oxidation, and halide reduction. Methods for depositing the precursor metal M on the support material (electrodeposition, electroless deposition, photodeposition) as well as the various options for the support are also reviewed

All-solid Nafion®-based amperometric sensors for monitoring gaseous oxygen

Oxygen reduction has been studied at three different designs of Au/Nafion®-based all-solid amperometric sensors. These consisted of a planar-type sensor (PTS) with all electrodes sputtered on the same face of the Nafion® membrane, a capillary planar type sensor (CPTS) with the introduction of a cover bearing a capillary, and a sandwich-type sensor (STS) with the working and counter electrode layers deposited on different sides of the polymer electrolyte membrane. The response of all sensors depended heavily on humidity and the degree of Nafion® hydration but, unlike the CPTS and STS configurations, no signs of a limiting current were recorded during voltammetric experiments at an open PTS device, indicating the absence of a significant diffusion barrier in that case. All three simple and inexpensive polymerbased sensors compared well with a commercial fuel cell-type sensor, regarding their response to gaseous oxygen concentration changes

Platinized titanium dioxide electrodes for methanol oxidation and photo-oxidation

Platinized deposits have been formed on TiO2 particulate films supported on Ti substrates, by means of galvanic replacement of pre-deposited metallic Cu and subsequent immersion of the Cu/TiO2 coatings into a chloroplatinic acid solution. The spontaneous replacement of Cu by Pt results in Pt(Cu)/TiO2/Ti electrodes. Both the platinized and the precursor TiO2/Ti electrodes have been characterized by SEM micro­scopy/EDS spectroscopy, their surface electrochemistry has been assessed by cyclic voltammetry in the dark and their photoelectrochemical properties by photovolta­m­metry under UV illumination. It has been found that, although platinized rutile-rich electrodes exhibit typical Pt surface electrochemistry, the anatase-rich electrodes show only traces of oxide formation and stripping. The latter has been translated to a suppression of methanol oxidation at anatase-rich electrodes. On the contrary, methanol oxidation at platinized rutile-rich electrodes occurs at significant rates and can be further enhanced upon UV illumination, as a result of Pt and TiO2 synergism in the photoelectrochemical oxidation of methanol

Carbon-supported Pt(Cu) electrocatalysts for methanol oxidation prepared by Cu electroless deposition and its galvanic replacement by Pt

Bimetallic Pt-Cu carbon-supported catalysts (Pt(Cu)/C) were prepared by electroless deposition of Cu on a high surface area carbon powder support, followed by its partial exchange for Pt; the latter was achieved by a galvanic replacement process involving treatment of the Cu/C precursor with a chloroplatinate solution. X-ray diffraction characterization of the Pt(Cu)/C material showed the formation of Pt-rich Pt-Cu alloys. X-ray photoelectron spectroscopy revealed that the outer layers are mainly composed of Pt and residual Cu oxides, while metallic Cu is recessed into the core of the particles. Repetitive cyclic voltammetry in deaerated acid solutions in the potential range between hydrogen and oxygen evolution resulted in steady-state characteristics similar to those of pure Pt, indicating the removal of residual Cu compounds from the surface (due to electrochemical treatment) and the formation of a compact Pt outer shell. The electrocatalytic activity of the thus prepared Pt(Cu)/C material toward methanol oxidation was compared to that of a commercial Pt/C catalyst as well as of similar Pt(Cu)/C catalysts formed by simple Cu chemical reduction. The Pt(Cu)/C catalyst prepared using Cu electroless plating showed more pronounced intrinsic catalytic activity toward methanol oxidation than its counterparts and a similar mass activity when compared to the commercial catalyst. The observed trends were interpreted by interplay between mere surface area effects and modification of Pt electrocatalytic performance in the presence of Cu, both with respect to methanol oxidation and poisonous CO removal. © 2013 Springer Science+Business Media Dordrecht.SCOPUS: ar.jinfo:eu-repo/semantics/publishe