Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase

Spektroelectrochemical investigations on electroactive materials in alcohol electrooxidation catalysis

Die vorliegende Arbeit behandelt die anodische Oxidation von kurzkettigen Alkoholen. Alkohole gehören zu den Biokraftstoffen und werden entweder gezielt durch Umwandlung von Biomasse hergestellt oder fallen als Nebenprodukte anderer chemischer Prozesse an. Die gezielte Verwertung bietet daher neben ökologischen auch ökonomische Vorteile. Die elektrochemische Umwandlung der alkoholischen Substrate erfordert die Verwendung heterogener Katalysatoren. Da Edelmetalle ökonomisch mit hohem Aufwand verbunden sind, müssen weniger edle Elemente hinsichtlich ihrer Aktivität untersucht werden. Dies ist eine Notwendigkeit, die neben konventionellen elektrochemischen Messtechniken auch den Einsatz von analytischen Verfahren erfordert. In dieser Arbeit wird die Entwicklung einer spektroelektrochemische Messzelle vorgestellt, die für die Bestimmung der Reaktionsprodukte verschiedener nicht-edler Katalysatormaterialen genutzt wird

Spectroelectrochemical studies on the effect of cations in the alkaline glycerol oxidation reaction over carbon nanotube-supported Pd nanoparticles

The effects of the alkali cations Na+ and K+ were investigated in the alkaline electrochemical oxidation of glycerol over Pd nanoparticles (NPs) deposited on functionalized carbon nanotubes (CNTs). The electrocatalytic activity was assessed by cyclic voltammetry revealing a lower overpotential of glycerol oxidation for nitrogen-functionalized Pd/NCNTs compared with oxygen-functionalized Pd/OCNTs. Whereas significantly lower current densities were observed for Pd/OCNT in NaOH than in KOH in agreement with stronger non-covalent interactions on the Pd surface, Pd/NCNT achieved an approximately three-times higher current density in NaOH than in KOH. In situ electrochemistry/IR spectroscopy was applied to unravel the product distribution as a function of the applied potential in NaOH and KOH. The IR spectra exhibited strongly changing band patterns upon varying the potential between 0.77 and 1.17 V vs RHE: at low potentials oxidized C3 species such as mesoxalate and tartronate were formed predominantly, and with increasing potentials C2 and C1 species originating from C–C bond cleavage were identified. The tendency to produce carbonate was found to be less pronounced in KOH. The less favored formation of highly oxidized C3 species and of carbonate is deduced to be the origin of the lower current densities in the cyclic voltammograms (CVs) for Pd/NCNT in KOH. The enhanced current densities in NaOH are rationalized by the presence of Na+ ions bound to the basic nitrogen species in the NCNT support. Adsorbed Na+ ions can form complexes with the organic molecules, presumably enhanced by the chelate effect. In this way, the organic molecules are assumed to be bound more tightly to the NCNT support in close proximity to the Pd NPs facilitating their oxidation