Last modified: 2014-10-08
Abstract
Heterogeneous catalysts are often non uniform at a microscopic level making their understanding extremely complicated. Particles can be of different sizes, compositions or interfaced with different sites of the support The support, in fact, can have a direct influence in the catalytic activity. When a deep analysis can be performed for each nanoparticle or on a reduced group of them, much progress can be made in heterogeneous catalysis.Scanning Kelvin probe (SKP) is a powerful technique which allows to correlates electroactivity to surface phenomena.SKP coupled with a Mass Spectrometer (MS) can be a powerful tool to understand kinetic mechanisms. Single point analysis imposing voltages, in fact, can be used to understand work functions, related to physical-chemical phenomena. Work function involves indicator of the surface condition, which can be affected by adsorbed layers, surface composition, structure, charging and the presence of contaminants.SKP is a non-contact technique measuring the contact potential difference (CPD) between an electrically vibrating reference probe (Kelvin Probe) and an electrically conductive sample. SKP allows obtaining high-resolution maps of the distribution of possible galvanic elements of the sample under analysis.The determination of CPD is an indirect method to measure the work function of a material. In this work we introduce a new method to identify catalytic activity of Pt-based catalysts for the oxygen reduction reaction, by studying the effects of carbon and metal separately, under different atmosphere conditions. Multiwall Nanotubes (MWCNTs) as support, Pt/MWCNT and Pt-Cu/MWCNT are investigated separately at different conditions, to obtain work functions responses.
A Kelvin probe Head (Mc Allister KP 6500) with two copper plates are interposed to generate a voltage different. A plane-ended cylindrical electrode vibrating perpendicular to a stationary sample with a frequency ω is considered, where both electrodes form a planar capacitor. The system can work either with atmosphere pressure or vacuum. Different gases can be introduced into the system in order to study different scenarios. The probe works as a second plate capacitor.
Performed measurements on MWCNT, Pt/MWCNT and Pt-Cu/MWCNT electrocatalyst under vacuum conditions in a fixed range of time show a CPD curve with a variation of the signal, suggesting superficial changes during time which can be attributed to superficial phenomena such as corrosion and adsorption process. Details will be presented in the extended abstract.