Influence of surface morphology on methanol oxidation at a glassy carbon-supported Pt catalyst

Abstract

Platinum supported on glassy carbon (GC) was used as a model system for studying the influence of the surface morphology of a Pt catalyst on methanol oxidation in alkaline and acidic solutions. Platinum was deposited by the potential step method on GC samples from H2SO4 + H2PtCl6 solution under the same conditions with loadings from 10 to 80 mg cm-2. AFM and STM images of the GC/Pt electrodes showed that the Pt was deposited in the form of 3D agglomerates composed of spherical particles. Longer deposition times resulted in increased growth of Pt forms and a decrease in the specific area of the Pt. The real surface area of Pt increased with loading but the changes were almost negligible at higher loadings. Nevertheless, both the specific and mass activity of platinum supported on glassy carbon for methanol oxidation in acidic and in alkaline solutions exhibit a volcanic dependence with respect to the platinum loading. The increase in the activity can be explained by the increasing the particle size with the loading and thus an increase in the contiguous Pt sites available for adsorption and decomposition of methanol. However, the decrease in the activity of the catalyst with further increase of loading and particle size after reaching the maximum is related to the decrease of active sites available for methanol adsorption and their accessibility as a result of more close proximity and pronounced coalescence of the Pt particles.

Platina istaložena na staklasti ugljenik (GC) korišćena je kao model sistem za ispitivanje uticaja morfologije površine Pt katalizatora za reakciju oksidacije metanola u alkalnim i kiselim rastvorima. Platina je taložena potenciostatskom pulsnom metodom na GC podlogu iz H2SO4 + H2PtCl6 rastvora pod istim uslovima u količini od 10 to 80 mg cm-2. AFM i STM slike GC/Pt elektroda pokazuju da je Pt istaložena u obliku 3D aglomerata koji se sastoje od sfernih čestica. Duže vreme taloženja dovodi do povećanog rasta Pt formi i smanjenja specifične površine nataložene platine. Realna površina istaložene platine raste sa povećanjem količine istaložene platine dostižući plato. Bez obzira na ovo, specifična i masena aktivnost platine istaložene na staklasti ugljenik za oksidaciju metanola kako u kiselim tako i u alkalnim rastvorima pokazuje vulkansku zavisnost od količine istaložene platine. Povećanje aktivnosti se može objasniti preko povećanja veličine čestica sa količinom istaložene platine, odnosno preko povećanja susednih Pt mesta dostupnih za adsorbciju i dekompoziciju metanola. Međutim, smanjenje katalitičke aktivnosti sa daljim povećanjem količine istaložene platine i veličine njenih čestica nakon postignutog maksimuma je povezano sa smanjenjem broja aktivnih mesta dostupnih za adsorpciju metanola i rezultat je smanjenja rastojanja i izraženog stapanja Pt čestica.