How do random superficial defects influence the electro-oxidation of glycerol on Pt(111) surfaces?

Abstract

The glycerol electrooxidation reaction (GEOR) has attracted huge interest in the last decade due to the very low price and availability of this polyol. In this work, we studied the GEOR on Pt(111) electrodes by introducing different densities of random defects. Our results showed that the generation of defects on Pt(111) slightly modified the GEOR onset potential, however it generates changes in the voltammetric oxidation charges and also in the relative production of CO to carbonyl containing compounds, C[double bond, length as m-dash]O. The voltammetric profiles in the forward scan show two oxidation peaks. FTIR data show that the first one is connected with the GlOH dissociative adsorption to form CO (and others intermediates) while the second one, at higher potentials, matches the onsets of the CO oxidation to CO and the C[double bond, length as m-dash]O production. FTIR also confirms that the lower activity of defected electrodes at lower potentials is connected to a higher CO poisoning. DFT calculations show that the presence of CO molecules on a Pt defected surface keeps water and GlOH molecules far from the surface and linked by H bonds. This paper is the last of a series of three works where we explore the GEOR on an important number of different Pt surfaces. These works show that it is difficult to oxidize GlOH at potentials lower than 0.6 V (under our experimental conditions) without suffering an important electrode poisoning (mainly by CO). Since the structure of nanoparticles might be mimicked by defected single crystals, these sets of reports provide a considerable amount of information concerning the influence of such surfaces towards GlOH reaction in acidic media. Therefore, if the well-known "nano"-effect does not produce substantial changes in the activity of Pt materials, they are not useful to be applied in a Direct Glycerol Fuel Cell (DGFC). On the other hand, it is very interesting that the density of electrode defects permits us to tune the relative production of CO to C[double bond, length as m-dash]O.

Electrocatalytic Oxidation of Glycerol on Platinum Single Crystals in Alkaline Media

“…Studying the GEOR on Pt basal plane electrodes Gomes and co‐workers showed that Pt(111) is less prone to catalyze the scission of C−C bond, while on Pt(110) and Pt(100) revealed the formation of adsorbed CO (CO ads ) at potential as low as 0.05 V. Similar results were reported by Fernandez et al . when atomic defects were systematic added under voltammetric conditions on well‐ordered surfaces.…”

“…In an recent communication is showed that after the first cycle the Pt(111) electrode has a dramatic deactivation, lowering its electroactivity to about 10 % of the highest faradaic response . In acidic media, the GEOR on Pt electrodes follow a similar trend as observed in basic media, in a much less extent …”

“…studying the GEOR on Pt(100) in acidic media showed that the presence of randomly distributed defects on the well‐ordered (100) surface does not influence significantly the general current signal response but shifts the main oxidation peak towards more positive potentials. On the other hand, for Pt(111) surface the authors observed a dramatic changes on GEOR response as defects were electrochemically created on the surface . Although it is not the goal of this work, the evaluation of surface defects and its role in GEOR mechanism has its practical importance.…”

“…From the previous references, it is showed that at low pH values the GEOR proceeds through successive dehydrogenation steps to form single bonded CO as a reaction intermediate (CO ads ) at potentials as low as 0.2 V, regardless the surface orientation . Although the carbon monoxide molecule coordinates strongly to Pt atoms and then could act as surface poison in acidic media, it is oxidized above 0.7 V so that the surface recovers its activity at those potentials.…”

“…The use of well‐ordered single crystal electrodes as model surfaces has provided an indispensable strategy when one wants to study specific reaction parameters in isolation. Despite the advantages of using single crystals to gather deeper insights about the kinetics and mechanism reaction, the impact of structure sensitivity of GEOR is a scarcely investigated issue …”