Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium

Abstract

In order to deepen the knowledge about the origin of the CO pre-oxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation were performed a series of CO stripping experiments on stepped Pt electrodes in acidic medium. For the occurrence of CO pre-oxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with CO ads ; (3) in presence of traces of CO in solution; (4) in presence of surface steps. If any of these four conditions is not satisfied, the CO pre-oxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO.By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials ~150 mV higher than the CO pre-oxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO pre-oxidation process. Once the sites responsible for the CO pre-oxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls This is a previous version of the article published by ACS in The Journal of Physical Chemistry C. 2015, 119(35): 20272-20282. doi:10.1021/acs.jpcc.5b05386 2 the whole process, rather than the diffusion of CO ads towards hypothetically "most active sites". Voltammetric and chronoamperometric experiments performed on partially covered CO adlayers suggest that adsorbed CO behave as a motionless species during its oxidation, in which the CO adlayer is removed piece by piece. By means in situ FTIR experiments the stretching frequency of CO selectively adsorbed on (110) step sites was examined. Band frequency results confirm that those molecules adsorbed on steps are fully coupled with the adsorbed CO on (111) terraces when the surface reaches full coverage.

Mobility and Oxidation of Adsorbed CO on Shape-Controlled Pt Nanoparticles in Acidic Medium

“…35 For the CO oxidative stripping in acid media, the current vs. time profile coming from multiple potential steps experiments provides information about the dynamic/mobility of CO ads in the adlayer during its oxidation. 18 The results shown in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 potential step protocol. Thus, when the potential is successively stepped from 0.10 V to 0.65 V, results in Figures 3C and 3F suggest that the oxidation of CO layer apparently continues exactly from the same point at which the process was stopped, regardless of the sequence of steps previously performed.…”

“…However, this fact does not appear to be consistent with recent contributions that show that the intrinsic catalytic activity of low coordinated sites (steps/kinks) to the CO electro-oxidation is smaller than the catalytic activity of the rest of sites present on catalyst surfaces, as the {111} terrace sites, no matter if such reaction is evaluated in acid or in alkaline solutions. [17][18] On the other hand, the Nucleation and Growth (N&G) model assumes that CO ads is a motionless species and that its oxidation takes place when domains of adsorbed oxygen-donors grow from the vacancies of a CO adlayer. 19 Hence, the presence of these CO ads superstructures on Pt single crystals has been evidenced by in situ STM.…”

Self‐Assembled Dendritic Pt Nanostructure with High‐Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction

“…Although this adsorbed OH species does not have the adequate geometry for the CO adsorbed on the same nanoparticle, it can interactr eadily with other CO adsorbed on neighboring Pt nanoparticles. [71,72] Therefore, the existence of high-index Pt facets with steps, edges, and kinks facilitates the CO oxidation reactione ven in the low-potential region.A ss hown in Figure3,P tn anodendrites urfaces showedt he broad but relatively high preoxidationp eak at the low-potential range between 0.4 and 0.65 V, suggesting the existence of lots of highindex facets and low-coordinated sites between {111}a nd {100} planes, whereas no peak was observed at the same potential range in the commercial Pt catalyst. [68] Interestingly,t he carbon-supported Pt nanodendrites showed totally different CO oxidation peaks for the commercial Pt/C catalyst, as shown in Figure 3, although the two catalysts have the same Pt loading (20 wt %) on carbon supports.…”

Probing adsorption on a nanoscale: field desorption microspectroscopy

“…3 Application examples 3.1 Weakly adsorbed CO species on Pt (111) The role of the weakly adsorbed CO species on Pt-catalyst surface, which are assumed to exist at elevated pressures, is often and controversially discussed in the literature (Freund et al 2011;Farias et al 2015). The experimental results are rather scarce due to the difficulties in detecting such layers in UHV.…”