par Ustarroz Troyano, Jon 
;Ornelas, Isabel;Zhang, Guohui;Perry, David D.S.;Kang, Minkyung;Walker, Marc
Référence ACS Catalysis, 8, 8, page (6775-6790)
Publication Publié, 2018-06-04
;Ornelas, Isabel;Zhang, Guohui;Perry, David D.S.;Kang, Minkyung;Walker, MarcRéférence ACS Catalysis, 8, 8, page (6775-6790)
Publication Publié, 2018-06-04
Article révisé par les pairs
| Résumé : | A major challenge in electrocatalysis is to understand the effect of electrochemical processes on the physicochemical properties of nanoparticle or nanocluster (NC) ensembles, especially for complex processes, such as the oxygen reduction reaction (ORR) considered herein. We describe an approach whereby electrocatalysis at a small number of well-defined mass-selected Pt NCs (Pt 923±37 , diameter, d ≈ 3 nm), deposited from a cluster beam source on carbon-coated transmission electron microscopy (TEM) grids, can be measured by a scanning electrochemical cell microscopy (SECCM) setup, in tandem with a range of complementary microscopy and spectroscopy techniques. The SECCM setup delivers high mass transport rates and allows the effects of transient reactive intermediates to be elucidated for different Pt surface coverages (NC spacing). A major observation is that the ORR activity decreases during successive electrochemical (voltammetric) measurements. This is shown to be due to poisoning of the Pt NCs by carbon-/oxygen-containing moieties that are produced by the reaction of reactive oxygen intermediates (RIs), generated by the ORR, with the carbon support. The effect is most prominent when the Pt surface coverage on the carbon support is low (textless6%). Furthermore, the NC deposition impact energy drastically affects the resulting Pt NC stability during electrochemistry. For lower impact energy, Pt NCs migrate as a consequence of the ORR and are rearranged into characteristic groups on the support. This previously unseen effect is caused by an uneven flux distribution around individual NCs within the ensemble and has important consequences for understanding the stability and activity of NC and nanoparticle arrays. ■ INTRODUCTION Understanding electrocatalytic processes in nanoparticle (NP) assemblies is very challenging because of the complex time-and history-dependent structure−activity−mechanism−stabil-ity relationships that operate in such systems. This is particularly true for the oxygen reduction reaction (ORR), for which the behavior of the most efficient (Pt and Pt alloy) catalysts is still not completely understood. 1−4 ORR processes on extended Pt surfaces have been extensively studied by employing well-defined Pt single crystals, 2,5,6 but highly dispersed supported NPs that provide large surface areas and enhanced mass-specific activities are needed for practical applications. The effects of catalyst loading and interparticle interactions on the ORR mechanism, selectivity, stability, and inherent activity remain under discussion and are undergoing revision. 1,4,7−14 A major consideration in the ORR is the balance between the 2e − process (leading to H 2 O 2) and 4e − process (yielding H 2 O) and the action of the reactive oxygen intermediates (RIs) produced. Most experimental studies have been performed on Pt NP dispersions, normally synthesized by wet-chemistry methods and deposited on high-surface-area carbon materials. 1,15−18 Although there have been important advances in understanding aspects of electrocatalysis, 1,3,15,16,19−21 such approaches incur difficulties, such as relatively poor control over the size, loading, and spatial distribution of the catalyst NPs. Moreover, solution-synthesized metal colloids are normally stabilized by ligands and may undergo unwanted aggregation during deposition onto activated carbon, inhibiting catalytic activity. 22 Subtle differences in NP size, loading, and |
