A Theoretical and Experimental In-Situ Electrochemical Infrared Spectroscopy Study of Adsorbed CO on Pt Dendrimer-Encapsulated Nanoparticles

Abstract

We present a combined experimental and theoretical study of CO ads on Pt 147 dendrimer-encapsulated nanoparticles (DENs). In-situ electrochemical IR spectroscopy reveals an 8 cm −1 redshift of the CO ads stretching frequency on Pt 147 DENs relative to a Pt (111) crystal. This value is in good agreement with the shift calculated by density functional theory. Importantly, the wavenumber shift observed in this study is significantly smaller than has been found previously. We attribute this primarily to the absence of support effects and the narrow size distribution of DENs. The agreement between experiment and theory validates the model nanoparticle system used for the calculations, and this will make it possible to use the CO ads frequency as a probe to study more complex DEN structures and as a descriptor of the catalytic activity of DENs toward reactions such as formic acid oxidation and methanol oxidation.We are interested in developing a better understanding of how small changes in the structure of nanoparticles in the 1-2 nm size range impact important electrocatalytic reactions like oxygen reduction and CO oxidation. We approach this problem by directly comparing theory and experiments. 1 This is important, because previous results from our groups and others have shown that nanoparticle structures in this size range are dynamic. For example, core@shell nanoparticles are able to invert, 2,3 ligands can drive shape changes, 4 and interactions with a substrate can result in major electronic and shape changes. [5][6][7] One effective method for gaining insight into nanoparticle surface structure is to measure changes in the vibrational frequency of adsorbed CO (CO ads ) using infrared (IR) spectroscopy. 8,9 Spectroscopic results from single-crystal surface models are relevant for understanding the structure of Pt nanoparticles (PtNPs) having size >4 nm, but for smaller nanoparticles the connection to bulk surfaces is more tenuous. 8 This is because the dominance of discrete facets is reduced, while the contribution of edge and corner sites increases. These latter sites can be identified because they bind CO more strongly, thereby decreasing its stretching frequencies. 8,10 It has previously been shown that the CO ads frequency decreases as the size of PtNPs supported on carbon (C/PtNP) decreases. 8,11,12 For example, frequency shifts on the order of 20 cm −1 have been reported as nanoparticle size decreases from ∼4 nm to ∼2 nm. 8 However, these studies have been carried out on commercial C/PtNP catalysts, which exhibit wide variations in the size and shape of the PtNPs and also effects exerted by the support. 12,13 This in turn makes it difficult to precisely correlate IR spectra to particular nanoparticle structures.We sought to better understand the interaction between CO and PtNPs by developing a more refined model system that could be directly compared to first principles theory. The model we chose are dendrimer-encapsulated nanoparticles (DENs), which are welldefined materials in the size range of 1-2 nm. 1,...

Effect of Intermediate Semiconducting TiOx Thin Films on Nanoparticle-Mediated Electron Transfer: Electrooxidation of CO

“…Specifically, anodic currents are present in both the forward and reverse scans, with the current in the forward scan being ~10× higher than the reverse scan. This behavior is consistent with the accepted Langmuir–Hinshelwood mechanism for CO electrooxidation at Pt catalysts [ 21 , 22 , 23 ]. Here, the reactant-pair is CO* and OH*, where OH* originates from water oxidation at the electrode (rather than being present at high concentrations in the electrolyte, as for the AuNP case) [ 24 ].…”

“…However, the depletion of CO near the electrode surface leads to a lower anodic current. The key point of this discussion is that the Pt 55 NPs are active toward CO electrooxidation, and the voltammetry is consistent with reports in the literature [ 21 , 22 , 23 , 24 , 25 ].…”

“…In the presence of the TiO x interlayer, no CO electrooxidation activity was observed for either metal. When the TiO x interlayer is absent, however, CO electrooxidation proceeds in accordance with prior literature reports [ 17 , 18 , 19 , 21 , 22 , 23 , 24 , 25 ]. We also replaced the semiconducting TiO x interlayer with insulating Al 2 O 3 and found that CO electrooxidation proceeds as expected.…”

“…We used acidic electrolytes for this experiment, rather than basic electrolytes, for two reasons. First, the CO electrooxidation at Pt catalysts in acidic media has been extensively characterized [ 21 , 22 , 23 , 24 , 25 ]. Second, key experiments involving Al 2 O 3 /PtNP supports, which are discussed later, require the use of acidic media due to the instability of Al 2 O 3 in bases [ 26 ].…”

Electrocatalytic Reduction of Oxygen on Platinum Nanoparticles in the Presence and Absence of Interactions with the Electrode Surface

“…In this paper, we report a method for cleanly removing dendrimers from surface-confined dendrimer-encapsulated Pt nanoparticles (Pt DENs), − without measurably changing their size or shape (Scheme ). The results of this study are important for two reasons.…”