In situ scanning tunneling microscopy studies of carbonate-induced restructuring of Ag-decorated Cu(100) electrodes.

Ag-decorated Cu electrocatalysts are of great interest for electrochemical CO2 reduction, because of an increased yield of multi-carbon products. Here, we present studies of well-defined AgCu electrodes by in situ scanning tunneling microscopy. These bimetallic model electrocatalysts are prepared by electrodepositing submonolayer Ag coverages on Cu(100) in 0.1 M H2SO4, resulting in monolayer islands with a hexagonal quasi-Ag(111) atomic lattice. Upon exchanging the solution at potentials in the double layer range to 0.1 M KHCO3, pronounced Ag island restructuring towards anisotropic shapes, the nucleation and growth of new islands, and a strong reduction in surface mobility are observed. In addition, high-resolution images reveal a highly disordered molecular adlayer, contrary to the case of Ag-free Cu(100) electrodes. These observations can be explained by interactions of metal adatoms with adsorbed (bi)carbonate and show that Ag redispersion on Cu electrocatalysts may occur even in the absence of CO2 reduction.

Structure of the (Bi)carbonate Adlayer on Cu(100) Electrodes.

(Bi)carbonate adsorption on Cu(100) in 0.1 M KHCO3 has been studied by in situ scanning tunneling microscopy. Coexistence of different ordered adlayer phases with ( 2 ${\sqrt{2}}$ ×6 2 ${\sqrt{2}}$ )R45° and (4×4) unit cells was observed in the double layer potential regime. The adlayer is rather dynamic and undergoes a reversible order-disorder phase transition at 0 V vs. the reversible hydrogen electrode. Density functional calculations indicate that the adlayer consists of coadsorbed carbonate and water molecules and is strongly stabilized by liquid water in the adjacent electrolyte.

Molybdenum tricarbonyl complex functionalised with a molecular triazatriangulene platform on Au(111): surface spectroscopic characterisation.

Transition metal complexes form the basis for small molecule activation and are relevant for electrocatalysis. To combine both approaches the attachment of homogeneous catalysts to metallic surfaces is of significant interest. Towards this goal a molybdenum tricarbonyl complex supported by a tripodal phosphine ligand was covalently bound to a triazatriangulene (TATA) platform via an acetylene unit and the resulting TATA-functionalised complex was deposited on a Au(111) surface. The corresponding self-assembled monolayer was characterised with scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS). The vibrational properties of the surface-adsorbed complexes were investigated with the help of infrared reflection absorption spectroscopy (IRRAS), and the frequency/intensity changes with respect to the bulk spectrum were analysed. A full vibrational analysis was performed with the help of DFT.

The Dynamic Nature of CO Adlayers on Pt(111) Electrodes.

CO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential-dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre-oxidation regime. We here employ in situ high-speed scanning tunneling microscopy for studying the surface phase behavior in CO-saturated 0.1 m H2 SO4 on the millisecond time scale. At potentials near the onset of CO pre-oxidation local fluctuations in the (2×2)-CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)-CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation.