Decomposition of Acetic Acid on Model Pt/CeO₂ Catalysts: The Effect of Surface Crowding

Adsorption and decomposition of acetic acid were studied by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature-programmed desorption on Pt/CeO₂(111) model catalysts prepared on Cu(111). Reference experiments under identical conditions were performed on stoichiometric CeO₂(111), partially reduced CeO_2–<i>x</i>, and oxygen pre-exposed O/Pt/CeO₂(111)/Cu­(111). The principal species formed on all samples during adsorption of acetic acid at 150 K were acetate and molecularly adsorbed acetic acid. On the basis of the differences in the splitting between the methyl and carboxyl/carboxylate groups in the C 1s spectra, we identified the adsorption sites for acetate and molecularly adsorbed acetic acid on Pt/CeO₂. During annealing, we detected an increase in the concentration of acetate on CeO₂(111) support exclusively in the presence of supported Pt particles. The effect is caused by the decomposition of molecularly adsorbed acetic acid on Pt particles followed by spillover of acetate to CeO₂(111) support. The following surface crowding by acetate on CeO₂(111) support alters the decomposition mechanism of acetate with respect to the Pt-free CeO₂(111). In particular, the formation of ketene and acetone was largely eliminated on Pt/CeO₂. We assume that the surface crowding by acetate triggers a switch in the adsorption geometry of acetate from the bidentate to the monodentate configuration. The acetates in both adsorption geometries were identified according to the different splitting between the methyl and carboxylate groups in the C 1s spectra. Decomposition of acetate did not leave behind any surface carbon on Pt-free CeO₂. In contrast, carbonaceous residues were found on CeO_2–<i>x</i> and Pt/CeO₂. The carbon residues were oxidatively removed above 500 K only from Pt/CeO₂

Chemical and Structural In-Situ Characterization of Model Electrocatalysts by Combined Infrared Spectroscopy and Surface X‑ray Diffraction

New diagnostic approaches are needed to drive progress in the field of electrocatalysis and address the challenges of developing electrocatalytic materials with superior activity, selectivity, and stability. To this end, we developed a versatile experimental setup that combines two complementary in-situ techniques for the simultaneous chemical and structural analysis of planar electrodes under electrochemical conditions: high-energy surface X-ray diffraction (HE-SXRD) and infrared reflection absorption spectroscopy (IRRAS). We tested the potential of the experimental setup by performing a model study in which we investigated the oxidation of preadsorbed CO on a Pt(111) surface as well as the oxidation of the Pt(111) electrode itself. In a single experiment, we were able to identify the adsorbates, their potential dependent adsorption geometries, the effect of the adsorbates on the surface morphology, and the structural evolution of Pt(111) during surface electro-oxidation. In a broader perspective, the combined setup has a high application potential in the field of energy conversion and storage

Functionalization of Oxide Surfaces through Reaction with 1,3-Dialkylimidazolium Ionic Liquids

Practical applications of ionic liquids (ILs) often involve IL/oxide interfaces, but little is known regarding their interfacial chemistry. The unusual physicochemical properties of ILs, including their exceptionally low vapor pressure, provide access to such interfaces using a surface science approach in ultrahigh vacuum (UHV). We have applied synchrotron radiation photoelectron spectroscopy (SR-PES) to the study of a thin film of the ionic liquid [C₆C₁Im]­[Tf₂N] prepared in situ in UHV on ordered stoichiometric CeO₂(111) and partially reduced CeO_2–<i>x</i>. On the partially reduced surface, we mostly observe decomposition of the anion. On the stoichiometric CeO₂(111) surface, however, a layer of surface-anchored organic products with high thermal stability is formed upon reaction of the cation. The suggested acid–base reaction pathway may provide well-defined functionalized IL/solid interfaces on basic oxides

Structure-Dependent Dissociation of Water on Cobalt Oxide

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H₂O on two atomically defined cobalt oxide surfaces, CoO(100) and Co₃O₄(111). Both surfaces are terminated by O^2– and Co²⁺ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H₂O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H₂O)_<i>n</i>(OH)_<i>m</i> clusters on Co₃O₄(111). We rationalize this structure dependence by the coordination number of surface Co²⁺. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H₂O whereas others do not. We propose that this structure dependence plays a key role in catalysis with cobalt oxide nanomaterials

A Versatile Approach to Electrochemical <i>In Situ</i> Ambient-Pressure X‑ray Photoelectron Spectroscopy: Application to a Complex Model Catalyst

We present a new technique for investigating complex model electrocatalysts by means of electrochemical in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Using a specially designed miniature capillary device, we prepared a three-electrode electrochemical cell in a thin-layer configuration and analyzed the active electrode/electrolyte interface by using “tender” X-ray synchrotron radiation. We demonstrate the potential of this versatile method by investigating a complex model electrocatalyst. Specifically, we monitored the oxidation state of Pd nanoparticles supported on an ordered Co3O4(111) film on Ir(100) in an alkaline electrolyte under potential control. We found that the Pd oxide formed in the in situ experiment differs drastically from the one observed in an ex situ emersion experiment at similar potential. We attribute these differences to the decomposition of a labile palladium oxide/hydroxide species after emersion. Our experiment demonstrates the potential of our approach and the importance of electrochemical in situ AP-XPS for studying complex electrocatalytic interfaces