CO Adsorption on PtRu/Ru(0001) Near Surface Alloys from Ultrahigh Vacuum to Millitorr Pressures

We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the adsorption of CO on PtRu/Ru(0001) near surface alloys (NSA) at 300 K and pressures from ultrahigh vacuum to 0.04 Torr. We observe differences in the fraction of Pt covered by adsorbed CO with changing Pt concentrations (0.36, 0.73, and 0.94 ML) in the NSA. For all alloy compositions and CO pressures, the amount of CO adsorbed on Pt sites in the alloy is less than what is observed on pure Pt(111). Further, the fraction of Pt sites covered by CO on the 0.36 and 0.73 ML Pt NSA surfaces are similar but lower than on the 0.94 ML Pt NSA surface. These observations support the concept of a decrease in the local adsorption energy of CO on Pt sites in alloy surfaces compared to pure Pt(111). We also found a correlation between the fraction of Pt covered by CO with the binding energy of the Pt 4f_7/2 core level: the fraction of Pt covered by CO decreases with increasing Pt 4f_7/2 binding energy. This observation may provide a simple analytical test for CO tolerance of PtRu alloy catalysts used in polymer electrolyte fuel cells

Monoethanolamine Adsorption on TiO₂(110): Bonding, Structure, and Implications for Use as a Model Solid-Supported CO₂ Capture Material

We have studied the adsorption of monoethanolamine (MEA, HO­(CH₂)₂NH₂), a well-known CO₂ capture molecule, on the rutile TiO₂(110) surface using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy measurements indicate that MEA adsorbs with the oxygen atom of the hydroxyl group and the nitrogen atom of the amine group bonded to adjacent 5-fold coordinated Ti-sites (Ti­(5f)) in the Ti-troughs, leading to a saturation coverage of 0.5 ML at room temperature. Density functional theory calculations confirm that this adsorption configuration is the most stable one with an adsorption energy of 2.33 eV per MEA molecule. The bonding of MEA to TiO₂(110) is dominated by local donor–acceptor bonds between the oxygen and nitrogen atoms of the MEA molecule and surface Ti­(5f) sites. Hydrogen bonds between adjacent MEA molecules stabilize the adsorption structure at saturation coverage. The implications of this bonding configuration for the use of MEA/TiO₂(110) as a model CO₂ capture material will be discussed

Key Structure–Property Relationships in CO₂ Capture by Supported Alkanolamines

Heterogeneous interfaces exhibit remarkable material properties resulting from their structural motifs, the judicious placement of functional chemical groups, etc. It has been a long-standing challenge to manipulate and design interface structures at the atomic level to achieve new functionalities. Here, we demonstrate that by modifying the length of the backbone in alkanolamines one can control the packing density of organic monolayers adsorbed on rutile TiO₂ and the interaction strength between their amine functional group and the substrate. As a result, we observed strikingly different activities in CO₂ capture by the amine functional group of different alkanolamines on TiO₂(110). Synchrotron photoelectron spectroscopy at near-ambient CO₂ pressures showed that adsorbed 2-amino-1-ethanol (monoethanolamine, MEA) is inactive, whereas the amine group in 3-amino-1-propanol (3AP)/TiO₂(110) readily reacts with and captures CO₂. Our results suggest that the geometry of the interface plays a decisive role in the reactivity of adsorbed functionalized organic molecules, such as solid-supported alkanolamines for CO₂ capture

Influence of Excess Charge on Water Adsorption on the BiVO₄(010) Surface

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure

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