Observation of Fano line shapes in infrared vibrational spectra of CO2 adsorbed on Cu(997) and Cu(111)

Low-temperature dissociation of CO2 molecules on vicinal Cu surfaces

The reaction of carbon dioxide on the vicinal Cu surfaces at low temperatures was investigated by infrared reflection absorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy, and quadrupole mass spectrometry. Dissociation of CO2 molecules into CO on the Cu(997) and Cu(977) surfaces was observed at temperatures between 80 K and 90 K, whereas it did not occur on Cu(111) under a similar condition. CO and physisorbed CO2 were the main adsorbates during the reaction. In contrast, the amount of atomic oxygen on the surface was small. The dissociation of CO2 was promoted by the small amount of oxygen produced by the CO2 dissociation on the Cu surfaces. This leads to the induction period in the CO2 reaction; the initial reaction rate on the clean Cu surfaces was low, and the coadsorbed oxygen enhanced the dissociation reactivity of CO2. Mass analysis of desorption species during the reaction revealed that the observed CO formation on the vicinal Cu surface is mainly caused by an oxygen-exchange reaction with residual CO in an ultra-high vacuum chamber.

Long Time CO2 Storage Under Ambient Conditions in Isolated Voids of a Porous Coordination Network Facilitated by the "Magic Door" Mechanism

A coordination network containing isolated pores without interconnecting channels is prepared from a tetrahedral ligand and copper(I) iodide. Despite the lack of accessibility, CO2 is selectively adsorbed into these pores at 298 K and then retained for more than one week while exposed to the atmosphere. The CO2 adsorption energy and diffusion mechanism throughout the network are simulated using Matlantis, which helps to rationalize the experimental results. CO2 enters the isolated voids through transient channels, termed "magic doors", which can momentarily appear within the structure. Once inside the voids, CO2 remains locked in limiting its escape. This mechanism is facilitated by the flexibility of organic ligands and the pivot motion of cluster units. In situ powder X-ray diffraction revealed that the crystal structure change is negligible before and after CO2 capture, unlike gate-opening coordination networks. The uncovered CO2 sorption and retention ability paves the way for the design of sorbents based on isolated voids.

State-resolved studies of CO2 sticking to CO2 ice

Internal vibrations may affect the adsorption, scattering, and reactions of molecules impinging onto a surface. The energy of the ν 3 antisymmetric stretch vibration of CO2 slightly exceeds the desorption energy of CO2 bound to CO2 ice. We use supersonic molecular beam techniques and rovibrationally state-resolved excitation to determine whether this vibration affects condensation of gas phase CO2 to its ice. We detect sticking and CO2 ice formation using RAIRS and quantify the sticking probability using the King and Wells method with modulation of the vibrational excitation and Fourier transform based detection. We find that the influence of this vibration on the structure of the formed ice and on the sticking probability is negligible under our conditions. Based on our detection limit, we quantify the weighted average sticking probability at approximately 0.9 and the difference between the state-resolved and weighted average sticking probability as below 0.03%.

Revealing CO2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO₂) activation. Despite extensive reactivity benchmarks, a correlation between CO₂ conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO₂(g). The CO₂ dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO₂ conversion to renewable energy sources during C₁ chemical reactions.

A Correction Scheme for Fano Line Shapes in Two-Dimensional Infrared Spectroscopy

The asymmetry of Fano line shapes observed for metal-adsorbate systems is reflected in two-dimensional infrared (2D IR) spectroscopy as a distorted spectrum. A phenomenological correction scheme is proposed that transforms distorted 2D IR spectra into conventional spectra. To that end, a phase correction factor is first derived from the IR absorption spectrum of the sample by symmetrizing the asymmetric line shape and subsequently applied to the distorted 2D IR spectra. The concept is illustrated for a model system consisting of an organic molecule (p-mercaptobenzonitrile) adsorbed on a sputter-coated metal layer (Au). The correction scheme reveals conventional, easily interpretable 2D IR spectra.

Cluster size effects on the adsorption of CO, O, and CO2and the dissociation of CO2on two-dimensional Cux(x1, 3, and 7) clusters supported on Cu(111) surface: a density functional theory study

In this study, we performed density functional theory based calculations to determine the effect of the size of Cux(x= 1 (adatom), 3 (trimer), 7 (heptamer)) clusters supported on Cu(111) toward the adsorption of CO, O, and CO2, and the dissociation of CO2. CO adsorbs with comparable adsorption energies on the different cluster systems, which are influenced by the reactivity of the Cu atoms in the cluster and the interaction of CO with the Cu atoms in the terrace. The O atom, on the other hand, will always favor to adsorb on hollow sites and is more stable on the hollow sites of smaller clusters. CO2dissociates with lower activation energy on the cluster region than on flat Cu(111). We obtained the lowest activation energy on Cu3due to its more reactive Cu atoms than the Cu7case and due to the possibility of O to adsorb on the cluster region, which is not observed in the Cu1case. The presented results will provide insights on future studies on supported cluster systems and their possible use as catalysts for CO2-related reactions.

The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS

The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO₂ and H₂) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface.

CO2 adsorption on the copper surfaces: van der Waals density functional and TPD studies

We investigated the adsorption of CO₂ on the flat, stepped, and kinked copper surfaces from density functional theory calculations as well as the temperature programmed desorption and X-ray photoelectron spectroscopy. Several exchange-correlation functionals have been considered to characterize CO₂ adsorption on the copper surfaces. We used the van der Waals density functionals (vdW-DFs), i.e., the original vdW-DF (vdW-DF1), optB86b-vdW, and rev-vdW-DF2, as well as the Perdew-Burke-Ernzerhof (PBE) with dispersion correction (PBE-D2). We have found that vdW-DF1 and rev-vdW-DF2 functionals slightly underestimate the adsorption energy, while PBE-D2 and optB86b-vdW functionals give better agreement with the experimental estimation for CO₂ on Cu(111). The calculated CO₂ adsorption energies on the flat, stepped, and kinked Cu surfaces are 20-27 kJ/mol, which are compatible with the general notion of physisorbed species on solid surfaces. Our results provide a useful insight into appropriate vdW functionals for further investigation of related CO₂ activation on Cu surfaces such as methanol synthesis and higher alcohol production.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.