Performing oxidation reactions at low temperatures using earth-abundant materials is crucial for advancing solutions for sustainable chemistry. CO oxidation serves as a benchmark reaction to characterize oxidation and to advance fundamental concepts in surface chemistry. While there are several examples of CO oxidation occurring on metal oxides at low temperatures, from 300 K to ∼200 K, reactivity in the cryogenic temperature regime typically requires a metal nanoparticle on a metal oxide. Here, we show oxygen atoms on the (111) facet of Cu₂O react with CO to form CO₂ at temperatures below 100 K. Combining spectroscopic experimental evidence with calculations, we propose a low barrier path for CO oxidation at reconstructed surface sites on Cu₂O(111). This finding is a rare example of an earth-abundant metal oxide, in this case copper, that can provide highly reactive multifunctional sites, enabling both adsorption and reaction fundamental steps toward the efficient heterogeneous oxidation of chemicals.

Single-atom catalysts have recently been subject to considerable attention within applied catalysis. However, complications in the preparation of well-defined single-atom model systems have hampered efforts to determine the reaction mechanisms underpinning the reported activity. By means of an atomic layer deposition method utilizing the steric hindrance of the ligands, isolated Fe1O3 motifs were grown on a single-crystal Cu2O(100) surface at densities up to 0.21 sites per surface unit cell. Ambient pressure X-ray photoelectron spectroscopy shows a strong metal-support interaction with Fe in a chemical state close to 3+. Results from scanning tunneling microscopy and density functional calculations demonstrate that isolated Fe1O3 is exclusively formed and occupies a single site per surface unit cell, coordinating to two oxygen atoms from the Cu2O lattice and another through abstraction from O-2. The isolated Fe1O3 motif is active for CO oxidation at 473 K. The growth method holds promise for extension to other catalytic systems.

Two local reactivity descriptors computed by Kohn-Sham density functional theory (DFT) are used to predict and rationalize interactions of nucleophilic molecules (exemplified by CO and H2O) with transition metal (TM) and oxide surfaces. The descriptors are the electrostatic potential, V-S(r), and the local electron attachment energy, E-S(r), evaluated on surfaces defined by the 0.001 e Bohr(-3) isodensity contour. These descriptors have previously shown excellent abilities to predict regioselectivity and rank molecular as well as nanoparticle reactivities and interaction affinities. In this study, we generalize the descriptors to fit into the framework of periodic DFT computations. We also demonstrate their capabilities to predict local surface propensity for interaction with Lewis bases. It is shown that E-S(r) and V-S(r) can rationalize the interaction behavior of TM oxides and of fcc TM surfaces, including low-index, stepped and kinked surfaces spanning a wide range of interaction sites with varied coordination environments. Broad future applicability in surface science is envisaged for the descriptors, including heterogeneous catalysis and electrochemistry.

The potential energy surfaces in gas phase and in aqueous solution for the nitration of benzene, chlorobenzene, and phenol have been elucidated with density functional theory at theM06-2X/6-311G(d,p) level combined with the polarizable continuum solvent model (PCM). Three reaction intermediates have been identified along both surfaces: the unoriented pi-complex (I), the oriented reaction complex (II), and the sigma-complex (III). In order to obtain quantitatively reliable results for positional selectivity and for modeling the expulsion of the proton, it is crucial to take solvent effects into consideration. The results are in agreement with Olah's conclusion from over 40 years ago that the transition state leading to (II) is the rate-determining step in activated cases, while it is the one leading to (III) for deactivated cases. The simplified reactivity approach of using the free energy for the formation of (III) as a model of the rate-determining transition state has previously been shown to be very successful for halogenations, but problematic for nitrations. These observations are rationalized with the geometric and energetic resemblance, and lack of resemblance respectively, between (III) and the corresponding rate determining transition state. At this level of theory, neither the sigma-complex (III) nor the reaction complex (II) can be used to accurately model the rate-determining transition state for nitrations.

Using local DFT-based probes for electrostatic as well as charge transfer/polarization interactions, we are able to characterize Lewis basic and acidic sites on copper, silver and gold nanoparticles. The predictions obtained using the DFT-probes are compared to the interaction energies of the electron donating (CO, H2O, NH3 and H2S) and the electron accepting (BH3, BF3, HCl [H-down] and Na+) compounds. The probes include the local electron attachment energy [E(r)], the average local ionization energy [% I(r)], and the electrostatic potential [V(r)] and are evaluated on isodensity surfaces located at distances corresponding to typical interaction distances. These probes have previously been successful in characterizing molecular interactions. Good correlations are found between Lewis acidity and maxima in V(r), appearing as a consequence of sigma-holes, as well as minima in E(r), of the noble metal nanoparticles. Similarly are Lewis basic sites successfully described by surface minima in V(r) and % I(r); the former are indicative of sigma-lumps, i.e. regions of enhanced sigma-density. The investigated probes are anticipated to function as reliable tools in nanoparticle reactivity and interaction characterization, and may act as suitable descriptors in large-scale screenings for materials of specific properties, e.g. in heterogeneous catalysis. Because of the similarity between the noble metal nanoparticle's interactions with Lewis bases and the concepts of halogen and hydrogen bonding, a new class of bonds is introduced - regium bonds - taking place between a sigma-hole of a Cu, Ag or Au compound and an electron donor.

The halogenation of monosubstituted benzenes in aqueous solvent was studied using density functional theory at the PCM-M06-2X/6-311G(d,p) level. The reaction with Cl-2 begins with the formation of C atom coordinated pi-complex and is followed by the formation of the sigma-complex, which is rate-determining. The final part proceeds via the abstraction of the proton by a water molecule or a weak base. We evaluated the use of the sigma-complex as a model for the rate-determining transition state (TS) and found that this model is more accurate the later the TS comes along the reaction coordinate. This explains the higher accuracy of the model for halogenations (late TS) compared to nitrations (early TS); that is, the more deactivated the substrate the later the TS. The halogenation with Br-2 proceeds with a similar mechanism as the corresponding chlorination, but the bromination has a very late rate-determining TS that is similar to the sigma-complex in energy. The iodination with ICl follows a different mechanism than chlorination and bromination. After the formation of the pi-complex, the reaction proceeds in a concerted manner without a sigma-complex. This reaction has a large primary hydrogen kinetic isotope effect in agreement with experimental observations.

The initial steps of Cu₂O sulphidation to Cu₂S have been studied using plane-wave density functional theory at the PBE-D3+U level of sophistication. Surface adsorption and dissociation of H₂S and H₂O, as well as the replacement reaction of lattice oxygen with sulphur, have been investigated for the most stable (111) and (100) surface facets under oxygen-lean conditions. We find that the (100) surface is more susceptible to sulphidation than the (111) surface, promoting both H₂S adsorption, dissociation and the continued oxygen–sulphur replacement. The results presented in this proceeding bridge previous results from high-vacuum experiments on ideal surface to more realistic corrosion conditions and set the grounds for future mechanistic studies. Potential implications on the long-term final disposal of spent nuclear fuel are discussed.

Adsorption and desorption of methanol on the (111) and (100) surfaces of  Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120–620 K, in combination with density functional theorycalculations and sum frequency generation spectroscopy. The bare (100) surfaceexhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (√3 × √3)R30°-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this  surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surfaceoxygen.

Crystalline surfaces of gold are chemically inert, whereas nanoparticles of gold are excellent catalysts for many reactions. The catalytic properties of nanostructured gold have been connected to increased binding affinities of reactant molecules to low-coordinated Au atoms. Here we show that the high reactivity at these sites is a consequence of the formation of σ-holes, i.e. maxima in the surface electrostatic potential (Vs,max) due to the overlap of mainly the valence s-orbitals when forming the bonding σ-orbitals. The σ-holes are binding sites for Lewis bases, and binding energies correlate with magnitudes of the Vs,max. For symmetrical Au clusters, of varying size, the most positive Vs,max are found at corners, edges, and surfaces (facets) and decreasing in that order. This is in agreement with the experimentally and theoretically observed dependence of catalytic activity on local structure. The density of σ-holes can explain the increasing catalytic activity with decreasing particle size also for other transition metal catalysts, such as platinum.