Structures and Energetics of Pt Clusters on TiO₂: Interplay between Metal–Metal Bonds and Metal–Oxygen Bonds

Depositing size-selected nanoclusters on a well-defined support surface provides a way to probe the metal–support interaction and the size dependence of the catalytic activity; however, the detailed structural information at such interface is often missing. Here we examine from density functional theory the interfacial structure of Pt₄ to Pt₈ clusters on rutile TiO₂(110). We find that Pt₄ prefers a flat, nearly square structure on TiO₂(110), while larger clusters such as Pt₅, Pt₆, Pt₇, and Pt₈ have a two-layer structure with the top layer not interacting with the support directly. The interaction strength generally increases with the contact area between Pt_<i>n</i> and TiO₂(110). The interfacial structure is a result of optimizing the Pt–Pt, Pt–O, and Pt–Ti interactions: Pt₄ prefers the square planar configuration on TiO₂(110) with more Pt–Ti interaction over a two-layer, bi-triangle configuration of more Pt–Pt bonds; Pt₈ prefers a hut-like two-layer structure over an edge-sharing bi-pyramid structure of greater internal strain. Our findings will be useful for understanding the interface of size-selected clusters on a typical reducible support such as TiO₂ and its catalytic activity for reactions such as CO oxidation

Interaction of Gold Clusters with a Hydroxylated Surface

We explore the interaction between gold nanoclusters and a fully hydroxylated surface, Mg(OH)₂’s basal plane, by using a density functional theory-enabled local basin-hopping technique for global-minimum search. We find strong interaction of gold nanoclusters with the surface hydroxyls via a short bond between edge Au atoms and O atoms of the −OH groups. We expect that this strong interaction is ubiquitous on hydroxylated support surfaces and helps the gold nanoclusters against sintering, thereby contributing to their CO-oxidation activity at low temperatures

Oxygen Vacancy-Assisted Coupling and Enolization of Acetaldehyde on CeO₂(111)

The temperature-dependent adsorption and reaction of acetaldehyde (CH₃CHO) on a fully oxidized and a highly reduced thin-film CeO₂(111) surface have been investigated using a combination of reflection–absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT+U) calculations. On the fully oxidized surface, acetaldehyde adsorbs weakly through its carbonyl O interacting with a lattice Ce⁴⁺ cation in the η¹-O configuration. This state desorbs at 210 K without reaction. On the highly reduced surface, new vibrational signatures appear below 220 K. They are identified by RAIRS and DFT as a dimer state formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules. This dimer state remains up to 400 K before decomposing to produce another distinct set of vibrational signatures, which are identified as the enolate form of acetaldehyde (CH₂CHO¯). Furthermore, the calculated activation barriers for the coupling of acetaldehyde, the decomposition of the dimer state, and the recombinative desorption of enolate and H as acetaldehyde are in good agreement with previously reported TPD results for acetaldehyde adsorbed on reduced CeO₂(111) [Chen et al. <i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 3385]. The present findings demonstrate that surface oxygen vacancies alter the reactivity of the CeO₂(111) surface and play a crucial role in stabilizing and activating acetaldehyde for coupling reactions

Adsorption and Reaction of Acetaldehyde on Shape-Controlled CeO₂ Nanocrystals: Elucidation of Structure–Function Relationships

CeO₂ cubes with {100} facets, octahedra with {111} facets, and wires with highly defective structures were utilized to probe the structure-dependent reactivity of acetaldehyde. Using temperature-programmed desorption (TPD), temperature-programmed surface reactions (TPSR), and <i>in situ</i> infrared spectroscopy, it was determined that acetaldehyde desorbs unreacted or undergoes reduction, coupling, or C–C bond scission reactions, depending on the surface structure of CeO₂. Room-temperature FTIR indicates that acetaldehyde binds primarily as η¹-acetaldehyde on the octahedra, in a variety of conformations on the cubes, including coupling products and acetate and enolate species, and primarily as coupling products on the wires. The percent consumption of acetaldehyde ranks in the following order: wires > cubes > octahedra. All the nanoshapes produce the coupling product crotonaldehyde; however, the selectivity to produce ethanol ranks in the following order: wires ≈ cubes ≫ octahedra. The selectivity and other differences can be attributed to the variation in the basicity of the surfaces, defects densities, coordination numbers of surface atoms, and the reducibility of the nanoshapes

Gold Nanoparticles Supported on Carbon Nitride: Influence of Surface Hydroxyls on Low Temperature Carbon Monoxide Oxidation

This paper reports the synthesis of 2.5 nm gold clusters on the oxygen free and chemically labile support carbon nitride (C₃N₄). Despite having small particle sizes and high enough water partial pressure these Au/C₃N₄ catalysts are inactive for the gas phase and liquid phase oxidation of carbon monoxide. The reason for the lack of activity is attributed to the lack of surface −OH groups on the C₃N₄. These OH groups are argued to be responsible for the activation of CO in the oxidation of CO. The importance of basic −OH groups explains the well documented dependence of support isoelectric point versus catalytic activity

Acid–Base Reactivity of Perovskite Catalysts Probed via Conversion of 2‑Propanol over Titanates and Zirconates

Although perovskite catalysts are well-known for their excellent redox property, their acid–base reactivity remains largely unknown. To explore the potential of perovskites in acid–base catalysis, we made a comprehensive investigation in this work on the acid–base properties and reactivity of a series of selected perovskites, SrTiO₃, BaTiO₃, SrZrO₃, and BaZrO₃, via a combination of various approaches including adsorption microcalorimetry, in situ FTIR spectroscopy, steady state kinetic measurements, and density functional theory (DFT) modeling. The perovskite surfaces are shown to be dominated with intermediate and strong basic sites with the presence of some weak Lewis acid sites, due to the preferred exposure of SrO/BaO on the perovskite surfaces as evidenced by low energy ion scattering (LEIS) measurements. Using the conversion of 2-propanol as a probe reaction, we found that the reaction is more selective to dehydrogenation over dehydration due to the dominant surface basicity of the perovskites. Furthermore, the adsorption energy of 2-propanol (Δ<i>H</i>_{<i>ads,</i>2<i>–propanol</i>}) is found to be related to both a bulk property (tolerance factor) and the synergy between surface acid and base sites. The results from in situ FTIR and DFT calculations suggest that both dehydration and dehydrogenation reactions occur mainly through the E_1cB pathway, which involves the deprotonation of the alcohol group to form a common alkoxy intermediate on the perovskite surfaces. The results obtained in this work pave a path for further exploration and understanding of acid–base catalysis over perovskite catalysts

Rational Design of Bi Nanoparticles for Efficient Electrochemical CO₂ Reduction: The Elucidation of Size and Surface Condition Effects

We report an efficient electrochemical conversion of CO₂ to CO on surface-activated bismuth nanoparticles (NPs) in acetonitrile (MeCN) under ambient conditions, with the assistance of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]­[OTf]). Through the comparison between electrodeposited Bi films (Bi-ED) and different types of Bi NPs, we, for the first time, demonstrate the effects of catalyst’s size and surface condition on organic phase electrochemical CO₂ reduction. Our study reveals that the surface inhibiting layer (hydrophobic surfactants and Bi³⁺ species) formed during the synthesis and purification process hinders the CO₂ reduction, leading to a 20% drop in Faradaic efficiency for CO evolution (FE_CO). Bi particle size showed a significant effect on FE_CO when the surface of Bi was air-oxidized, but this effect of size on FE_CO became negligible on surface-activated Bi NPs. After the surface activation (hydrazine treatment) that effectively removed the native inhibiting layer, activated 36-nm Bi NPs exhibited an almost-quantitative conversion of CO₂ to CO (96.1% FE_CO), and a mass activity for CO evolution (MA_CO) of 15.6 mA mg^–1, which is three-fold higher than the conventional Bi-ED, at −2.0 V (vs Ag/AgCl). This work elucidates the importance of the surface activation for an efficient electrochemical CO₂ conversion on metal NPs and paves the way for understanding the CO₂ electrochemical reduction mechanism in nonaqueous media

Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO₂ Supported Au₂₅(SCH₂CH₂Ph)₁₈ Nanoclusters

The effect of thiolate ligands was explored on the catalysis of CeO₂ rod supported Au₂₅(SR)₁₈ (SR = −SCH₂CH₂Ph) by using CO oxidation as a probe reaction. Reaction kinetic tests, in situ IR and X-ray absorption spectroscopy, and density functional theory (DFT) were employed to understand how the thiolate ligands affect the nature of active sites, activation of CO and O₂, and reaction mechanism and kinetics. The intact Au₂₅(SR)₁₈ on the CeO₂ rod is found not able to adsorb CO. Only when the thiolate ligands are partially removed, starting from the interface between Au₂₅(SR)₁₈ and CeO₂ at temperatures of 423 K and above, can the adsorption of CO be observed by IR. DFT calculations suggest that CO adsorbs favorably on the exposed gold atoms. Accordingly, the CO oxidation light-off temperature shifts to lower temperature. Several types of Au sites are probed by IR of CO adsorption during the ligand removal process. The cationic Au sites (charged between 0 and +1) are found to play the major role for low-temperature CO oxidation. Similar activation energies and reaction rates are found for CO oxidation on differently treated Au₂₅(SR)₁₈/CeO₂ rod catalysts, suggesting a simple site-blocking effect of the thiolate ligands in Au nanocluster catalysis. Isotopic labeling experiments clearly indicate that CO oxidation on the Au₂₅(SR)₁₈/CeO₂ rod catalyst proceeds predominantly via the redox mechanism where CeO₂ activates O₂ while CO is activated on the dethiolated gold sites. These results point to a double-edged sword role played by the thiolate ligands on Au₂₅ nanoclusters for CO oxidation

Complexity of Intercalation in MXenes: Destabilization of Urea by Two-Dimensional Titanium Carbide

MXenes are a new class of two-dimensional materials with properties that make them important for applications that include batteries, capacitive energy storage, and electrocatalysis. These materials can be exfoliated and delaminated to create high surface areas with interlayers accessibility. Intercalation is known to be possible, and it is critical for many applications including electrochemical energy storage, water purification, and sensing. However, little is known about the nature of the intercalant and bonding interactions between the intercalant within the MXene. We have investigated urea interaction within a titanium carbide based MXene using inelastic neutron scattering (INS) to probe the state of intercalated species. By comparison with reference materials, we find that under intercalation conditions urea decomposes readily, leading to intercalation of ammonium cations observable by INS and evolving carbon dioxide detected by infrared spectroscopy. Reactive molecular dynamics calculations were conducted to provide atomistic insights about reaction pathways and their energetics. These results have implications for understanding intercalation in active layered materials