The properties of amorphous solid water at and near the calorimetric glass transition temperature, Tg, of 136 K have been debated for years. One hypothesis is that water turns into a "true" liquid at Tg (i.e., it becomes ergodic) and exhibits all the characteristics of an ergodic liquid, including translational diffusion. A competing hypothesis is that only rotational motion becomes active at Tg, while the "real" glass transition in water is at a considerably higher temperature. To address this dispute, we have investigated the diffusive mixing in nanoscale water films, with thicknesses up to ∼100 nm, using infrared (IR) spectroscopy. The experiments used films that were composed of at least 90% H2O with D2O making up the balance and were conducted under conditions where H/D exchange was essentially eliminated. Because the IR spectra of multilayer D2O films (e.g., thicknesses of ∼3-6 nm) embedded within thick H2O films are distinct from the spectrum of isolated D2O molecules within H2O, the diffusive mixing of (initially) isotopically layered water films could be followed as a function of annealing time and temperature. The results show that water films with total thicknesses ranging from ∼20 to 100 nm diffusively mixed prior to crystallization for temperatures between 120 and 144 K. The translational diffusion had an Arrhenius temperature dependence with an activation energy of 40.8 ± 3.5 kJ/mol, which indicates that water at and near Tg is a strong liquid. The measured diffusion coefficient at 136 K is 6.25 ± 1.4 × 10-21 m2/s.

Single-atom catalysts (SACs) offer a promise of providing unique properties, superior selectivity, and maximum atomic efficiency compared to traditional nanoparticle catalysts. However, their stability under reaction conditions remains a critical challenge. This study examines the reactivity and structural evolution of a thermally stable ( 700 K) model Rh/Fe3O4(001) SAC, where Rh is substituted into the surface layer. Previously, we demonstrated that water formation via the Mars-van Krevelen mechanism during formic acid conversion destabilizes in-surface octahedral Rh, yielding active Rh adatoms and clusters that dynamically re-incorporate into the Fe3O4 lattice at 700 K. Here, we follow the evolution of the catalyst structure and changes in the CO and CO2 formation kinetics during multiple formic acid conversion cycles. Temperature-programmed reaction spectroscopy (TPRS) cycles to 700 K reveal that small Rh clusters formed during the first several cycles can re-incorporate into the Fe3O4(001) lattice. Over subsequent cycles, larger nanoparticles eventually form and persist. These effects are further accelerated when annealing is limited to only 550 K. Changes in the CO2 formation/desorption temperature in TPRS reveal that the activity for formic acid dehydrogenation increases progressively from single atoms to clusters and nanoparticles. This study provides fundamental insights into the dynamic behavior and performance of SACs during catalytic reactions.

Isotopically layered amorphous solid water films were used to measure the diffusivity of deeply supercooled liquid water near the glass transition. The films, composed of separate H218O and H216O layers, were grown by vapor deposition at low temperature and then heated to observe the intermixing of the isotopic layers. Very slow heating rates (as low as 10-4 K/s) were used to decouple the diffusion and crystallization processes to ensure that the observed intermixing occurred at temperatures that were well-separated from the onset of crystallization. Numerical simulations of the desorption spectra were used to extract the translational diffusivities. The diffusivities obtained in this paper are consistent with translational liquid-like motion at temperatures near and above the proposed Tg of 136 K. These findings support the idea that the melt of amorphous water, above its glass transition temperature is thermodynamically continuous with normal supercooled liquid.

Single metal atoms dispersed on oxides are a new emerging class of catalysts owing to their unique electronic and chemical properties. In this study, we have prepared a series of model single-atom catalysts possessing well-characterized Rh sites that include Rh adatoms (Rh-ad), mixed surface layers with octahedrally-coordinated Rh (Rh-oct), as well as metallic Rh clusters and nanoparticles (Rh-met) on Fe3O4(001). Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we investigated the activity of such model systems towards H-2 and their stability in reducing environments. Our results show that the atomically dispersed Rh-ad and Rh-oct species do not activate H-2,H- which would result in the formation of surface hydroxyls on Fe3O4(001). In contrast, the presence of Rh-met in H-2 results in the formation of hydroxyls and subsequent etching of the Fe3O4(001) at higher temperatures (>= 500 K) due to water formation via the Mars-van Krevelen mechanism. Additionally, such surface etching leads to the release of the Rh-oct from the surface lattice and their sintering to Rh-met. To bridge the material gap between the surface science models and high surface area catalysts, we perform parallel studies on powder Rh/Fe3O4 catalysts. The XPS characterization shows remarkable similarities between these systems. Further, our surface science studies provide an atomistic picture of the behavior of high surface area catalysts in the H-2 atmosphere.

Understanding the role of surface structure and hydroxylation in catalytic reactions on metal oxide surfaces is important for developing a mechanistic insight into the complex interface processes. Here, we investigate the reactivity of formic acid on reconstructed Fe3O4(001) using a combination of X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, temperature-programmed reaction spectroscopy, low energy electron diffraction, and electronic structure calculations. We find that formic acid initially dissociates at low temperatures (<80 K) into bidentate formate and a hydroxyl up to an initial dosed coverage of two HCOOH per Fe3O4(001) unit cell. At higher temperatures (>450 K), formate largely decomposes along the dehydration pathway, producing CO and H2O, with dehydrogenation to CO2 being a minority side reaction. As a first step, water formation leads to surface oxygen extraction via the Mars-van Krevelen mechanism. Computational studies reveal formate embedded in oxygen vacancies as a key intermediate in the CO formation mechanism. CO formation proceeds via two reaction pathways with desorption that peaks at 530 K on the hydroxyl-rich surface and 560 K on the hydroxyl-deficient surface. Atomic hydrogen coadsorption experiments and ab initio calculations reveal that the presence of surface hydroxyls reduces the CO formation barrier. These results highlight the complex interactions between substrate and intermediate species occurring during reactions on metal oxide surfaces.

The reaction coefficient for hydrogen/deuterium (H/D) exchange and the diffusion of hydrated excess protons within amorphous solid water (ASW) are characterized as a function of temperature. For these experiments, water films are deposited on a Pt(111) substrate at 108 K, and reactions with pre-adsorbed hydrogen atoms produce hydrated protons. Upon heating, protons diffuse within the water, and H/D exchange occurs when they encounter D2O probe molecules deposited in the films. The time-dependent concentration of D2O is monitored with infrared spectroscopy, and it indicates the protons diffusion from the substrate and establish an equilibrium distribution prior to significant H/D exchange for temperatures 114 K <= T <= 134 K. By controlling the distance between the D2O molecules and the substrate, we probe the distribution of protons within the film. It decays as x(-2) for the examined range of x (12-52 nm) due to the electric field that develops between the diffusing protons and their image charges in the metal substrate. This agrees with the theoretical distance scaling for the equilibrated proton concentration in a dielectric near a metal boundary. From the proton concentration and the measured D2O decay rate, a lower bound for the proton diffusion coefficient ranging from 10(-20) m(2)/s at 114 K to 10(-18) m(2)/s at 134 K is estimated. The diffusion coefficient has an activation energy of 0.40 eV, which is comparable to energies reported for molecular translations and rotations of H2O, suggesting they may play a critical role in the proton diffusion mechanism within ASW.

The stability and activity of supported single-atom catalysts (SACs) represent critical yet opposing factors limiting our ability to explore and exploit their unique properties. This study demonstrates the operation of a switchable catalyst that is activated in the presence of surface intermediates and reverts to a stable but inactive form when the reaction is completed. We employ atomically defined Rh/Fe3O4(001) catalysts to demonstrate how structurally stable Rh, bound in surface octahedral Fe sites, gets destabilized to form highly active Rh adatoms and small clusters. Conversion of formic acid, leading initially to surface formate and hydroxyl species, is employed as a model reaction to probe the dynamics of such processes. We find that surface hydroxyl recombination to water through the Mars-van Krevelen mechanism reduces Rh coordination, triggering its conversion to active Rh adatoms. Since such lattice oxygen exchange is observed in many acid-base and redox chemistries, the process can be broadly applicable to controlling the activation of the range of SACs.

Understanding the properties of supercooled water is important for developing a comprehensive theory for liquid water and amorphous ices. Because of rapid crystallization for deeply supercooled water, experiments on it are typically carried out under conditions in which the temperature and/or pressure are rapidly changing. As a result, information on the structural relaxation kinetics of supercooled water as it approaches (metastable) equilibrium is useful for interpreting results obtained in this experimentally challenging region of phase space. We used infrared spectroscopy and the fast time resolution obtained by transiently heating nanoscale water films to investigate relaxation kinetics (aging) in supercooled water. When the structural relaxation of the water films was followed using a temperature jump protocol analogous to the classic experiments of Kovacs, similar memory effects were observed. In particular, after suitable aging at one temperature, water's structure displayed an extremum versus the number of heat pulses upon changing to a second temperature before eventually relaxing to a steady-state structure characteristic of that temperature. A random double well model based on the idea of dynamic heterogeneity in supercooled water accounts for the observations.

Glassy solid forms of water can be made by vapor deposition, by the rapid cooling of small liquid droplets or by squashing ice. For the glass to be connected to normal water by a reversible path at atmospheric pressure its residual excess entropy relative to ice needs to be effectively zero. Measurement of the rates of evaporation of glassy water and ice near 150 K gives a measure of their free energy difference which implies an entropy difference of ΔS = 1.3±1.7 J/(K·mol) at 150 K. There are two interpretations of the small value of ΔS, depending on whether the measured free energy of the glass reflects the calorimetric or the statistical mechanical value of the entropy of a glass. An experiment is proposed to test that issue.

Experiments investigating the properties of deeply supercooled liquid water are needed to develop a comprehensive understanding of water's anomalous properties. One approach involves transiently heating nanoscale water films into the supercooled region for several nanoseconds at a time and then interrogating the water films after they have quenched to cryogenic temperatures. To relate the results obtained with this approach to other experiments and simulations on supercooled water, it is important to understand how closely the quenched structure tracks the (metastable) equilibrium structure of water as a function of the transient heating temperature. A key step involves quantifying the extent to which water that is transiently heated to ambient temperatures [hyperquenched water (HQW)] subsequently relaxes toward the structure of low-density amorphous (LDA) ice as it cools. We analyzed the infrared reflection-absorption spectra of LDA, HQW, and crystalline ice films to determine their complex indices of refraction. With this information, we estimate that HQW retains ∼50%-60% of a structural motif characteristic of water at high temperatures with the balance comprised of a low-temperature motif. This result, along with results from x-ray diffraction experiments on water and amorphous ices, allows one to quantify the fraction of the high-temperature motif at approximately zero pressure as a function of temperature from 150 to 350 K.

We have examined the structure of supercooled liquid D2O as a function of temperature between 185 and 255 K using pulsed laser heating to rapidly heat and cool the sample on a nanosecond timescale. The liquid structure can be represented as a linear combination of two structural motifs, with a transition between them described by a logistic function centered at 218 K with a width of 10 K. The relaxation to a metastable state, which occurred prior to crystallization, exhibited nonexponential kinetics with a rate that was dependent on the initial structural configuration. When the temperature is scaled by the temperature of maximum density, which is an isostructural point of the isotopologues, the structural transition and the non-equilibrium relaxation kinetics of D2O agree remarkably well with those for H2O.

The atomic-level characterization of active sites is essential to understanding the mechanisms behind catalytic reactions. In this study, using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we follow the morphological changes of a model Rh catalyst supported on Fe3O4(001) as a function of temperature and Rh coverage. We identify the preparation conditions leading to model catalysts containing homotopic or nearly homotopic Rh species bound as adatoms, substitutional octahedral sites within the Fe3O4(001), and nanoparticles. Adsorbates such as CO and CO2 are subsequently used to characterize the properties of different Rh sites. Using temperature-programmed desorption, we demonstrate that adatoms and nanoparticles exhibit high-temperature CO desorption (250-600 K). Strong binding on such sites further allows for CO oxidation to CO2 via the Mars-van Krevelen mechanism. In contrast, CO2 was found to interact weakly with all Rh sites. Differences in desorption temperature enable the use of CO and CO2 as titration methods for nanoparticles and Fe3O4(001), respectively. A small quantity of CO2 was found to be reduced to CO on Rh adatoms and small nanoparticles.

The reaction pathways of glycerol on TiO2(110) have been studied by molecular beam dosing and temperature-programmed desorption. The majority of adsorbed glycerol undergoes reactions to yield water, hydrogen, and carbon-containing products. For both water and hydrogen, we identify two distinct reaction channels resulting from the cleavage of O-H and C-H bonds. Quantification of the desorption yields reveals that the C-H bond scission channel, observed at temperatures higher than for the O-H bond scission, is a dominant reaction channel for hydrogen evolution. For the carbon-containing products, we identify gaseous allyl radical as the major product. Various minor carbon-containing products, including propylene, acrolein, allyl alcohol, acetone, formaldehyde, and ethylene, are also observed and quantified. The different surface chemistries observed between glycerol and simpler alcohols and glycols on TiO2(110), suggest that smaller oxygenates cannot serve as models for larger oxygenates in probing their reaction pathways on oxide surfaces.

The adsorption of the C2 hydrocarbons, including ethane, ethene, and ethyne, are studied on magnetite Fe3O4(001) by a combination of molecular beam dosing, temperature programmed desorption, and X-ray photoelectron spectroscopy. The ethane desorption profile has a single temperature invariant peak at 100 K, while ethene and ethyne exhibit additional peaks at similar to 120 -and similar to 135 K. An inversion analysis is used to extract coverage-dependent desorption energies as well as coverage-averaged prefactors for each molecule. Ethene and ethyne exhibit moderate coverage-dependent desorption energies decreasing from similar to 80 to similar to 30 kJ/mol at saturation, while ethane shows a relatively coverage invariant desorption energy similar to 28 kJ/mol. The desorption energies of the C2 hydrocarbons increase in the order of increasing bond order. This is likely due to the enhanced interaction of the ethene and ethyne pi system to the coordinatively unsaturated octahedral Fe sites (Fe-oct) on the oxide surface. The saturation coverages of each C2 hydrocarbon at 82 K were also determined. These match well with the 2-dimensional area determined from their liquid densities.

Significance Water plays a prominent role in many areas of physical and biological sciences. Water, though commonplace, is also highly unusual in comparison to most liquids. Promising models to explain water’s anomalous properties propose that it is a heterogeneous, temperature-dependent mixture of two structural motifs. Key phenomena related to this structural heterogeneity are predicted to occur for deeply supercooled water where fast crystallization has prevented experiments. We use a pulsed laser heating technique to examine the rate of structural relaxation for supercooled water at previously inaccessible temperatures. The results suggest that the complex relaxation kinetics are governed by a distribution of energetic barriers for rearrangement within a structurally heterogeneous environment. Furthermore, the structural relaxation is always fast compared to crystallization.

We measure the isothermal crystallization kinetics of amorphous acetonitrile films using molecular beam dosing and reflection adsorption infrared spectroscopy techniques. Experiments on a graphene covered Pt(111) substrate revealed that the crystallization rate slows dramatically during long time periods and that the overall kinetics cannot be described by a simple application of the Avrami equation. The crystallization kinetics also have a thickness dependence with the thinner films crystallizing much slower than the thicker ones. Additional experiments showed that decane layers at both the substrate and vacuum interfaces can also affect the crystallization rates. A comparison of the crystallization rates for CH3CN and CD3CN films showed only an isotope effect of ∼1.09. When amorphous films were deposited on a crystalline film, the crystalline layer did not act as a template for the formation of a crystalline growth front. These overall results suggest that the crystallization kinetics are complicated, indicating the possibility of multiple nucleation and growth mechanisms.

Crystalline acetonitrile has two polymorphs, a high-temperature (HT) phase that is stable between 217 K and its melting point at 229 K and a low-temperature (LT) phase that is stable below 217 K. Solid acetonitrile films can be prepared by vapor deposition in an ultrahigh vacuum chamber. To prevent sublimation of the film, temperatures are often kept below 150 K. While the LT phase is thermodynamically favored at these low temperatures, such preparation usually results in the formation of the metastable HT polymorph. In this work we use reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments to investigate the effects of the deposition temperature and underlying substrate on the morphology of acetonitrile films prepared with molecular beam deposition. We obtained the elusive LT phase when dosing at 120 K on a graphene substrate and on a crystalline decane layer. Dosing acetonitrile on other surfaces produced the HT phase, as did annealing of amorphous films. We used TPD experiments to determine the Gibbs energy difference between the HT and the LT phases. Our ΔG values agree with extrapolation of equilibrium calorimetry data. We also observed that acetonitrile films were amorphous when dosed at temperatures ≤ 60 K and porous for temperatures ≤ 50 K.

A fundamental understanding of the unusual properties of water remains elusive because of the limited data at the temperatures and pressures needed to decide among competing theories. We investigated the structural transformations of transiently heated supercooled water films, which evolved for several nanoseconds per pulse during fast laser heating before quenching to 70 kelvin (K). Water's structure relaxed from its initial configuration to a steady-state configuration before appreciable crystallization. Over the full temperature range investigated, all structural changes were reversible and reproducible by a linear combination of high- and low-temperature structural motifs. The fraction of the liquid with the high-temperature motif decreased rapidly as the temperature decreased from 245 to 190 K, consistent with the predictions of two-state "mixture" models for supercooled water in the supercritical regime.

Acetonitrile thin films were prepared on Pt(111) and on graphene on Pt(111) using molecular beam techniques. Temperature programed desorption (TPD) experiments of acetonitrile on Pt(111) displayed first-order kinetics for monolayer desorption and zero-order kinetics for multilayer desorption. We used reflection adsorption infrared spectroscopy (RAIRS) to characterize the orientation of the monolayer on Pt(111). Acetonitrile molecules have been shown to adopt a bridging structure where the C N bond breaks to form two single bonds to underlying platinum atoms. When T-deposition >= 120 K, our RAIRS spectra were consistent with this bridging configuration. However, when T-deposition was lowered to 60 K, molecules retained the C N bond and adopted a vertical orientation with the nitrogen closest to the platinum. On graphene, the TPD experiments show that both the monolayer and multilayer desorb with zero-order kinetics. Our RAIRS spectra suggest that the monolayer consists of acetonitrile molecules oriented with their molecular axes parallel to the graphene substrate. This orientation did not change when T(deposition)( )was varied from 60 to 120 K. Our results provide a more complete picture of how acetonitrile molecules interact with Pt(111) and with graphene surfaces.