H/D isotopic exchange between water molecules at ice surfaces

H/D isotopic exchange between H(2)O and D(2)O molecules was studied at the surface of ice films at 90-140 K by the technique of Cs(+) reactive ion scattering. Ice films were deposited on a Ru(0001) substrate in different compositions of H(2)O and D(2)O and in various structures to study the kinetics of isotopic exchange. H/D exchange was very slow on an ice film at 95-100 K, even when H(2)O and D(2)O were uniformly mixed in the film. At 140 K, H/D exchange occurred in a time scale of several minutes on the uniform mixture film. Kinetic measurement gave the rate coefficient for the exchange reaction, k(140 K)=1.6(+/-0.3) x 10(-19) cm(2) molecule(-1) s(-1) and k(100 K)< or =5.7(+/-0.5) x 10(-21) cm(2) molecule(-1) s(-1) and the Arrhenius activation energy, E(a)> or =9.8 kJ mol(-1). Addition of HCl on the film to provide excess protons greatly accelerated the isotopic exchange reaction such that it went to completion very quickly at the surface. The rapid reaction, however, was confined within the first bilayer (BL) of the surface and did not readily propagate to the underlying sublayer. The isotopic exchange in the vertical direction was almost completely blocked at 95 K, and it slowly occurred only to a depth of 3 BLs from the surface at 140 K. Thus, the proton transfer was highly directional. The lateral proton transfer at the surface was attributed to the increased mobility of protonic defects at the molecularly disordered and activated surface. The slow, vertical proton transfer was probably assisted by self-diffusion of water molecules.     

Fast thermal desorption spectroscopy study of H/D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.     

Spectroscopic and computational evidence for SO2 ionization on 128 K ice surface

The experimental part of this study focuses on FTIR spectroscopy of SO(2) adsorbate on the surface of ice nanoparticles at 128 K, in the 0.5-1 monolayer coverage range. In addition to the infrared spectroscopic features due to molecular SO(2), a structured band is observed at approximately 1030 cm(-1). A similar band was observed in past spectroscopic studies of SO(2) aqueous solutions, and assigned to anionic products of SO(2) ionization. Ab initio normal mode analysis of HSO(3)(-) yielded intense SO stretch bands in the vicinity of the observed "ionic" feature. The relative intensities of the molecular and the anionic bands indicate that 0.3 approximately 0.5 of the adsorbate is ionized. These results are consistent with the previously published data on isotopic exchange in SO(2)-covered ice nanoparticles (Devlin and Buch, J. Chem. Phys., 2007, 127, 091101) which pointed towards substantial SO(2) ionization at low temperatures. Density functional theory modeling of molecular and ionized adsorbate on a crystal ice slab suggests that anion solvation by molecular SO(2) adsorbate facilitates the SO(2) ionization process at the ice surface.     

Molecular-dynamics studies of surface of ice Ih

We performed molecular dynamics calculations of surface of ice Ih in order to investigate formation mechanism of melting layer on the surface. The results showed that the vibrational amplitude of the atoms in the surface layer greatly depends on the crystal orientation, whereas that in the ice bulk is isotropic. The anisotropy of the vibration is due to a dangling motion of the free O-H bonds exist at the surface layer. The dangling motion enhances the rotational motion of the water molecules. The vibrational density of state showed a coupling between the rotational vibration and the lattice vibration of the water molecules in the surface layer. The coupling of the vibrations causes a distortion of ice lattice. Through the hydrogen-bonding network, the distortion transmits to the interior of the crystal. We conclude that the dangling motion of the free O-H bonds exist at the surface layer is one of the dominant factors governing the surface melting of ice crystal.     

Proton mobility in thin ice films: a revisit

We have examined proton transport through an ice film in the temperature range 73-140 K by initially adding hydronium ions into the interior of the film and then monitoring the build-up of hydronium ion population at the film surface. The result confirms that the proton exhibits limited mobility in the ice film at low temperature, but it becomes highly mobile at temperature above 130 K. Based on this result we suggest an explanation of the anomalous experimental observations in the literature for the proton mobility in ice films.     

UV-induced protonation of molecules adsorbed on ice surfaces at low temperature

UV irradiation of ice films adsorbed with methylamine molecules induces protonation of the adsorbate molecules at low temperature (50-130 K). The observation indicates that long-lived protonic defects are created in the ice film by UV light, and they transfer protons to the adsorbate molecules via tunneling mechanism at low temperature. The methylammonium ion formed by proton transfer remains to be stable at the ice surface. It is suggested that this solid-phase protonation might play a significant role in the production of molecular ions in interstellar clouds.     

Segregation of hydroxide ions to an ice surface

Hydroxide ions that are initially buried within an ice film segregate to the ice film surface at elevated temperatures. This process was observed by conducting experiments with an ice film constructed with a bottom H(2)O layer and an upper D(2)O layer, with an excess of hydroxide ions trapped at the H(2)O/D(2)O interface as they were generated by Na hydrolysis. The transport of hydroxide ions from the interfacial layer to the surface was examined as a function of time using a low energy sputtering method. The progress of the H/D exchange reaction in surface water molecules was also monitored with the Cs(+) reactive ion scattering technique. At 90 K, only a small portion of buried hydroxide ions moved to the surface in the form of OD(-) species. This was due to hydroxide transport via proton hopping through a D(2)O layer, 3 BL thick, in the surface region. At 135 K, at which point water self-diffusion is active in the ice film, the majority of the buried hydroxide ions segregated to the surface after ～1 h. Both OH(-) and OD(-) species were produced at the surface, at an OH(-)/OD(-) population ratio ≥1. Based on kinetic measurements for the transport of OH(-) and OD(-) species and the H/D exchange of surface water molecules, we concluded that the major transport channel for hydroxide ions in this regime is the migration of molecular hydroxide species. H/D exchange reactions also occur between surface hydroxide ions and water molecules. No evidence was observed for the occurrence of the hop-and-turn process at 135 K, although it is known as an important mechanism of proton transport in ice.     

Quantitative determination of surface silanol groups in silicagel by deuterium exchange combined with infrared spectroscopy and chemometrics

Concentration of silanol groups on silica gel surface has been quantitatively determined using the deuterium exchange method. Simple and effective procedures have been used in pre-sample drying, deuterium exchange and extraction of resulting isotopic mixture from the exchange reaction. Each of four silica gel samples with varying surface area has been subjected to pre-drying to remove adsorbed water and then quantitatively mixed with deuterium oxide in a steel bomb for isotopic exchange. The resulting D(2)O/water mixture was then extracted by applying high pressure using infrared pellet press. The infrared spectrum of the isotopic mixture was measured and the composition was then determined by a multivariate calibration model established between infrared profiles of water in D(2)O standard mixtures and their composition. The results show that the silanol group concentration determined agrees with the values reported in the literature.     

Exceptional Isotopic‐Substitution Effect: Breakdown of Collective Proton Tunneling in Hexagonal Ice due to Partial Deuteration

Multiple proton transfer controls many chemical reactions in hydrogen‐bonded networks. However, in contrast to well‐understood single proton transfer, the mechanisms of correlated proton transfer and of correlated proton tunneling in particular have remained largely elusive. Herein, fully quantized ab initio simulations are used to investigate H/D isotopic‐substitution effects on the mechanism of the collective tunneling of six protons within proton‐ordered cyclic water hexamers that are contained in proton‐disordered ice, a prototypical hydrogen‐bonded network. At the transition state, isotopic substitution leads to a Zundel‐like complex, [HO???D???OH], which localizes ionic defects and thus inhibits perfectly correlated proton tunneling. These insights into fundamental aspects of collective proton tunneling not only rationalize recent neutron‐scattering experiments, but also stimulate investigations into multiple proton transfer in hydrogen‐bonded networks much beyond ice.     

Hydrogen peroxide formation following the vacuum ultraviolet photodissociation of water ice films at 90 K

The production of gaseous OH radicals from the 300-350 nm photodissociation of H(2)O(2) that was photolytically produced on a water ice surface following the 157 nm photolysis of water ice at 90 K was directly monitored using resonance-enhanced multiphoton ionization. The translational energy distribution estimated by the time-of-flight spectrum of the OH products is represented by a Maxwell-Boltzmann energy distribution with a translational temperature of 3750+/-250 K. The rotational temperature was estimated by a spectral simulation to be 225+/-25 K. Surface defects produced by HCl deposition on the water ice contributed to the higher production rate of H(2)O(2) in the 157 nm photoirradiation of water ice while surface coverage caused by CD(3)OH deposition decreased the H(2)O(2) production rate.     

Mechanistic study of proton transfer and HD exchange in ice films at low temperatures (100-140 K)

We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.     

Photo ejection of water molecules from amorphous ice films

Water molecules are photo-ejected upon laser irradiation from the surface of ice films grown on graphite (0001) and Pt(111). The films are deposited at temperatures between 40 and 140 K and irradiated with nanosecond laser pulses. The process is investigated in the wavelength range between 275 and 670 nm. The wavelength and photon flux dependence suggest a multi-photon process with energy threshold of around 9 eV. The photo-detachment is less effective or negligible from films annealed at temperatures above the amorphous-crystalline transition temperature of ice films. Coverage dependence of the phenomena relates the photo yield to surface roughness. Electronic excitation mechanism related to the defects in ice is proposed to explain the observations.     

Electrical processes upon the evaporation and condensation of water and ice

A mathematical model of electrical processes that take place upon the evaporation of water and sublimation of ice, as well as the condensation growth of water and ice phases from vapor, is proposed. The transfer of the main charge carriers, such as (i) protons and hydroxide ions (in ice, water, and vapor and (ii) orientational defects (in ice and water) is taken into account. Upon the evaporation of water and the sublimation of ice, the first carriers are accumulated before the phase front and cause positive charges in the surface of the water and ice, while the second carriers are depleted (their concentration becomes lower than the thermodynamic value) and impart a negative charge to water and ice. The contribution of protons and hydroxide ions dominates at a low rate of evaporation. In the condensation of vapor and relevant growth of water and ice phases, the polarity of surface charge is opposite to that observed upon evaporation. The values of interfacial current and signs of phase charges upon sublimation, evaporation, and condensation that are predicted in the model comply with experimental data.     

Pt/Ce0.75Zr0.25O2的催化性能及其氧移动性的 18O 同位素交换表征

 采用浸渍法制备了Pt/Ce0.75Zr0.25O2催化剂,考察了催化剂对乙醇及CO的氧化活性,并采用 18O 同位素交换、乙醇程序升温表面反应(C2H5OH-TPSR)、一氧化碳程序升温脱附(CO-TPD)和程序升温还原(H2-TPR)等技术对催化剂进行了表征. 结果表明, Pt/Ce0.75Zr0.25O2催化剂表现出较高的乙醇和CO氧化活性,其催化活性随着Pt负载量的增加而提高. 当Pt负载量为3%时,活性最高. 继续增加Pt负载量,催化剂活性下降. C2H5OH-TPSR和CO-TPD结果表明,催化剂对乙醇或CO的氧化活性与从催化剂表面脱附出来的CO2量有对应关系, CO2脱附量越大,催化剂活性越高. 18O 同位素交换结果表明,表面氧交换能力与其氧化活性有一定对应关系,催化剂的表面氧交换能力越高,氧化活性越高.     

H/D isotopic exchange in water interactions with the ferroelectric copolymer: Poly(vinylidene fluoride-trifluoroethylene) (70%:30%)

For both water and heavy water adsorption and absorption on crystalline poly(vinylidene fluoride with trifluoroethylene (30%)), P(VDF-TrFE 70:30), two distinctly different adsorption sites have been identified by thermal desorption spectroscopy. One adsorbed water species resembles ice and there is also an absorbed water species that interacts more strongly with the polymer thin film, and in addition, there is a polymer surface (polymer to ice interface) water species. We find that there is H/D exchange between the water or heavy water molecules and the ferroelectric polymer (largely -(CH2-CF2)-), particularly at the polymer surface.     

Detailed Description of Pulse Isotopic Exchange Method for Analyzing Oxygen Surface Exchange Behavior on Oxide Ion Conductors

A novel pulse ¹⁸O-¹⁶O isotopic exchange (PIE) technique for measurement of the rate of oxygen surface exchange of oxide ion conductors was presented. The technique employs a continuous flow packed-bed micro-reactor loaded with the oxide powder. The isothermal response to an ¹⁸O-enriched pulse passing through the reactor, thereby maintaining chemical equilibrium, is measured by on-line mass spectrometry. Evaluation of the apparent exchange rate follows from the uptake of ¹⁸O by the oxide at given reactor residence time and surface area available for exchange. The developed PIE technique is rapid, simple and highly suitable for screening and systematic studies. No rapid heating/quenching steps are required to facilitate ¹⁸O tracer anneal or analysis, as in other commonly used techniques based upon oxygen isotopic exchange. Moreover, the relative distribution of the oxygen isotopologues ¹⁸O₂, ¹⁶O¹⁸O, and ¹⁶O₂ in the effluent pulse provides insight into the mechanism of the oxygen exchange reaction. The PIE technique has been demonstrated by measuring the exchange rate of selected oxides with enhanced oxide ionic conductivity in the range of 350?900 oC. Analysis of the experimental data in terms of a model with two consecutive, lumped steps for the isotopic exchange reaction shows that for mixed conductors Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ(BSCF) and La₂NiO_4+δ the reaction is limited by the apparent rate of dissociative adsorption of O₂ molecules at the oxide surface. For yttria-stabilized zirconia (YSZ), a change-over takes place, from rate-limitations by oxygen incorporation below ∽800 oC to rate-limitations by O₂ dissociative adsorption above this temperature. Good agreement is obtained with exchange rates reported for these materials in literature.     

Supramolecular mechanisms responsible for the high activity of aerosol stimulants of atmospheric moisture nucleation

Water vapor nucleation preceding crystal nucleation was studied by computer simulation; the role of crystal defects on the surface with a structure complementary to the structure of ice was investigated. The Gibbs energy and microdrop formation energy were calculated at the molecular level. The structure and thermodynamic stability of nuclei in the fields of crystal defects of different types were analyzed. It was found that crystal point defects could not stimulate the formation of large nuclei. Extended defects such as wedge-shaped fractures were shown to be the most effective stimulants. The stage of monomolecular film formation in fractures was preceded by the formation of three-dimensional microdrops. The conclusion was drawn that microcrystallization was favored by a relatively large surface of contact with microfracture walls, the three-dimensional structure of nuclei, and their high thermodynamic stability in the transverse microfracture field; these factors drastically accelerated the growth of nuclei and their coalescence on the surface of aerosol particles.     

Isotopic Exchange in Porous and Dense Magnesium Borohydride

Magnesium borohydride (Mg(BH₄)₂) is one of the most promising complex hydrides presently studied for energy‐related applications. Many of its properties depend on the stability of the BH₄^? anion. The BH₄^? stability was investigated with respect to H→D exchange. In situ Raman measurements on high‐surface‐area porous Mg(BH₄)₂ in 0.3 MPa D₂ have shown that the isotopic exchange at appreciable rates occurs already at 373 K. This is the lowest exchange temperature observed in stable borohydrides. Gas–solid isotopic exchange follows the BH₄^?+D^.→BH₃D^?+H^. mechanism at least at the initial reaction steps. Ex situ deuteration of porous Mg(BH₄)₂ and its dense‐phase polymorph indicates that the intrinsic porosity of the hydride is the key behind the high isotopic exchange rates. It implies that the solid‐state H(D) diffusion is considerably slower than the gas–solid H→D exchange reaction at the surface and it is a rate‐limiting steps for hydrogen desorption and absorption in Mg(BH₄)_2.     

Kinetics of Heterogeneous Isotopic Exchange Reaction in Finite Bath

A general method to find the rate constant and particle self-diffusion coefficient is suggested for a heterogeneous isotopic exchange reaction which is controlled by surface mass reaction or controlled by a combination of surface mass reaction and intraparticle diffusion. The values of the kinetic parameter, ξ₁ (ratio of the forward surface mass reaction rate to the intraparticle diffusion rate), particle self-diffusion coefficient D and rate constant k are obtained by the proposed method for the isotopic exchange reaction systems CaCO₃(s)/Ca²⁺(aq) and CaC₂O₄(s)/Ca²⁺(aq).