The influence of water vapor on bentonites or smectites is of interest in many different fields of applied mineralogy such as nuclear-waste sealing or casting in the foundry industry. The water vapor affects the smectite surface and perhaps its structure probably leading to mostly unfavorable changes in its properties. In this first part of the present study, the influence of hot water vapor (200°C) on the physicochemical and mineralogical properties of smectite-group minerals was studied. After the steam treatment, turbidity measurements, methylene-blue sorption, water adsorption, and cation exchange capacity (CEC) were measured on both untreated and treated samples. Mineralogical changes were monitored by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) was used to measure O, Al, and Si. Only a few parameters showed differences between the untreated and vapor-treated samples. Sedimentation volumes (SV) decreased following the treatment. As shown by XRD and XPS, the crystalline structure of smectite remained unaffected by the steam treatment. Equivalent sphere diameters (ESD) were not affected systematically by the steam treatment. Differences in CEC values between untreated and treated samples were observed, but only for smectites with monovalent interlayer cations. From the variety of different measurements the conclusion of the present study was that steam treatment changes the charge properties at or near the smectite particle surface.

Adsorption isotherms and differential heats of adsorption of water vapor by muscovite were measured at 303 K. The heats of adsorption are stepwise and each step corresponds to the stoichiometric formation of adsorption complexes of H2O molecules with K+ ions, (H2O)n/K+, (n = 1–6), which are located on the basal and lateral (edge) faces. At saturation, the ditrigonal cavities of the basal faces are fully occupied by hexameric clusters. It is suggested that half of the K+ ions on the basal faces come from neighboring layers by migration under the influence of the adsorbed H2O at the initial stage of adsorption. Similar migration of K+ to the edges was also hypothesized, suggesting that only every second site can be occupied by a cluster. At the final stage of adsorption, H2O molecules are believed to form H-bonded bridges between the hexameric water/cation clusters on the basal faces, whereas on the edge faces no such effect is believed to occur. The mean molar integral adsorption entropy of water is ∼−7 J/mol K less than the molar entropy of the bulk liquid. The mobility of H2O on muscovite is slightly less than in bulk water. Migration of K+ cations under the influence of adsorbed H2O both on the basal and lateral faces of muscovite is reversible.

Vermiculites have the potential to serve as effective catalysts if pillared with Al, but their high charge presents an obstacle to the pillaring process. The purpose of this study was to submit natural vermiculite to thermal treatments in the presence of water vapor in order to effect a reduction in the global negative charge and thereby to enhance its susceptibility to pillaring. The process of charge reduction in vermiculite under the conditions selected involved the extraction of 25% of IVAl accompanied by the extraction of structural Mg and charge-compensating cations (Ca2+, Na+, and K+). The results indicate a reduction of 35% in the global negative charge in vermiculite by the end of the treatment. Some of the VIAl content was not removed during acid washing, and probably remained in the solid in structural positions in the octahedral sheet.

Protocols for conducting in situ transmission electron microscopy (TEM) reactions using an environmental TEM with dry gases have been well established. However, many important reactions that are relevant to catalysis or high-temperature oxidation occur at atmospheric pressure and are influenced by the presence of water vapor. These experiments necessitate using a closed-cell gas reaction TEM holder. We have developed protocols for introducing and controlling water vapor concentrations in experimental gases from 2% at a full atmosphere to 100% at ~17 Torr, while measuring the gas composition using a residual gas analyzer (RGA) on the return side of the in situ gas reactor holder. Initially, as a model system, cube-shaped MgO crystals were used to help develop the protocols for handling the water vapor injection process and confirming that we could successfully inject water vapor into the gas cell. The interaction of water vapor with MgO triggered surface morphological and chemical changes as a result of the formation of Mg(OH)2, later validated with mass spectra obtained with our RGA system with and without water vapor. Integrating an RGA with an in situ scanning/TEM closed-cell gas reaction system can thus provide critical measurements correlating gas composition with dynamic surface restructuring of materials during reactions.

We review our studies of the initial oxidation stages of Cu(001) thin films as investigated by in situ ultra-high vacuum transmission electron microscopy. We present our observations of surface reconstruction and the nucleation to coalescence of copper oxide during in situ oxidation in O2. We have proposed a semi-quantitative model, where oxygen surface diffusion is the dominant mechanism of the initial oxidation stages of Cu. We have also investigated the effect of water vapor on copper oxidation. We have observed that the presence of water vapor in the oxidizing atmosphere retards the rate of Cu oxidation and Cu2O is reduced when exposed directly to steam.

Electron beam damage is a significant limitation for transmission electron microscopy (TEM) studies of beam-sensitive samples. An approach for studying surface reactions on alkali halide crystals using 200 kV TEM is presented. Experiments were designed to monitor the reaction of NaCl crystals with HNO3 gas followed by water vapor to form solid NaNO3. During beam damage experiments, TEM micrographs record structural changes to both NaCl and NaNO3, including dislocation loops, void formation, and decomposition. Sample decomposition can be successfully minimized by a combination of commonly used techniques: (1) focusing the beam adjacent to the area of interest, (2) lowering the electron density, (3) choosing to image larger (micrometer- versus submicrometer-sized) alkali halide crystals, and (4) lowering temperature by the use of a liquid nitrogen cooling stage. From these results, additional studies were designed that monitored sequential experiments. Sensitive micrometer-sized sodium chloride single crystals before and after exposure to nitric acid vapor and water vapor and the subsequent growth of submicrometer-sized sodium nitrate single crystals could then be successfully imaged using TEM.