Dissolution of crystallites: surface energetic control and size effects.

Traditional understanding of dissolution assumes that the reaction is spontaneous and continues until equilibrium is reached. This paper presents theoretical and experimental data to support a dissolution mechanism that involves the existence of critical conditions for dissolution, in which the reaction is accompanied by the formation of pits and the subsequent displacement of pit steps. The accompanying increase in surface roughness leads to changes in surface energy with losses of crystal mass that are positive rather than negative and the existence of critical dissolution conditions. Critical pits and dissolution steps are verified experimentally and a relationship between the size and rate of displacement of steps is also demonstrated, in which the rate decreases with size and approaches zero at a critical size, r*. These microscopic step dynamics are consistent with the observed size-effects in bulk dissolution, which cannot be explained using traditional dissolution theories. The observed size effects include self-inhibition, in which the dissolution rate decreases with extent of reaction, dissolution suppression, and periodic resumption. These interesting dissolution phenomena are only readily displayed when the sizes of dissolving crystallites fall in the same range as the critical size (i.e., within 50r*). It is interesting to note that natural biominerals and many nanoparticles fall into this category, so that their suspensions can be dynamically stabilized without dissolution in undersaturated supporting media. The current research implies that dissolution kinetics cannot be understood well without appealing to fundamental physical concepts about the energetic control of dissolution steps on a molecular level. A new dissolution model for crystallites is introduced systemically.     

Stability of nanocrystals: thermodynamic analysis of oxidation and re-reduction of cobalt in water/hydrogen mixtures

The stability of nanosized materials differs significantly from the stability of bulk materials. In this study a thermodynamic analysis on the simultaneous oxidation and re-reduction of small metallic cobalt crystallites in the presence of water and hydrogen as a function of the crystallite diameter was performed as a model for catalyst deactivation in the Fischer-Tropsch synthesis. It is shown that spherical cobalt crystallites with a diameter less than 4.4 nm are likely to be oxidized under realistic Fischer-Tropsch synthesis conditions (p(H)(2)(O)/p(H)(2) < 1.5, T = 493 K).     

In situ surface oxidation study of a planar Co/SiO2/Si(100) model catalyst with nanosized cobalt crystallites under model Fischer-Tropsch synthesis conditions

The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study a planar Co/SiO(2)/Si(100) model catalyst with well-defined cobalt crystallites, close to the threshold value reported for oxidation in the literature (4-10 nm), was prepared by the spin coating method. The planar Co/SiO(2)/Si(100) model catalyst was characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering. The surface oxidation behavior of the nanosized metallic cobalt crystallites of 4-5 nm was studied using in situ near-edge X-ray absorption fine structure under model FTS conditions, i.e., H(2)/H(2)O = 1, P(Total) = 0.4 mbar, and 150-450 degrees C. No surface oxidation of metallic cobalt was observed under these model FTS conditions over a wide temperature range, i.e., 150-400 degrees C.     

The synthesis and magnetic properties of nanosized hematite (alpha-Fe2O3) particles

The synthesis of nanosized superparamagnetic hematite particles by dissolving ferric salts in hydrochloric acid and heating at 100 degrees C is described. A hydrolysis reaction causes the formation of hematite particles. The influence of the sequence of additions on the resulting precipitates was studied using TEM and XRD. The magnetic behavior was characterized by magnetization measurements. It was found that small changes in the reaction conditions led to remarkable changes in final size and shape of the hematite crystallites. A well-defined subrounded morphology and an average diameter of 41 nm were obtained for superparamagnetic hematite particles. This is the largest size reported thus far for superpara-magnetic hematite particles.     

A crystallite packing model for pseudoboehmite formed during the hydrolysis of trisecbutoxyaluminium to explain the peptizability

A model for pseudoboehmite crystallite packing formed during the hydrolysis of trisecbutoxyaluminium is postulated. The model describes platelike crystallites of pseudoboehmite stacked in a sharing edges only configuration. With this type of stacking, the pore sizes detected are approximately equal to the crystallite sizes of the hydrolysates. The hydrolysates age via a dissolution re-precipitation reaction. This increases the size of the crystallite size of the pseudoboehmite formed, speeding peptization by allowing nitrate ions to enter pores and access the surfaces of the crystallites. This type of model also allows an explanation for the peptization kinetics of systems containing sec-butanol formed during the hydrolysis of trisecbutoxyaluminium.     

Interaction between nanosized crystalline components of a composite based on <Emphasis Type="Italic">Acetobacter xylinum</Emphasis> cellulose and calcium phosphates

A composite consisting of two nanosized biocompatible components, Acetobacter xylinum cellulose and calcium phosphate, is prepared through aggregation in an aqueous suspension. The structures of initial components and composite are investigated by the methods of X-ray and electron diffraction and electron microscopy. The mineral component consists of two crystalline phases, hydroxyapatite and whitlockite (magnesium-containing tricalcium phosphate), which are nanosized platelike crystals. The composite preserves the crystalline structures of initial calcium phosphates and cellulose. In the course of composite formation, hydroxyapatite and whitlockite crystallites are adsorbed on the surfaces of nanofibrillar cellulose ribbons. Whitlockite nanocrystals are predominantly deposited on the surface of cellulose ribbons. The mutual orientation of the surfaces of crystalline structures of cellulose and two types of calcium phosphates, hydroxyapatite and whitlockite, is analyzed by means of computer simulation, and the variants of mutual arrangement of their surfaces during formation of the interfacial boundary are suggested.     

Microemulsion-based synthesis of CeO(2) powders with high surface area and high-temperature stabilities

Pure ceria powders, CeO(2), were synthesized in heptane-microemulsified aqueous solutions of CeCl(3) or Ce(NO(3))(3) stabilized by AOT (sodium bis(2-ethylhexyl) sulfosuccinate), DDAB (di-n-didodecyldimethylammonium bromide), or DDAB + Brij 35 surfactant mixtures. Micellar DTAB (n-dodecyltrimethylammonium bromide) and vesicular DDAB systems were also used as media for generating CeO(2). Characterization of the powders by X-ray powder diffractometry, laser-Raman spectroscopy, and Fourier transform infrared spectroscopy revealed that in the presence of surfactants almost-agglomerate-free nanosized crystallites (6-13 nm) of anionic vacancy-free cubic CeO(2) were produced. In the absence of surfactants 21-nm-sized crystallites were formed, comparing with the 85-nm-sized crystallites when cubic CeO(2) was created via thermal decomposition of cerium oxalate. Surface characterization, by X-ray photoelectron spectroscopy, N(2) sorptiometry, and high-resolution electron microscopy showed AOT- or (DDAB + Brij 35)-stabilized microemulsions to assist in formation of crystallites exposing surfaces of large specific areas (up to ca. 250 m(2)/g) but of low stability to high-temperature calcination (28-13 m(2)/g at 800 degrees C). In contrast, the double-chained DDAB was found to generate cubic CeO(2) crystallites of lower initial surface areas (144 (microemulsion) to 125 (vesicles) m(2)/g)) but of higher thermal stability (55-45 m(2)/g at 800 degrees C). Hence, the latter cerias could be considered as appropriate components for total oxidation (combustion) catalysts.     

The Early Stages of Native Enamel Dissolution Studied with Atomic Force Microscopy

Food-induced demineralization (erosion) is one of the key factors in surface structural changes of tooth enamel, with soft drinks being a significant etiological agent. The objective of this study was to measure early stages of enamel loss with high accuracy on native enamel surfaces combined with qualitative observations of changes in the surface morphology using the atomic force microscope (AFM). Native unerupted third molar surfaces were partly covered with a gold reference layer. Samples were imaged with the AFM before dissolution (at baseline) and after exposure to three different drinks (mineral water, a "toothkind" blackcurrant drink, and a lemon and lime juice drink) at five different exposure times (15 min, 30 min, 1 h, 2 h, and 3 h). The changes in the surface morphology were investigated qualitatively as well as quantitatively. This study showed that the maximum material loss occurred at the aprismatic parts of the enamel close to the perikymata. The maximum enamel loss was greatest for the lemon and lime juice drink and lowest for water. A two-way ANOVA of the transformed data, employing the natural logarithm, showed a statistically significant difference between both the drinks and the exposure time at a 95% confidence level (P=0.000). This demonstrates that the AFM is a suitable tool for measuring early stages of enamel demineralization. Copyright 2000 Academic Press.     

Enamel dissolution as a function of solution degree of saturation with respect to hydroxyapatite: a nanoindentation study

The objective of this study was to investigate human enamel dissolution as a function of degree of saturation (DS) of the surrounding solution with respect to hydroxyapatite. Nanoindentation was used to compare changes in enamel nanomechanical properties due to dissolution by two solutions. Citric acid solution (DS=0.000, pH 3.30) and citric acid solution containing calcium (299 mg/l) and phosphate (54.0 mg/l) (DS=0.032, pH 3.30) were compared with a control mineral water (DS=0.673, pH 7.48). Exposure times were 0, 120, 300, 600, 900, and 1200 s. Compared to untreated enamel, there was a statistically significant change in enamel hardness after 120 s exposure to both citric acid solutions, and in elastic modulus after 300 s exposure. The rate of change of both variables decreased with exposure time. This suggests that dissolution rate is diffusion-limited under these conditions, in agreement with previous studies. There was no statistically significant difference between the hardness or elastic modulus of enamel exposed to the two citric acid solutions at any time. This may be due to a change in solution composition during contact with the enamel.     

Constructing Intrapenetrated Hierarchical Zeolites with Highly Complete Framework via Protozeolite Seeding

Creation of intrapenetrated mesopores with open highway from external surface into the interior of zeolite crystals are highly desirable that can significantly improve the molecular transport and active sites accessibility of microporous zeolites to afford enhanced catalytic properties. Here, different from traditional zeolite-seeded methods that generally produced isolated mesopores in zeolites, nanosized amorphous protozeolites with embryo structure of zeolites were used as seeds for the construction of single-crystalline hierarchical ZSM-5 zeolites with intrapenetrated mesopores (mesopore volume of 0.51 cm³ g⁻¹) and highly complete framework. In this strategy, in contrast to the conventional synthesis, only a small amount of organic structure directing agents and a low crystallization temperature were adopted to promise the protozeolites as the dominant growth directing sites to induce crystallization. The protozeolite nanoseeds provided abundant nucleation sites for surrounding precursors to be crystallized, followed by oriented coalescence of crystallites resulting in the formation of intrapenetrated mesopores. The as-prepared hierarchical ZSM-5 zeolites exhibited ultra-long lifetime of 443.9 hours and a high propylene selectivity of 47.92 % at a WHSV of 2 h⁻¹ in the methanol-to-propylene reaction. This work provides a facile protozeolite-seeded strategy for the synthesis of intrapenetrated hierarchical zeolites that are highly effective for catalytic applications.     

Noncatalytic hydrogenation of naphthalene in nanosized membrane reactors with accumulated hydrogen and controlled adjustment of their reaction zone volumes

As part of ongoing studies aimed at designing the next generation of nanosized membrane reactors (NMRs) with accumulated hydrogen, the noncatalytic hydrogenation of naphthalene in pores of ceramic membranes (TRUMEM ultrafiltration membranes with D _av = 50 and 90 nm) is performed for the first time, using hydrogen preadsorbed in a hybrid carbon nanostructure: mono- and multilayered oriented carbon nanotubes with graphene walls (OCNTGs) that form on inner pore surfaces. In this technique, the reaction proceeds in the temperature range of 330–390°C at contact times of 10–16 h. The feedstock is an 8% naphthalene solution in decane. The products are analyzed via chromatography on a quartz capillary column coated with polydimethylsiloxane (SE-30). It is established for the first time that in NMRs, the noncatalytic hydrogenation of naphthalene occurs at 370–390°C, forming 1,2,3,4-tetrahydronaphthalene in amounts of up to 0.61%. The rate constants and activation energy (123.5 kJ/mol) of the noncatalytic hydrogenation reaction are determined for the first time. The possibility of designing an NMR with an adjustable reaction zone volume is explored. Changes in the pore structure of the membranes after their modification with pyrocarbon nanosized crystallites (PNCs) are therefore studied as well. It is shown that lengthening the process time reduces pore size: within 23 h after the deposition of PNCs, the average pore radius (r _av) falls from 25 to 3.1 nm. The proposed approach would allow us to design nanoreactors of molecular size and conduct hydrogenation reactions within certain guidelines to synthesize new chemical compounds.

     

Spatially resolved product formation in the reaction of formic acid with calcium carbonate (1014): the role of step density and adsorbed water-assisted ion mobility

The reaction of calcium carbonate (1014) single-crystal surfaces with formic acid (HCOOH) vapor was investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another and mirroring the rhombohedral structure of the calcite surface, while exposing unreacted carbonate surface. The size and surface density of the crystallites depend upon substrate step density, exposure time, and relative humidity. XPS data confirmed the crystallite composition as the expected calcium formate product. The AFM images show erosion and pit formation of the calcite surface in the vicinity of the product crystallites, clearly providing the spatially resolved characterization of the source of Ca ions. AFM experiments exploring the effects of water vapor on the reacted surface show that the calcium formate crystallites are mobile under conditions of high relative humidity, combining to form larger crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the alpha form, as confirmed by X-ray diffraction. The implications for the reactions described here are discussed.     

Theoretical studies on the ene reaction mechanisms of propene and cyclopropene with ethylene and cyclopropene: concerted or stepwise

The potential energy surfaces of the ene reactions of propene and cyclopropene with ethylene and cyclopropene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by CiLC method on the basis of CASSCF MOs. The concerted and stepwise reaction pathways of the ene reaction of propene with ethylene as the parent reaction were located. The energy barrier of the stepwise process is about 4 kcal/mol lower than that of the concerted one. The other reactions can be found only the stepwise mechanism. Although the endo-type reaction of propene with cyclopropene, where cyclopropene is the enophile, probably occurs through a one-step process, the mechanism is divided into the CC bond formations and the hydrogen migration as a stepwise reaction. The CiLC-IRC analysis of the concerted process of propene with ethylene shows the different patterns of the electronic state variation for the CC bond formation/breaking and the hydrogen migration.     

Synchrotron X-ray diffraction characterization of healthy and fluorotic human dental enamel

With the introduction of fluoride as the main anticaries agent used in preventive dentistry, and perhaps an increase in fluoride in our food chain, dental fluorosis has become an increasing world-wide problem. Visible signs of fluorosis begin to become obvious on the enamel surface as opacities, implying some porosity in the tissue. The mechanisms that conduct the formation of fluorotic enamel are unknown, but should involve modifications in the basic physical-chemistry reactions of demineralization and remineralisation of the enamel of the teeth, which is the same reaction of formation of the enamel's hydroxyapatite (HAp) in the maturation phase. The increase of the amount of fluoride inside of the apatite will result in gradual increase of the lattice parameters. The aim of this work is to characterize the healthy and fluorotic enamel in human tooth using Synchrotron X-ray diffraction. All the scattering profile measurements were carried out at the X-ray diffraction beamline (XRD1) at the Brazilian Synchrotron Light Laboratory—LNLS, Campinas, Brazil. X-ray diffraction experiments were performed both in powder samples and polished surfaces. The powder samples were analyzed to obtain the characterization of a typical healthy enamel pattern. The polished surfaces were analyzed in specific areas that have been identified as fluorotic ones. X-ray diffraction data were obtained for all samples and these data were compared with the control samples and also with the literature data.     

MOF/CC-derivatives with trace amount of cobalt oxides as efficient electrocatalysts for oxygen reduction reaction

Two MOF/CC-derivatives with trace amount of cobalt oxides exhibit excellent electrocatalytic activity for oxygen reduction reaction.     

Spatially Distributed Current Oscillations with Electrochemical Reactions in Microfluidic Flow Cells

The formation of spatiotemporal patterns is investigated by using a chemical reaction on the surface of a high‐aspect‐ratio metal electrode positioned in a flow channel. A partial differential equation model is formulated for nickel dissolution in sulfuric acid in a microfluidic flow channel. The model simulations predict oscillatory patterns that are spatially distributed on the electrode surface; the downstream portion of the metal surface exhibits large‐amplitude, nonlinear oscillations of dissolution rates, whereas the upstream portion displays small‐amplitude, harmonic oscillations with a phase delay. The features of the dynamical response can be interpreted by the dependence of local dynamics on the widely varying surface conditions and the presence of strong coupling. The patterns can be observed for both contiguous and segmented metal surfaces. The existence of spatially distributed current oscillations is confirmed in experiments with Ni electrodissolution in a microfluidic device. The results show the impact of a widely heterogeneous environment on the types of patterns of chemical reaction rates.     

Stabilization and acidic dissolution mechanism of single-crystalline ZnO(0001) surfaces in electrolytes studied by in-situ AFM imaging and ex-situ LEED

A combined approach of pH-dependent in-situ AFM topography and ex-situ LEED studies of the stability and dissolution of single-crystalline ZnO(0001)-Zn surfaces in aqueous media is presented. Hydroxide-stabilized and single-crystalline ZnO(0001)-Zn surfaces turned out to be stable within a wide pH range between 11 and 4 around the point of zero charge of pH PZC = 8.7 +/- 0.2. Hydroxide stabilization turned out to be a very effective stabilization mechanism for polar oxide surfaces in electrolyte solutions. The dissolution of the oxide surface started at an acidic pH level of 5.5 and occurred selectively at the pre-existing step edges, which consist of nonpolar surfaces. In comparison, the oxide dissolution along the ZnO(0001) direction proved to be effectively inhibited above a pH value of 3.8. On the basis of these microscopic observations, the mechanistic understanding of the acidic dissolution process of ZnO could be supported. Moreover, both the in-situ AFM and the ex-situ LEED studies showed that the stabilization mechanism of the ZnO(0001) surfaces changes in acidic electrolytes. At pH values below 3.8, the hydroxide-stabilized surface is destabilized by dissolution of the well-ordered radical3. radical3. R30 hydroxide ad-layer as proven by LEED. Restabilization occurs and leads to the formation of triangular nanoterraces with a specific edge termination. However, below pH 4 the surface structure of the crystal itself is ill-defined on the macroscopic scale because preferable etching along crystal defects as dislocations into the bulk oxide results in very deep hexagonal etching pits.     

Anisotropic demineralization and oriented assembly of hydroxyapatite crystals in enamel: smart structures of biominerals

It is interesting to note that the demineralization of natural enamel does not happen as readily as that of the synthesized hydroxyapatite (HAP), although they share a similar chemical composition. We suggest that the hierarchical structure of enamel is an important factor in the preservation of the natural material against dissolution. The anisotropic demineralization of HAP is revealed experimentally, and this phenomenon is understood by the different interfacial structures of HAP-water at the atomic level. It is found that HAP {001} facets can be more resistant against dissolution than {100} under acidic conditions. Although {100} is the largest surface of the typical HAP crystal, it is {001}, the smallest habit face, that is chosen by the living organisms to build the outer surface of enamel by an oriented assembly of the rodlike crystals. We reveal that such a biological construction can confer on enamel protections against erosion, since {001} is relatively dissolution-insensitive. Thus, the spontaneous dissolution of enamel surface can be retarded in biological milieu by such a smart construction. The current study demonstrates the importance of hierarchical structures in the functional biomaterials.     

Self-organization of rotaxane thin films into spatially correlated nanostructures: morphological and structural aspects

The self-organization of rotaxane thin films into spatially correlated nanostructures is shown to occur upon a thermal stimulus. The mechanism of formation of nanostructures and their organization has been investigated using atomic force microscopy, bright field transmission electron microscopy, selected area electron diffraction, and molecular mechanics simulations. The evolution of the nanostructures follows a complex pathway, where a rotaxane thin film first dewets from the substrate to form nanosized droplets. Droplets coalesce by ripening, generating spatially correlated motifs. In a later stage, the larger droplets change shape, nucleate, and coalesce to yield crystallites that grow into larger crystals by incorporating the surrounding droplets. The results show the following: (i) the nanostructures represent a metastable state of a crystallization process; (ii) spatial correlations emerge during ripening, but they are destroyed as stable nuclei are formed and crystallization proceeds to completion; iii) crystallization, either on graphite or amorphous carbon films, leads to a precise basal plane, viz. (010), which has minimum surface energy. The inherent degrees of freedom permitted in the rotaxane architecture favors the re-organization and nucleation of the film in the solid state. Low-energy trajectories leading to crystallites with stable surfaces and minimum energy contact plane are found to occur via concerted, small amplitude, internal motions without disruption of packing and intermolecular contacts.     

Water-catalyzed hydrolysis of the radical cation of ketene in the gas phase: theory and experiment

Both theoretical and experimental investigations are reported for the gas-phase hydrolysis of the radical cation of ketene, H(2)CCO(*+). Density functional theory (DFT) with the B3LYP/6-311++G(d,p) method indicates that a second water molecule is required as a catalyst for the addition of water across the C=O bond in H(2)CCO(*+) by eliminating the activation barrier for the conversion of [H(2)CCO.H(2)O](*+) to [H(2)CC(OH)(2)](*+). Theory further indicates that [H(2)CC(OH)(2).H(2)O](*+) may recombine with electrons to produce neutral acetic acid. Experimental results of flow-reactor tandem mass spectrometer experiments in which CH(2)CO(*+) ions were produced either directly from ketene by electron transfer or by the chemical reaction of CH(2)(*+) with CO are consistent with formation of an (C(2),H(4),O(2))(*+) ion in a reaction second-order in H(2)O. Furthermore, comparative multi-CID experiments indicate that this ion is likely to be the enolic CH(2)C(OH)(2)(*+) cation. The results suggest a possible mechanism for the formation of acetic acid from ketene and water on icy surfaces in hot cores and interstellar clouds.