Surface Reaction Products Research Articles

Mossbauer studies of stannous fluoride reactivity with synthetic tooth enamel: A model for the tooth cavity protection actions of novel dentifrices

SnF2 is an important toothpaste ingredient, added for the provision of clinical efficacy for hard and soft tissue diseases and in breath protection. Synthetic calcium hydroxyapatite powders were exposed to liquid supernates (25 w/w% toothpaste water slurries, centrifuged) of Crest Gum Care® (SnF2) dentifrice. One-minute treatments were followed by 3x water washing, centrifugation and lyophilization. Post treatment, powders were analyzed by Mossbauer spectroscopy with 0.5–1 gram of treated apatite powder. Results show that tooth mineral stannous fluoride interactions include: (1) formation of surface reaction products with both Sn(II) and Sn(IV) oxidation states; (2) Sn-F binding on mineral surfaces with no evidence of SnO. The surface binding is, however, not pure Sn-F but contains contributions of other ligands, probably oxygens from surface phosphates or hydroxyl groups. Results also suggest that surface reacted stannous tin is oxidized with time, even when bound as a layer on the tooth surface. This study demonstrates for the first time the presence of Sn-F on tooth enamel post treatment and the contribution of passivation to long term stannous chemistry on tooth surfaces. The study also illustrates the practical applications of the Mossbauer technique.

Propan-2-ol on Ni(1 1 1): identification of surface intermediates and reaction products

The adsorption and reaction of propan-2-ol on Ni(1 1 1) has been followed by reflection absorption IR spectroscopy and temperature programmed desorption. At 110 K, nondissociative molecular adsorption is observed in the monolayer, with randomly oriented multilayers observed at higher exposures. On increasing substrate temperature to 200 K, scission of the OH bond is observed with formation of a 2-propoxide surface species which is adsorbed with C s site symmetry and oriented upright with the metal–O–C held close to a 180° angle. The alkoxide species is stable to 320 K, above which scission of the α-CH bond occurs, with simultaneous formation and desorption of acetone. This selective dehydrogenation to acetone is the majority reaction pathway on the surface and is critically controlled by the high barrier to α-CH bond activation which ensures remarkable stability for the 2-propoxide intermediate. As a result, selective dehydrogenation occurs at a sufficiently high enough temperature so that acetone desorption competes very effectively with unselective decomposition to CO, H and C x H y . Acetone is, therefore, evolved in a reaction-limited process at 340 K, while the minority non-selective decomposition pathway evolves H 2 and CO in desorption-limited processes. The ease of bond breaking (O–H>α-CH>α-CC) identified for C 1 and C 2 alcohols on Ni(1 1 1) seems also to be valid for C 3 alcohol chemistry on this surface.

Spectroscopic diagnostics of organic chemistry in the protostellar environment

A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.

Temperature programmed desorption using time-of-flight mass spectrometry

A new method of thermal programmed desorption (TPD) using time-of-flight mass spectrometry (TOFMS) is developed for simultaneous detection, in wide mass ranges, of large molecules and their surface reaction products. It provides a three-dimensional (3D) TPD spectrum that contains a complete set of conventional two-dimensional (2D) TPD spectra of all the masses in the mass range of interest. As an example, we present a 3D TPD spectrum of (methylcyclopentadienyl)Ir(1,5-cyclooctadiene), molecular weight 379.53 AMU, on a rhodium surface. The 3D TPD spectrum comprises TOF mass spectra, from 1 to 385 AMU, accumulating 10 000 scans every 0.6 s. By covering the whole mass range of interest in one TPD experiment, the new TOFMS-TPD eliminates guesswork required in TPD using quadrupole mass spectrometry that limits the number of masses to follow.

Wood-Based Activated Carbons as Adsorbents of Hydrogen Sulfide: A Study of Adsorption and Water Regeneration Processes

Wood-based carbon was studied as a hydrogen sulfide adsorbent in three adsorption/regeneration cycles. The regeneration was done using a constant amount of either hot or cold water. The performance of carbon and efficiency of its regeneration were evaluated on the basis of the amount of hydrogen sulfide adsorbed and on the selectivity of the carbon surface for oxidation of H2S to sulfur oxides. The latter, as the products of surface reactions, made the regeneration feasible. To check the effect of aging/surface oxidation the tests were carried out on fresh and 3-year-stored carbon samples. The results obtained showed that the surface of fresh carbon can be regenerated to some extent, whereas in the case of aged materials the water regeneration is inefficient due to the deposition bulky sulfur polymers resistant to oxidation at temperatures up to 100 °C. Both hot and cold water treatment results in similar degree of regeneration of the carbon surface.

Growth of aluminium nitride on porous silica by atomic layer chemical vapour deposition

Aluminium nitride (AlN) was grown on porous silica by atomic layer chemical vapour deposition (ALCVD) from trimethylaluminium (TMA) and ammonia precursors. The ALCVD growth is based on alternating, separated, saturating reactions of the gaseous precursors with the solid substrate. TMA and ammonia were reacted at 423 and 623 K, respectively, on silica which had been dehydroxylated at 1023 K and pretreated with ammonia at 823 K. The growth in three reaction cycles was investigated quantitatively by elemental analysis, and the surface reaction products were identified by IR and solid state 27Al and 29Si NMR measurements. Steady growth of about 2 aluminium and 2 nitrogen atoms/nm silica 2/reaction cycle was obtained. The growth mainly took place through (i) the reaction of TMA which resulted in surface Al–Me and Si–Me groups, and (ii) the reaction of ammonia which replaced the aluminium-bonded methyl groups with amino groups. Ammonia also reacted in part with the silicon-bonded methyl groups formed in the dissociative reaction of TMA with siloxane bridges. TMA reacted with the amino groups, as it did with surface silanol groups and siloxane bridges. In general, the Al–N layer interacted strongly with the silica substrate, but in the third reaction cycle AlN-type sites may have formed.

The use of ternary phase diagrams in the study of high temperature corrosion products formed on Fe–5 wt% Al alloys in reducing and oxidizing environments

Ternary phase diagrams, in conjunction with microscopy techniques and reaction product chemistries, were used to describe the possible “diffusion paths” and resulting morphologies that may occur during formation of corrosion scales from high temperature gaseous exposure. Iron with 5 wt% of aluminum was exposed to reducing and oxidizing environments at 700°C. Characterization of the surface reaction products was conducted using microscopy techniques with energy dispersive spectroscopy and electron probe microanalysis. Under both conditions, iron diffused outward to form surface reaction products, either iron sulfide or iron oxide. The ingress of sulfur or oxygen through the previously formed reaction products was found to produce internal corrosion phases within the alloy. By plotting chemical information acquired from the corrosion scales on ternary phase diagrams, development of the phase layer sequence and morphologies of the multiphase corrosion scales was schematically explained. This analysis was conducted using conventions summarized by Clarke ( Trans. Am. Inst. Min. Engrs, 1963, 227, 1250) for plotting diffusion paths in multiphase ternary diffusion studies from solid-state reactions. By presenting the experimental data in this manner, the morphological development of the scales was related to the composition path on the ternary phase diagrams.

On the Abundance Gradients of Organic Molecules along the TMC‐1 Ridge

Gradients in molecular abundances along the TMC-1 ridge have been observed by several authors, most recently in a comprehensive study by Pratap et al. These can be explained by there being a difference in density, C/O ratio, or chemical evolutionary state along the ridge. The presence at the carbon-rich cyanopolyyne peak (CP) of specific oxygen-bearing molecules, believed to be products of grain surface reactions, leads us to investigate the possibility that the gradients are produced following the removal of icy grain mantles. We identify the mantle removal mechanism as the explosive desorption of photolyzed ices, induced by MHD waves as they propagate within the cloud. By identifying the protostellar object IRAS 04381+2540 as the source of these waves, we have constructed a dynamical-chemical model for the evolution of the TMC-1 ridge. The results of detailed chemical kinetic calculations are presented. We find that injection into the gas phase of the carbon-bearing species C2H2, C2H4 and CH4 can transiently enhance the production of many organic molecules, particularly the cyanopolyynes and polyacetylenes, and hence that the observed gradients can indeed be produced in this model. We suggest that other mantle-driven reaction pathways, involving formation of methylated chains (e.g., CH3C3N) by gas phase methyl cation transfer, as well as formation of carbenes and suboxides by fragmentation of surface-formed cumulenone compounds, might explain the presence of many other organic molecules in TMC-1. In our view the current chemical state of TMC-1 reflects a transient phase that accompanies the earliest epochs of low-mass formation and that regions rich in organic molecules should be observable around known Class 0 protostellar sources and be offset from them by about an ion-neutral damping length. Molecular clumps in dense cores should also show structure on this scale-length.

XPS study of reductive dissolution of birnessite by H2SeO3with constraints on reaction mechanism

Reductive dissolution of 7 A bimessite [Mi2+0.05Mn3+0.25Mn4+0.7O1.7(OH)0.25] by selenious acid (H2SeO3) produces Mn3+ and Mn2+ surface reaction products (here represented as S-MnOOH and S-MnO. respectively) and Mn2+,3+-selenite surface complexes at the solution-mineral interface. Mn2p3/2, Se3d. and O1s X-ray photoelectron spectra of reacted surfaces reveal that Mn4+ of birnessite is reduced simultaneously to Mn3+ and Mn2+ while Se6+ is oxidized to Se4+ according to the probable stoichiometric reactions:

Chemical Reactions of Organic Molecules Adsorbed at Ice: 2. Chloride Substitution in 2-Methyl-2-propanol

A new reaction is reported on the ice surface, namely the conversion of adsorbed 2-methyl-2-propanol [(CH3)3COH] to adsorbed 2-chloro-2-methylpropane [(CH3)3CCl] in the presence of absorbed HCl. The reaction was studied using temperature-programmed desorption mass spectrometry on ultrathin ice films under ultrahigh vacuum. Formation of (CH3)3CCl occurs at two temperatures: at or below 155 K and at 185 K. The lower-temperature process occurs at the surface of the film, but the origin of product at 185 K is unclear. The yield of the surface reaction product is low, approximately 0.04 monolayers. Importantly, adsorbed HCl is unreactive toward 2-methyl-2-propanol; reactive chlorine is derived from a dissociatively ionized state in the very near surface region of the film. The lower-temperature pathway for chloride substitution is observed only at the surface of an amorphous film, an observation that may have implications regarding the nature of surface reaction sites. Possible mechanisms of the surface react...

Cycloaddition Reactions of 1,3-Cyclohexadiene on the Silicon(001) Surface

[2+2] and [4+2] cycloaddition reactions of 1,3-cyclohexadiene on the Si(001) surface were studied. It is shown that not only the [4+2] cycloaddition reaction but also the [2+2] cycloadditions can occur on the Si(001) surface. Surface isomerization reactions connecting [4+2] and [2+2] are very unlikely due to a high energy barrier, implying that the surface reactions are kinetically controlled. Therefore the final surface reaction products are determined during the initial stage of the reactions in contrast with earlier assumptions that the “product distribution is thermodynamically determined”. Our interpretations are consistent with the new experimental results by the Hamers group. According to our CASSCF(6,6) calculations, the nonsymmetric π-complex transition states along the [2+2] cycloaddition mechanism, which has been suggested by many theoretical studies, seem to be an artifact. Nevertheless, the very soft Si dimer buckling motion of the Si(001) surface certainly facilitates the [2+2] reaction.

Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces

The interaction of water with two transition metal carbides, titanium carbide (TiC) and vanadium carbide (VC), has been investigated. The adsorption, reaction, and desorption of water on the (100) face of single-crystal samples of these materials have been studied as a function of substrate temperature over the range 100−600 K. The adsorption state of water on these surfaces has been probed with high resolution electron energy loss spectroscopy (HREELS). The reactivity of water has been directly measured with HREELS and X-ray photoelectron spectroscopy (XPS). The desorption of molecular water and the products of surface reactions has been followed with temperature programmed desorption. Collectively, these measurements indicate that water adsorbs both molecularly and dissociatively on TiC and VC; however, a greater degree of reactivity at cryogenic temperatures is observed on TiC. Dissociation of water produces surface bound hydrogen and hydroxyl groups on both surfaces and a fully dissociated surface oxide on TiC. Furthermore, a greater participation of the surface carbon atoms is observed at the TiC surface through the evolution of COx species at elevated temperatures. The differences in surface bonding and desorption profiles are discussed in terms of differences in electronic structure of the two metal carbides. Some possible implications of these studies for the use of TiC and VC as tribological materials are also discussed.

The attachment of the flame sheet to the carbon surface in a carbon combustion model: on the combustion rate

The behavior of the combustion rate of carbon, when the flame sheet attaches to or collapses at the carbon surface, is studied in a model in which the heterogeneous reactions 2C (s) + O 2 → 2CO [reaction (I)] and C (s) + CO 2 → 2CO [(IV)] and the homogeneous reaction 2CO + O 2 → 2CO 2 [(III)] occur in a stagnation flow of air. In this model the surface mass fraction of CO 2 increases with an increase in the combustion rate. In a restricted case in which the heterogeneous reaction is solely (IV), CO 2 is on the one hand the product of an overall surface reaction [reaction (III) plus (IV)] but on the other hand the reactant of reaction (IV) which determines the rate of this overall reaction. This causes peculiar variations of the combustion rate: (1) the combustion rate vanishes at a nonvanishing D s(IV) ( D s( n) is the surface Damköhler number of the nth reaction); (2) the combustion rate varies dramatically with D s(IV) in the D s(IV) range in which the combustion rate is nonvanishing. When reaction (I) occurs at all, CO 2 is made from CO by reaction (III); therefore, reaction (IV) occurs even in the D s(IV) range in which this reaction never occurs by itself. When D s(IV) is kept constant and D s(I) is increased, the combustion rate and the contribution of the gasification of carbon by reaction (I) naturally increase, but, because of an increase in the surface mass fraction of CO 2 with increasing combustion rate, the contribution by reaction (IV) also increases.

The role of adsorption heats and bond energies in the assignment of surface reaction products: ethyne and ethene on Ni{110}

In order to understand the complex dissociation processes that occur on adsorption of hydrocarbons on surfaces, it is necessary to understand the energetics involved. From investigations of the adsorption of various hydrocarbon species on surfaces, it has been possible to calculate M–C bond energies. These could then be used to calculate the expected adsorption heats for various possible adsorbed species on surfaces for the assignment of adsorbate states from measured heats in conjunction with spectroscopic data. Heats of adsorption and sticking probabilities for C 2H 2 and C 2H 4 on Ni{110} at 300 K have been measured. The initial sticking probability and heat of adsorption for C 2H 2 are 0.8 and 190 kJ mol −1, respectively, while those for C 2H 4 are 0.78 and 120 kJ mol −1. In both cases, CCH species are formed on the surface initially, and for the adsorption of C 2H 2, CH 2 and CH are formed on the surface at higher exposures. Assuming the presence of these species on the surface, a value of the Ni–C bond strength of 191 kJ mol −1 is found. This is in excellent agreement with the average value of 204 kJ mol −1 calculated for hydrocarbon adsorption on the Ni{100} surface previously.

Adsorption and coadsorption of water and glycine on TiO2.

Adsorption of water, ions, and biomolecules constitutes the first events occurring at biomaterial-biosystem interfaces. In this work, the adsorption and coadsorption of water and glycine on TiO2 were studied by thermal desorption spectroscopy (TDS). The first water monolayer desorbs in three peaks around 180K, 300K, and 400K, which are assigned to water molecularly adsorbed at oxygen sites, at Ti4+ sites, and to recombination of dissociated water, respectively. A fourth desorption peak (160K), appearing at coverages > 0.8 monolayer, is attributed to water clusters and multilayers. The water-TiO2 interaction is changed if the surface is annealed in vacuum, which leads to increased hydroxylation. Desorption spectra from glycine overlayers evaporated on TiO2 in situ show that around 40% of the first monolayer desorbs as intact molecules ( approximately 300-450 K) and the remainder as dissociation fragments and surface reaction products around 600 K. At coverages > 0.6 monolayers, intact molecules desorbing from cluster multilayers at 310 K are detected. The glycine desorption spectra are unaffected by coadsorbed water. In contrast, coadsorption of glycine displaces water from more strongly bound states in the monolayer to more weakly bound states and clusters, making the surface more hydrophobic. The study shows that TDS is a powerful method for characterizing biomaterial surfaces with regard to their interaction with biologically relevant molecules.

Mass Spectrometric Analysis of Reaction Products of Fast Oxygen Atoms-Material Interactions

To study the reaction products of atomic oxygen impingement on spacecraft external materials, a quadrupole mass spectrometer coupled to transfer ion optics has been integrated into a space simulation chamber, equipped with a laser-assisted source of fast oxygen atoms. The mass spectrometer is used in a time-of-e ight mode, which makes it possible to separate the effects of uv ‐vuv photons, oxygen ions, and oxygen atoms, all of which are present simultaneously in the pulsed neutral beam. Neutral surface reaction products are ionized either by charge transfer or by nonresonant laser multiphoton ionization. Initial studies on three polymer samples evidence a high analytical sensitivity, making it possible to detect and to differentiate the reaction products. Comparison between mass spectra data and direct mass loss measurements on several polymeric materials shows the semiquantitative aspect of the analysis technique that has been developed.

Condensed-Phase Products in Heterogeneous Reactions: N2O5, ClONO2, and HNO3 Reacting on Ice Films at 185 K

Heterogeneous reactions are important in a wide variety of chemical processes. In many cases reactions on a surface will change both the physical and chemical characteristics of the surface, which in turn will change the surface reactivity toward further gas/surface collisions. As a case study of relevance to the atmosphere, we have investigated the reactions of the NOy species ClONO2, N2O5, and HNO3 on thin ice films representative of water-ice polar stratospheric clouds (type II PSCs). Although these species are known to produce HNO3 upon reacting with the ice surface, the phase, composition, and state of adsorption (physical versus chemical) of the surface reaction product are not known. These reactions were studied using a Knudsen cell reactor to probe heterogeneous reaction rates, mass spectrometry to identify gas-phase reactants and products, and FTIR reflection−absorption spectroscopy to probe the phase and composition of the condensed phase. Under ice frost point conditions at 185 K, each NOy species reacted with ice to form a metastable supercooled H2O/HNO3 liquid layer. Although a crystalline 3:1 H2O:HNO3 hydrate is most thermodynamically stable under these conditions, a supercooled liquid with a composition slightly more dilute than 3:1 H2O:HNO3 continued to grow throughout the NOy exposure period. This product composition is similar to that expected for liquid type Ib PSCs in the atmosphere. ClONO2 and N2O5 reacted with the supercooled H2O/HNO3 liquid layer at 185 K with a reactive uptake coefficient of γ = 0.003 ± 0.002 and γ = 0.0007 ± 0.0003, respectively. These measured rate coefficients are about 2 orders of magnitude lower than the corresponding reaction rates on pure ice but are comparable to those measured on crystalline nitric acid trihydrate (NAT) or nitric acid dihydrate (NAD) surfaces representative of type Ia PSCs. HNO3 reacted with the supercooled liquid layer with γ > 0.02. When H2O vapor pressures were decreased to below the ice frost point, the supercooled H2O/HNO3 liquid layer became more concentrated in HNO3 as H2O preferentially desorbed. Only during desorption when stoichiometric ratios of 3:1 or 2:1 H2O:HNO3 were obtained did the supercooled liquid layer crystallize to NAT or NAD, respectively. These results suggest that water-ice particles in the polar stratosphere may be initially coated with a supercooled H2O/HNO3 liquid layer and that heterogeneous nucleation of NAT on ice from either the gas phase or the H2O/HNO3 supercooled liquid phase is slow. The implications of a supercooled H2O/HNO3 liquid layer on ice will be discussed in the context of polar ozone depletion.

Decomposition of P(CH 3) 3 on Ru(0001): comparison with PH 3 and PCl 3

The decomposition of P(CH 3) 3 adsorbed on Ru(0001) at 80 K is studied by soft X-ray photoelectron spectroscopy using synchrotron radiation. Using the chemical shifts in the P 2p core levels, we are able to identify various phosphorus-containing surface reaction products and follow their reactions on Ru(0001). It is found that P(CH 3) 3 undergoes a step-wise demethylation on Ru(0001), P(CH 3) 3 → P(CH 3) 2 → P(CH 3) → P, which is complete around ∼450 K. These results are compared with the decomposition of isostructural PH 3 and PCl 3 on Ru(0001). The decomposition of PH 3 involves a stable intermediate, labeled as PH x , and follows a reaction of: PH 3 → PH x → P, which is complete around ∼190 K. The conversion of chemisorbed phosphorus to ruthenium phosphide is observed and is complete around ∼700 K on Ru(0001). PCl 3 also follows a step-wise decomposition reaction, PCl 3 → PCl 2 → PCl → P, which is complete around ∼300 K. The energetics of the adsorption and the step-wise decomposition reactions of PH 3, PCl 3 and P(CH 3) 3 are estimated using the bond order conservation Morse potential (BOCMP) method. The energetics calculated using the BOCMP method agree qualitatively with the experimental data.

Formation of surface reaction products on bioactive glass and their effects on the expression of the osteoblastic phenotype and the deposition of mineralized extracellular matrix

The objective of the study was to examine the effect of alkali ion release, pH control and buffer capacity on the expression of the osteoblastic phenotype. In addition, we determined the importance of modifications of the surface of porous bioactive glass (BG) on the activity of rat calvaria osteoblasts in vitro. We found that at a low tissue culture medium (TCM) volume to BG surface area (Vol/SA) ratio, the products of glass corrosion elevated the pH of the TCM to a value that adversely affected cellular activity; thus, the matrix synthesized by the cells was non-mineralized. On the other hand, when the Vol/SA was high and the buffer capacity of the medium was not exceeded, the cells generated a mineralized extracellular matrix. Addressing the second issue, we observed that modification of the composition of the BG surface markedly influenced osteoblast activity. BG that was coated with either a calcium phosphate-rich layer only or a serum protein layer changed the phenotypic characteristics of the osteoblasts. The presence of either of these surfaces lowered the alkaline phosphatase activity of the attached cells; this finding indicated that the osteoblast phenotype was not conserved. However, when the BG was coated with a bilayer of calcium phosphate and serum proteins, the alkaline phosphatase (AP) activity was elevated and the extracellular matrix contained characteristic bone markers. Our findings indicate that the calcium phosphate-rich layer promotes adsorption and concentration of proteins from the TCM, and it is utilized by the osteoblasts to form the mineralized extracellular matrix.

Identification of a surface reaction product by detection of a vibrational combination band

NCO molecules formed by the reaction of adsorbed CO and nitrogen atoms are unambiguously identified by means of infrared absorption spectroscopy. This identification of a surface reaction product is achieved by the detection of the combination band of two intramolecular modes. Isotopic substitution experiments using 12C 18O and 13C 16O are used to confirm the mode assignments through the unequal shifts of the various vibrational modes. The intermode anharmonicity is accurately determined to be 25.5–26 cm −1 which establishes the first reliable number of this type for an adsorbate species.