Bulk OH Research Articles

Study of Al2O3 thin films by ALD using H2O and O3 as oxygen source for waveguide applications

Atomic Layer Deposition (ALD) is an advantageous technique to obtain alumina (Al2O3) optical waveguides based on the trimethylaluminum-water (TMA-H2O) process. However, the influence of ozone (O3) as oxygen source on pure Al2O3 ALD waveguides has not been studied yet. In this work, the optical design, fabrication and characterization of slab optical waveguides based on Al2O3 thin films using water (Al2O3–H2O) or ozone (Al2O3–O3) as oxygen source is presented. Ellipsometry measurements show a refractive index of 1.64 for Al2O3–H2O and of 1.65 for Al2O3–O3 waveguides. The intensity distribution at the waveguides output demonstrates propagation of visible light (λ = 632.8 nm) for both type of waveguides. Optical propagation losses are smaller for O3 than for H2O waveguides, the lowest value for Al2O3–O3 is 0.51 dB/cm and 3.03 dB/cm for Al2O3–H2O. Fourier transform infrared spectroscopy (FTIR) analysis shows a prominent band for bulk OH group in water oxidized samples, which may be associated with the increase of propagation losses. X-ray photoelectron spectroscopy (XPS) show no differences in chemical composition (supplementary material).

A biochar modified nickel-foam cathode with iron-foam catalyst in electro-Fenton for sulfamerazine degradation

A nickel-foam cathode modified by a self-nitrogen-doped biochar derived from waste giant reed was synthesized. The fabricated cathode (B@Ni-F) proved to be with high oxygen reaction reactive (ORR) reactivity and H2O2 selectivity (70.41%) owing to the enrichment of oxygen functional groups and pyridinic N when low-temperature pyrolyzed biochar was incorporated. The charge transfer resistance of B@Ni-F decreased to 7.18 Ω, which was 95.7 Ω for the original nickel-foam, proving by electrochemical impedance spectroscopy (EIS). Expectedly, Its H2O2 accumulation improved 14 times, thus making it comparable with commonly used electrodes like carbon cloth and graphite plate. Subsequently, B@Ni-F cathode and iron-foam (Fe-F) catalyst were firstly used in the electro-Fenton (EF) process for sulfamerazine (SMR) degradation. Double-functional polyphosphate electrolytes including tetrapolyphosphate (4-TPP), tripolyphosphate (3-TPP), pyrophosphate (PP) and Na3PO4 were compared with the conventional Na2SO4 electrolyte in EF for SMR degradation. The absolute rate constant for oxidation of SMR by OH was determined to be (3.4 ± 0.09) × 109 M−1 s−1. SMR degradation enhancement in the presence of polyphosphate-based electrolytes is associated with bulk OH generation from Fe2+- polyphosphate ligand complexes via O2 activation. The Fe2+-3-TPP complexes have relatively higher oxidation ability compared to Fe2+-PP, Fe2+-PO4 species. A plausible SMR oxidation pathway is proposed based on the by-products detected by UPLC-MS/MS and density functional theory (DFT) calculations. The dominant SMR degradation pathway was hydroxylation of aniline residue of SMR, followed with the cleavage of SN and then breakage of aromatic rings.

Bulk Hydroxylation and Effective Water Splitting by Highly Reduced Cerium Oxide: The Role of O Vacancy Coordination

Reactions of reduced cerium oxide CeOx with water are fundamental processes omnipresent in ceria-based catalysis. Using thin epitaxial films of ordered CeOx, we investigate the influence of oxygen vacancy concentration and coordination on the oxidation of CeOx by water. Upon changing the CeOx stoichiometry from CeO2 to Ce2O3, we observe a transition from a slow surface reaction to a productive H2-evolving CeOx oxidation with reaction yields exceeding the surface capacity and indicating the participation of bulk OH species. Both the experiments and the ab initio calculations associate the effective oxidation of highly reduced CeOx by water to the next-nearest-neighbor oxygen vacancies present in the bixbyite c-Ce2O3 phase. Next-nearest-neighbor oxygen vacancies allow for the effective incorporation of water in the bulk via formation of OH– groups. Our study illustrates that the coordination of oxygen vacancies in CeOx represents an important parameter to be considered in understanding and improving the reactivity of ceria-based catalysts.

Distinguishing Surface versus Bulk Hydroxyl Groups of Cellulose Nanocrystals Using Vibrational Sum Frequency Generation Spectroscopy.

In plant cell walls and cellulose-containing composites, nanocrystalline cellulose interacts with water molecules or matrix polymers through hydrogen bonding of the hydroxyl groups at the cellulose surface. These interactions play key roles in cellulose assembly in plant cell walls and mechanical properties of cellulose composites; however, they could not be studied properly due to the spectroscopic difficulty of selectively detecting the surface hydroxyl groups of nanocrystalline domains. This study employed the sum frequency scattering principle to distinguish the hydroxyl groups inside of the crystalline nanodomain of cellulose and those exposed at the surface of crystalline domains. The comparison of the spectra at various scattering angles revealed that the OH peak near ∼3450 cm-1 comes from the weakly hydrogen-bonded OH groups at the surface of crystalline cellulose. Also, a time delay measurement found that the sharp vibrational features observed near 3700 cm-1 can be attributed to isolated OH groups not accessible by ambient water molecules. These findings allow the distinction of surface versus bulk OH groups in sum frequency generation vibrational spectroscopy.

CO2 Reduction on the Pre-reduced Mixed Ionic-Electronic Conducting Perovskites La0.6 Sr-0.4 FeO3-δ and SrTi0.7 Fe0.3 O3-δ.

The activity of the pre-reduced perovskites La0.6 Sr0.4 FeO3-δ (LSF64) and SrTi0.7 Fe0.3 O3-δ (STF73) for the CO2 reduction to CO was investigated with special focus on the reactivity of oxide-dissolved hydrogen. This is of particular interest in hydrogen solid-oxide electrolysis cell (H-SOEC) technology, where proton-conducting ceramics are used and the reaction 2e- +2H+ +CO2 →CO+H2 O is of central importance. To clarify if hydrogen dissolved in LSF64 and STF73 partakes in the CO2 reduction, temperature-programmed reduction (TPR) in H2 , followed by temperature-programmed reoxidation (TPO) in CO2 and, moreover, temperature-programmed desorption (TPD) of ad- and absorbed species were utilized. The experiments reveal that 50 mol % of the CO2 is converted by hydrogen dissolved in STF73 and reacts quantitatively. On the other hand, LSF64 converts less than 20 mol % of CO2 via dissolved hydrogen and a residual of bulk OH is still detectable after CO2 -TPO.

Surface and Bulk Thermal Dehydroxylation of FeOOH Polymorphs.

In this study, bulk and surface thermal decomposition of synthetic iron oxyhydroxides to iron oxides was followed using the temperature-programmed desorption (TPD) technique. Submicron-sized akaganéite (β-FeOOH), rod- and lath-shaped lepidocrocite (γ-FeOOH), and goethite (α-FeOOH) particles were heated in vacuo in the 30-400 °C range, and their OH vibrational modes were monitored by Fourier transform infrared (FTIR) spectroscopy while H2O(g) release was monitored by quadrupole mass spectrometry. Peak thermal dehydroxylation temperatures were larger in the order of lath lepidocrocite (200 °C) < akaganéite (200/260 °C) < rod lepidocrocite (268 °C) < goethite (293 °C). Pre-equilibration of these particles to aqueous solutions of HCl increased dehydroxylation temperatures of all minerals except goethite by 13-40 °C. These shifts were explained by (1) the dissolution of particles or regions of particles of lower degree of crystallinity by HCl, as well as (2) the strengthening of the hydrogen bond environment in the akaganéite bulk. The latter is a means of facilitating H2O(g) formation via interactions between two adjacent OH groups. Strongly analogous forms of interactions at the FeOOH particle surfaces were also shown to facilitate the release of singly coordinated (-OH) hydroxo groups to the gas phase at temperatures lower than 125 °C, thus creating OH vacancies that may be actively involved in the transfer of bulk to surface OH groups during thermal dehydroxylation. Doubly- (μ-OH) and triply- (μ3-OH) coordinated hydroxo groups were however resilient to exchange under those conditions, and their dehydroxylation was strongly congruent with that of bulk OH groups. By resolving the bulk and surface thermal decomposition of FeOOH polymorphs, this work provides clearer insight into the fate of these materials in natural and technological settings where important thermal gradients are commonplace.

Molecular Understanding of the Bulk Composition of Crystalline Nonstoichiometric Hydroxyapatites: Application to the Rationalization of Structure–Reactivity Relationships

Crystalline hydroxyapatite samples (HAps) have been prepared by using the co‐precipitation method under various pH conditions, leading to nonstoichiometric solids (1.65 &lt; Ca/P &lt; 1.77). The aim of this study was to rationalize the sensitivity of the catalytic activity of HAps to their bulk compositions going from the macroscopic level expressed by the Ca/P ratio to the molecular level properties of the bulk. From DRIFT, 31P NMR and Raman characterizations, hydroxyapatites were obtained with a range of structural defects compared with the ideal stoichiometric compound. If the amount of HPO42– and B‐type carbonates directly impacts the Ca/P ratio, it is not the case for A‐type carbonates. All these defects, and especially the A‐type carbonates, participate in the modulation of OH content inside the channels. Irrespective of the Ca/P values, the OH concentration appears to be perfectly related to the surface basic reactivity measured through 2‐methyl‐3‐butyn‐2‐ol (MBOH) conversion. Thus, except for the similar carbonate content (in cases of low Ca/P values), the Ca/P ratio is not sufficient to predict the catalytic behavior of all HAps synthesized under various conditions: in the case of variable carbonate content monitored under different pH conditions, a larger range of Ca/P ratio can be obtained including over‐stoichiometric HAps samples (Ca/P &gt; 1.67), and the bulk OH concentration becomes a much better descriptor than the Ca/P ratio to account for the basic reactivity.

Synthesis and characterization of Ti(IV)-substituted calcium hydroxyapatite particles by forced hydrolysis of Ca(OH)2-Na5P3O10-TiCl4 mixed solution

Ti(IV)-substituted calcium hydroxyapatite (TiHap) particles were prepared by aging Ca(OH)2, TiCl4, and sodium triphosphate (sodium tripolyphosphate, Natpp: Na5P3O10) mixed solution at 100 °C for 18 h. The ellipsoidal secondary TiHap particles with ca. 100∼150 nm in length composing by aggregation of small ellipsoidal primary particles with ca. 20 nm in length were produced at atomic ratio of Ti/(Ca+Ti) [XTi]≦0.2. The in situ IR spectra of these TiHap particles exhibited very small bulk OH− band at 3,570 cm−1. This result indicated that the TiHap particles were formed by aggregation of fine primary particles and OH− ions along with c-axis in the primary particles were disordered. The TiHap particles with Ca/P atomic ratio larger than theoretical value of 1.67 did not exhibit surface P–OH groups at 3,659 and 3,682 cm−1. The diffuse reflectance UV spectra of TiHap particles revealed that these particles have a UV absorption property, especially fabricated at XTi = 0.1. The particles prepared at XTi = 0.6 and 0.8 were amorphous and nanoparticles with 5∼10 nm in diameter, but those precipitated at XTi = 1.0 were poorly crystallized anataze-type TiO2 nanoparticles.

Discrimination of infrared fingerprints of bulk and surface POH and OH of hydroxyapatites

The identification of the nature of the surface acid base pairs present on hydroxyapatites surface is a key point to rationalize their catalytic properties. To investigate the possible involvement of acidic protonated phosphate (POH) and/or of basic OH emerging from the channels, the spectroscopic fingerprints of the surface sites have to be discriminated from bulk HPO42− and OH− groups. This was monitored by infrared spectroscopy (i) using isotopic H–D exchanges, either implemented in mild conditions (353–373K) or at higher temperature (573K) to selectively label the lone surface or to achieve deep deuteration, respectively (ii) following then the perturbation induced upon adsorption probe molecules. Two bulk contributions are observed in the νOH region, one at 3572cm−1 that is associated to OH− from the channels, and one at 3657cm−1 that was assigned to HPO42−. Five surface contributions perturbated upon CO adsorption at 77K can be ascribed to protonated forms of the various terminated phosphates. The formation of hydrogenocarbonate species upon CO2 adsorption evidences the presence of basic surface OH−. However, due to complex material restructuring occurring upon CO2 adsorption that greatly modifies the H-bonding interactions, the related νOH fingerprint could not be formally identified. Whatever, from the formation of low intense band at 2633 or 3572cm−1 under soft deuteration or protonation conditions respectively, it is inferred that the related bands could be ascribed to both surface and bulk OD or OH.

Using pulsed wave ultrasound to evaluate the suitability of hydroxyl radical scavengers in sonochemical systems

Hydroxyl radical (OH) scavengers are commonly used in sonochemistry to probe the site and nature of reaction in aqueous cavitational systems. Using pulsed wave (PW) ultrasound with comparative sonochemistry we evaluated the performance of OH scavengers (i.e., formic acid, carbonic acid, terephthalic acid/terephthalate, iodide, methanesulfonate, benzenesulfonate, and acetic acid/acetate) in a sonochemical system to determine which OH scavengers react only in bulk solution and which OH scavengers interact with cavitation bubbles. The ability of each scavenger to interact with cavitation bubbles was assessed by comparing the pulse enhancement (PE) of 10μM of a probe compound, carbamazepine (CBZ), in the presence and absence of a scavenger. Based on PE results, acetic acid/acetate appears to scavenge OH in bulk solution, and not interact with cavitation bubbles. Methanesulfonate acts as reaction promoter, increasing rather than inhibiting the degradation of CBZ. For formic acid, carbonic acid, terephthalic acid/terephthalate, benzenesulfonate, and iodide, the PE was significantly decreased compared to in the absence of the scavenger. These scavengers not only quench OH in bulk solution but also affect the cavity interface. The robustness of acetic acid/acetate as a bulk OH scavenger was validated for pH values between 3.5 and 8.9 and acetic acid/acetate concentrations from 0.5 to 0.1M.

Comparative study of visible-light-driven photocatalytic mechanisms of dye decolorization and bacterial disinfection by B–Ni-codoped TiO2 microspheres: The role of different reactive species

The controversy of mechanism still exists over whether photocatalytic decontamination proceeds via photon-generated h+, e−, OH, O2− or H2O2. This study aims to investigate the roles of these reactive charges and oxidative species in the photocatalytic dye decolorization and bacterial disinfection processes in the presence of a visible-light-driven (VLD) photocatalyst, B–Ni-codoped TiO2 microsphere, by employing various scavengers in the photocatalytic system and utilizing a novel partition system. Significant differences between VLD photocatalytic dye decolorization and bacterial disinfection are found. For photocatalytic dye decolorization, the reaction mainly occurs on the photocatalyst surface with the aid of surface-bounded reactive species (h+, OHs and O2−), while bacterial cell can be inactivated by diffusing reactive oxidative species such as OHb and H2O2 without the direct contact with the photocatalyst. The diffusing H2O2 plays the most important role in the photocatalytic disinfection, which can be produced both by the coupling of OHb in bulk solution and OHs on the surface of photocatalyst at the valence band. Furthermore, the O2−, which is detected by using the electron spin resonance technique, is found to have direct function for the photocatalytic disinfection process. This study establishes a facile and versatile research methodology to investigate the VLD photocatalytic mechanism in different photocatalytic system.

Properties of atomic layer deposited HfO 2 thin films

The growth, composition and morphology of HfO 2 films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H 2O at 250° and ii) tetrakis dimethyl amino hafnium and H 2O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO 2 peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO 2 films. The index of refraction for all films is very close to that for bulk crystalline HfO 2.

The use of multiple probe molecules for the study of the acid–base properties of aluminium hydroxyfluoride having the hexagonal tungsten bronze structure: FTIR and [36Cl] radiotracer studies

The combination of several probe molecules has enabled the construction of a detailed picture of the surface of aluminium hydroxyl fluoride, AlF(2.6)(OH)(0.4), which has the hexagonal tungsten bronze (HTB) structure. Using pyridine as a probe leads to features at 1628 cm(-1), ascribed to very strong Lewis acid sites, and at 1620-1623 cm(-1), which is the result of several different types of Lewis sites. This heterogeneity is indicated also from CO adsorption at 100 K; the presence of five different types of Lewis site is deduced and is suggested to arise from the hydroxylated environment. Brønsted acid sites of medium strength are indicated by adsorption of lutidine and CO. Adsorption of lutidine occurs at OH groups, which are exposed at the surface and CO reveals that these OH groups have a single environment that can be correlated with their specific location inside the bulk, assuming that the surface OH group may reflect the bulk OH periodicity. A correlation between the data obtained from CO and pyridine molecules has been established using co-adsorption experiments, which also highlight the inductive effect produced by pyridine. Adsorption of the strong Brønsted acid, anhydrous hydrogen chloride, detected by monitoring the beta(-) emission of [(36)Cl]-HCl at the surface, indicates that surface hydroxyl groups can behave also as a Brønsted base and that H(2)O-HCl interactions, either within the hexagonal channels or at the surface are possible. Finally, the formation of strongly bound H(36)Cl as a result of the room temperature dehydrochlorination of [(36)Cl]-labelled tert-butyl chloride provides additional evidence that HTB-AlF(2.6)(OH)(0.4) can behave as a Lewis acid.

A vibrational study of the nature of hydroxyl groups chemical bonding in two aluminium hydroxides

Lengths, strengths and valences of OH bonds in the two aluminium hydroxides gibbsite and bayerite were determined on the basis of vibrational spectral data. The uncoupled OD stretching modes in the range 2400–2800 cm −1 were recorded by means of infrared diffuse reflectance, thereby avoiding effects of surface, vibrational coupling or particle shape. The assignment of the corresponding Raman spectra resulted in the determination of harmonic wavenumbers, force constants and anharmonicity coefficients of bulk OH groups in the two minerals. OH bond lengths deduced from these data varied from 0.964 Å to 0.975 Å in gibbsite and 0.962 Å to 0.973 Å in bayerite. These lengths appear to correspond to weak H-bonds contrary to previously recognized data from X-ray diffraction and neutron diffraction studies. Finally, bond valences were calculated on the basis of these new bond lengths and discussed as a function of crystallographic structures and the nature of hydrogen bonding in these two structures.

Indentation crack initiation behavior of vitreous silica glasses containing different hydroxyl concentration

The indentation crack initiation behavior of eight vitreous silica specimens containing bulk OH concentrations ranging from 0.2 to 754 wt ppm was investigated. A recording microindentation instrument equipped with optical observation and acoustic emission detection was used to study, in situ, the cracking behavior from indentation with a Vickers diamond. No significant differences in the threshold loads for various types of cracking behavior of the specimens were found. In addition, the polishing medium was found to have little influence on the cracking behavior. The lengths of median-radial cracks around indentations varied little between specimens. The Vickers hardness of the specimens measured at 0.98 N ranged from 6.6 ± 0.3 GPa to 7.5 ± 0.7 GPa, and no trend with the OH concentration was apparent. In addition, the Vickers hardness of the specimens measured while under a 9.81 N maximum load (LVHmax), showed little variation, and no apparent trend with the OH concentration.

Effects of H2O, pH, and oxidation state on the stability of Fe minerals on Mars

On Mars, there is evidence that solutions may have been present on the surface episodically in the past. These solutions were derived by weathering minerals such as olivine and sulfides found in mafic‐ultramafic rocks, and possibly sulfates and meteorite fragments. Upon removal of water and/or supersaturation of solutions, secondary minerals formed. We present a theoretical model for the formation of the solutions and the subsequent precipitation of Fe‐bearing phases. The first Fe‐bearing phases to weather via oxidation are sulfides (troilite or pyrrhotite) which produce secondary Fe (hydr)oxides or FeS2(pyrite or marcasite) and Fe2+sulfates such as melanterite. Melanterite may be replaced by Fe sulfates that have decreasing H2O/(3Fe3++ 2Fe2+), increasing oxidation, and increasing bulk OH/(OH + 2SO4). At Meridiani Planum the presence of jarosite indicates that the solutions were oxidized with pH &lt; 4.5. The solutions were likely Fe‐Mg‐(Ca)‐SO4‐(Cl)‐rich and precipitated Fe (hydr)oxides, Fe phosphates, Fe sulfates with low OH/(OH + SO4), Ca‐Mg sulfates, and possible halides, along with Si‐rich phases. The minerals indicating an acid environment were preserved by either removing components such as salts or water from the system (e.g., in dust storms or episodic aqueous events); isolating the mineral surfaces from fluids; using disequilibrium processes; or a combination. At other localities, jarosite is below detection limits, and schwertmannite may be present. If schwertmannite is present, the solution had near‐neutral pH (∼4 &lt; pH &lt; 10) and Fe (hydr)oxides would have crystallized rapidly, leaving Mg‐Na‐(Ca)‐SO4‐Cl‐rich solutions that likely precipitated Ca phosphates, Ca‐Mg‐Na sulfates, Fe sulfates with moderate‐high OH/(OH + SO4), halides, Si‐rich phases, and possibly Fe‐Mg‐Ca carbonates.

Basicity of potassium-salt modified hydrotalcite studied by 1H MAS NMR using pyrrole as a probe molecule

Potassium nitrate is supported on calcined hydrotalcite by a wet mixing method to produce strong basic sites. X-ray powder diffraction patterns reveal that dropping of potassium salt on calcined hydrotalcite promotes the recovery of the layer structure on rehydration. Potassium species are dispersed on the surface homogeneously up to a loading amount of about 10 wt.% KNO 3. 1H MAS NMR results reveal that there are at least three kinds of OH groups on calcined hydrotalcite; strong basic groups, weak basic groups and bulk OH groups. Introduced potassium ions preferentially replace the protons of the weak basic OH groups first and then the protons of the strong basic OH groups. Pyrrole is used as a probe molecule for 1H MAS NMR observation to characterize basicity. The basicity of calcined hydrotalcite is comparable to that of KX zeolite. On the other hand, KNO 3 modified hydrotalcite shows stronger basicity after calcination than the calcined hydrotalcite. It adsorbs pyrrole dissociatively.

Vibrational spectrum of brucite, Mg(OH) 2: a periodic ab initio quantum mechanical calculation including OH anharmonicity

The harmonic frequency spectrum of bulk Mg(OH) 2, brucite, has been computed with the CRYSTAL periodic code, using four different hamiltonians, namely Hartree–Fock (HF), local density (LDA), gradient corrected PW91 and hybrid B3LYP. The anharmonicity of the OH stretching frequency has also been evaluated, as well as the transverse/longitudinal optical (TO/LO) splitting. Comparison with the frequencies of the single layer shows that the bulk OH stretching splitting is due to interlayer interaction. Agreement with experiment is excellent for B3LYP (less than 10 cm −1 differences), less so for PW91 and much less satisfactory for both LDA and HF.

Acidity and basicity of metal oxide catalysts for formaldehyde reaction in supercritical water at 673 K

Formaldehyde (HCHO) reactions in supercritical water (673 K and 25–40 MPa) with and without acid and base catalysts (homogeneous: H 2SO 4 and NaOH, and heterogeneous: CeO 2, MoO 3, TiO 2, and ZrO 2) were conducted by use of batch reactors. Cannizzaro reaction (2HCHO+H 2O→CH 3OH+HCOOH) and self-decomposition of HCHO (HCHO→CO+H 2) were found to be primary reactions for all the cases and the contribution of each reaction depended on the condition. In the case of the homogeneous systems, Cannizzaro reaction became dominant with increasing bulk hydroxyl ion (OH −). The simple network model can well express the experimental results in the homogeneous conditions. We correlated the ratio of the yield of CH 3OH to that of CO (at 15 min) against bulk OH − in the homogeneous system. For elucidating acidity and basicity of metal oxide catalysts on HCHO reaction in supercritical water, OH − concentration on the metal oxide surface was calculated by use of the above correlation and the following order was found: CeO 2>ZrO 2>MoO 3>TiO 2 (rutile)>TiO 2 (anatase). At the reaction condition, CeO 2 and ZrO 2 were base catalysts, and MoO 3 and TiO 2 were acid catalysts. The experimental results with the metal oxides can be expressed by the model that was developed under homogeneous systems, with the values of the OH − concentrations that were calculated from the correlation about the CH 3OH/CO ratio at 15 min of reaction time.

Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study

Quenching of the 4-carboxybenzophenone triplet (3CB*) by H2N−CH2−CO2−Et and by amino acid anions of the general formula NR2−CR2−CO2-, where R is H or Me, has been investigated in basic aqueous solution. Spectral analysis of the primary products on the microsecond time scale showed that the major quenching process was electron transfer to 3CB*, producing the CB•- radical anion and the R2N•+−CR2−CO2- zwitterion aminium radical. However, on a nanosecond time scale, a small amount of (CBH)• was also formed, and this was attributed to a rapid proton transfer from about 10% of the aminium radicals to the CB•- anion radicals within the primary solvent cage. The values of the overall primary quenching rate constants were (8.5 ± 0.9) × 108 M-1 s-1 for N,N-dimethylglycine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine, (1.5 ± 0.2) × 108 M-1 s-1 for alanine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine ethyl ester, and (0.3 ± 0.03) × 108 M-1 s-1 for α-methylalanine. The introduction of methyl groups into the glycine structure resulted in a pattern of reactivity similar to that observed for amines. Except in the case of glycine ethyl ester, there were strong secondary growths of CB•-. This was attributed to the reduction of CB by the R2N−•CR2 species produced from the decarboxylation of the R2N•+−CR2−CO2- aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ± 0.4) × 108 M-1 s-1 for H2N−•CH2, (1.7 ± 0.3) × 109 M-1 s-1 for H2N−•C(Me)H, and (1.8 ± 0.3) × 109 M-1 s-1 for H2N−•CMe2. The transfer of protons from aminium radicals within the solvent cage gives rise to •NR−CR2−CO2- aminyl radicals, and these are known to undergo β-elimination of CO2•-. There was evidence for the presence of aminyl radicals in the case of α-methylalanine, where a small tertiary growth of CB•-, due to the reduction of CB by CO2•-, was observed. The magnitude of this growth matched the yield of (CBH)• from the spectral analysis of the primary products. In further experiments, the R2N•+−CR2−CO2- zwitterion aminium radicals were deprotonated by bulk OH- when NaOH was added at concentrations in the range of 1−4 M. As expected, this produced a lowering of the CB•- yield from the R2N−•CR2 radicals and an increase from the CO2•- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 × 1010 M-1 s-1 for the transfer of the proton from R2N•+−CR2−CO2- to OH-. It indicated that the rate constant for transfer of the proton from R2N•+−CR2−CO2- to CB•- within the solvent cage was (6.9 ± 1.5) × 109 s-1. The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ± 0.5) × 1010 s-1. The latter rate is at least 1 order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.