Active OH Groups Research Articles

Low Temperature Water Gas Shift: Evaluation of Pt/HfO2 and Correlation between Reaction Mechanism and Periodic Trends in Tetravalent (Ti, Zr, Hf, Ce, Th) Metal Oxides

It is well-known that catalysts containing small quantities of noble metals (e.g., Pt, Au) that are highly dispersed on Group IV and related tetravalent metal oxides (e.g., TiO2, ZrO2, CeO2, ThO2) display, once activated, high conversion for water-gas shift (WGS). The mechanism suggested for this by many authors is bimolecular in nature. In one view, adsorption of CO on the partially reducible oxide produces formates by reaction with active bridging OH groups, followed by water-assisted formate decomposition by dehydrogenation, with this second step being facilitated by metal particles at the metal–support interface. Considering this group of oxides, HfO2, as far as we are aware, had not yet been studied as a potential candidate for low temperature water-gas shift (LTWGS). Thus, in this contribution, catalysts composed of Pt supported on HfO2 were prepared and tested. Catalyst testing results suggest that this system is, like Pt/ZrO2 and Pt/CeO2, active for the WGS reaction, although additional studies ar...

Al 2O 3 supported Ru catalysts prepared by thermolysis of Ru 3(CO) 12 for catalytic wet air oxidation

Low loading catalysts Ru/γ-Al 2O 3 and Ru-Ce/γ-Al 2O 3 were prepared by thermolysis of Ru 3(CO) 12 on γ-Al 2O 3. The catalysts were characterized by XPS, XRD and SEM. Two new Ru species (Ru A and Ru B) were detected during the Ru 3(CO) 12 decomposition process due to chemical interaction with the active OH groups on the surface of Al 2O 3 support, and the reduction of them can lead to more dispersed metallic phases. The sample was completely decomposed at 673 K in H 2, and RuO 2 was formed with minor amounts of Ru 0. When the temperature was increased to 773 K to heat the sample, the ratio of Ru 0 to RuO 2 increased. However, after the addition of CeO 2, only RuO 2 was detected on surface. The catalysts exhibited high activities in Catalytic Wet Air Oxidation (CWAO) of different organic compounds at high concentration such as isopropyl alcohol, phenol, acetic acids and N,N-dimethylformamide, which is attributed to the better dispersion of Ru particles and the addition of CeO 2 further enhanced number of effectively active sites on the cluster-derived catalyst surface.

Natural polyphenols as chain‐breaking antioxidants during methyl linoleate peroxidation

AbstractA technique based on monitoring oxygen consumption was applied to study 11 natural and model polyphenols (PP, QH2) as well as four typical monophenolics as a chain‐breaking antioxidant during the controlled chain oxidation of methyl linoleate (ML) in bulk at 37°C. The antioxidant activities of QH2 were characterized by two parameters: the rate constant k1 for reaction of QH2 with the peroxy radical ${\rm LO}_{2}^{{\bf .}} $: (i) QH2 + ${\rm LO}_{2}^{{\bf .}} $ → ${\rm QH}^{{\bf .}} $ + LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH2. The rate constant k1 were reduced significantly by factor of 4 –28 as compared to these determined during the oxidation of styrene in bulk; the effect was typically more pronounced for catechol derivatives than for pyrogallol derivatives. At the same time, f for QH2 was found to be close to two independent of the number of active OH groups, similar to that determined earlier during the inhibited oxidation of styrene. The formation of H bond between OH group of QH2 and carboxyl group of ML is suggested as a reason for reducing effect of ML on k1.Practical applications: This work reports rate constants for the reaction of lipid peroxyl radical with phenolics and stoichiometric coefficient of inhibition, which characterize the antioxidant activity (AOA) of 15 natural and model PP, QH2 during the controlled peroxidation of ML. The reactivity of PP, QH2 during the oxidation of ML is routinely lower than the reactivity during the oxidation of non‐polar model hydrocarbons. This information may be useful to estimate the AOA of natural PP, QH2 in real systems of practical significance including plant oils, fats, food‐stuffs, biological objects, and similar.

Redox agents modulate neuronal activity and reproduce physiological aspects of neuronal aging

The high oxygen consumption and post-mitotic nature of the central nervous system (CNS) makes it particularly susceptible to oxidative stress, the impact of which is widely regarded as a root cause of functional impairment of the aging brain in vertebrates and invertebrates alike. Using an invertebrate model system we demonstrate that the lipid soluble antioxidant α-tocopherol can both reverse 2,2-azobis(2-methylpropion-amidine) dihydrochloride (AAPH) induced decline in excitability in young neurons as well as restore the electrical activity and excitability of aged neurons not unlike the level of their younger equivalents. Furthermore, using two analogs of α-tocopherol where either the acyl chain has been removed (Trolox) or the hydroxyl group of the chromanol ring has been methylated we were able to assert that the restorative effect of α-tocopherol requires both insertion into the plasma membrane as well as an active OH group. Thus, our results indicate peroxidation is an important modulator of neuronal excitability as well as support the growing body of evidence suggesting α-tocopherol's actions may extend well beyond its established role as a lipid domain preventative antioxidant.

Beckmann Rearrangement of 15N-Cyclohexanone Oxime on Zeolites Silicalite-1, H-ZSM-5, and H-[B]ZSM-5 Studied by Solid-State NMR Spectroscopy

By means of solid-state 15N NMR spectroscopy, evidence for the formation of nitrilium ions as intermediates of the Beckmann rearrangement of 15N-cyclohexanone oxime to epsilon-caprolactam on silicalite-1, H-ZSM-5, and H-[B]ZSM-5 is reported. The zeolites under study are characterized by different acid strengths (silicalite-1 < H-[B]ZSM-5 < H-ZSM-5). Depending on the nature of catalytically active surface OH groups, reactant and product molecules exist in the nonprotonated or protonated state. In addition, formation of byproducts such as 5-cyano-1-pentene and epsilon-aminocapric acid as a result of dehydration and hydrolysis of the reactant and product molecules, respectively, were observed.

Microstructure, topology and X-ray diffraction in Ag-metal reinforced polymer of polyvinyl alcohol of thin laminates

A polymer composite of Ag-metal reinforced polyvinyl alcohol (PVA) is synthesized in shape of thin laminates of 200–300 μm thickness. The process involves a chemical Ag+ dispersion in PVA and in-situ \({\rm Ag}^{+} \to {\rm Ag}\) reduction-reaction with active PVA molecules under hot conditions (with stirring) in water at 60–70°C temperature. The product results in a metal Ag-polymer complex dispersed in the solution. After evaporating part of water, a derived viscous solution is casted (in hot conditions) in shape of a thin laminate in a glass mould. In addition to chemical reducer, active OH-groups (free from H-bonding) in PVA molecules of refreshed surfaces act as head groups to adsorb Ag+ and drive a directional \({\rm Ag}^{+} \to {\rm Ag}\) growth. Short fibrils of Ag-metal thus occur in reaction over the PVA molecules. Casting thin laminates from a liquid sample Ag-PVA allows the fibrils (also the polymer molecules) to align along the surface. Selected Ag-contents up to 5.0 wt.% in Ag-PVA laminates are studied in terms of scanning electron micrograph, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. Average size, morphology and aspect ratio (ϕ) vary in Ag-metal depending on the Ag-content. As long Ag-metal fibrils as 2–5 μm, ϕ=35, occur in a sample of 2.0 wt.% Ag. The Ag-metal reflects in two characteristic 3d5/2 and 3d/3/2 XPS bands of 368.3 and 374.1 eV respectively.

Water-gas shift: an examination of Pt promoted MgO and tetragonal and monoclinic ZrO2 by in situ drifts

In situ DRIFTS measurements on unpromoted and Pt promoted MgO and ZrO2 (both tetragonal and monoclinic) indicate that at high H2O/CO ratios, where the reaction rate has been reported to be zero order in H2O and first order in CO, the mechanism involved in the catalysis of water-gas shift is likely a surface formate mechanism, in agreement with Shido and Iwasawa. Pt was found to catalyze the removal of surface carbonates and to facilitate the generation of active OH groups relative to the unpromoted catalyst. Comparison with Pt/ceria revealed that the OH groups involved in the catalysis of magnesia and zirconia may be those of the bridging variety which occur at defect sites. That is, water dissociated over vacancies to produce bridging OH groups, as observed by infrared spectroscopy. The existence of such an adsorbed species is implied in the zero reaction order for water, where kinetics suggests that the surface should be saturated by an adsorbed water species. The lower extent of vacancy formation for magnesia and zirconia-based materials in comparison with ceria could explain a lower surface population of active bridging OH groups. CO was used as a probe molecule of the reduced centers, as it reacts with bridging OH groups to generate surface formates, a proposed WGS intermediate, and the decomposition of which is proposed to be the rate-limiting step. The trends in formate intensity by CO adsorption and CO conversion in WGS catalytic testing both followed the order: Pt/ceria>Pt/m-zirconia>Pt/t-zirconia>Pt/magnesia. In all cases, a normal kinetic isotope effect was observed in switching from H2O to D2O, consistent with a link between the rate-limiting step and the decomposition of surface formates, as noted previously by Shido and Iwasawa for Rh/ceria, MgO, and ZnO.

Low-temperature water–gas shift: impact of Pt promoter loading on the partial reduction of ceria and consequences for catalyst design

Partial reduction of ceria generates catalytically active bridging OH groups on the surface of ceria. Pt facilitates this surface reduction process, and in this work, the impact of the Pt promoter loading on catalyst structural-property relationships was explored. XANES spectra were recorded under H 2 treatment for a series of Pt/ceria catalysts with increasing Pt loading at both the Pt and Ce L III edges. Reduction of Pt oxide was hindered by metal–support interactions, such that higher Pt loadings facilitated reduction of Pt oxide to Pt 0. Two routes of bridging OH group formation are as follows: (1) once it is reduced, Pt 0 dissociates H 2, which spills over to the ceria surface to generate the bridging OH group active sites directly, accompanied by a change in the oxidation state of the Ce atoms involved with the sites from Ce 4+ to Ce 3+; and (2) H 2 or CO removes ceria surface capping oxygen atoms to generate vacancies (and surface Ce 3+), followed by H 2O dissociation at the vacancies to generate the bridging OH groups. Either route highlights the direct link between the extent of ceria partial reduction and the active site density of the bridging OH group active sites. The relative Ce 3+ and Ce 4+ concentrations from XANES were quantified and at low temperature; the greatest degree of ceria reduction was obtained for the Pt/ceria catalysts with higher Pt loadings, correlating with a higher bridging OH group active site density. Using in situ DRIFTS, we used CO as a probe molecule, as it reacts with the bridging OH groups to generate surface formates, the proposed intermediates of the WGS reaction. While addition of CO to the unpromoted catalyst reduced at 250 °C led to only very weak formate bands due to a lack of bridging OH groups on the ceria surface at that temperature, strong formate bands arose on the surface of the Pt/ceria catalysts at 250 °C. In situ DRIFTS was also utilized to probe the dynamics of the surface formate coverages under low-temperature WGS reaction conditions over the Pt/ceria series. A high H 2O/CO feed ratio was employed, and the surface formate coverages were found to be more limited by the WGS rate for the heavily loaded Pt/ceria catalysts. This indicates that Pt may not only serve to facilitate the generation of the bridging OH group active sites at low temperature, but may also be involved in accelerating surface formate decomposition, the elementary step of the mechanism that is proposed to be rate limiting. A clear trend of higher CO conversion with higher Pt loading was established in reaction testing. HR-TEM carried out on the 5%Pt/ceria catalyst indicated well-dispersed Pt clusters in the diameter range of 1–2 nm.

Low Temperature Water–Gas Shift: Role of Pretreatment on Formation of Surface Carbonates and Formates

Recently, the role of ceria vacancies on water–gas shift activity has been explained in terms of a redox mechanism, whereby CO adsorbed on a metal reduces the ceria surface to generate CO2, and water reoxidizes the ceria surface to CeO2, liberating hydrogen in the process. In this study, we examine the possibility of a ceria-mediated redox mechanism by examining more closely the evolution of carbonate and formate bands under different controlled treatment environments, and utilizing different reduction procedures. Earlier it was claimed that the decomposition of carbonates by water was consistent with a redox process, whereby the CO2 product could spillover to the support. We found that the observation of carbonate formation and decomposition by water was a result of the treatment procedure used in the earlier work, and that, once bridging OH groups are produced in the presence of water, the reaction more likely proceeds via a formate intermediate, which is produced by reaction of CO with the active bridging OH groups. However, the vacancies appear to play an important role in generating these active sites. Possible pathways to active site generation are discussed.

Complexation of Zn Arylporphyrinates with Leucine Methyl Ester

Extra coordination of L-leucine methyl ester with seventeen different Zn arylporphyrinates is studied by spectrophotometric titration and capability of Zn porphyrinates with the active OH groups to recognize LeiOCH3 in toluene at 20°C is determined. The formation of associates of the composition amino acid ether–porphyrinate depending on the substituent positions in a macrocycle is studied by the 1H NMR method. The most strong donor–acceptor bonds between Zn porphyrinate and LeiOCH3 are observed in the case of pyridine-substituted porphyrins and porphyrins with phenyl rings containing electron-donor substituents in the m-position. The best recognizing capabilities with respect to leucine are shown by Zn porphyrinates with di- and tetra-4-OH-phenyl substitution in the meso-positions of a macrocycle.

Nature of strontium extraction by synergistic mixtures of chlorinated cobalt dicarbollide and polyethers

Extraction of strontium by synergistic mixtures of chlorinated cobalt dicarbollide (DCC) with different polyethers was studied. In the acidic media the distribution coefficients (DSr) decreases in the order 15-crown-5>PEG-400>18-crown-6 and does not correspond to the row of constants stability of the strontium complexes with these polyethers. When passing to the salt media (Li and Na nitrates) the row of DSr is change: PEG-400>18 crown 6>15 crown 5. IR-spectroscopy study has shown that in synergistic mixtures the proton forms [H5O2 +PEG], [H5O2 +(15-crown-5)2] and [H3O3 +(18 crown 6)] cations, which are exchange on Sr2+ resulting to formation [Sr2+PEG], [Sr2+(15-crown-5)2] and [Sr2+(18-crown-6)(H2O) n ] cations correspondingly. The PEG-400 is the best among polyethylene glycols, since its all six COC groups and two even more active OH groups complete the first co-ordination sphere of Sr2+. Extremely low extractability of Sr2+ from acidic media in the presence of 18-crown-6 as compared with 15-crown-5 is mainly caused by the high stability of the [H3O+(18-crown-6)] cation that is responsible for less extractable hydrated Sr complex formation.

Vacancies and Electron Deficient Surface Anions on the Surface of MgO Nanoparticles

In the course of surface defect characterization on MgO nanoparticles three different types of O- species were observed by electron paramagnetic resonance (EPR) spectroscopy. They are a result of UV excitation under high-vacuum conditions/in O2 atmosphere or of surface color center bleaching by N2O, respectively. On the other hand, the O- species induce homolytic H2 splitting which was evidenced for the first time by infrared (IR) spectroscopy. The resulting products are a so far unknown IR active OH group and an uncharged surface mobile H atom. The latter one acts as a reducing agent for either one of two types of anion vacancies which are transformed into EPR active surface color centers FS+. They are fundamentally different from the classical FS+(H) center obtained in predominant abundance by UV irradiation of MgO in H2 atmosphere. One of the above-mentioned O- species is created by bleaching of FS+(H) by N2O. The respective EPR signal is exactly the same as that obtained by thermal decomposition of a ...

Acid-diffusion suppression in chemical amplification resists by controlling acid-diffusion channels in base matrix polymers

We have investigated the suppression of acid diffusion during postexposure baking (PEB) of chemical amplification resists in terms of the molecular control of base matrix polymers to control acid-diffusion channels. According to a previously proposed model, acid diffuses along acid-diffusion channels in chemical amplification resists. The acid-diffusion channels are composed of OH groups outside the base matrix polymers (active OH groups) and vacancies in the matrices. The negative-type chemical amplification resists used in this work consisted of cresol novolak with controlled molecular-weight distributions as the base matrix polymers, an acid-catalyzed crosslinker of melamine resin, and acid generators of onium salt. Measurements of the dependence of the pattern sizes of isolated lines on the PEB time made it clear that acid diffusion defines the resist pattern sizes based on Fick’s law. The densities of the active OH groups play a critical role in acid diffusion, and a base matrix polymer with fewer active OH groups is expected to result in less acid diffusion. Therefore, we can suppress acid diffusion by reducing the number of oligomers in the base matrix polymers by a precipitation method, since these oligomers stereochemically include many active OH groups. We have obtained high resolution by using a resist with an oligomer-free resin. The acid-diffusion model was also confirmed based on infrared absorption spectroscopy and thermal analysis. We conclude that acid diffusion can be suppressed by controlling the acid-diffusion channels.

Suppression of Acid Diffusion in Chemical Amplification Resists by Molecular Control of Base Matrix Polymers

Suppression of acid diffusion during post-exposure baking (PEB) of chemical amplification resists is investigated from the standpoint of molecular control of base matrix polymers. Negative-type chemical amplification resists composed of cresol novolak-based matrix polymers, acid-catalyzed crosslinkers of melamine resins, and acid generators of onium salts are prepared. The molecular weight distributions of the base matrix polymers are controlled by means of a precipitation method. The resists are exposed with electron beams in isolated lines to evaluate the acid diffusion characteristics. Dependence of pattern sizes on the PEB time clearly shows that acid diffusion determines the resist pattern sizes based on Fick's law. The diffusion coefficients of resists with base matrix polymers with small polydispersities are smaller than those of resists with base matrix polymers with large polydispersities. Acid diffusion can still be suppressed by applying base matrix polymers with small weight-average molecular weights and small polydispersities. Diffusion coefficients can be further decreased by using base matrix polymers with more p-cresol components. A diffusion mechanism is proposed based on acid diffusion channels composed of active OH-groups and vacancies in the base matrix polymers.

Effects of dry grinding on P retention properties of andosols

Abstract Dry grinding of Andosols resulted in an initial increase and subsequent decrease, in some cases, of the P retention capacity and pH(KF) values, along with changes in the amount of P extracted by Bray's 2nd method from Andosols with the addition of P. These phenomena were ascribed to the increase of the amount of active OH groups caused by structural destruction during grinding for a short period of time and decrease associated with reaggregation upon prolonged grinding. The formation of P-bearing substances by dry grinding of Andosols was assumed based on a comparison of the decrease of the amount of P extracted with Bray's 2nd solution from Andosols to which P had been added before and after grinding.

In-situ IR investigation of coke formation on dealuminated mordenite catalysts

Coke formation over a series of dealuminated mordenites was investigated by in-situ IR spectroscopy using dealkylation/disproportionation of ethylbenzene as a model reaction. The rates of coke formation and catalyst deactivation increased with increasing acidity, i.e. number and/or strength of the acidic sites. Comparison of deactivation by ethylbenzene and ethylene reaction suggested that deactivation during conversion of ethylbenzene is more likely due to partial pore blocking than to poisoning or consumption of active OH groups. Sensitivity to coke formation and increase of selectivity for para-diethylbenzene with increasing coke deposition seemed to be correlated to an increase in acid strength of the active centres.

Nature of active sites of aluminosilicate catalysts

The catalytic activity of aluminosilicates with respect to the hydrogen redistribution reaction is shown to be due exclusively to exchangeable ions and hydrolytic acidity. There is no important difference between the two kinds of acidity. Silica gel is found to contain active, “hydrolytic,” and nonactive OH groups. Silica gel acquires ion-exchange capacity and catalytic properties exclusively as a result of substitution of aluminum for the protons in hydrolytic OH groups. There is a linear relationship between the content of hydrolytic ions of aluminum and activity. It is shown that H-aluminosilicates are unstable and spontaneously convert by inner exchange to Al-aluminosilicates. Consequently, the exchange acidity of aluminosilicates is due solely to the presence of exchangeable ions of aluminum on their surface. The exchangeable aluminum is a strong Lewis acid. The ability of aluminosilicates to acquire catalytic activity only in the presence of traces of moisture is assumed to be a result of complex formation of the exchangeable aluminum with water. The ionization of this complex leads apparently to the appearance of acid sites of the Brönsted type. Evidentally, the active site is the exchangeable aluminum ion in combination with one water molecule. In catalytic conversion experiments aluminum ions prove to be equal in activity, even though they differ in their ability for exchange in an aqueous medium. The active sites are completely accounted for by the value of hydrolytic acidity. The true value of this acidity may be obtained by the carbonate method. Those methods of titration in anhydrous media by butylamine or ethyl acetate fail to take into account just that part of aprotonic acidity that is responsible for the activity of the catalyst.