Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Enhancing the surface binding stability of chromophores, catalysts, and chromophore–catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1–12) by atomic layer deposition of an overlayer of TiO₂. Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases and with a pathway identified in which O-atom transfer to OH⁻ occurs with a rate constant increase of 10⁶ compared to water oxidation in acid.Heterogeneous catalysis plays an important role in industrial chemical processing, fuel reforming, and energy-producing reactions. Examples include the Haber–Bosch process, steam reforming, Ziegler–Natta polymerization, and hydrocarbon cracking (1–8). Research in heterogeneous catalysis continues to flourish (9–15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16–23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.Procedures are available for immobilization of organometallic and coordination complexes on the surfaces of solid supports. Common strategies include surface derivatization of metal oxides by carboxylate, phosphonate, and siloxane bindings (24–27), carbon-grafted electrodes (28–30), and electropolymerization (31–33). These approaches provide a useful bridge to the interface and a way to translate mechanistic understanding and ease of synthetic modification of solution catalysts to heterogeneous applications with a promise of higher reactivity under milder conditions.A significant barrier to this approach arises from the limited stability of surface binding. Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic solutions (27, 34). For water oxidation catalysis this is particularly detrimental given the accelerated rates that are accessible for catalytic water oxidation as the pH is increased due to the intervention of base-catalyzed pathways with concerted atom–proton transfer accompanying O—O bond formation (35).We report here the results of a designed strategy for the systematic surface stabilization of molecular catalysts on solid oxide surfaces. In the strategy we use indium tin oxide (ITO) electrodes and first bind a phosphonate-derivatized molecular water oxidation catalyst to the surface of the electrode. The derivatized electrode is then coated with a conformal nanoscale TiO₂ overlayer applied by layer-by-layer atomic layer deposition (ALD). The overlayer of TiO₂ acts to block hydrolysis of the phosphonate groups from the surface, Fig. 1. ALD stabilization has been used previously to demonstrate significantly enhanced photostability of surface-bound chromophores in acidic and neutral solutions (36–38). In this article we apply the ALD stabilization procedure to surface stabilization of a known water oxidation catalyst and show remarkably enhanced surface binding stability even in basic solutions. Retention of electrocatalytic reactivity on the surface is demonstrated and water oxidation catalysis investigated over a wide pH range. Clear evidence is found in these studies that added proton acceptor bases enhance the kinetic pathways in the key, rate-limiting step (O—O bond formation) via an atom–proton transfer (APT) mechanism (22, 35). In addition, a facile pathway has been identified with direct attack by OH⁻ on an activated oxo form of the catalyst with rate enhancements of up to 10⁶ for water oxidation.Open in a separate windowFig. 1.Schematic representation of the ALD overlayer protection strategy for a catalyst surface-attached to nanoITO protected by TiO₂. (A) Illustrating the electrochemical device architecture showing the surface derivatized electrode and water oxidation. (B) Underivatized electrodes exposed to basic aqueous conditions showing detachment of the catalysts from the electrode surface. (C) ALD protection of surface attachment even basic aqueous conditions. This figure was adapted from ref. 38.     

The oral glucose tolerance test: A study of the influence of age on the response to the standard oral 50 g glucose load

Summary 160 women and 133 men over 50 were chosen for the experiment. They were divided into three age groups. In order to study a group of subjects who were normal for their age, caution was taken to discard known overt diabetics and to avoid any interfering factor that might influence glucose tolerance. The results are expressed by the median and the 5th and 95th percentiles. The median increases progressively with age. There is a highly significant difference between subjects over and under 65, but none between sexes. The 5th percentile varies very little whereas the 95th percentile shows a large increase. This is more evident in women than in men. This would indicate an increased prevalence of diabetes with age rather than a general reduction of glucose tolerance. The therapeutic problems are discussed.

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Beeinflussung der oralen Glucosebelastung (mit 50 g Glucose) durch das Alter

Zusammenfassung Die Untersuchung bezog sich auf 160 Frauen und 133 Männer über 50, verteilt auf drei Altersgruppen. Um ein normales Bevölkerungsmuster zu untersuchen, wurden bekannte, manifeste Diabetiker und Patienten mit diabetischer Veranlagung ausgeschlossen. Die Resultate wurden ausgedrückt in Medianen und den Perzentilen 5 und 95. Der Median steigt allmählich mit dem Alter. Es existiert ein hoch signifikanter Unterschied zwischen den Altersklassen über und unter 65, aber nicht zwischen beiden Geschlechtern. Das Perzentil 5 schwankt wenig, das Perzentil 95 hingegen steigt stark. Dies ist ausgesprochener bei Frauen als bei Männern der Fall. Dieses würde dann eher eine steigende Frequenz des Diabetes mit dem Alter bedeuten, als eine allgemein verringerte Grlucosetoleranz. Die therapeutische Bedeutung wird besprochen.

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Influence de l'âge sur l'épreuve d'hyperglycémie par administration orale de 60 g de glucose

Résumé L'expérimentation porte sur 160 femmes et 133 hommes de plus de 50 ans répartis en 3 classes d'âge. L'étude d'une population âgée normale a nécessité l'exclusion des diabétiques avérés et de tout facteur diabétogène possible. Les résultats sont exprimés par la médiane et les percentiles 5 et 95. La médiane augmente progressivement avec l'âge; la différence entre les sujets de plus et de moins de 65 ans est hautement significative, sans variation statistique entre les sexes. Le percentile 5 évolue très peu, par contre on observe une augmentation importante des valeurs du percentile 95; cet aspect est plus marqué chez les femmes. Ces données démontreraient une fréquence croissante du diabète avec l'âge plutôt qu'une diminution générale de la tolérance au glucose. L'implication thérapeutique est discutée.

     

Lipofuscin in lymphocytes and plasma cells in aging

The age-related changes in spleen and lymph node lymphocytes and plasma cells of Wistar rats and in the peripheral lymphocytes of healthy human individuals were examined. Similar changes were described in human and rat lymphocytes and plasma cells consisting of mitochondrial lesions. The cristae of the mitochondria disappeared and were replaced by myelin-like structure, lamellar structure, electron-dense and electron-translucent material. It is supposed that this material corresponds to lipofuscin.     

Colloids with high-definition surface structures

Compared with the well equipped arsenal of surface modification methods for flat surfaces, techniques that are applicable to curved, colloidal surfaces are still in their infancy. This technological gap exists because spin-coating techniques used in traditional photolithographic processes are not applicable to the curved surfaces of spherical objects. By replacing spin-coated photoresist with a vapor-deposited, photodefinable polymer coating, we have now fabricated microstructured colloids with a wide range of surface patterns, including asymmetric and chiral surface structures, that so far were typically reserved for flat substrates. This high-throughput method can yield surface-structured colloidal particles at a rate of approximately 10(7) to 10(8) particles per operator per day. Equipped with spatially defined binding pockets, microstructured colloids can engage in programmable interactions, which can lead to directed self-assembly. The ability to create a wide range of colloids with both simple and complex surface patterns may contribute to the genesis of previously unknown colloidal structures and may have important technological implications in a range of different applications, including photonic and phononic materials or chemical sensors.     

Relationship between plasma lipids and all-cause mortality in nondemented elderly

OBJECTIVES: To investigate the relationship between plasma lipids and risk of death from all causes in nondemented elderly. DESIGN: Prospective cohort study. SETTING: Community-based sample of Medicare recipients, aged 65 years and older, residing in northern Manhattan. PARTICIPANTS: Two thousand two hundred seventy-seven nondemented elderly, aged 65 to 98; 672 (29.5%) white/non-Hispanic, 699 (30.7%) black/non-Hispanic, 876 (38.5%) Hispanic, and 30 (1.3%) other. MEASUREMENTS: Anthropometric measures: fasting plasma total cholesterol, triglyceride, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), and non-HDL-C, body mass index, and apolipoprotein E (APOE) genotype. clinical measures: neuropsychological, neurological, medical, and functional assessments; medical history of diabetes mellitus, heart disease, hypertension, stroke, and treatment with lipid-lowering drugs. Vital status measure: National Death Index date of death. Survival methods were used to examine the relationship between plasma lipids and subsequent mortality in younger and older nondemented elderly, adjusting for potential confounders. RESULTS: Nondemented elderly with levels of total cholesterol, non-HDL-C, and LDL-C in the lowest quartile were approximately twice as likely to die as those in the highest quartile (rate ratio (RR)=1.8, 95% confidence interval (CI)=1.3-2.4). These results did not vary when analyses were adjusted for body mass index, APOE genotype, diabetes mellitus, heart disease, hypertension, stroke, diagnosis of cancer, current smoking status, or demographic variables. The association between lipid levels and risk of death was attenuated when subjects with less than 1 year of follow-up were excluded (RR=1.4, 95% CI=1.0-2.1). The relationship between total cholesterol, non-HDL-C, HDL-C, and triglycerides and risk of death did not differ for older (>or=75) and younger participants (>75), whereas the relationship between LDL-C and risk of death was stronger in younger than older participants (RR=2.4, 95% CI=1.2-4.9 vs RR=1.6, 95% CI=1.02-2.6, respectively). Overall, women had higher mean lipid levels than men and lower mortality risk, but the risk of death was comparable for men and women with comparable low lipid levels. CONCLUSION: Low cholesterol level is a robust predictor of mortality in the nondemented elderly and may be a surrogate of frailty or subclinical disease. More research is needed to understand these associations.     

Surface Chemistry in Nanoscale Materials

Although surfaces or, more precisely, the surface atomic and electronic structure, determine the way materials interact with their environment, the influence of surface chemistry on the bulk of the material is generally considered to be small. However, in the case of high surface area materials such as nanoporous solids, surface properties can start to dominate the overall material behavior. This allows one to create new materials with physical and chemical properties that are no longer determined by the bulk material, but by their nanoscale architectures. Here, we discuss several examples, ranging from nanoporous gold to surface engineered carbon aerogels that demonstrate the tuneability of nanoporous solids for sustainable energy applications.