The Role of Impurities on the Low Temperature CO Oxidation on Au/TiO2

A number of Au/TiO2 catalysts have been prepared by a variety of methods in order to assess the affect of catalysts preparation methods on performance, catalyst contamination and the interplay between them. Their activity was studied in a pulse flow microreactor and it was found that preparation methods designed to eliminate impurities lead to more active samples. The effect of chlorine is often cited to be detrimental in the literature, but we have quantified it using XPS. It was found that the activity decreased in a nearly linear fashion with an increasing amount of this element at the surface. It is generally considered that catalysts prepared by the incipient wetness (IW) technique are ineffective for CO oxidation, but we show here that, by appropriate preparation methods, high activity IW catalysts can be made.     

CO Oxidation and the CO/NO Reaction on Pd(110) Studied Using “Fast” XPS and a Molecular Beam Reactor

We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO–NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal “light-off” phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O₂ dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2CO_a + O_2g + 2S → 2CO_2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. “Fast XPS”, carried out in situ during the course of the reaction, shows domination of the surface by CO_a below 390 K and domination by O_a above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO–NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. “Light-off” is dictated by the onset of the dissociation of NO_a, which occurs at ～400 K. Once N_a and O_a are formed, N₂O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing O_a as CO_2g. Again, the reaction self-accelerates and there is a rapid change of surface coverage from NO_a to O_a at ～450 K. The overall self acceleration is due to the following overall reaction, 2NO_a + CO_g + S → N₂O_g + CO_2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by O_a and to the reduced coverages of the molecular species.     

Accommodation response during a prolonged visual search task

The visual accommodative response is measured during dark field viewing and while engaged in a visual search task. A laser optometer is used to compare performance using microfiche and hard-copy displays. Results indicate a marked bias in the direction of the individual's resting accommodative state during all conditions of display viewing. Implications of these results and those from other studies are discussed with respect to future research in display design.     

Insights into surface reactivity: formic acid oxidation on Cu(110) studied using STM and a molecular beam reactor

Using a combination of STM and molecular beam reactor data we summarise some important features of a model reaction (formic acid oxidation on Cu(110)) which is of general significance to surface reactivity and to catalysis. Three such features are highlighted here. The first concerns the role of weakly held species (possibly physisorbed) in surface reactions. These species, although of very short lifetime on the surface, can, nevertheless, diffuse over long distances to “find” a sparse distribution of active sites. Thus a very low coverage of oxygen on the surface of Cu(110) increases the sticking probability of all the formic acid molecules which strike the surface to high value (0.82), even though the clean surface is relatively unreactive. The important concept here is the “diffusion circle” or “collection zone” which represents the area of surface visited by the molecule in its short sojourn in the weakly held state. The second theme concerns the concept of the “flexible surface”. We show that the involvement of surface atoms in reactions directs the structure and reactivity for a particular reaction. For formic acid oxidation the liberation of Cu atoms during the removal of oxygen as water leads to gross restructuring of the surface and can lead to “compression” of one reactant (the oxygen in this case) into a lower area, higher local coverage, unreactive state (the c(6×2) oxygen structure). Thirdly, and finally, it is proposed that, for many surface reactions, the surface acts in an analogous way to a solvent, supporting a “dissolved” (highly mobile and fluxional) phase of intermediates at low coverage, which crystallise out above a critical coverage (the 2D “solubility limit”). This revised version was published online in July 2006 with corrections to the Cover Date.     

Myosin heavy chain isoforms influence myofibrillar ATPase activity under simulated postmortem pH, calcium, and temperature conditions

The pH and Ca(2+) sensitivity of myofibrillar ATPase activity plays an integral role in regulating postmortem muscle ATP utilization and likely paces postmortem glycolysis. The objective of this study was to determine the influence of pH and Ca(2+) concentration on the ATPase activity of myofibrils from red semitendinosus (RST) and white semitendinosus (WST) porcine muscles. Myofibrillar ATPase was measured at 39 °C over a pH range 5-7.5 and a [Ca(2+)] range pCa 4-9 (10(-4)-10(-9)M). At maximum Ca(2+)-dependent activation (pCa 4), RST myofibrils had lower (p<0.0001) ATPase activity than WST myofibrils. This maximum activity of myofibrils from both muscle regions was not influenced from pH 7.5 to 6.5, declined between pH 6.5 and 5.75 (Hill coefficient, n(H)=2.7-3.4; pH at half maximum activity, pH(50)=5.97) and was near zero at pH 5.5. At pH 7, pCa-activity relationships showed that RST required less Ca(2+) for half-maximum activation (higher pCa(50); 6.50) than WST myofibrils (pCa(50)=6.35) but had no difference in n(H). At pH 7, both RST and WST myofibrils had maximum Ca(2+)-dependent, actin-activated ATPase activity at pCa ?6 and Ca(2+)-independent myosin ATPase activity at pCa ?6.75. pCa-activity relationships at different pH levels indicated that pCa(50) decreased with pH from pH 6.5 to 6.125 in both RST and WST myofibrils. At pH <5.75, [Ca(2+)] did not influence ATPase activity in RST or WST myofibrils. These data show that myofibrils with predominantly fast MyHC (WST) have a higher actin-activated myosin ATPase activity than myofibrils with primarily slow MyHC isoforms (RST) at Ca(2+) concentrations and pH values characteristic of postmortem muscle.     

Impact of carcass scalding and chilling on muscle proteins and meat quality of broiler breast fillets

The objective of this study was to determine the effects of broiler carcass scalding and chilling methods on meat quality and muscle proteins. During processing, carcasses were hard (60 °C, 1.5 min) or soft (52.8 °C, 3 min) scalded, and either immersion chilled (IC: 0.5 °C, 40 min) or air chilled (AC: 0.5 °C, 120 min). Breast fillets were deboned at 4 h postmortem and used for measuring meat quality and muscle protein characteristics. Scalding by chilling treatment interaction effects on meat quality were not observed. Air chilled carcasses had greater pH₂₄, and reduced drip loss and shear force compared to IC carcasses. Cook yield, color (L*a*b*), and moisture content were not different between chilling treatments. Scalding treatments did not influence quality traits. Sarcoplasmic protein solubility was not influenced by chilling treatment, but was greater in hard versus soft scalded carcasses. Myofibrillar protein solubility was greater in fillets from soft scalded IC carcasses. Alterations in the electrophoretic profiles of the myofibrillar and sarcoplasmic proteins due to treatments indicated minor changes in protein degradation and solubility. Data suggest that while only chilling method influenced meat quality, both scalding and chilling methods influenced protein solubility and degradation in breast fillets deboned 4 h postmortem.     

Myosin heavy chain isoform composition influences the susceptibility of actin-activated S1 ATPase and myofibrillar ATPase to pH inactivation

The objectives of this study were to determine the influence of pH and MyHC isoforms on myofibrillar and actin-activated myosin subfragment 1 (S1) ATPase activity and the protective effect of actin. Red (RST) semitendinosus and white (WST) semitendinosus myofibrils were incubated at pH 7, 6, or 5.5 with 0 or 2mM ATP. RST and WST S1 isolates were incubated at pH 7, 6, or 5.5 in the presence or absence of actin. Maximum calcium-activated myofibrillar and actin-activated S1-ATPase activity were then assayed at pH 7. Incubation of myofibrils with ATP caused ATPase activity of myofibrils to decrease (p<0.05) with the pH of the incubation. RST myofibrils maintained a higher (p<0.0001) relative activity than WST myofibrils after incubation at pH 6 with ATP. Myofibrils incubated without ATP exhibited higher (p<0.001) activities than those incubated with ATP following pH 5.5 treatments. WST myofibrils had a lower (p<0.05) relative activity than RST following incubation at pH 5.5 without ATP. S1 ATPase activities decreased (p<0.05) with incubation pH in WST samples, but not in RST samples. WST S1 activity was higher (p<0.01) in samples exposed to pH 6 and 5.5 with actin bound compared to those incubated without actin. RST S1 exhibited a higher (p<0.01) relative activity than WST samples following pH 5.5 treatment with bound actin. These data show that low pH inactivates myofibrils by altering actin-activated S1 ATPase. Furthermore, these results suggest that muscles with high proportions of fast fibers are more susceptible to pH inactivation of ATPase activity and that the protective effect of actin binding to myosin is less in fast fibers.