The direct synthesis of hydrogen peroxide with Au-Pd catalysts is described and discussed: in particular, the roles of the support and promoters. Catalysts prepared by co-impregnation on various supports with calcination at 400 • C were stable and could be re-used several times without loss of metal. Catalysts calcined at lower temperatures were found to be unstable and could not be successfully re-used. Au-Pd/carbon and Au-Pd/silica catalysts gave the highest rate of H 2 O 2 production, and the order of reactivity observed for the support materials investigated is: carbon ∼ silica > TiO 2 > Al 2 O 3 . Bimetallic Au-Pd particles on TiO 2 and Al 2 O 3 were found to exhibit a core-shell structure, Pd being concentrated on the surface. It is considered that the Au-Pd/silica catalysts have a similar core-shell morphology based on X-ray photoelectron spectroscopy studies, whereas, in contrast, the calcined Au-Pd/carbon catalysts are observed to be homogeneous alloys. TEM studies showed that the silica contains impurities of carbon and that the Au-Pd alloys all preferentially interact with these carbonaceous impurities, hence resulting in a rather similar catalytic performance to that of the carbon supported Au-Pd catalysts. The origin of the enhanced activity for the silica and carbon supported catalysts is a result of higher H 2 selectivity for the formation of hydrogen peroxide, which is due to the surface composition and size distribution of the nanoparticles. The effect of promoters is investigated, and it is shown that addition of Br − and PO 4 3− is deleterious under our conditions, which contrasts markedly with Pd catalysts for which such species are essential. Furthermore, we show the acid solution formed by CO 2 in water increases the rate of H 2 O 2 synthesis, and thereby the CO 2 diluent in our experiments acts as green in situ acid promoter.
Variations in dissolved cations, total alkalinity, sulphate, and field pH are recorded for samples of precipitation, supraglacial melt, and bulk melt waters (those emerging at the portal) from Fjallsjökull (south-east Iceland). The composition of supraglacial melt indicates derivation from precipitation by the acquisition of solutes from the weathering and dissolution of rock debris, in a system open to atmospheric carbon dioxide. Bulk melt waters have variable compositions, approaching saturation with calcite, which can be derived from supraglacial melt water by continued solute acquisition in a system closed to atmospheric carbon dioxide. Solute–discharge relationships suggest that chemical evolution of the bulk melt waters occurs when aggressive englacial waters are mixed with the solute and sediment-rich subglacial component. Simple mixing behaviour occurs over intermediate discharges as long as variations in the extent of post-mixing evolution are small compared to changes in the mixing ratio.
Contact stress transducers w er e p.1 aced in subglac ial b edrock a nd used to monitor continuously shear stress and normal pressur e changes at the contact with the overriding glacier sole 100 m beneath the surface of the Glacier d'Argentiere during periods in summer 1973 and spri ng 1975. The measured flu ctuations in normal press ure and shear st ress do n ot appear to be r elated to changes in sliding velocity. Analysis of the data reveals short-term flu ctuations in normal pressure a nd sh ea r stress which appear to be related to the passage of individual la rge debris pa rticl es or groups of p a rticl es over the transducer. The shea r stress appears to be a fun ction of the volume conce ntration of debris in the ice. The volume concentration at any point appears to be partially d ependent o n a "streami ng" process b y which basal debris-rich ice tends to flow a round the lateral flanks of hummocks on the glacier bed. Where su b-glacial cav iti es occur, this streaming effect appears to be d ependent on the extent of cavitation and thus on ice overburden pressure a nd velocity. It is suggested that this process can account for an apparent lag between changes in normal press ure and sh ear stress.The maximum ra tio b etween shear and normal stress averaged over a period of 10 min was 0.44. This is equivalent to a spatial average over 0.3 cm. D ebris concen trat ions in basal ice of up to 43% by volume occurred. It is suggested that concentrations of this order are commo n a t the base of temperate glaciers and thus that a signifi cant part of the drag a t the base of a glacier may be con tributed by frictional interactions between the basal-d ebris load and the bed. REsUME. M esures directes du cisaillement a la base d'un glacier. D es j a uges de contrainte pa r contact ont ete placees sur le lit sous-glaciaire et utilisees pour relever en continu les va ria tions de la pression normale et de la contrainte d e cisailleme nt au contact avec la fa ce inferieure du glacier a 100 m sous la surface du glacier d' Argenti ere pendant des p eriodes de I' ete 1973 et du printemps 1975. Les flu ctuations mesurees d ans la pression norma le et la contrainte de cisaillem ent ne semblent pas e tre correlees a ux variations de la vitesse d e glissement. L'analyse d es donnees revele d es flu ctuations de courte duree dans la pression normal e e t la contrainte de cisailleme nt qui sem blent liees au passage de gros blocs morainiques isoles ou de groupes de blocs au-dessus du capteur. Le cisaill ement parait etre fonct ion de la concentration en volume des materiaux m orainiques dans la glace. La concentra tion e n volume en un point sembl e dependre e n panie d'un processus d e "courant" par lequel la glace basale rich e en moraine tend a s'ecouler autour des fl a ncs la teraux d es protube rances du lit glaciaire. Lorsqu' il se produit des cavites sous-glaciaires, cet effet d e courant parait lie a I'impo rtan ce de la cavite et done a la vitesse et a la pression de la glace d e couverture. On sugbere que ce process...
ABSTRACT. Contact stress transducers w er e p.1 aced in subglac ial b edrock a nd used to monitor continuously shear stress and normal pressur e changes at the contact with the overriding glacier sole 100 m beneath the surface of the Glacier d'Argentiere during periods in summer 1973 and spri ng 1975. The measured flu ctuations in normal press ure and shear st ress do n ot appear to be r elated to changes in sliding velocity. Analysis of the data reveals short-term flu ctuations in normal pressure a nd sh ea r stress which appear to be related to the passage of individual la rge debris pa rticl es or groups of p a rticl es over the transducer. The shea r stress appears to be a fun ction of the volume conce ntration of debris in the ice. The volume concentration at any point appears to be partially d ependent o n a "streami ng" process b y which basal debris-rich ice tends to flow a round the lateral flanks of hummocks on the glacier bed. Where su b-glacial cav iti es occur, this streaming effect appears to be d ependent on the extent of cavitation and thus on ice overburden pressure a nd velocity. It is suggested that this process can account for an apparent lag between changes in normal press ure and sh ear stress.The maximum ra tio b etween shear and normal stress averaged over a period of 10 min was 0.44. This is equivalent to a spatial average over 0.3 cm. D ebris concen trat ions in basal ice of up to 43% by volume occurred. It is suggested that concentrations of this order are commo n a t the base of temperate glaciers and thus that a signifi cant part of the drag a t the base of a glacier may be con tributed by frictional interactions between the basal-d ebris load and the bed. REsUME. M esures directes du cisaillement a la base d'un glacier. D es j a uges de contrainte pa r contact ont eteplacees sur le lit sous-glaciaire et utilisees pour relever en continu les va ria tions de la pression normale et de la contrainte d e cisailleme nt au contact avec la fa ce inferieure du glacier a 100 m sous la surface du glacier d' Argenti ere pendant des p eriodes de I' ete 1973 et du printemps 1975. Les flu ctuations mesurees d ans la pression norma le et la contrainte de cisaillem ent ne semblent pas e tre correlees a ux variations de la vitesse d e glissement. L'analyse d es donnees revele d es flu ctuations de courte duree dans la pression normal e e t la contrainte de cisailleme nt qui sem blent liees au passage de gros blocs morainiques isoles ou de groupes de blocs au-dessus du capteur. Le cisaill ement parait etre fonct ion de la concentration en volume des materiaux m orainiques dans la glace. La concentra tion e n volume en un point sembl e dependre e n panie d'un processus d e "courant" par lequel la glace basale rich e en moraine tend a s'ecouler autour des fl a ncs la teraux d es protube rances du lit glaciaire. Lorsqu' il se produit des cavites sous-glaciaires, cet effet d e courant parait lie a I'impo rtan ce de la cavite et done a la vitesse et a la pression de la glace d e couverture. On sugbere que ...
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