Surface Vycor glass

Similar, very detailed studies were made by Ebert [112] on water adsorbed on alumina with similar conclusions. Water adsorbed on zeolites showed a dielectric constant of only 14-21, indicating greatly reduced mobility of the water dipoles [113]. Similar results were found for ammonia adsorbed in Vycor glass [114]. Klier and Zettlemoyer [114a] have reviewed a number of aspects of the molecular structure and dynamics of water at the surface of an inorganic material. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

Vycor glass (Corning, 7930) is porous silica processed in the same way as CPG except that the last step of base etching is missing. Therefore the surface is rough. Only a 40-A pore is available. When porous silica with a pore size smaller than 75 A is needed, Vycor glass is the choice. Bulk pieces of Vycor glass are available commercially and need to be crushed into small particles before use. 

We have also obtained measurements for the relaxation time of octafluorocyclo-butane (c-C4F8) gas in porous Vycor glass. Vycor (Corning Glass, -96.5 wt-% Si02) has a specific surface area of approximately 120 m2 g-1, and was cleaned using H202 and evacuated to 10-8 Torr before all measurements. The Vycor was placed in... 

Solid surfaces nature of the surface of colloidal silica, clays, zeolites, silica gels, porous Vycor glasses, alumina rigidity, polarity and modification of surfaces... 

Surface diffusion has been extensively studied in literature. An overview of experimental data is given in Table 6.1. Okazaki, Tamon and Toei (1981), for example, measured the transport of propane through Vycor glass with a pore radius of 3.5 nm at 303 K and variable pressure (see Table 6.1). The corrected gas phase permeability was 0.69 m -m/m -h-bar, while the surface permeability was 0.55 m -m/m -h-bar, and so almost as large as the gas phase permeability (Table 6.1). It is clear from Table 6.1, that the effects of surface diffusion, especially at moderate temperatures, can be pronounced. At higher temperatures, adsorption decreases and it can be expected that surface diffusion will become less pronounced. 

Fig. 1. Magnetic field dependences of the proton spin-lattice relaxation time of water in Bioran B30 and Vycor glasses at temperatures above 27°C and below the temperature where the non-surface water freezes ( —25°C and —35°C). The solid lines represent the power law in the Larmor frequency with an exponent of 0.67 (34).

The interaction of Ru3(CO)i2 with Si02 seems to proceed via a hydride surface carbonyl species. An ulterior decomposition under vacuum mainly gives Ru particles of 1.4nm and Ru(II) carbonyl species ]90]. Similarly, [HRu3(CO)io( t-OSi)]surface specics are generated by the reachon of Ru3(CO)i2 with a porous Vycor glass at 65 °C under air ]103]. Above 130 °C it was reported that the surface cluster breaks down with formation of ]Ru(CO) (OSi)2]surface species (n = 2 and/or 3). When the initial surface species was heated in air at T> 250°C, decomposition of the cluster and formation of RUO2 nanoparticles were observed. 

Irradiation of porous Vycor glass with UV light (< 330 nm) in the presence of oxygen (256) leads to photoadsorption and an EPR signal with g = 2.0310, 2.0109, and 2.0053 this is again consistent with an ionic O2 on the surface. 

Porous vycor glass Surface-bound, octahedrally coordinated Fe3+ species obtained in the photolysis of Fe(CO)5 physisorbed onto the glass Magnetically ordered materials were obtained which exhibited magnetic hyperfine fields of 370 and 425 kG 798... 

The separation of dioxygen from air has been carried out using a cobalt porphyrin complex tethered to the surface of a Vycor glass mem-... 

Studies of inorganic photochemistry in unusual environments has attracted considerable attention. Photochemical studies conducted in organized assemblies such as micelles, microemulsions and vesicles,217 on surfaces such as porous Vycor glass,218 in a lamellar solid,219 and in the gas phase have been reported.220... 

Shiotani et at. (65b) have reported the formation of several oxygen species on titanium-supported surfaces. Two of these are produced by UV irradiation of the titanium-porous Vycor glass system in the presence of N20 or 02 at 77 K and one is believed to be an O- ion. Their g tensors are not complete and there is no further data from 17 O-labeled 02 or N20 or on their reactivity to support the assignment to O" ions. 

Coke formed on solid surfaces during the pyrolyses of acetylene, ethylene, ethane, propylene, and butadiene were examined by using a scanning electron microscope. Seven types of coke have been identified braided filament, uniform diameter filament, needle or spike, ribbon, fluffy or cottonlike fibers, knobby, and amphorous. The first four types contained metal (especially iron) and were magnetic. Magnetic cokes formed sometimes on Incoloy 800, stainless steel 304, stainless steel 410, and Hastelloy X surfaces, but never on Vycor glass or aluminized Incoloy 800 surfaces. Conditions at which each type of coke was formed are discussed. 

Ethane was pyrolyzed in several tubular reactors having internal diameters of about 0.47 cm and a heated length of 107 cm. The reactors used were constructed of Incoloy 800, stainless steel 304 (SS 304), stainless steel 410 (SS 410), Hastelloy X, and Vycor glass. Each reactor was maintained at almost isothermal conditions by suspending it in a fluidized sand bath. More details on the reactors are described by Dunkleman and Albright (12) and Herriott, Eckert, and Albright (13). After suitable pyrolysis, the reactor was cut to expose the coke on the inner surfaces. 

Variables Affecting Type of Coke. The material of construction of the solid on which the coke formed, the temperature, and the space time all affected the type of coke formed. Cokes formed on Incoloy 800, SS 304, SS 410, and Hastelloy X were sometimes magnetic. Cokes formed in an alonized surface were always nonmagnetic, and no metals were detected by EDAX except for a trace of aluminum. Cokes formed in Vycor glass reactors were also nonmagnetic. 

The results for Run 19 (Vycor glass reactor), Run 21 (alonized Incoloy 800 reactor), and Run 14 (coke-covered Incoloy 800 reactor) were similar to both the kinetics and type of products obtained. Although neither oxygen or hydrogen pretreatments were tried in Vycor glass or alonized Incoloy 800 reactors prior to acetylene pyrolyses, it is thought that such pretreatments would have little or no effect on acetylene reactions. This conclusion is based on such pretreatments prior to pyrolysis with other hydrocarbons in these two reactors. It has been concluded that all increases in acetylene conversions above those of Runs 14, 19, and 21 were in some way caused by surface reactions. Based on this assumption, surface reactions were of major importance in Runs 15, 18, and 23. 

Visual observation of the Vycor glass reactor immediately following the butadiene run at 700°C resulted in important information. The reactor was cut to permit inspection of the black deposits thought to be primarily coke. The last two-thirds of the reactor and a short section of the unheated tube that extended beyond the furnace were covered on the inner surface with a smooth layer of coke. This deposit, when viewed from the outside of the reactor, appeared as a black mirror. It is of special interest that the inlet section of the tubular reactor did not have any coke deposits. This section was the one that was subjected to increasing temperatures in the furnace. The start of the coke deposits occurred approximately in the section where maximum temperatures occurred during a run. Most of the deposits appeared to occur in the... 

Methane was formed in significant amounts in such cases it is thought that hydrogen reacts with the surface coke or metal carbides on the surface, such as has been shown earlier to occur (1). At the same temperatures (600°-700°C), little or no reactions occurred in the Vycor glass or in the alonized Incoloy 800 reactors. Clearly the Incoloy 800 surfaces were promoting significant coking reactions at these temperatures. 

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