Surface area effect

Surface area effects in mass transfer or heterogeneous reactions... 

SURFACE AREA EFFECTS IN MASS TRANSFER OR HETEROGENEOUS REACTIONS 51... 

Substances Hazardous to Health, See also Toxic chemicals, 19, 138, 140 Substitution, 133 Sulphur, 30 Sulphur dioxide first aid, 280, 306 physical properties, 306 physiological properties, 69, 71, 304, 305 precautions, 305, 306 vapour pressure, 305 Suppression, 135 Surface area effects, 50, 199 Symbols for dangerous substances, 446, 452 Synthetic lubricants, 159 Synthetic resins, 172 Systemic poisons, 19, 77... 

Surface area effect on the relaxation time constant In a previous communication (17) we have developed a simple dynamic model which allows one to predict the change in the mole number of silver oxide S Ac, therefore AV, in terms of the imposed current, Pq and The dynamic equation of the model was... 

Matrix composition can also overcome surface area effects. A high silica matrix of moderate surface area was found to have essentially the same contaminant selectivities as a moderate silica matrix having only very low surface area. 

Smooth Pt and Rh electrodes have been compared [105, 269] with electrodepos-ited layers to investigate the effect of roughness (which may be of the order of 103). While electrodeposited Pt absorbs hydrogen and bright Pt does not, no surface area effect has been observed as for hydrogen evolution. This indicates that the internal surface (pores) of rough electrodes does not work because of exclusion due to gas formation. Thus, the porosity of the active layers also needs to be characterized. It has been shown that in the case of Raney Ni [270] this can be conveniently done by means of impedance measurements [271]. 

While no effects of simple roughness (except surface area effects) has been observed for the electrocatalytic activity of Ni, the situation appears to differ with mild steel for which a decrease in Tafel slope has been observed in the low overpotential range [272] (Fig. 9). An investigation of the effect of temperature and NaOH concentration has suggested [273] that in fact this may be related to the appearance, at low overpotential, of surface oxides, whose reduction leads to the break in Tafel line. Again, these results warn against interpreting certain apparent Tafel slopes only in terms of kinetic mechanisms. 

The catalytic activity of these oxometalates is well documented [360, 361]. An inactive surface of Ti02 becomes an efficient catalyst for H2 evolution as it is derivatized with silicotungstic acid [362, 363]. However, while real electrocatalytic effects seem likely for a pigmented Ni surface in view of the lower Tafel slope observed (which can also be due to some activation of the Ni itself), these are not completely established for the surface of pure oxometalates surface area effects could be entirely responsible for the apparent activation. The real surface state of these electrodes deserves to be further investigated since these materials might fall into the category of amorphous phases. 

Nanocomposites are already making an impact on the choice and use of polymeric materials. As the dimensions of the particles diminish into the range of a few nanometers, surface area effects dominate, changing fundamentally the interactions between particle and polymer. Often nanocomposites containing less than 5% additive have substantially improved properties with no adverse effects. 

Higgins 171) warned against misinterpretation of promoter effects in catalysts prepared by incipient wetness and coprecipitation using acidic solutions. Acidic solutions can cause the formation of V0P04 2H20, which has a detrimental effect on the catalyst performance that can be mistakenly attributed to the promoter. These authors recommend that these methods of introducing promoters be used with care, and in particular they recommended that the acidity of the impregnation solution be carefully monitored. It is also clear that to imderstand the promotional effects of dopants, variables such as preparation methods and surface area effects must be taken into accoimt. 

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