Chemisorption contamination effects

In studying the chemisorption of hydrogen on carefully reduced nickel the author has actually observed that a minute quantity of the vapor of stop-cock grease or of mercury vapor from a pressure gage appreciably affect the rate of chemisorption in so far as these contaminants reduce considerably the rate of adsorption and produce the effects typical for the so-called activated adsorption. Incomplete reduction of nickel oxide to the metal leads to a similar result. This can be avoided by repeated reduction and subsequent evacuations of the metal sample at 400°C. for a week. A typical result obtained with an exhaustively reduced nickel specimen is shown in Fig. 1. In view of these findings, the activated adsorption of hydrogen on other reduced metal catalysts frequently reported in the earlier literature might have been caused by contamination effects. 

At the higher metal level (2.0-4.5% Ni with up to 2% Sb) used to study artificially contaminated materials, XRD results have shown the formation of Ni-Sb alloys (NiSb x<0.08) whereas XPS data have indicated that a non-reducible antimony oxide, a well dispersed reducible Sb phase together with reducible Sb (that form an alloy with reducible Ni), were present. Selective chemisorption data for unsupported Ni-powders showed that one surface structure can effectively passivate 2-3 Ni atoms with respect to H2 chemisorption. XPS examination confirmed that Sb segregates at the surface of Ni particles where it can drastically affect the electron properties of neighboring Ni atoms thus reducing their activity. 

The remaining vacuum pumps to be discussed in this chapter fall into a group which remove gas particles from systems by sorption effects such as adsorption, chemisorption/gettering and implantation. They tend to be used on systems where any contamination of the vacuum by pump fluids, lubricants, etc. must be avoided. However, those pumps that remove gas particles exclusively by temperature-dependent gas adsorption on molecular sieves or A1203 (adsorption pumps) will not be discussed. 

Although part of the data for activated adsorption, particularly on metal surfaces, can be ascribed to contamination or diffusion effects, there are some experimental results which cannot be explained in this way. The theoretical reasons for this type of behaviour have been discussed by Bond and by Hayward and Trapnell. In general, such chemisorptions obey the Elovich equation... 

Oxygen Chemisorption.—Most studies on Pt have used polycrystalline wires, foils, or specially cut single-crystal faces. In all cases, samples are initially heavily contaminated with graphitic surface layers. Pignet, Schmidt, and Jarvis, for example, showed by AES that heating at 400 °C in 1 atm of oxygen effectively removed surface C and S and that the resulting surface was not... 

So, clean surfaces tend to restructure to satisfy the unbalanced atomic forces. When foreign atoms adsorb onto such surfaces, further reconstruction is possible. For example, the chemisorption of contaminant atoms can destroy the clean surface reconstructions described above. Alternatively, new structures may form, as when carbon is chemisorbed on nickel (100) surfaces. If such carbon-coated surfaces were brought together in an adhesion experiment, the carbon would have to diffuse out before full Ni-Ni adhesion could be attained. Such diffusion and restracturing effects could explain the observed changes of adhesion with time. Also, hysteresis in adhesion values could then be accounted for. 

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