Chemisorption pressure decreases

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

CO2, N2 and N2O production as a function of the catalyst potential, UWR> obtained at 62IK for fixed inlet pressures of NO and CO. A sharp increase in reaction rate and product is observed as the catalyst potential is reduced below 0 V, i.e., upon Na supply to the Pt catalyst. The selectivity to N2, Sn2, is enhanced from 17% to 62%. This dramatic enhancement in catalytic performance is due to (a) enhanced NO vs CO chemisorption on Pt with decreasing potential and (b) Na-induced dissociation of chemisorbed NO. 

A third empirical criterion is based on the effect of temperature on the amount adsorbed. For physical adsorption the amount of gas adsorbed always decreases monotonically as the temperature is increased. Significant amounts of physical adsorption should not occur at temperatures in excess of the normal boiling point at the operating pressure. Appreciable chemisorption can occur at temperatures above the boiling point and even above the critical temperature of the material. Because chemisorption can be an activated process that takes place at a slow rate, it may be difficult to determine the amount of chemisorption corresponding to true equilibrium. Moreover, the process may not be reversible. It is also possible for two or more types of chemisorption or for chemical and physical adsorption to occur simultaneously on the same surface. These facts make it difficult to generalize with regard to the effect of temperature on the amount adsorbed. Different behavior will be observed for different adsorbent-adsorbate systems. 

Adsorbent regeneration is normally accomplished by reversing the adsorption process, either by decreasing the system pressure or, more commonly, by increasing the system temperature. In some cases, particularly in chemisorption systems, the adsorbent activity can be restored by reaction with a suitable reagent. 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

The adsorption of oxygen on diamond was studied by Barrer (156). Essentially no chemisorption was observed at —78°. From 0 to 144° oxygen was chemisorbed, but no carbon oxides were liberated. Some carbon dioxide was formed as well from 244 to 370° by interaction of oxygen and diamond surface not covered with surface oxides. Surfaee oxide formation was observed at low pressures. The coefficient of friction of diamond increases considerably after heating in a high vacuum. The measurements by Bowden and Hanwell (157) showed a decrease in the friction on access of oxygen, even at very low pressures. 

Typical magnetization-volume and pressure-volume isotherms obtained by this method are shown in Fig. 12. The magnetization-volume isotherm is linear i.e., each hydrogen molecule adsorbed causes the same decrease in magnetization. In the interpretation of these results, and those for other gases to which this method has been applied, it has been assumed on the basis of reasonable, but not conclusive, evidence that the slope of this isotherm corresponds to the entry of one electron into the nickel d-band for each chemisorhed H atom. From Fig. 12 it is evident that chemisorption of hydrogen is not complete at one atmosphere, and recent measurements (Vaska and Selwood, 102) indicate that it continues to much higher pressures. 

Here (no- ), is the equilibrium concentration of chemisorbed oxygen at constant temperature and oxygen pressure. In Fig. 13 the decrease of the heat of chemisorption with increasing surface concentration of chemisorbed particles is schematically represented. Frankenburg (75) proposed a similar interpretation for the decrease of heats of chemisorption of hydrogen on tungsten. 

Before 1916, adsorption theories postulated either a condensed liquid film or a compressed gaseous layer which decreases in density as the distance from the surface increases. Langmuir (1916) was of the opinion that, because of the rapidity with which intermolecular forces fall off with distance, adsorbed layers are not likely to be more than one molecular layer in thickness. This view is generally accepted for chemisorption and for physical adsorption at low pressures and moderately high temperatures. 

Equation (72) can, of course, represent 6 values as a function of the pressure p (or the concentration c) provided that these values are not too close to zero or too close to unity. In the range of medium values of 6, it adequately represents many cases of chemisorption (369). This isotherm equation will always fit the experimental data when the heat of chemisorption shows an approximately linear decrease with increasing 0 values it is not necessary that an activation energy be present. This may be seen from Eqs. (71) and (72) even when h = 0, Eq. (72) results. 

The upper part of Figure 3.11 shows data on the total chemisorption of hydrogen (at 100-torr equilibrium pressure). The lower part shows data on the strongly chemisorbed fraction, that is, the amount which cannot be removed by evacuation to 10 6 torr at room temperature. Both total and strong chemisorption decrease as the Cu/Ru intensity ratio increases, but the percentage decrease is greater for the strong chemisorption. 

---