Chemisorption capacity

Regardless of method, desorption is never complete. Adsorbent capacity is always less following regeneration than it is on initial loading of adsorbent. Some adsorbable materials undergo chemisorption they chemically combine with the adsorbent. An example is the Reinluft process (52) for removing SO2 from flue gas on activated carbon. The SO2 is attached to the carbon as sulfuric acid. Desorption occurs only upon heating to 370°C a mixture of CO2, evolved from the chemically bound carbon, and SO2 are driven off. 

The presence of alkali promoters on the substrate surface can affect both the rate of chemisorption, (e.g. on K/Rh(100))55 and the adsorptive capacity... 

The type catalyst, Pt/Ti02 reduced at 300 C behaves much like Pt/Si02, but reduction at 500°C largely eliminates its capacity for the chemisorption of H2 and hydrogenation while inducing activity for the hydrogenation of CO. Ti suboxide formed by reduction encapsulates the Pt particles. 

The amount of coke formed as a function of the number of turnovers is shown in Fig. 2. The steeper slopes of these curves for Pt/y-Al203 and Pt/Ti02 indicate the higher selectivity of Pt/y-AI2O3 and Pt/TiOj to form coke than Pt/ZrOj Hydrogen chemisorption capacity decreased nwkedly after some time on stream (see Table 2), but could be completely restored by oxidative treatment. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

The surface area and the dimensions and volume of the pores can be determined in many ways. A convenient method is based on measurement of the capacity for adsorption. The experimental techniques do not differ from those used for chemisorption (see Section 3.6.3). The fundamental difference between physi.sorption and chemisorption is that in chemisorption chemical bonds are formed, and, as a consequence, the number of specific sites is measured, whereas in physisorption the bonds are weak so that non-chemical properties, in particular the surface area, are determined. 

Figure 3.51. CO chemisorption by pulse response of a reduced 5 wt% Pt/Al203 a. Thermal Conductivity Detector (TCD) signals after the CO pulses, b. Cumulative amount of CO chemisorbed. The monolayer capacity is 0.06 mmol/g Pt, corresponding with a dispersion of 24%.

It should be noted that excitons can annihilate on surface defects as well, in particular on chemisorbed particles participating in the reaction. This involves a change in the charged state of these particles and, as a result, the chemisorption capacity of the surface with respect to these particles and the rate of the reaction in which these particles participate are also changed. This case requires a special investigation since the quantities p and involved in the theory are of a different form (8) than in the case of the electronic mechanism of light absorption to which our attention was restricted in the present article. 

FTS activity (215-232°C, 19-28 bar, H2/CO = 1.98-2.28) in a two-stage slurry bubble column using a Co/A1203 catalyst, (b) Correlation between amounts of residual carbon after 02 treatment and H2 chemisorption capacity. (Drawn from data provided in Gruver et al.34)... 

Hydrogen is chemisorbed by diamond at temperatures from 400° upwards as was shown by Barrer (134b). Apparently, surface hydrides are formed as is indicated by the decrease in the capacity for potassium chemisorption (Table XIII). A significant decrease was also measured for the heat of immersion in water after hydrogen treatment at 800° [(55), Table XIV]. Methane is liberated when hydride-covered diamond is heated in a vacuum (153c). 

The capacity of isolated Ta hydrides to fully cleave the NsN bond is thus original with respect to surface science [the dissociative chemisorption of dinitrogen on... 

---