The Fundamentals of Adsorption
1. The first eight problems are linked to the content of this chapter. To tackle the remainder, the reader is recommended to consult either some later chapters or the relevant reference cited.
Discuss with the aid of a potential-energy diagram, the process of dissociative chemisorptions of a diatomic gas at a metal surface. What relationship exists between the enthalpy of chemisorptions and the activation energy of desorption?
Thermal desorption spectra of atomic oxygen bound to the surface of a ruthenium catalyst show symmetrical peaks with a characteristic shift to lower temperatures at higher initial coverage, suggesting second-order kinetics. Using the data given in the table, confirm the reaction order and estimate the activation energy of desorption.
|
Relative desorption rate (peak height)
|
Coverage
|
Temperature(oC)
|
|
0.83
|
1
|
448
|
|
1.6
|
0.92
|
468
|
|
2.07
|
0.8
|
476
|
|
2.93
|
0.66
|
494
|
|
2.4
|
0.32
|
522
|
|
0.89
|
0.14
|
540
|
|
0.13
|
0.04
|
548
|
2. At a temperature of 190 k and a pressure of CO of 48 Torr, 1/10th of a monolayer of adsorbate is present ate a nobale metal surface. At 250 k a partial pressure of 320 Torr is required at a noble metal surface. At 250 k a partial pressure of 320 Torr is required to achieve the same surface coverage. Estimate the molar enthalpy of adsorption, and comment upon whether the gas is likely to be physically adsorbed or chemisorbed. What other measurements would you perform to identify the nature of the adsorption?
3. It is alleged that the desorption of xenon from a graphite surface obeys zero-order kinetics (as does the desorption of certain metals from the surface of silicon). How would you (i) establish experimentally that this is indeed so and (ii) interpret such a result?
4. Estimate how long it would take to cover an atomically clean surface of a nickel catalyst with a monolayer of oxide if it were exposed to O2 at a pressure of(a) 7.60 x10-4 Torr or 1.33 x 10-8 Pa, or (c) to a good laboratory vacuum.
5. What information can you glean from the following sets of data?
a) Acidic mordenite takes up the following amounts x of n-hexane as a function of pressure p at a temperature of 100oC.
| x(mmol g-1) |
p(Torr) |
| 0.531 |
4.4 |
| 0.555 |
10.8 |
| 0.587 |
21.2 |
| 0.6 |
41.5 |
| 0.607 |
54.6 |
| 0.612 |
57.4 |
b) A pillared clay catalyst takes up the following amounts of N2 at 78K:
| 102 g g-1 |
p(Torr) |
| 7.63 |
21.1 |
| 8.75 |
130.9 |
| 9.05 |
195 |
| 9.46 |
287.2 |
| 10 |
422.4 |
| 11 |
490.3 |
| 9.6 |
276.6 |
| 9.1 |
159 |
| 7.95 |
31.8 |
| 7.19 |
9.4 |
6. It is known that the enthalpies of adsorption of an atom bound to sites at (a) a flat surface, (b) a ledge site and (c) a kink site are, respectively, 10, 20 and 30 kcal mo1-1 (42, 84 and 126 kJ mo1-1). Estimate the residence time of the atom on each of these sites at 300 K.
7 From the information in the table below pertaining to the adsorption of CO on Ca2+ -exchanged zeolite-Y, determine the isosteric heats of adsorption at each of the coverages.
| T(k) |
Pressure(N m-2) |
|
Amount Adsorbed (106 m3 g-1) |
|
|
0.05 |
0.1 |
0.2 |
0.4 |
0.6 |
| 273 |
|
15.2 |
34.2 |
116.2 |
310.5 |
| 290.3 |
|
40 |
100 |
356.2 |
834.3 |
| 304.4 |
28.6 |
80 |
247.6 |
|
|
| 322.6 |
914.4 |
247.6 |
685.7 |
|
|
This table has been compiled from Figure 4 of Egerton and Stone (1970).
These workers establish that CO is a sensitive probe molecule for investigating the location of unshielded multivalent cations in zeolitic catalysts.