Page 137 - Introduction to chemical reaction engineering and kinetics

6.2 Description of Elementary Chemical Reactions 119

l Electron attachment:
e - + X - + X -

* Electron-impact ionization:
e- + X -+ X+ + 2e-

l Ion-molecule reactions:

CsHT+ + CbHs + CsH, + C4H9+ (bimolecular exchange)
H+ + C3H, -+ CsH,+ (bimolecular association)

6.2.1.4 Elementary Reactions on Surfaces
Surface reactions are important in heterogeneous reactions and catalysis.
l Adsorption/desorption-molecules or fragments from gas or liquid bond to solid sur-
face.
l Simple adsorption-molecule remains intact.

? - 9
C
c +- I
Ni Ni Ni Ni Ni Ni

l Dissociative adsorption-molecule forms two or more surface-bound species.

H-H ----c H H
+- I
cu cu cu cu cu cu

l Site hopping-surface-bound intermediates move between binding sites on surface.
l Surface reactions-similar to gas-phase arrangements, but occur while species
bonded to a solid surface.
l Dissociation:

H
H\I/H HyH
?
F - E ‘I;’
Pt Pt Pt Pt Pt Pt Pt Pt Pt

l Combination:

Rearrangements of the adsorbed species are also possible.