Page 178 - Principles of Catalyst Development

166 CHAPTER 7
7.4.3.2. Reaction Titration

In cases not readily amenable to direct chemisorption, titration of
specific sites is possible with probe reactions. The reaction must be an
irreversible interaction between a gas and surface site, such that only one
event occurs per site. The first example of this technique was the titration
of Pt-O sites.(241) The surface of supported platinum is first oxidized to
form no more than a monolayer. Then follows reaction with hydrogen

(7.16)

Uptake of hydrogen is increased by a factor of 3 over hydrogen
chemisorption. This results in greater precision for catalysts with low
loading.
Another example is copper surface area measurement in Raney Cu
and Ni-Cu alloys. When reacted with N2 0, most conveniently in a pulse
apparatus, Cu reacts as follows:

(7.17)

Measurement of eluted nitrogen pulses gives the surface copper concentra-
tion directly.(244) The same technique has been used with supported silver
catalysts.(245)

7.4.3.3. Poison Titration
Poison titration is a convenient way to measure the concentration of
active sites. The best procedure is to use a simple pulse reactor, such as
that in Fig. 7.26. Pulses of a poisoning agent are injected between reactant
pulses. If all the poison adsorbs irreversibly, then activity declines with
each pulse. Typical results are shown in Fig. 7.27, in which hydrogen sulfide
poisons metal sites. (246) Extrapolation of the activity curve to zero gives the
amount of poison necessary to neutralize the active sites. A knowledge of
surface stoichiometry is necessary to proceed further. For example, in Fig.
7.27 the assumed ratio was two nickel for each sulfur.(246) This technique
has the potential for innovative application to many systems.

7.4.4. Acidity
The nature of acidity on insulating oxides is discussed in Chapter 4.
Important factors are (1) type of acidity, (2) acid strength, and (3) distribu-
tion of acid strengths. It is not easy to distinguish between Lewis or Bronsted