Page 238 - Introduction to chemical reaction engineering and kinetics

220 Chapter 8: Catalysis and Catalytic Reactions

Consider two cases: (i) the product is strongly adsorbed and inhibits the reaction; and (ii)
it is very weakly adsorbed.
(b) The decomposition of acetaldehyde on Pt at temperatures between 960 and 12OO”C, and
at pressures between 3.33 and 40.0 kPa, appears to be a bimolecular reaction with no
inhibition by reaction products.
(c) A study of the kinetics of ethanol dehydrogenation over Cu in the presence of water vapor,
acetone, or benzene showed that any one of these three inhibited the reaction.
(d) In the reaction of nitrous oxide (NzO) with hydrogen over Pt (SOi’-580°C pnZ = 7 to 53
kPa, pQo = 40 to 53 kPa), it has been observed that NzO is weakly adsorbed and HZ is
very strongly adsorbed.
8-6 For the surface-catalyzed gas-phase reaction A(g) + B(g) -+ products, what is the form of
the rate law, according to the LH model, if A is strongly adsorbed and B is weakly adsorbed?
Assume there is no adsorption of product(s). Interpret the results beyond what is already spec-
ified.
8-7 For the reaction in problem 8-6, suppose there is one product P which can be adsorbed. Derive
the form of the rate law according to the LH model, if
(a) A, B, and P are all moderately adsorbed;
(b) A and B are weakly adsorbed and P is strongly adsorbed. (Interpret the result further.)
8-8 Consider the reaction mechanism for methanol synthesis proposed in Figure 8.3:

CO.s+H.s -+ HCO.s+s (1)
HCO.s+H.s --z H$ZO.s+s (2)
H2C0.s +H.s + HaC0.s + s (3)
HsCO.s+H.s -+ HsCOH.s+s (4)

Assume that the coverages of H, CO, and methanol are given by the Langmuir adsorption
isotherm in which CO, Hz, and methanol adsorption compete for the same sites, and the in-
termediates H,CO@s are present in negligible quantities.
(a) Assume that step (1) is rate limiting, and write the general rate expression.
(b) Assume that (3) is rate limiting (steps (1) and (2) are in equilibrium), and write the general
rate expression.
(c) Experimental data are represented by = kpC$5di;

v
rCH,OH
“OP
0 (d) How would the rate law change if the H,CO*s intermediates were allowed to cover a
To obtain this rate law, which of the surface steps above is rate limiting?
substantial fraction of sites? (This can be attempted analytically, or you may resort to
simulations.)
8-9 (a) Rate laws for the decomposition of PHa (A) on the surface of MO (as catalyst) in the
temperature range 843-9 18 K are as follows:

pressure, p&Pa rate law
+O (-IA) = +A
8 x 1O-3 (-rA) = kPd@ + bpA)
2.6 x 1O-2 (-rA) = constaut

Interpret these results in terms of a Langmuir-Hinshelwood mechanism.
(b) In the decomposition of N20 on Pt, if NzO is weakly adsorbed and 02 is moderately
adsorbed, what form of rate law would be expected based on a Langmuir-Hinshelwood
mechanism? Explain briefly.