Chemistry - A Molecular Science

(Nora) #1

Chapter 14 Inorganic Chemistry


presence of finely divided Ni, Pt, or Pd and a high pressure of H


, the reaction proceeds 2


rapidly even at ordinary temperatures. The m


echanism for the reaction is shown in Figure


14.15. Hydrogen molecules approach the catalyst and interact with the surface (Figure 14.15a). The H-H bonds break, and the H atom


s adsorb to the catalyst surface (Figure


14.15b), where they are free to migrate. The


bonds of ethene molecules are also broken π


when they interact with the surface (Figur


e 14.15c) as two Pt-C bonds are formed (Figure


14.15d). Migrating hydrogen atoms collide with


the adsorbed ethene molecules and form


C-H bonds, breaking a Pt-C and a Pt-H bond


in the process (Figure 14.15e). A second


collision with another migrating hydrogen atom results in the product (Figure 14.15f).


Controlling the emission products in gasolin


e engine exhaust is another use of


heterogeneous catalysts. At the high operati


ng temperatures of an automobile engine,


nitrogen can react with oxygen to form NO and NO


, which are responsible for the brown 2


color of smog. If the car is incorrectly tuned,


incomplete combustion may result in unused


hydrocarbons and CO instead of CO


in the exhaust. To alleviate this problem, automobile 2


manufacturers now install catalytic converters in the exhaust system. The


catalytic


converter


is a heterogeneous catalyst that functions in much the same manner as the


platinum surface described for the hydrogenation of ethene. The hot exhaust from the combustion of gasoline is passed over a mixtur


e of Pt, Pd, and Rh. The mixture serves as a


heterogeneous catalyst for the complete com


bustion of CO and unspent hydrocarbons to


CO


and H 2


O and for the decomposition of NO and NO 2


to N 2


and O 2


. Tetraethyl lead, 2


Pb(C


H 2


) 54
, was once a common gasoline additive that was used to make ‘leaded’ gasoline

burn ‘better’ (increase the octane rating). Howeve


r, Pb atoms in the exhaust also adsorb to


the catalyst’s surface, but they do not react. Instead, they remain on the surface, blocking active sites and ‘poisoning’ the catalyst. Therefore, only ‘unleaded’ gasoline is used in the US, which has the added benefit of di


scontinuing poisonous lead emission (PbO


). 2


Homogeneous catalysts


function in the same phase as the reactants, and a


commercially important example is the transition metal-catalyzed polymerization of alkenes into polymers such as polyethylene* and polystyrene. This is a multibillion dollar industry in which millions of tons of polymers are produced annually. Consequently, even a small improvement in the functioning of a catalyst can reduce costs significantly.


The inorganic complex titanocene (Figure 14.16a) is a four-coordinate Ti(IV) complex
with one chloride ion, two organic ligands

(designated simply as L) that participate only


indirectly in the reaction, and another organic ligand (C


H 2


in Figure 14.16a) that serves as 5


the foundation upon which the polymer forms. The fact that the titanium is in a high,


Pt

Pt

Pt

Pt

HH

Pt

Pt

Pt

Pt

HH

Pt

Pt

Pt

Pt

Pt

HH

CC

H

HH

H

Pt

Pt

Pt

Pt

Pt

HH

CC
HH

HH

Pt

Pt

Pt

Pt

C

H C

H H

HHH

CC

H

H HH

HH

Pt

Pt

Pt

Pt

Pt

(a)

(b)

(c) (d)

(e)

(f)
Figure 14.15 Hydrogenation of ethene using a platinum surface as a heterogeneous catalyst
* The accepted name for C

H 2

is ethene. However, its older name, 4

ethylene, is also in common use. Thus, the polymer formed from ethene units is called polyethylene.

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