Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 199

molecular Cl 2 . The required reaction temperature of 720 K is dictated by the
high activation for the re-oxidation of CuCl 2 so that the conversion of HCl is
limited to 75% for thermodynamic reasons. Even worse the catalyst is not
stable since the CuCl 2 formed is volatile at such high temperatures leading
to rapid and uncontrolled loss of the active component during the Deacon
reaction.
Alternative catalysts for the Deacon process have been proposed but
most of them have not been industrialized. Instead electrolysis or simple
neutralization to salts have been the standard ways to get around the waste
disposal problem of the toxic HCl. 3,4 Recently (1999), Sumitomo Chemical
introduced and commercialized the first stable and active catalyst for the
Deacon process which is based on RuO 2 . The reaction temperature can be
kept as low as 600 K so that the equilibrium conversion is 90-95%. 5,6 In
addition, RuO 2 is stable, partly due to the low reaction temperature and
partly due to the surface chlorination of RuO 2 . Under reaction conditions a
fraction of the surface lattice O atoms is replaced by chlorine 7 ; bridging
chlorine is further stabilized by Ti doping. 8
The scarcity and the high cost of Ru have initiated an intensive search for
alternative materials as potential catalysts for the HCl oxidation. 4,9,10
Recently, it was demonstrated that CeO 2 represents an interesting option
for replacing RuO 2 . 11,12 The activity of CeO 2 -based catalysts in the HCl
oxidation reaction was proposed to be governed by the oxygen storage
capacity (OSC). Unfortunately, pure CeO 2 is chemically not stable under HCl-
rich reaction conditions, 12,13 and currently the activity of CeO 2 is too low to
be economically competitive with RuO 2 -based catalysts. 10,12 However, it is
known that doping isovalent elements like Zr 41 into the CeO 2 lattice strongly
affects the redox properties of ceria 14-17 and improves the thermal stability
of CeO 2 against sintering. 18 Therefore, Zr doping may be a promising
strategy to stabilize CeO 2 against in-depth chlorination in the Deacon pro-
cess via formation of a solid solution of CeO 2 and ZrO 2 which may have
higher activity as well.

d n 9 r 4 n g | 8

.

8.2 Why Model Catalysis?
In heterogeneous catalysis the reaction mixture comes in contact with the
surface of the catalyst, which consists of supported particles of the active
component dispersed and immobilized over high surface carriers, often
oxides such as MgO, SiO 2 ,orAl 2 O 3 . The particles expose various facets with
defects such as edges and kinks. Therefore, from a structural point of view
heterogeneous catalysts represent a quite intricate situation. In general, the
reaction mechanism consists of the adsorption of the reactants and the
subsequent formation of the desired product in a surface reaction some-
times including lattice atoms such as oxygen in oxides. 19 The interaction of
the active component with the support may improve the activity and se-
lectivity of the catalyst. 20,21 But also particle size and particle morphology