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such as catalysts (NiO, Cr
2
O
3
,Co
3
O
4
,Fe
2
O
3
, Mg(OH)
2
and 1%Pt/g-Al
2
O
3
),
were carried out. A planetary ball mill (Fritsch Pulverisette 6) was filled
with the solid (1 g) and three balls (12 g, d
¼
10 mm, hardened steel),
and pressurized to 10 MPa with a mixture of reaction gases (CO/O
2
/
He
¼
1 : 20 : 79 v/v%). The gas/solid mixture was milled for 2-4 h at room
temperature and the gas phase was analyzed at the end of the experiment.
CO conversion was observed for NiO (22% after 2 h), Cr
2
O
3
(10%), Pt/Al
2
O
3
(48%) and Co
3
O
4
(7.3% after 4 h). The use of Fe
2
O
3
and Mg(OH)
2
did not lead
to any conversion after 2 h.
Selected active oxidants were then studied in more detail in continuous
experiments to provide closer insight into the process. For this purpose, a
commercial shaker mill (Retsch MM200) was modified to allow for the
continuous operation of heterogeneously catalyzed gas-phase reactions,
which led to a dramatic increase in the reaction rate. Without milling, no CO
conversion into CO
2
was observed at room temperature over Cr
2
O
3
. The
nature of the highly active sites is not clear, although defects induced by the
high mechanical-energy input probably contribute strongly. Although it is
very dicult to obtain insight into the processes that occur during milling, a
recent study has introduced a method for in situ XRD (X-ray diffraction)
experiments during ball milling that may be able to shed light onto the
catalytic reactions during milling. Ball milling is possible on a production
scale (in the cement industry, mills with volumes of 100 m
3
are in use) and if
improvements in catalytic effectiveness outweigh the more complex process
and potentially high running costs, which are due to milling energy re-
quirements, it may become suitable for industrial applications.
20
3.3 Mechanochemical Reduction
Castricum et al. have found that the creation of various types of defects when
zinc oxide (ZnO) is milled (in a vibratory ball mill under vacuum or in the
presence of oxygen) results in an increased amorphous fraction, as well as
higher surface area.
21
Mechanochemical reactions occur when copper and
copper oxides are milled together with zinc oxide; the oxidation of copper
and copper oxides takes place in the presence of oxygen, whereas reduction
takes place in vacuum. These reactions are promoted by the presence of
ZnO. The formation of a Cu
2
O-like intermediate is suggested, which is not
observed when ZnO is not present.
The high-intensity milling of ZnO results in an increased fraction of
amorphous ZnO and in a higher surface area, probably due to surface
roughening or increased dispersion. The milling of mixtures of Cu, Cu
2
Oor
CuO and ZnO results in the oxidation of Cu precursors when milling occurs
in synthetic air, and reduction when milling occurs under vacuum. These
mechanochemical reactions are promoted by the presence of ZnO. The
formation of a Cu
2
O-like intermediate, which is closely attached to ZnO, has
been observed as well as several copper oxide species that reduce at different
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