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Fig. 9 Representative TEM images with EDS mapping of Cu-Mn-O precursors prepared
by SAS (left hand side) and conventional co-precipitation (right hand side).
62
Reproduced
in part from ChemCatChem, (2009), 1, page 248 with permission of John Wiley and Sons.
CuMn
2
O
4
from precursors produced by more conventional co-
precipitation of metal nitrate salts with Na
2
CO
3
requires temperatures in
excess of 400 1C, and additional by-product phases such as CuO and
Mn
2
O
3
are commonly present.
65
Transmission electron microscopy
(TEM) analysis of the SAS precipitated mixed metal acetates showed the
exceptional degree of Cu and Mn mixing afforded by the high diffusion
rates present in the SAS process (Fig. 9). In contrast, Cu and Mn phase
separation was observed in the co-precipitated material, due to the far
lower diffusion rates observed in the liquid phase mixing of the nitrate
salt solution and the Na
2
CO
3
solution.
The lower calcination temperature required to form CuMn
2
O
4
from
the SAS precursor can be attributed to the high degree of intimate mixing
of Cu and Mn phases, limiting the required metal ion migration to form
CuMn
2
O
4
. A further publication focusing on the heat treatment of SAS
and mechanically mixed Cu and Mn acetates found much greater inter-
action between Cu and Mn phases in SAS prepared materials, supporting
this hypothesis.
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This study further showed that decomposition of the
mixed metal acetates under an inert environment resulted in reduction of
the Cu
2
รพ
phases to nanoparticles of metallic Cu supported on MnO. In a
similar manner to that observed for the vanadium phosphate system, the
catalytic activity normalised for the surface area of the SAS CuMn
2
O
4
material was higher than that of a standard coprecipitated Hopcalite and
an industrial catalyst (Fig. 10). STEM analysis of the calcined SAS
prepared hopcalite showed that the activity correlated with the degree of
homogeneity of the copper and manganese mixing. Higher activity was
observed for samples that were well mixed and poorer activity in samples
that showed segregated single oxide phases. However, the surface areas
of standard materials were considerably higher than that produced by
SAS, and this resulted in a lower mass normalised activity for the SAS
material.
Further studies that investigated the addition of 0-15 vol% water as
a co-solvent to the Cu and Mn acetate in ethanol solution, found
that water had a profound effect on the SAS precipitated materials.
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