combination to CO or CO2 is that bound to the NiO.
Thus, when carbon is used as a milling additive, the
NiO surface is partially reduced after 800 ℃. As a
result, a distinct endothermic melting event is observed
between 1425 and 1454 ℃ in the differential scanning
calorimetry for the NiO milled with C (Figs. 5(b) and
6(b)). The interpretation of this event as melting is
supported by its reversibility, manifested in an
exothermic solidification event, observed from 1265 to
1230 ℃ during the cooling curve (Fig. 6(b) inset). That
the melting event is attributable to metallic or reduced
Ni formed by CO/CO2 desorption is further supported
by the observation that the melting peak is not
observed for the same differential scanning calorimetry
experiment performed in air where oxygen is present
to replenish any abstracted from the surface (data not
shown). Finally, the melting point of metallic nickel is
1455 ℃, quite close to the temperature observed in the
present study [47].
NiO milled with methanol shows a mass loss
of 0.7% (Fig. 5(a)) indicating that the alcohol
milling residue is present at the beginning of
the thermogravimetric analysis experiment. This
observation is in accord with the X-ray photoelectron
spectroscopy data which suggests the presence of a
C–O containing species. As in the case of atomic
carbon, the presence of the alcohol milling residue
results in partial reduction of the NiO surface, and an
endothermic melting transition between 1425 and
1454 ℃ (Figs. 5(b) and 6(b)). As in the case of NiO
milled with carbon, this melting transition is shown to
be reversible in the cooling curve (Fig. 6(b) inset). The
partial surface reduction and resulting melting
transition are in accord with previous studies of
methanol adsorbed on NiO. In these works, upon
heating, molecular desorption of the alcohol competes
with surface decomposition to carbon, hydrogen, and
oxygen [48]. The carbon-containing decomposition
products may abstract surface oxygen upon desorption.
In contrast to NiO milled with amorphous carbon or
methanol, the mass loss of NiO milled with Vertrel is
significantly less (0.3 wt%, Fig. 5(a)) and there are no
marked melting transitions or other significant
endothermic or exothermic events. This suggests that
Vertrel is a fairly inert milling additive that does not
decompose appreciably on the NiO surface. This
observation is in accord with the differences in the
bond dissociation energies of C–H (338 kJ/mol) versus
C–F (513 kJ/mol) [47].