In most cases, this reaction requires a temperature in the range 1200–1500 K, but
adding water to the system leads to a signifi cant reduction in the reaction temperature:
MeCl x þ
x
2
H 2 O þ
2y À x
2
O 2 ) MeO y þ x HCl
ð4: 13Þ
However, in many cases, the advantage of a reduced reaction temperature is
outweighed by the disadvantage of having highly corrosive hydrochloric acid as a
byproduct in the system. If available, the use of a carbonyl is recommended; for
example, in the synthesis of maghemite (c-Fe 2 O 3 ) it would be:
2FeðCOÞ 5 þ
13
2
O 2 ) Fe 2 O 3 þ 10CO 2
ð4: 14Þ
For the available carbonyls, carbonyl chlorides or nitrosyl carbonyls, this process
will operate successfully in most cases at temperatures below 600 or 700 K. In many
cases, the advantage of a low temperature for evaporation of the precursor and for
the reaction is more important than the high price and dif ficult handling. Handling
is difficult because most carbonyls are highly toxic and have limited stability in the
open air. However, by selecting appropriate temperatures, the use of carbonyls will
provide much more freedom in terms of the morphology of the intended powder.
Low temperatures lead to extremely fi ne and fluffy – in most cases amorphous –
powders, whereas the products tend to be more crystalline when using a higher
reaction temperature. A typical example of such a fine powder, in this case
amorphous Fe 2 O 3 , is shown in Figure 4.14. These fine powders also tend to
Figure 4.14 Fe 2 O 3 powder prepared by
decomposition of Fe(CO) 5 and subsequent
oxidation. (Reproduced with permission by
MACH I Inc., King of Prussia, PA, USA;
www.machichemicals.com). This product
consists of fractal agglomerates of amorphous
3-nm particles. The high surface product has
excellent catalytic properties.
4.3 Physical and Chemical Vapor Synthesis Processes j59
adding water to the system leads to a signifi cant reduction in the reaction temperature:
MeCl x þ
x
2
H 2 O þ
2y À x
2
O 2 ) MeO y þ x HCl
ð4: 13Þ
However, in many cases, the advantage of a reduced reaction temperature is
outweighed by the disadvantage of having highly corrosive hydrochloric acid as a
byproduct in the system. If available, the use of a carbonyl is recommended; for
example, in the synthesis of maghemite (c-Fe 2 O 3 ) it would be:
2FeðCOÞ 5 þ
13
2
O 2 ) Fe 2 O 3 þ 10CO 2
ð4: 14Þ
For the available carbonyls, carbonyl chlorides or nitrosyl carbonyls, this process
will operate successfully in most cases at temperatures below 600 or 700 K. In many
cases, the advantage of a low temperature for evaporation of the precursor and for
the reaction is more important than the high price and dif ficult handling. Handling
is difficult because most carbonyls are highly toxic and have limited stability in the
open air. However, by selecting appropriate temperatures, the use of carbonyls will
provide much more freedom in terms of the morphology of the intended powder.
Low temperatures lead to extremely fi ne and fluffy – in most cases amorphous –
powders, whereas the products tend to be more crystalline when using a higher
reaction temperature. A typical example of such a fine powder, in this case
amorphous Fe 2 O 3 , is shown in Figure 4.14. These fine powders also tend to
Figure 4.14 Fe 2 O 3 powder prepared by
decomposition of Fe(CO) 5 and subsequent
oxidation. (Reproduced with permission by
MACH I Inc., King of Prussia, PA, USA;
www.machichemicals.com). This product
consists of fractal agglomerates of amorphous
3-nm particles. The high surface product has
excellent catalytic properties.
4.3 Physical and Chemical Vapor Synthesis Processes j59