Chemistry Reference
In-Depth Information
electron structure of Au
x
+
hampers the interaction of the gold cluster valence
electrons with the unpaired electrons of the 2
ˀ
g
* antibonding O
2
orbitals. As the
temperature of the ion trap is raised the types of product ions observed for Au
4
+
and
Au
6
+
change. At 300 K, Au
4
H
2
O
+
and Au
6
H
4
O
+
are observed, which signifies that
the oxygen has been activated. Detailed DFT calculations were carried out on the
co-adsorption of H
2
and O
2
and their chemical activation on the Au
4
+
and Au
6
+
clusters. These suggest a mechanisms for dissociation of O
2
involving the adsorp-
tion at adjacent sites to give [(Au)
x
2
Au(H
2
)Au(O
2
)]
+
structures, which can then
undergo intermolecular hydrogen atom transfer to form [(Au)
x
2
Au(H)Au(O
2
H)]
+
intermediates possessing the mixed hydride, hydroperoxide sites, which in turn can
eliminate water to form a gold oxide site.
Reactions with N
2
/H
2
and N
2
/O
2
The reactions of Au
x
+
(
x
3 and 5) with mixtures of N
2
and H
2
and N
2
and O
2
have
been studied in a variable-temperature ion trap [
281
]. As noted above, at low temper-
atures (100 and 200 K) in the presence of pure N
2
,bothAu
3
+
and Au
5
+
both give
Au
x
(N
2
)
y
+
.InpureH
2
,Au
3
+
and Au
5
+
both give Au
x
(H
2
)
y
+
. In contrast, no reactions
are observed between Au
x
+
and O
2
. At 100 K, mixtures of N
2
and H
2
react withAu
3
+
to
yield Au
3
(H
2
)
2
(N
2
)
+
,Au
3
(H
2
)(N
2
)
2
+
and Au
3
(N
2
)
3
+
.Au
5
(H
2
)
3
(N
2
)
+
,Au
5
(H
2
)
2
(N
2
)
2
+
and Au
5
(H
2
)(N
2
)
3
+
are observed under the same conditions. The authors suggested
that these reactions involve competitive co-adsorption. Au
3
+
and Au
5
+
co-adsorb N
2
and O
2
at 100 K, at longer reactions times and with low concentrations of N
2
to,
respectively, give: Au
3
(N
2
)
3
+
,Au
3
(N
2
)
2
(O
2
)
+
,Au
3
(N
2
)(O
2
)
2
+
and Au
3
(O
2
)
3
+
;
Au
5
(N
2
)
3
+
,Au
5
(N
2
)
2
(O
2
)
+
,Au
5
(N
2
)(O
2
)
2
+
and Au
5
(O
2
)
3
+
;aswellasAu
5
(N
2
)
4
+
,
Au
5
(N
2
)
2
(O
2
)
2
+
,Au
5
(N
2
)(O
2
)
3
+
and Au
5
(O
2
)
4
+
. Both cooperative and competitive
co-adsorption appears to operate for N
2
/O
2
mixtures.
¼
Reactions with CH
4
and O
2
Lang and Bernhardt studied the reactions of Au
x
+
(
x
2-4) with CH
4
and O
2
in a
variable ion trap [
289
]. Upon reaction with CH
4
only, these clusters appear to
adsorb a CH
4
molecule initially to form Au
x
(CH
4
)
+
. Interestingly, at 300 K and
upon reaction with another molecule of CH
4
,Au
2
+
forms Au
2
(C
2
H
4
)
+
indicating
dehydrogenation of CH
4
, shown in this study to be catalytic. Larger Au
x
+
clusters
studied adsorb more CH
4
. To study the possibility of methane oxidation, the
reaction of CH
4
and O
2
on the gold dimer cation Au
2
+
was also examined at
210 K. Two peaks corresponding to Au
2
(CH
4
)O
2
+
and Au
2
(C
3
H
8
O
2
)
+
were
detected in the mass spectrum. The latter ion corresponds to a dehydrogenated
product and was tentatively assigned as Au
2
(CH
2
O)
2
(CH
4
)
+
¼
(i.e. formation of
formaldehyde) or a further oxidation product Au
2
(CO
2
)(CH
4
)
2
+
.
