Chemistry Reference
In-Depth Information
In a similar method to that described above, the reactions of the bare gold cluster
cations Au
x
+
(
x
1-3, 5, 7) with CH
3
Br have been investigated [
286
]. Adsorbates
of molecular methyl bromide were observed for each of the gold clusters. When
the mass-selected cluster was held in the ion trap for longer reaction times, the
products of methyl elimination would successively increase to also produce
Au
x
(CH
3
Br)
y
(Br)
z
+
.
¼
Reactions with Other Neutral Substrates
H¨ckendorf found that small gold cluster anions Au
x
(
x
2-4) were unreactive to
a wide range of substrates (methanol, acetonitrile, acetaldehyde, acetone, dimethyl
sulphide, methyl mercaptan, benzene, ethynylbenzene and difluoroacetic acid) in an
FT-ICR mass spectrometer [
283
]. The only reactive substrates were dimethyl-
disulphide (discussed above) and trifluoroacetic acid, which reacts via adduct
formation for
x
¼
¼
2 and 4 (Eq. (
33
)) and via Au displacement for
x
¼
2
(Eq. (
34
)). Au
was unreactive towards CF
3
CO
2
H:
Þ
Au
x
þ
CF
3
CO
2
H
!
Au
x
CF
3
CO
2
H
ð
ð
33
Þ
Þ
þ
!
Au
x
1
CF
3
CO
2
H
ð
Au
ð
34
Þ
4.1.2 Reactions of Bare Gold Cluster Ions with More than One
Substrate: 'Cooperative Effects'
Reactions with CO and O
2
The bare gold cluster anions Au
2
and Au
3
, are able to form co-adsorption
complexes, Au
a
(CO)
b
(O)
c
(
a
1, 2), when exposed to small
partial pressure of CO and O
2
in a temperature-controlled ion trap at cryogenic
temperatures [
287
]. This type of cluster complex has been proposed theoretically to
be a key intermediate in a catalytic CO oxidation cycle [
288
].
¼
2, 3;
b
¼
1;
c
¼
Reactions with H
2
and O
2
Lang et al. have used a variable-temperature ion trap to examine the H
2
/O
2
co-adsorption behaviour of cationic gold clusters Au
x
+
2-7) at 100 K
[
271
]. They found a striking odd-even alternation reactivity. For the even
x
cluster sizes (
x
(
x
¼
2,4,6), cooperative co-adsorption of only one oxygen molecule
was observed, with Au
2
H
4
O
2
+
,Au
4
H
6
O
2
+
and Au
6
H
6
O
2
+
being observed. No
co-adsorption of multiple O
2
molecules occurs, even at higher O
2
pressures and
extended reaction times. For the odd
x
cluster sizes (
x
¼
3,5,7) no measurable
co-adsorption of O
2
occurred. The lack of reactivity of Au
x
+
¼
odd) can be
explained via a valence electron structure model in which the spin-paired valence
(
x
¼
