Chemistry Reference
In-Depth Information
Table 10 Bimolecular
collision rates for the
reactions of Au
x
;
x
k
exp
a
k
theor
b
Efficiency (%)
c
Cluster
Au
0.20
6.74
0.3
7
Au
2
0.22
6.53
0.3
with CO
Au
3
0.18
6.46
0.3
Au
4
0.23
6.42
0.4
Au
5
0.96
6.40
1.5
Au
6
1.26
6.38
2.0
Au
7
2.43
6.38
3.8
a
k
exp
Experimental bimolecular rate constant
b
Calculated theoretical collision rate [
277
]
c
k
exp
/
k
theor
100
Table adapted from [
273
]
first CO was described in Sect.
3.7
. Multiple CO adsorptions have been observed for
gold cluster anions and cations (Eq. (
15
)):
ðÞ
=þ
y
Au
=þ
x
þ
y
CO
!
Au
n
CO
ð
15
Þ
Ervin and Lee studied the reactions of gold cluster anions (Au
x
;
x
7) with O
2
and CO in a flow tube reactor [
273
]. Unlike in the case of O
2
(see below), no even-
odd alteration of the reactivity was observed with CO, and gold clusters Au
x
with
x
4) (Table
10
).
Wallace and Whetten have published several studies on the reaction of CO with
size-selected gold clusters at room temperature [
276
,
278
-
280
]. For instance, in a
study aimed at understanding the size-dependent reactivity of Au
x
(
x
>
4 were more reactive than the smaller ones (
x
4-19)
[
280
], they concluded that initial products formed seem to correspond mainly to
clusters with a gold electron shell filling at 8, 14, 18 and 20 electrons (i.e. Au
5
CO
,
Au
11
CO
,Au
15
CO
,Au
15
(CO)
2
). When the concentration of CO is increased,
cluster saturation is observed with 4-8 molecules of CO adsorbed (e.g. Au
5
(CO)
4
,
Au
8
(CO)
5
,Au
9
(CO)
6
,Au
12
(CO)
8
). In another study, Whetten and Wallace
reported that Au
2
and Au
3
were unreactive [
276
] at room temperature. This
contradicted Ervin's results that these small clusters react with CO, albeit slowly
[
273
]. This was later explained when they reported that preadsorbed water on Au
2
and Au
3
allowed the subsequent adsorption of the carbonyl and displacement of
water [
278
].
The temperature-dependent reaction kinetics of the bare gold cluster anions
Au
x
(
x
¼
1-3) to CO (and O
2
discussed below) adsorption have been studied by
Bernhardt et al. in a variable-temperature ion trap [
268
]. At room temperature Au
x
(
x
¼
1-3) was found to be unreactive with CO, as previously described by Whetten
et al. [
280
]. For an ion-trap temperature of 250 K, the gold clusters Au
x
(
x
¼
¼
2, 3)
are able to adsorb only one molecule of CO. At 100 K Au
x
(CO)
y
(
x
,
y
2,2; 3,2)
are also detected, and Au
remains unreactive even at lower temperatures. Kinetic
measurements reveal that Au
2
(CO)
and Au
2
(CO)
2
reach an equilibrium when
reaction times are extended. In contrast Au
3
(CO)
2
is almost the sole product of
allowing Au
3
(CO)
to react with CO (Eq. (
15
)) over extended reaction times.
¼
