Chemistry Reference
In-Depth Information
Scheme 7 Au
2
-catalysed
oxidation of CO [
287
,
321
].
Step 1 is oxygen addition;
step 2 is CO addition; step
3 is oxygen atom transfer
Þ
3
!
Au
3
þ
Au
3
CO
2
ð
3CO
2
ð
75
Þ
Ag
3
O
n
þ
Þ
n
þ
n
N
2
O
þ
n
CO
!
Ag
3
O
n
CO
2
ð
n
N
2
ð
76
Þ
Bernhardt's group has used the same experimental set-up to study the related
oxidation of CO (Eq. (
77
)) by O
2
catalysed by the gold dimer anion Au
2
over a
temperature range of 100-300 K (Scheme
7
)[
324
,
325
]. DFT calculations were used
to examine key intermediates associated with this cycle [
288
,
325
].Thefirststepof
the cycle involves adsorption of O
2
onto Au
2
, with the resultant complex stabilised
by further collisions with the helium bath gas. DFT calculations predict that Au
2
O
2
consists of dioxygen bound molecularly to Au
2
, with partial electron transfer to the
adsorbed O
2
molecule resulting in a 'superoxo-like' species (cf. Sect.
3.4
). In step
2, Au
2
O
2
reacts with CO to yield the metastable complex Au
2
CO
3
,whichwasnot
observed at room temperature but could be detected at lower temperatures (100 K).
DFT calculations of potential isomers of 'Au
2
CO
3
' revealed the digold-carbonate
species of connectivity [Au-Au-OCO
2
]
to be the most stable species, although a
second structural isomer was found and suggested to be relevant to the catalytic
process. The final step of the cycle in which Au
2
CO
3
reacts with CO resulted in the
liberation of two molecules of CO
2
and the regeneration of Au
2
, thus closing a
three-step catalytic cycle for the oxidation of CO to CO
2
with O
2
as the terminal
oxidant (Scheme
7
). Related work by other groups on the reactions of gold clusters
with O
2
and CO was highlighted in previous sections.
2CO
þ
O
2
!
2CO
2
ð
77
Þ
Two competing, temperature-dependent catalytic cycles for the oxidation of
methane by O
2
catalysed by Au
2
+
have been described (Scheme
8
)[
326
,
327
].
Variable-temperature ion-trap mass spectrometry experiments were carried out in
which the partial pressures of CH
4
and O
2
were varied and product distributions at
different temperature were examined. In combination with these detailed experi-
ments, kinetic modelling of the product ion abundances was used to establish the
order of the individual steps of each of the catalytic cycles, and DFT calculations
were used to shed light on possible mechanisms. Reversible absorption of methane
onto Au
2
+
(step 1 of Scheme
8
) represents the key first step and provides the shared
entry point into both catalytic cycles, which also share the same next step involving
