Supporting Information
Direct Formation of Acetate from the Partial Oxidation of
Ethylene on a Au/TiO2 Catalyst
Isabel Xiaoye Green1, Monica McEntee1, Wenjie Tang2, Matthew Neurock1,2, and
John T. Yates, Jr1,2.
1. Department of Chemistry, University of Virginia, Charlottesville, VA 22904
2. Department of Chemical Engineering, University of Virginia, Charlottesville,
VA 22904
1. Calibration for Gas Sample Analysis for Ethylene Oxide
The samples were transferred to a Thermoquest CE Instruments Trace GC 2000 Series gas
chromatography mass spectrometer (GC-MS) instrument with a 30 m GS-Q capillary column (Agilent
Technologies, ID of 0.32 mm) for gas separation. Helium (Praxair, Ultralift) was filtered through a
helium purifier and used as the carrier gas with a column flow rate set at 1.4 mL/min. The GC oven
temperature program was set to 303 K for 10 minutes, and then heated at a rate of 5 K/min up to 373 K.
Ions were detected in a Thermoquest Finnigan Voyager MS in electron impact positive ion (EI+) mode
with a scan range from 10-250. The GC-MS was calibrated against standard ethylene oxide (C2H4O)
solutions in dimethyl sulfoxide (10-3, 5x10-4, 10-4, 5x10-5, and 10-5 M solutions using a 0.5 μL sample
volume) as shown in Figure S1(a). The number of C2H4O molecules detected for each concentration of
the standard solution was acquired from the integrated area of the C2H4O elution peak for each gas
chromatogram. One of the chromatograms, 5x10-4 M, is shown in Figure S1(b) with its corresponding
mass spectrum averaged over 11.65-11.97 minutes of C2H4O elution in Figure S1(c). The limit of
detection of the GC-MS was estimated to be ~3x1012 molecules, corresponding to 3x1015 molecules in the
gas phase in the IR cell.
Assuming an average Au particle size of ~3 nm and knowing the Au loading is ~8 wt%, we calculated
the total amount of Au atoms at the perimeter to be ~1017 atoms in the 5 mg catalyst sample employed. If
~3x1015 ethylene oxide molecules were detected in our experiment in the ~2.3 L cell volume the detection
limit would correspond to only ~3% of the total perimeter sites on the catalyst.
2. Measurement of Ethylene Oxide Production from Ethylene Oxidation on Au/TiO2 at 400K
Gas samples were extracted from the vacuum cell using a 1 mL gas-tight syringe (Hamilton, Model
1001 SL SYR). The syringe enters through a PTFE rubber septum (Wilmad-LabGlass) which is screwed
onto a homemade glass tube attached to the cell. A schematic of the tube and syringe is shown in Figure
S2. The samples were extracted at 6, 20, 40, 60, 80 and 90 minutes during the ethylene partial oxidation
experiment and transferred to the GC-MS for analysis. The GC oven temperature program is described
above.
3. Adsorption of Ethylene
The oxidation of ethylene proceeds by the initial adsorption of ethylene at potential sites at the Au/TiO 2
interface.
Ethylene was examined at the Au, Ti5c and bridging oxygen sites at the interface. Ethylene
was found to bind weakly in di-σ and π bound modes to the Au and and Ti5c sites, respectively (Figure
S4). The adsorption energies for ethylene at Au (coordination number = 7) and Ti 5c were calculated to be
-0.48 eV and -0.24 eV, respectively.
# C2H4O molecules
-1
Intensity (c s )
Calibration for C2H4O
14
1.5x10
14
1.0x10
13
5.0x10
0.0
0.5 L liquid samples
-4
5x10 M
0.0
2.0x10
-4
4.0x10
-4
6.0x10
a
-4
8.0x10
-4
1.0x10
-3
[C2H4O] (M)
6.0x10
6
4.0x10
6
-4
5x10 M C2H4O
b
6
2.0x10
10.0
10.4
10.8
11.2
11.6
12.0
12.4
12.8
-1
Intensity (c s )
Elution Time (min)
1.5x10
6
1.0x10
6
29
Averaged Mass Spectrum
from 11.65-11.97 min
15
5
5.0x10
0.0
44
25
50
c
75
100
125
m/z
150
175
200
225
250
Figure S1. Calibration of ethylene oxide in DMSO solution. (a). Calibration curve of ethylene oxide
concentrations at 1x10-5 M, 5x10-5 M, 1x10-4M, 5x10-4 M and 1x10-3 M. (b). Gas chromatogram of 5x10-4
M ethylene oxide solution which was used for (a). (c). Mass spectrum averaged over 11.65-11.97 minutes
of ethylene oxide elution, corresponding well to the literature cracking pattern for ethylene oxide (NIST
Mass Spec Data Center SES, director "Mass Spectra" in NIST Chemistry WebBook, NIST Standard
Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD,
20899 http://webbook.nist.gov. Accessed 8 Jan 2013).
To vacuum chamber
Gas Extraction Setup
Swaglok
valve
PTFE stop valve
(Gas Tight)
Glass
Tube
1 mL Syringe
Plunger
with PTFE tip
PTFE rubber septum
w/plastic screw cap
Figure S2. Schematic drawing of the gas extraction system for transfer of gas samples to the GC-MS.
Figure S3. The model Au/TiO2 interface involving a 3x3 Au nano-rod covalently bonded to the bridging
oxygen sites on the rutile TiO2 (110) surface support. A) top view, B) side view.
Figure S4. Ethylene adsorption at a) Au site; b) perimeter Ti5c site.