Ozone-activated nanoporous gold: A stable and storable material for catalytic oxidation

Article abstract of DOI:10.1021/acscatal.5b00330

We report a new method for facile and reproducible activation of nanoporous gold (npAu) materials of different forms for the catalytic selective partial oxidation of alcohols under ambient pressure, steady flow conditions. This method, based on the surface cleaning of npAu ingots with ozone to remove carbon documented in ultrahigh vacuum conditions, produces active npAu catalysts from ingots, foils, and shells by flowing an ozone/dioxygen mixture over the catalyst at 150 °C, followed by a temperature ramp from 50 to 150 °C in a flowing stream of 10% methanol and 20% oxygen. With this treatment, all three materials (ingots, foils, and shells) can be reproducibly activated, despite potential carbonaceous poisons resulting from their synthesis, and are highly active for the selective oxidation of primary alcohols over prolonged periods of time. The npAu materials activated in this manner exhibit catalytic behavior substantially different from those activated under different conditions previously reported. Once activated in this manner, they can be stored and easily reactivated by flow of reactant gases at 150 °C for a few hours. They possess improved selectivity for the coupling of higher alcohols, such as 1-butanol, and are not active for carbon monoxide oxidation. This ozone-treated npAu is a functionally new catalytic material.

Full text of DOI:10.1021/acscatal.5b00330

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Ozone-Activated Nanoporous Gold: A Stable
and Storable Material for Catalytic Oxidation
Michelle L Personick, Branko Zugic, Monika M Biener, Juergen Biener, Robert J. Madix, and Cynthia M. Friend
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00330 • Publication Date (Web): 28 May 2015
Downloaded from http://pubs.acs.org on June 11, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 16
ACS Catalysis
1
2
3
4
5
6
7
8
9
OzoneꢀActivated Nanoporous Gold: A Stable and
Storable Material for Catalytic Oxidation
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Michelle L. Personick,† Branko Zugic,† Monika M. Biener,⊥ Juergen Biener,⊥ Robert J. Madix ,‡
Cynthia M. Friend*,†‡
† Department of Chemistry and Chemical Biology, Harvard University, Cambridge,
Massachusetts 02138, United States, and ‡ School of Engineering and Applied Sciences, Harvard
University, Cambridge, Massachusetts 02138, United States.
⊥Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National
Laboratory, Livermore, California 94550, United States.
KEYWORDS Nanoporous gold, energyꢀefficient catalysis, selective oxidation, activation, ozone
ABSTRACT
We report a new method for facile and reproducible activation of nanoporous gold (npAu)
materials of different forms for the catalytic selective partial oxidation of alcohols under ambient
pressure, steady flow conditions. This method, based on the surface cleaning of npAu ingots with
ozone to remove carbon documented in ultrahigh vacuum conditions, produces active npAu
catalysts from ingots, foils and shells by flowing an ozone/dioxygen mixture over the catalyst at
150 °C, followed by a temperature ramp from 50ꢀ150 °C in a flowing stream of 10% methanol
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 16
1
2
3
4
5
6
7
8
9
and 20% oxygen. With this treatment all three materials (ingots, foils, and shells) can be
reproducibly activated, despite potential carbonaceous poisons resulting from their synthesis, and
are highly active for the selective oxidation of primary alcohols over prolonged periods of time.
The npAu materials activated in this manner exhibit catalytic behavior substantially different
from those activated under different conditions previously reported. Once activated in this
manner, they can be stored and easily reactivated by flow of reactant gases at 150 °C for a few
hours. They possess improved selectivity for the coupling of higher alcohols, such as 1ꢀbutanol,
and are not active for carbon monoxide oxidation. This ozoneꢀtreated npAu is a functionally new
catalytic material.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
INTRODUCTION
There is widespread interest in developing new catalytic materials for largeꢀscale chemical
transformations, such as selective oxidation processes, in order to meet the global challenge of
reducing energy use. The overall objective is to design materials that continuously function as
catalysts at moderate operating temperature and that have high reaction selectivity for desired
products. A major objective in catalysis science is to create materials that can be reproducibly
prepared and that have catalytic activity and selectivity sustained over time.
Although considerable advances have been made in the synthesis of exotic nanomaterials with
tailored shapes, sizes and compositions,1,2 there is an ongoing need to determine how to
reproducibly control their catalytic behavior and enhance their longevity. Materials that blend
metal compositions and bridge multiple length scales open up a wealth of opportunities to design
more energyꢀefficient catalysts that also maximize the economical use of resources such as
precious metals.3 However, new materials raise fresh challenges, especially in activating these
materials for catalytic process and maintaining their activity and selectivity.4
ACS Paragon Plus Environment
2

Page 3 of 16
ACS Catalysis
1
2
3
4
5
6
7
8
9
Nanoscale Au supported on metal oxides has been widely investigated as a catalyst material
for selective oxidation because the relative inertness of the gold can render high selectivity.5ꢀ8
One important factor impeding the use of supported nanoscale gold catalysts is their propensity
to agglomerate and to rapidly lose activity.9ꢀ11 Recently, nanoporous metal materials have been
prepared using a wide variety of methods, including dealloying of bulk alloys as well as bottomꢀ
up synthetic approaches.12ꢀ23 In particular, freeꢀstanding nanoporous Au (npAu) etched from bulk
AgꢀAu alloys via a method that results in a dilute AgꢀAu alloy (~1ꢀ3% Ag) has been investigated
as a catalyst for oxidative processes, including the selective partial oxidation of alcohols.5,7,19,20
Nanoporous Au sustains activity over a relatively extended period,19,20 most likely due to the
nanoporous morphology which does not readily agglomerate at moderate temperatures. The 1ꢀ
3% Ag that remains in the material after this particular etching procedure is key to the activity of
npAu for oxidative catalysis. The residual Ag dissociates molecular oxygen (O2) to form
adsorbed O, and the amount of Ag regulates the oxidative strength of the material.5,18,20,24,25
Two major challenges for the use of these nanoporous materials for catalysis are (1) activating
them readily and (2) sustaining their activity. The current method for activating npAu materials
for catalytic partial oxidation involves flowing a mixture of reactant gases, such as CO and O2,
over the catalyst at approximately 75 °C until the material becomes active for CO oxidation.19,20
Though previous work has reported successful activation of npAu catalysts using this
method,5,7,19,20 our experience is that this activation procedure is highly inconsistent and
irreproducible;26 for example, some ingots of npAu activate easily, while others do not activate at
all, even if kept under a steady flow of reactants for many days. This variability points to a strong
influence of npAu material preparation on the consistency of this approach. In addition, npAu
materials which have been activated for methanol selfꢀcoupling using this method deactivate for
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 16
1
2
3
4
5
6
7
8
9
that same reaction after exposure to higher alcohols, such as ethanol and 1ꢀbutanol. It is of key
importance to develop a reliable procedure for reproducibly activating npAu materials,
regardless of their method of preparation.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We have recently shown that it is necessary to remove adventitious carbon from npAu ingots
in order to effect alcohol selfꢀcoupling reactions on npAu ingots under ultrahigh vacuum (UHV)
conditions.26 Cycles of ozone doses to the npAu surface followed by heating to approximately
600 K burn off carbon and yield ingots active for the dissociation of O2.26,27 However, the high
annealing temperature used in these experiments results in coarsening of the npAu ingots and
some accumulation of Ag at the surface. The principles developed from these studies suggest a
protocol for using ozone treatment to reproducibly activate npAu materials at atmospheric
pressure (including materials that cannot be activated by the previously published procedure) for
catalytic oxidation. This procedure activates not only npAu ingots, but also hollow npAu shells
and thin npAu foils, even though the latter materials are particularly challenging to activate as a
consequence of carbon incorporation resulting from their method of preparation; they could not
be activated for alcohol oxidation using previously published procedures. Comparison of the
catalytic behavior of ozoneꢀactivated materials with those derived from the previously published
procedure suggests that these ozoneꢀactivated npAu materials are a functionally different
catalytic material.
EXPERIMENTAL
Ingots of npAu were prepared via chemical etching of Ag70Au30 bimetallic alloy disks in
concentrated nitric acid as previously reported (Figs. 1a,b; see Supporting Information for
additional details).5,12,13 These ingots have a diameter of approximately 5 mm and a thickness of
200ꢀ300 µm. Due to the high tortuosity of the npAu ingots and their known mass transport
ACS Paragon Plus Environment
4

Page 5 of 16
ACS Catalysis
1
2
limitations for catalytic reactions,25 we also generated thin (~100 nm thick) npAu foils (Figs.
1c,d) by dealloying commercially available 6 karat white gold leaf12,13 (Ag85Au15 alloy) and
hollow spherical shells of nanoporous Au (Figs. 1e,f and S1) by adapting a procedure for
synthesizing hierarchicallyꢀporous Au monoliths (Supporting Information).28 The shells have an
average diameter of approximately 8 µm and a shell thickness of 350ꢀ450 nm. Generally, the
pore and ligament dimensions of the ingots, foils, and shells are very similar, ranging from 30ꢀ75
nm, but the characteristic pore depths of the foils and shells are 50 nm and 200 nm, respectively,
compared to the half thickness of the ingots, 150 µm. Furthermore, the hollow shells have a
BET surface area of 7.2 m2/g, which significantly exceeds the surface area of the ingots (4.6
m2/g).
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Photographs (a,c) and scanning electron microscopy (SEM) images (b,dꢀf) of freshlyꢀ
prepared nanoporous Au (npAu) materials used in this work: ingots (a,b), foils (c,d), and hollow
shells (e,f). Images in the bottom row (b,d,f) are higher magnification SEM images of the
materials in the top row (a,c,e). All scale bars in (b,d,f) are the same size.
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 16
1
2
Based on experiments in UHV26 we developed the following procedure for activating these
npAu materials. First, the npAu ingots, foils, or shells were independently loaded into a flow
reactor tube furnace in a glass reactor tube with a quartz frit (0.4 inch internal diameter), with the
npAu lightly sandwiched between two pieces of deactivated glass wool. The temperature in the
reactor was then ramped from 30 °C to 150 °C (10 °C/min) in a flow of 30 g/Nm3 of ozone in a
50% O2/He gas mixture (approximately 3% ozone) at a total flow rate of 50 mL/min. The npAu
materials were then held at 150 °C for one hour under the same flow conditions. Subsequently,
the reactor was cooled to 50 °C and then ramped from 50 °C to 150 °C at 10 °C/min in flowing
stream of 10% methanol and 20% O2 (total flow rate of 50 mL/min in He). Any residual atomic
oxygen from the ozone treatment is removed after a very short time in the methanol/O2 reactant
mixture. A small amount of CO2 was evolved during this activation process as the catalyst
became active for the oxidative coupling of methanol to yield methyl formate.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Because of their method of synthesis, the initial condition of the foils and shells could present
extra challenges for their activation as catalytic materials. The precursor white gold leaf used to
generate the foils is manufactured for use in craft processes and, thus, is not prepared with
concern for residue (eg, carbon or sulfur) on the surface of the material. The hollow npAu shells
are synthesized using polyvinylpyrrolidone as a stabilizing agent and necessitate the removal of
the polystyrene core templates by calcination at 450 °C, both of which are likely to leave
complex carbonaceous residue on the surface of the material. That these potential surface
poisons are apparently removed during the activation process, facilitating O2 dissociation and
leading to high activity and selectivity, is a key advance of this method. These materials could
not be activated for alcohol oxidation using a flowing stream of O2 with methanol and/or CO—
the activation procedure previously reported.5,19,20
ACS Paragon Plus Environment
6

Page 7 of 16
ACS Catalysis
1
2
3
4
RESULTS AND DISCUSSION
5
6
7
8
9
Reproducible activation of all three forms of npAu (ingots, foils, and shells) was achieved
using the procedure described above (Figs. 2 and S4). The specific activity and selectivity of the
three types of npAu materials for oxidative catalysis was evaluated using the oxidation of
methanol to methyl formate as a test reaction, because the catalytic activity of npAu for this
reaction is already wellꢀcharacterized for monolithic npAu ingots.5 Fresh samples of each of
these materials could be activated without fail, whereas the previously reported method of
activation of the ingots in CO/O2 or methanol/O2 mixtures (sometimes with added CO) at
temperatures from 30ꢀ80° C failed more often that it succeeded.5,26,29 All three ozoneꢀactivated
materials catalyzed the selective oxidative coupling of methanol to methyl formate in flowing
gas composed of 10% methanol and 20% O2 in He, with a total flow rate of 50 mL/min at 150 °C
with 100 % selectivity (Table 1). No other partial oxidation products were detected, including
formaldehyde or formic acid (Figs. S2 and S3). There is no quantifiable production of the
combustion product, CO2, except during activation (Figs. 2 and S4).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Activity of ozoneꢀactivated npAu materials for the selective oxidation of methanol.a
Catalyst
material
Ingots
Foils
Rate of conversion Selectivity to
of methanol
methyl formate
100%
0.017 mmol sꢀ1 gꢀ1
0.091 mmol sꢀ1 gꢀ1
0.083 mmol sꢀ1 gꢀ1
100%
100%
Shells
a Conditions: 10% methanol and 20% O2 in He, flow rate: 50 mL/min, 150 °C
.
These catalytic materials, activated in the manner described, show sustained and reproducible
catalytic activity and selectivity. The catalysts exhibited an initial equilibration period of
approximately 24 hours during which the activity of the catalyst increased and then gradually
decreased to a stable value. This activation and stabilization period is shown in Fig. 2 for ~ 10
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 16
1
2
3
4
5
6
7
8
9
mg of the npAu shell catalyst, which was well dispersed in the reactor bed and thus exhibited a
high rate of methanol conversion, as reported in Table 1. Corresponding data for the ingots and
foils is presented in the Supporting Information (Fig. S4). The data in Fig. 3, which shows
sustained conversion of methanol over time, was obtained using five times the amount of the
npAu shell catalyst used in Fig. 2 (~ 50 mg), and thus yielded a higher conversion. The stable
conversion rates of methanol for the foils and wellꢀdispersed shells were 0.091 mmol sꢀ1 gꢀ1 and
0.083 mmol sꢀ1 gꢀ1, respectively, which are nearly five times higher than the specific reaction rate
on the ingots (0.017 mmol sꢀ1 gꢀ1). The higher rates confirm that, as a result of their thinner and
more open morphologies, the shells and foils do remove some of the mass transport limitations
inherent to the ingots. Further, the fact that the reaction rates on the foils and shells are very
similar suggests that mass transport limitations are negligible for these materials, since their
characteristic pore depths differ significantly. The reaction rate on the ingots is comparable to the
activity previously reported at 80 °C for ingots activated in methanol and O2 at room temperature
(0.022 mmol sꢀ1 gꢀ1).5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Conversion to methyl formate (black squares) and to CO2 (red circles) versus time for
the initial activation and stabilization period of the npAu shell catalyst in the oxidation of
methanol after ozone treatment and conversion to methyl formate (blue triangles) after exposure
ACS Paragon Plus Environment
8

Page 9 of 16
ACS Catalysis
1
2
3
4
5
6
7
8
9
to air for four months (previously activated catalyst) (~ 10 mg of catalyst). The temperature ramp
from 50ꢀ150 °C takes place between the first and second time points. No carbon dioxide is
produced during the reactivation after exposure to air. Similar graphs of the activation of ingots
and foils are shown in Fig. S4 in the Supporting Information.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. The steady conversion of methanol to methyl formate over the course of greater than
seven days on activated npAu shells, after stabilization (~ 50 mg of catalyst). Conditions: 10%
methanol and 20% O2 in He, flow rate: 50 mL/min, 150 °C.
The initial activation of npAu creates a very robust catalyst material that can be removed from
the reactor and stored in air for at least four months without losing activity. Once returned to the
reactor, a few hours of exposure to the reaction conditions used for the methanol reaction (10%
methanol and 20% O2 in He, 50 mL/min, 150 °C) were required for the material to return to a
stable conversion; no reactivation in ozone was required (Fig. 2). Together, these results
demonstrate the utility of this activation method for stably activating a variety of npAu catalysts,
regardless of their architecture or the differences of their preparation method.
After stabilization of the catalyst following activation, no loss of catalytic activity was
observed for the production of methyl formate from methanol and O2 over an extended period of
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 16
1
2
3
4
5
6
7
8
9
time even though the surface area of the catalyst decreased due to coarsening of the ligament
structures under reaction conditions. After one month of use, the surface area of the npAu shells
decreased by approximately a factor of 10 – from 7.2 m2/g to 0.7 m2/g (BET), presumably due to
agglomeration and a change in morphology; the ligament diameter increased from 80 ± 20 nm to
250 ± 80 nm (SEM) (Fig. 4). Over the same period, the surface area of the npAu ingots
decreased from 4.6 m2/g to 2.8 m2/g (BET).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. SEM images of the ligament sizes of the npAu shells before exposure to reactant gases
(a) and after one month on stream (b). Inset in (a): highꢀmagnification image of the ligament
structure of the npAu shell in (a). A more detailed study of this coarsening process using the
npAu foils is underway.
A minor amount of coarsening of the ligaments (from 80 ± 20 nm to 115 ± 25 nm) occurred on
the npAu shells during the initial activation in ozone and one to two hours under reaction
ACS Paragon Plus Environment
10

Page 11 of 16
ACS Catalysis
1
2
3
4
5
6
7
8
9
conditions. After four days of reaction the ligament size of the shells had increased to 140 ± 40
nm. These observations indicate that a significant amount of coarsening occurs during the early
stages of reaction30 and that the resulting rearrangement of the structure and composition of the
catalyst surface may be important for the activity of the catalyst for methanol coupling in these
materials. Notably, there is not a significant amount of coarsening as a result of the initial ozone
treatment31—the coarsening occurs under reaction conditions creating the active catalyst. Studies
are underway exploring the detailed relationship between coarsening and reactivity on these
materials.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
There are key differences between the ozoneꢀactivated npAu catalysts studied here and
materials activated by simply flowing reactant mixtures over the npAu at lower temperature
without ozone treatment. First, the ozoneꢀactivated npAu operates at 150 °C and above, whereas
the reactantꢀactivated ingots function (though irreproducibly) at temperatures as low as 20 °C.5
Second, the ozoneꢀactivated catalysts show no activity for CO oxidation under standard
operation conditions (no CO2 production observed by GCꢀMS), whereas the other catalysts
readily oxidize CO.19,20,32,33 This difference could present an advantage for the ozoneꢀactivated
materials for selective oxidation in the presence of CO.
Further, previous studies with npAu ingots activated solely with the flowing reactant stream
reported only the formation of butyraldehyde from the oxidation of 1ꢀbutanol, with no evidence
of the coupling product, butyl butyrate,34 but the ozoneꢀactivated materials readily selfꢀcouple
butanol. This reaction was revisited under catalytic conditions after controlled studies in UHV
showed that npAu can effectively catalyze the selfꢀcoupling of 1ꢀbutanol and ethanol (see
below).35 Under a flow of 5.36% 1ꢀbutanol and 20% O2 in He (50 mL/min), butyl butyrate was
ACS Paragon Plus Environment
11

ACS Catalysis
Page 12 of 16
1
2
3
4
5
6
7
8
9
produced with selectivities of 20.6%, 11.9% and 16.4%, for the ingots, foils, and shells,
respectively, with butyraldehyde as the other only other reaction product (Table 2 and Fig. S5).
Table 2. Rate and selectivity for the formation of the selfꢀcoupled esters in the selective
oxidation of and 1ꢀbutanol on ozoneꢀactivated npAu materials.a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Catalyst
material
Ingots
Foils
Ethanol self-coupling
rate (selectivityb)
1-Butanol self-coupling
rate (selectivityb)
0.010 mmol sꢀ1 gꢀ1 (36.1%) 0.008 mmol sꢀ1 gꢀ1 (20.6%)
0.014 mmol sꢀ1 gꢀ1 (22.1%) 0.021 mmol sꢀ1 gꢀ1 (11.9%)
0.010 mmol sꢀ1 gꢀ1 (20.2%) 0.016 mmol sꢀ1 gꢀ1 (16.4%)
Shells
a Conditions: ~5% ethanol or 1ꢀbutanol and 20% O2 in He, flow rate: 50 mL/min, 150 °C
b The only other product in each case is the aldehyde.
.
There is another important difference between the performance of the npAu catalysts activated
with these two methods. On ozoneꢀactivated npAu the selectivity to ethyl acetate in the selfꢀ
coupling of ethanol (5.23% ethanol/20% O2 in He, 50 mL/min) was stable at 36.1% for the
ingots, 22.1% for the foils, and 20.2% for the shells, with the remaining product being
acetaldehyde (Table 2 and Fig. S6), whereas it has been reported that the selfꢀcoupling of ethanol
by reactantꢀactivated npAu shows low and variable selectivity for ethyl acetate production from
day to day.34 Both ethanol and 1ꢀbutanol selfꢀcouple at rates comparable to methanol selfꢀ
coupling on the ingots (Table 1). Collectively, these observations indicate that npAu catalysts
generated using the ozone activation treatment reported here are functionally different from those
generated by activation in the flowing reactant stream despite having the same general bulk
composition and structure.
CONCLUSIONS
Exposure of npAu ingots, foils or shells to a flowing stream 50% O2/He containing
approximately 3% ozone followed by ramping the temperature from 50 to 150 °C in a stream of
10% methanol and 20% O2 reproducibly activates the material for the catalytic oxygenꢀassisted
ACS Paragon Plus Environment
12

Page 13 of 16
ACS Catalysis
1
2
3
4
5
6
7
8
9
selfꢀcoupling of primary alcohols. This procedure is effective even though the initial state of the
surfaces of these different nanoporous varies significantly due to the methods by which they are
prepared. The method is simple and reproducible. All three materials have been shown to
maintain stable catalytic activity over a period of time of at least one week and catalyze the
formation of both esters and aldehydes from ethanol and 1ꢀbutanol, which confirms the Auꢀlike
surface reactivity of these catalysts. Due to their increased accessible surface area, the npAu foils
and shells generally have a higher specific rate for the production of methyl formate than do the
ingots, indicating that these two morphologies mitigate the mass transport limitations inherent to
npAu ingots.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The npAu materials activated in the manner reported here show functionally different catalytic
behavior than reported in previous studies of npAu ingots activated using O2 with methanol
and/or CO.5,19,20 The activation procedure previously reported, in which ozone was not used,
activated neither the foils nor the shells, and did not yield reproducible activation of the ingots
for coupling reactions. The activity, selectivity, and conditions for catalytic alcohol coupling are
also considerably different for the ozoneꢀactivated materials: (1) the ozoneꢀactivated npAu is
inactive for CO oxidation; (2) the ozoneꢀtreated catalysts retain their activity for methanol selfꢀ
coupling even when exposed to higher alcohols, unlike previous materials, which become
inactive.; (3) the oxidation of 1ꢀbutanol over the new npAu catalysts leads to selfꢀcoupling to
form butyl butyrate, whereas the npAu ingots activated without prior ozone treatment produced
only butyraldehyde. The ozone activation has the potential to be widely applicable in activating
npAu materials produced via other synthetic approaches, as well as materials with other
nanoporous Au alloy compositions.
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 16
1
2
3
4
ASSOCIATED CONTENT
5
6
7
8
9
Supporting Information. Additional experimental details, large area SEM image of hollow
npAu shells, representative GCꢀMS data, and graphs of the activation of ingots and foils. This
material is available free of charge via the Internet at http://pubs.acs.org.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: friend@fas.harvard.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported as part of the Integrated Mesoscale Architectures for Sustainable
Catalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office
of Science, Basic Energy Sciences under award #DEꢀSC0012573. The microscopy work was
performed at the Center for Nanoscale Systems (CNS), a member of the National
Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science
Foundation under NSF award no. ECSꢀ0335765. CNS is part of Harvard University. Work at
LLNL was performed under the auspices of the U.S. Department of Energy by LLNL under
Contract DEꢀAC52ꢀ07NA27344.
REFERENCES
1. Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem. Int. Ed. 2008, 48, 60ꢀ103.
ACS Paragon Plus Environment
14

Page 15 of 16
ACS Catalysis
1
2
3
4
2. Personick, M. L.; Mirkin, C. A. J. Am. Chem. Soc. 2013, 135, 18238ꢀ18247.
5
6
7
3. Hemminger, J.; Crabtree, G.; Sarrao, J. From Quanta to the Continuum: Opportunities for
Mesoscale Science. http://science.energy.gov/bes/news-and-resources/reports.
8
9
4. Carrero, C. A.; Schloegl, R.; Wachs, I. E.; Schomaecker, R. ACS Catal. 2014, 4, 3357ꢀ3380.
5. Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Bäumer, M. Science 2010, 327, 319ꢀ322.
6. Baker, T. A.; Liu, X.; Friend, C. M. Phys. Chem. Chem. Phys. 2011, 13, 34ꢀ46.
7. Wittstock, A.; Biener, J.; Bäumer, M. Phys. Chem. Chem. Phys. 2010, 12, 12919ꢀ12930.
8. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405ꢀ408.
9. Kolmakov, A.; Goodman, D. W. Catal. Lett. 2000, 70, 93ꢀ97.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10. Sykes, E. C. H.; Williams, F. J.; Tikhov, M. S.; Lambert, R. M. J. Phys. Chem. B 2002, 106,
5390ꢀ5394.
11. Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811ꢀ814.
12. Ding, Y.; Kim, Y. J.; Erlebacher, J. Adv. Mater. 2004, 16, 1897ꢀ1900.
13. Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450ꢀ
453.
14. Chen, L. Y.; Chen, N.; Hou, Y.; Wang, Z. C.; Lv, S. H.; Fujita, T.; Jiang, J. H.; Hirata, A.;
Chen, M. W. ACS Catal. 2013, 3, 1220ꢀ1230.
15. Ma, A.; Xu, J.; Zhang, X.; Zhang, B.; Wang, D.; Xu, H. Sci. Rep. 2014, 4, 4849.
16. Lee, M. N.; SantiagoꢀCordoba, M. A.; Hamilton, C. E.; Subbaiyan, N. K.; Duque, J. G.;
Obrey, K. A. D. J. Phys. Chem. Lett. 2014, 5, 809ꢀ812.
17. Déronzier, T.; Morfin, F.; Lomello, M.; Rousset, J. J. Catal. 2104, 311, 221ꢀ229.
18. Déronzier, T.; Morfin, F.; Massin, L.; Lomello, M.; Rousset, J. Chem. Mater. 2011, 23,
5287ꢀ5289.
19. Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42ꢀ
43.
20. Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Bäumer, M.
Angew. Chem. Int. Ed. 2006, 45, 8241ꢀ8244.
21. Lang, X. Y.; Guo, H.; Chen, L. Y.; Kudo, A.; Yu, J. S.; Zhang, W.; Inoue, A.; Chen, M. W.
J. Phys. Chem. C 2010, 114, 2600ꢀ2603.
22. Wang, H.; Ge, X. Electroanalysis 2012, 24, 911ꢀ916.
23. Lee, D.; Jang, H. Y.; Hong, S.; Park, S. J. Colloid Interface Sci. 2012, 388, 74ꢀ79.
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 16
1
2
3
4
5
24. Moskaleva, L. V.; Röhe, S.; Wittstock, A.; Zielasek, V.; Klüner, T.; Neyman, K. M.;
Bäumer, M. Phys. Chem. Chem. Phys. 2011, 13, 4529ꢀ4539.
6
7
8
9
25. Wittstock, A.; Neumann, B.; Schaefer, A.; Dumbuya, K.; Kübel, C.; Biener, M. M.; Zielasek,
V.; Steinrück, H.; Gottfried, J. M.; Biener, J.; Hamza, A.; Bäumer, M. J. Phys. Chem. C 2009,
113, 5593ꢀ5600.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
26. Stowers, K. J.; Madix, R. J.; Friend, C. M. J. Catal. 2013, 308, 131ꢀ141.
27. Schaefer, A.; Ragazzon, D.; Wittstock, A.; Walle, L. E.; Borg, A.; Bäumer, M.; Sandell, A. J.
Phys. Chem. C 2012, 116, 4564ꢀ4571.
28. Nyce, G. W.; Hayes, J. R.; Hamza, A. V.; Satcher, J. H. Chem. Mater. 2007, 19, 344ꢀ346.
29. Röhe, S.; Frank, K.; Schaefer, A.; Wittstock, A.; Zielasek, V.; Rosenauer, A.; Bäumer, M.
Surf. Sci. 2013, 609, 106ꢀ112.
30. Qian, L. H.; Chen, M. W. Appl. Phys. Lett. 2007, 91, 083105.
31. Biener, J.; Wittstock, A.; Biener, M. M.; Nowitzki, T.; Hamza, A. V.; Baeumer, M.
Langmuir 2010, 26, 13736ꢀ13740.
32. Wittstock, A.; Wichmann, A.; Biener, J.; Bäumer, M. Faraday Discuss. 2011, 152, 87ꢀ98.
33. Fujita, T.; Tokunaga, T.; Zhang, L.; Li, D.; Chen, L.; Arai, S.; Yamamoto, Y.; Hirata, A.;
Tanaka, N.; Ding, Y.; Chen, M. Nano Lett. 2014, 14, 1172ꢀ1177.
34. Kosuda, K. M.; Wittstock, A.; Friend, C. M.; Bäumer, M. Angew. Chem. Int. Ed. 2012, 51,
1698ꢀ1701.
35. Stowers, K. J.; Friend, C. M.; Madix, R. J.; Biener, M. M.; Biener, J. Catal. Lett. 2015,
ASAP, http://dx.doi.org/10.1007/s10562ꢀ015ꢀ1525ꢀ4.
TOC Graphic
ACS Paragon Plus Environment
16