Microstructured Au/Ni-fiber catalyst for low-temperature gas-phase selective oxidation of alcohols

Article abstract of DOI:10.1039/c1cc12964c

Galvanic deposition of Au onto a thin-sheet sinter-locked 8 μm Ni-fiber delivers a high-performance Au/Ni-fiber catalyst for alcohol oxidation, due to the unique combination of excellent heat conductivity, remarkable low-temperature activity, and good stability/regenerability. The special NiO@Au ensembles formed during the reaction contribute to promoting the low-temperature activity. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc12964c

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COMMUNICATION
Microstructured Au/Ni-fiber catalyst for low-temperature gas-phase
selective oxidation of alcoholsw
Guofeng Zhao, Huanyun Hu, Miaomiao Deng and Yong Lu*
Received 20th May 2011, Accepted 18th July 2011
DOI: 10.1039/c1cc12964c
Galvanic deposition of Au onto a thin-sheet sinter-locked 8 lm
Ni-fiber delivers a high-performance Au/Ni-fiber catalyst for
alcohol oxidation, due to the unique combination of excellent
heat conductivity, remarkable low-temperature activity, and
good stability/regenerability. The special NiO@Au ensembles
formed during the reaction contribute to promoting the low-
temperature activity.
Considering the special properties of the SMF and the
extraordinary activity of Au NPs, a novel catalyst would
emerge if Au NPs are embedded in a metallic microfibrous
structure. Here, we discovered a microfibrous-structured
Au/Ni-fiber catalyst with the aid of a galvanic exchange
reaction (3Ni + 2HAuCl4 = 2Au + 3NiCl2 + 2HCl), which
is active, selective, and stable for the gas-phase oxidation of
alcohols under mild conditions. The formation of NiO@Au
active ensembles on the Ni-fiber induces an unappreciated
synergistic effect by substantially promoting the low-temperature
activity. Previously, a highly active Au/Cu-fiber catalyst was
developed in our group using the galvanic deposition method
but with a definitely different surface nanostructure and a
dehydrogenation mechanism.3b
Oxidation of alcohols to the carbonyl compounds is an
important process in organic chemistry.1 However, there needs
to be an exigent fundamental shift from methods based on the
toxic and expensive inorganic oxidants to greener and more
atom-efficient methods that adopt recyclable catalysts and O2
as an oxidant.1,2
Recently, several gold catalysts, including Au/SiO2,2b
Au–Cu/SiO2,2c and Au/OMMs,2e have been developed for
the gas-phase oxidation of alcohols, which offers many
advantages over the liquid-phase oxidation including the
convenience of catalyst separation, solvent-free conditions
and much higher efficiency.1 However, their weak thermal
conductivity will induce hotspots, which not only cause catalyst
degradation but are dangerous in industrial applications.2c
Additionally, a high reaction temperature (generally 4280 1C)
is required for most catalysts to achieve satisfactory product
yield, which is a main cause of catalyst deactivation.2b,c,3
Therefore, identifying a robust catalyst with a unique combi-
nation of high thermal conductivity, excellent low-temperature
activity, and durability is highly desirable from academic and
industrial standpoints.3
A typical sinter-locked metal-nickel-fiber (SMF-Ni) was
prepared,3,4b with characteristic irregular 3D networks and a
thin-sheet structure (Fig. S1a and S1b, ESIw). Gold particles
were then deposited onto the SMF-Ni via galvanic exchange
reaction (denoted as galvanic deposition [GD]), attributable to
the large electrode potential difference between the Ni2+/Ni0
(À0.22 V) and Au3+/Au0 (1.5 V) pairs (Fig. S1cw). An optical
photograph of a typical catalyst, Au-4/Ni-fiber (Au: 4 wt%),
indicated a well-preserved thin-sheet structure (Fig. S1dw).
After calcining at 300 1C, very uniform coverage consisting
of lamellar-shaped NiCl2@Au composites was formed along
with the Ni-fiber (Fig. S1e and S1fw), with an increase in the
BET surface area from 0.6 m2 gÀ1 of fiber to 4.5 m2 gÀ1 of
catalyst. Fig. S2w illustrates such a GD method, quite distinct
from the common preparation methods (e.g., deposition
precipitation5a and Au–Ag alloy etching).5b
A support with high thermal conductivity is crucial for this
new type of catalyst. Recently, a new class of thin-sheet sinter-
locked metal-fiber (SMF) supports has received growing
attention, due to their special three-dimensional (3D) network,
open structure, and high thermal conductivity,4 which is
promising for the development of high-performance catalysts,
especially for strongly endothermic or exothermic reactions.3,4
Our initial tests focused on the gas-phase oxidation of
benzyl alcohol to benzaldehyde. Interestingly, the low-
temperature activity of the Au/Ni-fiber catalysts was drama-
tically promoted after undergoing 1 h reaction at 380 1C in
advance (termed pre-activation, Fig. S3, ESIw). Additionally,
the conversion could be tuned by adjusting the Au loading and
catalyst calcination temperatures with high benzaldehyde
selectivity (498%; Fig. S3w). At 280 1C, the highest conver-
sion of 99% could be obtained over the pre-activated Au-4/
Ni-fiber, using a high WHSV of 20 hÀ1 (Table S1w). Notably, a
conversion of 85% could still be obtained even at 230 1C,
which is only 25 1C higher than the boiling point of benzyl
alcohol and much lower than the reported results.2b,c,3 For
reference, a neat SMF-Ni support calcined at 300 1C in air was
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Department of Chemistry, East China Normal University,
3663 North Zhongshan Road, Shanghai 200062, China.
E-mail: ylu@chem.ecnu.edu.cn; Fax: +86 21-62233424
w Electronic supplementary information (ESI) available: Catalysts
testing and characterization, data analysis and discussion; Fig. S1 to
Fig. S11; Tables S1 to S4. See DOI: 10.1039/c1cc12964c
c
9642 Chem. Commun., 2011, 47, 9642–9644
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also tested and induced conversion of o4% with selectivity of
99% under identical reaction conditions. The above results
indicate that Au embedment modification of SMF-Ni remark-
ably promoted the low-temperature activity.
a
transformation from NiCl2 to NiO occurred during
pre-activation (Fig. S5 and S6w), and the small NiO segments
were visualized by TEM (Fig. 1a) to partially cover the Au
NPs to form the active NiO@Au-NPs ensembles (structurally
illustrated in Fig. 1b; see Discussion 3 in ESIw for details).
Especially, the surface-phase junction (Fig. 1a and b) in
NiO@Au-NPs is very essential to form a special interaction
at their interfaces, thereby leading to considerable promotion
of low temperature activity (see Discussion 1–3 in ESIw for
details). Indeed, if the fresh catalyst Au-4/Ni-fiber was washed
by deionized water in the stage just after galvanic reaction
before drying and calcining, the benzyl alcohol conversion
dramatically decreased from 99% to 39% (Table S2w), because
the NiCl2 was washed away and no apparent NiO would be
formed during the pre-activation process (Fig. S7 and S8w).
Analogously, only NiO supported on Ni-fiber (Table S2w),
and furthermore, only Au or NiO NPs supported on Ti-fiber
(Tables S2 and S3w) and stainless-steel 316L-fiber (Tables S2
and S3w) just induced a very low benzyl alcohol conversion of
o20%, whereas their activity was remarkably promoted after
partially covering Au with NiO by post-doping treatments
(Tables S2 and S3w: NiO@Au/Ni-fiber, NiO@Au/Ti-fiber and
NiO@Au/SS-fiber). Additionally, the highest benzyl alcohol
conversion of only 39% was obtained on the Au@NiO/Ni-fiber
We then extended the tests using a range of large volatile
alcohols (Table S1w). This catalyst could oxidize 1-phenyletha-
nol to acetophenone at a high conversion of 99% at 300 1C,
but only 49% for 2-phenylethanol even at a higher tempera-
ture of 340 1C. In the case of octanol, 1-octanol showed
greater activity and selectivity than 2-octanol, consistent with
a previous report.2a Notably, cyclohexanol and cyclopropyl
carbinol were very reactive among the aliphatic alcohols.
Furthermore, 1,2-propylene glycol, which favors the forma-
tion of hydroxylketone rather than methyl glyoxal, could also
be oxidized to the target product with a selectivity of 66% at a
conversion of 99%.
Stability is a very important practical consideration for a
heterogeneous catalyst. For the benzyl alcohol oxidation at
250 1C (Fig. S4, ESIw), the pre-activated Au-4/Ni-fiber
delivered a much longer single-run lifetime of 230 h with very
good activity and selectivity maintenance than the reported
catalysts.2b–d,3 The spent catalyst was able to restore its
activity after calcining in air at 380 1C to burn away the
deposited carbon (6.4 wt% of carbon after a 230 h run). The
catalyst remained active with almost unchanged selectivity
of 499% after two regenerations (660 h time-on-stream), thus
showing promising stability characteristics. A high benzaldehyde
yield that approached 10 kg gÀ1 Au-4/Ni-fiber (or 250 kg gÀ1
Au) could be achieved after a 660 h run, which indicates a
much higher benzaldehyde productivity than the reported
catalysts, such as Au/SiO2 (600 g/g Cat.),2b K–Cu–TiO2
(120 g/g Cat.),2d Au/OMM (360 g/g Cat.),2e and Au/Cu-fiber
(840 g/g Cat.).3b
Another advantage of this catalyst is that its excellent heat
conductivity can rapidly dissipate a large quantity of reaction
heat from such strongly exothermic oxidation reactions.
A very low DT (between the bed and reactor external wall)
of o10 1C was observed through the entire 660 h test. In
contrast, very severe heat transfer limitations exist over the
oxides-supported catalysts because of their poor thermal
conductivity (e.g., Au–Cu/SiO2:2c DT = B53 1C, WHSV =
10 hÀ1; K–Cu–TiO2:2d DT = B17 1C, WHSV = 0.6 hÀ1).
Notably, the heat liberation rates with Au–Cu/SiO2 and
K–Cu–TiO2 were 50% and 3%, respectively, of that of
Au-4/Ni-fiber (WHSV = 20 hÀ1). Isothermal operation is
not only a safety consideration but a very important feature
that benefits activity and selectivity maintenance.4
An important issue is the origin of low-temperature activity
for the pre-activated Au/Ni-fiber. Generally, small gold
particles (o5nm) were considered to be crucial to its activity.6
However, the pre-activated Au-4/Ni-fiber has a Au particle
size of 25 nm (Fig. S5, ESIw). To resolve this problem, a series
of contrastive experiments were designed and performed. The
results from the contrastive catalysts with different particle
sizes (10–30 nm) and supports demonstrated that in this new
type of catalyst the particle size (410 nm) and support
property are not the key factors in determining the catalyst
performance (see Discussion 1–2 in ESIw for details). Interestingly,
XRD and X-ray photoemission (XPS) analysis indicated that
Fig. 1 (a) TEM image of the NiO@Au-NPs ensembles for the pre-
activated Au-4/Ni-fiber catalyst. (b) Structural illustration of the
partial coverage of Au particles with NiO segments. (c) O2-TPD
profiles of the pre-activated Au/Ni-fiber catalyst samples with various
Au loadings and the relationship between the relative intensity of TPD
signals plotted against the benzyl alcohol conversion (inset). (d) XPS
spectra in O1s, Ni2p, and Au4f regions on the surface of the
pre-activated Au-4/Ni-fiber sample before and after undergoing TPD.
c
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(Au particles dispersed on NiO; Table S2w). This experimental
observation strongly indicates that NiO@Au ensembles
are formed during the reaction and their synergistic effect
contributes to promoting the low-temperature activity. Such a
special synergistic effect has also been reported with the partial
coverage of Au with TiO2 and CeO2 or Pt with FeO catalyst
systems.7
and stabilizing them, but also in serving as an active O
specimens reservoir, due to the fact that XPS peaks from
Ni2O3 rather than NiO are apparently decreased along with
the disappearance of the Au4f peak of Au+ after TPD
(Fig. 1d). Similarly, it has been reported that CeO2 could
stabilize Aud+ in Au/CeO2 catalyst, and conversely, the Aud+
could increase the amount of surface active oxygen.9 There-
fore, the above results suggest that O species coming from the
Ni2O3–Au+ hybrid sites rather than the lattice O of Ni2O3 are
active for the low-temperature selective oxidation of alcohols
by the direct-oxidation dehydrogenation route.1d This is quite
different from the catalytic dehydrogenation mechanism on
our previously-developed Au/Cu-fiber catalyst.3b
Temperature-programmed desorption (TPD) was used to
obtain information about the synergistic effect of NiO@Au-NPs
anchored on Ni-fiber. The pre-activated catalysts with various
Au loadings all showed a single O2 desorption signal at
B510 1C, with maximum over the Au-4/Ni-fiber (Fig. 1c).
The signal ratio (i.e., the peak area of each sample divided by
the peak area of Au-1/Ni-fiber) correlated linearly well with
the benzyl alcohol conversion. Moreover, the conversion
tended to approach zero when extrapolating to a zero TPD
signal (Fig. 1c, inset). This coincided strongly with the
observation that benzyl alcohol conversion on the pre-
activated Au-4/Ni-fiber decreased to o1% within a few
minutes after switching off the O2 gas, and the TPD signal
of the catalyst dramatically decreased, nearly to zero accord-
ingly (Fig. S9, ESIw). Additionally, the extremely weak signal
for Au-4/Ti-fiber significantly increased after post NiO-doping
(NiO@Au/Ti-fiber) to B80% of that for Au-4/Ni-fiber
(Fig. S10w), consistent with the promotion of benzyl alcohol
conversion from 8% to 87% (Table S2w). In contrast,
Au@NiO/Ni-fiber provided a much weaker TPD signal at
B580 1C (Fig. S11w), thereby obtaining a much lower
conversion (39%) of benzyl alcohol.
Galvanic deposition of Au onto a sinter-locked Ni-fiber
delivers an excellent Au/Ni-fiber catalyst for low-temperature
gas-phase alcohol oxidation. Partial coverage of Au particles
with NiO segments (NiO@Au-NPs ensembles) formed on the
Ni-fiber during the reaction contributed to a high surface
concentration of Ni2O3–Au+ hybrid sites thereby leading to
more highly active O species. This significantly promotes the
low-temperature activity of gas-phase selective oxidation of
alcohols.
We thank the NSF of China (20973063, 21076083), the
MOST of China (2011CB201403), the Fundamental Research
Funds for the Central Universities, the Shanghai Rising-Star
Program (10HQ1400800), the Shanghai Leading Academic
Discipline Project (B409), and the Shanghai Synchrotron
Radiation Facility for helpful discussion.
Fig. 1d shows the XPS spectra of O1s, Au4f, and Ni2p for
the pre-activated Au-4/Ni-fiber samples. A large amount of
Ni2O3 specimens were formed, attributable to the strong peaks
for O1s at 531.8 eV and Ni2p at 856.2 eV. Notice that Au+
specimens were clearly detected with the Au4f (7/2) peak at
84.8 eV (surface Au+/Ni3+ ratio: 1/45), and OH species were
generally adsorbed to provide an O1s peak also at approxi-
mately 532 eV.8 Interestingly, the O1s peak at 531.8 eV
decreased significantly while the 529.7 eV O1s peak (lattice
O of NiO (Table S4, ESIw)) almost remained unchanged after
the TPD experiments. Meanwhile, the Au4f (7/2) peak for
Au+ at 84.8 eV disappeared completely, and the Ni2p peak
area for Ni2O3 at 856.2 eV decreased by B56%, which nearly
stoichiometrically agreed with reduction of the O1s peak for
Ni2O3 (Discussion 4 and Table S4w). As expected, on the same
sample after the TPD, as shown in Fig. 1d, an extremely low
conversion of o1% was obtained when reacting with benzyl
alcohol at 280 1C in the absence of O2, indicating that only
Ni2O3 in the absence of Au+ is inactive for the alcohol
oxidation. Moreover, disappearance of Au+ was also observed
by XPS on the reactive samples after switching off the O2 feed
followed by a 15 min reaction with benzyl alcohol alone, in
which the benzyl alcohol conversion decreased from 99%
to o1% (Fig. S9w). Therefore, the interaction between NiO
segments and Au–NPs, by nature, not only induces a high
surface concentration of Au+ specimens (surface Au+/Au0
ratio: 1/2) but also creates unique Ni2O3–Au+ hybrid sites.
Among such hybrid sites, Ni2O3 specimens likely play a key
role not only in promoting the formation of Au+ specimens
Notes and references
1 (a) T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037;
(b) A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,
2006, 45, 7896; (c) A. Corma and H. Garcia, Chem. Soc. Rev., 2008,
37, 2096; (d) J. L. Gong and C. B. Mullins, Acc. Chem. Res., 2009,
42, 1063.
2 (a) D. I. Enache, J. K. Edwards, P. Landon, B. Solsna-Espriu,
A. F. Carely, A. A. Herzing, M. Watanabe, C. J. Kiely,
D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362;
(b) S. Biella and M. Rossi, Chem. Commun., 2003, 378;
(c) C. D. Pina, E. Falletta and M. Rossi, J. Catal., 2008, 260, 384;
(d) J. Fan, Y. H. Dai, Y. L. Li, N. F. Zheng, J. F. Guo, X. Q. Yan
and G. D. Stucky, J. Am. Chem. Soc., 2009, 131, 15568; (e) G. Ma,
X. Yan, Y. Li, L. Xiao, Z. Huang, Y. Lu and J. Fan, J. Am. Chem.
Soc., 2010, 132, 9596.
3 (a) M. M. Deng, G. F. Zhao, Q. S. Xue, L. Chen and Y. Lu, Appl.
Catal., B, 2010, 99, 222; (b) G. F. Zhao, H. Y. Hu, M. M. Deng and
Y. Lu, Green Chem., 2011, 13, 55.
4 (a) D. K. Harris, D. R. Cahela and B. J. Tatarchuk, Composites,
Part A, 2001, 32, 1117; (b) Y. Lu, H. Wang, Y. Liu, Q. S. Xue,
L. Chen and M. Y. He, Lab Chip, 2007, 7, 133.
5 (a) R. Zanella, S. Giorgio, C. R. Henry and C. Louis, J. Phys. Chem.
B, 2002, 106, 7634; (b) A. Wittstock, V. Zielasek, J. Biener,
C. M. Friend and M. Baumer, Science, 2009, 327, 319.
¨
6 (a) M. S. Chen and D. G. Goodman, Science, 2004, 306, 252;
(b) B. K. Min and C. M. Friend, Chem. Rev., 2007, 107, 2709.
7 (a) T. Fujitani, I. Nakamura, T. Akita, M. Okumura and
M. Haruta, Angew. Chem., Int. Ed., 2009, 48, 9515; (b) C. M. Y.
Yeung, K. M. K. Yu, Q. J. Fu, D. Thompsett, M. I. Petch and
S. C. Tsang, J. Am. Chem. Soc., 2005, 127, 18010; (c) Q. Fu,
W. X. Li, Y. Yao, H. Liu, H. Y. Su, D. Ma, X. K. Gu, L. Chen,
Z. Wang, B. Wang and X. Bao, Science, 2010, 328, 1141.
8 L. C. Wang, L. He, Q. Liu, Y. M. Liu, M. Chen, Y. Cao, H. Y. He
and K. N. Fan, Appl. Catal., A, 2008, 344, 150.
9 Q. Fu, H. Saltsburg and M. Flytzani-Stephanopoulos, Science,
2003, 301, 935.
c
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