Gold supported on graphene oxide: An active and selective catalyst for phenylacetylene hydrogenations at low temperatures

Article abstract of DOI:10.1021/cs5002724

A constraint to industrial implementation of gold-catalyzed alkyne hydrogenation is that the catalytic activity was always inferior to those of other noble metals. In this work, gold was supported on graphene oxide (Au/GO) and used in a hydrogenation appl

Full text of DOI:10.1021/cs5002724

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Article
Gold supported on graphene oxide: an active and selective
catalyst for phenylacetylene hydrogenations at low temperatures
Lidong Shao, xing huang, Detre Teschner, and Wei Zhang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2014
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Gold supported on graphene oxide: an active and
selective catalyst for phenylacetylene
hydrogenations at low temperatures
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*, †
‡, §
§
∥
Lidong Shao, Xing Huang,
Detre Teschner, and Wei Zhang
†
College of Environmental and Chemical Engineering, Shanghai University of Electric Power
Shanghai 200090, PR China
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
‡
Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun East Road,
100190 Beijing, PR China
§
Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society,
Faradayweg 4ꢀ6, 14195 Berlin, Germany
∥ Department of Energy Conversion and Storage, Technical University of Denmark, Risø
campus, Frederiksborgvej 399, 4000 Roskilde, Denmark
KEYWORDS. Gold, graphene oxide, phenylacetylene hydrogenation, activity, selectivity, low
temperatures, coordination sites
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ABSTRACT. A constraint to industrial implementation of goldꢀcatalyzed alkyne hydrogenation
is that the catalytic activity was always inferior to those of other noble metals. In this work, gold
was supported on graphene oxide (Au/GO) and used in a hydrogenation application. 99%
selectivity towards styrene with a 99% conversion in the hydrogenation of phenylacetylene was
obtained at 60°C, which is 100°C to 200°C lower than optimal temperatures in most previous
reports on Au catalysts. A series of gold and palladium based reference catalysts were tested
under the same conditions for phenylacetylene hydrogenation, and the performance of Au/GO
was substantiated by studying the role of functionalized GO in governing the geometrical
structure and thermal stability of supported Au nanoparticles under reaction conditions.
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In polymerization industry, styrene and ethylene feedstock usually contains small quantities
(<2%) of acetylene compounds, which poison olefin polymerization catalysts. Thus, preliminary
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,2
removal of alkynes via selective hydrogenation is necessary. Despite being the most applicable
catalyst in alkyne selective hydrogenation, palladiumꢀbased catalysts suffer fast deactivation and
selectivity loss due to the accumulated carbonaceous deposition and the delicate subsurface
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chemistry under hydrogenation conditions.
In contrast to the chemically inert nature of bulk gold, nanosized gold displays rich surface
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chemistry.
Alkyne selective hydrogenation has been reported using supported gold
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ꢀ12
nanoparticles (AuNPs) as catalyst materials in the last decades.
Although AuNPs exhibited
high selectivities on various supporting materials, a constraint to practical applications was that
the catalytic activity was always inferior to those of other noble metals. The optimal temperature
windows in previous studies on goldꢀcatalyzed alkyne hydrogenation have been commonly
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reported to be above 150°C.
Thus elevated temperature is usually required for goldꢀbased
catalyst to achieve meaningful magnitudes of reactivity. Accordingly, designing goldꢀbased
catalysts for alkyne hydrogenation with stabilized selectivity at lower temperatures is a decisive
step for industrial implementations. To the best of our knowledge, gold supported on graphene or
graphene oxide has rarely been reported in hydrogenation applications to date.
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Table 1. Phenylacetylene hydrogenations over Au/GO, Au/CNTs, Pd/GO, Pd/C and Pd/Al O
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and Pd/CNTs
[
a,b]
Catalysts
Temperature
Conversion%
Styrene
[c]
Selectivity%
o
Au/GO(2wt%)
Au/GO(2wt%)
Au/GO(2wt%)
Au/GO(2wt%)
Au/CNTs(2wt%)
Au/CNTs(2wt%)
Au/CNTs(2wt%)
Pd/GO(2wt%)
Pd/GO(2wt%)
Pd/C(1wt%)
40 C
5
99
99
99
99
90
90
90
0
o
60 C
99
99
99
75
80
95
99
99
99
99
99
99
o
80 C
o
100 C
o
60 C
o
80 C
o
100 C
o
40 C
o
100 C
0
o
40 C
0
o
Pd/C(1wt%)
100 C
0
o
Pd/Al O (1wt%)
40 C
0
2
3
o
Pd/Al O (1wt%)
100 C
0
2
3
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Pd/CNTs(2wt%)
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100 C
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[
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Au/GO(2wt%)
Au/GO(2wt%)
Au/GO(2wt%)
60 C
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[d]
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80 C
[
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100 C
[
a] The metal loadings of selfꢀprepared Au/GO, Au/CNTs Pd/CNTs and Pd/GO are measured by
XꢀRay fluorescence analysis (XFA), Pd/C and Pd/Al O are supplied from SigmaꢀAldrich.
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[b] For each catalyst, the same amount (30 mg) of catalyst material was tested for
hydrogenations.
[c] Product mixtures were analyzed by HPLC, no byproducts from polymerization or ethanol
dehydration were detected.
[
d] Au supported on GO with prolonged reduction time.
In the present work, phenylacetylene hydrogenation was conducted over gold supported on
graphene oxide (Au/GO). Gold supported on carbon nanotubes (Au/CNTs), palladium supported
on graphene oxide (Pd/GO) and palladium supported on CNTs (Pd/CNTs) were prepared and
applied under the same conditions for references. Commercial available catalysts of palladium
on charcoal (Pd/C) and palladium on alumina (Pd/Al O ) were applied for catalytic comparisons.
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The morphologies of Pd/GO, Pd/C, Pd/Al O and Pd/CNTs are shown in Figure S1, where the
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dispersion of Pd nanoparticles is displayed for each catalyst. For comparing catalytic properties
of several catalysts under the same hydrogenation conditions, the continuous flow system
permits a fast optimization of reaction conditions and aids catalyst collections without filtration
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steps. Au/GO was studies in both continuous flow and batch systems. Other catalysts were
studied in the flow system only.
Hydrogenations were carried out with a stock 0.02 M solution of phenylacetylene using ethanol
as solvent. Reactant solution was pumped into the capsule of flow reactor at a flow rate of 0.2
mL/min. Each capsule carries a same amount of catalysts. At each temperature, effluent solution
was collected after 15 minutes stabilization under reaction conditions. Each collection lasted for
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minutes. After collection, reaction temperature in reactor was increased directly to the next
target and same procedures were repeated. After one cycle of running at 40, 60, 80, 100°C
respectively, reactor was cooled to room temperature and washed by solvent. Such
hydrogenation cycle was repeated for three times for each catalyst for evaluating the catalytic
stabilities. After proper stabilizations at each temperature (15 minutes under reaction conditions),
collected catalytic results are within 2% deviation.
Reactant conversion and selectivity values versus temperature are shown in Table 1. Full
conversions were obtained at 40 and 100°C when Pd/GO, Pd/C, Pd/Al O and Pd/CNTs were
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used. Despite the various supports and metal loadings, no selectivity towards styrene was
obtained for any of the Pdꢀbased catalysts. In contrast to the Pdꢀbased catalysts, a conversion of
5% with a selectivity of 99% towards styrene was obtained at 40°C for Au/GO. Increasing
temperature (60, 80, and 100°C) resulted in full phenylacetylene conversion (99%) with
stabilized selectivities of 99%. The estimated turnꢀover frequency (TOF) at the applied
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conditions and at full conversion is 360 h , which is consistent with a recent work using gold
as catalyst for phenylacetylene hydrogenations. Please note that this is the lower limit to the real
TOF, as at full conversion we do not probe the kinetic regime of the catalyst. Reaction over
Au/CNTs under the same conditions exhibited lower activities and selectivities at each
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temperature. Decrease of activity and selectivity was also observed on Au/GO that underwent
prolonged reduction time during sample preparation.
The observed fullꢀhydrogenation towards ethylbenzene on Pd/GO, Pd/C, Pd/Al O and Pd/CNTs
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are consistent with reported low selectivities over monometallic Pdꢀbased catalysts in alkyne
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hydrogenation. On the other hand, achieving high activities and selectivities at 60°C over Auꢀ
based catalysts in alkyne selective hydrogenation can be considered as a novel result as the
optimal temperature window in previous studies of goldꢀcatalyzed alkyne hydrogenation has
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mostly been above 150°C.
To confirm the improved catalysis over Au/GO in phenylacetylene selective hydrogenation,
conventional batch system was also applied (See supporting information for experimental
details). Full conversion was obtained after 2 hours at 60°C, and high styrene selectivities were
observed at the applied batch conditions (Figure S2). The same results from both continuous
flow and batch systems indicate that catalyzing phenylacetylene hydrogenation with high
activities and selectivities at low temperature is an intrinsic property of Au/GO.
Compared to Pdꢀbased catalysts, significantly improved catalytic results over Au/GO at low
temperatures prompted an inꢀdepth examination of its electronic properties under reactive
atmosphere. Inꢀsitu Xꢀray photoelectron spectroscopy (XPS) was applied to study the surface
and subsurface regions of catalysts. We characterized the state after storing the fresh "asꢀis"
sample in air, as well as under H at room temperature and 100°C. The Au4f profiles of Au/GO
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(
Figure 1a) exhibited metallic phases in the fresh state as well as under H at room temperature
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and 100°C (no surface and subsurface modification or newꢀphase formation were traced),
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whereas palladium can transform into the wellꢀknown palladium hydride under H at room
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temperature (Figure 1b).
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Figure 1 In-situ XPS analysis of a), Au4f region of Au/GO in fresh (as is) status and under 1
mbar of hydrogen at 100°C. b), Pd3d region of Pd foil in fresh (as is) status and under hydrogen
at room temperature in the βꢀhydride state.
Subsurface chemistry governs the catalytic properties of Pdꢀbased catalysts in selective
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hydrogenation applications.
Pdꢀhydride (PdH ), Pdꢀcarbide(PdC ), or coꢀexistence of both
x y
phases PdH C may influence catalysis by affecting and generating active phases under reaction
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y
conditions. Therefore, the activity and selectivity over Pdꢀbased catalysts during hydrogenations
are highly dependent on the applied conditions and the catalyst prehistory. As for monometallic
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Pd catalysts used in the current work, fullꢀhydrogenation took place due to the strong affinity of
palladium towards H . As for AuNPs supported on GO, the absence of surface and subsurface
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dynamics indicated much weaker interaction with H , in line with previous DFT calculations
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and the preferential adsorption towards C≡Cs. Moreover, the high styrene selectivity was
independent of reaction temperatures, which suggests that the product readily desorbed from the
catalyst without undergoing secondary hydrogenation or other side reactions. Thus the enhanced
preferences of adsorbing phenylacetylene and desorbing styrene explains the improved
selectivity towards styrene over supported AuNPs on GO. In fact, the lack of styrene
hydrogenation even at full phenylacetylene conversion (see Figure 1 or Figure S2) suggests very
weak interaction of styrene with Au/GO and/or the barrier of its further hydrogenation being far
above the potential energy of styrene desorption.
Compared to Pdꢀbased catalysts, fullꢀhydrogenation is not favored over nanoscale gold, which is
also in line to the absence of surface and subsurface dynamics of Au under reaction conditions.
However, detailed analyses of active phases on Au/GO are required to interpret the obtained high
activities and selectivities at much lower temperatures than previous reports. Before we
investigate the geometrical structures of supported AuNPs, studying the supporting effect of GO
is crucial, because it may affect the size, shape, crystalline structure, as well as the thermal
stability of supported AuNPs under reactive atmosphere. CNTs, as reference materials, were
applied as support of AuNPs which were tested in phenylacetylene hydrogenations under the
same conditions.
Distribution and intensity of functional groups over the carbon support is proportional to the
degree of surface functionalization. In the process of functionalizing carbon nanomaterials,
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functionalities are introduced by oxidative treatments due to their high efficiency and
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simplicity.
In the present work, strong oxidation was applied to attack and expand the
graphitic layers of graphite. Consequently, the obtained GO possesses a heavily functionalized
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surface with large surface area. As for functionalized CNTs, oneꢀstep HNO treatment has not
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led to a morphology change on CNTs. Microscopic analysis showed that the inner walls of the
CNTs were graphitic, suggesting that the inner walls were not attacked or oxidized during the
oxidation process. The XPS analysis of Au/GO and Au/CNTs (Figure S3) reflects a stronger
functionalization on GO than on CNTs, the O/C ratio being 0.12 on GO and 0.06 on CNTs.
Figure 2 a), HRTEM image of an AuNP on GO, with an inset of the Fast Fourier Transform. b),
HRTEM image of an AuNP on GO after prolonged reduction treatment, with an inset of the Fast
Fourier Transform.
In Figure S4, a STEM image displays the wellꢀdispersed character of AuNPs on GO with a
mean particle size of 5.2 ± 0.2 nm. Some aggregates of AuNPs are also visible on a large area of
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GO. A close observation of a supported AuNP on GO is shown in Figure 2a; the lattice fringes
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(
002) and (111) with a characteristic acute angle of 54.7 were identified on a rounded AuNP
based on the JCPDS card (No. 04ꢀ0784) with space group Fmꢀ3m. Nevertheless, the rounded
AuNPs on the present GO possess enriched low coordination sites (edge, stepped and kink) next
to the exposed facets. As for Au/CNTs, high resolution microscopic studies revealed multiply
twinned Au particles (MTPs) supported on CNTs (Figure S5) with an increased mean particle
size (8.4 ± 0.2 nm).
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The kind of metal and the local atomic structure of the surface both affect the ease by which the
hydrogen molecule becomes dissociatively chemisorbed. Over coinage metals, hydrogen
dissociation is either not preferred or happens with a low rate. Sljivancanin and Hammer pointed
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out the importance of vacancy and kink sites for this reaction over Cu. They attributed the high
reactivity of these defects due to the broken CuꢀCu bridge bonds, which cause reduced Cu(4sp)ꢀ
H (σ) repulsion but unchanged Cu(4sp)ꢀH (σ*) attraction. We expect the same situation for Au.
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Now, in general, decreasing the coordination number of surface atoms (creating steps, kink and
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corners), the dꢀband width becomes necessarily smaller.
This, in turn, shifts the dꢀband center
towards the Fermi level. Therefore these lower coordination sites bind adsorbates stronger than
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those on perfect low Miller index planes.
Since hydrocarbon adsorption of low Miller index
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gold facets is very weak,
this indicates that sites with low coordination number not only
activate hydrogen, but also play an important role in adsorbing hydrocarbon reactants. With the
proper choice of support and reduction temperature, nonꢀionic gold particles can be formed with
high density of low coordination sites, enhancing alkyne partial hydrogenation.
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Figure 3 a), STEM image revealing the Au dispersion of the used Au/GO. b), Energy dispersion
Xꢀray in the STEM mode (STEMꢀEDX) maps of AuNPs supported on GO after reaction. c),
STEM image of fresh Au/CNTs. d), STEM image of the used Au/CNTs.
In our work, the high density of surface functionalities on GO gives rise to a strong anchoring
effect and thus produces thermodynamically nonꢀequilibrium particle shapes at appropriate
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reduction temperatures. On the other hand, the somewhat lower functionalization of CNTs
allowed for goldꢀgold interaction and clustering during reductions in sample preparation. Thus,
twinning occurred while growing of particles on CNTs. Accordingly, Au/CNTs exhibited lower
activities and selectivities in phenylacetylene hydrogenation compared to Au/GO under the same
conditions.
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In the AuNPs applications, oxidized species are commonly reported as coꢀexisting species for
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influencing catalytic performance. Oxidized Au species may be present on the surface before
reductive treatment. Furthermore, preꢀabsorbed species may influence catalytic applications by
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affecting surface adsorbing strength towards other reactant molecules.
Therefore, sufficient
reduction is required for obtaining the metallic phase of gold. However, special care during
reduction process is necessary because prolonged reduction time may lead to an increased
twinning of AuNPs (Figure 2b). AuNP size distribution was observed to increase to 18.6 ± 0.2
nm due to the progressive removal of functionalities under reductive atmosphere with prolonged
reduction time. Tested in phenylacetylene hydrogenation, Au/GO after prolonged reduction
exhibited significantly lower activity and styrene selectivity under the same conditions. The
purpose of carrying out prolonged reduction and testing in hydrogenation was to prove the role
of functionalities in affecting catalysis by influencing the geometrical structures of supported
AuNPs.
Au/GO has not only exhibited enhanced activities and selectivities, but also a high stability after
several running cycles in flow reactor. To reach an understanding of the obtained high stability
of Au/GO, used Au/GO was compared to fresh samples. Because no filtration is required for
collecting or separating used catalyst from reactant solution, running reactions in continuous
flow aids a friendly platform for sample analyses. Another important character of using flow
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system is the continuous flow reaction pathway. In batch systems, leached metal particles can be
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reabsorbed onto the support surface during catalytic reaction. As for in flow system, such
readsorption is unlikely taking place due to the oneꢀdirection sample scouring with continuous
flows. Thus, the catalytic stability can be directly related to the information obtained on collected
catalysts. The STEM imaging of used Au/GO (Figure 3a) suggests that the AuNPs retained a
good dispersion with the mean size of 5.3 ± 0.2 nm (Figure S6). GO displays a similar
morphology with no obvious surface and structural alternations compared to fresh Au/GO in
Figure S4. Consistent with Figure 3a, a local view using the STEMꢀEDX mode displays a
homogeneous distribution of Au, O and C elements indicating the wellꢀdispersed status, as seen
in Figure 3b. Confirmed by Xꢀray fluorescence analysis (XFA), Au loading remained the same
after hydrogenation. As for Au/CNTs, leaching of AuNPs was observed as deduced from Figure
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3d in comparison to the fresh catalyst (Figure 3c). Confirmed by XFA, 0.4 wt% of Au loading
was leached from the CNTs surface. Compared to Au/CNTs, the preservation of AuNPs on GO
further confirmed the role of support by anchoring AuNPs. Thereby, a high stability towards
selective phenylacetylene hydrogenation was achieved over Au/GO.
The role of functionalization on carbon supports in affecting hydrogenation was assessed by
comparing Au/GO with Au/CNTs and Au/GO after prolonged reduction treatment. The XPS
analysis revealed a higher functionalization profile on GO than on oxidized CNTs, suggesting
that GO was advantageous in possessing more abundant functional groups that could serve as
nucleation and anchored sites for nanoparticle growth. After metal ions coordinating with
oxygen functional groups, hydrolysis is applied to crystallize the precursors nucleated on the
oxidized carbon supports to form nanocrystals. The temperature and reduction time can tune the
size, shape and crystallinity of the nanocrystals. Small gold nanoparticles with nonꢀequilibrium
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particle shape and exposing large number of low coordination sites are crucial for both
dissociative hydrogen activation and phenylacetyelene adsorption. Designing a strongly coupled
AuNPs–GO hybrid material with optimal geometrical structure of gold nanoparticles is the key
factor for improved hydrogenation performance.
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In this work, gold was supported on GO and utilized in a hydrogenation application. 99% styrene
selectivity with a 99% conversion in the hydrogenation of phenylacetylene was obtained at 60°C,
which is far lower than the optimal temperature in most previous reports. Such catalytic
0
performance was achieved in both continuous flow and batch systems over highly dispersed Au
nanoparticles on GO. In the flow system, Au/GO exhibited stable performance during several
running cycles. Compared to Pdꢀbased catalysts, the enhanced selectivities obtained over Au/GO
were interpreted by the absence of the subsurface dynamics of metallic phases under reactive
atmosphere and the preference for the desorption of partial hydrogenation product. Highly
dispersed rounded AuNPs with enriched lowꢀcoordination sites were active and selective for
phenylacetylene hydrogenation. These sites are crucial for both hydrogen activation and alkyne
adsorption. The established correlation between the shape of AuNPs and improved catalytic
performance demonstrated the influence of support functionalization on the geometrical structure
of supported AuNPs. Prolonged reduction treatment further confirmed the support effect of GO
on the shape and size of the AuNPs. Functionalized surfaces of GO synthesized from expanding
and oxidizing graphite layers can hinder Au–Au interaction during catalyst preparation and tune
the geometrical structures of thus formed AuNPs, retard thermal dynamics (crystalline changes,
thermal aging and leaching) of supported AuNPs under reaction conditions, and enhance the
catalytic stability through metalꢀsupport interaction.
ASSOCIATED CONTENT
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Experimental procedures including Material Synthesis, Electron microscopy, In situ xꢀray
photoelectron spectroscopy and Catalysis, Figures S1ꢀ6 are supplied as Supporting Information.
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*
Eꢀmail: lidongshao@outlook.com
Notes
The authors declare no competing financial interest.
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The following illustration is used as the TOC.
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