Plasmonic Switching of the Reaction Pathway: Visible-Light Irradiation Varies the Reactant Concentration at the Solid–Solution Interface of a Gold–Cobalt Catalyst

Article abstract of DOI:10.1002/anie.201904452

Product selectivity of alkyne hydroamination over catalytic Au₂Co alloy nanoparticles (NPs) can be made switchable by a light-on/light-off process, yielding imine (cross-coupling product of aniline and alkyne) under visible-light irradiation, but 1,4-diphenylbutadiyne in the dark. The low-flux light irradiation concentrates aniline on the catalyst, accelerating the catalytic cross-coupling by several orders of magnitude even at a very low overall aniline concentrations (1.0×10^?3 mol L^?1). A tentative mechanism is that Au₂Co NPs absorb light, generating an intense fringing electromagnetic field and hot electrons. The sharp field-gradient (plasmonic optical force) can selectively enhance adsorption of light-polarizable aniline molecules on the catalyst. The light irradiation thereby alters the aniline/alkyne ratio at the NPs surface, switching product selectivity. This represents a new paradigm to modify a catalysis process by light.

Full text of DOI:10.1002/anie.201904452

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Plasmonic Switching of Reaction Pathway: Visible-Light
Irradiation Varies Reactant Concentration at the Solid-Solution
Interface of a Gold-Cobalt Catalyst
Authors: Sarina Sarina, Erandi Peiris, Eric R. Waclawik, Godwin A.
Ayoko, Pengfei Han, Jianfeng Jia, and Huai-Yong Zhu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904452
Angew. Chem. 10.1002/ange.201904452
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Plasmonic Switching of Reaction Pathway: Visible-Light
Irradiation Varies Reactant Concentration at the Solid-Solution
Interface of a Gold-Cobalt Catalyst
Erandi Peiris[a], Sarina Sarina*[a], Eric R. Waclawik[a], Godwin A. Ayoko[a], Pengfei Han[a], Jianfeng Jia[b],
Huai-Yong Zhu*[a]
Abstract: Product selectivity of alkyne hydroamination over catalyst
Au2Co alloy nanoparticles (NPs) can be made switchable by a light-
on, light-off process, yielding imine (cross-coupling product of aniline
and alkyne) under visible-light irradiation, but 1,4-diphenylbutadiyne
in the dark. The low-flux light irradiation concentrates aniline on the
catalyst, accelerating the catalytic cross-coupling by several orders
even at a very low overall aniline concentration (1.0×10-3 mol/L). A
tentative mechanism is that Au2Co NPs absorb light, generating an
intense fringing electromagnetic field and hot electrons. The sharp
field-gradient (plasmonic optical force) can selectively enhance
adsorption of light-polarizable aniline molecules on the catalyst. The
light irradiation thereby alters the aniline/alkyne ratio at the NPs
inducing selective adsorption of reactant molecules on the
catalyst surface. This finding is of fundamental significance to
photocatalysis.
surface, switching product selectivity. This represents
paradigm to modify a catalysis process by light.
a new
Controllable product selectivity is of equal importance to efficient
reactant conversion in chemical synthesis.[1,2] Recently, we
found that alloying small amounts of Cu into Au NPs can alter
the reaction pathway of the catalytic reduction of nitro-aromatics
under visible-light irradiation, directly transforming them into
anilines in a highly selective manner.[2] Using a monometallic Au
NP photocatalyst instead generates azobenzene as the final
product, indicating the reaction pathway changes, due to the
alloy effect in this case. If light irradiation applied in
photocatalytic selective synthesis could determine the reaction
pathways with the same catalyst material, it could prove useful
to addressing the challenge of product selectivity control in
catalysis generally.
Figure 1. Product selectivity of alkyne hydroamination reaction. (A) The
reaction catalysed by Au2Co/ZrO2 under visible-light irradiation and (B) in the
dark. (C) The influence of temperature under visible-light irradiation and (D) in
the dark.
Previously we have found that Au NPs supported on ZrO2
can catalyse the hydroamination of alkyne with aniline when
irradiated by visible-light at ambient temperature.[3] This reaction
provides an atom-economical route for the synthesis of various
organic nitrogen molecules that are important fine chemicals and
synthetic intermediates.[4] In the present study we conduct
screening test of various alloy NPs of gold with a transition metal
for alkyne hydroamination reaction and observe an interesting
photo-switchable product selectivity on Au2Co alloy NPs. As
illustrated in Figure 1 product selectivity of the reaction can be
readily switched by a light-on or light-off process, yielding imine
under low-flux (~0.5 W/cm2), visible-light irradiation (Figure 1A),
and 1,4-diphenylbutadiyne (homo-coupling product of alkyne) in
the dark (Figure 1B). We have found that visible-light irradiation
can change this reaction’s pathway and reaction kinetics by
The photo-switchable reaction pathway was not observed
with supported monometallic catalysts of either Au/ZrO2 or
Co/ZrO2, respectively (Figure S1 in Supporting information). The
hydroamination reaction rate over Au2Co/ZrO2 catalyst was
increased, compared with the rate over Au/ZrO2 catalyst. The
catalytic performances of other Au alloy catalysts (e.g. Au-Ni,
Au-Cu alloy) for the hydroamination were investigated under
visible-light irradiation and in the dark as well. In terms of the
reactant conversion and selectivity towards the cross-coupling
imine product, the Au2Co/ZrO2 catalyst exhibited a superior
photocatalytic performance compared to other alloy catalysts
(Table S1) and to AuCo alloy catalysts with other Au:Co ratios
(Figure S2). Light irradiation of Au/ZrO2 catalyst and other gold
alloy catalysts did not result in substantial change in product
selectivity. Further investigation revealed that the Au2Co/ZrO2
catalyst maintained the optical-switching capability at elevated
temperatures as illustrated in Figure 1C and 1D. Notably, the
product selectivity abruptly changed upon light irradiation even
at low light intensity (0.1 Wcm-2), demonstrating a reaction
pathway switch due to light irradiation (Figure S3).
[a]
Dr. E. Peiris, Dr. S. Sarina, A/Prof. E. R. Waclawik, Prof. G. Ayoko,
Dr. P. F. Han, Prof. H. Y. Zhu
School of Chemistry, Physics and Mechanical Engineering
Queensland University of Technology
2 George Street, Brisbane 4001 Australia
E-mail: s.sarina@qut.edu.au; hy.zhu@qut.edu.au
Prof. J. F. Jia
[b]
The scope of the photo-switchable product selectivity was
explored by using various anilines and alkynes as the reactants.
The results in Table 1 clearly show that the optical switch of
product-selectivity occurs when the reactants have different
School of Chemical and Material Science, Shanxi Normal University,
Linfen 041004, China
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201904452
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substituent groups. We were pleased to see that the optical
switch-effect was observed for a wide range of reactants, where
cross-coupling pathway prevails under light irradiation, while
homo-coupling does in the dark. The optimal reaction conditions
are given Table S2.
electrons and reactant and, thus, the reactions. However, the
hot electron mechanism cannot explain the photo-switchable
product selectivity phenomenon.
Table 1. Substrate scope for the hydroamination of alkyne
Entry
R1
R2
Cross coupling %
Homo coupling %
Photo
Dark
25
40
29
13
11
15
13
21
20
19
15
30
Photo
≤5
16
20
≤5
≤4
9
Dark
75
60
71
87
89
85
87
79
80
81
85
70
1
Ph
Ph
95
2
Ph-CH2
Ph-C2H4
Ph-Br
Ph-I
Ph
84
Figure 2. Characterization of the Au2Co/ZrO2 photocatalyst. (A) TEM image of
the catalyst. (B) HR-TEM image of an Au2Co alloy NP. The inset shows a line
profile analysis providing information on Au/Co distribution of the NP (C)
Diffuse reflectance ultraviolet-visible (DR-UV-Vis) spectra of samples.
3
Ph
80
4
Ph
95
5
Ph
96
Since the first step of all heterogeneous catalysis
processes is reactant adsorption at the catalyst surface we
investigated whether light irradiation induced changes in the
reactant adsorption. As shown in Figure 3A, light irradiation on
Au2Co/ZrO2 catalyst suspended in bulk liquid phase causes
substantial changes to the concentration of aniline and
phenylacetylene (denoted as alkyne). In dark conditions, the
alkyne adsorption to the catalyst surface is observed as a 20%
concentration decrease detected in the bulk liquid. With aniline,
a 10% concentration decrease in the bulk liquid was observed.
Upon visible-light irradiation, the measured aniline concentration
in the bulk liquid decreased from 90% to 70% of the initial
concentration, which could be explained if aniline was
concentrated near the catalyst NPs, by visible-light irradiation.
In contrast, the measured alkyne concentration in the bulk
liquid phase increased from 80% to 90% of the initial
concentration under irradiation: the amount of the adsorbed
alkyne decreased from 20% (in the dark) to 10% under the light
irradiation. The irradiation weakened the alkyne adsorption on
the catalyst. Moskovits and co-worker have observed similar
6
Ph-OCH3
Ph-CH3
Ph
Ph
91
7
Ph
93
7
8
Ph-Cl
Ph-Br
Ph-OCH3
Ph-CH3
Ph
96
4
9
Ph
94
6
10
11
12
Ph
90
10
12
20
Ph
88
C3H7
80
50 mg of catalyst, 1 mmol alkyne, 1.2 mmol amine, 1.0 ml toluene as solvent,
0.5 W/cm2 light intensity, 24 h, argon atmosphere, 80 °C temperature.
Structure analysis of the catalyst is shown in Figure 2 and
Figure S4. The Au2Co alloy NPs with a mean particle size of 7
nm were dispersed on a ZrO2 support (Figure 2A, S4A and 4B),
slightly larger than the Au/ZrO2 (5 nm in average, Figure S4C).
The actual Au:Co mass ratio is 2:0.84 (Figure S5 and Table S3),
very close to the designed ratio 2:1. Energy dispersive X-ray
spectroscopy line scan analysis indicates that both Au and Co
coexist in the particle (Figure 2B), confirming the formation of
alloy NPs. X-ray photon spectra (XPS) analysis indicates that Au
is in metallic state, while both metallic Co0 and oxidized Co2+
coexist in the catalyst (Figure S4D).
The catalyst of Co NPs on ZrO2 support, Co/ZrO2(3 wt%),
exhibited limited activity with a 20% conversion under light
(Table S4, entry 7). The Co2+ species alone have no catalytic
activity to this reaction. This conclusion is corroborated by the
results of the experiment to further clarify the contribution of the
metallic Co in the Au2Co/ZrO2 catalyst (Table S4).
photodesorption of molecule with
a quinoline ring from
plasmonic silver NPs.[6] Since light irradiation does not cause
any obvious change in either reactant concentration in the
presence of ZrO2 alone (≤4%), it is concluded that aniline was
attracted-to and alkyne was released-from the illuminated alloy
NPs of the catalyst in solution. The concentration changes are
reversible: when the light was switched off, the reactant
concentration restored, the
schematic illustration of
concentration change is shown in Figure 3C. The light induced
adsorption changes on Au/ZrO2 and Co/ZrO2 samples are
provided in Figure S7 for comparison.
In the dark, alkyne adsorption on the alloy catalyst was
twice that of aniline adsorption (Figure 3A), thus, most alkyne
molecules adsorbed on the surface of the alloy NPs were
activated to yield the diyne, homo-coupling product. The actual
aniline/alkyne molar ratio at the catalyst surface determines the
reaction pathway, but this ratio may be significantly different
under conditions of visible-light irradiation. Under the same
conditions in the dark the initial adsorption of the two reactants
on Au/ZrO2 catalyst were similar (Figure S7B), which is
favouring the cross-coupling reaction in the dark. Visible-light
irradiation did not switch the selectivity on Au/ZrO2.
The Au2Co/ZrO2 catalyst absorbed visible-light over a
broad range with a peak at 520 nm, similar to the absorption of
Au/ZrO2 (Figure 2C). The Au NPs can efficiently absorb visible-
light due to the localized surface plasmon resonance (LSPR)
effect and conduction electrons are rapidly excited to higher
energy levels.[1d,5] These excited, energetic electrons (hot
electrons) can activate reactant molecules to induce chemical
reactions.[1d,2,3] Silica coated Au2Co@ZrO2 catalysts (TEM
images are provided Figure S6), did not exhibit catalytic activity.
The SiO2 layer presents an impermeable barrier to reactant
molecules, preventing the interaction between the light-excited
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Au2Co catalyst discussed above. The aforementioned results
suggest that light irradiation altered the product selectivity by
significantly changing the ratio of adsorbed aniline to adsorbed
alkyne on Au2Co NPs.
The changes in reactant adsorption on Au2Co/ZrO2 should
affect reaction kinetics. In Figure 4A the reaction rates are
expressed in turnover frequency (TOF, mmol·g-1·h-1) and plotted
against the initial concentration of aniline. In the dark, the rate
increased as aniline concentration was increased. Under
irradiation of 1.0 W/cm2 intensity light, at a very low initial aniline
concentration of 5×10-3 M a TOF of 200 mmol·g-1·h-1 was
achieved. It is also noted that the product selectivity towards
imine (the cross-coupling product) increased significantly with
the increasing aniline concentration when aniline concentration
was above 0.5 M (Figure 4B) even in the dark.
Figure 3. Results and schematic illustrations of selective trapping aniline to
alloy NPs by irradiation of Au2Co/ZrO2 photocatalyst. (A) Visible-light
irradiation induced reactant concentration change on Au2Co alloy NPs in the
light-on and light-off cycle process (which were measured by gas
chromatography, GC). (B) The UV-Vis spectra of aniline and phenylacetylene
in toluene solvent respectively (1 mmol in 1 mL). (C) Schematic adsorption
mechanism and relation to the product selectivity based on data in panel A.
The alkyne adsorption on Au2Co alloy NPs in the dark is
much larger than that on Au NPs, the irradiation destabilizes the
strong alkyne adsorption on Au2Co/ZrO2 catalyst. The UV-VIS
absorption spectra of aniline and phenylacetylene adsorbed
Au/ZrO2 and Au2Co/ZrO2 catalysts are shown Figure S8. A
damping of the surface plasmon absorption band (at ~ 530 nm)
is observed only for phenylacetylene adsorbed onto the
Au2Co/ZrO2 catalyst, indicating strong interaction between
phenylacetylene and the alloy particles. This damping induced
by strong adsorption is not observed on other Au alloys. The
strong interaction is consistent with the high alkyne adsorption of
Au2Co/ZrO2 catalyst in the dark and should facilitate activation of
the alkyne molecules at the Au2Co NP surface. However this
strong interaction was not reflected in FTIR spectra (Figure S9),
which indicated that the phenylacetylene adsorption proceeds by
a multilayer physical adsorption on the AuCo surface (although
not as strong as chemisorption, this amount is sufficient to
induce LSPR damping). A strong interaction between aniline and
Co sites at alloy NP surface was observed in the FTIR spectra.
When light irradiation increased aniline adsorption on the alloy
catalyst this effect may benefit to activation of aniline for reaction.
The unique role of Co in the alloy is the increased adsorption to
alkyne molecules. The enhanced adsorption subsequently
increased the catalytic activity of Au and induced the photo-
switchable reaction pathway of alkyne and aniline.
Figure 4. Influence of reactant concentration on reaction rate on Au2Co/ZrO2
catalyst. (A) Phenylacetylene concentration was kept constant at 0.5 M while
aniline concentration was continually increased from 0.5 mM to 10 M. The
reactions were conducted at the same temperature with and without irradiation.
(B) Product selectivity analysis of panel A.
The unusually high TOF at very low concentration (Figure
4A) and the selectivity difference in Figure 4B support the
conclusion that the aniline surface concentration at the
illuminated catalyst solid-solution interface was considerably
higher than in the bulk of the solution. This would accelerate the
reaction as we have observed, when higher aniline
concentrations resulted in faster reaction rates in the dark
(Figure 4A). The increased aniline concentration at the catalyst
surface and the photodesorption of alkyne from the catalyst work
together, to switch the product selectivity.
The selective attraction of aniline molecules to the catalyst
surface is generated under illumination of Au2Co NPs. We tried
to understand the nature of this force. It is known that when
irradiated with light, plasmonic metal NPs generate an intense
electromagnetic (EM) field in close proximity to the NPs and hot
spots (plasmon field enhancement),[5,7,8] which is accompanied
by generating a force [8] similar to the optical force generated by
laser.[9] It has been predicted that laser irradiation of closely
spaced silver NPs can result in steep field gradients that may
trap small particles and large biomolecules.[8] We measured
Surface Enhanced Raman Scattering (SERS) spectra of aniline
adsorbed on catalysts (Figure S10). The enhanced SERS
signals can be attributed to two factors: EM enhancement and
chemical enhancement (interaction between adsorbed
molecules and the surface).[10] Vibration bands of aniline
adsorbed onto the Au2Co/ZrO2 catalyst are observed with
considerably higher intensity compared to the weak SERS
We applied alkyne as the sole reactant to conduct the
homo-coupling reaction on Au2Co/ZrO2 (Table S5). When
irradiated with light (photo-reaction), the alkyne conversion rate
(30%) on Au2Co catalyst was lower than that of the reaction in
the dark (45%). The reason for this reduction of catalytic
performance can be explained by
a decreased alkyne
adsorption on Au2Co NPs during irradiation. This result is
consistent with the irradiation induced release of alkyne from
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peaks of aniline adsorbed on Au/ZrO2 or ZrO2. This
demonstrated that Au2Co alloy NPs of the catalyst system can
generate a stronger EM field, compared to Au/ZrO2, under the
same conditions. More aniline molecules adsorbed on
Au2Co/ZrO2 catalyst can also lead to the higher SERS signal
intensity. However the photo-responsive reversible adsorption
and desorption of aniline on Au2Co indicated that the higher
aniline adsorption is due to the enhanced EM field of Au2Co.
started to decline after the fourth cycle, however, the product
selectivity switching-effect remained. The Co is slightly oxidised
after each cycle. A reduction treatment can reduce the oxidized
Co. There was no change in NP size or morphology after recycle
tests (Figure S13).
We have demonstrated that low intensity visible-light
irradiation of Au2Co alloy NPs can significantly change the
adsorption of both reactants of alkyne hydroamination on
Au2Co/ZrO2 catalyst and switch the product-selectivity of the
reaction, in addition to the generation of hot electrons in metal
NPs to induce the reaction. This function allows us to achieve
high turnover frequency at very low concentration in catalytic
reactions and to tune the product selectivity readily by visible-
light irradiation. A number of evidences suggest that the light
irradiation of Au2Co NPs generates an optical plasmon force that
can add to van der Waals force and selectively attract aniline
molecules to the catalyst. The light irradiation also weakens
adsorption of alkyne. The simultaneously selective trapping and
releasing of reactants by light may provide a new technology to
engineer product selectivity for chemical syntheses as illustrated
with the substrate scope.
The low flux visible-light irradiation employed in this study
(0.5-1.0 W/cm2) is usually considered insufficient to produce a
plasmon-generated optical plasmon force.[8a] However, any alloy
surface-molecule interaction is also strongly dependent on the
molecular polarizability.[8c-e] When a molecule can be promoted
to an excited electronic state by absorption of incident light, the
molecule’s polarizability increases and so its response to the
optical plasmon field-gradient will increase significantly.[8c,8d] The
absorption spectra of reactants aniline and alkyne in toluene
solvent are shown in Figure 3B, aniline absorbs light at
wavelengths 400-440 nm. Electronically excited aniline, having
greater polarizability, may experience
a stronger optical
plasmon-based force of attraction. The alkyne which does not
absorb in the visible-light range is not affected. Thus, this
compound-selective force generated by irradiation can
superpose with the ubiquitous van der Waals force to overcome
the Brownian motion of the excited molecules. The excited
aniline molecules should, by this tentative mechanism, be
attracted from the bulk of the solution to the metal NP surface,
while the light is on. In the absence of the metal NPs, the
excitation of aniline due to the light absorption cannot induce
chemical transformation under the conditions applied for the
photocatalytic hydroamination.
Experimental Section
The catalysts were prepared by reducing the metal salts in the presence
of ZrO2 powder with NaBH4. The photocatalytic reaction were conducted
in Pyrex glass tube irradiated with light source at controlled temperatures.
The light irradiation induced reactant adsorption and desorption on
catalyst in the light-on and light-off cycle process was conducted using a
Nelson halogen lamp with
a
light intensity of 0.5 W/cm2. The
concentration changes of each reactant in the bulk liquid phase were
measured by a gas chromatography (GC). Details of the methods and
catalyst characterisation are provided in SI.
According to the proposed mechanism, the higher light
intensity should result in stronger force and faster cross-coupling
reaction. Indeed, the cross-coupling proceeds faster under 1.0
w/cm2 irradiation than under 0.5 W/cm2 irradiation (Figure 4A). It
is also noted that the fastest reaction was observed faster at 400
nm wavelength (Figure S11), this should be attributed to the
more intense light absorption of aniline in the range of 400-440
nm (Figure 3B), which induces higher activation of aniline.
Acknowledgements
We gratefully acknowledge financial support from Australian
Research Council (ARC DP150102110).
Keywords: visible light photocatalysis
• plasmonic metal
nanoparticles • reaction pathway • product selectivity • selective
We also conducted the reaction using catalysts with
different Au2Co alloy contents. As shown in Table S6 the cross-
coupling product selectivity of the reaction over the catalyst with
1 wt% of Au2Co alloy NPs is significantly lower than occurs with
the catalysts with 3 wt% and 5 wt% of alloy content, and the
homo-coupling product selectivity is higher, compared with those
of the other two catalysts. The number of hot spots in the
catalyst with 1 wt% of alloy NPs is lower than those in the
catalysts with 3 wt% and 5 wt% of alloy content. The optical
plasmon force attracting aniline to the catalyst with 1 wt% of
Au2Co alloy NPs is the weakest among the catalyst. Hence,
under light irradiation aniline adsorption on this catalyst is less
than that on the catalyst with higher alloy NP contents, resulting
in less cross-coupling product but more homo-coupling product.
The results support that optical plasmon effects play a key role
in switching the product selectivity.
adsorption
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Entry for the Table of Contents (Please choose one layout)
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Product selectivity of alkyne
hydroamination over catalyst Au2Co
alloy nanoparticles, can be made
switchable by a light-on, light-off
process.
Erandi Peiris, Sarina Sarina,* Eric R.
Waclawik, Godwin A. Ayoko, Pengfei
Han, Jianfeng Jia, Huai-Yong
Zhu*Page No. – Page No.
Plasmonic Switching of Reaction
Pathway: Visible-Light Irradiation
Varies Reactant Concentration at the
Solid-Solution Interface of a Gold-
Cobalt Catalyst
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