Impacts of organic stabilizers on catalysis of Au nanoparticles from colloidal preparation

Article abstract of DOI:10.1021/cs501161c

Metal nanoparticles (NPs) from colloidal synthesis are advantageous in fundamental catalysis research because of their precisely controlled size and morphology but unfortunately are usually contaminated with residues from the organic stabilizer essentially required in the synthesis. These residues could modify the surface property and disturb the catalysis intrinsic to "clean" NPs. Herein, polyvinylpyrrolidone (PVP)-stabilized Au NPs (4.7 ± 1.0 nm) from colloidal synthesis were immobilized on SiO₂ support and subjected to ultraviolet-ozone (UVO) treatment to remove the residues. Hydrogenation reactions of p-chloronitrobenzene (p-CNB) and cinnamaldehyde (CAL) were conducted to probe consequences of the stabilizer removal on the catalytic properties of Au NPs. Measurements by FTIR and XPS revealed a controlled removal and degradation of PVP according to the UVO-treatment duration. Careful HRTEM analysis disclosed that both the size and morphology of Au NPs remained unchanged after the UVO-treatment. Residual PVP significantly improved the activity of Au NPs for p-CNB hydrogenation but lowered the activity for CAL hydrogenation. Continued selectivity changes of CAL hydrogenation to favor the reaction at the C=C bond were observed on increasing the removal degree of the residues. The UVO-cleaned Au NPs were also "restabilized" with PVP and other stabilizers by adsorption in aqueous solution. Comparison of the catalytic properties of these Au NPs involving different stabilizers with those of the UVO-cleaned ones enabled a comprehension of the stabilizer impacts on the hydrogenation catalysis of Au NPs.

Full text of DOI:10.1021/cs501161c

Research Article
pubs.acs.org/acscatalysis
Impacts of Organic Stabilizers on Catalysis of Au Nanoparticles from
Colloidal Preparation
Ru-Yi Zhong, Ke-Qiang Sun, Yong-Chun Hong, and Bo-Qing Xu*
Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua
University, Beijing 100084, China
S
* Supporting Information
ABSTRACT: Metal nanoparticles (NPs) from colloidal
synthesis are advantageous in fundamental catalysis research
because of their precisely controlled size and morphology but
unfortunately are usually contaminated with residues from the
organic stabilizer essentially required in the synthesis. These
residues could modify the surface property and disturb the
catalysis intrinsic to “clean” NPs. Herein, polyvinylpyrrolidone
(PVP)-stabilized Au NPs (4.7
1.0 nm) from colloidal
synthesis were immobilized on SiO2 support and subjected to
ultraviolet-ozone (UVO) treatment to remove the residues.
Hydrogenation reactions of p-chloronitrobenzene (p-CNB)
and cinnamaldehyde (CAL) were conducted to probe
consequences of the stabilizer removal on the catalytic
properties of Au NPs. Measurements by FTIR and XPS revealed a controlled removal and degradation of PVP according to
the UVO-treatment duration. Careful HRTEM analysis disclosed that both the size and morphology of Au NPs remained
unchanged after the UVO-treatment. Residual PVP significantly improved the activity of Au NPs for p-CNB hydrogenation but
lowered the activity for CAL hydrogenation. Continued selectivity changes of CAL hydrogenation to favor the reaction at the
CC bond were observed on increasing the removal degree of the residues. The UVO-cleaned Au NPs were also “restabilized”
with PVP and other stabilizers by adsorption in aqueous solution. Comparison of the catalytic properties of these Au NPs
involving different stabilizers with those of the UVO-cleaned ones enabled a comprehension of the stabilizer impacts on the
hydrogenation catalysis of Au NPs.
KEYWORDS: catalysis by gold, metal colloids, selective hydrogenation, stabilizer effect, ultraviolet-ozone, chloronitrobenzene,
cinnamaldehyde
morphology compared to conventional preparation approaches
like impregnation or deposition-precipitation.3 However, a part
of the organic stabilizer would remain as surface residue in the
final catalysts and impact the property and catalysis of their
stabilized Au NPs,3,5,13−17 which could significantly disturb our
study on the true structure−property relationship intrinsic to
“clean” Au NPs. Possible effects of the stabilizer-residue would
include steric blocking of the catalytic sites,3,13−17 modifying
the electronic state5,18 and interaction13 of Au NPs with the
support, and so forth. However, most work regarding the
catalysis of Au NPs from colloidal synthesis has focused on the
size and/or morphology feature, the role of stabilizers has been
seldom discussed.2,4,5,7 It remains unclear how the stabilizer
residues would disturb the catalysis of Au NPs at the molecular
scale.
1. INTRODUCTION
Au nanoparticles (NPs) have attracted extensive interest since
the late 1980s for their unique size-dependent catalytic
properties and versatile applications in chemical syntheses,
energy conversion, and environmental protection.1 Investiga-
tions on the catalytic properties of Au NPs with narrow size
distribution2,3 and/or specific facets4 would gain insight on the
fundamental nature of active sites. By varying the type5,6 or
dosage3,7 of stabilizers in colloidal synthesis, the metal particle
size and/or morphology can be precisely tailored. Typical
stabilizers used in controlled synthesis of Au NPs include small
molecules such as citrate (Citr)5,8 and cetyltrimethyl-
ammonium bromide (CTAB),4 or polymers such as polyvinyl-
pyrrolidone (PVP)2,3,5,7,9 and poly(vinyl alcohol) (PVA).3,5,10
Among the various stabilizers, PVP has received a broad
attention for its chemical stability, nontoxicity, and excellent
solubility.11
Several protocols have been proposed in earlier investigations
of the stabilizer effects on catalysis by metal NPs. The
Au NPs from colloidal synthesis can be immobilized onto a
wide range of supports, such as SiO2,10,12 Al2O3,12,13 and
carbon.5,12 This two-step strategy (colloidal synthesis and then
immobilization) allows for a finer control of the Au size and
Received: August 8, 2014
Revised: September 24, 2014
© XXXX American Chemical Society
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seemingly convenient one is to compare the catalytic activities
of metal NPs synthesized using different stabilizers.3,5,13,19
Because the size and morphology of the resulting metal NPs
may differ significantly, it is often difficult to isolate and assess
the stabilizer effects on the catalytic performance. Another
approach is via postsynthesis modification of Au NPs,13,20,21
like adding stabilizers to supported Au NPs from deposition-
precipitation,13 exchanging CTAB-stabilized20 or Citr-stabi-
lized21 Au NPs with other stabilizers. The introduction of a
stabilizer to supported Au NPs from “stabilizer-free” prepara-
tions could lead to unwanted modification to the support
surface,13 whereas the stabilizer-exchange approach would have
difficulties in ensuring similar coverage and complete exchange
for different stabilizers.22,23
whose catalytic consequences are assessed by comparing the
catalytic properties of these Au NPs with those of the UVO-
treated ones. The data entitle a comprehensive discussion on
the nature of active Au sites for the hydrogenation catalysis.
2. EXPERIMENTAL SECTION
2.1. Preparation and Immobilization of Au NPs
Stabilized by PVP and Citr. The PVP-stabilized Au NPs of
4.7 1.0 nm (Au-PVP) were synthesized via a seed-mediated
method, as detailed in our earlier publications.5,9,37 Immobiliza-
tion of the Au-PVP NPs on SiO2 powders (Degussa, AEROSIL
90, SBET = 90 m2·g−1) was conducted using the procedure
reported previously.9,21,36 Unless otherwise specified, the
content of Au was fixed at 1.00 0.05 wt%, as confirmed by
ICP-AES analysis. The resultant Au-PVP/SiO2 powders were
dried in air at 110 °C and were stored in a refrigerator for later
use.
2.2. UVO-Treatment. UVO-treatment of Au-PVP/SiO2
was carried out using a U-shape low pressure mercury lamp
(BIOZONE, 10-11100, 20 w) at ambient temperature in
flowing air (70 mL·min−1), for varying periods (0.1 to 12 h), in
order to control the removal of PVP residues. The lamp emits
mainly ultraviolet radiations at 184.9 and 253.7 nm; the former
can generate highly oxidative ozone and atomic oxygen from
ambient oxygen (air), and the latter can excite organics and
promote their oxidation to H2O, CO2, and N2.26 The lamp was
mounted in a tubular glass reactor (i.d., 60 mm; length, 500
mm), as shown in Figure S1 (Supporting Information). The
reactor was wrapped with an aluminum foil to prevent leak of
UV light. The sample (Au-PVP/SiO2) powders were dispersed
at a thickness of ca. 0.6 mm on a glass slide placed 6 mm apart
from the lamp surface.27,28 The resulting sample was coded as
Au-PVP/SiO2-tUVO according to the UVO-treatment duration
t in hour.
2.3. Restabilization of UVO-Cleaned Au-PVP/SiO2
Samples. UVO-treatment for 12 h was found to be a suffcient
duration to completely clean the Au-PVP/SiO2 sample from
organic residues. Restabilization with PVP of this UVO-cleaned
sample (Au-PVP/SiO2-12UVO) led to RePVP(Au-PVP/SiO2-
12UVO), which was obtained by immersing Au-PVP/SiO2-
12UVO in an aqueous solution of PVP (molar PVP-monomer/
Au = 100) for 2 h under stirring, followed by separation of the
solids by filtration and then drying without washing at 110 °C.
A sample coded as RePVP(Au-PVP/SiO2-12UVO)-W was also
made by adding an extensive washing step (with deionized
water) prior to the drying step. Similar restabilization with PVP
of the Au-PVP/SiO2-tUVO samples (t = 1 and 3 h) produced
the RePVP(Au-PVP/SiO2-tUVO samples, respectively. Alter-
natively, a restabilization with PVA, CTAB or Citr of Au-PVP/
SiO2-12UVO produced the RePVA(Au-PVP/SiO2-12UVO),
ReCTAB(Au-PVP/SiO2-12UVO), or ReCitr(Au-PVP/SiO2-
12UVO) sample.
2.4. Characterization Methods. TEM measurements
were conducted on a Tecnai G2 F20 U-Twin microscope at
200 kV. At least 300 particles were randomly measured to
determine the mean diameter of Au NPs, using the equation d
= Σ(nidi)/Σni, where ni is the number of particles with a
diameter of di. High-resolution TEM (HRTEM) images were
obtained on a Tecnai G2 F20 S-Twin microscope at 300 kV.
Further details of the TEM/HRTEM measurements were given
previously.37
A straightforward approach would be to compare the
catalytic performance of Au NPs before and after removal of
the surface stabilizers.14−17 Many attempts were made to
eliminate stabilizer-residue from the metal NPs but usually with
little attention paid to avoiding undesirable side effects. The
commonly used high-temperature (oxidation) treatment could
compromise the size and morphology of the metal NPs,14,15,24
the metal−support interaction,14,15 and perhaps lead to
secondary residues poisonous to the catalytic sites.25 Wet
chemical methods like washing with different solvents16,21−24 or
17
oxidation by KMnO4 could only partly remove the stabilizer
residues, induce aggregation of the metal NPs, and even cause
further contamination from the reagents.16,17,24 Recently,
ultraviolet-ozone treatment (UVO-treatment) that combines
excitation by UV light with the oxidation by ozone26 was
applied to eliminate the PVP stabilizer from PVP-stabilized Pt27
and Pd28 NPs under ambient conditions. The sizes and
morphology of the noble metal NPs almost showed no change
before and after UVO-treatment.27,28 Though more or less
passivation of the NP surface by chemisorbed oxygen was
detected during the UVO-treatment,27,28 a follow up mild
reduction with hydrogen could eliminate such passivation,28
making UVO-treatment a promising technology for the
removal of organic stabilizers from immobilized metal colloids.
However, it remains unknown if the same technology could be
applicable for quantitative removal of organic stabilizers from
the less stable Au NPs. Also, the cleanness or removal degree of
the UVO-treated metal NPs was usually not comprehensively
probed with surface-sensitive techniques (e.g., XPS).
The UVO-treatment technology is used herein to systemati-
cally vary the removal degree of PVP and its derived secondary
residues on nearly monodisperse PVP-stabilized Au NPs.
Comprehensive characterizations using transmission electron
microscopy (TEM), Fourier-transform infrared spectroscopy
(FTIR), and X-ray photoelectron spectroscopy (XPS) are
conducted to ascertain that both the size and morphology of Au
NPs remain unchanged after the UVO-treatment. The UVO-
treated samples are then used to assess impacts of the stabilizer
on the catalysis by Au NPs using hydrogenation of p-
chloronitrobenzene (p-CNB) and cinnamaldehyde (CAL) as
the probe reactions.21 SiO2 is used to immobilize the Au NPs
because it is a widely accepted neutral support material for
metal NP catalyst.29 Previously, Au NPs were found highly
selective for p-chloroaniline (p-CAN) production in p-CNB
hydrogenation,30−32 and showed an interesting structure
sensitivity in CAL hydrogenation.33−36 “Restabilization” of
the UVO-cleaned Au NPs with different stabilizers (i.e., PVP,
PVA, CTAB, and Citr) is also done to prepare samples
containing the same Au NPs but involving different stabilizers,
FTIR studies were made on a PerkinElmer 2000 system. For
rigorous comparison of the intensity of the absorption bands,
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the sample powders were ground with 12-fold KBr powders (by
weight) and then pressed into a wafer of 36.0 mg (diameter 13
mm) for the measurement. The spectra were recorded at a
resolution of 4 cm−1 from 4000 to 500 cm−1, accumulating 10
scans per spectrum.
Overall, these PVP-associated IR bands became less intensified
on increasing the duration, indicating a removal and/or
degradation of PVP-residue from the sample.
A new band at 1698 cm−1 appeared shortly after exposure to
the UVO-treatment (t ≤ 0.2 h), accompanied by a distinct
intensification of the 1385 cm−1 band. These two bands can be
ascribed to the CO stretching (1698 cm−1) and C−O−H in-
plane bending (1385 cm−1) modes of carboxylic groups.40
These two bands became most intensified at t = 0.5 h and then
decreased gradually on further t increasing, indicating that the
PVP-residue was first oxidized to intermediates featuring the
carboxylic groups. When t was extended to 12 h, the spectrum
became essentially the same as that of SiO2, suggesting a
complete removal of PVP and its derived surface residues; the
remaining band at 1630 cm−1 signified the H−O−H bending
vibration of adsorbed H2O on SiO2, whereas the ones at 2009
and 1870 cm−1 are the overtones of Si−O stretching
vibrations.41,42
XPS measurements were performed on a PHI 5300
ESCA1610 SAM spectrometer equipped with a Mg Kα
radiation (1253.6 eV); the precision in measuring the binding
energy (BE) was within 0.1 eV. The spectra for N 1s and Au
4f were scanned 60−200 times to improve the signal-to-noise
ratio. The BE for C 1s in adventitious contaminants on “pure”
SiO2 support was set at 284.8 eV for charge correction, and
then used to calibrate the BEs for the other elements (Au 4f, N
1s, Si 2p and O 1s). The atomic fraction data for different
elements (Au, N, O, Si, and C) at the sample surface were
obtained according to their XPS peak areas after subtraction
from Shirley-type backgrounds, using empirical cross section
factors. The C 1s and Au 4f spectra were analyzed by curve-
fitting using Gaussian-type peaks. Deconvolution of the Au 4f
spectra into Au 4f5/2 and 4f7/2 doublets was done with a BE
separation of 3.7 eV and an area ratio of 0.75.
2.5. Catalytic Hydrogenation Reactions of p-CNB and
CAL. The hydrogenation reactions were performed in a 25 mL
stainless steel autoclave under magnetic stirring (900 rpm).27,33
No mass-transfer (diffusion) limitation during the reactions was
confirmed by Madon−Boudart tests10,38 of Au-PVP/SiO2
samples with various Au loadings (Figure S2). Blank tests
with bare SiO2 and PVP-loaded SiO2 but without Au NPs
detected no reaction of either p-CNB or CAL.
Figure 1B presents the band deconvolution curves in the
region of 1550−1750 cm−1, involving the CO stretching and
H−O−H bending vibrations. Although the band intensities
(i.e., peak area) could measure the concentration of their
associated species because the spectra were recorded on
carefully prepared wafers with consistently weighted composi-
tion and mass (as indicated by the invariant overtones of the
Si−O stretching modes in Figure 1A), the invariant intensity of
the 1630 cm−1 band (bending vibration of adsorbed water on
SiO2) from the deconvolution was taken as a reference to
accurately quantify the intensities of the two CO bands
arising, respectively, from the carboxylic (1698 cm−1) and
carbonylic (1655 cm−1) groups, which are shown as a function
of t in Figure 2. Clearly, the PVP-specific carbonylic groups
(1655 cm−1) decreased monotonically with t, demonstrating a
gradual and continuous degradation of PVP-residue during the
treatment.
3. RESULTS AND DISCUSSION
3.1. Removal of PVP Residues by UVO-Treatment.
3.1.1. FTIR and XPS Studies. Figure 1A shows the IR spectra of
Figure 2. Intensities of the IR bands for carboxylic (1698 cm−1) and
carbonyl (1655 cm−1) CO stretching vibrations, in relation to that
for the H−O−H bending vibration (1630 cm−1) for adsorbed H2O on
the SiO2 support.
Figure 1. FTIR spectra for Au-PVP/SiO2 samples of different UVO-
treatment durations; the up-most curve (blue) shows the spectrum for
the “pure” SiO2 support (A). Deconvolution of the overlapped bands
in the range of 1550−1750 cm−1 (B).
The effect of UVO-treatment on the Au-PVP/SiO2 sample
was also characterized with XPS; the results are given in Figure
3 and Table S2. The Au 4f7/2 and 4f5/2 doublets were observed
at 83.1 and 86.9 eV for the as-prepared Au-PVP/SiO2, which
were 0.9 eV lower than those for metallic Au reference (84.0
and 87.7 eV). This is indicative of electron transfer from PVP
to Au, in agreement with earlier reports that as-synthesized
PVP-stabilized Au NPs were negatively charged.2,5,18,43
Apparently, the negative charge on these Au NPs came from
Au-PVP/SiO2 with different UVO-treatment durations. The
absorption bands and their assignments are summarized in
Table S1. The bands in the region of 3000−2800 cm−1 were
CH2 stretching vibrations in the hydrocarbon backbone and the
pyrrolidone rings, and the ones in 1500−1400 cm−1 were C−N
stretching and C−H scissoring vibrations.39,40 The 1655 cm−1
band was mainly associated with CO stretching vibration.
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bonding between the SiO2-based silanols and the carbonyl-
based oxygen atoms in PVP-residue.44 The BEs for O 1s and Si
2p increased to their “normal” positions for “pure” SiO2 at t ≥ 1
h, though their intensities generally increased until t = 12 h
(Figure 3).
A reference PVP-SiO2 sample was also prepared by PVP
immobilization (adsorption) from an aqueous solution
containing the same amount of PVP but without any Au
NPs. Its IR spectra on exposure to the UVO-treatment are
shown as Figure S3. Though the overall features of the spectra
were similar to their Au-PVP/SiO2 counterparts, the signifi-
cantly less intensified signal for the carboxylic groups on PVP-
SiO2-1UVO than on Au-PVP/SiO2-1UVO would indicate that
the carboxylic groups in the intermediate associated with the
Au NPs were more stable than those on the support (SiO2)
surface. This would explain why the XPS BEs for Au did not,
but those for Si and O almost returned to their normal
positions when the UVO-treatment duration was t = 1 h
(Figure 3).
Therefore, our IR and XPS data are consistent in revealing a
fast degradation and removal of the PVP stabilizer and its
derived residues during the first hour of the UVO-treatment.
Both sets of data also demonstrate a complete removal of the
residual organics from both the Au NPs and their supporting
SiO2 by conducting the UVO-treatment for 12 h.
3.1.2. Sizes and Morphologies of Au NPs before and after
the UVO-Treatment. Figure 4 shows the representative TEM
Figure 3. XPS spectra of Au 4f, C 1s, N 1s, O 1s, and Si 2p for Au-
PVP/SiO2 samples of different UVO-treatment durations.
electron donation from the oxygen atoms in the CO groups
of PVP molecules because the XPS signal for N 1s (399.9 eV)
appeared at the same position as in pure PVP.43 The BEs of Au
4f increased gradually upon the UVO-treatment and became
equal to those for the reference Au when the UVO-treatment
was continued for longer than ca. 3 h.
The changes of the XPS signals for C 1s and N 1s, also
shown in Figure 3, demonstrate a simultaneous removal of C-
and N-containing species during the UVO-treatment. The C 1s
signal consisted of two components with BEs at 284.8 and
287.8 eV, which can be assigned to the C0 atoms in the aliphatic
parts and Cδ+ atoms bound to the O and N atoms in the
residues, respectively. The ratio Cδ+/C0 increased with the
UVO-treatment duration up to t = 0.5 h and then kept on
decreasing with t increasing (Table S2), which coincides well
with the intensity change of the CO stretching band (1698
cm−1) (Figures 1B and 2). These data disclose the degradation
of PVP-residue by photochemical oxidation into surface
intermediates involving carboxylic groups, whose further
oxidation would lead to volatile products like CO2, H2O and
etc. The small C 1s signal at 284.8 eV on the Au-PVP/SiO2-
12UVO sample was arisen from adventitious hydrocarbon
contaminants during the sample storage and preparation for
XPS measurement.
Figure 4. Representative TEM images for Au-PVP (A), Au-PVP/SiO2
(B), Au-PVP/SiO2-1UVO (C), and Au-PVP/SiO2-12UVO (D).
images of Au-PVP, Au-PVP/SiO2, Au-PVP/SiO2-1UVO, and
Au-PVP/SiO2-12UVO. The sizes and shapes of Au NPs in the
as-prepared Au-PVP/SiO2 without UVO-treatment (4.7 1.0
nm, Figure 4B) were essentially indistinguishable from those in
Au-PVP/SiO2-1UVO (4.9 1.1 nm, Figure 4C) and Au-PVP/
SiO2-12UVO (4.8 1.2 nm, Figure 4D); they all resembled
their colloidal precursor particles (Au-PVP) prior to immobi-
lization (4.7 1.0 nm, Figure 4A). These observations clearly
indicate that the sizes and shapes of Au NPs were kept invariant
throughout the UVO-treatment.
As shown in the last two columns of Figure 3, increasing the
UVO-treatment duration also resulted in shifts to higher
binding energies (BEs) of the O 1s (from 532.5 to 533.1 eV)
and Si 2p (from 103.3 to 103.8 eV) electrons. Their lower BEs
in the as-prepared Au-PVP/SiO2 were probably due to H-
Au NPs in the as-prepared Au-PVP sample are a mixture of
single crystalline (S) and 5-twinned particles, according to our
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earlier particle structure analysis by HRTEM.37 Figure 5 shows
some representative HRTEM images for the Au NPs in Au-
Table 1. Particle Structure Selectivity for Au NPs in Au-PVP,
Au-PVP/SiO2, Au-PVP/SiO2-1UVO, and Au-PVP/SiO2-
12UVO Samples
a
b
sample
Au-PVP
Au size (nm)
(Ih + Dh) (%)
S (%)
4.7 1.0
4.7 1.0
4.9 1.1
4.8 1.2
87.2
86.6
89.2
87.4
12.8
13.4
10.8
12.6
Au-PVP/SiO2
Au-PVP/SiO2-1UVO
Au-PVP/SiO2-12UVO
a
Ih and Dh denote, respectively, icosahedra and decanedra featuring 5-
b
fold twinned particle structures. S denotes single crystals.
degree of the stabilizer could be controlled simply by changing
the UVO-treatment duration.
For references, the as-prepared Au-PVP/SiO2 was also
treated by the reflux method described by Hutchings et al.16
According to our FTIR evaluation of the CO stretching
absorption (Figure S4), the refluxing in water under stirring at
90 °C of Au-PVP/SiO2 for 1 h (Au-PVP/SiO2-R1) resulted in a
removal of the PVP-residue by 25−30%, but an extension of
the refluxing time to 4 h (Au-PVP/SiO2-R4) did not lead to any
further removal. Except their lower intensity, the overall feature
of the IR spectra for these Au-PVP/SiO2-R1 and -R4 samples
appeared essentially the same as for the as-prepared Au-PVP/
SiO2, disclosing that the chemical state (structure) of the PVP
stabilizer was kept intact during the reflux treatment. This may
imply that the stabilizer removed during the reflux treatment
was adsorbed mainly on the surface of SiO2 (support).
Accordingly, the sizes of Au NPs in Au-PVP/SiO2-R4 were
found by TEM in 4.9 1.0 nm, showing no detectable size
change during the reflux treatment.
Therefore, the present UVO-treatment method is advanta-
geous to the reflux method16 because it can lead to controlled
removal of the residual stabilizer, up to a complete cleaning of
the sample from the stabilizer. The control of the removal
degree, while inducing essentially no change in the Au sizes and
shapes, would also make the UVO-treatment method advanta-
geous over the high-temperature calcination,14,15,24 and
chemical etching (e.g., using KMnO417) methods that were
documented earlier for eliminating stabilizer-residue from
immobilized metal NPs. It is expected that this UVO-treatment
method could also be promising for removal of other stabilizers
from supported metal NPs derived from colloidal synthesis.
The removal of PVP and its derived residues during the
UVO-treatment is further assessed in Figure 6 according to the
calibrated overall intensity of CO (carbonylic and carboxylic)
stretching vibrations (It/I0) in the IR spectra (Figure 1) and the
Figure 5. Representative HRTEM images of Au NPs for Au-PVP/SiO2
(A), Au-PVP/SiO2-1UVO (B), Au-PVP/SiO2-12UVO (C), and
RePVP(Au-PVP)/SiO2-12h (D). Drawings in the bottoms show the
model particle structures of the single (S), decahedral (Dh), and
icosahedral (Ih) crystallites.
PVP/SiO2-tUVO (t = 0, 1, and 12 h) samples; the
multitwinned particles were identified in two shapes, decahedra
(Dh) and icosahedra (Ih). The observation that Dh and Ih
particles were slightly larger than the S ones agrees with our
earlier study on the particle morphology of colloidal and
carbon-immobilized Au NPs.37 Clearly, the Au NPs after UVO-
treatment (e.g., in Au-PVP/SiO2-1UVO and Au-PVP/SiO2-
12UVO, Figure 5B,C) also remained as a mixture of S, Dh, and
Ih particles. The particle-shape selectivity data from careful
HRTEM measurements are listed in Table 1. The overall
selectivity for 5-fold twinned particles was 85−89% (by number
of particles) for every sample, independent of the UVO-
treatment duration. These data demonstrate that not only the
Au particle sizes but also their internal particle structures were
kept unchanged during the UVO-treatment.
3.1.3. Degradation of PVP Residues by UVO-Treatment.
The preceding data from FTIR, XPS, and TEM analyses are
consistent in demonstrating that UVO-treatment can be
developed as a method to clean the SiO2-supported Au NPs
(Au-PVP/SiO2) from the stabilizer (PVP) without changing
the sizes and shapes of the Au NPs. Moreover, the removal
Figure 6. (A) Effects of UVO-treatment duration on the normalized
CO stretching vibration intensity (It/I0) and XPS N/Au ratio. (B)
Correlation between logarithm of It/I0 and the UVO-treatment
duration.
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atomic N/Au ratio from XPS measurements (Figure 3),
respectively. After UVO-treatment of 1 h, the N/Au ratio
declined by ca. 88%, and the intensity of CO declined by ca.
60%. The more rapid decline in the N/Au ratio might mean a
faster elimination of N-containing species than those involving
CO groups.
overall amounts of PVP stabilizer in the RePVP(Au-PVP/SiO2-
tUVO) samples appeared very similar. However, the spectrum
for the washed sample (i.e., RePVP(Au-PVP/SiO2-12UVO)-
W) was ca. 20% less intensified than those for the unwashed
RePVP(Au-PVP/SiO2-tUVO) ones but was essentially the
same as that for the as-prepared Au-PVP/SiO2. These facts
would indicate that the unwashed samples could involve some
(ca. 20%) reversibly adsorbed PVP molecules that could
become removed (via desorption) by extensive washing with
deionized water.
Shown in Figure 7B are the IR spectra for RePVA(Au-PVP/
SiO2-12UVO), ReCTAB(Au-PVP/SiO2-12UVO), and ReCitr-
(Au-PVP/SiO2-12UVO), which were obtained by “restabiliza-
tion” of Au-PVP/SiO2-12UVO with PVA, CTAB and Citr,
respectively. The PVA stabilizer in RePVA(Au-PVP/SiO2-
12UVO) was characterized by the absorptions at 2925 and
2855 cm−1 (CH2 stretching), 1460 cm−1 (broad, CH2
scissoring), and 1385 cm−1 (C−O−H in-plane bending),
though the signal characteristic of the stretching vibration for
the polar C−O bond (ca. 1050 cm−1) was overlapped by strong
absorptions from the SiO2 support. In the spectrum for
ReCTAB(Au-PVP/SiO2-12UVO), the absorptions at 2960,
2925, and 2855 cm−1 (C−H stretching), 1460 and 1385 cm−1
(C−H bending) denoted the alkyl chains of the CTAB
stabilizer. The spectrum for ReCitr(Au-PVP/SiO2-12UVO)
gave two absorptions at 1580 (asymmetric) and 1400 cm−1
(symmetric) for R-COO− vibrations; the absorptions in the
region of 3000−2800 cm−1 were due to C−H stretching
vibrations.
3.3. Catalytic Hydrogenation Study of Au NPs with
and without Stabilizer Residues. Catalytic hydrogenation
reactions of p-CNB21,30,31 and CAL10,21,33−35 were conducted
respectively to assess the effects of PVP removal by UVO-
treatment and the stabilizer nature on the catalytic property of
Au NPs. The reactant conversion levels were controlled below
20% for rigorous comparison, and the catalytic activity was
expressed by turnover frequency (TOF, h−1) on surface Au
atoms. The dispersion or exposed percentage of Au in each
sample was taken as the reciprocal of their average Au particle
size in nanometer. The reported TOF numbers (Figures 8 and
9, Tables 2 and 3) were obtained as the average rates from 3 to
5 replicate measurements; the relative errors were within 10%,
mostly less than 6%.
To gain a kinetic image about the removal/degradation of
the organic residues, the curve of It/I0 versus t in Figure 6A was
fitted in Figure 6B with a first-order removal kinetics, which
gives a rate constant of 0.86 h−1 at t ≤ 1 h and 0.23 h−1 at t =
1−12 h. Recalling that the most prominent changes to the
surface residues during the first hour of the UVO-treatment
were photochemical oxidations of the PVP-residue to surface
species involving carboxylic groups and a preferred loss of
nitrogen from the residues (Figures 1, 2, 3, and 6), it would be
reasonable to speculate that in the initial stage of the UVO-
treatment (t < 1 h) the PVP-residue experienced a fast
photochemical degradation by simultaneously breaking the C−
N bonds49 and oxidizing the carbonyls in the pyrrolidone rings
and (more or less) some terminal carbons to form the
carboxylic-involving species. The significantly slower removal
rate at t > 1 h could hint that these “secondary” surface species
involving carboxylic groups could transform to some different
residues more resistant to UVO-treatment. It is known that
vinylcarbonates are cross-linkers in photopolymerization of N-
vinylpyrrolidone under UV irradiations.45 Thus, the slower
removal rate at t > 1 h could be due to that such different
residues were somehow cross-linked, as the surface species
involving carboxylic groups can easily lead to formation of vinyl
carbonates at the surface.
3.2. “Restabilization” of Au NPs with Different
Stabilizers. The IR spectra for RePVP(Au-PVP/SiO2-tUVO)
of t = 1, 3, and 12 h are compared in Figure 7A with those for
3.3.1. Selective Hydrogenation of p-CNB. The hydro-
genation of p-CNB produced p-CAN as the sole product
according to GC analysis, as documented earlier on catalytic
hydrogenation of nitroaromatics over Au NPs supported on
ZrO2, TiO2 and SiO2.32 Plotted in Figure 8 is the
dependence on the UVO-treatment duration (t) of the Au
activity according to p-CNB consumption (TOFp‑CNB) for the
Au-PVP/SiO2-tUVO catalysts. Apparently, TOFp‑CNB declined
rapidly with t up to 1 h (from 178 to 101 h−1) and then
remained in the vicinity of 100 h−1 on further t increasing.
Considering the unaltered sizes and morphologies of the Au
NPs during the UVO-treatment, these results are therefore
strong indications that the presence of PVP21 and its derived
residues, especially those removed/reacted within the first hour
of the UVO-treatment, were very beneficial to the Au catalysis
for p-CNB hydrogenation.
30
31
Figure 7. (A) Comparison by FTIR spectroscopy of RePVP(Au-PVP/
SiO2-tUVO) (t = 1, 3, and 12 h) and RePVP(Au-PVP/SiO2-12UVO)-
W samples with as-prepared Au-PVP/SiO2 and Au-PVP/SiO2-12UVO.
(B) FTIR spectra of ReStabilizer (Au-PVP/SiO2-12UVO) samples.
Au-PVP/SiO2-12UVO and Au-PVP/SiO2. The RePVP(Au-
PVP/SiO2-tUVO) samples were obtained by readsorption of
PVP on Au-PVP/SiO2-tUVO. Apparently, all the three
RePVP(Au-PVP/SiO2-tUVO) samples showed very similar
spectra resembling to that for the as-prepared Au-PVP/SiO2.
Therefore, in spite of the differences in the UVO-treatment
duration and thus the amount of the organic residues, the
The blue stars in Figure 8 report the Au TOFp‑CNB data for
RePVP(Au-PVP/SiO2-tUVO) of t = 1, 3, and 12 h. In reference
to the numbers for their Au-PVP/SiO2-tUVO counterparts, the
Au activity in RePVP(Au-PVP/SiO2-1UVO) was slightly
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than that for RePVP(Au-PVP/SiO2-12UVO) (Table 2). These
differences reveal that the PVA stabilizer was at least as effective
as PVP in enhancing the Au activity for p-CNB hydrogenation,
but CTAB and Citr were detrimental to the Au catalysis. These
results indicate that the nature of the stabilizer molecules
impacts significantly the catalysis of Au NPs.
3.3.2. Selective Hydrogenation of CAL. The hydrogenation
of CAL could occur at either CC or CO bond, or both
CC and CO bonds, leading to hydrocinnamaldehyde
(HCAL) or cinnamyl alcohol (COL), or hydrocinnamyl
alcohol (HCOL).10,36 Figure 9 shows the activity and selectivity
Figure 8. Effect of UVO-treatment duration on the Au activity by
TOFp‑CNB of Au-PVP/SiO2 catalyst for p-CNB hydrogenation. The
blue stars show the TOFp‑CNB data for the Au in RePVP(Au-PVP/
SiO2-tUVO) (t = 1, 3, and 12 h); the red circle gives the TOF number
for the Au-PVP/SiO2-R1 catalyst.
higher, but those in RePVP(Au-PVP/SiO2-3UVO) and
RePVP(Au-PVP/SiO2-12UVO) were distinctly higher (see
also Table 2). Interestingly, the RePVP(Au-PVP/SiO2-
Table 2. Catalytic Data for Au NPs in Au/SiO2 Samples
Involving Different Stabilizers for p-CNB Hydrogenation
a
Reaction
Figure 9. Effects of UVO-treatment duration on the Au activity by
TOFCAL and product selectivity of Au-PVP/SiO2 catalyst for CAL
hydrogenation.
b
Au size
(nm)
p-CNB
TOF
catalyst
Au-PVP/SiO2
conv. (%)
(h−1
)
4.7 1.0
4.8 1.2
4.8 1.1
18.1
10.3
17.1
18.2
21.5
0.4
178
101
168
179
212
4
Au-PVP/SiO2-12UVO
RePVP(Au-PVP/SiO2-12UVO)
RePVP(Au-PVP/SiO2-12UVO)-W
RePVA(Au-PVP/SiO2-12UVO)
ReCitr(Au-PVP/SiO2-12UVO)
ReCTAB(Au-PVP/SiO2-12UVO)
data of Au-PVP/SiO2-tUVO for this reaction. The Au activity
according to CAL consumption (TOFCAL) decreased slightly
when the UVO-treatment duration t was no longer than 1 h;
however, the activity increased profoundly at t > 1 h and
became 3-fold that for the as-prepared Au-PVP/SiO2 (t = 0 h)
at t = 12 h. These results indicate that the PVP stabilizer and its
derived residues involving carboxylic groups (Figure 2) had a
negative effect on the Au catalysis in the hydrogenation of CAL.
The variation in t of the Au-PVP/SiO2-tUVO catalyst also
led to remarkable change in the product selectivity (Figure 9).
The selectivity for HCAL increased from 60% at t = 0 h to 87%
at t = 12 h at the expense of COL, which was reduced from
30% to 9%. Therefore, increasing the overall removal degree of
the organic residues by increasing the UVO-treatment duration
enhanced the selectivity for the hydrogenation at the CC
bond. Note that the selectivity change toward HCAL formation
was most pronounced at t ≤ 1 h.
Table 3 presents the results of CAL hydrogenation over the
ReStabilizer(Au-PVP/SiO2-12UVO) samples, together with
those over Au-PVP/SiO2 and Au-PVP/SiO2-12UVO. The
catalytic data for RePVP(Au-PVP/SiO2-12UVO) were more
or less close to those for the as-prepared Au-PVP/SiO2,
providing a further piece of support for the negative effects of
PVP and its derived residues on Au catalysis for CAL
hydrogenation.
2.0
20
c
Au-Citr/SiO2
11.1 6.5
4.9 1.0
3.1
71
Au-PVP/SiO2-R1
Au-PVP/SiO2-R4
18.4
18.4
181
181
a
Rxn conditions: 50 mg catalyst, 2 mmol p-CNB, 5 mL toluene
(solvent), PH = 2 MPa, p-CNB/Au (molar) = 800, 150 °C, 4 h. The
TOF rates were based on the number of surface Au atoms, which were
obtained from the average size of Au NPs assuming spherical shapes;
the measurement errors were within 10% in repeated measurements or
b
2
c
a deviation from these reported rates by 5%. Preparation details of
this sample were described in ref 21.
12UVO) became 94% active, and its washed counterpart
RePVP(Au-PVP/SiO2-12UVO)-W was already as active as the
as-prepared Au-PVP/SiO2, which further support the very
beneficial effect of PVP to the Au catalysis for p-CNB
hydrogenation.21 The significant lower Au TOFp‑CNB numbers
for both RePVP(Au-PVP/SiO2-1UVO) and RePVP(Au-PVP/
SiO2-3UVO) than that for RePVP(Au-PVP/SiO2-12UVO)
clearly demonstrate an “inhibiting” effect of the PVP-derived
“secondary” residues involving carboxylic groups and/or vinyl
carbonates, which could either prevent or shield the Au NPs
from direct interaction or “restabilization” with the readsorbed
PVP polymer. This indicates that not only the amount but also
the nature of the organic residues impacted greatly the catalytic
property of Au NPs.
The catalytic data for RePVA(Au-PVP/SiO2-12UVO) were
in the middle of those for Au-PVP/SiO2 and Au-PVP/SiO2-
12UVO. Change of the ReStabilizer to CTAB (ReCTAB(Au-
PVP/SiO2-12UVO)) lowered the Au activity to the level for the
as-prepared Au-PVP/SiO2, but the selectivity for HCAL
production was improved at the expense of COL. However,
the use of Citr as the ReStabilizer (ReCitr(Au-PVP/SiO2-
The Au TOFp‑CNB for ReCTAB(Au-PVP/SiO2-12UVO) and
ReCitr(Au-PVP/SiO2-12UVO) appeared much lower, but that
for RePVA(Au-PVP/SiO2-12UVO) was even a little higher
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a
Table 3. Catalytic Data for Au NPs in Au/SiO2 Catalysts Involving Different Stabilizers for the CAL Hydrogenation Reaction
product sel. (%)
b
catalyst
Au-PVP/SiO2
CAL conv. (%)
HCAL
COL
HCOL
TOF (h−1
)
6.4
18.0
5.7
60
87
63
72
79
14
74
72
59
65
30
9
10
4
125
352
112
168
200
25
Au-PVP/SiO2-12UVO
RePVP(Au-PVP/SiO2-12UVO)
RePVP(Au-PVP/SiO2-12UVO)-W
RePVA(Au-PVP/SiO2-12UVO)
ReCitr(Au-PVP/SiO2-12UVO)
ReCTAB(Au-PVP/SiO2-12UVO)
27
21
15
52
15
20
33
29
10
6
8.6
10.2
1.3
5
35
11
7
6.7
131
263
119
112
b
Au-Citr/SiO2
5.8
Au-PVP/SiO2-R1
Au-PVP/SiO2-R4
6.1
8
5.7
6
a
b
Rxn conditions: 50 mg catalyst, 4 mmol CAL, 5 mL xylene (solvent), PH = 2.0 MPa, CAL/Au (molar) = 1600, 150 °C, 4 h. See footnotes of Table
2
1.
12UVO)) made the Au NPs least active but distinctively more
selective toward COL and HCOL.
3.4. Impacts of Stabilizers on Au Catalysis. A simple
comprehension of the above characterization and catalytic
hydrogenation results would point to a rich chemistry of the
stabilizer effects on catalysis of Au NPs from colloidal synthesis.
Not only the amount but also the nature of the stabilizer and its
derived residues could impact significantly on the activity and/
or selectivity of the Au catalyst.
Shown in the last two rows of Tables 2 and 3 are the catalytic
data obtained over Au-PVP/SiO2-R1 and -R4, which were
essentially the same as or very close to those for the as-prepared
Au-PVP/SiO2 catalyst in the selective hydrogenation of both p-
CNB and CAL. The indistinguishable catalytic performance of
Au NPs in these three samples thus clearly demonstrates that
the catalytic properties of the PVP-stabilized Au NPs were not
affected during the reflux treatment in water, providing an
indirect piece of evidence that the reflux treatment16 could only
remove PVP molecules being weakly adsorbed on SiO2 surface
but had no influence on those serving as the “real” stabilizer to
the Au NPs. This strongly contrasted with the impacts of the
UVO-treatment on the catalytic performance of the same Au-
PVP/SiO2 (Figures 8 and 9). Therefore, the removal and
change of PVP and its derived residues due to the UVO-
treatment would be responsible for the catalytic performance
variations. In combination with the “intactness” of Au NPs to
the UVO-treatment (Figures 4 and 5), it is therefore possible to
address the impacts on Au catalysis of PVP and its derived
residues without considering the size and morphology changes
of Au NPs.
3.4.1. PVP Promotion of Au Catalysis in p-CNB Hydro-
genation. The promotional role of PVP to the catalysis of Au
NPs in the hydrogenation of p-CNB is well-supported by the
distinctly higher TOFp‑CNB numbers of the PVP-involving Au
NPs in Au-PVP/SiO2, RePVP(Au-PVP/SiO2-12UVO) and
RePVP(Au-PVP/SiO2-12UVO)-W in comparison with that of
the UVO-cleaned Au NPs in Au-PVP/SiO2-12UVO (Table 2).
To gain a deeper insight into this catalysis chemistry, we
compare in Figure 10 the overall reaction kinetics over Au-
PVP/SiO2 (no UVO-treatment) and its UVO-cleaned counter-
part Au-PVP/SiO2-12UVO by showing the H2 pressure drops
as a function of the reaction time, using a calibrated Parr
reactor. The pressure drops in the starting stage (<0.5 h) were
coupled with unintended/unavoidable temperature fluctuation
in calibrating the working pressure during the reactor heat up.
Figure 10. Hydrogen pressure drop during the catalytic hydrogenation
of p-CNB over Au-PVP/SiO2 and Au-PVP/SiO2-12UVO catalysts.
Au-PVP/SiO2 always led to faster pressure drops than Au-PVP/
SiO2-12UVO. The uniform pressure drops over both catalysts
after ca. 0.5 h, when the reaction temperature became stabilized
at 150
1 °C, would mean uniform reaction rates or zero
partial order in p-CNB because the reaction was conducted
with ca. 4-fold excess of H2, and the accumulated H2
consumption was below ca. 15%. It would thus be reasonable
to directly correlate the reaction rate constants with the
TOFp‑CNB numbers. The apparent activation energies (Ea)
derived from the Arrehenius plots measured in the temperature
range of 120−190 °C (Figure S5) were Ea1 = 56.1 and Ea2
=
45.0 kJ·mol−1 for Au-PVP/SiO2 and Au-PVP/SiO2-12UVO,
respectively; their corresponding pre-exponential factors were
A1 = 1.7 × 109 and A2 = 4.5 × 107 h−1.
According to earlier studies,49,50 H2 activation would be the
rate-determining step for p-CNB hydrogenation over oxide-
supported Au NPs. Hydrogen activation by chemisorption is
difficult on Au NPs or clusters,47−49 but neutral Au NPs would
be relatively more active than their negatively charged
counterparts for H2 activation by chemisorption.5,33,46 The
higher Ea for Au-PVP/SiO2 (Ea1) is thus not surprising because
the negatively charged Au-PVP NPs (without UVO-treatment)
would become neutral when the PVP-stabilizer was eliminated
by the UVO-treatment, as evidenced by the Au XPS spectra
(Figure 3).
A seemingly plausible explanation for the low activity of Au-
PVP/SiO2-12UVO would thus be that the number of active
sites on this catalyst was only a fraction (1/37) of that on the
as-prepared Au-PVP/SiO2 catalyst. This would hint that the
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density of active sites in Au-PVP/SiO2 was remarkably higher
than that in Au-PVP/SiO2-12UVO, which should not be the
case because the Au sizes, their morphologies, and internal
particle structures were essentially not changed before and after
UVO-treatment (Figures 4 and 5, Table 1). We thus assume no
significant difference in the number of active Au sites for Au-
PVP/SiO2 and Au-PVP/SiO2-12UVO. The pre-exponential
factors would also arise from the differences in the entropies of
activation (ΔS) for the hydrogenation reaction over the two
catalysts according to the transition state theory, that is R × ln
(A1/A2) = ΔS1 − ΔS2 = 30.1 J·mol−1·K−1. This would point to
different mobility of activated complexes in the reaction
transition state on the two catalysts. A highly mobile activated
complex on Au-PVP/SiO2 could thus be assumed with the
benzene ring of p-CNB being perpendicular to the catalytic Au
surface. This orientation of the activated complex would refer
to a steric function of the Au-associated PVP stabilizer in
regulating the adsorption mode of the reacting CNB molecules,
as depicted in Figure S6. On the other hand, the benzene ring
in the activated complex on Au-PVP/SiO2-12UVO could “lie
on” or assume an orientation in parallel to the surface of the
UVO-cleaned Au NPs (Figure S6), its interaction with the
“open and clean” Au NPs would be significantly stronger than
its perpendicularly orientated counterpart on the PVP-involving
Au NPs in Au-PVP/SiO2, and should thus have much less
mobility. Thus, the higher activity of the as-prepared Au-PVP/
SiO2 catalyst would be due to that the presence of PVP could
offer the reaction with a much higher entropy of activation,
which compensated the energy penalty in H2 activation.
Quantum chemical calculations showed that the adsorption
energies on Au13 of nitrostyrene through the nitro group and of
N-vinylpyrrolidone through the carbonyl group were 64.2 and
42.5 kJ·mol−1, respectively.51−53 Thus, the adsorption of p-
CNB on Au NPs could be stronger than that of any single PVP
monomer. But, in reality, the polymeric PVP molecules would
interact with the Au NPs via more than one carbonyl groups or
by multiple coordination,5,13,43 which could lower the
adsorption strength of p-CNB. All of these factors could
contribute to the high mobility of the activated complex during
the hydrogenation of p-CNB on the as-prepared Au-PVP/SiO2
catalyst.
Besides the low mobility of the activated complex over the
PVP-free Au-PVP/SiO2-12UVO catalyst, the prevailing lying-
on adsorption of p-CNB molecules at the “clean” Au NPs
(Figure S6) could also result in “blockage” of some catalytically
active Au sites for the reaction. This seems also consistent with
the much smaller pre-exponential factor (A2) for the reaction
over this PVP-free catalyst. The activity decline of Au-PVP/
SiO2-tUVO at t < 1 h (Figure 8) would thus be mainly related
to the removal of the PVP stabilizer from Au NPs since ca. 80%
of the “stabilizing” PVP-residue was removed during this period
(Figure 2). The very similar activity numbers by TOFp‑CNB of
all Au-PVP/SiO2-tUVO at t ≥ 1 h (including t = 12 h) might
imply that the lying-on adsorption of p-CNB molecules was not
sensitive to the secondary carboxylic residues. Also, it would be
possible that only a small percentage of the lying-on p-CNB
molecules could have enough reactivity, and the presence of
such secondary residues could hardly change this percentage.
Further discussion on effect of the secondary residues would
require a complete piece of future study.
of the Au NPs for the hydrogenation of CAL. The remarkably
lower TOFCAL of the as-prepared Au-PVP/SiO2 than that of the
UVO-cleaned Au-PVP/SiO2-12UVO catalyst clearly demon-
strates a negative effect of PVP stabilizer on Au catalysis for this
reaction. The continuous increases in TOFCAL of Au-PVP/
SiO2-tUVO with the UVO-treatment duration at t > 0.5 h
further indicate that the PVP-derived carboxylic residues
(secondary residues) also played negatively to the Au activity
(Figure 9). These observations could be explained by
envisaging a partial blockage of active Au sites by the organic
residues (PVP and its derived carboxylic residues), either by
direct interaction with or steric hindering of catalytic Au sites
from being accessed by the reactant CAL.
However, the Au activity for CAL hydrogenation did not
increase monotonously with increasing the removal degree of
the organic residues. The TOFCAL numbers for the samples of t
≤ 0.5 h were even slightly lower than that for Au-PVP/SiO2
without UVO-treatment. This would suggest that the carboxylic
residues were more negative to the Au activity than PVP
stabilizer; otherwise, the partial removal of PVP stabilizer by
these short UVO treatments (e.g., Figures 2 and 6) would more
or less enhance the Au activity. Thus, the activity changes
induced by these short UVO treatments (i.e., when the removal
degree of the PVP-residue was not sufficiently high) could
mainly be due to that the carboxylic residues exerted a stronger
electronic effect than did the original PVP stabilizer. In
agreement with this explanation, the sample maximized in the
carboxylic residues (t = 0.5 h, Figure 2) exhibited the lowest Au
activity (Figure 9).
The product selectivity of CAL hydrogenation also varied
remarkably on increasing the removal degree of the organic
residues. The “UVO-cleaned” Au NPs (Au-PVP/SiO2-12UVO)
showed the maximum selectivity (87%) toward HCAL or the
hydrogenation at the CC bond. A significantly higher
selectivity toward the hydrogenation at the CC rather than
the CO bond seems to be an intrinsic propensity of Au NPs
on noninteracting SiO2 support because Au/SiO2 catalysts from
other preparations also showed this kind of chemoselectivity in
hydrogenation of α,β-unsaturated aldehydes.34,36,54 Previously,
a UVO-treatment of PVP-stabilized Pd NPs was found effective
in enhancing the catalytic activity of Pd NPs for ethylene
hydrogenation.28
The remarkable changes in the product selectivity of CAL
hydrogenation according to the removal degree of the organic
residues (or UVO-treatment duration t) could be explained as a
main consequence of the steric effect of the PVP stabilizer and
its derived carboxylic residues. A preferred activation of the
terminal CO bond for the hydrogenation of CAL would
become relatively unfavorable on increasing the removal degree
of the residues since this CO bond should be much less
sensitive to the steric hindrance than the CC bond. The
observed continuous decline in the COL selectivity in Figure 9
is in line with this explanation. A very recent study also showed
that a deposition of thiolate self-assembled monolayers on a
conventional Pt/Al2O3 catalyst can significantly enhance the
CO hydrogenation selectivity of CAL for COL formation.55
From an energy perspective, low-coordinated surface Au sites
(AuLC) were shown to induce a high selectivity toward the C
C bond hydrogenation of α,β-unsaturated aldehydes,34,56
probably due to their preference to the πCC adsorption
mode.34 At the molecular level, the PVP stabilizer and its
derived residues would most likely interact with these AuLC
sites via their carbonyl or carboxylic groups.33,56 Extending the
3.4.2. Modification by PVP and its Derived Residues of Au
Catalysis in CAL Hydrogenation. The UVO-treatment
produced a quite different impact on the catalytic performance
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UVO-treatment duration should then increase the extent of
uncovering/exposing of these AuLC sites, making the Au surface
more suitable for selective CC bond activation. Therefore,
both the steric and electronic/energetic factors became to favor
a higher selectivity toward HCAL formation when the removal
degree of the organic residues was increased by extending the
UVO-treatment duration.
The AuLC sites are located at the corners, edges and Au(100)
facets with coordination numbers (CN) of 6, 7 and 8,
respectively.37 We propose that those organic residues bound
to the Au sites at the corners (CN = 6) and edges (CN = 7)
were removed mainly at the early stage of the UVO-treatment
(i.e., t < 1 h, Figures 2 and 6) because their removal resulted in
the most pronounced changes both in the Au activity
(TOFCAL) and the product selectivity (Figure 9).
Removal of the organic residues would also change the
overall electronic state of the Au NPs (Figure 3). The as-
prepared PVP-stabilized Au NPs could be more electron-rich
and could become neutral upon increasing the removal degree
of the organic residues. This change would not only benefit a
preferred CAL activation at the CC bond35 but also promote
the activation of H2,46,48 though the present data did not allow
a discrimination between the catalytic sites responsible for CAL
activation from those for H2 activation.
3.4.3. Impact of Different Stabilizers on the Hydro-
genation Catalysis of Au. The catalytic consequences of the
restabilization with different stabilizers of the UVO-cleaned
“bare” Au NPs in Au-PVP/SiO2-12UVO on the hydrogenation
reactions of both p-CNB (Table 2) and CAL (Table 3) clearly
demonstrate that the nature of the stabilizer also greatly
impacted the hydrogenation catalysis of Au. The even higher
TOFp‑CNB and TOFCAL numbers for RePVA(Au-PVP/SiO2-
12UVO) than those for the as-prepared Au-PVP/SiO2 would
be mainly due to that H2 activation over the PVA restabilized
Au NPs was easier than over the Au-PVP NPs because the PVA
restabilized Au NPs would be essentially neutral but those in
Au-PVP/SiO2 negatively charged.5,18,21,36
The very low TOFp‑CNB numbers of the CTAB and Citr
restabilized Au NPs (ReCTAB(Au-PVP/SiO2-12UVO) and
ReCitr(Au-PVP/SiO2-12UVO), Table 2) could be due to that
these ionic stabilizers (CTAB and Citr) could not form around
Au NPs dynamic and loosely adsorbed shells for regulating the
orientation of p-CNB molecules toward the desirable
adsorption state. In particular, CTAB would form via a self-
assembling dense cationic CTA-layer on the Au surface57 and
impose a severe blockage to the catalytic Au sites for p-CNB
hydrogenation. A severe blockage to catalytic Au sites for
benzyl alcohol oxidation in toluene (solvent) was reported
previously by a close-packed self-assembled dodecylamine layer
on the Au surface.13 On the other hand, the CTAB restabilized
Au NPs not only showed a TOFCAL number close to that of Au-
PVP/SiO2 but also a higher HCAL selectivity (Table 3). These
results imply that the cationic CTA-layer had actually a lower
steric hindrance toward the CC bond activation than the
polymeric PVP layer, which is in line with an earlier finding that
CTAB was prone to occupy terrace sites58 or Au(100) (CN =
8) and Au(111) (CN = 9) facet sites on the present Au NPs,37
leaving most of the corner and edge sites for the hydrogenation
toward the CC bond. This suggests that the active Au sites
for CAL hydrogenation were somehow different from those for
p-CNB hydrogenation.
obtained with a reference Au-Citr/SiO2 sample, which were
prepared by SiO2-immobilization of Au-Citr NPs (4.7
0.9
nm) obtained by reduction of HAuCl4 with citrate as the Au-
stabilizer,21 were also included in Tables 2 and 3. The IR
spectrum for this Au-Citr/SiO2 is shown in the bottom of
Figure 7B; the very weak Citr signals agree well with earlier
documentations that citrate anion is a weak stabilizer for Au
NPs.6,12−15,21 The much higher TOFp‑CNB and TOFCAL
numbers of Au-Citr/SiO2 than that of ReCitr(Au-PVP/SiO2-
12UVO) could arise from the much lower Citr coverage on the
Au NPs in the former sample, as evidenced in their IR spectra
(Figure 7B). In the catalytic hydrogenation of CAL, these two
catalysts also produced quite different product selectivity;
ReCitr(Au-PVP/SiO2-12UVO) exhibited very higher selectivity
for COL and HCOL formation, even much higher than every
other catalysts in this work (Table 3). These results would be
explained by assuming that the Citr anions prefer to selectively
occupy the AuLC sites at the corners and edges as these surface
sites could be more active for the hydrogenation toward the
CC bond.34,56 The product selectivity data over Au-Citr/
SiO2 were in the middle between those over Au-PVP/SiO2-
12UVO and Au-PVP/SiO2, in line with the low coverage of
Citr. It is thus inferred that the catalytic sites for the
hydrogenation toward the CO bond of CAL would be
those of lower energy (CN = 8 or 9) on exposed Au (100) or
(111) facets.37 The adsorption of excessive Citr on the UVO-
cleaned “bare” Au NPs in Au-PVP/SiO2-12UVO would result
in severe blockage of the high energy AuLC sites (CN = 6 and
7), thus leading to remarkably lowered selectivity for the
hydrogenation toward the CC bond. Among all the samples
involving a kind of the stabilizers, Au-Ctri/SiO2 catalyst showed
the highest activity for CAL hydrogenation (TOFCAL, Table 3),
but its activity for p-CNB hydrogenation (TOFp‑CNB, Table 2)
was even significantly lower than the UVO-cleaned Au-PVP/
SiO2-12UVO catalyst. This also points to that the active Au
sites and mechanism for CAL hydrogenation would be different
from those for p-CNB hydrogenation, though their exact nature
remains unclear at this stage.
Finally, we discuss the similarity and differences in catalytic
performance of the PVP-involving Au NPs in Au-PVP/SiO2,
RePVP(Au-PVP/SiO2-12UVO) and Au-PVP/SiO2-R. The Au
TOFp‑CNB numbers were essentially the same for p-CNB
hydrogenation but TOFCAL for CAL hydrogenation in the
order of RePVP(Au-PVP/SiO2-12UVO)-W > Au-PVP/SiO2 ≥
RePVP(Au-PVP/SiO2-12UVO) ≈ Au-PVP/SiO2-R1 ≈ Au-
PVP/SiO2-R4, showing that the latter reaction was more
sensitive to the amount and subtle state of the polymeric PVP
stabilizer. This sensitivity of CAL hydrogenation was further
featured by the product selectivity data. The higher selectivity
for HCAL of the extensively washed RePVP(Au-PVP/SiO2-
12UVO)-W could be due to that in addition to the elimination
of ca. 20% PVP the extensive washing of the RePVP(Au-PVP/
SiO2-12UVO) would also make the “re-adsorbed” PVP-
stabilizer looser or more flexible, which led to less hindrance
to the reactant CAL molecules and improved the relative
competitiveness of the CC activation. On the other hand, the
product selectivity data over the two Au-PVP/SiO2-R samples
were very similar to those over the as-prepared Au-PVP/SiO2
though the amount of residual PVP in both Au-PVP/SiO2-R
was ca. 25−30% lower (Figure S4). This would be expected
because the reflux treatment mainly removed those PVP
molecules adsorbed on the surface of SiO2 (support).
In order to better understand the effect of Citr stabilizer in
the ReCitr(Au-PVP/SiO2-12UVO) catalyst, the catalytic data
3991
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ACS Catalysis
Research Article
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ASSOCIATED CONTENT
* Supporting Information
■
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510−514.
S
Assignments of infrared absorptions associated with PVP-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: bqxu@mail.tsinghua.edu.cn.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Financial supports from national Natural Science Foundation of
China (grant nos. 21033004 and 21221062), National Basic
Research Program of China (2013CB933103) and Tsinghua
University (20131089311) are acknowledged.
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