Size Specific Activity of Polymer Stabilized Gold Nanoparticles for Transfer Hydrogenation Catalysis

Article abstract of DOI:10.1007/s10562-016-1760-3

Abstract: [Poly (N-Vinyl Pyrrollidone)] (PVP) (K30, average Mol. Wt., 40?kDa) stabilized gold nanoparticles with four different size (mean diameter) of 1.4?±?0.2, 3.8?±?0.4, 5.5?±?0.8 and 7.8?±?1?nm were synthesized using solution based chemical reduction method and used as catalysts for transfer hydrogenation reaction. Transfer hydrogenation of several organic functional groups (>C=O, –CH=O, –NO₂, –CH=CH₂, –C≡C–H, –C≡N) were studied using potassium formate (HCOOK) as hydrogen source in water. The major focus of this work was on catalytic activity, product selectivity and recycling ability of Au:PVP nanoparticles as a transfer hydrogenation catalyst. The size specific catalytic activity showed that up to 3.8?nm sized gold nanoparticles are highly active catalyst and afterwards its activity diminishes sharply with increasing particle size. Mechanistic insight showed that formate ions are activated on gold surface and produced bound hydride species, which is the active intermediate for this catalytic transformation. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1760-3

Catal Lett
DOI 10.1007/s10562-016-1760-3
Size Specific Activity of Polymer Stabilized Gold Nanoparticles
for Transfer Hydrogenation Catalysis
1
1
1
•
Nikeshsinh Chavda
Yadvendra K. Agrawal
•
Abhishek Trivedi
1
•
Jaydev Thakarda
1
•
Prasenjit Maity
Received: 14 April 2016 / Accepted: 1 May 2016
Ó Springer Science+Business Media New York 2016
Abstract [Poly (N-Vinyl Pyrrollidone)] (PVP) (K30,
average Mol. Wt., 40 kDa) stabilized gold nanoparticles
with four different size (mean diameter) of 1.4 ± 0.2,
Graphical Abstract
3
.8 ± 0.4, 5.5 ± 0.8 and 7.8 ± 1 nm were synthesized
using solution based chemical reduction method and used
as catalysts for transfer hydrogenation reaction. Transfer
hydrogenation of several organic functional groups
(
[C=O, –CH=O, –NO , –CH=CH , –C:C–H, –C:N)
2
2
were studied using potassium formate (HCOOK) as
hydrogen source in water. The major focus of this work
was on catalytic activity, product selectivity and recycling
ability of Au:PVP nanoparticles as a transfer hydrogenation
catalyst. The size specific catalytic activity showed that up
to 3.8 nm sized gold nanoparticles are highly active cata-
lyst and afterwards its activity diminishes sharply with
increasing particle size. Mechanistic insight showed that
formate ions are activated on gold surface and produced
bound hydride species, which is the active intermediate for
this catalytic transformation.
Keywords Gold nanoparticle Á Transfer hydrogenation Á
Chemoselectivity Á Size effect Á Water
1
Introduction
First demonstration of catalytic activity of gold nanopar-
ticles by Haruta and Hutchings [1, 2] was a seminal find-
ing, as gold is traditionally known as an inert metal, and
their work has inspired researchers through out the globe to
explore more in this domain. Afterwards a variety of
heterogeneous [3], homogeneous [4] and quasi-homoge-
neous [5] based gold nanocatalysts have been studied with
a major emphasis of their application in oxidation catalysis
[5, 6]. Apart from oxidation reaction, nanogold is also
proven as an active catalyst for hydrogenation [7–9],
transfer hydrogenation [10, 11], de-oxygenation [12],
N-alkylation [13] and carbon–carbon coupling reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1760-3) contains supplementary
material, which is available to authorized users.
&
Prasenjit Maity
pmaity@gfsu.edu.in
1
Institute of Research and Development, Gujarat Forensic
Sciences University, Gandhinagar 382007, Gujarat, India
123

N. Chavda et al.
[
14]. Catalytic transfer hydrogenation (TRH) is an alter-
native way to hydrogenate organic unsaturation with
-propanol or formate as hydrogen source under mild and
acetylene, Styrene and Ethyl benzene were obtained from
Sigma-Aldrich. All aromatic nitro compounds, aniline
derivatives and alcohols were purchased from Finer
Chemicals, India. Milli–Q grade water was used in all
preparations, measurements and as catalysis medium. UV–
Vis spectra of aqueous Au:PVP hydrosols were obtained
using a JASCO V-670 spectrophotometer. FT-IR spectra of
aqueous Au:PVP hydrosols were recorded on a JASCO-
4200 FTIR spectrophotometer using ATR (Attenuated
Total Reflectance) probe. X-ray photoelectron spec-
troscopy (XPS) measurement of Au-a and Au-b samples
were was performed on a JEOL JPS-9010MC instrument
using Mg-Ka radiation source and the spectrum was cali-
brated with respect to C1s peak, which was adjusted at
285 eV value. TEM images were recorded using a Philips
JEM 2000FX microscope operated at 200 kV. A water
dispersion of gold nanoparticle was drop-casted onto
hydrophilic carbon-coated copper grid followed by drying
in open air for 2 h. Powder XRD pattern was measured in a
Bruker C-100 model using Cu-Ka radiation source. The
gold content of Au:PVP samples were determined on a
Plasma Lab 8440 ICP-AES instrument. The reaction mix-
tures were analyzed in a Shimadzu gas chromatograph (GC
14A) coupled with a FID detector by using a silica capil-
lary column (RTX-1).
2
safe reaction conditions as compared to the use of highly
flammable molecular hydrogen [15–19]. Transfer hydro-
genation of carbonyl and nitro derivatives by solid metal
oxide (TiO , Fe O , Al O , CeO ) supported nanogold
2
2
3
2
3
2
using isopropanol and potassium formate as hydrogen
source were reported by Cao and coauthors [10, 11]. They
have also proposed a mechanistic model for ceria sup-
ported gold nanocatalyst catalyzed transfer hydrogenation
using potassium formate as hydrogen source [11]. Formate
ions first adsorb and produce bound hydride species on
redox active CeO surface, which is then transferred to
2
gold surface through reverse hydrogen spillover. In case of
hydrogenation (using molecular H ) of aromatic nitro
2
derivatives by TiO /Au catalyst, the active role of metal
2
oxide support was found to be preferential adsorption of
-
NO group adjacent to gold sites [7]. Thus both hydro-
2
genation and transfer hydrogenation by solid supported
gold nanocatalyst proceeds through active involvement of
metal oxide supports towards adsorption or activation of
substrates/reducing agent. It would be interesting to find
out the activity of bare gold nanoparticles towards TRH
catalysis in absence of any support’s role. However the use
of bare metal nanoparticle is not feasible in real world
catalysis. PVP protected gold nanoparticle (Au:PVP) is an
ideal choice for such studies, as PVP is an organic polymer
with little scope to directly influence the TRH catalysis
apart from donating little negative charge to the gold core
2.2 Preparation of Au:PVP Nanoparticles
Size specific preparation of Au:PVP nanoparticles were
performed using reported protocol [14, 21]. For Au-a sam-
[
20]. Size specific catalytic activity is also another funda-
ple, an aqueous solution of HAuCl (1 mM, 30 mL) was
4
mental and very important characteristics, which is difficult
to study with metal-oxide supported Au (heterogeneous
catalysts) due to limited precession over their size specific
synthesis. Thus, the main objectives of this work are as
follows: first, to find out the activity of Au:PVP nanopar-
ticles towards TRH catalysis for a variety of organic
functional groups. Second, to understand the size specific
activity and selectivity of Au nanoparticle towards TRH
catalysis. Lastly, understand the reaction mechanism with
main emphasis on the role of gold, towards activation of
formate ion to generate hydride species.
mixed with PVP (K30, 1.2 mmol in monomer unit) and the
mixture was stirred for 15 min at 273 K. Next, an aqueous
solution of NaBH (0.1 M, 3 mL) was added to it with vig-
4
orous stirring at 273 K and kept for another 30 min. The
Final purified Au-a was obtained by lyophilizing the deion-
ized dispersion of the as-prepared Au:PVP nanoparticles by
ultrafiltration (MWCO = 10 kDa) with water followed by
drying using fridge dryer.
Au-b, Au-c and Au-d: The larger Au:PVP nanoparticles
were prepared by seed mediated growth method using a
mild reducing agent Na SO . In brief, these larger Au:PVP
2
3
nanoparticles were prepared by reduction of HAuCl by
4
Na SO in the presence of previously synthesized Au-a
3
2
2
Experimental
(1.4 nm) nanoparticle as seed. The mixture of de-aerated
aqueous solutions of HAuCl (30 mM), Na SO (90 mM)
4
2
3
2
.1 Materials
and PVP were quickly added to that of Au-a seed particles
with vigorous stirring at room temperature for 6 h. The Au
All the reagents were commercially available and used
without further purification. Hydrogen tetrachloroaurate
tri-hydrate (HAuCl Á3H O), sodium tetrahydroborate,
atomic ratio of HAuCl to Au-a (1.4 nm) was kept at 3:1,
4
7.5:1 and 10:1 for Au-b, Au-c and Au-d samples respec-
tively where as the amount of Na SO was three times with
4
2
2
3
Ethyl acetate, Polyvinylpyrollidone (PVP; K30, average
molecular weight: 40 kDa), Carbonyl compounds, Phenyl
respect to the amount of HAuCl in all cases and the
4
amount of added PVP was 20 molar equivalent with
1
23

Size Specific Activity of Polymer Stabilized Gold Nanoparticles for Transfer Hydrogenation…
respect to HAuCl . The final solutions were purified by
4
size histogram of four samples show their monodispersity
centrifugation using molecular wt. cut-off membrane filter
with mean particle diameter of 1.4 ± 0.2, 3.8 ± 0.4,
5.5 ± 0.8 and 7.8 ± 1 nm for Au-a, Au-b and Au-c and
Au-d samples respectively. Optical absorption spectra
show gradual increase of intensity of Surface Plasmon
Resonance (SPR) band at 520 nm with increase of particle
size for samples Au-a to Au-d. XRD diffraction profiles are
also in accordance with the Fm3 m fcc crystal structure of
bulk gold with dominant contribution from 111 plane [22].
The mean crystallite sizes of Au nanoparticles obtained
from XRD analysis using Debye–Scherrer equation [23]
are 1.05, 3.3, 4.95 and 7.12 nm for Au-a, Au-b, Au-c and
Au-d respectively (See supporting information). XPS
spectral analysis of Au-a and Au-b samples showed well-
resolved double structure where the two components have
83.7 eV (4f7/2) and 87.6 eV (4f5/2) binding energy (Eb)
values, respectively (See supporting information, Fig S2).
In comparison to the bulk gold, which has 4f7/2 binding
energy of 84.1 eV, these Au:PVP samples have little lower
4f orbital binding energy due to accumulation of electron
density from PVP [20]. ICP analysis showed that all
Au:PVP samples contain 4 ± 0.1 wt% Au.
(
MWCO = 10 kDa) and finally dried in a lyophilizer to
make them fine powder. A simplified schematic diagram
for synthesis of Au:PVP nanoparticles is shown in
Scheme 1.
2
.3 Catalytic Test
A 10 mL aqueous solution containing 25 mg Au:PVP
nanoparticles (5 lmol of Au) and 420 mg HCOOK
(
5 mmol) was prepared in a two neck round bottom flask
fitted with reflux condenser and placed in temperature
controllable oil bath with a magnetic stirrer bar inside. A
0
.5 mmol portion of substrate (substrate/catalyst = 100)
was added to this solution either in pure form (for liquids)
or as a ethyl acetate solution (2 mL). The reaction was
continued at 80 °C with a stirring rate of 900 rpm for a
specific amount of time. After a given amount of time
(
12 h), the reaction mixture was transferred to a conical
flask and 10 mL of ethyl acetate was added, and the mix-
ture was stirred for 20 min to extract the reactants and
products. The ethyl acetate phase was analyzed in a Shi-
madzu gas Chromatograph (GC 14A) coupled with a FID
detector and using a silica capillary column (RTX 1). For
recycling experiments, the aqueous Au:PVP solution was
phase separated and reused in next batch of reaction under
identical set of conditions.
3.2 Au:PVP Catalyzed TRH Reaction
The purified and dried (powder form) Au-PVP nanoparti-
cles were used as TRH catalyst using water as the reaction
medium and potassium formate as hydrogen source. All the
reactions were performed in aqueous-organic biphasic
medium by adding either liquid substrates or ethyl acetate
solution of solid substrates to water solution of Au:PVP
and HCOOK. A variety of organic functional groups, such
as [C=O, –CH=O, –NO , –CH=CH , –C:C–H, and
3
Results and Discussion
3
.1 Catalyst Characterization
2
2
–C:N were tested for TRH catalysis and the results are
Au:PVP nanoparticles were synthesized using solution
based chemical reduction method and were purified
accordingly using membrane filters in a high speed cen-
trifuge followed by drying in a lyophilizer [14, 21]
summarized in Table 1. As can be seen, benzaldehyde
(entry 1), acetophenone (entry 2) and nitrobenzene (entry
3) are efficiently transformed to corresponding alcohol
derivatives and aniline respectively where as other func-
tional groups are very poorly reactive. Blank tests were
performed, which showed that either the absence of Au
catalyst or potassium formate or both results no conversion
at all for benzaldehyde and nitrobenzene (entries 1 and 3,
Table 1). Also, as reported previously, Au-PVP activates
Phenyl acetylene easily via breaking of its acetylic carbon-
(
Scheme 1). The TEM images (Fig. 1), optical absorption
spectra (Fig. 2), powder XRD patterns and (Fig. 3) of gold
nanoparticles are shown below. TEM image and particle
hydrogen (:C–H) bond. Afterwards phenyl acetelyde
-
(
Ph–C:C ) attached to the gold clusters to form the
corresponding alkyne protected gold clusters and hence its
hydrogenation was not successful using Au:PVP nanopar-
ticles [24]. The result clearly shows that gold nanoparticles
are able to activate formate ions followed by catalytic
hydrogenation of organic functional groups. A range of
different aliphatic and aromatic carbonyl and aromatic
nitro derivatives were hydrogenated using this catalytic
Scheme 1 Synthetic scheme for four different sized Au:PVP
nanoparticles
123

N. Chavda et al.
Fig. 1 TEM images along with particle size histograms of Au-a (a), Au-b (b), Au-c (c) and Au-d (d) nanocatalysts
Fig. 2 UV–Vis absorption spectra of aqueous solution of four
different sized Au:PVP nanoparticles
Fig. 3 Powder X-ray Diffraction profile of Au-a, Au-b, Au-c and Au-
d samples along with the diffraction pattern of standard bulk fcc gold
system and the results are summarized in Table 2. All car-
bonyl compounds were converted to corresponding alcohol
derivatives with high conversion and complete selectivity. It
was found that aldehydes (entries 1–7) are more reactive
than ketones (entries 8–9) due to less steric crowding around
carbonyl carbon center as well as its superior electrophilic
character compared to ketones. Among carbonyl
compounds the increase of conversion from entry 2 to 4 can
be ascribed due to favorable steric effect. The influence of
electronic effect is also evident as we could find better
reactivity with electron withdrawing group substituted
aldehydes (entry 6) than substrates with alkyl substituents
(entries 2–4). However, 4-methoxy benzaldehyde (entry 5)
unexpectedly showed high activity in spite of its electron
1
23

Size Specific Activity of Polymer Stabilized Gold Nanoparticles for Transfer Hydrogenation…
Table 1 TRH of selected organic functional groups using Au-a as
quasi-homogeneous catalyst in water
11–13) and nitro phenols (entries 14–16) to corresponding
aniline derivatives were also clearly evident from this study.
The transfer hydrogenation result discussed so far clearly
%
Conversion
Entry Substrate
Product
shows that aldehyde (–CHO) and nitro (–NO ) groups are
2
most reactive. Hence to find out their comparative reactiv-
ity, 4-nitro benzaldehyde (entry 18) was hydrogenated under
identical conditions. The study resulted 90 % conversion
with product selectivity of 50 % 4-amino benzaldehyde,
30 % 4-amino benzyl alcohol and 20 % 4-nitro-benzyl
b
c
CHO
2
CH OH
1
92 (0 , 0 )
O
O
2
40
b
c
NO
2
NH
2
3
4
88 (0 , 0 )
<1
alcohol. The result indicates that –NO group hydrogenation
2
is faster than –CHO group but due to their comparable
reactivity, selective hydrogenation of one in presence of
other is not feasible.
<
<
1
1
5
6
3.3 Recycling Test
N
2 2
CH NH
<1
Aqueous biphasic catalysis has the advantages of easy
product isolation and recycling of the used catalyst if the
catalyst is soluble/dispersible in water and reactants/prod-
ucts are in water immiscible organic solvent. The recycling
ability of Au:PVP catalysts was studied up to fourth cycle
using nitrobenzene as the substrate of choice with very
consistent reactivity (Fig. 4a) and total TON value of 285.
The recycling study demonstrates the high efficacy of
Au:PVP nanoclusters as quasi-homogeneous catalyst
towards transfer hydrogenation reactions.
Au-a = 25 mg (5 l mol Au), substrate = 0.5 mmol, HCOOK = 5 m-
mol, water = 10 mL, Temperature = 353 K, Time = 12 h
a
Same conditions, except addition of Au-a
b
Same conditions, except addition of HCOOK
donating substituent. A variety of aromatic nitro derivatives
were also hydrogenated with good conversion and complete
selectivity (Table 2 entries 10–17). The influence of steric
effect on conversion of alkyl nitro derivatives (entries
Table 2 TRH of selected carbonyl and nitro compounds to corresponding alcohol and aniline derivatives using Au-a as quasi-homogeneous
catalyst in water
Carbonyl-
derivatives
%
Nitro-
%
Entry
1
Entry
10
Conversion
derivatives Conversion
92
CHO
NO2
88
5
5
NO2
CHO
2
3
11
12
85
5
6
8
5
CHO
CHO
NO
2
85
NO
2
4
5
6
13
14
15
70
90
80
H
3
CO
CHO
90
HO
HO
NO2
85
NO
2
Cl
CHO
95
NO
2
7
CHO
16
60
OH
O
4
5
0
0
NC
NO2
8
9
17
18b
90
90
O
OHC
NO2
Au-a = 25 mg (5 l mol Au), substrate = 0.5 mmol, HCOOK = 5 mmol, water = 10 mL, Temperature = 353 K, Time = 12 h, % conversion
values are average of 3 readings with experimental error of ± 3 %
a
Conversion = 90 %, with selectivity of 50 % 4-amino benzaldehyde, 30 % 4-amino benzyl alcohol and 20 % 4-nitro-benzyl alcohol
123

N. Chavda et al.
Fig. 4 a Bar-chart diagram showing recycling results for nitrobenzene transfer hydrogenation using Au-a as catalyst, b Size specific catalytic
activity of Au:PVP catalysts towards TRH of benzaldehyde and nitrobenzene
Scheme 2 Equation 1 and 2 are
controlled experiments as
described in Sect. 3.5 and eq. 3
is proposed decomposition route
of formate by Au catalyst in
absence of substrate
3
.4 Size Specific Catalytic Activity
nanoparticles were calculated (See supporting information)
and then activity (TOF) per unit surface area was calculated
and plotted against the size of gold nanoparticles (Fig. 4b).
The result is more accurate indication of superiority of small
sized Au particles on TRH catalysis. We can see decrease of
TOF per unit surface area values for both substrates with
increasing particle size. However, there is no appreciable
activity difference between Au-a (1.4 nm) and Au-b
(3.8 nm) catalysts for both the substrates. It is also interesting
that Au-b (3.8 nm) catalyst showed slight better activity than
Au-a (1.4 nm) for TRH of nitrobenzene (Fig. 4b). Thus, we
can postulate that for TRH catalysis Au particles up to 4 nm
size is highly active, after that its activity suddenly shrinks.
The result also shows that metal oxide (CeO , TiO ) support
Size specific catalytic activity of metal nanoparticles is an
important study to understand the reaction mechanism and
rational design of active and selective catalysts [21, 25]. Four
different sized Au:PVP nanoparticles (Au-a, Au-b, Au-c,
Au-d) were used to study the size specific catalytic conver-
sion of benzaldehyde and nitrobenze towards their hydro-
genation products benzyl alcohol and aniline respectively. In
all these studies molar ratio of substrate to Au was kept
constant and it was similar to conditions e.g., Table 1. As
expected, steady decrease of % of conversion was observed
for both substrates with increasing particle size (Fig. 4b).
However, only surface atoms of a nanoparticle are active in
catalysis, hence it is necessary to find out normalized cat-
alytic activity per unit surface area for their comparison.
Thus, the total surface area for these different sized Au:PVP
2
2
may be advantageous but not essential for –NO group
2
adsorption and subsequent hydrogenation of aromatic nitro
derivatives [7, 11].
1
23

Size Specific Activity of Polymer Stabilized Gold Nanoparticles for Transfer Hydrogenation…
Fig. 5 a Time monitored UV–
Vis spectra of Au-a and
HCOOK solution in water at
3
53 K (0–3 h) and then keeping
the same solution in air for 2 h.
b Plot Abs at 509 nm (Abs509
vs. time showing increase up to
h (during reaction) and then it
)
3
decreases on keeping the
solution in open air. c FTIR
spectra of same solution using
ATR probe. d Change of pH of
aqueous Au-a ? HCOOK
solution and only HCOOK
solution after keeping at 353 K
for 2 h
3
.5 Mechanistic Insight
nanoparticles were dispersed in water along with HCOOK
under identical conditions like Table 1 but no substrate
were added. After 12 h, substrates were added and then
after 24 h, product was analyzed. The results showed only
6 % conversion of nitrobenzene to aniline, but the same
reaction when studied by adding fresh HCOOK, resulted
81 % yield (Scheme 2, Eq. 1). This experiment shows that
in absence of substrate, the formate ions are activated and
gets consumed probably by forming molecular hydrogen
and bicarbonate. In next experiment, nitrobenzene and
styrene were together added as substrates (Scheme 2,
Eq. 2) for catalysis under identical conditions like Table 1.
The analysis shows that only 30 % and 1 % conversion of
nitrobenzene and styrene respectively as opposed to 88 %
conversion when only nitrobenzene was used (entry 2,
Table 1). This result suggest that both styrene and
nitrobenzene compete for adsorption on gold surface, but
Next, we were interested to shed light on possible mech-
anistic pathway of Au:PVP catalyzed TRH catalysis. We
can envision that catalytic transfer hydrogenation sequen-
ces on gold surface proceed through following steps. Both
formate ions and substrate molecules first get adsorbed on
metal surface. Formate ions are than activated and produce
active hydride ions, which are transferred to substrate
molecules [26]. In the last stage, after accepting pro-
ton(s) from water, substrate forms hydrogenated products
and bicarbonate ions are formed as a byproduct. However,
there are two possible mechanistic pathway of hydride
transfer to substrate molecule [26]. In the first case, formate
ions are adsorbed on gold surface and produce gold-hy-
dride intermediate species, which are then transferred to
substrate (metal hydride route). In second case, adsorbed
and activated formate donates hydride ions to substrate
molecules with out forming metal hydride intermediate
-
H transfer to nitrobenzene is favored over styrene and this
step makes the difference between their reactivity. How-
ever because of this competition and high activation energy
for styrene hydrogenation, a high proportion of formate
ions are decomposed and conversion of nitrobenzene is
low. These two experiments hint the possibility of metal
(
direct hydride transfer). We did couple of experiments to
ascertain the correct mechanistic scenario on Au:PVP
catalyzed TRH reaction. Control experiments with
nitrobenzene as substrate were studied where initially Au-a
123

N. Chavda et al.
Scheme 3 Proposed catalytic
cycle for Au:PVP catalyzed
TRH of carbonyl compounds
-
2200 cm region confirms the existence of a valley and in
1
hydride route as the preferred mechanism operative here.
These short-lived intermediate is easily decomposed by
forming molecular hydrogen and hence leaves no excess
formate in solution for substrate hydrogenation. Detection
of these intermediate gold-hydride species was our next
focus point and for that we were investigating the FTIR
spectral signature, UV–Vis spectroscopy and pH of reac-
tion mixture of Au:PVP and potassium formate solution.
Recently Tsukuda et al., has demonstrated that surface
Plasmon resonance band (SPR) of Au clusters in solution
can be amplified by addition of hydride ions (NaBH4)
through electron donation to Au core from hydride ions
-
1
some cases a weak peak at 2139 cm . Although, the peak
is very weak but its presence can be assigned due to the
presence of gold-hydride intermediate species. The pH of
the above reaction solution increases from 6.8 (at t = 0) to
10.1 (at t = 2 h) (Fig. 5d). This is another evidence that
formate ions are activated on gold surface and produce
dihydrogen and potassium hydrogen carbonate (or potas-
sium hydroxide and carbon dioxide) according to Eq. 3
(Scheme 2). Critical monitoring of reaction solution also
allowed us observing few small bubbles that could be due
to evolution of dihydrogen or carbon dioxide. Thus, from
the above three set of experiments, we concluded that the
Au:PVP catalyzed TRH proceeds via metal hydride route
and accordingly a plausible catalytic cycle is presented in
Scheme 3.
[
27]. Thus, monitoring SPR band of Au:PVP hydrosol is an
option to detect the formation of gold-hydride species. An
aqueous solution (30 mL) of Au-a (50 mg) and HCOOK
(
500 mg) was kept under stirring at 353 K and its UV–Vis
spectrum was monitored at different time interval (Fig. 5a
and 5b). The absorbance at 509 nm increases up to 4 h and
then decreases upon keeping it in air, although the final
absorbance is still higher than t = 0. We conjecture that
the increased absorbance value of SPR band at 509 nm is
the result of two simultaneous phenomenons; Au-H species
formation and Au nanoparticle agglomeration. Subtraction
of final absorption value (Abs509 at 6 h) from absorption
value at 4 h (Abs509 at 4 h) gives positive (rAbs) value
4 Conclusions
In summary, Organic polymer (PVP) stabilized gold
nanoparticles are active catalyst for transfer hydrogenation
of organic-carbonyl and aromatic nitro derivatives using
potassium formate as hydrogen source in water medium. A
variety of substituted aldehyde and nitro derivatives were
converted to corresponding alcohol and aniline derivatives
in presence of other functional groups. The size specific
catalytic activity patterns show that Au particles up to 4 nm
sizes are highly active, after that its activity suddenly
shrinks. It is also found that aldehyde hydrogenation is
more sensitive and decreases steadily with increasing gold
particle size compared to nitrobenzene hydrogenation. The
mechanistic insight hints the pathway of intermediate gold-
and it clearly indicates the formation of gold-hydride (Au -
n
H ) intermediate.
n
Garcia et al. and Silverwood et al. have reported the
formation of intermediate Au-H species by FT-IR spectral
-
1
signature at 2125–2130 cm region [28, 29]. We mea-
sured the FT-IR spectrum of above reaction solution at
different time interval using an ATR probe (Fig. 5c).
Critical observation of the FT-IR spectra at 2100 to
1
23

Size Specific Activity of Polymer Stabilized Gold Nanoparticles for Transfer Hydrogenation…
hydride formation step, which either transfers to substrate
for its hydrogenation or decomposes by forming hydrogen
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(
H ). The recycling process is very easy due to aqueous-
2
organic biphasic catalysis medium and the catalytic activity
in repetitive cycles was found to be quite consistent.
14. Tsunoyama H, Sakurai H, Ichikuni N, Negishi Y, Tsukuda T
(2004) Langmuir 20:11293
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Acknowledgments This work was supported by the funding from
Science and Engineering Research Board (SERB), Government of
India (project no. SERB/FT/007/2014).
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