Activity and selectivity of colloidal platinum nanocatalysts for aqueous phase cyclohexenone hydrogenation

Article abstract of DOI:10.1016/j.apcata.2011.05.033

The effect of varying the particle size of poly(vinylpyrrolidone)-capped colloidal platinum nanocatalysts, synthesized in the 1-10 nm size range, on the activity and selectivity during the aqueous-phase hydrogenation of cyclohexenone was investigated. For all particle sizes utilized, products observed during the reaction in order of increasing selectivity were cyclohexenol, cyclohexanol, and cyclohexanone. Selectivity for the unsaturated alcohol (cyclohexenol) was found to be highest for the smallest-sized nanocatalyst, whereas selectivity for the saturated ketone (cyclohexanone) was found to be highest for the largest-sized nanocatalyst. Activities for cyclohexenone hydrogenation were observed to increase with increasing particle size, yet apparent activation energies were determined to be similar for 2.9-7.1 nm nanocatalysts, likely indicating that the same surface reaction is kinetically favored on these nanocatalyst surfaces. Attenuated total reflectance infrared (ATR-IR) spectroscopic measurements on adsorbed cyclohexenone suggest the η₁(CO) form is more prevalent on the smaller sized nanocatalysts, while the η₂(CC) and η₄(CO, CC) forms are more preferred on the larger sized nanocatalyst. The findings presented herein provide further evidence that structure sensitivity in ketone hydrogenation reactions is linked with catalyst surface structure and adsorbate orientation.

Full text of DOI:10.1016/j.apcata.2011.05.033

Applied Catalysis A: General 402 (2011) 104–109
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Activity and selectivity of colloidal platinum nanocatalysts
for aqueous phase cyclohexenone hydrogenation
Nathan E. Musselwhite, Sarah B. Wagner, Kimberly A. Manbeck,
Lindsay M. Carl, Kyle M. Gross, Anderson L. Marsh∗
Department of Chemistry, Lebanon Valley College, 101 N. College Ave., Annville, PA 17003, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of varying the particle size of poly(vinylpyrrolidone)-capped colloidal platinum nanocata-
lysts, synthesized in the 1–10 nm size range, on the activity and selectivity during the aqueous-phase
hydrogenation of cyclohexenone was investigated. For all particle sizes utilized, products observed dur-
ing the reaction in order of increasing selectivity were cyclohexenol, cyclohexanol, and cyclohexanone.
Selectivity for the unsaturated alcohol (cyclohexenol) was found to be highest for the smallest-sized
nanocatalyst, whereas selectivity for the saturated ketone (cyclohexanone) was found to be highest for the
largest-sized nanocatalyst. Activities for cyclohexenone hydrogenation were observed to increase with
increasing particle size, yet apparent activation energies were determined to be similar for 2.9–7.1 nm
nanocatalysts, likely indicating that the same surface reaction is kinetically favored on these nanocat-
alyst surfaces. Attenuated total reflectance infrared (ATR-IR) spectroscopic measurements on adsorbed
cyclohexenone suggest the ꢀ1(C O) form is more prevalent on the smaller sized nanocatalysts, while
the ꢀ2(C C) and ꢀ4(C O, C C) forms are more preferred on the larger sized nanocatalyst. The findings
presented herein provide further evidence that structure sensitivity in ketone hydrogenation reactions
is linked with catalyst surface structure and adsorbate orientation.
Received 14 January 2011
Received in revised form 20 May 2011
Accepted 23 May 2011
Available online 30 May 2011
Keywords:
Ketone
Chemoselective hydrogenation
Colloidal platinum
Nanocatalyst
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
such as poly(vinylpyrrolidone), or PVP. Reports of hydrogenations
involving these nanocatalysts dispersed in water have been limited,
Selective hydrogenations using platinum catalysts play a valu-
able role in many chemical industries, ranging from petrochemicals
to pharmaceuticals [1,2]. Chemoselective hydrogenations involv-
ing unsaturated ketones and aldehydes are of particular interest
[3]. For example, the C C bond in oxosteriods may be hydrogenated
over TiO2 supported Pt catalysts [4]. Many of these reactions
are operated in the liquid phase using organic solvents and at
much lower temperatures than those for gas-phase reactions, thus
increasing selectivity due to a decrease in the occurrence of side
reactions [5]. To operate under more environmentally friendly con-
ditions it would be desirable to utilize aqueous media instead
of organic solvents [6]. Improvement of catalyst performance
under these reaction conditions requires tailoring of material prop-
erties through synthetic methods, such as those from colloidal
nanoscience [7,8]. The nanocatalysts prepared using these meth-
ods are composed of transition metal nanoparticles with a narrow
size distribution and typically are capped with organic polymers or
dendrimers to prevent aggregation in solution. The solubility of the
nanocatalyst may be tailored for water by selecting a capping agent
however [9,10].
In a prior study by our group, platinum nanocatalysts pre-
pared with colloidal nanoscience methods were used to examine
the effect of changing nanocatalyst properties (particle size and
capping agent molecular weight) and reaction conditions (temper-
ature, hydrogen pressure, and reactant concentration) during the
hydrogenation of cyclohexanone [10]. In all reactions carried out,
100% selectivity for cyclohexanol formation was observed, with no
side products from decomposition reactions detected. Measured
turnover frequencies were seen to increase with increasing parti-
cle size, but were found to show no trend with regards to capping
agent molecular weight. From the results of temperature stud-
ies, similar apparent activation energies in the 21–24 kJ/mol range
were determined for the particle sizes employed. A rate law was
derived from pressure and concentration studies in which the reac-
tion order with respect to hydrogen is 0.3 and the reaction order
with respect to cyclohexanone is 0.8. This rate law is consistent with
a Langmuir–Hinshelwood mechanism in which both molecules
have low adsorption and one of the molecules (hydrogen) exhibits
dissociative adsorption. Attenuated total reflectance infrared (ATR-
IR) spectroscopic measurements support the determined rate law,
with two peaks observed in the spectrum in the C O stretching
region, suggesting that cyclohexanone in solution exists in equilib-
∗
Corresponding author. Tel.: +1 717 867 6149; fax: +1 717 867 6075.
E-mail address: marsh@lvc.edu (A.L. Marsh).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.05.033

N.E. Musselwhite et al. / Applied Catalysis A: General 402 (2011) 104–109
105
rium with cyclohexanone adsorbed on the Pt nanocatalyst surface.
It is evident that results from experimental work done using these
prepared nanocatalysts increase the molecular-level knowledge of
catalyst properties that influence reaction selectivity.
TOFs were calculated by dividing the moles of cyclohexenone
reacted by the moles of platinum, and then normalized through
division by dispersion factors calculated using TEM particle sizes,
as done for our cyclohexanone hydrogenation studies [10].
Cyclohexenone adsorption employing ATR-IR spectroscopy was
performed using a Nicolet 380 Fourier-Transform infrared spec-
trometer fitted with a Specac Gateway ATR optics assembly and
flow cell containing a ZnSe crystal, where 64 scans were acquired
at 4 cm−1 resolution with an ATR correction. Nanocatalyst solutions
were pumped through the cell from a reservoir using a peristaltic
pump as a small volume (<1 mL) cyclohexenone was added to the
solution in the reservoir. To remove contributions from water and
PVP, difference spectra were carefully calculated by subtracting
the spectra for the corresponding platinum colloid solutions from
the spectra for the platinum colloid solutions after cyclohexenone
addition.
In the work presented here, we extend our investigations
of aqueous phase hydrogenations using PVP-capped platinum
nanocatalysts to the hydrogenation of the unsaturated ketone
cyclohexenone, which has been identified as a product of the
gasification of biomass [11]. Prior studies of the hydrogena-
tion of cyclohexenone have been limited [12–19], particularly
those under more environmentally friendly reaction conditions
[16–19]. After synthesis and characterization, Pt nanocatalysts
in the 1–10 nm size range were employed in aqueous phase
hydrogenations. Reaction compositions were found from gas chro-
matography/mass spectrometry (GC/MS) measurements, which
allowed turnover frequencies (TOFs) and reaction selectivities to
be determined as a function of particle size. In addition, appar-
ent activation energies were calculated from the temperature
dependence of TOFs. ATR-IR spectroscopy was used to elucidate
preferred adsorption modes of cyclohexenone on the nanocat-
alyst surfaces. The experimental findings were then utilized in
developing a molecular level model of the effects of changing par-
ticle size on selectivity towards specific reaction pathways during
hydrogenation.
3. Results and discussion
Displayed in Fig. 1 are the TEM image and the corresponding
histogram for platinum nanocatalysts synthesized with methanol
as a reducing agent. The average size of these nanocatalyst parti-
cles (together with one standard deviation) was determined to be
2.9 0.1 nm. TEM images and corresponding histograms for plat-
inum nanocatalysts synthesized via other routes may be found in
Figs. S-1–S-3 of the supplementary data. Average sizes of these
nanocatalyst particles (together with one standard deviation) were
calculated to be 1.7 0.2 nm, 3.6 0.2 nm, and 7.1 0.3 nm, respec-
tively. All of the TEM particle sizes reported in this work agree well
with those reported in the literature for platinum nanoparticles
prepared using these colloidal-based synthetic methods [10,21].
Hereafter these colloidal platinum nanocatalysts are referred to by
these TEM sizes.
After characterization, the synthesized nanocatalyts were dis-
persed in water and subsequently utilized in the hydrogenation
of cyclohexenone (CHEO), the reaction pathways of which are
shown in Scheme 1. In the upper pathway cyclohexanone (CHO)
is formed, whereas in the lower pathway cyclohexenol (CHEOL)
is produced. Both of these products may undergo further hydro-
genation to cyclohexanol (CHOL). Displayed in Fig. 2 is a plot of
compositions in mole percents as a function of reaction time for a
hydrogenation reaction using 7.1 nm colloidal Pt nanocatalysts that
was carried out at a temperature of 323 K, a hydrogen pressure of
2.1 bar, and a cyclohexenone to platinum mole ratio of 1000:1. As
seen in the figure, the fraction of cyclohexenone decreases steadily
over the course of the reaction, while the fraction of cyclohexanone
increases gradually. During the initial stage of the reaction small
fractions of cyclohexenol and cyclohexanol are formed. At longer
reaction times, the fraction of cyclohexenol decreases to zero and
the fraction of cyclohexanol increases, likely due to the hydrogena-
tion of both cyclohexenol and cyclohexanone. Still, based on the
data, as presented in the figure, the dominant reaction channel
appears to be the hydrogenation of cyclohexenone to cyclohex-
anone.
2. Experimental
Poly(vinylpyrrolidone) of 29,000 g/mol molecular weight,
hexachloroplatinic acid (H2PtCl6·6 H2O), ethylene glycol, and 2-
cyclohexen-1-one (≥95%), were obtained from Aldrich. ACS grade
methanol and acetone were acquired from PHARMCO, while
research purity hydrogen was purchased from Airgas. All chemicals
were used without purification. A Millipore Direct-Q system was
used to generate 18 Mꢁ deionized water for nanocatalyst synthesis
and reaction studies.
Specific details for the synthesis of each size of nanocata-
lyst are described in our prior publication [10] and are based on
published literature methods [20,21]. Nanocatalysts were charac-
terized at The Penn State College of Medicine’s Electron Microscopy
Laboratory using a JEOL JEM-1400 transmission electron micro-
scope. For TEM measurements, colloidal platinum nanocatalyst
solutions were placed dropwise onto carbon-coated copper grids.
A minimum of 200 particles were counted during size determina-
tions.
Cyclohexenone hydrogenation reactions were performed in a
Parr 4566 mini benchtop reactor with constant stirring at 1000 rpm,
as previously described for cyclohexanone hydrogenation [10]. To
charge the reactor, a selected volume of the Pt colloid solution was
mixed with a solution of cyclohexenone in deionized water to give
a 1000:1 mole ratio of cyclohexenone to platinum. The reactor
was heated to the selected temperature, flushed with hydrogen,
and then pressurized to 2.1 bar. At selected reaction times small
aliquots (<1 mL) of the reaction mixture were sampled and the
platinum nanocatalysts were removed by subsequent centrifuga-
tion. Using integrated peak areas obtained from GC/MS (Agilent
6890/5973, Supelcowax 10 column, 15.0 m × 100 ␮m × 0.10 ␮m)
analysis of reaction samples, the composition in mole percent of
the reaction mixture was calculated for each kinetic run. The reac-
tion selectivity of each product, Si, was found by taking a ratio of
the mole percent of that product, xi, to the total mole percent of
products, as shown by Eq. (1):
To better understand the effect of particle size on the distri-
bution of products during the course of the reaction, experiments
were performed using nanocatalysts with selected particle sizes
synthesized over the 1–10 nm size range. Turnover frequencies
based on the amount of cyclohexenone reacted and product selec-
tivities were calculated using mole percents determined at around
40% conversion. Summarized in Table 1 are percent selectivities for
the products cyclohexanone, cyclohexenol, and cyclohexanol cal-
culated for hydrogenation reactions performed at 308 K and 2.1 bar
hydrogen with colloidal platinum nanocatalysts of selected sizes.
In general, the percent selectivity for cyclohexanone increases from
xi
S =
(1)
ꢀ
i
xj
j

106
N.E. Musselwhite et al. / Applied Catalysis A: General 402 (2011) 104–109
Table 1
Percent selectivities, turnover frequencies, and activation energies for selected sizes
of 29,000 g/mol PVP-capped Pt nanocatalysts. Reaction conditions were 2.1 bar H2
and a 1000:1 mole ratio for cyclohexenone to Pt. The percent selectivities and
turnover frequencies are values at 40% conversion for reactions carried out at 308 K.
dTEM
(nm)
Dispersion
factor
(D)
%
SCHO
%
%
SCHOL
TOF
Ea
SCHEOL
(molCHEO
reacted molsurface
(kJ/mol)
−1 s−1
)
Pt
1.7
2.9
3.6
7.1
0.80
0.47
0.38
0.19
86
89
88
91
4
0
1
2
10
0.26
0.33
0.40
0.67
42
21
20
21
11
11
7
86% to 91% with an increase in particle size from 1.7 nm to 7.1 nm,
whereas the percent selectivity for cyclohexenol decreases from
4% to 2% with an increase in particle size from 1.7 nm to 7.1 nm.
The percent selectivity for cyclohexanol also decreases, going from
around 10% to around 7% with an increase in particle size from
1.7 nm to 7.1 nm. The observed percent selectivities for cyclohex-
anone are similar to those reported previously in the literature
for hydrogenations using platinum catalysts [12,14,17,18]. In each
instance the authors report cyclohexanol as the only other major
product; no unsaturated alcohol was found. Conversely, increased
selectivity to the unsaturated alcohol was seen during the gas phase
hydrogenation of crotonaldehyde for larger particle sizes [22,23].
Here the authors attribute this trend to an increased number of
active sites on the Pt particle surfaces since larger particles would
be more capable of interacting with both the C O and C C groups.
For cyclohexenone hydrogenation, a similar trend was also
observed with regards to measured TOF values. In comparing the
TOFs described in this work across nanocatalyst particle sizes, as
shown in Table 1, it is apparent that the TOF for the hydrogenation
of cyclohexenone increases with an increase in particle size at this
reaction temperature. This same increase in TOF with an increase in
Fig. 1. Representative (a) TEM image and (b) corresponding histogram for 2.9 nm
colloidal platinum nanocatalysts capped with 29,000 g/mol PVP.
Fig. 2. Compositions in mole percents during hydrogenation of cyclohexenone
using 7.1 nm colloidal platinum nanocatalysts. Reaction conditions were 323 K,
2.1 bar hydrogen, and a cyclohexenone to platinum mole ratio of 1000:1.
Scheme 1. Reaction pathways in the hydrogenation of cyclohexenone.

N.E. Musselwhite et al. / Applied Catalysis A: General 402 (2011) 104–109
107
the temperature range employed in the studies. Linear regres-
sion analysis of the data in the Arrhenius plots resulted in similar
apparent activation energies over the 2.9 to 7.1 nm particle size
range, as seen in Table 1. The calculated apparent activation ener-
gies of around 20 kJ/mol for particle sizes of 2.9, 3.6, and 7.1 nm
are close to the value of 20.9 kJ/mol reported for the hydrogena-
tion of cyclohexenone on a supported Rh catalyst [13] and are
slightly lower than the value of 25 kJ/mol measured for the liquid-
phase hydrogenation of cyclohexene over a Pt foil catalyst [25].
The value of 42 kJ/mol determined for hydrogenations using the
1.7 nm nanocatalysts indicates that the reaction on these particles
is more sensitive to changes in temperature. It is probable that
this change in apparent activation energy results from a change in
adsorption energy on the smaller sized nanocatalysts. In an effort
to understand the details of the reactions described by these appar-
ent activation energies, a kinetic analysis of the reaction data was
carried out.
In order to perform the kinetic analysis, the reaction data
presented in Fig. 1 were modeled using Euler’s method of approxi-
mation [26]. The following reactions were considered in the model:
k
1
Cyclohexenone−→ Cyclohexanone
k
2
Cyclohexenone−→ Cyclohexenol
(2)
k
3
Cyclohexanone−→ Cyclohexanol
k
4
Cyclohexenol−→ Cyclohexanol
Fig. 3. Arrhenius plots for cyclohexenone hydrogenation over the 308–338 K
temperature range using nanocatalysts with selected Pt particle sizes. Reaction
conditions were 2.1 bar hydrogen and a cyclohexenone to platinum mole ratio of
1000:1.
These reactions were then used to create the following set of
differential equations to describe the reaction rates:
dC1
= −k1C1 − k2C1
dt
dC2
dt
dC3
dt
= k1C1 − k3C2
particle size was observed for the hydrogenation of cyclohexenone
over Pt nanoparticles supported on bentonite [14], as well as for
the hydrogenation of cyclohexanone using these colloidal PVP-Pt
nanocatalysts [10] and for the hydrogenation of crotonaldehyde
over supported Pt catalysts [22,23]. In the case of crotonaldehyde
hydrogenation, the authors postulate that an increase in parti-
cle size leads to an increase in the number of active sites for
hydrogenation, mainly (1 1 1) oriented terrace sites. Nanocatalysts
synthesized with largely (1 1 1) terraces exposed were reported to
be more active for aldehyde hydrogenation than those with largely
(1 0 0) terraces exposed [24]. In the case of PVP-capped Pt nanocat-
alysts suspended in water, it is expected that (1 1 1) terrace sites
are open on the platinum surface for all particle sizes prepared
[8]. The PVP groups might be interacting with the defect sites on
the surface, leaving the terrace sites open, as indicated by previ-
ous ATR-IR measurements of adsorbed CO [10]. The smaller sized
nanocatalysts would be expected to contain a higher concentra-
tion of defect sites, which would not be active for hydrogenation,
than a higher concentration of terrace sites. Although the change
in TOF with a change in Pt nanocatalyst particle size perhaps sug-
gests the reaction is structure sensitive, dispersion factors for the
smaller sized nanocatalysts may be overestimated due to the lower
concentration of terrace sites.
(3)
= k2C1 − k4C3
dC4
= k3C2 + k4C3
dt
In these equations, C1, C2, C3, and C4 are the concentrations
of cyclohexenone, cyclohexanone, cyclohexenol, and cyclohexanol,
respectively. The initial conditions were set such that, at time = 0,
C1 =/ 0 and C2 = C3 = C4 = 0. The model provides reasonable fits to
the experimental data, as seen by the dashed lines shown in Fig. 2.
The values for the rate constants were then multiplied by the mole
ratio factor of 1000 and normalized by dispersion factors to esti-
mate TOFs for the various reaction channels. For the first pathway,
the hydrogenation of cyclohexenone to cyclohexanone, a TOF of
1.4 molCHO molsurface −1 s−1 was estimated. For the second path-
Pt
way, the hydrogenation of cyclohexenone to cyclohexenol, a TOF
of 0.068 molCHEOL molsurface −1 s−1 was predicted. This TOF for
Pt
C
O hydrogenation is lower than that determined in the case of
cyclohexanone hydrogenation [10], which likely indicates that the
presence of the C C bond slows down the rate of C O hydro-
genation. The sum of the two calculated values is higher than the
experimental value of 0.79 molCHEO reacted molsurface −1 s−1 for the
Pt
Additional experiments were also conducted over the temper-
ature range 308–338 K for each nanocatalyst particle size in order
to determine apparent activation energies. Hydrogenations were
carried out at selected temperatures with a hydrogen pressure of
2.1 bar and a cyclohexenone to platinum mole ratio of 1000:1. In
general, the results of these experiments indicate that the TOF
increased as the reaction temperature was increased for all of
the particle sizes studied. Apparent activation energies for hydro-
genations carried out with nanocatalysts of selected particle sizes
were determined using the Arrhenius plots shown in Fig. 3. As
seen in the figure, the reactions exhibited Arrhenius behavior over
total hydrogenation calculated at 40% conversion of cyclohexenone.
It is interesting to note the TOF for the first channel is around
20 times higher than that for the second channel, consistent with
a reported observation for the hydrogenation of crotonaldehyde
on a Sn/Pt(1 1 1) single crystal catalyst [27]. In the case of further
hydrogenation, a TOF of 0.13 molCHO
molsurface −1 s−1 for
Pt
reacted
cyclohexanone to cyclohexanol was calculated, whereas a TOF of
13 molCHEOL
molsurface −1 s−1 for cyclohexenol to cyclohex-
Pt
reacted
anol was determined. The value for cyclohexanone hydrogenation
is close to that observed in our previous study mentioned earlier
[10]. These findings clearly suggest that relative surface reaction

108
N.E. Musselwhite et al. / Applied Catalysis A: General 402 (2011) 104–109
Scheme 2. Possible adsorption modes for cyclohexenone on a Pt surface.
CH wagging modes. Similar results were also found for the 1.7 nm
and 3.6 nm Pt nanocatalysts; however, a much weaker band for
the C O stretching mode was observed for the 7.1 nm Pt nanocata-
lysts. These spectroscopic results will be combined with the kinetic
results to develop a molecular-level picture of the factors influenc-
ing activity and selectivity for this catalytic reaction.
Reported findings from theoretical calculations suggest that
adsorption modes of the aldehydes play a role in influenc-
ing reaction selectivity towards the unsaturated alcohol versus
the saturated aldehyde [31,32]. For cyclohexenone three possi-
ble adsorption configurations may be considered, as depicted in
Scheme 2. The first adsorption mode, an ꢀ1(C O) configuration
shown in Scheme 2a, consists of the carbonyl group interacting
with the Pt nanocatalyst surface with the ring normal to the sur-
face. The second binding configuration, an ꢀ2(C C) configuration
shown in Scheme 2b, consists of the vinyl group interacting with
the Pt nanocatalyst surface with the ring normal to the surface. The
third adsorption mode, an ꢀ4(C O, C C) configuration shown in
Scheme 2c, consists of the carbonyl and vinyl groups interacting
with the Pt nanocatalyst surface with the ring parallel to the sur-
face. Using surface vibrational selection rules [33], the C O stretch
should only be visible in the ATR-IR spectrum in the configuration
shown Scheme 2a. Based on the ATR-IR data presented here, this
adsorption mode is likely present in higher concentrations on the
smaller sized nanocatalyst surfaces, whereas the other two modes
may be found in higher concentrations on the 7.1 nm nanocatalyst
surface. The configuration in Scheme 2b would almost certainly
lead to hydrogenation of the vinyl group, and since results from
the kinetic modeling show that the rate of C C hydrogenation is
much faster than the rate of C O hydrogenation, it is likely that only
the vinyl group would be hydrogenated in Scheme 2c. Any unsat-
urated alcohol that forms, then, would be the result of the reaction
of cyclohexenone while adsorbed in the configuration shown in
Scheme 2a, particularly since the kinetic modeling results indicate
this reaction step to be the slowest out of the four. The kinetic mod-
eling results also seem to suggest that any unsaturated alcohol
that forms would be immediately hydrogenated to the saturated
alcohol. Findings from previous studies showed that the addition
of a small amount of NaOH to the reaction mixture inhibits this
pathway during cinnamaldehyde hydrogenation, thereby increas-
ing unsaturated alcohol selectivity [34,35]. The lower selectivity of
cyclohexenol in the present work, then, may likely be the result of
further hydrogenation to form the saturated alcohol.
Fig. 4. ATR-IR spectrum of the carbonyl stretching region for cyclohexenone in the
presence of 2.9 nm Pt nanocatalysts capped with 29,000 g/mol PVP.
rates play a key role in determining reaction selectivity during the
hydrogenation of cyclohexenone.
In addition to a range of kinetic studies, spectroscopic experi-
ments were also carried out to verify preferred adsorption modes
for cyclohexenone on the Pt nanocatalyst surfaces. Displayed in
Fig. 4 is the carbonyl region of an ATR-IR spectrum of cyclohexenone
in the presence of 2.9 nm Pt nanocatalysts dispersed in water. Peak
assignments were made based on comparisons with literature data
[28,29] and vibrational modes calculated at the B3LYP level of the-
ory with the 6-31G basis set and water as the solvent using Gaussian
03 [30]. These peak assignments are summarized in Table 2. As
seen in the figure, there is a broad strong peak at 1651 cm−1, which
is assigned to the C O stretching and C C stretching modes. It is
likely that this band is the result of the overlap of the two, and
that the C O stretching mode is the dominant contribution. The
C
O stretching vibrational frequency is also likely red-shifted from
the value for the gas phase molecule, suggesting that the molecule
is interacting with the Pt nanocatalyst surface through the car-
bonyl group. The bands at 1458 and 1425 cm−1 are assigned to
CH2 scissoring modes, while the band at 1396 cm−1 is assigned to
Table 2
Experimentally observed vibrational frequencies in the 1750–1250 cm−1 region of
the ATR-IR spectrum for cyclohexenone in the presence of 2.9 nm colloidal platinum
nanocatalysts capped with 29,000 g/mol PVP and dispersed in water. Also included
are vibrational frequencies, calculated at the B3LYP level of theory with the 6-31G
basis set and water as the solvent, and normal mode assignments.
4. Conclusions
PVP-capped colloidal platinum nanocatalysts were synthe-
sized in the 1–10 nm size range and characterized using TEM.
The prepared nanocatalysts were active in the aqueous-phase
hydrogenation of cyclohexenone and displayed high selectivity
for the formation of cyclohexanone. Experiments in which the
temperature-dependence of the reaction rate was investigated
yielded similar activation energies for particle sizes of 2.9–7.1 nm,
indicating that the same reaction channel, cyclohexanone for-
mation, is favored on these nanocatalysts. Results from kinetic
ꢂ˜exp (cm−1
)
ꢂ˜calc (cm−1
)
Normal mode assignment
1651
1736
1679
1517
1485
1476
1424
C
C
O stretching
C stretching
CH2 scissoring
CH2 scissoring
CH2 scissoring
CH wagging
1458
1425
1396

N.E. Musselwhite et al. / Applied Catalysis A: General 402 (2011) 104–109
109
modeling of the experimental data provide further evidence for
this observation. Still, smaller sized nanocatalysts were more selec-
tive for unsaturated and saturated alcohol formation. Findings from
ATR-IR experiments clearly suggest that adsorbed configurations of
cyclohexenone likely play a role in determining selectivity. Taken
together, these results may be used to enhance the knowledge of
designing environmentally friendly and efficient routes for manu-
facturing desired chemical commodities.
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Acknowledgments
This material is based upon work supported by the National Sci-
ence Foundation under Grant No. 0719160. The authors would like
to thank Mr. Roland Myers for his help in obtaining TEM images.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.05.033.
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