Establishing a Au nanoparticle size effect in the oxidation of cyclohexene using gradually changing Au catalysts

Article abstract of DOI:10.1021/cs400701j

The effect of the size of gold nanoparticles on their catalytic activity in aerobic oxidation of cyclohexene was established using supported gold nanoparticles that gradually undergo a change in size during the catalytic reaction. Two triphenylphosphine-s

Full text of DOI:10.1021/cs400701j

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Article
Establishing Au nanoparticle size effect in the oxidation
of cyclohexene using gradually changing Au catalysts
Baira Donoeva, Daniil Ovoshchnikov, and Vladimir B. Golovko
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 06 Nov 2013
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Establishing Au nanoparticle size effect in the
oxidation of cyclohexene using gradually changing
Au catalysts
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†
§‡
†‡
†§*
Baira G. Donoeva, Daniil S. Ovoshchnikov, Vladimir B. Golovko
†
Department of Chemistry, University of Canterbury, 20 Kirkwood Ave, Christchurch 8041,
New Zealand
§
The MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand
KEYWORDS Gold nanoparticles, gold clusters, oxidation, catalysis, size effect.
ABSTRACT The effect of gold nanoparticles’ size on their catalytic activity in aerobic oxidation
of cyclohexene was established using supported gold nanoparticles that gradually change their
size during catalytic reaction. Two triphenylphosphineꢀstabilized clusters, Au (PPh ) (NO ) and
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3 3
Au (PPh ) Cl , were synthesized and deposited on SiO . The clusters did not retain their
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structure during the catalytic reaction – larger particles with mean diameters of ca. 5ꢀ10 nm
gradually formed. By combining kinetic experiments with the monitoring catalyst
transformations using TEM, diffuseꢀreflectance UVꢀvisible spectroscopy and XPS, we showed
0
that catalytic activity appeared only after Au particles larger than 2 nm had formed, while intact
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clusters and phosphineꢀfree subꢀ2 nm particles were inactive in cyclohexene oxidation under the
studied conditions.
INTRODUCTION
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Gold was long considered chemically inactive, until Haruta and Hutchings discovered
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remarkable catalytic properties of gold nanoparticles in their pioneering works. Since that
discovery, gold nanoparticles were shown to be active in various reactions, including lowꢀ
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temperature CO oxidation, CꢀC bond formation, selective hydrogenation and oxidation, etc.
One of the crucial parameters determining the catalytic activity of gold is the size of gold
particles, e.g. only particles with sizes below 5 nm are active in the lowꢀtemperature CO
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oxidation and their activity increases with decrease of the particle size. Attempts were also
made to establish particle size effects for the liquidꢀphase oxidation reactions catalyzed by
supported gold nanoparticles. However, opposite trends in the effects of Au particle size on their
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catalytic activity in the same reactions are frequently reported.
Typically, particle size effects
are studied by comparing catalytic activities within a series of supported gold nanoparticles with
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different mean diameters which are assumed to be constant during the reaction.
Gold
particles within such series are often prepared using different wet chemistry methods, which
could affect the catalytic activity thus obscuring the effect of the particle size. Such influence
could be eliminated by employing gold particles that gradually change their size during the
catalytic reaction. Because catalytic activity can be observed only when active sites are present,
simultaneous monitoring of the catalytic activity and the state of catalyst in such gradually
changing system could provide insight regarding the optimal catalyst morphology. In this work,
we employed silicaꢀsupported phosphineꢀstabilized Au clusters with initial sizes below 2 nm and
studied their activity in aerobic oxidation of cyclohexene used as a model reaction. We found
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that the clusters were unstable during the reaction and gradually agglomerated to form larger
particles. By combining the investigation of cyclohexene oxidation kinetics with the monitoring
catalyst transformations we established an Au nanoparticles size effect on their catalytic activity.
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RESULTS AND DISCUSSION
Triphenylphosphineꢀstabilized gold cluster, Au (PPh ) (NO ) (Au ), and gold
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nanoparticles with an estimated composition of Au (PPh ) Cl (Au ), with mean gold core
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diameters of 0.9 and 1.5 nm, respectively, were immobilized on SiO . The catalysts were
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denoted by xAu /SiO , where x indicates Au loading (wt%). The catalysts were tested in the
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aerobic solventꢀfree oxidation of cyclohexene at 65 ˚C. Four major products formed during the
reaction: cyclohexene oxide, 2ꢀcyclohexeneꢀ1ꢀol, 2ꢀcyclohexeneꢀ1ꢀone and cyclohexenyl
hydroperoxide with the latter being the main product. We observed an induction period in the
oxidation of cyclohexene, catalyzed by as made Au /SiO . The length of the induction period
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depended on the Au loading of the catalyst (Figure 1A). The decrease of the Au loading of
Au /SiO from 0.5 to 0.02 wt% led to the elongation of the induction period from ca. 1 to 6.5 h.
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The length of the induction period also depended on the type of Au cluster: 0.1Au /SiO
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showed a significantly shorter induction period of 50 minutes compared with 3.5 hours for
.1Au /SiO (Figure 1B).
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Figure 1. Cyclohexene oxidation catalyzed by A) Au /SiO with gold loadings of 0.5, 0.1 and
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0.02% and B) 0.1Au /SiO and 0.1Au /SiO . Conditions: solventꢀfree cyclohexene, 10 mL, O
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atm, 65 °C, catalyst 0.1 g.
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Change of the Au particle size during the reaction was monitored using TEM and diffuseꢀ
reflectance UVꢀvisible spectroscopy (DR UVꢀvis). TEM micrographs of 0.5Au /SiO sampled
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from the reaction after 0.5, 1 and 16 h are shown in Figures 2BꢀD. Intact Au clusters of as made
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.5Au /SiO are not visible on brightꢀfield TEM images because of the poor contrast for the
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supported Au clusters smaller than 1 nm (Figure 2A). During the reaction larger particles,
detectable by TEM, gradually form and their number increases, as evidenced by the increase of
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the surface density of visible particles: ca. 40, 170 and 370 particles/ꢁm were detected in TEM
micrographs of 0.5Au /SiO sampled from the reaction after 0.5, 1 and 16 h, respectively. The
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mean diameter of detectable particles was 3.1 ± 1.2 nm after 1 h of reaction and it increased
further to 5.4 ± 1.4 nm during the next 15 h. The development of the particle size as a function of
reaction time for 0.5Au /SiO is shown in Figure S7.
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Figure 2. 1) TEM micrographs of 0.5Au /SiO as made (A) and sampled from the reaction after
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0.5 h (B), 1 h (C) and 16 h (D); 2) DR UVꢀvis spectra of 0.5Au /SiO (E), 0.1Au /SiO (F) and
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.02Au /SiO (G) sampled at reaction times indicated along the rightꢀhand ordinates.
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DR UVꢀvis study of 0.5Au /SiO showed the appearance of the surface plasmon
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resonance (SPR) band at 520 nm for samples of catalyst recovered after 1 h of reaction. The
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appearance of SPR band indicates the formation of gold particles larger than 2 nm (Figure 2E).
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Hence, both TEM and DR UVꢀvis studies of 0.5Au /SiO show the similar time of formation
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of particles larger than 2 nm. DR UVꢀvis studies of 0.1Au /SiO and 0.02Au /SiO showed that
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particles larger than 2 nm form only after ca. 4 and 6.5 h, respectively. Slower gold
agglomeration for catalysts with lower Au loadings is most likely due to the lower density of Au
particles on SiO surface. As seen from the kinetics of cyclohexene oxidation (Figure 1A) and
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DR UVꢀvis data, in all cases formation of sufficient number of particles larger than 2 nm was
accompanied with the appearance of catalytic activity, which indicates that such particles are
active in cyclohexene oxidation. The absence of an induction period for Au /SiO , recycled after
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6 h catalytic run, supports this conclusion (Figure 3).
Figure 3. Cyclohexene oxidation catalyzed by as made, recycled and preꢀcalcined 0.1Au /SiO .
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Importantly, the cyclohexene used for catalytic oxidation did not contain 2,6ꢀdiꢀtertꢀ
butylꢀ4ꢀmethylphenol, which is typically added as a stabilizer by chemical suppliers. When we
used cyclohexene which contained the stabilizer, the Au clusters did not agglomerate after 16
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hours under typical reaction conditions (65 °C, 1 atm O ) and no cyclohexene conversion was
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observed. Stirring 0.5Au /SiO for 16 hours under typical reaction conditions, but in nꢀhexane
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instead of cyclohexene, also did not lead to cluster agglomeration. These results imply that
thermal treatment at 65 °C alone is not enough to induce agglomeration of Au supported on
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SiO . Recently, Kilmartin et al. reported the removal of phosphine ligands from silica supported
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Au (Ph P(oꢀtolyl)) (NO ) by tertꢀbutyl hydroperoxide. We suggest that in our case phosphine
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ligands were removed from the cluster via reaction with cyclohexenyl hydroperoxide (CyOOH),
which is present in trace amounts in the stabilizerꢀfree cyclohexene (ca. 0.015 wt% by GC).
Agglomeration did not occur in nꢀhexane or stabilized cyclohexene because neither of them
contained even traces of peroxides. Thus, cluster agglomeration is possible only when PPh3
ligands are removed from the gold core of Au . An indication of such stepꢀbyꢀstep in situ
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transformation of gold clusters, which includes ligand removal from the cluster cores and
subsequent particle agglomeration, can also be seen in Figure 2F. DRꢀUVꢀvis spectra of catalyst
sampled in the initial stages of the reaction possessed a weak band at 450 nm, originating from
intact Au clusters (Figure S1C). The features in the optical spectra of small phosphineꢀstabilized
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gold clusters are known to be due to the effects of the ligand shell bound to the gold core of the
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cluster. Therefore, the disappearance of the band at 450 nm before the SPR band emerges
indicates that agglomeration occurs after the ligand shell is at least partially removed. Because
sintering is not possible when ligands are still bound to the gold cores of the clusters, we
conclude that particles forming during the reaction are predominantly phosphineꢀfree.
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Pure Au clusters lose phosphine ligands upon calcination at 230°C for 40 min in Ar flow
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according to TGA (Figure S3). Calcination of 0.1Au /SiO under the same conditions in order to
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remove phosphine ligands from supported Au led to the formation of 8.0 ± 2.6 nm particles
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(
Figure S10). Catalyst treated this way showed a cyclohexene conversion profile similar to that
of the recycled 0.1Au /SiO (particle size 9.6 ± 3.9 nm, SI), which indicates that phosphineꢀfree
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particles of similar size and larger than 2 nm obtained in different ways have similar activity in
the reaction (Figure 3).
Inasmuch as two catalyst parameters change during the reaction, i.e. Au particle size and
the presence of phosphine ligands, the initial inactivity of the as made catalysts could be due to
one of the following: a) stabilizing ligands cover active gold sites, thus hindering the catalytic
activity; once the phosphine ligands are removed, the particles become active b) subꢀ2 nm gold
particles, even without phosphine ligands, are inactive in cyclohexene oxidation and activity
appears only after the particle size reaches a certain threshold. Results of our experiments
indicate that removal of phosphine ligands is not a sufficient condition for the appearance of
catalytic activity and particle size plays a significant role:
a) As seen from Figure 2F, the PPh ligand shell was at least partially removed from Au after
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hours, as indicated by disappearance of the band at 450 nm, but no activity was observed until
particles larger than 2 nm had formed after ca. 4 hours of reaction (Figure 1A).
b) Because the core of Au101 is larger than that of Au , gold particles derived from Au need
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significantly less time to overcome the threshold in particle size, which leads to shorter induction
period for Au101 catalyst (Figure 1B).
To confirm that small (subꢀ2 nm) and phosphineꢀfree Au particles are inactive in
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cyclohexene oxidation, we deposited Au on SBAꢀ15, a mesoporous SiO ꢀbased material with
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large surface area (see SI for details). We used a mixture of ethanol and dichloromethane to
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ensure homogeneous distribution of clusters throughout the surface of the support. To remove
phosphine ligands from the cluster core, the catalyst was calcined at 230 °C for 40 min in Ar
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flow (denoted as 0.1Au /SBA_c230). The large surface area of the support and the homogeneous
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distribution of clusters prevented particles from sintering during calcination, as evidenced by the
absence of the SPR band in the DR UVꢀvis spectrum of 0.1Au /SBA_c230 (Figure 4). On the
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other hand, the absence of absorption bands at 450, 380 and 315 nm, characteristic for the Au9
cluster, indicates successful removal of ligands during calcination. TEM of 0.1Au /SBA_c230
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also confirmed very minimal agglomeration – only a few particles of 2ꢀ3 nm were detected,
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which correlates with the results obtained for Au11 clusters in the similar system. Thus,
phosphineꢀfree subꢀ2 nm particles supported on SBAꢀ15 were obtained.
Figure 4. UVꢀvis spectrum of Au dissolved in CH Cl (a) and DR UVꢀvis spectra of as made
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.1Au /SBAꢀ15 (b) and 0.1Au /SBA_c230 (c).
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Although the Au particles were phosphineꢀfree, the kinetics of cyclohexene oxidation in
the presence of 0.1Au /SBA_c230 showed an induction period of ca. 3.5 hours. The appearance
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of catalytic activity correlated with the formation of plasmonic particles (Figure 5), similarly to
the behavior observed for SiO ꢀbased catalysts, discussed earlier. Recycled 0.1Au /SBA_c230
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did not show an induction period in cyclohexene oxidation. This result confirmed that
phosphineꢀfree Au particles with the sizes below 2 nm are inactive in the cyclohexene oxidation.
The change of catalytic behavior of Au particles once they become larger than 2 nm could be due
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to the transition from nonꢀmetallic to metallic properties, which occurs at sizes of ca. 2 nm.
Figure 5. A) Cyclohexene oxidation catalyzed by 0.1Au /SBA_c230 (black) and recycled
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.1Au /SBA_c230 (red). B) DR UVꢀvis spectra of 0.1Au /SBA_c230 sampled from cyclohexene
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oxidation at different reaction times.
Numerous examples of catalysis, driven or enhanced by the surface plasmon resonance of
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Au nanoparticles, have been recently reported.
Because catalytic activity in our study
appeared only when gold nanoparticles larger than 2 nm, i.e. plasmonic particles, were formed,
we investigated whether the observed activity was enabled by the SPR. The activity of
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.5Au /SiO in cyclohexene oxidation under ambient light was similar to that in the absence of
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light, which indicated that catalytic activity was not enhanced by the SPR in this study (Table
S2).
XPS spectrum of a postꢀreaction 0.5Au /SiO showed the Au4f signal centered at 84.0
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eV, a binding energy characteristic of bulk neutral gold (Figure 6). No signal at 85.0 eV from
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pristine Au clusters was detected in the postꢀreaction catalyst, indicating that most of the gold
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present in the catalytic system is in the metallic state and no intact clusters are left after the
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reaction. Unlike the recently reported activity of positively charged 20ꢀ150 nm Au nanoparticles
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in styrene oxidation, catalytic activity in our case was solely due to the metallic Au particles,
as evidenced by the activity of the postꢀreaction catalysts in the second catalytic cycle.
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Figure 6. XPS Au4f7/2 spectra of as made 0.5Au /SiO and 0.5Au /SiO recovered after 16 h
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catalytic cycle of cyclohexene oxidation. Dotted vertical line indicates Au4f7/2 peak position
corresponding to metallic gold.
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Finally, colloid stabilizerꢀfree
and citrateꢀstabilized
Au nanoparticles were
synthesized and deposited onto SiO with the total gold loadings of 0.1 wt%. Four catalysts
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0.1Auꢀ9.1/SiO , 0.1Auꢀ13.7/SiO , 0.1Auꢀ33.9/SiO and 0.1Auꢀ47.4/SiO with particle sizes of
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9.1, 13.7, 33.9 and 47.4 nm, respectively, were obtained (Figure S11). All the catalysts were
active in cyclohexene oxidation and no induction period was observed (Figure 7). Comparison of
initial reaction rates and turnover frequencies (TOFs) of the Au colloidꢀbased catalysts is shown
in Table 1. Catalytic activity of 0.1Au/SiO gradually decreased with the increase of Au particle
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size, which is in accordance with the smaller fraction of surface Au atoms for larger particles.
Indeed, TOF values normalized by surface Au are very close for differently sized supported Au
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nanoparticles (Table 1). This result confirms that Au particles larger than 2 nm are responsible
for the observed catalytic activity in the oxidation of cyclohexene.
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Figure 7. Cyclohexene oxidation catalyzed by 9.1, 13.7, 33.9 and 47.4 nm Au particles
supported on SiO with the total gold loading of 0.1 wt%.
2
Table 1. Comparison of initial reaction rates and TOFs of Au/SiO catalysts in
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cyclohexene oxidation.
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ꢀ1
ꢀ1
Catalyst
Reaction rate × 10 ,
TOF , s
TOFsurface Au, s
Au
molsubstrate/s
a
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2
3
4
0.1Auꢀ9.1/SiO2
0.1Auꢀ13.7/SiO2
0.1Auꢀ33.9/SiO2
0.1Auꢀ47.4/SiO2
0.58
1.14
0.78
0.53
17.5
a
0.39
18.0
b
0.28
21.7
b
0.20
0.40
18.9
a
b
Fraction of surface Au atoms was estimated assuming spherical and cuboctahedral
shape of Au nanoparticles (based on TEM observations).
0
In this work we showed that only Au particles larger than 2 nm were active in aerobic
cyclohexene oxidation; however oxygen adsorption and its dissociative activation on large
3
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metallic gold particles were shown to be extremely impeded.
On the other hand, extended
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gold surfaces were long known to catalyze hydrogen abstraction from hydrocarbons, and thus
activation of cyclohexene rather than O by gold nanoparticles is the more probable pathway in
2
our case. This suggestion is also supported by the fact that the main product of cyclohexene
oxidation in this work is cyclohexenyl hydroperoxide (Figure S12), which is formed in the
reaction between dissolved oxygen and cyclohexenyl radicals generated via allylic hydrogen
abstraction from cyclohexene. When a sufficient amount of cyclohexenyl hydroperoxide is
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formed it further acts as a radical chain propagator in cyclohexene autooxidation. A similar
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mechanism was recently described for cyclohexane oxidation catalyzed by Au/MgO. Higher
12,37,38
activity of Au catalysts in cyclohexene oxidation compared to cyclohexane oxidation
can
be explained by more facile Hꢀabstraction from an allylic position because allylic CꢀH is weaker
than alkylic CꢀH bond.
Thus, we showed that at low temperatures subꢀ2 nm particles are not able to catalyze
hydrogen abstraction from the cyclohexene and metallic Au particles larger than 2 nm are the
active sites associated with the observed catalytic activity. Such a size effect is opposite to that
discovered for oxidation reactions where O activation is involved, e.g. styrene oxidation, for
2
1
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11
which either subꢀ2 nm particles
active sites.
or large, positively charged particles were suggested as the
In summary, we established relationship between the size of supported gold nanoparticles
and their activity in solventꢀfree aerobic oxidation of cyclohexene by employing gradually
changing catalysts. We showed that phosphineꢀstabilized gold clusters and phosphineꢀfree Au
particles smaller than 2 nm are inactive in this reaction, and that catalytic activity appears only
upon formation of sufficient number of metallic particles larger than 2 nm. Further increase in
Au particle size results in gradual decrease in catalytic activity, which correlates with the
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reduction of the Au surface area. The sizeꢀdependency observed in this study is in agreement
with the suggested mechanism of substrate activation through the abstraction of hydrogen
catalyzed by metallic gold nanoparticles.
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EXPERIMENTAL SECTION
Materials
Gold (99.99%), sodium borohydride (>97%), hydrochloric acid (37%, AR grade), nitric
acid (65%, AR grade), triphenylphosphine (98%), Pluronic P123 (Mn ~5,800), tetraethyl
orthosilicate (98%), cyclohexene (99%, inhibitorꢀfree), 2ꢀcyclohexeneꢀ1ꢀol (95%), 2ꢀ
cyclohexeneꢀ1ꢀone (98%) and cyclohexene oxide (98%) were purchased from SigmaꢀAldrich
and Merck. Oxygen (99.7%) was obtained from BOC gases. Silicon oxide (Aerosil OX 50) was
purchased from Evonik. All materials were used as received.
Catalyst preparation
Au (PPh ) (NO ) (Au ) and Au (PPh ) Cl (Au ) were synthesized according to the
9
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3 3
9
101
3 21
5
101
3
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published procedures.
As made gold clusters were deposited onto SiO from CH Cl solution.
2 2 2
A calculated amount of gold cluster dissolved in CH Cl (10 mL, 1ꢀ2 mg/mL) was added dropꢀ
2
2
wise to a vigorously stirred suspension of SiO (500 mg) in CH Cl (15 mL). The mixture was
2
2
2
stirred for 30 min and the solid was collected by centrifugation. Colorless supernatant solution
confirmed complete cluster deposition. The catalysts were washed with CH Cl (20 mL) and
2
2
dried under vacuum at room temperature. Deposition of Au cluster onto SBAꢀ15 was performed
9
41
using the methodology similar to the one described by Liu et al. SBAꢀ15 (500 mg) was
suspended in the mixture of C H OH and CH Cl (5 mL, 1:4). Solution of Au cluster (2.3 mg)
2
5
2
2
9
in the same solvent mixture (5 mL) was slowly added to the suspension of SBAꢀ15 under
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vigorous stirring. The mixture was stirred for 2.5 h. The solid was collected via centrifugation,
washed with CH Cl (10 mL) and dried under vacuum at room temperature.
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Stabilizerꢀfree gold nanoparticles were synthesized using method reported by Martin et
al. After deposition onto SiO the average diameter of Au particles was 9.1 nm. Citrateꢀ
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stabilized Au nanoparticles were prepared following a protocol described by Turkevich et al.
Briefly, 450 mL of 0.24 mM HAuCl and 1.6 mM sodium citrate solution was heated at 70 ˚C
4
for 1 hour. The resulting red solution containing 13.9 nm Au particles was cooled down using ice
bath. 33.9 nm particles were prepared by heating 50 mL of 0.3 mM HAuCl and 0.58 mM
4
sodium citrate solution at 90 ˚C for 5 minutes. 47.4 nm particles were prepared by heating 50 mL
1
.2 mM HAuCl and 1.9 mM sodium citrate at 60 ˚C for 2 hours. The resulting colloid Au
4
particles were deposited on SiO from aqueous solutions. A calculated volume of fresh Au
2
nanoparticles solution in water was added to SiO (0.5 g). Mixture was sonicated for 1 minute
2
and water was slowly removed from the suspension using rotary evaporator. The resulting solid
was collected, dried under vacuum at room temperature and characterized using TEM.
Catalyst characterization
Catalysts were characterized using transition electron microscopy (TEM), Xꢀray
photoelectron spectroscopy (XPS), diffuseꢀreflectance UVꢀvis (DR UVꢀvis). Gold loadings were
established using atomic absorption spectroscopy (AAS) on a VARIAN SpectrAA 220FS. TEM
analysis was performed using Philips CM200 operating at 200 kV. Samples for TEM analysis
were suspended in diethyl ether and deposited on holeyꢀcarbon coated Cu grids. At least 100
particles were measured to plot size distributions. DR UVꢀvis spectra were recorded using Cintra
404 (GBC Scientific Equipment) spectrometer. XPS was conducted on a Kratos Axis DLD
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spectrometer with a monochromated AlꢀKα Xꢀray source. The samples were pressed into indium
as the mounting medium. A supply of low energy electrons was used for charge neutralization.
Survey spectra were recorded with a pass energy of 80 eV and highꢀresolution spectra with a
pass energy of 40 eV. Binding energies were normalized with respect to the position of the
adventitious C1s peak at 285.0 eV.
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Catalytic experiments
Catalytic activity of Au catalysts was tested in aerobic solventꢀfree cyclohexene
oxidation without addition of radical initiator. Kinetic studies of cyclohexene oxidation were
performed in a glass reactor equipped with a reflux condenser. 10 mL of cyclohexene containing
0.2 M of nꢀdecane as an internal standard, 100 mg of catalyst and Teflonꢀcoated magnetic stirrer
were loaded into 100 mL 2ꢀnecked glass round bottom flask. System was flushed with O three
2
times and connected to a rubber balloon filled with O . The mixture was magnetically stirred at
2
65 °C. Samples of reaction mixture were taken using glass syringe. Reaction was stopped by
cooling the reactor to room temperature; condenser was rinsed with 5 mL of diethyl ether and the
reaction mixture was separated from the solid catalyst by centrifugation. Catalysts were washed
with diethyl ether and dried under vacuum at room temperature before recycling. Each
experiment was reproduced at least three times. The typical standard errors of independent
catalytic tests were below 1.5 %. Turnover frequencies were calculated from the initial rates of
cyclohexene oxidation.
The liquid samples were analyzed by gas chromatography (GCꢀFID) using Shimadzu
GCꢀ2010 equipped with an Rxiꢀ5SilMS capillary column (30 m × 0.25 mm × 0.25 ꢁm). The
products of cyclohexene oxidation were identified by GCꢀMS (Shimadzu GCMSꢀQP2010) and
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quantified using solutions of reference compounds with known concentrations. The
4
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concentration of cyclohexenyl hydroperoxide was established using iodometric titration.
Addition of PPh to the reaction mixture prior to GC analysis did not change the concentration of
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2ꢀcyclohexeneꢀ1ꢀone, determined from GC analysis, which implies that there was no
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cyclohexenyl hydroperoxide decomposition during GC analysis.
Concentration of
cyclohexenyl hydroperoxide was calibrated by comparison of GC data and iodometric titration
results and was further calculated directly from GC results.
ASSOCIATED CONTENT
Supporting Information.
Cluster and catalyst characterization details, additional catalytic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: vladimir.golovko@canterbury.ac.nz
Author Contributions
These authors contributed equally
‡
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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We would like to thank Dr Colin Doyle from RCSMS for XPS analysis; Campbell McNicoll and
Dr Tim Kemmitt from Callaghan Innovation NZ for their help with surface area measurements.
We thank Prof Bryce Williamson for valuable discussions. This work was supported by the
University of Canterbury and the MacDiarmid Institute. B.D. thanks the MacDiarmid Institute
for the research scholarship and D.O. thanks the University of Canterbury for UC Doctoral
scholarship.
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