Influence of Dioxygen on the Promotional Effect of Bi during Pt-Catalyzed Oxidation of 1,6-Hexanediol

Article abstract of DOI:10.1021/acscatal.6b00972

A series of carbon-supported, Bi-promoted Pt catalysts with various Bi/Pt atomic ratios was prepared by selectively depositing Bi on Pt nanoparticles. The catalysts were evaluated for 1,6-hexanediol oxidation activity in aqueous solvent under different dioxygen pressures. The rate of diol oxidation on the basis of Pt loading over a Bi-promoted catalyst was 3 times faster than that of an unpromoted Pt catalyst under 0.02 MPa of O₂, whereas the unpromoted catalyst was more active than the promoted catalyst under 1 MPa of O₂. After liquid-phase catalyst pretreatment and 1,6-hexanediol oxidation, migration of Bi on the carbon support was observed. The reaction order in O₂ was 0 over Bi-promoted Pt/C in comparison to 0.75 over unpromoted Pt/C in the range of 0.02-0.2 MPa of O₂. Under low O₂ pressure, rate measurements in D₂O instead of H₂O solvent revealed a moderate kinetic isotope effect (rateH₂O/rateD₂O) on 1,6-hexanediol oxidation over Pt/C (KIE = 1.4), whereas a negligible effect was observed on Bi-Pt/C (KIE = 0.9), indicating that the promotional effect of Bi could be related to the formation of surface hydroxyl groups from the reaction of dioxygen and water. No significant change in product distribution or catalyst stability was observed with Bi promotion, regardless of the dioxygen pressure.

Full text of DOI:10.1021/acscatal.6b00972

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Article
Influence of Dioxygen on the Promotional Effect of
Bi during Pt-catalyzed Oxidation of 1,6-Hexanediol
Jiahan Xie, Benjamin Huang, Kehua Yin, Hien N. Pham,
Raymond R. Unocic, Abhaya K. Datye, and Robert J Davis
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00972 • Publication Date (Web): 24 May 2016
Downloaded from http://pubs.acs.org on June 4, 2016
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Influence of Dioxygen on the Promotional Effect of Bi
during Pt-catalyzed Oxidation of 1,6-Hexanediol
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a
a
a
b
Jiahan Xie , Benjamin Huang , Kehua Yin , Hien N. Pham ,
c
b
a, *
Raymond R. Unocic , Abhaya K. Datye , Robert J. Davis
Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, PO Box
00741, Charlottesville, VA 22904-4741, United States
4
a: Department of Chemical Engineering, Charlottesville, University of Virginia, VA
22904-4741, United States
b: Department of Chemical Engineering and Center for Microengineered Materials,
University of New Mexico, Albuquerque, NM 87131, United States
c: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
TN 37831, United States
*Author to whom correspondence is addressed
Phone: 1 - 434 - 924 - 6284
Email: rjd4f@virginia.edu
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Abstract:
A series of carbon-supported, Bi-promoted Pt catalysts with various Bi/Pt atomic
ratios was prepared by selectively depositing Bi on Pt nanoparticles. The catalysts were
evaluated for 1,6-hexanediol oxidation activity in aqueous solvent under different dioxygen
pressures. The rate of diol oxidation based on Pt loading over a Bi-promoted catalyst was
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three times faster than an unpromoted Pt catalyst in 0.02 MPa O
catalyst was more active than the promoted catalyst in 1 MPa O
catalyst pretreatment and 1,6-hexanediol oxidation, migration of Bi on the carbon support
was observed. The reaction order in O was zero over Bi-promoted Pt/C compared to 0.75
over unpromoted Pt/C in the range of 0.02 – 0.2 MPa O . Under low O pressure, rate
measurements in D O instead of H O solvent revealed a moderate kinetic isotope effect
푟푎푡푒 /푟푎푡푒 ) on 1,6-hexanediol oxidation over Pt/C (KIE = 1.4) whereas a negligible
2
, whereas the unpromoted
2
. After liquid-phase
2
2
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2
2
(
퐻 푂
2
퐷 푂
2
effect was observed on Bi-Pt/C (KIE = 0.9), indicating the promotional effect of Bi could
be related to the formation of surface hydroxyl groups from reaction of dioxygen and water.
No significant change of product distribution or catalyst stability was observed with Bi
promotion, regardless of the dioxygen pressure.
Key Words:
Pt; Bi; Heterogeneous catalysts; Alcohol oxidation; Catalyst restructuring; Promotional
effect; Dioxygen pressure; Isotope effect;
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1
. Introduction
Selective oxidation of alcohols to carbonyl compounds is an important reaction in
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organic synthesis1,2 and will likely play a significant role in the utilization of biomass
resources.3 In particular, the oxidation of biomass-derived alcohols such as
5-hydroxymethylfurfural (HMF), glycerol and 1,6-hexanediol (HDO), using supported
metal catalysts and dioxygen has received attention because it offers a green and
sustainable path to value-added chemicals.2,4–18 Among supported metal catalysts, Au has
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demonstrated the highest activity for alcohol oxidation under basic conditions. The use of
base, however, is not attractive because of the cost and waste involved in neutralizing the
product stream to recover the free acid. In contrast, alcohol oxidation over Pt does not
require high concentrations of base.15,19 Unfortunately, severe deactivation was observed
during the oxidation of HDO over Pt catalysts.20
Previous work has shown that the addition of Bi can promote the catalytic
performance of Pt catalysts. Bismuth-promoted Pt catalysts have demonstrated enhanced
rate, stability and/or selectivity, compared to monometallic Pt catalysts.21–33 For example,
enhancements in activity by a factor of 1.5 to 66 have been reported for the oxidation of
various primary alcohols, secondary alcohols and polyols over Bi-promoted Pt
catalysts.21,23–28 Addition of Bi to Pt has also been claimed to make a catalyst more resistant
to deactivation.25,26,34 During glycerol oxidation over Pt, the presence of Bi promoted the
oxidation of the secondary hydroxyl group.21,30,35 Based on a kinetic model proposed by
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Wörz et al., the theoretical yield of dihydroxyacetone (DHA) can be as high as 70%, but
only 4% DHA selectivity was obtained at 37% conversion over unpromoted Pt/C.36
The promotional effects of Bi are often attributed to preventing the over-oxidation of
Pt and the leaching of Pt into solution,1,21,23,25,26,37 suppressing the production and
adsorption of poisoning species,1,25,28,38–40 favoring the formation of adsorbed hydroxyl
species23,25–27,38,41,42 and forming complexes of Bi with reaction intermediates.21,27,35 Mallat
et al. used electrochemical methods to investigate the in-situ catalyst potential of Bi-Pt and
Pt catalysts during the oxidation of various alcohols and found that the Pt in the
Bi-promoted catalyst remained reduced under conditions that would oxidize an unpromoted
Pt catalyst.26–28 Results from in-situ X-ray absorption spectroscopy (XAS) further
confirmed that addition of Bi helped maintain Pt in a metallic state.23 The enhanced
stability of a Bi-Pt catalyst was also attributed to small ensembles of Pt atoms in the
promoted catalyst,23–28 which suppressed the formation and adsorption of hydrocarbon
fragments and CO generated from C-C cleavage.23,25–27,38,40 Another role of Bi might be to
form new active centers. Using a laser-induced temperature jump method, Feliu et al.
studied the interaction between water and the Bi-Pt surface and proposed that Bi might
induce a surface dipole that leads to a reorientation of water and favors the formation of
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surface hydroxyl groups.41,42 The presence of surface hydroxyl groups might participate in
the activation of alcohol, and the removal of adsorbed remnants on the surface,10,15,18 thus
promoting the activity and stability of the alcohol oxidation reaction.24–27,38,43,44 During
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polyol oxidation, Bi has been proposed to form complexes with the substrate and promote
the selective oxidation of the secondary hydroxyl group, altering the product distribution
compared to an unpromoted Pt catalyst.21,30,35
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One potential complication with investigating the promotional effects of
multi-component catalysts is restructuring under reaction conditions. In fact, changes in the
structure of PtM (M = Rh,45,46 Ru, Pd,
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Co,50,51 Au and Sn53–55) bimetallic catalysts
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have been reported during reaction, as revealed by in-situ or ex-situ characterization using
X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), sum
frequency generation vibrational spectroscopy (SFG), (scanning) transmission electron
microscopy ((S)TEM) and infrared spectroscopy (IR). Although leaching of Pt and Bi
from supported Bi-Pt catalysts was not detected under alcohol oxidation conditions,21,27 the
structure of Bi-Pt particles under reaction conditions is still a matter of debate. Therefore, it
is necessary to examine the influence of the catalytic reaction on catalyst structure to
elucidate the promotional effect of Bi on Pt oxidation catalysts.
In this study, a series of carbon-supported, Bi-promoted Pt catalysts was prepared and
evaluated in the oxidation of HDO in liquid water under different dioxygen pressures
without the addition of base. Extensive characterization of as-prepared and used catalysts
was performed in an attempt to correlate reactivity results to catalyst structure. In addition,
the influence of reaction conditions (HDO concentration, dioxygen pressure, temperature)
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and the kinetic isotope effect of deuterated oxide solvent were investigated over Pt/C and
Bi-Pt/C.
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. Experimental Methods
2.1.Catalyst preparation
The 2.69 wt% Pt/C (Sigma-Aldrich) catalyst was used as received. The Bi-promoted
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Pt catalysts were prepared by the selective deposition of Bi on Pt nanoparticles. First, 2 g
3
-1
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of Pt/C was pre-reduced by H
2
(GT&S 99.999%) flow (100 cm min ) in 250 cm distilled
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and deionized water stirred at 600 rpm at room temperature. After 30 min, 0.429 cm acetic
acid (Sigma-Aldrich, 99.9%) was added and an appropriate amount of Bi(NO
(Sigma-Aldrich, 99.999%) in 2 wt% aqueous acetic acid solution was dropped into the
slurry within 30 min under H flow. After waiting an additional 15 min, the catalysts were
3
)
3
∙5H O
2
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filtered, washed thoroughly with distilled and deionized water, and dried at room
temperature. The as-prepared catalysts were denoted as xBi-Pt/C, where x is the nominal
atomic ratio of Bi to Pt.
To study the effect of reaction conditions on xBi-Pt/C, a catalyst was recovered after
HDO oxidation at 328 K under 0.02 MPa O in 0.1 M HDO solution, washed with distilled
2
and deionized water and dried at room temperature. The used catalyst after reaction was
denoted as xBi-Pt/C-R. To study the effect of the pretreatment of the catalyst prior to
reaction, fresh xBi-Pt/C that was treated at 328 K under He atmosphere in 0.1 M HDO
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solution for 30 min was recovered, washed and dried at room temperature and was denoted
as xBi-Pt/C-P.
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2
.2.Catalyst characterization
The specific surface area was measured by N
2
sorption using a Micromeritics ASAP
sorption, after
2
020. The samples were evacuated for 600 min at 393 K prior to the N
2
which adsorption isotherms were measured at 77 K. The Brunauer–Emmett–Teller (BET)
method was used to determine the specific surface area. The Barret, Joyner and Halenda
(BJH) model was used to investigate the pore size distribution.
The fraction of available Pt on the surface of a catalyst was estimated by
H chemisorption using the Micromeritics ASAP 2020. The catalysts were heated to 473 K
2
−1
at 4 K min under flowing H (GTS Welco 99.999%) and reduced for 2 h. The samples
2
were then evacuated for 2 h at 473 K before being cooled to 308 K for analysis in the
pressure range of 1.33 – 59.9 kPa. The amount of exposed Pt was evaluated by the total
amount of H
2
adsorbed, extrapolated to zero pressure, assuming a stoichiometry (H/Ptsurf)
equal to unity.
The X-ray diffraction (XRD) patterns were recorded using a PANalytical X'Pert Pro
MPD (Multi-Purpose Diffractometer) instrument with Cu Kα radiation (45 kV, 40 mA) and
scanning of 2θ from 10° to 70° with a step size of 0.0025° at a rate of 1° min−1.
To estimate the metal particle sizes, scanning transmission electron microscopy
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(
STEM) was performed on an FEI Titan 80-300 operating at 300 kV. A sample was also
characterized with a JEOL 2010 F operating at 200 kV that is equipped with an Oxford
Aztec energy dispersive spectrometer (EDS) system for single nanoparticle EDS and with
an Hitachi HF-3300 that is equipped with a Bruker SDD-EDS system for elemental
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mapping. To prepare a sample, ∼1 mg of catalyst was suspended in 10 cm of ethanol by
agitating the mixture for 30 min in a sonication bath. A copper grid was dipped into the
solution to capture the particles and the ethanol was thoroughly evaporated before
microscopy.
Scanning electron microscopy (SEM) was performed on an FEI Quanta 650
FEG-SEM operated at 2 kV and a working distance of 10 mm under the secondary electron
imaging mode. The sample was prepared by adhering the catalyst powders to a copper
conductive tape on a sample stage.
The X-ray photoelectron spectroscopy (XPS) was performed utilizing a Thermo
Scientific ESCALAB 250 spectrometer equipped with a focused monochromatic Al Kα
X-ray radiation source (1486.6 eV) and a hemispherical analyzer with a 6-element
multichannel detector. The incident X-ray beam was 45° off normal to the sample while the
X-ray photoelectron detector was normal to the sample. A large area magnetic lens with a
5
00 µm spot size in constant analyzer energy mode was utilized with a pass energy of 20
eV for region scans. Charge compensation was employed during data collection with an
internal electron flood gun (2 eV) and a low energy external Ar ion flood gun. All spectra
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were deconvoluted with a curve fitting routine in CasaXPS software. The background was
corrected using the linear method and the binding energy of the C 1s peak assigned at 284.5
eV, which is attributed to the support, was used to reference the peak positions.56 The
integral of each peak was calculated after subtracting the background and fitting with a
combination of Gaussian and Lorentzian curves of variable proportion. The Pt:Bi surface
atomic ratios were calculated from peak areas normalized by atomic subshell
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photoionization cross sections of the corresponding element . The detailed XPS curve
fitting parameters are listed in Table S1.
Elemental analysis was used to determine some of the Pt and Bi weight loadings on
the catalysts and the concentration of leached Pt and Bi in the reaction solution after 4 h of
HDO oxidation. Elemental analysis was performed by Galbraith Laboratories (2323
Sycamore Drive, Knoxville, TN 37921) using inductively coupled plasma – atomic
emission spectroscopy (ICP-AES).
X-ray absorption spectroscopy at the Pt LIII and Bi LIII edges was performed with a
storage ring operating at 2.8 GeV and 300 mA. The (NH
3
)
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Pt(NO
3
) (Sigma-Aldrich,
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9.995%), Bi(NO (Sigma-Aldrich, 99.999%), Pt/C and Bi-Pt/C samples were ground
)
3 3
into fine powders and spread over Kapton tape. The XAS data were processed using the
Demeter software package.58
2
.3. 1,6-Hexanediol oxidation reaction
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The high-pressure, semi-batch alcohol oxidation reactions were performed in a
3
3
50 cm Parr Instrument Company 4592 batch reactor with a 30 cm glass liner. An aqueous
.1 M HDO solution (10 mL) and 0.0132 g catalyst were added to the glass liner. The glass
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liner was inserted into the reactor, sealed, purged with He (GTS Welco, 99.999%), heated
to 328 K, and held for 20 min. The reaction was initiated by pressurizing the reactor with
O
2
(GTS Welco, 99.993%) in the range of 0.2 – 1 MPa. In the low O
2
pressure region (0.02
–
0.1 MPa), the reactions were performed in a 100 mL flask reactor equipped with gas inlet
and outlet, reflux condenser, thermometer and magnetic stir bar. In a typical run, an
aqueous 0.1 M HDO solution (50 mL) and 0.0660 g catalyst were added to the reactor,
-1
heated to 328 K under He flow (100 mL min ) and held for 20 min. The reaction was
-1
initiated by the flow of a He and O
2
mixture with a total flow rate of 100 mL min . The O
2
pressure in the reactor was adjusted by varying the flow rates of He and O
2
. Liquid samples
were periodically removed and the catalyst was removed using a 0.2 μm PTFE filter before
product analysis with a Waters e2695 high performance liquid chromatograph (HPLC)
equipped with refractive index detector. Product separation in the HPLC was accomplished
on an Aminex HPX-87H column (Bio-Rad) operating at 318 K with an aqueous 5 mM
3
−1
H
2
SO solution as the mobile phase flowing at 5 cm min . The retention times and
4
calibration curves were determined by injecting known concentrations of standards. Carbon
balances were always greater than 90%.
The maximum O
2
transport rate from the gas phase to the aqueous phase in the
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pressurized reaction system was determined by the sulfite oxidation method. The amount
of catalyst added to the reactor was chosen so that the alcohol oxidation rate would not be
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limited by O mass transfer from the gas to the liquid. Selectivity to a specific product is
2
defined as moles of that product formed divided by moles of all products produced. The
initial turnover frequency (TOF) [mol alcohol converted (mol metalsurface)-1 s-1] was
calculated from the initial conversion of the alcohol, usually within the first 5 min of the
reaction. A sample was also taken at 10 min to confirm that significant deactivation had not
occurred in the initial rate determination. An estimate of surface Pt is needed to calculate a
turnover frequency because Bi is not active for oxidation catalysis. Since Bi is highly
mobile under reaction conditions (to be discussed later) whereas Pt nanoparticles are stable,
we normalized the initial rate of HDO conversion to the estimated fraction of Pt atoms that
can be exposed to the surface during reaction, which is determined from H
2
-chemisorption
of Pt/C before the Bi deposition.
3
3
3
. Results
.1. Characterization of the Bi-promoted Pt catalysts
.1.1. As-synthesized materials
During the preparation of Bi-Pt/C catalysts, the goal was to selectively deposit Bi on
carbon-supported Pt nanoparticles in the presence of H because Bi(III) cannot be reduced
2
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particles to form adatoms.60 To study the structure of Bi-Pt/C prepared in this work,
commercial Pt/C and Bi-promoted Pt/C were extensively characterized.
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The Pt/C and as-prepared 0.6Bi-Pt/C samples were examined by SEM and N sorption
2
to explore whether the carbon support was stable under conditions used for Bi
incorporation. As shown in Figures S1a and S1b, the size of the carbon support particles
was approximately the same for Pt/C and 0.6Bi-Pt/C. The majority of carbon particles were
below 15 μm while some larger particles (~25μm) were still observed. The similar surface
area and pore size distribution of Pt/C and 0.6Bi-Pt/C indicate that the porous structure of
the carbon support was maintained after the deposition of Bi on Pt/C.
The Pt/C and Bi-Pt/C catalysts were also characterized by XPS to investigate the
oxidation states and distributions of Pt and Bi throughout the carbon support particles. The
Pt 4f7/2 and Bi 4f7/2 spectra of 0.6Bi-Pt/C are shown Figure 1 and the fitting results are
summarized in Table 1. The average Bi:Pt atomic ratio from XPS (2.4) is much higher than
that from ICP-AES analysis (0.6) as shown in Table 2, suggesting an enrichment of Bi in
the surface region of the carbon support particles. The XPS analysis also revealed the
oxidation states of Pt and Bi. In the high vacuum chamber of the spectrometer, Pt was in the
metallic state, with the binding energy of Pt 4f7/2 at approximately 71 eV while the majority
of Bi (86 %) was present as Bi(III). Similar results from XPS were reported by Mallat et
al.27
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The XAS results of Pt/C and Bi-Pt/C are shown in Figure S2. The addition of Bi to the
Pt catalyst resulted in a decrease in the relative white line intensity at the Pt LIII edge, which
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was attributed to Bi species deposited on the Pt particles. In contrast to XPS, which was
measured in high vacuum, air-exposed Pt/C and 0.3Bi-Pt/C revealed Pt to be partially
oxidized, as supported by both a higher relative white line intensity at the Pt LIII edge
compared to a reference Pt foil (Figure S2a) and a Pt-O feature shown in the Fourier
transform of the extended X-ray absorption fine structure (EXAFS) (Figures S2b and S2c).
The oxidized Pt measured in XAS was most likely due to chemisorption of oxygen on the
Pt nanoparticles. As reported by Gracia et al., the fraction of oxidized Pt increased
significantly with the decreasing particle size under oxidizing conditions.61 Note that
although the EXAFS and the relative white line intensity revealed partially oxidized Pt in
Pt/C and 0.6Bi-Pt/C, the Pt LIII edge threshold energy of Pt/C and 0.6Bi-Pt/C was consistent
with the Pt metal foil and significantly shifted from the edge of (NH
3
)
4
Pt(NO
3
2
) in which Pt
is in the 2+ oxidation state (Figure S2a). The inconsistency of white line intensity and the
23
low edge energy was also reported elsewhere, which may be related to adsorbents on
nanoparticles, such as water, resulting in a shift of XANES to a lower energy.62
The metal particle sizes of Pt/C, 0.3Bi-Pt/C and 0.6Bi-Pt/C catalysts were determined
by STEM. The dark-field STEM images of carbon-supported Pt and Bi-Pt catalysts showed
a narrow size distribution of observed nanoparticles, with the majority of them in the range
of 0.5 – 3.0 nm (Figures S3 and S4). As summarized in Table 3, the number average
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diameter for catalysts with different Bi loading were similar (1.4 ± 0.5 nm for Pt/C and
0.3Bi-Pt/C; 1.5 ± 0.5 nm for 0.6Bi-Pt/C). The similar metal particle sizes in this study
eliminated size as a factor that might influence the observed catalytic performance. The
surface-weighted average diameters 63 were also similar for Pt/C and Bi-Pt/C (Table 3),
indicating that the fraction of exposed metal atoms for catalysts with different Bi loadings
was similar.
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The X-ray diffraction patterns of Pt/C and Bi-Pt/C catalysts are presented in Figure S5.
o
The peak at around 2θ = 25 was attributed to graphitic carbon (JCPDS 23-0064) while the
o
peak close to 2θ = 40 was attributed to the (111) reflection of Pt (JCPDS 04-802). No
significant shift in the diffraction patterns of Bi-Pt/C samples was observed compared to
the unpromoted Pt catalyst, suggesting that Pt-Bi alloys 64 were not likely formed under the
mild conditions used to prepare the Bi-promoted catalysts. The broad, low-intensity Pt
features in the diffraction patterns indicate that Pt particles were well-dispersed on the
support, which is in agreement with the small particle sized observed in STEM (Figure S3).
6
5
Although features of Bi
2
O
3
and Bi
2
Pt
2
O were observed on 0.9Bi-Pt/C, the diffraction
7
patterns of bismuth metal and bismuth oxide 66 were not detected for xBi-Pt/C (x = 0 – 0.6),
indicating that Bi species were well-dispersed over the carbon support and/or Pt particles
when the Bi:Pt atomic ratio was less than 0.6.
Chemisorption of H was utilized to probe the surface composition of the Bi-promoted
2
Pt nanoparticles. The ratio of adsorbed hydrogen atoms to total platinum atoms is shown in
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Table 3. With an increasing amount of Bi, the H/Pt ratio decreased from 0.63 for Pt/C to
0.18 for 0.6Bi-Pt/C (Figure S6). Because the fraction of exposed metal atoms, estimated
from STEM, was similar for Pt/C and Bi-promoted Pt/C catalysts (Table 3), the decrease in
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H chemisorption capacity indicates that the Bi promoter (Bi metal or Bi oxide) was present
2
on the surface of Pt particles and blocked hydrogen uptake on Pt sites.
To further investigate the composition of nanoparticles, Bi-Pt/C was examined by
energy-dispersive X-ray spectroscopy (EDS) operating in the STEM mode.
Single-nanoparticle EDS was first performed on twenty nanoparticles, which were chosen
randomly (Figures S7a and S7b), and the elemental composition of each particle is listed in
Table S2. As shown in Figure S7a and Table S2, both Pt and Bi signals were detected in
most particles and the average elemental composition of particles on 0.6Bi-Pt/C is 72 ±
1
5 mol% Pt and 28 ± 15 mol% Bi. The relatively narrow distribution of elemental
compositions indicates that during the preparation, the Bi was deposited selectively on the
Pt particles rather than the carbon support. The overall elemental composition of 0.6Bi-Pt/C
from ICP-AES analysis was 61 mol% Pt and 39 mol% Bi, which is in the confidence
interval of elemental composition obtained from EDS. Additional STEM-EDS mapping
further supported the conclusion that Bi species were selectively deposited on the Pt
particles in Bi-Pt/C. The dark-field image, presented in Figure 2a, contained relatively large
nanoparticles (~ 3 nm) to ensure sufficient EDS counts. The corresponding EDS map of Pt
(blue), Bi (red) and the overlayed map of Pt and Bi are shown in Figures 2b, 2c and 2d,
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respectively. Both the Pt and Bi EDS maps overlapped well with the dark field image. In
the regions where nanoparticles appeared, Pt and Bi EDS signals appeared simultaneously.
An STEM-EDS mapping study over a larger area was also performed and confirmed that Pt
and Bi signals appeared in the same region (Figure S8).
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.1.2. Characterization of the Bi-promoted Pt catalysts after pretreatment and reaction
Extensive characterization suggests that xBi-Pt/C (x = 0 – 0.6) catalysts with oxidized
Bi selectively deposited on Pt particles were successfully prepared. As discussed in the
Introduction, structural changes of bimetallic catalysts might happen during the reaction. In
the current study, a catalyst was first pretreated in alcohol solution under He at 328 K for 20
min to reduce nanoparticles in-situ and then reacted in alcohol solution under various O
2
pressures. To investigate the influence of reaction environment on the structure of Bi-Pt/C
catalysts, samples recovered after 20 min of the aqueous alcohol pretreatment
(0.6Bi-Pt/C-P) and after 4 h of the alcohol oxidation reaction (0.6Bi-Pt/C-R) were
extensively characterized.
The stability of the carbon support was examined first. An SEM image together with
the measured surface area and pore size distribution of 0.6Bi-Pt/C-R is presented in Figure
S1c. The size of the carbon particles after reaction was similar to the as-prepared sample
2
2
-1
and the surface area of the recovered catalyst was 9.24×10 m g , which is slightly lower
3
2
-1
than the as-prepared 0.6Bi-Pt/C sample (1.10 × 10 m g ). The similar pore size
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distribution also suggests that the porous structure of carbon support was relatively stable
and was largely maintained under the reaction conditions used in this study.
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Fresh 0.6Bi-Pt/C and used 0.6Bi-Pt/C-R were analyzed by ICP-AES to check for
leaching. As shown in Table 2, the loadings of Pt and Bi measured by ICP-AES were
slightly lower on the catalyst recovered from 4 h HDO oxidation compared to those of fresh
catalyst. Because the atomic ratio of Bi to Pt remained constant at 0.6, we suspect the
slightly lower loadings on the recovered catalyst is the result of adsorbed species on the
3
0
catalyst after reaction, a conclusion also reached by Wörz et al. The reaction medium was
also tested for leached metals after 4 h of reaction. The concentrations of Pt and Bi were
both below 6.3 ppm, the detection limit of ICP-AES, confirming that less than 2% of
available Pt or Bi leached into the solution.
The 0.6Bi-Pt/C-R was also characterized by STEM to examine whether or not
sintering of nanoparticles occurred during the reaction. The STEM image, the
corresponding particle size distribution and the statistical analysis of the nanoparticle sizes
are presented in Figures S3d, S4d and Table 3, respectively. The mean particle diameter of
0
.6Bi-Pt/C-R is 1.6 ± 0.5 nm, which is similar to that of the as-prepared 0.6Bi-Pt/C (1.5
0.5 nm). Likewise, the surface average diameter, which is related to the fraction of
±
exposed metal sites, was also similar for 0.6Bi-Pt/C (1.8 nm) and 0.6Bi-Pt/C-R (2.0 nm).
Prior work by Ide et al. has also shown that a monometallic Pt/C catalyst does not sinter
during the selective oxidation of alcohols under similar conditions.20 The negligible
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aggregation of Pt was also supported by results from XRD, as shown in Figure S5. The
very weak diffraction peak associated with Pt(111) for 0.6Bi-Pt/C-R was relatively
unchanged from that for as-prepared 0.6Bi-Pt/C, indicating the excellent dispersion of the
Pt nanoparticles was maintained.
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.6Bi-Pt/C-P and 0.6Bi-Pt/C-R were interrogated by XPS and compared to as-prepared
.6Bi-Pt/C. The Pt 4f7/2 and Bi 4f5/2 core level spectra of 0.6Bi-Pt/C-P and 0.6Bi-Pt/C-R are
shown in Figures 1c - f and the analysis of the spectra is summarized in Table 1. For both
.6Bi-Pt/C-P and 0.6Bi-Pt/C-R, the Pt remained metallic and the Bi remained oxidized. In
0
addition, the binding energy of the features associated with 4f7/2 Pt(0), 4f5/2 Bi(0) and 4f5/2
Bi(III) were similar among the 0.6Bi-Pt/C, 0.6Bi-Pt/C-P and 0.6Bi-Pt/C-R samples. The
XPS results revealed that Pt was not over-oxidized irreversibly and that there was no
significant change of the electronic interaction of Pt and Bi after the pretreatment and
reaction compared to the as-prepared catalyst.
Although leaching, sintering or change of oxidation state was not detected after the
reaction, severe migration of Bi species was revealed by XPS and EDS. The atomic ratio of
Pt:C obtained from XPS was similar for fresh 0.6Bi-Pt/C and the catalyst after pretreatment
and reaction, suggesting that Pt particles did not substantially migrate on the carbon support.
However, a significant decrease in Bi:Pt atomic ratio was observed on 0.6Bi-Pt/C-P (0.87)
and 0.6Bi-Pt/C-R (0.66), compared to the fresh 0.6Bi-Pt/C (2.4). The Bi:Pt atomic ratio of
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.6Bi-Pt/C-P and 0.6Bi-Pt/C-R determined by XPS was similar to the overall Bi:Pt ratio
(0.6) determined from ICP-AES. Since neither Bi nor Pt leached during the reaction, the
results from XPS indicate a substantial migration of Bi on the carbon support and a more
uniform distribution of Bi species throughout the carbon particles as a result of
pretreatment and reaction.
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The migration of Bi on the carbon support was also supported by the EDS study. For
fresh 0.6Bi-Pt/C, single-nanoparticle EDS study shows that nanoparticles have a relatively
uniform composition of Pt and Bi, indicating the selective deposition of Bi. After the
pretreatment, the relatively uniform composition of Pt and Bi in the individual
nanoparticles was maintained, as indicated in Figures S7c and S7d and Table S2. The
average elemental composition of 0.6Bi-Pt/C-P is 73 ± 12 mol% Pt and 27 ± 12 mol%
Bi, which is similar to that of as-prepared 0.6Bi-Pt/C. However, some segregation of Bi and
Pt was observed on 0.6Bi-Pt/C-P in the EDS mapping study. As shown in Figures 2e-h,
although Bi was present in the Pt-rich region, it also appeared in regions without Pt, most
notably in the lower right area of the figure. The Pt and Bi EDS maps in a larger region are
presented in Figure S8. Areas containing only Bi (no Pt) were observed in these maps as
well. After 4 h of reaction, the segregation of Pt and Bi was more evident. The
single-nanoparticle analysis showed a broader distribution of compositions of individual
particles after reaction, which results in 45 ± 30 mol% Pt and 55 ± 30 mol% Bi, as
shown in Table S2. As with 0.6Bi-Pt/C-P, regions that contained only Bi were detected on
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species migrated substantially on the carbon support, no diffraction pattern of Bi species
was observed by XRD (presented in Figure S5) suggesting that Bi species remained well
dispersed in the vicinity of the Pt nanoparticles or on the support.
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.2. Oxidation of 1,6-hexanediol
.2.1. Tests for transport artifacts
The potential influence of external mass transport limitations on reaction rates was
first examined by testing the effect of the stirring rate on initial activity of HDO oxidation
over 0.6Bi-Pt/C, the most active catalyst, under 0.02 MPa O . As shown in Figure 3, the
2
initial activity (TOF) first increased with stirring rate and then reached a plateau when the
stirring rate exceeded 1000 rpm. In subsequent work, the stirring rate was chosen to be
1
200 rpm. In addition, the maximum O
phase in the pressurized reaction system was determined by the sulfite oxidation method 59
and for the dioxygen pressure used here, the reaction rate did not exceed the O transfer rate.
Hence, at 1200 rpm, neither transport of dioxygen from the gas phase into the bulk of the
liquid nor the film diffusion of HDO or O to the catalyst surface limited the reaction rate.
Internal transport limitations did not affect the measured rates over Bi-Pt/C in the
range of 0.02 – 1 MPa O and 0.05 – 0.2 M HDO or Pt/C in the range of 0.05 – 0.2 M HDO
2
transport rate from the gas phase to the aqueous
2
2
2
under high O
2
pressure (0.2 – 1 MPa) because the reaction order in both HDO and O is
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close to zero (Figure 4a). However, a reaction order of 0.75 with respect to O pressure was
2
observed over Pt/C in the range of 0.02 – 0.2 MPa O
2
. The Weisz-Prater criterion was used
on the observed rate,67
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to estimate the role of internal mass transport of O at 0.02 MPa O
2
2
which was adapted from the work on hindered diffusion in liquid-phase glycerol oxidation
over a PtBi catalyst by Wörz et al.30 The details of the dimensionless Weisz-Prater
parameter (훷 ) calculation are described in the supplementary information. Under the
푖
conditions in our study, 훷 was less than 0.3, indicating a negligible diffusional resistance
푂
2
in our porous catalysts.67
3
.2.2. Enhanced activity of 1,6-hexanediol oxidation over Bi-promoted Pt catalyst
The series of xBi-Pt/C (x = 0, 0.03, 0.09, 0.18, 0.30, 0.45, 0.60 and 0.90) catalysts was
tested in the semi-batch oxidation of HDO under 0.02 MPa O . The conversion of HDO
2
after 15 min and 4 h is shown in Figure 5. Addition of Bi clearly promoted activity for
alcohol oxidation relative to Pt/C. For example, the HDO conversion at 15 min increased
from 2.2 % over Pt/C to 6.1% over 0.6Bi-Pt/C. A similar promotional trend was also
observed after 4 h of reaction.
Among the Bi-Pt/C catalysts with different Bi loadings, the highest conversion was
obtained over 0.6Bi-Pt/C. At high catalyst loading in the reactor, the 0.6Bi-Pt/C catalyst
allowed for nearly stoichiometric conversion of 1,6-hexanediol to adipic acid. More
specifically, a 98% yield of adipic acid was obtained after 36 h of reaction over 0.6Bi-Pt/C
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The Pt/C and Bi-Pt/C catalysts were also tested under different dioxygen pressures and
interestingly the effect of Bi addition on activity appeared to depend on O
shown in Figure 7. Although the addition of Bi had a promotional effect on the rate in 0.02
MPa O (Figure 7a), the presence of Bi inhibited the rate on Pt/C under high dioxygen
pressure (1 MPa), as shown in Figure 7d. The influence of Bi addition on the product
distribution was also examined under high (1 MPa) and low (0.02 MPa) pressure of O . As
shown in Figure 8, the selectivities to 6-hydroxyhexanal, 6-hydroxyhexanoic acid and
-oxohexanoic acid were similar over Pt/C, 0.3Bi-Pt/C and 0.6Bi-Pt/C at similar levels of
conversion (20 %), indicating that the main influence of Bi is on the rate of HDO oxidation.
2
pressure, as
2
2
6
3
.2.3. Influence of reaction conditions on HDO oxidation
Figure 4a summarizes the promotional effect of Bi on Pt as a function of O pressure.
2
The TOF in Figure 4 is based on the surface Pt atoms associated with Pt/C since our
characterization indicates that Bi is a highly mobile species during pretreatment and
reaction. Under low O
increased significantly with O
of 0.02 to 0.2 MPa O . At high pressure (0.2 to 1 MPa O
(zero order) over Pt/C. When the O pressure exceeds 0.2 MPa, the TOF over Pt/C
2
pressures, the TOF for HDO oxidation over unpromoted Pt/C
pressure, resulting in a reaction order of 0.75 over the range
), the reaction was independent of
2
2
2
O
2
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actually exceeded that over 0.6Bi-Pt/C. The reaction order in O for HDO oxidation over
2
0.6Bi-Pt/C was close to zero over the whole range of pressures from 0.02 to 1 MPa.
Evidently, the observed promotional effect of Bi on Pt-catalyzed HDO oxidation is directly
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related to the observation that the reaction is zero order in O
even at low pressure.
2
over the promoted system
The effect of substrate concentration and temperature on the reaction rate was also
investigated, as shown in Figures 4b and 4c. In the range of 0.05 – 0.2 M, the oxidation of
HDO was found to be nearly zero order with respect to HDO concentration at 328 K, under
either 0.02 or 1 MPa O . Zero order behavior with respect to alcohol was also reported in
2
other studies, indicating the saturated adsorption of HDO on the catalyst surface.15,17,68 The
apparent activation energy of HDO oxidation was also calculated from rates measured from
3
18 to 338 K, as shown in Figure 4c. The apparent activation energies (E
a
) reported in
Table 4 were experimentally indistinguishable over Pt/C and 0.6Bi-Pt/C. At low O
2
-1
pressure (0.02 MPa), the apparent activation energies were 22 and 23 kJ mol over Pt/C
and Bi-promoted Pt/C, respectively. Although the apparent activation energy values were
higher at high O
2
pressure (1 MPa O ), no discernable difference between the promoted and
2
unpromoted Pt catalysts was observed.
3
.2.4. Kinetic isotope effect of D O over Pt/C and Bi-Pt/C and the promotional effect of Bi
2
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The significantly different O
2
reaction order under low O pressure condition over Pt/C
2
and Bi-Pt/C suggests that the promotional effect of Bi is likely related to the activation of
0
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O
2
. For Pt/C, the rate of O
2
dissociation is fast so that it can be assumed to be at equilibrium
pressure, the dissociation of O might be a
15
under high O
2
pressure whereas under low O
2
2
kinetically-relevant step. From this perspective, the role of Bi might be to accelerate O
2
dissociation so that the O
Figure 4a.
2
reaction order is zero, even under low O pressure, as shown in
2
Zope et al. reported the calculated activation barriers for O
2
dissociation over Pt(111)
1
0
determined to be 64 kJ mol-1,
in water, with E
a
for direct dissociative adsorption of O
2
-1
which was lowered to 18 kJ mol when O
2
dissociation was assisted by adsorbed water
dissociation step was also supported
(ꢀ ∗ +ꢂ ꢀ ∗→ ꢀꢀꢂ ∗ +ꢀꢂ ∗). A water-assisted O
2
ꢁ
ꢁ
by the detection of peroxide during HDO oxidation over Pt/C as described in our previous
15
work. Since water may be an active participant in the dissociation of O
2
, the effect of
replacing H
The initial rates (TOF) of HDO oxidation over Pt/C and 0.6Bi-Pt/C under 0.02 MPa
and 1 MPa O were measured in H O and D O, respectively. The measured TOF and
TOF /TOF ratio are reported in Table 5. Under high O pressure, a TOF /TOF ratio was
determined to be 1.1 over both Pt/C and 0.6Bi-Pt/C. The negligible kinetic isotope effect
with D O under high O pressure was also observed during ethanol oxidation by Ide et al.,
2
O with D
2
O was evaluated over Pt/C and 0.6Bi-Pt/C.
2
2
2
H
D
2
H
D
2
2
suggesting that the dissociative adsorption of alcohol and formation of surface hydroxyl
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species from dioxygen and water are not the kinetically-relevant steps. Under low O
pressure (0.02 MPa), however, a modest kinetic isotope effect of D O was observed over
Pt/C in this work. The initial TOF of HDO oxidation over Pt/C was 0.063 and 0.044 s in
O and D O, respectively, resulting in a TOF /TOF ratio of 1.4. In contrast to the results
over Pt/C, a negligible kinetic isotope effect was observed over 0.6Bi-Pt/C under 0.02 MPa
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-1
H
2
2
H
D
-1
O
2
, with the TOF
the presence of Bi allows the Bi-Pt/C system to have similar reaction kinetics during
alcohol oxidation at low and high pressures of O . Without Bi, there is a strong dependence
of the reaction rates on O pressure at low pressure as well as a key role of water.
H
and TOF
D
measured at 0.17 and 0.20 s (TOF
H
/TOF = 0.9). Evidently,
D
2
2
3.2.5. Influence of Bi on the stability of Pt catalyst during 1,6-hexanediol oxidation
Others have reported that Bi might promote the stability of the catalyst in alcohol
oxidation by suppressing the formation or adsorption of contaminating species or
preventing Pt from over-oxidation.23–25,28,34,40 Ide et al. suggested that severe deactivation
of Pt catalysts during alcohol oxidation was possibly related to strongly bonded species
20
generated from intermediates. However, the nature of the deactivating species remains
unknown. In an attempt to quantitatively compare the deactivation behavior of our Pt/C and
Bi-Pt/C catalysts, the experimental results were fitted by an empirical deactivation model
proposed by Froment and Bischoff, which was applied to the deactivation caused by
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continuous formation of carbon on active sites. As shown in Figure S9, the following
model fit the results well:
0
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0
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0
ꢃ0
푟 =
1
1ꢄ푘 ∙ꢅ
푑
with r as the measured reaction rate; r
0
as the initial reaction rate; k
d
as the deactivation
factor and t as reaction time. The deactivation factors, k
d
, for HDO oxidation over Pt/C and
0
.6Bi-Pt/C at 328 K under different O
Under different O pressures, k
with O pressure was observed over Bi-Pt/C and Pt/C.
2
pressure (0.02 – 1 MPa) are shown in Figure 9.
-1
2
d
varied from 0.007 to 0.07 s . However, no obvious trend in
k
d
2
4
. Discussion
Extensive characterization of 0.6Bi-Pt/C, 0.6Bi-Pt/C-P and 0.6Bi-Pt/C-R revealed a
significant structural change of Bi-Pt/C under pretreatment and reaction conditions. A
schematic model for the evolution of Bi-Pt/C during alcohol pretreatment and alcohol
oxidation is summarized in Scheme 1. During catalyst preparation, Bi was selectively
deposited on Pt particles in the outer layer of the carbon support, involving both atomic or
monolayer adsorption as well as multilayer deposits of Bi at high Bi/Pt ratios (Scheme
2
7
1
a). However, under conditions of pretreatment and reaction, substantial migration of Bi
species on the carbon support was observed, which likely involved the redistribution of the
Bi present in the multilayer deposits. Although some areas on the carbon support containing
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only Pt or Bi were detected on Bi-Pt/C after pretreatment and reaction, Bi and Pt were still
in the vicinity of each other in most regions, even after a long reaction time.
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Most importantly, the addition of Bi only increased the rate of HDO oxidation under
low O
order in O
conditions. Evidently, O
pressure and promotion with Bi facilitated O
likely to involve a water-assisted O dissociation path. This speculation was supported by
the moderate kinetic isotope effect of D O on HDO oxidation over Pt/C at low O pressure,
with TOF /TOF being 1.4, which is substantially higher than TOF /TOF = 0.9 seen over
0.6Bi-Pt/C. Bismuth is reportedly oxidized faster than Pt under alcohol oxidation
2
pressure. The increased rate was related to fact that the promoted system was zero
whereas the unpromoted catalyst was 0.75 order in O under the same
activation became kinetically-relevant over Pt/C at low O
activation. The promotional effect of Bi is
2
2
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2
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2
H
D
H
D
2
3
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conditions and BiO
Bi to act as a redox site to facilitate O
In addition, Feliu et al. proposed that BiO
adsorption on the surface and favor the formation of oxygenated species.41,42 Since Bi itself
has negligible activity for H -chemisorption and HDO oxidation, Pt is believed to be the
active site for alcohol oxidation involving the dissociative adsorption of alcohol and β-H
x
is readily reduced when it is in contact with Pt, which may enable
activation and subsequent transfer to the Pt surface.
on a Pt surface is likely to change the water
2
x
2
1
0
elimination. Combining our reaction kinetics and characterization results, we propose that
the BiO species adjacent to Pt nanoparticles form active sites that accelerate the
water-assisted dissociation of O in liquid water and the subsequent formation of hydroxyl
x
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species on Pt surface, which promotes the rate of HDO oxidation at low O pressure. The
2
apparent activation energies measured over Bi-promoted and unpromoted Pt are
experimentally indistinguishable, which is likely the result of a similar activation barrier for
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-1
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water-assisted O
as determined from DFT calculations on a Pt(111) surface. At higher O
dissociative adsorption of O is kinetically irrelevant, which is evidenced by the zero
reaction order with respect to O over Pt/C (Figure 4a). Indeed, the promotional effect of Bi
disappeared when the O pressure exceeded 0.2 MPa.
2
dissociation (18 kJ mol ) and β-H elimination from alcohol (15 kJ mol )
1
0
2
pressure, the
2
2
2
5
. Conclusions
Bismuth was successfully deposited onto carbon-supported Pt nanoparticles, but liquid
phase pretreatment of the Bi-promoted Pt catalyst caused migration of Bi. The presence of
Bi adjacent to Pt significantly promoted the rate of 1,6-hexanediol oxidation under low O
pressure without influencing the product distribution or catalyst deactivation. In the range
of 0.02 to 0.2 MPa O , the order of reaction in O changed from 0.75 over Pt/C to zero
over Bi-promoted Pt/C, which accounts for the observed promotion in rate at low O
pressure. Results from the kinetic isotope effect with D O vs H O suggest that
water-assisted O activation is not kinetically-relevant on Bi-promoted Pt at low O
pressure whereas it is relevant on unpromoted Pt. All of the promotional effects of Bi
disappear at higher O pressure.
2
2
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2
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Supporting Information:
The supporting information includes: XPS curve fitting parameters; the calculation of
Weisz-Prater parameters for dioxygen in alcohol oxidation reaction; XAS, XRD and H
chemisorption of as-prepared Bi-Pt/C catalysts; SEM images, STEM images, EDS analysis,
sorption, size-counting of fresh Bi-Pt/C and catalysts after pretreatment and reaction;
2
N
2
curve fitting of deactivation model of 1,6-hexanediol oxidation over Pt/C and Bi-Pt/C.
This material is available free of charge via the Internet at http://pubs.acs.org
Acknowledgements
This material was based upon the work supported by the National Science Foundation
under award number EEC-0813570. Use of the National Synchrotron Light Source was
supported by the US Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DE-AC02-98CH10886. Beamline X18B at the NSLS is
supported in part by the Synchrotron Catalysis Consortium, US Department of Energy
Grant No. DE-FG02-05ER15688. A portion of microscopy research was conducted at the
Center for Nanophase Materials Sciences in Oak Ridge National Lab, which is a DOE
Office of Science User Facility. The authors would like to acknowledge Dr. Dmitry Pestov
and Professor Eric Carpenter in the Nanomaterials Characterization Center at Virginia
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