1
12
M.B. Griffin et al. / Journal of Catalysis 307 (2013) 111–120
mechanistic details of each of these systems. On clean Pd surfaces,
the classic model for dehydrogenation of alcohols and polyols be-
gins with adsorption through the oxygen atoms, O–H scission,
and the formation of an alkoxide intermediate. This is followed
by C–H bond scission in the rate-limiting step [4,17,18]. More
uncertainty exists on Au surfaces. One explanation, supported by
the increased activity under alkaline conditions, suggests that
O–H bond scission is the rate-limiting step [19]. However, work
done with Au electrodes shows that C–H bond scission is rate-
determining [20]. Little is known about the behavior of polyols
on Au–Pd bimetallic alloys, and much could be gained through
increased insight into the reactivity of these systems.
This work investigates the oxidation of ethylene glycol and 1,2-
propanediol under alkaline conditions on Pd/C, Au/C, and Au–Pd/C.
Electroless deposition was used to prepare catalysts with various
Au/Pd ratios at the catalyst surface, and experiments were carried
out using selectively deuterated reagents to determine the
rate-limiting step by monitoring for a kinetic isotope effect. The
advantage of electroless deposition is that it has been shown to
selectively and controllably deposit one metal (in this case Au)
onto the surface of another (Pd), creating truly bimetallic catalyst
surfaces. A number of experimental and theoretical techniques
were utilized to better understand the catalyst composition and
the reactions that take place on its surface. The results address fun-
damental issues associated with the oxidation of polyols on metal
surfaces and provide routes toward the development of improved
industrial catalysts.
preparation. The bath was stirred for 4 h to ensure complete depo-
sition. The catalysts were then washed with 1 L of deionized water,
filtered, and dried at room temperature overnight.
A 1 wt% Au/C catalyst was made using the deposition–precipita-
tion procedure described by Prati et al. [10]. Saturated sodium car-
bonate was added to a mixture of 0.050 g sodium gold chloride and
10 mL deionized water until a fixed pH of 10 was reached. The mix-
ture was then added to a stirred slurry of 2.55 g of carbon and
25 mL of deionized water. After being stirred at room temperature
for 1 h, the slurry was heated to 343 K and 3 mL of formaldehyde
was added drop wise over 3 min to ensure complete reduction of
Au. This solution was stirred for 1 h before being filtered and dried.
Since the Au/C catalysts were prepared using a different technique
than the Pd/C and Au–Pd/C catalysts, factors beyond the type of
metal may contribute to differences in reactivity.
2.3. Catalyst characterization
The concentration of accessible surface Pd sites was determined
by chemisorption using hydrogen titration of oxygen pre-covered
Pd. For this technique, the catalysts were heated to 473 K at a rate
of 10 K/min under a 10% H /90% Ar mixture. The temperature was
2
held at 473 K for 1 h, and then, the catalysts were purged with ar-
gon for 30 min before being cooled in argon to 313 K. Next, the cat-
alyst surface was saturated with oxygen by exposing it to 10% O
2
/
90% Ar for 30 min. It was then purged with argon for 10 min before
3
2
being exposed to pulse doses of 0.52 cm of 10% H /90% Ar until all
of the surface oxygen was converted to water. Hydrogen consump-
tion was monitored using a high-sensitivity thermal conductivity
detector downstream from the sample cell. This technique pro-
vides a good indication of the number of active sites exposed on
the Pd/C catalyst. Also, since only Pd can dissociate oxygen at
2
. Materials and methods
2.1. Materials
3
13 K, the Au coverage on the bimetallic catalysts can be estimated
Ethylene glycol and 1,2-propanediol, as well as reagents used
by calculating the ratio of active sites before and after electroless
deposition [22]. While oxygen can potentially ‘‘spillover’’ [23]
and bind to surface Au atoms, the low adsorption energy of O on
Au surfaces (Section 3.5) suggests that room-temperature coverage
should be low. Moreover, assuming negligible O coverage on Au is
consistent with the results presented below, where low amounts of
deposited Au are found to eliminate approximately one adsorption
site per Au atom deposited [21,24,25].
for product identification (acetic acid, acetone, formic acid, glycolic
acid, glyoxal, glyoxalic acid, hydroxyacetone, lactic acid, methyl
glyoxal, oxalic acid, and pyruvic acid) were obtained in high purity
from Sigma Aldrich. High-purity deuterated reagents were ob-
tained from Sigma Aldrich (DOCH
Fisher Scientific (HOCD CD OH). CP-97 carbon (BASF Catalysis
Inc), sodium gold chloride (NaAuCl O, Alfa-Aesar), potassium
ꢁ3H
gold cyanide (KAu(CN) , STREM Chemicals), sodium carbonate pel-
lets (Na CO , J.T. Baker), formaldehyde (CH O, Sigma Aldrich 37% in
water), and hydrazine (N , Sigma Aldrich 37% in water) were
used in the preparation of the Au/C and Au–Pd/C catalysts. Sodium
hydroxide pellets (NaOH, J.T. Baker) and 18.2 M -cm deionized
2 2 2 2
CH OD, DOCD CD OD) and
2
2
4
2
2
The Au/C catalysts were characterized using X-ray diffraction
2
3
2
(
XRD). These experiments were performed on a Rigaku Miniflex
2 4
H
II desktop X-ray diffractometer equipped with a D/tex Ultra detec-
tor with a Cu K-alpha radiation source; scans were performed from
X
1
0 degrees to 80 degrees, with a step size of 0.020 degrees and a
water (Thermo Scientific Barnstead Nanopure system) were used
in the reaction media and for catalyst preparation. Ultra-high-pur-
ity oxygen, argon, and hydrogen were obtained from Airgas.
scan speed of 0.500 degrees/min. The PDXL software package from
Rigaku was used to analyze data and calculate average particle
sizes.
X-ray photoelectron spectroscopy (XPS) experiments were car-
ried out on the bimetallic Au–Pd/C catalysts using a Kratos Axid Ul-
tra DLD XPS system equipped with a hemispherical energy
2.2. Catalyst preparation
A series of Au–Pd/C bimetallic catalysts was prepared by depos-
analyzer. The monochromatic Al Ka X-ray source was operated
iting Au onto a 5 wt% Pd/C catalyst (BASF Catalysis Inc) using an
electroless deposition procedure described previously [21]. Potas-
sium gold cyanide was used as the metal salt and hydrazine was
used as the reducing agent. Hydrazine is toxic and unstable in its
anhydrous form and should be handled in solution if possible.
For deposition baths in which the Au concentration was less than
at 15 keV and 120 W, incident at 45 degrees to the surface normal.
The pass energy was fixed at 40 eV for detailed scans of the Au 4f
region. Scans were performed on catalysts as synthesized and after
in situ reduction under hydrogen at 473 K for 1 h. For comparison,
ꢀ
ꢀ
AuCl3 and AuðCNÞ were used to prepare 1 wt% Au/C catalysts as
2
references for the Au(III) and Au(I) states respectively and were
analyzed as prepared. Transmission electron microscopy (TEM)
was conducted using a JEOL 2100F high-resolution instrument.
4
4 ppm, a metal salt-to-reducing agent molar ratio of 1:10 was
used. Baths with Au concentrations over 44 ppm were subject to
higher decomposition rates of the reducing agent, so a molar ratio
of 1:20 was used. The electroless bath volume was held at 200 mL
and 0.500 g of 5 wt% Pd/C was used for all catalyst compositions.
The bath temperature was held at 313 K and concentrated sodium
hydroxide was used to maintain pH 12 for the duration of the
2.4. Reaction studies
The aqueous-phase oxidation of ethylene glycol and 1,2-pro-
panediol was performed in a 100 mL EZE-Seal™ batch reactor
Griffin, Michael B.
