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H. Nair, C.D. Baertsch / Journal of Catalysis 258 (2008) 1–4
alysts of molybdenum, tungsten, and vanadium. Using ethanol as
a model reactant, the number of active redox sites for ethanol ox-
idative dehydrogenation to acetaldehyde at 453 K is determined by
anaerobic reaction at 453 K, during which oxygen is removed from
the reactant stream and the transient decay in activity from steady
state is monitored. Ethanol oxidation to acetaldehyde is known to
occur through a Mars–van Krevelen-type mechanism using oxy-
gen from the catalyst surface. Reoxidation of the catalytic sites
reduced by ethanol can be suppressed if oxygen is not present
in the reactant stream. Thus, the presented anaerobic reaction
method is a stoichiometric titration, during which each molecule
of acetaldehyde produced after oxygen removal has a direct cor-
respondence to the number of active redox sites available at the
surface. Only active redox sites are counted, because redox prod-
uct formation is used for the quantification. This is in contrast to
ethanol chemisorption techniques, which quantify the total surface
coverage of ethanol, some of which reacts via a redox path to form
acetaldehyde and some of which reacts via an acid-catalyzed path-
way to form diethyl ether (at 453 K). In principle, this methodology
of anaerobic titration for estimating active redox site densities on
metal oxides catalysts using the reactant as the probe molecule can
be extended to any similar oxidation or oxidative dehydrogenation
reaction using lattice oxygen in the catalytic cycle. This technique
of measuring redox site density should be extremely accurate, be-
cause the reactant is used as the probe molecule and active sites
are measured at reaction temperature with all conditions similar
except for the presence of oxygen in the gas phase.
was maintained in the reactant stream through controlled injec-
tion of liquid ethanol (0.000022 cm /s, AAPER, absolute 200 proof)
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into a flowing mixture of oxygen and helium with a Cole Palmer
single-syringe infusion pump (EW-74900-00). Reactant and prod-
uct concentrations were measured using the thermal conductivity
detector of an Agilent 3000A micro gas chromatograph (HP-PLOT
Q column).
Anaerobic reactions were carried out to measure the amount
of lattice oxygen available to ethanol during reaction. Oxygen was
removed from the reactant stream using a 4-port Valco switching
valve under total reactant flow rates and ethanol partial pressure
identical to those under steady-state reaction conditions. Acetalde-
hyde and ether formation were monitored simultaneously using a
combination of a Hiden Analytical HPR20 mass spectrometer and
an Agilent 3000A micro gas chromatograph (PlotQ column). Inten-
sities at m/e of 44 (acetaldehyde) and 59 (diethyl ether) were used
in conjunction with the micro gas chromatograph to quantify prod-
uct formation rates as a function of time. The rate of acetaldehyde
formation is known to decay with time once oxygen is cut off in
the reactant stream. Rapid mass spectrometry analysis indicated
that the oxygen in the outlet stream was reduced to zero almost
immediately after it was removed from the feed.
Activation energies for alumina-supported Mo, W, and V cat-
alysts with surface densities of 8 metal atoms/nm2 were ob-
tained from Arrhenius plots using acetaldehyde formation rates
from steady-state reaction at 443 K, 453 K, and 463 K. This nar-
row temperature range was chosen to obtain an accurate estimate
of the activation energy at 453 K without disturbing either the
catalyst selectivity or the number of redox sites on the catalyst ob-
served with more drastic changes in temperature. Error estimates
for activation energies are based on 95% confidence intervals.
2. Experimental
2.1. Catalyst preparation
Alumina-supported catalysts containing tungsten oxide (WOx–
3. Results and discussion
Al2O3), molybdenum oxide (MoOx–Al2O3), and vanadium oxide
(
VOx–Al2O3) with surface densities varying from 0.5 to 8 metal
Reaction of ethanol at 453 K over all supported molybdenum,
tungsten, and vanadium oxide catalysts led to the formation of
both acetaldehyde via oxidative dehydrogenation and diethyl ether
via dehydration on what can be classified as redox and acid sites,
respectively. Product selectivity is independent of conversion over
the range studied (<10% conversion) and is representative of the
relative redox and acidic character of the catalyst. Diethyl ether is
the only product formed over alumina; thus, reactions on the sup-
port should not affect the quantification of redox sites.
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atoms/nm were prepared by incipient wetness impregnation of
γ -alumina (Alcoa 151 m /g, 98.8%) with aqueous solutions of am-
2
monium tungsten oxide (Alfa Aesar, 99.999%), ammonium molyb-
date (Alfa Aesar, 99.999%), and ammonium vanadium oxide (Alfa
Aesar, 99.995%), respectively. Oxalic acid, at an oxalic acid: pre-
cursor weight ratio of 2:1, was used to aid dissolution of the
precursor at higher loadings. Metal atom surface densities were
based on the initial alumina surface area available for impregna-
2
tion (151 m /g). BET results obtained by N2 physisorption at 77 K
Fig. 1 shows rates of acetaldehyde (redox product) forma-
tion per metal atom as a function of surface density for WOx–
Al2O3, MoOx–Al2O3, and VOx–Al2O3 catalysts. Acetaldehyde forma-
tion rates were substantially lower over acidic WOx–Al2O3 than
over the more reducible MoOx–Al2O3 and VOx–Al2O3 catalysts. Ac-
etaldehyde formation rates varied with surface density and showed
slight maxima near the reported monolayer coverage for MoOx–
on a Micromeretics ASAP 2000 instrument confirm that the surface
area did not decrease significantly at the surface densities studied
here, as also was shown in previous studies [14]. Catalysts were
dried overnight at 393 K and then calcined in flowing air (zero
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grade, 0.5 cm /s) at 923 K (0.33 K/s) for 3 h.
UV–visible spectroscopy was used to confirm good dispersion
and domain growth of supported catalysts with increasing surface
density. UV–vis methods and results for the catalysts in this pa-
per have been reported previously [15]. Absorption edge energies
decreased with increasing surface density, confirming that oxide
domain size systematically increased with increasing surface den-
sity.
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2
Al2O3 (∼4 atoms/nm ) and VOx–Al2O3 (∼8 atoms/nm ) [16].
This observed maximum in the oxidative dehydrogenation rate per
metal atom near monolayer coverage has been attributed to a re-
quired balance between oxide domain reducibility (which increases
with increasing surface density) and oxide domain accessibility
(which decreases with increasing surface density beyond mono-
layer capacity) [17].
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.2. Ethanol reaction
Using total metal atoms to calculate the turnover frequency is
known to be an incorrect approach, because each metal site does
not act as an active site for ethanol oxidation [18,19]. To prop-
erly identify the density of active redox sites for ethanol oxidative
dehydrogenation, the reaction mechanism, which parallels that of
methanol, must be taken into account [18,19]. Ethanol initially ad-
sorbs as an ethoxy species that can then form either acetaldehyde
or diethyl ether. The main difference in the mechanistic pathways
for the two products is the use of lattice oxygen. Acetaldehyde
The activity and selectivity of prepared catalysts were mea-
sured using continuous-flow, fixed-bed, vertical U-tube quartz re-
actors containing ∼0.025 g of catalyst (125–250 μm particles) dis-
persed on a quartz frit. Before reaction, all samples were treated in
3
1.67 cm /s of simulated air (22% O2/He) at 773 K for 2 h. Reactions
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were carried out at 453 K with a total gas flow rate of 0.83 cm /s.
A constant partial pressure of 0.5 kPa ethanol and 1.5 kPa oxygen