J. Quesada et al.
AppliedCatalysisA,General551(2018)23–33
particles were slightly smaller than those previously observed in the
case of the Au/mTiO2 catalyst, the particle mean size (5.0 nm) were
quite similar, enabling the direct comparison of both gold-containing
catalysts without taking into account structure-sensitivity aspects. XPS
analysis of this material is shown in Fig. S2. No relevant signals asso-
ciated to Au 4f5/2 nor 4f7/2 were detected with enough resolution, being
not possible the determination of the actual metal loading. This result is
explained by the low amount of Au expected and the proximity of main
signals related to Si2p that can disrupt the general spectrum. The
plasmon band was also analysed for this material, being the spectrum
added to the supplementary information (Fig. S3), in order to compare
it with the SPR of Au/mTiO2. In general terms, main differences be-
tween both materials are related to the signal position (with a max-
imum at 522 nm for the SiO2 support) as well as the lower intensity
observed for this silica. These differences are justified by the higher
dielectric constant of the surrounding medium of mTiO2. The decon-
volution of this spectrum is congruent with the particle mean size
previously determined (5 nm).
balances closures were observed for the TiO2 at all the studied tem-
peratures, whereas no relevant differences were observed between va-
lues obtained for mTiO2 and Au/mTiO2, except at the highest tem-
perature studied. At these conditions, the balance closure is
significantly lower with the metallic catalyst (68.1%, Au/mTiO2 at
673 K).
The selectivities to the different reaction products obtained with the
studied catalysts are also shown in Fig. 5. Acetaldehyde selectivities are
considerably higher with the Au/mTiO2 at 523 K (TiO2: 37.7%; mTiO2:
45.9%; Au/mTiO2: 68.5%) and 573 K (TiO2: 16.4%; mTiO2: 15.6%; Au/
mTiO2: 57.6%). Although at the highest temperatures acetaldehyde
selectivity is lower for the Au/mTiO2 than for mTiO2, the sum of the
selectivities to acetaldehyde and all of the long chain chemicals pro-
duced from acetaldehyde (crotonaldehyde, crotyl alcohol, 1butanol,
butanal and ethyl acetate) are always the highest (TiO2: 19.3 and
29.9%, mTiO2: 25.1 and 36.9%, Au/mTiO2: 34.2 and 38.0%, at 623 and
673 K respectively). This fact corroborates the promoting effect of the
metal on the ethanol dehydrogenation step, especially at low tem-
peratures. However, at higher temperatures the values of acetaldehyde
selectivities are more similar among the three different materials, be-
cause subsequent condensation steps take place at larger extent. This
fact suggests that the alcohol dehydrogenation becomes less relevant as
temperature increases, comparing to other competitive reactions (de-
hydration to diethyl ether, and mainly to ethylene), masking the effect
of the metal on the dehydrogenation reactions.
Highest selectivities to diethyl ether (side product) were observed
for TiO2 and mTiO2 catalysts, leading to a decrease of acetaldehyde
formation (main route). This effect is more remarkable at lowest tem-
peratures (72.9 and 71.2% diethyl ether selectivity at 573 K with the
TiO2 and mTiO2, respectively). These values are also higher than those
reported in the literature for other catalysts tested in this reaction
(hydroxyapatites, MgAl mixed oxides, MgO) [54–56]. Therefore, a
strong interaction between ethanol and the surface with these materials
is suggested, as diethyl ether is produced from two ethanol molecules
by reaction coupling followed by a dehydration. This is in agreement
with the strongest interaction between ethanol and titania, comparing
with other materials (magnesia and hydroxyapatites) previously re-
ported by Young and coworkers [20]. In the case of the Au/mTiO2
material, the presence of the active metal phase on the catalytic surface,
which promotes the ethanol dehydrogenation, leads to lower diethyl
ether selectivity at low temperatures (94% lower than the
mTiO2support at 523 K). However, the diethyl ether selectivity at the
highest temperatures - especially at 673 K - are similar comparing the
three catalysts (27.3, 23.0, and 26.5% with the TiO2, mTiO2, and Au/
mTiO2, respectively), which is in agreement with the faster reaction
rates of dehydration steps at the highest temperature. Likewise, ethy-
lene selectivities increase with the temperature for the three catalysts,
being lower with the Au/mTiO2 for the considered temperature range,
but mainly at the lowest temperatures (9.8, 14.0, and 6.4% with the
TiO2, mTiO2, and Au/mTiO2 at 573 K, respectively). The selectivity to
ethylene is higher with the mTiO2 because of its higher concentration of
acid and strong basic sites. The former sites promote dehydration
3.2. Catalytic activity
The evolution of the conversion and carbon balance closure working
under reductive conditions for the TiO2, mTiO2, and Au/mTiO2 cata-
lysts are shown in Fig. 5. At working conditions and considering the
small size of both, reactant and products, the mass transfer effects can
be discarded, being the experimental results directly related to the
catalytic activity of each material. This hypothesis was previously
checked in same reaction at similar conditions [45]. Both the conver-
sion and the carbon balance follow the same trends for the three con-
sidered materials (increasing conversions at increasing temperatures;
decreasing carbon balances). Conversions follow the order
TiO2 < mTiO2 < Au/TiO2, being the differences more important as
temperature increases, reaching a difference of 20% between the con-
version with the Au/TiO2 and the unsupported material. These results
involve an improvement because of the metal but also an improvement
because of the modification of the original surface, introducing the ir-
regularities needed to maximize the active sites available. Thus, the
conversion obtained with Au/TiO2 at 673 K (74.2%) is, to the best of
our knowledge, among the highest conversions reported for this reac-
tion at these reaction conditions [51–53]. In order to corroborate the
absence of diffusional limitations, these results (the worst scenery) were
used to determine the Thiele modulus (obtaining a value of 0.09 that
corresponds to a ηφ2 of 5.18·10−2) and the Carberry value (with a re-
sult of 1.15·10-6). According to these values, both diffusional limitations
are far from the threshold values leading to relevant effects of mass
transfer on the overall kinetics. If conversions are normalized by basic
sites (the active sites for this reaction in the cases of the non-metallic
catalysts), activity of the Au/mTiO2 at 523 K is 3.6 and 4.5 times higher
than the corresponding to TiO2 and mTiO2 catalysts, respectively. This
suggests a relevant role of gold and reducing conditions in the final
conversion, since the Au/mTiO2 catalyst has a lower concentration of
active sites and specific surface area (Table 1). The highest carbon
Fig. 5. Conversion, carbon balance, and selectivity
evolution in the gas phase ethanol condensation
under reducing conditions over: (a) TiO2, (b) mTiO2,
and (c) Au/mTiO2. (WHSV = 7.9 h1; He-H2 flow).
Symbols: conversion (●), carbon balance (▲). Bars:
acetaldehyde (yellow), ethylene (blue), diethyl ether
(purple), 1butanol (red), 1,3butadiene (green), and
others (grey). (For interpretation of the references to
colour in this figure legend, the reader is referred to
the web version of this article.)
80
60
40
20
0
523
573
623
673
523
573
623
673
523
573
623
673
28