Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO2 Interfaces

Article abstract of DOI:10.1002/anie.201703669

Recent efforts to design selective catalysts for multi-step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO₂ films of regulated porosity to gain a new level of control over catalyst performance, resulting in essentially 100 % HDO selectivity for two biomass-derived alcohols. This catalyst also showed exceptional reaction specificity in HDO of furfural and m-cresol. In addition to improving HDO activity by maximizing the interfacial contact between the metal and metal oxide sites, encapsulation by the nanoporous oxide film provided a significant selectivity boost by restricting the accessible conformations of aromatics on the surface.

Full text of DOI:10.1002/anie.201703669

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201703669
German Edition: DOI: 10.1002/ange.201703669
Heterogeneous Catalysis
Directing Reaction Pathways through Controlled Reactant Binding at
Pd–TiO2 Interfaces
Jing Zhang+, Bingwen Wang+, Eranda Nikolla,* and J. Will Medlin*
Abstract: Recent efforts to design selective catalysts for multi-
step reactions, such as hydrodeoxygenation (HDO), have
emphasized the preparation of active sites at the interface
between two materials having different properties. However,
achieving precise control over interfacial properties, and thus
reaction selectivity, has remained a challenge. Here, we
encapsulated Pd nanoparticles (NPs) with TiO2 films of
regulated porosity to gain a new level of control over catalyst
performance, resulting in essentially 100% HDO selectivity for
two biomass-derived alcohols. This catalyst also showed
exceptional reaction specificity in HDO of furfural and m-
cresol. In addition to improving HDO activity by maximizing
the interfacial contact between the metal and metal oxide sites,
encapsulation by the nanoporous oxide film provided a sig-
nificant selectivity boost by restricting the accessible confor-
mations of aromatics on the surface.
I
nterfacial sites between metals and metal oxides have been
proposed to be important in many catalytic reactions.[1] Such
interfaces can form the basis of bifunctional catalysis, where
each material contributes its unique functionality to the active
site. For example, bifunctional catalysts have shown promise
for selective hydrodeoxygenation (HDO) of alcohols/alde-
hydes with aromatic substituents, a desirable path for
production of fuels or chemicals from biomass (Figure 1a).[1–3]
While the mechanism for interfacial reactivity is under
debate, reducible metal oxides (e.g. TiO2, CeO2) have been
suggested to provide deoxygenation sites, while the noble
metals (e.g, Pd, Ru) provide hydrogenation sites.[4–8]
The importance of interfacial sites in catalysis suggests
that catalyst design should emphasize novel methods for
tailoring these interfaces that go beyond using the oxide as
a simple catalyst carrier.[9,10] A promising approach is to
employ inverted systems in which the metal oxide is deposited
as a film onto the metal nanoparticles (NPs). While encapsu-
lated structures have been applied to enhance the stability of
Figure 1. a) Reaction pathways of benzyl alcohol and furfuryl alcohol
on Pd catalysts. b) Controlled binding as well as interfacial sites
induced by nanoporous TiO2 film.
metal NPs[11–14] or improve the selectivity and/or activity for
selective oxidation,[15–17] hydrogenation,[18–20] and decarbon-
ylation,[21] HDO provides an important probe reaction for
testing their utility for reactions that occur at metal/oxide
interfaces. Moreover, the unique geometry of the sites in an
encapsulated structure could result in a new way to control
interfacial properties via biasing the reactant binding orien-
tations. For aromatic alcohols, flat-lying adsorbates are prone
to decarbonylation and ring hydrogenation while upright
adsorption geometries favor HDO.[22–28] Here, we report on
a method for controlling the encapsulation of Pd NPs with
nanoporous TiO2 to produce interfacial sites where accessible
conformations of aromatics are restricted. The resulting
catalysts exhibited high selectivity toward HDO with
[*] Dr. J. Zhang,[+] Prof. Dr. J. W. Medlin
Department of Chemical and Biological Engineering
University of Colorado Boulder
Boulder, CO 80303 (USA)
E-mail: will.medlin@colorado.edu
B. Wang,[+] Prof. Dr. E. Nikolla
Department of Chemical Engineering and Materials Science
Wayne State University
Detroit, MI 48202 (USA)
E-mail: erandan@wayne.edu
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201703669.
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improved activity during reactions of furfuryl alcohol, benzyl
alcohol, furfural and m-cresol. The improvements were
attributed to a combined effect of controlling binding
orientation of the aromatics and the presence of metal–
support interfacial sites, as illustrated in Figure 1b.
Using a method modified based on Klabunde and co-
workers,[29] we synthesized Pd0 particles which were uniform
in size ( ꢀ 4 nm) and well-dispersed in solution, as confirmed
by transmission electron microscopy (TEM) and X-ray
diffraction (XRD) (Figure S1 in the Supporting Information).
Prior to synthesis of TiO2 films, a layer of surfactant
molecules (cetyl trimethylammonium bromide (CTAB))
was deposited on the dispersed Pd NPs. The microstructure
of the TiO2 films was controlled by adjusting the rate of
hydrolysis of the titanium precursor via varying the rate of the
water addition[30] or lowering the reactivity of the titanium
precursor using acetone.[30–32]
Figure 2a shows the scanning transmission electron
micrographs (STEM) along with the bright field TEM (BF-
TEM) of the Pd@TiO2 structure synthesized by instantaneous
addition of water during synthesis (Pd@TiO2-WI). Lattice
fringes of anatase TiO2 (101) are observed on top of the Pd
NPs (circled in black) with measured values of 0.318 nm,
consistent with the calculated value from the XRD spectrum
(Figure S2), confirming that the Pd NPs are encapsulated by
the TiO2 film. Similar STEM and BF-TEM results were
obtained for the case when Pd@TiO2 were synthesized using
a dropwise injection rate for water (Pd@TiO2-WD) and when
water was replaced by acetone during synthesis (Pd@TiO2-
AI), as shown in Figure 2b and c, respectively. Electron
energy loss spectroscopy (EELS) measurements (Figure S3)
on Pd@TiO2-WI were also consistent with Pd NPs being
surrounded by the TiO2 film. Narrow troughs in the TiO2
EELS signal along the line were detected, perhaps due to the
porous structure of the film (Figure S3c). The pore size
distribution of the Pd@TiO2 materials showed that most of
the pores (82%) of Pd@TiO2-WI were less than 5 nm in width
with an average pore size of 4.8 nm (Figure 2a3) while the
TiO2 film of Pd@TiO2-WD had larger pores (ca. 50% of the
pores > 5 nm in width, Figure 2b3). A further increase in the
pore size was observed for Pd@TiO2-AI, with about 70% of
the pores larger than 15 nm (Figure 2c3). We also synthesized
4 nm Pd NPs supported on pre-synthesized TiO2 (Pd/TiO2)
and on a commercial a-Al2O3 support (Pd/Al2O3) by incipient
wetness impregnation. The STEM image of the former is
shown in Figure 2d. Unlike the encapsulated systems (Fig-
ure 2a–c), as expected the TiO2 (101) fringes were only found
on the support and not on top of the Pd NPs (circled in black)
(Figure 2d). The Pd NPs were dominated by the Pd (111)
lattice fringes, measured to be 0.225 nm, in agreement with
the calculated value from the XRD spectrum (Experimental
Procedure and Figure S4b in the Supporting Information).
The weight loading and apparent dispersion of Pd was
determined by ICP-OES and CO chemisorption, respectively.
The results in Table S1 suggest that the active sites for HDO
were minimally blocked by the TiO2 film in all cases, likely
due to the high porosity of the film (0.12 cm3 gÀ1 for Pd@TiO2-
WI). Table S1 also shows that Pd@TiO2-WI had the lowest Pd
loading, likely due to the fast hydrolysis of the titanium
Figure 2. Characterization of a) Pd@TiO2-WI, b) Pd@TiO2-WD,
c) Pd@TiO2-AI: 1) STEM images, 2) BF-TEM images, 3) N2 physisorp-
tion isotherm plots and pore size distribution of TiO2 films on Pd
NPs; d) STEM image of Pd/TiO2.
precursor. Infrared spectra measured after CO adsorption
indicated that TiO2 encapsulation had little effect on the types
of Pd sites available for adsorption (Figure S5).
Catalysts were evaluated for benzyl alcohol HDO; the
catalyst performance as a function of time on stream is shown
in Figure S6 in the Supporting Information. Figure 3 summa-
rizes the effects of support and encapsulation on the catalyst
performance. For impregnated Pd NPs, changing the support
from Al2O3 to TiO2 led to an increase in HDO selectivity from
36% to 50% (Figure 3a). Encapsulation with TiO2 further
increased the selectivity to 84 and 88% for Pd@TiO2-WD and
Pd@TiO2-AI, respectively. For Pd@TiO2-WI which contained
narrower pores, a HDO selectivity of approximately 100%
was obtained. We found that the improved selectivity was not
only owed to suppression of the undesired reaction pathways
but higher rates for toluene formation (Figure 3b).
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We conducted kinetic studies to evaluate the effect of the
TiO2 film on activation energies for benzyl alcohol conversion
(Figure S8). We found similar activation energies of 90–
93 kJmolÀ1 for benzene formation on all the encapsulated Pd
and the supported Pd catalysts, suggesting that the decrease in
decarbonylation rate was caused by lower active site density,
rather than a change in the mechanism. The apparent
activation energy for HDO, however, varied greatly among
different catalysts. For Pd@TiO2-WI, a value of 87 kJmolÀ1
was measured, which is similar to the ca. 100 kJmolÀ1
measured on a crowded Pd (111) single crystal.[22,23] For
impregnated Pd and encapsulated Pd with larger pores, the
apparent activation energy was less than 30 kJmolÀ1, which is
À
much lower than a typical C O dissociation barrier. Low or
negative apparent activation energies have frequently been
observed for HDO on metal catalysts, and have been
attributed to poisoning of active sites by dehydrogenated
adsorbates, the effect of which is strong at high temper-
ature.[26,33] Encapsulation of Pd within the smallest-pore TiO2
structures thus appears to have decreased the effects of these
side reactions on the surface chemistry, in line with the high
observed HDO selectivity.
We hypothesized that the improved performance of TiO2-
encapsulated Pd as compared to supported catalysts was due
to two effects: the larger concentration of Pd-TiO2 interfacial
sites, and the restriction of unfavorable adsorbate binding
geometries. It has been reported that using reducible metal
oxides as a support could promote HDO since the partially
reduced oxide creates oxygen vacancies to facilitate adsorp-
[4–6]
À
tion and C O activation of hydroxylated reactants.
The
presence of metals capable of hydrogen spillover was found to
enhance the reducibility of the metal oxides.[34,35] For exam-
ple, Prins and co-workers[34] found that for Pt/TiO2 catalysts,
reduction of TiO2 in the near vicinity of Pt began by 500 K.
We performed temperature programmed reduction (TPR) to
evaluate the reducibility of the metal oxide supports (note
that the Pd NPs were already in a reduced state) for the
various catalysts. To limit interference from moisture desorp-
tion, deuterium (D2) was used as reducing gas during the TPR
and the signal of D2 (m/z = 4) and D2O (m/z = 20) was
recorded to detect reduction. As shown in Figure S9 in the
Supporting Information, the onset reduction temperature,
represented by a simultaneous decrease in the D2 signal and
increase of the D2O signal, followed a sequence of Pd@TiO2 <
Pd/TiO2 < TiO2, which is consistent with the HDO selectivity
and activity for the Pd-containing catalysts (Al2O3 is consid-
ered non-reducible in this temperature range). Pd@TiO2
showed the lowest onset reduction temperature of ca.
1608C; in contrast, consistent with prior work for Pt/TiO2,[34]
the reduction of a conventional TiO2-supported system began
close to 2308C. These results suggest that intimate contact
between Pd and reducible TiO2 (as achieved in the Pd@TiO2
materials) is key for HDO performance.
Figure 3. Selectivity and turnover frequency (TOF) for catalytic conver-
sion of benzyl alcohol (a,b) and furfuryl alcohol (c,d). Reaction
conditions are shown in Experimental Procedures in the Supporting
Information. Decarbonylation products refer to furan and THF.
Catalyst performance was also evaluated for HDO of
furfuryl alcohol; the catalyst performance with time on
stream is shown in Figure S6 in the Supporting Information.
Unlike benzyl alcohol, two ring hydrogenation products,
tetrahydrofuran (THF) and tetrahydrofurfuryl alcohol
(THFA), were formed from furfuryl alcohol. Similar to the
case for benzyl alcohol, encapsulation of the Pd NPs with
TiO2 dramatically improved the selectivity towards HDO
(Figure 3c). Furthermore, when the TiO2 film mainly con-
tained small pores (Pd@TiO2-WI), HDO became the only
observed pathway. The comparison of the turnover frequen-
cies for the different catalysts (Figure 3d) shows that the
improved selectivity for the Pd@TiO2 was again due to both
an increase in HDO rate and a decrease in other pathways.
Post-reaction characterization using STEM and BF-TEM
(Figure S7) showed that unlike the Pd/TiO2, the Pd@TiO2-WI
retained its particle size and encapsulation structure after
reactions.
Better reducibility of TiO2 alone could not explain the
100% selectivity to toluene and 2-methyl furan over
Pd@TiO2-WI, since its TPR profile was similar to Pd@TiO2-
WD (Figure S9). Moreover, the negligible turnover frequency
for decarbonylation over Pd@TiO2-WI (Figure 3c–d) suggests
the elimination of sites associated with that reaction. The
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classical strong metal-support interaction (SMSI) induced by
high temperature reduction could alter the HDO selectivity,
but is unlikely in our case since none of the catalysts went
through the high temperature reduction necessary to induce
the SMSI. We hypothesized that the TiO2 film consisting
mainly of small pores (Pd@TiO2-WI) discouraged “flat-lying”
adsorption of the reactant molecules, while favoring
“upright” adsorption. To support this hypothesis, we inves-
tigated the hydrogenation of two probe molecules, styrene
and 1-hexene, over the catalysts. The planar structure of
styrene is expected to occupy a larger collection of contiguous
Pd atoms at the catalyst surface, with a similar “footprint” to
flat-lying benzyl alcohol and furfuryl alcohol, while the non-
aromatic 1-hexene would occupy fewer sites on the surface
due to the absence of an aromatic substituent.
Figure 5. Mass spectra from a pulse of benzyl alcohol over Pd/TiO2
that was precovered by D2: a) ions for toluene, b) ions for benzene;
conditions: temperature: 1908C, pressure: 1 atm; mole fraction of the
pulse: YD2 =20%, Ybenzyl alcohol =0.03%; gas flow rates: 140 mLminÀ1
.
The relative rates of the hydrogenation probe reactions
are plotted in Figure 4. Pd@TiO2-WI exhibited a much lower
activity for hydrogenation of styrene, while the other
encapsulated and supported Pd materials showed comparable
activity for this reaction. In contrast, all the Pd catalysts
showed similar activities for hydrogenation of 1-hexene.
These results suggest a significant restriction for styrene
adsorption and hydrogenation on Pd@TiO2-WI, supporting
the hypothesis that suppression of the flat-lying adsorption of
the reactant molecules contributes to the high HDO selec-
tivity in the case of Pd@TiO2-WI, as depicted in Figure 1b.
To further verify how reactants bind to the surface and the
resultant product from different binding orientations, we
performed isotopic studies using D2 instead of H2 for HDO of
benzyl alcohol. Figure 5b shows that H/D exchange occurred
for all six aromatic H atoms of benzene, which verified
benzene formation was likely through a “flat-lying” adsorbate
configuration, which enables facile exchange of aromatic H
with surface D. To the contrary, aromatic H/D exchange was
not observed for toluene (Figure 5a), suggesting its formation
occurred through a more “upright” adsorption, which only
enabled the H/D exchange on the methyl group. No aromatic
H/D exchange was observed for Pd@TiO2-WI, with toluene
being the only product (Figure S10), evidencing the restric-
tion of “flat-lying” adsorption on Pd@TiO2-WI. We also
conducted the isotopic study for hydrogenation of styrene
(Figure S11), which showed that the H/D exchange occurred
for all aromatic H atoms, suggesting its adsorption pattern is
also “flat-lying”. The similar restriction for both benzene and
ethylbenzene formation from benzyl alcohol and styrene,
respectively, strongly suggests that the Pd@TiO2-WI
restrained “flat-lying” adsorption of aromatic reactants
thereby improving HDO selectivity. We propose two possible
mechanisms for the apparent restriction of flat-lying adsorp-
tion. First, the confined pores of TiO2 films on Pd@TiO2-WI
may provide a more crowded surface to disrupt contiguous
arrays of active sites (“ensembles”) necessary for flat-lying
adsorption. It should be noted that the detailed connection
between bulk pore size and the structure of available sites
around the interface is not clear; the results reported here
suggest that the surface crowding on Pd@TiO2-WI may be
more severe than would be expected based on the average
(bulk) pore size. Alternatively, chemical interactions between
TiOx and Pd may contribute to the selectivity boost for
Pd@TiO2-WI, for example via more extensive electronic
perturbation of Pd sites within more highly confined pores.
To investigate the generality of this approach, the Pd
catalysts were used for HDO of furfural and m-cresol to
produce 2-methyl furan and toluene, respectively. For these
reactions, noble metals suffer poor reaction specificity despite
their high activity/stability; alloys and base metals have been
considered as substitutes.[36–38] As with the model reactants,
we observed high HDO selectivity using the Pd@TiO2-WI
(Figure 6). Figure S12 shows that the improved selectivity was
due to faster HDO rates and suppression of decarbonylation
and ring hydrogenation. It may be interesting to use a similar
encapsulation approach to enhance properties of more
inherently selective alloy and base metal catalysts.
Figure 4. Relative TOF of hydrogenation of olefins with different sizes
(styrene and 1-hexene) versus catalytic conversion of benzyl alcohol.
Reaction conditions are shown in Experimental Procedures in the
Supporting Information. The relative TOF for Pd/TiO2 was normalized
to 1 for both reactions using scaling factors. The same scaling factors
were used for all other Pd catalysts to ensure quantitative comparison
among them.
In conclusion, we showed that the HDO selectivity of
aromatic alcohols/aldehydes and phenolics can be signifi-
cantly enhanced by encapsulating Pd NPs with porous TiO2,
while maintaining high catalytic activity. While all the
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Manuscript received: April 9, 2017
Final Article published: && &&, &&&&
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Communications
Heterogeneous Catalysis
Nanoscale morphology control over
active sites consisting of Pd and TiO2
specifies binding orientation of reactant
molecules to provide unprecedented
reaction specificity toward hydrodeoxy-
genation during catalytic conversion of
biomass-derived aromatic alcohols/alde-
hydes and phenolics.
J. Zhang, B. Wang, E. Nikolla,*
J. W. Medlin*
&&&& — &&&&
Directing Reaction Pathways through
Controlled Reactant Binding at Pd–TiO2
Interfaces
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