Strong Metal-Support Interactions for Palladium Supported on TiO2 Catalysts in the Heterogeneous Hydrogenation with Parahydrogen

Article abstract of DOI:10.1002/cctc.201500618

Parahydrogen-induced polarization (PHIP) was successfully utilized to demonstrate the strong metal-support interaction (SMSI) effect for palladium supported on titania catalysts. Heterogeneous hydrogenation of 1,3-butadiene over Pd/TiO₂ catalysts led to the formation of 1- and 2-butenes and butane, and hyperpolarized products were obtained if parahydrogen was used in the reaction. However, if the catalysts were reduced in H₂ flow at 500°C before the hydrogenation reaction, the observed polarization levels were significantly lower or even zero, which was indicative of the suppression of the pairwise addition of hydrogen route. This observation indicated the possibility to detect the SMSI effect by the PHIP technique. Moreover, by using X-ray photoelectron spectroscopy it was shown that Pd is partially present as Pd^δ+ after reduction under a hydrogen atmosphere at 500°C. These results were confirmed by transmission electron microscopy, which revealed the formation of Pd^δ+ and the dissolution of Pd in the titania lattice.

Full text of DOI:10.1002/cctc.201500618

DOI: 10.1002/cctc.201500618
Communications
Strong Metal–Support Interactions for Palladium
Supported on TiO2 Catalysts in the Heterogeneous
Hydrogenation with Parahydrogen
Kirill V. Kovtunov,*[a, b] Danila A. Barskiy,[a, b] Oleg G. Salnikov,[a, b] Dudari B. Burueva,[a, b]
Alexander K. Khudorozhkov,[b, c] Andrey V. Bukhtiyarov,[b, c] Igor P. Prosvirin,[b, c]
Evgeny Y. Gerasimov,[b, c] Valerii I. Bukhtiyarov,[b, c] and Igor V. Koptyug[a, b]
Parahydrogen-induced polarization (PHIP) was successfully uti-
lized to demonstrate the strong metal–support interaction
(SMSI) effect for palladium supported on titania catalysts. Het-
erogeneous hydrogenation of 1,3-butadiene over Pd/TiO2 cata-
lysts led to the formation of 1- and 2-butenes and butane, and
hyperpolarized products were obtained if parahydrogen was
used in the reaction. However, if the catalysts were reduced in
H2 flow at 5008C before the hydrogenation reaction, the ob-
served polarization levels were significantly lower or even zero,
which was indicative of the suppression of the pairwise addi-
tion of hydrogen route. This observation indicated the possibil-
ity to detect the SMSI effect by the PHIP technique. Moreover,
by using X-ray photoelectron spectroscopy it was shown that
Pd is partially present as Pdd+ after reduction under a hydrogen
atmosphere at 5008C. These results were confirmed by trans-
mission electron microscopy, which revealed the formation of
Pdd+ and the dissolution of Pd in the titania lattice.
cessfully observed by the PHIP technique,[5–7] contrary to the
implications of the Horiuti–Polanyi mechanism[8] widely accept-
ed for a broad range of supported metal nanoparticles. There-
fore, understanding the nature of active sites that can add two
hydrogen atoms from the same hydrogen molecule as a pair
to a substrate molecule is a highly important task.[9–11]
Recently, it was shown that the nature of the catalyst sup-
port can have a significant impact on the rate of the pairwise
addition of hydrogen.[12,13] Remarkably, titania-supported metal
catalysts were shown to exhibit much higher levels of the pair-
wise addition of hydrogen (and larger PHIP effects) relative to
that exhibited by metals on other supports.[9,14] Therefore, ex-
amination of the nature of the active sites for titania-supported
nanoparticles in the context of the pairwise addition of hydro-
gen may open up new possibilities for the rational preparation
of catalysts that are able to maximize the observed PHIP ef-
fects and for the production of hyperpolarized molecular con-
trast agents potentially suitable for in vivo magnetic resonance
imaging investigations.[14–16]
The hydrogenation of unsaturated compounds with parahy-
drogen leads to the observation of the parahydrogen-induced
polarization (PHIP) phenomenon.[1,2] This results in a significant
enhancement in the NMR signals of the corresponding reac-
tion products or intermediates. In the case of homogeneous
catalytic hydrogenation, a single metal center often plays the
role of the active site for hydrogen activation, and therefore,
the pairwise hydrogen addition route is the major mechanism
for such processes.[3,4] In contrast, heterogeneous pairwise ad-
dition of hydrogen is uncommon. Nevertheless, it can be suc-
In catalysis, it is well known that metals supported on titania
exhibit the strong metal–support interaction (SMSI) effect.[17,18]
This phenomenon is known to modify the selectivity and activ-
ity of a catalyst.[19,20] Therefore, significant alteration in the se-
lectivity of the pairwise addition of hydrogen for titania-sup-
ported catalysts described above is not surprising.[11–13] One
plausible explanation for the nature of the SMSI effect is the
electronic interaction between the metal and the support,
which leads to the formation of an electron-rich metal.[21,22] An-
other possible explanation is migration of the support material
and encapsulation of metal nanoparticles. Importantly, titania
is able to rapidly migrate over considerable distances and to
appear on initially pristine metal surfaces.[23] However, the first
observation and evidence for the SMSI effect was related to
a significant loss in chemisorption ability for noble metal cata-
lysts supported on titania after hydrogen treatment at high
temperatures.[17] Such dramatic changes in the chemisorption
properties are closely linked to the catalytic activity and selec-
tivity of supported nanoparticles. Therefore, the SMSI effect
plays a very important role in many heterogeneous catalytic
processes.
[a] Dr. K. V. Kovtunov, D. A. Barskiy, O. G. Salnikov, D. B. Burueva,
Prof. I. V. Koptyug
Laboratory of Magnetic Resonance Microimaging
International Tomography Center, SB RAS
3 A Institutskaya St., Novosibirsk 630090 (Russia)
E-mail: kovtunov@tomo.nsc.ru
[b] Dr. K. V. Kovtunov, D. A. Barskiy, O. G. Salnikov, D. B. Burueva,
A. K. Khudorozhkov, Dr. A. V. Bukhtiyarov, Dr. I. P. Prosvirin,
Dr. E. Y. Gerasimov, Prof. V. I. Bukhtiyarov, Prof. I. V. Koptyug
Novosibirsk State University
2 Pirogova St., Novosibirsk 630090 (Russia)
[c] A. K. Khudorozhkov, Dr. A. V. Bukhtiyarov, Dr. I. P. Prosvirin,
Dr. E. Y. Gerasimov, Prof. V. I. Bukhtiyarov
The major differences in the PHIP effects observed for metal
nanoparticles supported on titania relative to those previously
observed for metals on other supports[11–13] were tentatively at-
tributed to low-temperature SMSI effects. Therefore, in this
communication the behavior of titania-supported catalysts is
Boreskov Institute of Catalysis SB RAS
5 Acad. Lavrentiev Pr., Novosibirsk 630090 (Russia)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500618.
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explored over a much broader temperature range to systemat-
ically address the SMSI effect and its influence on the efficiency
of the pairwise addition of hydrogen in hydrogenation reac-
tions with parahydrogen. The decrease in the overall catalytic
activity of Pd/TiO2 catalysts under SMSI conditions (H2 treat-
ment at 5008C) and significant (down to zero) loss of PHIP
levels during heterogeneous hydrogenation of 1,3-butadiene
with parahydrogen were observed, and the Pdd+ state was de-
tected as confirmation of the presence of the SMSI effect.
Two series of Pd/TiO2 supported catalysts reduced under an
atmosphere of H2 at 100 and 5008C were used in the hetero-
geneous hydrogenation of 1,3-butadiene (Figure 1). Strong
PHIP signals were observed if the Pd/TiO2 catalyst reduced at
fact that PHIP effects were not detected for butane if the Pd/
TiO2 catalyst reduced at 5008C was used (see Figure S1 in the
Supporting Information).
There are two possible reasons for these observations. First,
the significant loss in polarization may be caused by agglomer-
ation of metal nanoparticles, which without doubt occurs
during reduction at high temperatures. Second, the decrease
in the polarization levels may be directly related to the nature
of the active sites responsible for the pairwise addition of hy-
drogen. To determine the particle-size distributions for the Pd/
TiO2 catalysts reduced at various temperatures, transmission
electron microscopy (TEM) was utilized. For the Pd/TiO2 cata-
lyst reduced at 1008C, the average particle size was found to
be approximately 4.5 nm, whereas for the catalyst reduced at
5008C it was approximately 5.5 nm (see Figure S2). This insig-
nificant particle size increase is not expected to have such
a dramatic impact on the rate of the pairwise addition of hy-
drogen. Moreover, for the Pd/TiO2 catalyst with a 9.4 nm metal
particle size reduced at 1008C, PHIP effects in the heterogene-
ous hydrogenation of 1,3-butadiene with parahydrogen were
successfully detected, whereas the same catalyst reduced at
5008C under a hydrogen atmosphere did not produce signifi-
cant PHIP effects (see Figure S3). All of these observations
imply that the change in the metal particle size has an insignif-
icant influence on the levels of the observed PHIP effects and
that the main mechanism of polarization loss is blockage of
the catalytically active sites responsible for the pairwise addi-
tion of hydrogen.
The origin of the SMSI effect for palladium catalysts support-
ed on titania upon reduction under a hydrogen atmosphere at
temperatures of approximately 5008C, which leads to a moder-
ate loss in the overall catalytic activity along with a significant
decrease in polarization levels, was further studied by using
TEM. Importantly, for the Pd/TiO2 catalyst reduced at 5008C,
unique TEM images were obtained that provided an opportu-
nity to observe the SMSI between TiO2 and Pd directly
(Figure 2).
Figure 1. 1HPHIP NMR spectra for the heterogeneous hydrogenation of 1,3-
butadiene at 1008C over Pd/TiO2 with a 4.5 nm average particle size previ-
ously reduced under an atmosphere of H2 at various temperatures (100, 300,
and 5008C).
1008C was used. Remarkably, for the Pd/TiO2 catalyst reduced
under a H2 atmosphere at 5008C, the observed polarization
levels diminished dramatically: the polarization dropped to
almost zero for butane, and for 1-butene and 2-butene it
became more than one order of magnitude lower than that
observed with the Pd/TiO2 catalyst reduced at 1008C. Such
a dramatic decrease in polarization intensity may be explained
in terms of the SMSI effect. With the aim to investigate the in-
fluence of the reduction temperature on the intensities of the
PHIP signals and the underlying SMSI effect, the samples of
the Pd/TiO2 catalyst were reduced at various temperatures in
the range of 100 to 5008C at 1008C increments. As a result, it
was established that the intensities of the PHIP signals de-
creased progressively and significantly with an increase in the
reduction temperature (Figure 1).
It was found by TEM that a thin (ꢀ2 nm) layer of Pd is
formed on the support surface, and the interplanar distances
of Pd metal change from 2.25 to 2.17 Š. Moreover, Pd signals
are observed in the energy-dispersive X-ray (EDX) spectrum for
catalyst areas in which no palladium particles are visible by
Importantly, the decrease in intensities of the polarized sig-
nals was also accompanied by a decrease in the overall hydro-
genation activity of the Pd/TiO2 catalysts. This demonstrates
the presence of the SMSI effect, as defined in the context of
catalytic research. At the same time, the overall activity of the
Pd/TiO2 catalyst in the heterogeneous hydrogenation of 1,3-
butadiene decreased by only 20–30% if the reduction temper-
ature was increased to 5008C. This is in stark contrast to the
Figure 2. TEM image of the Pd/TiO2 catalyst reduced at 5008C under a hydro-
gen atmosphere, visualizing the formation of a thin layer of Pd.
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TEM (see Figure S4). Consequently, some part of Pd is present
in the catalyst sample as small atomic clusters. These observa-
tions confirm that some interaction between Pd and TiO2
during high-temperature reduction occurs. One plausible ex-
planation for this effect is the partial oxidation of Pd0 caused
by its interaction with the surface of TiO2.
(2%) and therefore an insignificant contribution of Ti3+ species
to the total intensity of the Ti 2p photoelectron spectra leads
to the absence of the Ti3+ characteristic line in the Ti 2p photo-
electron spectrum (see the Supporting Information). Moreover,
it is not possible to detect the Ti atoms located under Pd parti-
cles. On the basis of these results, most Pdd+ species detected
by XPS are located at the interface between the Pd nanoparti-
cles and titania.
To verify the electronic state of palladium, X-ray photoelec-
tron spectroscopy (XPS) was utilized. Ex situ XPS studies for
which reduction of the Pd/TiO2 catalyst samples occurred out-
side the XPS setup were performed (see the Supporting Infor-
mation). Figure 3a shows the Pd3d photoelectron spectra
measured for Pd/TiO2 after sample reduction and/or oxidation
at various temperatures.
Therefore, we may suggest that the formation of the Pdd+
state, as confirmed by TEM and ex situ and in situ XPS, may
prevent the pairwise addition of hydrogen. We note that the
amount of the Pdd+ state is approximately 25%, and the over-
all catalytic activity decreased by 20–30%. Therefore, dissolu-
tion (encapsulation) of Pd metal in the lattice of TiO2, which
was successfully detected by TEM (Figure 2), leads to the for-
mation of the Pdd+ state, as detected by XPS. During encapsu-
lation of Pd nanoparticles, an entirely new state of palladium is
formed. Given that the overall activity of the catalyst decreased
proportionally to the amount of Pdd+ states produced, where-
as the activity in the pairwise addition of hydrogen decreased
dramatically, the formation of the Pdd+ state on titania corre-
lates with the disappearance of catalytically active sites respon-
sible for the pairwise addition of hydrogen and the observa-
tion of PHIP effects. The combination of the results obtained
by TEM and XPS allows us to suggest that Pdd+ sites are
formed at the metal–support interfaces, which provides direct
confirmation of the presence of the SMSI effect. The combina-
tion of the XPS data with the NMR spectroscopy results ob-
tained during heterogeneous hydrogenation with parahydro-
gen (Figures 1 and 3a) also allows us to conclude that maxi-
mum levels of polarization were achieved for the partially re-
duced catalyst for which the total amount of metal–support or
metal–oxide interfaces was maximized.
Figure 3. XPS spectra of the Pd/TiO2 catalyst obtained during the a) ex situ
and b) in situ experimental protocols.
In conclusion, the strong metal–support interaction (SMSI)
effect for palladium supported on titania catalysts is associated
with the formation of the Pdd+ state during high-temperature
(5008C) hydrogen treatment. In the context of the observed
parahydrogen-induced polarization (PHIP) effect, the high-tem-
perature SMSI effect dramatically decreases the activity of the
Pd/TiO2 catalyst in terms of the pairwise addition of hydrogen.
Therefore, utilization of the catalyst reduced at 5008C in the
heterogeneous hydrogenation of 1,3-butadiene with parahy-
drogen leads to a dramatic (more than one order of magni-
tude) loss in polarization. Formation of the Pdd+ state by en-
capsulation of palladium metal nanoparticles, as confirmed by
X-ray photoelectron spectroscopy (XPS) and transmission elec-
tron microscopy (TEM), correlated with the decrease in the
number of active sites responsible for the pairwise addition of
hydrogen. Utilization of the PHIP technique to investigate SMSI
effects along with TEM and XPS analysis was demonstrated
here for the first time and demonstrates that PHIP effects are
much more sensitive to the onset of SMSI effects than the cat-
alytic activity or chemisorption measurements. The reported
results also imply that the true origin of the much larger PHIP
effects observed for titania-supported metal catalysts reduced
at low temperatures relative to those observed for metals on
other supports needs to be explored further.
Three different electronic states of Pd are detected depend-
ing on the sample treatment procedure. The state with a bind-
ing energy of 336.7 eV, which is common for palladium(II)
oxide (PdO), was obtained after treatment of the catalyst
sample under an oxygen atmosphere at 4008C.[24–26] Further
treatment at 1008C under an atmosphere of H2 led to the ap-
pearance of a second state with a Pd3d5/2 binding energy of
334.9 eV, which can be attributed to the metallic Pd0 form.[24–27]
After heating Pd/TiO2 at 3008C under a H2 atmosphere, only
the metallic Pd0 state was present at the surface of titania. Fi-
nally, hydrogen treatment at 5008C (SMSI conditions) allowed
us to observe an additional Pd state (binding energy of
336.1 eV), assigned to Pdd+, which was formed as a result of
the interaction between Pd and TiO2.
Further confirmation of the formation of the Pdd+ state for
the Pd/TiO2 catalyst was obtained by using an in situ H2 treat-
ment at 5008C along with XPS detection (Figure 3b). A clearly
visible Pdd+ state was found to form during reduction at
5008C. To the best of our knowledge, this is the first direct ob-
servation of the formation of Pdd+ for a catalytic metal system
supported on titania (see the Supporting Information). We
note that under such conditions any possibility for the pres-
ence of PdO is completely excluded. Low concentrations of Pd
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Acknowledgements
O.G.S., D.A.B., V.I.B., and I.P.P. thank Russian Science Foundation
(RSCF) (grant 14-13-00445) for support of catalysts preparation
and XPS studies. I.V.K., K.V.K., and E.Y.G. thank Russian Founda-
tion for Basic Research (RFBR) (grants 14-03-00374-a, 14-03-
31239-mol-a, and 14-03-93183 MCX_a) for support of the high-
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Received: May 2, 2015
Published online on August 3, 2015
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