Superior activity of rutile-supported ruthenium nanoparticles for HCl oxidation

Article abstract of DOI:10.1039/c3cy00372h

The activity of supported Ru-based catalysts in the oxidation of HCl to Cl₂ is shown to be strongly affected by active phase-support interactions and by the way of incorporation of the Ru species. Depositing colloidally prepared Ru nanoparticles on a rutile carrier results in catalysts attaining very high and stable HCl conversion levels under industrially-relevant conditions using ruthenium loadings as low as 0.16 wt%.

Full text of DOI:10.1039/c3cy00372h

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Superior activity of rutile-supported ruthenium
nanoparticles for HCl oxidation†
Cite this: Catal. Sci. Technol., 2013,
3, 2555
Evgenii V. Kondratenko,a Amol P. Amrute,b Marga-Martina Pohl,a
Received 29th May 2013,
Accepted 5th July 2013
a
b
b
´
´
Norbert Steinfeldt, Cecilia Mondelli and Javier Perez-Ramırez*
DOI: 10.1039/c3cy00372h
www.rsc.org/catalysis
The activity of supported Ru-based catalysts in the oxidation of HCl to Therefore, the use of NPs for developing novel catalysts is also
Cl2 is shown to be strongly affected by active phase–support inter- attractive from economic and environmental perspectives.
actions and by the way of incorporation of the Ru species. Depositing
The gas-phase oxidation of HCl to Cl2 comprises a recent
colloidally prepared Ru nanoparticles on a rutile carrier results in industrial process that would benefit from a decreased catalyst cost
catalysts attaining very high and stable HCl conversion levels in order to achieve a wider application. This reaction represents a
under industrially-relevant conditions using ruthenium loadings sustainable route to recycle by-product HCl streams in the phosgene-
as low as 0.16 wt%.
mediated manufacture of polyurethanes and polycarbonates.15,16
RuO2/TiO2/SiO2 and RuO2/SnO2–Al2O3 are the state-of-the-art
Catalysts featuring supported metals or metal oxides are applied in catalysts, developed by Sumitomo and Bayer, respectively.17–19
numerous commercial processes to convert abundant raw materials The choice of a support with identical structure (rutile type)
into higher-value products ranging from fuels to plastics, resins, and similar lattice parameters to the active phase has been
and pharmaceuticals. Their performance is typically determined indicated as one main reason for the outstanding performance
by the chemical or structural nature of the deposited species, their of these materials, as maximum RuO2 dispersion and stabilisa-
electronic and/or acid–base properties, and the active phase– tion are expected through its epitaxial growth.17 Due to the high
support interactions.1–5 Therefore, synthetic control over all these and fluctuating market price of ruthenium (peak price at
parameters is critical for designing optimised catalysts, from the 600 USD per ounce in 2007),15 considering a typical Ru loading
viewpoints of performance and active phase utilisation. Recently, of 1–2 wt%18,20 leads to a metal cost associated to the catalyst for
various routes have been developed to prepare metal nanoparticles a world-scale HCl recycling facility (ca. 100 kton Cl2 per year) of
(NPs) with well-defined size, shape, and chemical composi- several million euros. Thus, catalysts containing a lower amount of
tion.1,4,6,7 Noteworthily, the size of tailored NPs usually prepared ruthenium are highly desired but their activity and robustness
in colloidal solutions8,9 remains unaltered upon deposition onto should not be compromised. A first step in this direction is
typical catalyst carriers.10,11 This enabled us to systematically represented by the recent development by some of us of a RuO2/
investigate the effect of size and even shape of supported noble SnO2–Al2O3 system with 0.5 wt% Ru which displays excellent
metal NPs on the activity and selectivity in both low-temperature stability and an even double Ru-specific activity compared to
hydrogenation4,7,12 and high-temperature oxidation10,11,13,14 the counterpart catalyst with 2 wt% Ru, likely due to an enhanced
reactions. Moreover, owing to the high metal utilisation and, dispersion of the active phase.18,19 As all these catalysts have
thus, activity attained, it was possible to generate catalysts based been prepared by conventional impregnation of the carriers
on NPs with drastically reduced metal loadings, even down to with a metal precursor solution, followed by calcination,18–20
0.005 wt%, which still exhibited satisfactory performance.11,14 further catalyst optimisation might be achieved by variations in
the synthetic strategy.
Herein, we show that the deposition of tailored Ru NPs on
a
¨
¨
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock,
TiO2-rutile with a metal loading as low as 0.16 wt% produces a
highly active and stable material, while an identically prepared
Al2O3-supported catalyst is virtually inactive. This result high-
lights the importance of the active phase–support interactions.
Moreover, our novel rutile-based system exhibits a HCl conver-
sion level close to that of a classically prepared reference RuO2/
TiO2-rutile catalyst with a Ru loading in the range of those
Albert-Einstein-Str. 29 A, D-18059 Rostock, Germany
b Institute for Chemical and Bioengineering, Department of Chemistry and Applied
Biosciences, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland.
E-mail: jpr@chem.ethz.ch; Fax: +41 44 6331405; Tel: +41 44 6337120
† Electronic supplementary information (ESI) available: Details of catalyst pre-
paration, characterisation, and catalytic evaluation. Results from XPS of the
as-prepared catalyst featuring Ru NPs on TiO2-r and H2-TPR of selected samples
after use in HCl oxidation. See DOI: 10.1039/c3cy00372h
c
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reported for the industrial systems (2 wt%).18,20 The metal maintained upon deposition on the support, as confirmed using
utilisation in this NP-based catalyst is significantly enhanced, X-ray photoelectron spectroscopy (XPS, Fig. S2, ESI†). As expected,
as its Ru-related activity is 2 times higher than that of an the conventional impregnation method resulted in the formation of
impregnated catalyst with the same metal content (0.16 wt%) bigger (2–3 nm) nanostructures for the same metal loading (Fig. 1c).
and 9 times higher than the reference catalyst. Its exceptional In this case, their nature is oxidic due to the calcination step.
performance is explained by the extraordinarily high dispersion
To elucidate the effect of the carrier nature and of the prepara-
of the catalytically active RuO2 species and by electronic effects tion method on the activity of the supported Ru NP-containing
of the TiO2-rutile support. The former is demonstrated to catalysts in the gas-phase oxidation of HCl to Cl2, catalytic tests
originate from the reaction-induced oxidation of the initially were performed in a continuous-flow micro-reactor (details are
metallic spherical Ru NPs and restructuring into thin plate-like provided in the ESI†). In view of the presence of excess O2 in the
RuO2 structures, owing to the lattice matching with the support. reaction mixture, we questioned whether the metallic Ru NPs
The latter is shown to determine the generation of RuO2 species underwent oxidation upon use. To clarify this point, selected
featuring highly labile oxygen atoms.
catalysts applied for 2 h in the reaction were characterised by
The Ru NPs were prepared by the ethylene glycol (EG) method21 temperature-programmed reduction with hydrogen (H2-TPR).
and deposited on TiO2-rutile (r), TiO2-anatase (a), or g-Al2O3 carriers Hydrogen consumption below 550 K was observed (Fig. S3,
with a metal loading of 0.16–0.4 wt%. For benchmarking purposes, ESI†), which corresponds to the reduction of RuO2 to Ru,22 thus
we applied the dry impregnation (I) method to produce indicating the in situ formation of an oxidic phase. This
TiO2-r-supported catalysts with 0.16 (RuO2/TiO2-I) and 2 wt% implies, from a kinetic viewpoint, that the rate of interaction
Ru (RuO2/TiO2-I-ref). Further details on these synthetic proce- of gaseous O2 with metallic Ru is significantly higher than the
dures are provided in the ESI.† Table 1 shows the actual metal rate of removal of adsorbed oxygen species by HCl. Thus,
loading and surface area of the catalysts. The double Ru irrespective of the method of catalyst preparation, RuO2 species
content of the alumina-supported catalyst compared to the are active for the oxidation of HCl to Cl2. This justifies the usage
other catalysts attained using the EG route is likely due to the of RuO2 instead of Ru in the catalysts’ nomenclature.
much larger surface area of this support.
The HCl conversion was strongly affected by both the structure
The size and distribution of the Ru NPs in the synthesis of the supporting oxide and the synthetic procedure (Table 1). With
solution were determined by small angle X-ray scattering (SAXS) regard to the first aspect, among the catalysts prepared via the
in combination with the indirect Fourier transformation (IFT) colloidal route, RuO2/TiO2-r-EG showed the highest HCl conver-
technique. The resulting function of volume-weighted particle sion, which was approximately 2 and 13 times higher than for
radius distribution is relatively narrow with a maximum at RuO2/TiO2-a-EG and RuO2/g-Al2O3-EG, respectively. This activity
ca. 0.5 nm (Fig. S1 in the ESI†). Since the same colloidal solution order resembles the previously reported relative activity of classi-
was used for preparing the supported catalysts, the size of the cally prepared catalysts featuring supported RuO2 species.17 Thus,
Ru NPs deposited on the carriers was not expected to differ. The the role of the carrier is decisive regardless of the method of
distribution and morphology of the supported ruthenium phase preparation and deposition of the catalytically active species.
was analysed using high-angle annular dark field scanning trans- On the other hand, the activity of the TiO2-r-supported catalysts
mission electron microscopy (HAADF-STEM). Accordingly, the Ru is remarkably influenced by the way of incorporation of the
nanostructures appear bright in contrast to the background, Ru-containing phase. In fact, RuO2/TiO2-r-EG displayed more
which was confirmed by elemental energy-dispersive X-ray (EDX) than double HCl conversion compared to the corresponding
analysis of selected regions of the images. Very small and well- impregnated catalyst with identical metal loading (Table 1).
distributed spherical NPs were obtained via the EG route on all Actually, the difference in activity would be even greater bearing
of the carriers, as exemplified by the micrograph of the fresh in mind that (i) the catalytic tests were performed in an integral
Ru/TiO2-r-EG catalyst (Fig. 1a). In line with the SAXS measure- reactor and (ii) the negative effect of Cl2 and H2O on the HCl
ments, the size distribution of the supported species is also conversion (product inhibition) increases with the conversion. It
narrow (Fig. 1b). Their average diameter was estimated to be must be highlighted that the RuO2/TiO2-r-EG catalyst exhibited
ca. 1.2 Æ 0.25 nm. The originally metallic state of the NPs was only a slightly lower HCl conversion, but a Ru-specific rate of one
order of magnitude higher, than the reference catalyst with a Ru
loading of 2 wt% (RuO2/TiO2-r-I-ref, Table 1).18
Table 1 Characterisation and catalytic data of supported RuO2 catalysts
In order to obtain kinetic insights into the superior perfor-
Rua
(wt%) (m2 gÀ1
SBET
XHCl r c
(%) )
(mol Cl2 minÀ1 mol RuÀ1
b
c
mance of the novel NPs-based catalyst, we investigated the effects
of temperature and O2/HCl ratio on its activity. Interestingly, the
apparent activation energy and reaction order in O2 for RuO2/
TiO2-r-EG (66 kJ molÀ1 and 0.5) were very similar to those
for RuO2/TiO2-r-I (64 kJ molÀ1 and 0.5) (Fig. 2a and b). These
results indicate that the deposition method of the RuO2 species
does not alter the representative kinetic fingerprints of HCl
oxidation. Furthermore, the stable HCl conversion of RuO2/
TiO2-r-EG during a 50 h catalytic run (Fig. 2c) demonstrated that
Catalyst
)
RuO2/TiO2-r-EG 0.16
RuO2/TiO2-a-EG 0.17
RuO2/g-Al2O3-EG 0.40
29 (28)d 51
54 (51) 22
149 (156)
33 (26) 23
30 (29) 70
48
19
1.4
21
5.3
4
RuO2/TiO2-r-I
0.16
RuO2/TiO2-r-I-ref 2.00
a
b
c
Determined by ICP-OES. N2 sorption at 77 K. Conditions: W = 0.25 g,
Tbed = 623 K, O2/HCl = 2, FT = 166 cm3 STP minÀ1, t = 2 h, and P = 1 bar.
d
Surface area of the samples after Deacon reaction in parentheses.
c
2556 Catal. Sci. Technol., 2013, 3, 2555--2558
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Fig. 1 HAADF-STEM of RuO2/TiO2-r-EG (a and d), RuO2/TiO2-r-I (c and f), and RuO2/Al2O3-EG (e) in fresh form (a and c) and after HCl oxidation (d–f). Size distribution
of the supported particles in fresh RuO2/TiO2-r-EG (b).
Fig. 2 Comparative performance of RuO2 (0.16 wt% Ru)/TiO2-r prepared by the ethylene glycol route (EG) and by dry impregnation (I): Arrhenius plot at
Tbed = 543–653 K and O2/HCl = 2 (a), dependence of the HCl conversion on the feed O2/HCl ratio at Tbed = 623 K (b), and long catalytic run at Tbed = 623 K and
O2/HCl = 2 (c). Other conditions: W = 0.25 g, FT = 166 cm3 STP minÀ1, and P = 1 bar. Data were acquired after 1 h under each condition for (a) and (b).
this synthetic procedure also leads to a catalyst with potential form, while those on TiO2-r were partially transformed into
technical relevance.
plate-like RuO2 structures. The restructuring of the Ru NPs on
In order to rationalise the support effect, since all systems TiO2-r is explained on the basis of the matching between TiO2-r
studied feature RuO2 species, we first explored whether their and RuO2 in terms of lattice symmetry and parameters (see
distinctive performance could be related to differences in scheme in the graphical abstract). The structural analogy of
morphology of the active phase. Based on the HAADF-STEM carrier and active phase has been already shown to drive
images of the used samples (Fig. 1d–f), it appears that the the deposition of RuO2 as an epitaxial layer over the carrier
initially spherical Ru NPs on Al2O3 did not change their original in classically prepared catalysts with higher Ru loading.17,22
c
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This effect is further supported by the identification of plate-like stability. In view of a potential practical application, it is also
RuO2 structures also on the surface of RuO2(0.16 wt% Ru)/TiO2-r-I relevant to note that, although the synthetic approach utilised
(Fig. 1d). Although a statistically relevant quantitative determina- here to prepare and deposit Ru nanoparticles on carriers is not as
tion of the thickness of the RuO2 platelets in RuO2/TiO2-r-EG is standard in industrial practice as the classical impregnation
challenging, the thickness and size of the RuO2 layers formed via methods, the procedure is amenable to large-scale production.
impregnation appeared significantly larger than those attained
We thank Bayer MaterialScience AG for permission to publish
from the well-defined Ru NPs. This is probably due to the fact that these results.
the latter species transform easier into thin RuO2 layers than
larger aggregates already in oxidic form.
Notes and references
The described morphology change of the TiO2-r-supported Ru
NPs is expected to determine a somewhat higher Ru dispersion
for RuO2/TiO2-r-EG than for the other catalysts obtained via the
EG route. Possibly, a larger fraction of the intrinsically more active
and stable RuO2 (110) facet is also exhibited in the platelets in line
with the higher exothermic adhesion energy on TiO2-r calculated
for this compared to other exhibited RuO2 surfaces.22,23 Never-
theless, the TiO2-r support should exert additional promoting
effect(s) to account for the much higher HCl oxidation activity of
RuO2/TiO2-r-EG. Based on the H2-TPR analysis of the used cata-
lysts, we put forward the existence of electronic interactions. The
reduction profile of RuO2-/TiO2-r-EG (Fig. S3, ESI†) in fact evi-
dences an additional H2 consumption peak with a maximum at
368 K, which suggests the presence of RuO2 species featuring
labile lattice oxygen species. The great relevance of these species
for the catalytic activity certainly deserves a precise elucidation of
their nature and impact on the HCl oxidation at a molecular level.
However, this would require the application of advanced char-
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c
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2558 Catal. Sci. Technol., 2013, 3, 2555--2558