Effects of the Support-Crystal Size on the Catalytic Performance of RuO2/TiO2 in the Deacon Process

Article abstract of DOI:10.1007/s10562-020-03493-5

RuO₂/TiO₂ catalysts with different TiO₂ crystal sizes were prepared via a dry impregnation method, and these prepared catalysts were applied in the oxidation of HCl. The results show that decreasing the support-crystal size is an effective method to enhance the dispersion of RuO₂ on TiO₂, which is helpful to increase the catalytic activity significantly. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03493-5

Catalysis Letters
https://doi.org/10.1007/s10562-020-03493-5
Efects oꢀ the Support‑Crystal Size on the Catalytic Perꢀormance
oꢀ RuO /TiO in the Deacon Process
2
2
Xue Wang · Yupei Liu · Chunhui Xu · Xinqing Lu · Rui Ma1,2 · Yanghe Fu1,2 · Shuhua Wang · Weidong Zhu
1
1
1
1,2
3
1,2,3
Received: 13 August 2020 / Accepted: 3 December 2020
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
RuO /TiO catalysts with diferent TiO crystal sizes were prepared via a dry impregnation method, and these prepared cata-
2
2
2
lysts were applied in the oxidation oꢀ HCl. The results show that decreasing the support-crystal size is an efective method
to enhance the dispersion oꢀ RuO on TiO , which is helpꢀul to increase the catalytic activity signiꢁcantly.
2
2
Graphic Abstract
Keywords TiO · Support-size efect · RuO · Dispersion · Deacon process
2
2
Supplementary inꢀormation The online version oꢀ this article
https://doi.org/10.1007/s10562-020-03493-5) contains
supplementary material, which is available to authorized users.
(
2
3
*
*
Xinqing Lu
Key Laboratory oꢀ the Ministry oꢀ Education
ꢀor Advanced Catalysis Materials, Institute oꢀ Physical
Chemistry, Zhejiang Normal University, Jinhua 321004,
People’s Republic oꢀ China
xinqinglu@zjnu.cn
Weidong Zhu
weidongzhu@zjnu.cn
National Engineering Technology Research Center
oꢀ Fluoro-Materials, Zhejiang Juhua Technology Center Co.,
Ltd, Quzhou 324004, People’s Republic oꢀ China
1
Zhejiang Engineering Laboratory ꢀor Green Syntheses
and Applications oꢀ Fluorine-Containing Specialty
Chemicals, Institute oꢀ Advanced Fluorine-Containing
Materials, Zhejiang Normal University, Jinhua 321004,
People’s Republic oꢀ China
Vol.:(0
1
123456789)
3

X. Wang et al.
1
Introduction
same Ru content prepared by a conventional impregnation
method. Additionally, it is expected that the dispersion
oꢀ RuO2 might be enhanced by decreasing the support-
crystal size, because the smaller the support-crystal size,
the higher the speciꢁc surꢀace area ꢀor carrying the active
species. However, the efects oꢀ the support-crystal size on
the catalytic properties oꢀ the Deacon catalyst RuO2/TiO2
have not been systematically investigated yet.
Chlorine is widely used as a reactive intermediate ꢀor man-
uꢀacturing industrial and consumer products, especially
polyurethane and polycarbonate. However, approximately
5
0% oꢀ the Cl produced worldwide ends up as HCl or
2
chloride salts [1]. The most representative example is the
production oꢀ toluene diisocyanate (TDI), in which 4 mol
oꢀ HCl are produced ꢀor each mole oꢀ TDI production
while the produced HCl is environmentally undesirable
and has a limited market. Consequently, it is attractive to
In this work, RuO2/TiO2 catalysts with diferent support-
crystal sizes were prepared and characterized with various
techniques in detail. These prepared catalysts were then eval-
uated in the Deacon process and the efects oꢀ the support-
crystal size on the catalytic properties oꢀ the resultant RuO2/
TiO2 in the HCl oxidation were systematically investigated.
The results show that the decrease oꢀ the support-crystal
size can be an efective method to enhance the dispersion
oꢀ RuO2 on TiO2, leading to a signiꢁcant increase in the
catalytic activity.
ꢁ
nd eꢂcient methods ꢀor converting the by-product HCl
in waste streams back into the reactive intermediate Cl .
2
The heterogeneously catalyzed gas-phase oxidation oꢀ
HCl by molecular oxygen to H O and Cl , the so-called
2
2
Deacon process, is a sustainable route to meet the world’s
growing demand ꢀor Cl and to recycle the by-product HCl
2
in the chlorine-related chemical industry [2, 3], showing
a lower energy consumption than that by an electro-cata-
lyzed chlorine evolution reaction. In the Deacon process,
it has been repeatedly demonstrated that supported RuO
2 Experimental
2
catalysts are efective because oꢀ their high activities at
low temperatures and resistance to bulk chlorination [4].
Both TiO (rutile) and SnO (cassiterite) have been iden-
2.1 Catalyst Preparation
2
2
tiꢁed as the best supports ꢀor preparing supported RuO
Based on the recipe and synthesis procedure reported in the
literature [22], the rutile-type TiO2 support with a crystal
size oꢀ ca. 2000 nm was prepared as ꢀollows, 0.6 g oꢀ TiCl4
was dissolved in 100 mL oꢀ HCl aqueous solution (3.5 M)
with stirring in an ice-water bath, ꢀollowed by adding 0.07 g
oꢀ NaF. The obtained reaction mixture was then hydrother-
mally crystallized at 493 K ꢀor 12 h and cooled to room tem-
perature. The product was washed with distilled water and
then dried at 373 K overnight. Finally, the dried sample was
calcined in static air at 773 K ꢀor 5 h with a heating rate oꢀ
2
Deacon catalysts, because RuO is deposited in the ꢀorm
2
oꢀ epitaxially grown structures as a result oꢀ lattice match-
ing, leading to an enhanced dispersion and stabilization
oꢀ the active phase, which is quite diferent ꢀrom other
supports, such as SiO , Al O , and TiO with an anatase-
2
2
3
2
type structure [5–10]. Additionally, rutile-TiO supported
2
RuO catalysts are also highly active in many other impor-
2
tant reactions, such as photocatalytic water splitting [11],
trichloroethylene oxidation [12], CO oxidation [13], dehy-
drogenation oꢀ NH [14] and CH OH [15], and electro-
−
1
5 K min , and the calcined sample was named TiO2-2000.
For comparison, two commercially available rutile-type
TiO2 samples with average crystal sizes oꢀ ca. 200 and
50 nm in diameter, purchased ꢀrom Shanghai Xushuo Bio-
technology Co., Ltd., were calcined under the same condi-
tions as those ꢀor the synthesized one to ensure their rutile-
type structures, and these calcined samples were named
TiO2-200 and TiO2-50, respectively.
3
3
catalysis [3]. Furthermore, a transient mechanistic study
has demonstrated that the surꢀace RuO Cl oxychloride
2
-x
x
ꢀ
ormed via the partial chlorination oꢀ the surꢀace Ru atoms
(
coordinatively unsaturated and bridge sites) is the active
phase, indicating that the catalytic perꢀormance oꢀ sup-
ported RuO catalysts is closely related to the amount
2
oꢀ the surꢀace Ru atoms [16–18]. Hence, it is attractive
to increase the percentage oꢀ the surꢀace Ru atoms by
The RuO2/TiO2 catalysts (nominal 2.0 wt% Ru with
diꢀꢀerent support-crystal sizes) were prepared by a dry
impregnation method as ꢀollows, 1.0 g oꢀ TiO2 support
was impregnated with 0.1 mL oꢀ RuCl3 aqueous solution
increasing the dispersion oꢀ RuO on the supports.
2
Strengthening the active phase-support interactions by
optimizing the synthesis parameters during the catalyst
preparation has been demonstrated as an efective way
−
1
(0.1 g mL ) at room temperature ꢀor 6 h. In addition, the
RuO2/TiO2-50 catalysts with nominal 1.5 and 2.5 wt% Ru
loadings were also prepared by the dry impregnation method
as ꢀollows: 1.0 g oꢀ TiO2-50 was impregnated with 0.1 mL oꢀ
RuCl3 aqueous solution (a RuCl3 concentration oꢀ 0.075 or
to increase the dispersion oꢀ RuO on the supports [8,
2
1
9–21]. Pérez-Ramírez et al. [21] have ꢀound that depos-
iting colloidally prepared Ru nanoparticles on a rutile
carrier results in catalysts attaining a higher activity in
the Deacon process, compared to the catalyst with the
−
1
0.125 g mL ) at room temperature ꢀor 6 h. Then the above-
obtained slurry was dried at 343 K ꢀor 12 h and calcined in
1
3

Efects oꢀ the Support‑Crystal Size on the Catalytic Perꢀormance oꢀ RuO /TiO in the Deacon…
2
2
−
1
static air at 523 K ꢀor 16 h with a heating rate oꢀ 5 K min .
Iꢀ not mentioned, the nominal Ru loading in the RuO /TiO
every 15 min to determine the produced Cl and unconverted
2
HCl amounts. Consequently, the chlorine balances were
calculated by comparing the total mole oꢀ chlorine in the
reactant stream with the one in the absorbed solution and
the conversion oꢀ HCl as a ꢀunction oꢀ time on stream was
determined as well.
2
2
catalysts reꢀers to 2.0 wt%.
2.2 Characterization
The powder X-ray diꢀꢀractometry (XRD) patterns were
recorded on a Philips PW3040/60 powder difractometer
using Ni-ꢁltered Cu Kα radiation (λ = 0.1541 nm) in the
3 Results and Discussion
−
1
2
θ range oꢀ 20°–80° with a scanning rate oꢀ 2° min . The
morphologies and sizes oꢀ the support crystals were deter-
mined by scanning electron microscopy (SEM) on a ZEISS
3.1 Catalyst Characterization
GeminiSEM 300 microscope. The RuO particle sizes on
The morphology and phase purity oꢀ the TiO supports used
2
2
the supports were determined by TEM on a JEOL JEM-
were examined by the SEM and XRD techniques. As shown
2
100 microscope at 200 kV aꢀter the samples were deposited
in Fig. S1, the SEM images indicate that the three diferent
onto a holey carbon ꢀoil supported on a copper grid. The
TiO supports have average crystal sizes oꢀ ca. 50, 200, and
2
textural properties oꢀ the samples were determined by N
2000 nm, respectively. Fig. S2 shows the XRD patterns oꢀ
2
adsorption–desorption at 77 K using a Micromeritics ASAP
these three TiO supports in comparison with the standard
2
2
020 instrument aꢀter the samples were degassed under a
pattern oꢀ rutile-type TiO (JCPDS No. 21-1276). All the
2
vacuum at 523 K ꢀor 6 h. The Raman spectra were collected
on a Renishaw In-Via Reꢃex Raman microscope equipped
difraction peaks in the XRD patterns can be indexed as a
pure rutile phase and are ꢀully consistent with the standard
−
1
with Renishawdiode laser 532 nm at a resolution oꢀ 3 cm .
The X-ray photoelectron spectra (XPS) were obtained with a
Thermo Scientiꢁc ESCALAB250xi instrument using mono-
chromatic Al Kα radiation and operating at a constant power
oꢀ 200 W. The reduction temperature oꢀ metal oxides was
ones oꢀ rutile TiO . Moreover, a signiꢁcant diference in the
2
intensity oꢀ peaks is observed, mainly due to their diferent
crystal sizes. The Brunauer–Emmett–Teller (BET) speciꢁc
surꢀace area is the total oꢀ the internal and external speciꢁc
surꢀace areas. As shown in Table S1, the external speciꢁc
surꢀace areas are very closely to the BET ones ꢀor the three
determined by H -TPR (temperature-programmed reduction)
2
on a Micromeritics AutoChem II 2920 instrument equipped
with a thermal conductivity detector (TCD). Typically, 0.1 g
oꢀ the sample was pretreated in ꢃowing helium with a rate
supports, indicating that all the investigated TiO supports
2
are non-porous. In other words, only the external surꢀaces oꢀ
the supports can ofer the platꢀorm ꢀor the loading oꢀ RuO .
2
−
1
oꢀ 30 mL min at 473 K ꢀor 1 h. The H -TPR proꢁle was
In addition, the measured external speciꢁc surꢀace area, S ,
2
ext
then recorded ꢀrom 323 to 1073 K with a heating rate oꢀ
oꢀ the support decreases with increasing the crystal size
−
1
1
3
0 K min in 5 vol.% H in Ar with a total ꢃow rate oꢀ
2
signiꢁcantly.
−
1
0 mL min .
The TEM experiments were carried out to determine the
particle size oꢀ the active phase RuO in the three diferent
2
2.3 Catalytic Oxidation of HCl
catalysts. As shown in Fig. 1, with the same nominal Ru
loading in the three diferent catalysts, the mean diameter
oꢀ RuO particles decreases ꢀrom ca. 2.6 nm in RuO /TiO -
The catalytic oxidation oꢀ HCl was carried out in a ꢁxed-bed
reactor (8 mm inner diameter) at atmosphere pressure. The
catalyst was pressed at 8.0 MPa and then crushed into parti-
cles ranging ꢀrom 0.18 to 0.25 mm in diameter. The reactor
was loaded with 0.2 g oꢀ the pellet catalyst. Aꢀter the reactor
2
2
2
2000 to ca. 2.3 nm in RuO /TiO -200 and ca. 2.0 nm in
2
2
RuO /TiO -50, implying that the active phase RuO is better
2
2
2
dispersed on the surꢀace oꢀ the TiO support with a small
2
crystal size. Additionally, it is also observed that the mean
temperature was heated up to 623 K in ꢃowing N with a rate
diameter oꢀ RuO particles in RuO /TiO -50 increases with
2
2
2
2
−
1
oꢀ 30 mL min , the N ꢃow was switched of and a mixture
increasing the nominal Ru loading (Fig. S3). Hence, both
2
oꢀ HCl and O was introduced into the reactor. In the cur-
the support TiO crystal size and the Ru loading dominate
2
2
rent investigation, the weight hourly space velocity (WHSV)
the particle size oꢀ the active phase RuO in the resultant
2
−
1
oꢀ HCl was maintained at 39.1 h , and the molar ratio oꢀ
catalyst.
O to HCl varied ꢀrom 2:8 to 4:8. The same amount oꢀ the
The Raman spectroscopy was employed to investigate
2
pellet support as that oꢀ the catalyst loaded in the reactor
was used ꢀor a blank experiment to check whether the sup-
port was also active in the oxidation. The reaction eꢄuent
was absorbed by an excessive KI solution and the absorbed
solution was analyzed by iodometry and acid–base titration
the electronic interactions between the active phase RuO
2
and the support TiO . As shown in Fig. 2, the characteristic
2
bands oꢀ the TiO supports are observed at 143, 234, 441,
2
−
1
and 606 cm , which can be assigned to B , two-photon
1
g
scattering process, E (planar O–O vibration), and A (Ti–O
g
1g
1
3

X. Wang et al.
Fig. 1 TEM images oꢀ RuO /
2
TiO -50 (a), RuO /TiO -200
2
2
2
(
b), and RuO /TiO -2000 (c)
2 2
1
3

Efects oꢀ the Support‑Crystal Size on the Catalytic Perꢀormance oꢀ RuO /TiO in the Deacon…
2
2
Fig. 2 Raman spectra oꢀ rutile-
RuO (a), supports TiO -x (b),
2
2
2
2
and catalysts RuO /TiO -x (c).
2
The x represents a TiO crystal
size oꢀ ca. 50 nm (a), 200 nm
(
b), and 2000 nm (c), respec-
tively, and the same below
stretch) Raman-active modes, respectively [23–25]. In the
support. In addition, RuO /TiO -2000 shows a new band
2 2
−
1
case oꢀ RuO /TiO -2000 (Fig. 2c), the shiꢀt oꢀ the E peak to
emerged at 515 cm assigned to the E mode oꢀ rutile-type
2
2
g
g
a lower wavenumber suggests that the electron transꢀer ꢀrom
RuO crystals, implying that the RuO is poorly dispersed
2
2
RuO to TiO results ꢀrom the ꢀormation oꢀ Ru–O–Ti linkage
and inclines to aggregate to ꢀorm microcrystallites. Fortu-
2
2
−
1
[
ꢀ
13]. More interestingly, it can be seen that the Raman shiꢀt
nately, the band at 515 cm is absent ꢀor RuO /TiO -50,
2 2
rom TiO to RuO /TiO increases with decreasing the sup-
ꢀurther demonstrating that the TiO support with the small-
2
2
2
2
port-crystal size, indicating that more Ru–O–Ti linkages are
est crystal size will be the best ꢀor the dispersion oꢀ RuO .
2
ꢀ
ormed. In other words, the active phase RuO is better dis-
The XPS technique was also employed to investigate the
2
persed on the surꢀace oꢀ a relatively smaller crystal-size TiO
state oꢀ the Ru species in the catalysts. It is well known
2
1
3

X. Wang et al.
that XPS is a surꢀace-sensitive technique ꢀor the analysis oꢀ
elements and their oxidation states, and the binding energy
(
BE) is closely related to the estimated charges on the cen-
tral atoms. As shown in Fig. 3, the Ru 3d bands at bind-
5
/2
ing energies oꢀ around 282 and 281 eV are assigned to
4
+
3+
4+
Ru and Ru , respectively [26]. The percentage oꢀ Ru
increases ꢀrom 53.7% ꢀor RuO /TiO -2000 to 70.2% and
2
2
1
00% ꢀor RuO /TiO -200 and RuO /TiO -50, respectively,
2 2 2 2
indicating that more electron transꢀer ꢀrom RuO to TiO
2
2
results ꢀrom the ꢀormation oꢀ more Ru–O–Ti linkages
in the catalysts with the relatively smaller crystal-size
support.
The H -TPR analysis was carried out to study the inter-
2
actions between the active phase RuO and the support
2
TiO . The H -TPR proꢁles oꢀ the TiO supports and the
2
2
2
corresponding RuO /TiO catalysts are shown in Figs. 4
2
2
and S4, respectively. In a temperature range ꢀrom 323 to
1
073 K, all the peaks in the catalysts are assigned to the
reduction oꢀ the active phase RuO rather than that oꢀ the
2
support TiO , because no reduction peak is observed in
2
all the TiO supports at the conditions investigated. It is
2
reported that the reduction temperature oꢀ Ru–O–Ti is
higher than that oꢀ Ru–O–Ru [27]. In this study, it is note-
worthy that the reduction temperature oꢀ RuO increases
2
ꢀ
rom 397 K ꢀor RuO /TiO -2000 to 498 K ꢀor RuO /TiO -
2
2
2
2
5
0. This could be concluded that more Ru–O–Ti species
are ꢀormed in RuO /TiO -50, which is in good accordance
2
2
with the observation ꢀrom the Raman spectra.
In summary, all oꢀ the TEM, Raman, XPS, and H -TPR
2
characterization results show that the support with a
smaller crystal size will be helpꢀul to ꢀorm more Ru–O–Ti
species, leading to a better dispersion oꢀ RuO on the sup-
2
port surꢀace.
3
.2 Catalytic Oxidation of HCl
The catalytic activities oꢀ the TiO supports and the RuO /
2
2
TiO catalysts were investigated in the gas-phase oxidation
2
oꢀ HCl. The supports do not show any activity, as expected.
It has been demonstrated that the surꢀace RuO Cl oxy-
2
–x
x
chloride ꢀormed by the partial chlorination oꢀ the surꢀace
Ru atoms (coordinatively unsaturated and bridge sites) is the
active phase. Thereꢀore, the catalytic activity oꢀ RuO /TiO
2
2
is closely related to the amount oꢀ the surꢀace Ru atoms [16].
As shown in Fig. 5, RuO /TiO catalysts with the diferent
2
2
TiO crystal sizes have almost identical TOF values in the
2
catalytic reaction. However, the conversion oꢀ HCl increases
in the order oꢀ RuO /TiO -2000<RuO /TiO -200<RuO /
2
2
2
2
2
TiO -50, which could be ascribed to the efects oꢀ the sup-
2
port-crystal size. TiO with a smaller crystal size, possessing
2
a larger external surꢀace area, will promote the ꢀormation oꢀ
the more surꢀace RuO , resulting in the enhanced catalytic
2
activity in the oxidation oꢀ HCl. It is also worthy oꢀ note
Fig. 3 Ru 3d XPS spectra oꢀ RuO2/TiO2-50 (a), RuO2/TiO2-200 (b),
and RuO /TiO -2000 (c)
2
2
1
3

Efects oꢀ the Support‑Crystal Size on the Catalytic Perꢀormance oꢀ RuO /TiO in the Deacon…
2
2
Fig. 4 H -TPR proꢁles oꢀ RuO /
2
2
TiO catalysts
2
that the conversion oꢀ HCl over RuO /TiO -50 is 82.3%,
4 Conclusions
2
2
close to its equilibrium conversion (87.0%) at the applied
reaction conditions. Moreover, the molar ratio oꢀ O to HCl
The RuO /TiO catalysts with diferent support-crystal sizes
2
2
2
plays a critical role in the reaction (Fig. 6), i.e., a high O par-
were prepared via the impregnation method and these pre-
2
tial pressure is beneꢁcial ꢀor Cl production. Furthermore, the
pared catalysts were used in the HCl oxidation to Cl . The
2
2
conversion oꢀ HCl over the RuO /TiO -50 catalyst increases
results show that the dispersion oꢀ RuO on the surꢀace oꢀ
2
2
2
with increasing Ru loading while these catalysts have almost
identical TOF values (Fig. S5). A similar observation has also
been ꢀound in the hydrogenation oꢀ cyclohexene over the sup-
ported platinum catalysts [28, 29].
TiO with a rutile structure can be efectively controlled
2
by adjusting the support-crystal size. TiO with a smaller
2
crystal size possesses a larger geometric surꢀace area ꢀor
the loading oꢀ RuO and enhances the ꢀormation oꢀ more
2
Ru–O–Ti species, resulting in a better dispersion oꢀ RuO
2
1
3

X. Wang et al.
Fig. 5 Comparative results
on the oxidation oꢀ HCl over
diferent RuO /TiO catalysts.
2
2
Reaction conditions: WHSV
−
1
(
HCl)=39.1 h , molar ratio oꢀ
O to HCl=4:8, and reaction
2
temperature=623 K
Fig. 6 Efects oꢀ the molar ratio
oꢀ O to HCl on the oxidation
2
oꢀ HCl over RuO /TiO -50.
2
2
Reaction conditions: WHSV
−
1
(
HCl)=39.1 h and reaction
temperature=623 K
1
3

Efects oꢀ the Support‑Crystal Size on the Catalytic Perꢀormance oꢀ RuO /TiO in the Deacon…
2
2
on the surꢀace oꢀ the support and a higher catalytic activity
in the oxidation oꢀ HCl. The current work could stimulate
14. Nagaoka K, Eboshi T, Takeishi Y, Tasaki R, Honda K, Imamura
K, Sato K (2017) Sci Adv 3:e1602747
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5. Zarei M, Davarpanah A, Mokhtarian N, Farahbod F (2020) Energy
Sources Part A 42:89
ꢀ
undamental research and industrial applications oꢀ highly
active Deacon catalysts.
16. Hevia MAG, Amrute AP, Schmidt T, Pérez-Ramírez J (2010) J
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Acknowledgements The authors grateꢀully acknowledge ꢁnancial
support ꢀrom Zhejiang Provincial Key R&D Project (2019C03118)
and Zhejiang Provincial Natural Science Foundation (Q20B030021).
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