- Email: [email protected]
anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen
Journal Pre-proof Origin of high stability of Pt/anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen Hassnain Abbas Khan, Sunghoon Kim, Kwang-Deog Jung
PII:
S0920-5861(19)30601-7
DOI:
https://doi.org/10.1016/j.cattod.2019.10.037
Reference:
CATTOD 12542
To appear in:
Catalysis Today
Received Date:
28 June 2019
Revised Date:
12 September 2019
Accepted Date:
28 October 2019
Please cite this article as: Khan HA, Kim S, Jung K-Deog, Origin of high stability of Pt/anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.10.037
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Origin of high stability of Pt/anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen Hassnain Abbas Khana,b, Sunghoon Kima, Kwang-Deog Jung*a,b,
a
Clean Energy and Chemical Engineering, University of Science and Technology, 217, Gajeong-ro Yuseong-gu, Daejeon, Republic of Korea
Clean Energy Research Centre, Korea Institute of Science and Technology, P.O. Box 131,
ro of
b
Cheongryang, Seoul 136-791, Republic of Korea
-p
*Corresponding author: Kwang-Deog Jung, Email: [email protected]
Jo
ur
na
lP
re
Graphical Abstract
Highlights
Anatase-TiO2 supported Pt catalysts show the high stability in SA decomposition
Pt is encapsulated by TiO2 at the temperature higher than 650 oC.
Pt is anchored to anatase-TiO2 due to the strong binding of Pt to anatase-TiO2
The tightly anchored Pt is the origin of the high stability of Pt/anatase-TiO2.
Abstract Here, anatase- and rutile TiO2 supported Pt catalysts are prepared for sulfuric acid (SA)
ro of
decomposition in 650~850 oC. The anatase-supported Pt catalysts (Pt/G5) retains the pure anatase phase even at 850 oC and rutile-supported catalyst (Pt/P25) is prepared by thermal treatment using P25. The prepared Pt/G5 catalyst is highly stable at the temperature of 650 oC850 oC and a WHSV of 85 gH2SO4 gcat-1 h-1 and there is no Pt loss even in SA decomposition at both 650 oC and 850 oC for 100 h. However, the Pt/P25 is steadily deactivated and 35% Pt
-p
loss is observed at 850 oC for 100 h. On both Pt/G5 and Pt/P25, Pt is encapsulated by TiO2 during the thermal heat treatment and SA decomposition occurs through the exposure of Pt by
re
SA: encapsulation anchored Pt exposure by SA sulfuric acid decomposition on Pt anchored to anatase TiO2. The encapsulation of Pt by TiO2 and no CO chemisorption, resulted
lP
from typical strong metal support interaction (SMSI) phenomena, are observed. The stronger binding of Pt to anatase TiO2 is pronounced in the condition of SMSI. The reported DFT calculation shows that the binding strength between Pt and TiO2 is pronounced under the
na
condition of the thermal treatment at the temperature higher than 450 oC or the H2 reduction. Therefore, it is proposed that the strong binding of Pt to anatase TiO2 is the origin of the high stability of anatase-TiO2 supported catalysts in SA decomposition. These findings afford a
ur
practical solution for the catalysts developments in SA decomposition in a wide reaction
Jo
temperature ranges of 650~850 oC.
Key words; sulfuric acid decomposition, anatase-TiO2 supported catalysts, encapsulation of Pt, the binding energy of Pt to TiO2.
1. Introduction Hydrogen energy economy has been focused to fulfill the clean energy society. In that aspect, hydrogen production from water is primarily important for the purpose. Photovoltaic
ro of
(PV)-electrolysis [1] and photoelectrolysis [2,3] have been improved for several decades, but they need more improvements in technological readiness level as well as in economic feasibility. On the other hand, thermochemical technologies such as high temperature electrolysis (HTE) [4] and thermochemical cycles (typically, sulfur-iodine cycle (SI)) [5-8]
-p
have also been developed to produce hydrogen from water using high temperature. The heat source of high temperature can be obtained from the solar energy and high temperature nuclear
re
reactor. Recent progresses toward the very high temperature reactor (VHTR) makes the VHTR the most viable option in aspect of the technological advancement [9]. So, the SI cycle using
lP
the VHTR can be a good technical, political and economic candidates to produce hydrogen in mass, because the VHTR is very safe as compared with the existing nuclear power plant. The SI cycle consists of three reaction steps [10]: (1) sulfuric acid (SA) decomposition, (2) HI
na
decomposition and (3) Bunsen reaction. In the process of the SI cycle, SA decomposition should absorb heat from a wide temperature ranges from 550 oC to 850 oC. Therefore, the stability and activity of catalysts in very corrosive condition at a wide temperature of 550~850 o
ur
C becomes a main hurdle for the SA decomposition [11-13].
Jo
Metal oxides have been considered as catalysts for the SA decomposition. The mechanism of sulfuric acid decomposition on metal oxide proceeds as follows [14]: MxOy + ySO3 Mx(SO4)y
(1)
Mx(SO4)y MxOy +y SO2 + y/2O2
(2)
In this scheme, metal sulfate is an intermediate for the reaction, and oxygen and sulfur dioxide are produced by decomposition of sulfate. Although the catalytic activity of metal oxides are closely related to the decomposition temperature of metal sulfates, the
decomposition temperature of metal sulfates is not only one factor to choose an active catalysts [14-17]. The bimetallic oxide catalysts and supported bimetallic oxide catalysts from CuO, Fe2O3, Cr2O3 and V2O5 are reported to enhance catalytic activity [18-21]. If reaction temperature is lower than temperature of the metal sulfate decomposition, the metal oxides can be corroded so that the catalysts cannot be used in the wide temperature ranges. On the other hand, because noble metals such as Pt, Pd, Rh, Ir and Ru are mostly resistant to the sulfuric acid, platinum metals were examined for SA decomposition [22]. The experimental and theoretical estimation exhibits that Pt is the most active for the reaction based on the following
2SO3 + 2Pt 2PtO + 2SO2 2 PtO 2 Pt + 1/2 O2
ro of
mechanism [22-23]. (3) (4)
SO3 decomposition occurs via SO3 adsorption on the metal surface. The selection of
-p
supports is very important for dispersing active metal or metal oxides as rules. Although BaSO4 is resistant to SO3, BaSO4 can be applicable only in dry SO3 decomposition [24]. BaSO4 can
re
be dissolved by steam generated during SA decomposition. SiC support is also resistant to sulfuric acid, but it is suspicious as a stable support in a longer term [23]. Oxygen from SA
lP
decomposition oxidizes SiC to SiO2. Among SiO2, Al2O3 and TiO2, SiO2 is the most resistant to the sulfuric acid, but the binding of Pt to SiO2 is the weakest [25]. PtOx can be evaporated at high temperature [26-27], which is the cause of severe sintering at the high temperature in
na
O2 environment [28]. So, the weak binding of Pt to SiO2 results in severe deactivation by the Pt loss and Pt sintering at 850 oC [23]. The higher reaction temperature deactivates Pt catalysts the more, reversely from metal oxide catalysts. The structure and surface modification to
ur
enhance the binding of Pt to SiO2 can prevent the Pt loss and Pt sintering effectively [29, 30], but the schemes of the catalyst preparations are sophisticated. Rutile-TiO2 supported Pt and Fe
Jo
catalysts exhibited lower activity than Al2O3 supported catalysts at 850 oC, but exhibited much higher stability [31]. Decomposition temperatures of Ti-sulfate and Al-sulfate are observed at 580 oC and 800 oC, respectively [14], which can be a reason of the higher stability of rutileTiO2. Petkovic et al [31] reported that rutile TiO2 supported catalysts have the relatively high activity and selectivity at 700 oC-850 oC. On the other hand, Nur et. al [32] reported that the rutile-TiO2 supported Pt catalyst had unusually very low activity at 600 oC-700oC, emphasizing the importance of the high activity and stability of anatase-TiO2 supported catalysts at the relatively low reaction temperature lower than 650 oC.
Recently, we found that the commercial G5 support retained the anatase phase even at 850 oC. Therefore, we attempted to compare the catalytic activity of rutile and anatase supported Pt catalysts to clarify the structural effects on the SA decomposition in 650oC and 850 oC. For comparison, rutile phase can be obtained by the calcination of commercially available Degussa P25 at the temperature higher than 650 oC. Here, we scrutinized the effects of anatase and rutile phase in SA decomposition on the supported Pt catalysts using these supports. 2. Experiment
ro of
2.1 Preparation of Pt/TiO2 catalysts Ultrafine & Specialty Titanium Dioxide Products CristalACTiV™ G5 (anatase TiO2), Lot number (6450000615) was used as a support of Pt. Aqueous solution of 1.0 wt% H2PtCl6 was prepared and TiO2 was added into the aqueous H2PtCl6 solution under stirring. After the
-p
impregnation for 3 h, the suspension was dried at 80 oC with a rotary evaporator. The dried samples were ramped up to 120 oC for 4 h, followed by the calcination at 450 oC in air for 4 h
re
with ramping 4 oC/min in muffle furnace. Catalyst was pretreated at the reaction temperature for 2 h under a nitrogen flow in a tube furnace before sulfuric acid decomposition.
lP
G5 (Ultrafine & Specialty Titanium Dioxide Products CristalACTiV™) was used as an anatase support and P25 (TiO2, Degussa, Rutile: Anatase with ratio 85:15, 99.9%, 20 nm) was
na
used as a rutile support.
2.2 Characterizations of prepared catalysts
ur
The X-ray diffraction (XRD) patterns were recorded on a diffractometer (M/S, Shimadzu Instruments, Japan) operated at 40 kV voltage and 30 mA current using Ni-filtered Cu Kα (λ =
Jo
0.15418 nm). The surface areas, pore volumes, and pore size distributions of the samples were obtained by N2 physisorption analysis. The N2 adsorption–desorption measurements were performed at −196 °C using an automated gas sorption system (Belsorp II mini, BEL Japan, Inc.). The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used for calculating the surface areas and pore size, respectively. Each catalyst was degassed in vacuum 130 °C for 4 h before the analysis. TEM and STEM images were obtained with transmission electron microscopy (TEM-Talos; F 200X system). TEM-Talos was operated at an accelerating voltage of 80–200 kV using a LaB6 source.
ICP-OES (inductively coupled plasma-optical emission spectrometry) analysis was conducted to find the Pt metal content in the pristine as well as a spent catalyst on (ICP-OES, iCAP 6000 series, Thermo, USA). The chemisorption of CO was performed in a pulse mode (BELCAT,II BEL Japan, Inc.). Prior to the measurements, 0.05 g of the sample was thermally treated under a He stream at 450 ℃ for 50 min, to remove physically adsorbed water and other impurities. The sample was cooled down to room temperature, and heated to 500 ℃ with a heating rate of 10 ℃/min in pure H2 at a flow rate of 50 mL/min. The sample was then reduced in H2 at 450 ℃ for 2 h.
ro of
After the reduction, the sample was purged with He gas at the same temperature for 1 h. After cooling to 40 ℃. 1.0% CO/He gas was introduced into the probe cell for CO chemisorption. The CO loop gas was used for each pulse (a sample loop of 1 mL), and the pulse injections were repeated till saturation. The amount of CO was measured using a thermal conductivity
-p
detector. The metal dispersion in each catalyst was calculated from the amount of CO adsorbed, assuming the stoichiometry factor (SF) for Pt/CO to be 1.0:
100×V𝑠 ×S𝐹 ×M𝑊 S𝑊 ×F𝑛 ×22,414
re
Dispersion (%) =
where VS is the cumulative volume of adsorbed CO (cm3 at STP), MW is the molecular weight
lP
of Pt metal (g/mol), SW is the weight of the sample, and Fn is the Pt fraction in relation to the total weight of the catalyst sample. CO chemisorption with the spent catalysts was performed using above procedure after (1) purging the sample in N2 (50 mL/min) for 3h at the reaction
na
temperature and (2) cooling down the reactor temperature to room temperature in N2. The X-ray photoelectron spectra (XPS) of all calcined and reduced catalysts were obtained using an Ulvac-PHI spectrometer (PHI 5000 Versa Probe). Deconvolution of the C1s, Ti3s, and
ur
Pt peaks was performed using a sum of Lorentzian–Gaussian functions. The binding energies
Jo
were corrected by the C1s peak from carbon contamination to 284.6 eV. 2.3 Activity measurements for sulfuric acid decomposition The catalytic activity was evaluated in a bayonet type quartz reactor at the temperature of
650, 750, or 850 °C, and the stability of catalysts was tested for 100 h at 650 and 850 oC and a WHSV of 85 gH2SO4 gcat-1 h-1. A quartz reactor was made with a bayonet type (total length: 55 cm) with the diameter of 8.0 mm of an inner tube and the dia. of 28 mm of an outer tube for sulfuric acid evaporation. The feed with a 1:1 molar ratio of SA (85 wt%) to N2 was introduced
into the reactor through a quartz preheater. The flow rates of aqueous sulfuric acid and N2 were 0.25 g/min (liquid phase, 25 °C) and 95 mL min-1 (gas phase), respectively. The WHSV at the typical reaction condition is 85 gH2SO4 gcat-1 h-1. The prepared catalyst was pelletized and crushed into 120–180 mesh, and placed in the inner tube of the bayonet type quartz reactor. The amount of catalyst is 0.15 g and size of the catalyst bed is 8.0 mm in thickness and 10 mm to height. The catalysts were fixed with quartz wool. A thermocouple was placed in the catalyst bed. Sulfuric acid (SA, purity:85 wt%) was introduced into the reactor through an evaporator by a liquid micro-pump. The product O2 from the H2SO4 trap was analyzed by a gas
ro of
chromatography (YoungLin Inc. - M600D).
3. Results and discussion
3.1. Physico-chemical characterizations of pristine TiO2 supported Pt catalysts
-p
Table 1 shows physical properties for differently pretreated catalyst samples. BET surface area (SBET) of Pt/G5-450 was approximately two times larger than that of Pt/P25-450. SBET
re
decreased in an increase in temperature of thermal treatment as usual. SBET of Pt/G5-850 became approximately five times larger than that of Pt/P25-850. The measured average pore
lP
volume and average pore diameter is mainly from interparticles. Fig. 1 shows the XRD patterns of the prepared catalysts. A P25 support has both anatase (JCPDS #71-1166) and rutile (JCPDS # 73-1763) phases, while A G5 support has anatase phase
na
only. When pristine Pt/G5 and Pt/P25 catalysts were thermally treated at 650 oC and 850 oC in N2 for 2h, most of anatase phase in P25 changed to rutile even at 650 oC (Fig.1(C)), but anatase phase of G5 was retained even in the thermal treatment at 850 oC (Fig.1(H)). The appearance
ur
of Pt characteristic peaks in the XRD patterns indicates that PtOx is thermally reduced on the Pt-TiO2 catalysts treated in N2 at the temperature higher than 650 oC. The thermal reduction of
Jo
PtOx was also observed on Pt/Al2O3 [26] and Pt/TiO2 [25]. The reduction temperature of PtOx on Pt/Al2O3 was 637 oC from O2 TPD analysis. It is needed to note that the reduction of PtOx on Al2O3 with small Pt oxide particles occurs at a significantly higher temperature. So, the appearance of the metallic Pt on the Pt-TiO2 catalysts treated in N2 at the temperature higher than 650 oC should be due to the thermal reduction of PtOx. From XRD patterns, we can name the Pt/P25-650 and 850 catalysts to rutile-TiO2 supported Pt catalysts and we named the Pt/G5650 and 850 catalysts to anatase-TiO2 supported Pt catalysts.
CO chemisorption was attempted to measure metal surface area. Pt/P25-450 and Pt/G5450 (samples calcined at 450 oC) have the metal dispersions of 50.9% and 23.7 %, respectively. The metal sizes of Pt/P25-450 and Pt/G5-450 from CO chemisorption are corresponding to 2.2 and 4.8 nm, respectively. The Pt/P25-450 sample with the lower SBET has higher metal dispersion than the Pt/G5-450 with the larger SBET. However, it should be noted that no CO chemisorption was measured on the Pt/TiO2 catalysts treated in N2 at the temperature higher than 650 oC. No CO chemisorption of the catalysts treated at the temperature higher than 650 o
C should not mean no Pt presence, because XRF analysis and XRD patterns show the presence
of Pt clearly (Table 1 and Fig.1). This kind of phenomenon was also previously observed on
ro of
Pt/SiO2 catalytic system [30]. On the Pt/SiO2 system, the encapsulating Pt with SiO2 support on the surface was observed. Therefore, it is proposed that TiO2 encapsulates Pt at the high temperature treatment. Ono et al. [25] also showed that the encapsulation of Pt by TiOx on TiO2/Ti thin film was not only observed at the temperature above 425 oC in ultrahigh vacuum,
-p
but Pt-Ti-O was also observed on the surface. So, it is plausible that Pt is encapsulated by TiO2 and Pt-Ti-O phase can be formed on the thermally treated Pt/TiO2 at 850 oC, because the
re
reducibility of meal oxide is decided by thermodynamics.
Fig.2 shows TEM and STEM images. The morphology of both Pt/G5-850 and Pt/P25-850
lP
is not so much different. From the statistics of Pt particles, the average Pt particle sizes of Pt/G5-850 and Pt/P25-850 are calculated to be 4.7 nm and 3.0 nm, respectively. It should be noted that there are no CO chemisorbed on both Pt/G5-850 and Pt/P25-850. As observed with
na
CO chemisorption of Pt/G5-450 and Pt/P25-450, the CO chemisorption with the 1 wt% Pt catalysts with the size of 3.0-5.0 nm should be measured. It is clear there are little Pt loss from XRF analysis (Table 1) during the heat treatment up to 850 oC. Therefore, no CO chemisorption
ur
of Pt/G5-850 and Pt/P25-850 with the respective average Pt particle sizes of 4.7 nm and 3.0 nm does not means the severe Pt sintering or Pt loss. On the other hand, the selected area
Jo
electron diffraction (SAED) patterns of Pt particles on TiO2 show the mixed crystal structure of Pt and TiO2 on both Pt/P25 and Pt/G5 (Fig.2(a) and Fig.2(b)). The clear Pt and TiO2 Laue spots in SAED of a Pt particle suggest that TiO2 may cover Pt particles, supporting that Pt particles are encapsulated. It was reported that the suppression of CO chemisorption on Pt/TiO2 was ascribed to the formation of Pt-Ti-O via the strong metal support interaction (SMSI) [35]. Another phenomena from the SMSI is the Pt encapsulation [35]. Therefore, it can be concluded that no CO chemisorption on both Pt/G5-850 and Pt/P25-850 with the respective Pt particle size of 3.0 and 4.8 nm and the Laue spots of both Pt and TiO2 in SAED are the clear indication
of the Pt encapsulation via the SMSI. 3.2. Sulfuric acid decomposition on Pt-TiO2 catalysts Fig.3(a) shows initial SA conversions with respect to reaction temperature at a WHSV of 84 gH2SO4 gcat-1 h-1. The averaged SA conversion for 2 h was used as the initial SA conversion at the reaction temperature. Pt/P25 exhibits a little higher activity than Pt/G5 at the initial reaction time, which can be ascribed to the higher Pt dispersion of Pt/P25. Nur et.al [32] compared the catalytic activity at a WHSV of 110 gH2SO4 gcat-1 h-1 and the temperature lower than 800 oC on anatase-TiO2 (JRC-TIO-8, Japan) and rutile-TiO2 (JRC-TIO-6, Japan)
ro of
supported Pt catalysts. The SA conversion with rutile-TiO2 support Pt catalyst was one fourth of that of anatase-TiO2 supported Pt catalyst even at 800 oC. On the other hand, Petkovic et. al [31] reported the initial SA conversion of 65% on rutile (Johnson Matthey, West Deptford, NJ) supported Pt catalyst at ca. 840 oC and a WHSV of 49.5 gH2SO4 gcat-1 h-1, agreeing on our
-p
results on the rutile-TiO2 supported catalysts.
Fig.3(b) shows the stability of Pt/G5 and Pt/P25 catalysts. Pt/G5 catalyst, anatase-TiO2
re
supported Pt catalyst, exhibits very high stability on both 650 oC and 850 oC. On the other hand, Pt/P25, rutile-TiO2 supported catalyst, deactivated steadily up to 100 h, similarly to the result
lP
of Petkovic et. al. [31]. The main cause of deactivation of Pt catalyst has been assigned to the Pt loss as well as the Pt sintering. ICP analysis was performed to measure the Pt loss. The quantities of Pt with the pristine Pt/G5-850 and Pt/P25-850 catalysts were 1.06 wt% and 0.82
na
wt%, respectively, while those with the spent Pt/G5-850 and Pt/P25-850 catalysts were 1.02 and 0.54 wt%, respectively. It is clear that anatase phase can prevent the Pt loss via Pt oxidation. However, rutile phase cannot prevent the Pt loss, resulting in steady deactivation on the rutile
ur
supported Pt catalyst. Therefore, it can be concluded that the anatase supported Pt catalysts is highly stable in the wide temperature range of 650-850 oC.
Jo
In Pt/SiO2, the encapsulation of Pt by SiO2 at the inner wall was proposed to be a reason
for the Pt stabilization in SA decomposition at high temperature [30]. Reaction can proceed as the exposure of Pt by sulfuric acid SA decomposition re-encapsulation of Pt by SiO2. However, it was observed that Pt at the outer wall was lost due to the weak binding of Pt to SiO2. The Pt encapsulation cannot prevent the Pt loss on Pt/SiO2 at 850 oC, because the exposed PtOx is inevitably evaporated in the weak binding of Pt to SiO2. On Pt/TiO2, it is obvious from CO chemisorption and TEM analysis that Pt is also encapsulated on both anatase and rutile.
However, the calculation of the binding energies showed that the binding energy of Pt was much higher on stoichiometric anatase-TiO2 (2.80 eV) than on stoichiometric rutile-TiO2 (2.14 eV) [33]. Here, the stoichiometry means no generation of vacant sites or no reduction of TiO2. However, the binding energy of Pt to the reduced TiO2 increased a lot. It was revealed that the binding energy of Pt became 4.8 eV on the reduced anatase-TiO2 and 3.52 eV on the reduced rutile-TiO2. The binding of Pt to the reduced anatase-TiO2 increased much more than that to the reduced rutile-TiO2. The TiO2 reducibility even at the temperature higher than 450 oC was clearly observed on Pt on well fabricated TiO2/Ti thin film in thermal treated Pt-TiO2 [32]. Both Pt/P25 and Pt/G5 catalysts were thermally treated in N2 at 850 oC. Therefore, the
ro of
reduction of TiO2 near Pt can be probable at the temperature of 650~850 oC. Therefore, the thermal reduction of anatase-TiO2 can be an origin of the high thermal stability of Pt/G5 catalyst, increasing the relative binding energy of Pt to anatase-TiO2.
Fig. 4 shows XPS analysis of pristine and spent Pt/G5-850 and Pt/P25-850 catalysts. Both
-p
pristine Pt/G5-850 and Pt/P25-850 catalyst indicates that PtO is mostly reduced at high temperature. Ti(3s) at 75.9 eV is not negligible due to the Pt encapsulation by TiO2, which is
re
overlapped with Pt2+(4f5/2). After 100 h reaction, PtO is little present on the spent Pt/G5-850 sample in XPS analysis, while the low intensity of PtO is observed on the spent Pt/P25-850.
lP
The XPS spectra are qualitatively representative for the real Pt valence state during the reaction, because the samples are collected after cooling down to room temperature in N2 environment. Unfortunately, the low concentration of Pt-TiO2 interface cannot clarify the electron transfer in
na
XPS, but XPS analysis supports that Pt particles are covered by TiO2 from the high intensity of Ti3S.
ur
The experimental results show the remarkable two observations: (a) CO chemisorption was not detected with the catalysts pretreated at the temperature higher than 450 oC (b) Pt was
Jo
encapsulated. The SMSI effect has been pronounced on thermally treated Pt/TiO2 or H2reduced Pt/TiO2 [34]. The above observations are the peculiar phenomena of the SMSI effect [35]. The encapsulation came from the reduction of TiO2 to TiOx and the active metal was covered by the migration of the reduced TiO2. So, the encapsulation can be described as one of evidences for the SMSI effect on Pt/TiO2. Furthermore, it has been reported that SMSI (strong metal support interaction) between metals and TiO2 is predominant on anatase-TiO2 [36-39]. Rutile-TiO2 supported catalysts were more active than anatase-TiO2 supported catalysts in CO oxidation and methane combustion,
while anatase-TiO2 supported catalysts showed the higher thermal stability. The same behavior was also observed in sulfuric acid decomposition here. The higher initial activity of Pt/P25-850 in Fig.3 can be due to the higher metal dispersion or lower SMSI, as compared to Pt/G5. However, it is clear that the Pt/G5 is much more stable than Pt/P25. It was shown that the stronger binding of anatase-Pt than rutile-Pt could be more pronounced in the reduced TiO2 [33]. Because the origin of SMSI comes from the reduction of TiO2 in the vicinity of metal [34], it is proposed that the stronger binding of Pt to the reduced anatase-TiO2 can be the origin of the high stability of Pt/G5 in sulfuric acid decomposition.
ro of
Additionally, no Pt loss was observed on Pt/G5 at the reaction tempereature of 650 oC and 850 o
C for 100 h in sulfuric acid decomposition even for 100 h. The catalytic system with no Pt
loss for 100 h at both 650 oC and 850 oC is rarely reported in sulfuric acid decomposition. Therefore, it is suggested that the high stability of the anatase-TiO2 supported catalytic system in the wide temperature range of 650 oC-850 oC can afford the practical solution for the catalyst
-p
development in the decomposition of sulfuric acid: high stability, reasonable activity and easy
re
massive preparation of catalysts.
lP
4. Conclusion
We prepared anatase and rutile supported Pt catalysts using P25 and G5, respectively. Pt was dispersed better on P25 than on G5. The apparent initial activity of the Pt/P25 catalyst is
na
slightly higher than that of the Pt/G5 catalyst, while the stability of the Pt/P25 is much lower than that of Pt/G5. There is no Pt loss on Pt/G5 after the SA reaction at 650 oC and 850 oC for
ur
100 h, while there is 35% Pt loss on Pt/P25. The Pt encapsulation and anchoring on both anatase- and rutile-TiO2 are observed from CO chemisorption, TEM images and XPS analysis. The encapsulation and no CO chemisorption on Pt/TiO2 are typical peculiar phenomena of
Jo
SMSI interaction. It is shown on the well defined thin TiO2/Ti film that the surface of TiO2, thermally treated at the temperature higher than 450 oC, was reduced in the vicinity of Pt [25]. Therefore, the SMSI on both Pt/P25 and Pt/G5 is very probable, when the catalysts are thermally treated at 850 oC in N2. On the other hand, the Pt binding to anatase-TiO2 is stronger than that to rutile-TiO2 [33]. However, the Pt binding to anatase-TiO2 become much stronger than that rutile-TiO2, when TiO2 is reduced in the vicinity of Pt-SMSI. So, it is proposed that SA decomposition occurs on Pt anchored to anatase-TiO2 through Pt exposure of encapsulated
Pt in contact with sulfuric acid. The outstanding high stability without Pt loss can be ascribed to the strong binding of Pt to the anatase support.
Acknowledgment This work was financially supported by the Korea Institute of Science and Technology (KIST) and by Ministry of Science and ICT through the Korea CCS 2020 R&D program
ro of
References [1] S.S. Kumar, V. Himabindu, Materials Science for Energy Technologies 2 (2019) 442
[2] M. Reuβ, J. Reul, T. Grube, M. Langemann, S. Calnan, M. Robinius, R. Schlatmann, U. Rau, D. Stolten, Sustainable Energy & Fuel, 3 (2019) 801.
-p
[3] J.H. Kim, D. Hansora, P. Sharma, J.W. Jang, J.S. Lee, Chem. Soc. Rev. 48 (2019) 1855.
[4] A. Pandiyan, A. Uthayakumar, R. Subrayan, S.W. Cha, S.B. Moorthy, Nanomaterials and Energy 8 (2019) 1.
re
[5] M. Soser, M. Pecchi, T. Fend, energies 12 (2019) 352
lP
[6] F. Yilmaz, R. Selbas, Thermodynamic performance assessment of solar based Sulfur-Iodine thermochemical cycle for hydrogen generation, Energy 140 (2017) 520-529.
na
[7] J. Chang, Y.W. Kim, K.Y. Lee, Y.W. Lee, W.J. Lee, J.M. Noh, M.H. Kim, H.S. Lim, Y.J. Shin, K.K. Bae, K.D. Jung, A study of A nuclear hydrogen production demonstration plant. Nucl. Eng. Tech. 39 (2007) 111-122.
ur
[8] Y. Shin, T. Lee, K. Lee, M. Kim, Modeling and simulation of HI and H2SO4 thermal decomposers for a 50 NL/h sulfur-iodine hydrogen production facility. Appl. Energy 173 (2016) 460.
Jo
[9] K. Jedrzejewski, M. Hanuszkiewicz-Drapala, Analysies of the Efficiency of a HighTemperature Gas-Cooled Nuclear Reactor Cogeneratino System Generating Heat for the Sulfur-Iodine Cycle, J. Energy Resour. Technol, 140 (2018) 112001. [10] K. Onuki, S. Kubo, A. Terada, N. Sakaba, R. Hino, Thermochemical water-splitting cycle using iodine and sulfur, Energy & Environmental Science, 2 (2009) 491-497. [11] J. Park, J.H. Cho, H. Jung, S. kumar, IL. Moon, Simulation and experimental study on the sulfuric acid decomposition process of SI cycle for hydrogen production, Int. J. Hydrogen 38 (2013) 5507. [12] P. Zhang, C. Xhou, H. Guo, S. Chen, L. Wang, J. Xu, Design of integrated laboratoryscale iodine sulfur hydrogen production cycle at INET Int. J. Energy Res. 40 (2016) 1509-1517. [13] S. Kasahara, Y. Imai, K. Suzuki, J. Iwatsuki, A. Terada, X.L. Yan. Conceptual design of
the iodine-sulfur process flowsheet with more than 50% thermal efficiency for hydrogen production Nuclear Eng. Design 329 (2018) 213-222. [14] K.D. Jung, T.H. Kim, K.T. Gong, H.Y. Jeon, C.H. Shin, H. Kim, B.G. Lee, SO3 decomposition on Alumina and Titania Supported Catalysts in IS cycle to Produce Hydrogen. Proceedings of ICAPP’05 Seoul Korea Paper 5117 (2005). [15] Tagawa, T. Endo, Catalytic decomposition of sulfuric acid using metal oxides as the oxygen generating reaction in thermochemical water splitting process. Int. J. Hydrogen Energy 14 (1989) 11-17. [16] Neri G., Rizzo G., S. gavagno, Appl. Catal.A: General 274 (2004) 243-251.
ro of
[17] Rosen M.A., Scott D.S. “Comparative efficiency assessment for a range of hydrogen production process” Int. J. Hydrogen 23 (1998) 653. [18] T.H. Kim, G.T. Gong, B.G. Lee, K.Y. Lee, H.Y. Jeon, C.H. Shin, H. Kim, K.D. Jung, Catalytic decomposition of sulfur trioxide on the binary metal oxide catalysts of Fe/Al and Fe/Ti, Appl. Catal A: General 305 (2006) 39-45.
-p
[19] H. Abimanyu, K.D. Jung, K.W. Jun, J. Kim, K.S. Yoo, Preparation and characterization of Fe/Cu/Al2O3-composite granules for SO3 decomposition to assist hydrogen production Appl. Catal A: General 343 (2008) 134-141.
lP
re
[20] A.M. Banerjee, M.R. Pai, S.S. Meena, A.K. Tropathi, S.R. Bharadwaj, Catalytic acitivities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles. Int. J. Hydrogen 36 (2011) 47684780.
na
[21] M. Machida, T. Kawada, S. Hebishima, S. Hinokuma, S. takeshima Macroporous Supported Cu-V oxide as a promising substitute of the Pt catalyst for sulfuric acid decomposition in solar thermochemical hydrogen production. Chem. Mater. 24 (2012) 557561. [22] Rashkeev S.N., Ginosar D.M., Petkovic L.M., Farrel H.H. Catalytic activity of supported metal particles for sulfuric acid decomposition reaction. Catal Today 139 (2009) 291-298.
ur
[23] H.A. Khan, P.Natarajan, K.D. Jung Synthesis of Pt/mesoporous SiC-15 and its catalytic performance for sulfuric acid decomposition. Catal. Today 303 (2018) 25.
Jo
[24] B.M. Nagaraja, K.D. Jung, B.S. Ahn, H. Abimanyu, K.S. Yoo, Catalytic decomposition of SO3 over Pt/BaSO4 Materials in Sulfur-Iodine cycle for hydrogen production. Ind. Eng. Chem. Res. 48 (2009) 1451-1457. [25] L.K.Ono, B. Yuan, H. Heinrich, B.R. Cuenya, Formation and Thermal Stability of Platinum Oxides on Size-Selected Platinum Nanoparticles: Support effects. J. Phys. Chem. C 114 (2010) 22119. [26] E.S. Putna, J.M. Vohs, R.J. Gorte, Osygen desorption from α-AL2O3(0001) supported Rh, Pt and Pd particles. Surf. Sci. 391 (1997) L1178-L1182. [27] J.M. Bray, I.J. Skavdahl, J.-S. McEwen, W.F. Schneider, First-principles reaction site
model for coverage-sensitive surface reactions: Pt(111)-O temperature programmed desorption. Surf. Sci. 622 (2014) L1-L6. [28] P.N. Plessow, F.Abild-Pedersen, Sintering of Pt Nanoparticles via Volatile PtO2: Simulation and Comparison with Experiments, ACS catal. 6 (2016) 7098-7108. [29] H.A. Khan, M.I. Iqbal, A. Jaleel, I. Abbas, S.A. Abbas, K.D. Jung, Pt encapsulated hollow mesoprous SiO2 sphere catalyst for sulfuric acid decomposition reaction in SI cycle, Int. J. Hydrogen, 44 (2019) 2312-2322. [30] H.A. Khan, P. Natarajan, K.D. Jung, Stabilization of Pt at the inner wall of hollow spherical SiO2 generated from Pt/hollow spherical SiC for sulfuric acid decomposition, Appl.Catal.BEnviron. 231 (2018) 151-160.
ro of
[31] L.M. Petkovic, D.M. Ginosar, H.W. Rollins, K.C. Burch, P.J. Pinhero, H.H. Farell, Pt/TiO2(rutile) catalysts for sulfuric acid decomposition in sulfur-based thermochemical watersplitting cycles, Appl. Catal. A: General 338 (2008) 27-36.
-p
[32] A.S.M. Nur, T. Matsukawa, S. Hinokuma, and M. Machida, Catalytic SO3 Decomposition Activity and Stability of Pt Supported on Anatase TiO2 for Solar Thermochemical Water-Splitting Cycles, ACS Omega 2 (2017) 7057.
re
[33] H. Iddir, V. Skavysh, S. Ogut, H.D. Browning, M.M. Disko, Preferential growth of Pt on rutile TiO2, Physical Rev. B 73 (2006) 041403. [34] U. Diebold, The surface science of titanium dioxide, Surf, Sci. Reports, 48 (2003) 53-229.
lP
[35] C. Ocal, S. Ferrer, The strong metal-support interaction (SMSI) in Pt-TiO2 model catalysts, A new CO adsorption state on Pt-Ti atoms, J. Chem. Phys. 84 (1986) 6474-6478.
na
[36] Y. Li, Y. Fan, H. Yang, B. Xu, L. Feng, M. Yang, Y. Chen, Strong metal–support interaction and catalytic properties of anatase and rutile supported palladium catalyst Pd/TiO2, Chem. Phys. Lett. 372 (2003) 160–165.
ur
[37] A. Nobile, M.W. Davis, Importance of the Anatase-Rutile Phase Transition and Titania Grain Enlargement in the Strong Metal-Support Interaction Phenomenon in Fe/TiO2 Catalysts, J. Catal. 116 (1989) 383-398.
Jo
[38] H. Tang, Y. Su, B. Zhang, A.F. Lee, M.A. Isaacs, K. Wilson, L. Li, Y. Ren, J. Huang, M. Haruta, B. Qiao, X. Liu, C. Jin, D. Su, J. Wang, T. Zhang, Classical strong metal–support interactions between gold nanoparticles and titanium dioxide, Sci. Adv. 3 (2017) e1700231. [39] Z. Rui, S. Wu, C. Peng, H. Ji, Comparison of TiO2 Degussa P25 with anatase and rutile crystalline phases for methane combustion, Chem. Eng. J. 243 (2014) 254-264.
Table 1. Physical properties of differently pretreated Pt-TiO2 catalysts
Pt/P25-450 Pt/P25-650 Pt/P25-850 Pt/G5-450 Pt/G5-650
Calcined in air at 450 oC for 2 h Thermally treated in N2 at 650 oC for 2 h Thermally treated in N2 at 850 oC for 2 h Calcined in air at 450 oC for 2 h Thermally treated in N2 at 650 oC for 2 h Thermally treated in N2 at 850 oC for 2 h
SBET, m2 g-1
Av. pore vol. cm3 g-1
XRF Pt, wt%
Av. pore dia., nm
49.3
0.5
0.85
42.1
17.6
0.3
0.83
66.1
1.4
0.6
0.82
163.2
91.8
0.4
1.09
18.8
45.9
0.5
1.05
46.2
6.9
0.1
0.99
Jo
ur
na
lP
re
-p
Pt/G5-850
Pretreatment
ro of
Catalyst
65.1
ro of
Jo
ur
na
lP
re
-p
Fig. 1. XRD patterns of (A) P25 support, (B) Pt/P25-450, (C) Pt/P25-650, (D) Pt/P25-850, (E) G5 support, (F) Pt/G5-450, (G) P/G5-650, and (H) P/G5-850.
(a)
Pt (111) TiO2 A(004)
TiO2 A(004)
Pt (200)
lP
re
-p
(b)
ro of
4.7 nm
TiO2 R(002) 3.0 nm
na
Pt (200)
ur
Pt (200)
Jo
Fig. 2. TEM/STEM images and particle size distribution of (a) Pt/G5-850 and (b) Pt/P25850
100
(a)
SA converson (%)
80 60 40
0 600
650
700
750
800
ro of
20
850
900
o
Reaction temperature ( C) (b)
-p
80
re
60 40 20 0
10 20 30 40 50 60 70 80 90 100
na
0
lP
SA conversion (%)
100
Time (h)
Jo
ur
Fig. 3. Catalytic performance of sulfuric acid decomposition at WHSV of 85 gH2SO4 gcat-1 h-1: (a) reaction temperature effect of Pt/G5 () and Pt/P25 (), and (b) stability of Pt/G5 at 650 oC (), Pt/G5 at 850 oC (), Pt/P25 at 650 oC () and Pt/P25 at 850 oC ().
(b1)
(a1)
Ti 3s
Ti 3s Pt0
Pt0
Pt2+
(a2)
(b2)
Ti 3s
Ti 3s Pt0
ro of
Pt2+
Pt2+
lP
re
-p
Pt0
Jo
ur
na
Fig. 4. XPS analysis of pristine and spent Pt/TiO2 catalysts: (a1) pristine Pt/G5-850, (a2) spent Pt/G5-850, (b1) pristine Pt/P25-850, and (b2) spent Pt/P25-850.
ro of
-p
re
lP
na
ur
Jo
PII:
S0920-5861(19)30601-7
DOI:
https://doi.org/10.1016/j.cattod.2019.10.037
Reference:
CATTOD 12542
To appear in:
Catalysis Today
Received Date:
28 June 2019
Revised Date:
12 September 2019
Accepted Date:
28 October 2019
Please cite this article as: Khan HA, Kim S, Jung K-Deog, Origin of high stability of Pt/anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.10.037
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Origin of high stability of Pt/anatase-TiO2 catalyst in sulfuric acid decomposition for SI cycle to produce hydrogen Hassnain Abbas Khana,b, Sunghoon Kima, Kwang-Deog Jung*a,b,
a
Clean Energy and Chemical Engineering, University of Science and Technology, 217, Gajeong-ro Yuseong-gu, Daejeon, Republic of Korea
Clean Energy Research Centre, Korea Institute of Science and Technology, P.O. Box 131,
ro of
b
Cheongryang, Seoul 136-791, Republic of Korea
-p
*Corresponding author: Kwang-Deog Jung, Email: [email protected]
Jo
ur
na
lP
re
Graphical Abstract
Highlights
Anatase-TiO2 supported Pt catalysts show the high stability in SA decomposition
Pt is encapsulated by TiO2 at the temperature higher than 650 oC.
Pt is anchored to anatase-TiO2 due to the strong binding of Pt to anatase-TiO2
The tightly anchored Pt is the origin of the high stability of Pt/anatase-TiO2.
Abstract Here, anatase- and rutile TiO2 supported Pt catalysts are prepared for sulfuric acid (SA)
ro of
decomposition in 650~850 oC. The anatase-supported Pt catalysts (Pt/G5) retains the pure anatase phase even at 850 oC and rutile-supported catalyst (Pt/P25) is prepared by thermal treatment using P25. The prepared Pt/G5 catalyst is highly stable at the temperature of 650 oC850 oC and a WHSV of 85 gH2SO4 gcat-1 h-1 and there is no Pt loss even in SA decomposition at both 650 oC and 850 oC for 100 h. However, the Pt/P25 is steadily deactivated and 35% Pt
-p
loss is observed at 850 oC for 100 h. On both Pt/G5 and Pt/P25, Pt is encapsulated by TiO2 during the thermal heat treatment and SA decomposition occurs through the exposure of Pt by
re
SA: encapsulation anchored Pt exposure by SA sulfuric acid decomposition on Pt anchored to anatase TiO2. The encapsulation of Pt by TiO2 and no CO chemisorption, resulted
lP
from typical strong metal support interaction (SMSI) phenomena, are observed. The stronger binding of Pt to anatase TiO2 is pronounced in the condition of SMSI. The reported DFT calculation shows that the binding strength between Pt and TiO2 is pronounced under the
na
condition of the thermal treatment at the temperature higher than 450 oC or the H2 reduction. Therefore, it is proposed that the strong binding of Pt to anatase TiO2 is the origin of the high stability of anatase-TiO2 supported catalysts in SA decomposition. These findings afford a
ur
practical solution for the catalysts developments in SA decomposition in a wide reaction
Jo
temperature ranges of 650~850 oC.
Key words; sulfuric acid decomposition, anatase-TiO2 supported catalysts, encapsulation of Pt, the binding energy of Pt to TiO2.
1. Introduction Hydrogen energy economy has been focused to fulfill the clean energy society. In that aspect, hydrogen production from water is primarily important for the purpose. Photovoltaic
ro of
(PV)-electrolysis [1] and photoelectrolysis [2,3] have been improved for several decades, but they need more improvements in technological readiness level as well as in economic feasibility. On the other hand, thermochemical technologies such as high temperature electrolysis (HTE) [4] and thermochemical cycles (typically, sulfur-iodine cycle (SI)) [5-8]
-p
have also been developed to produce hydrogen from water using high temperature. The heat source of high temperature can be obtained from the solar energy and high temperature nuclear
re
reactor. Recent progresses toward the very high temperature reactor (VHTR) makes the VHTR the most viable option in aspect of the technological advancement [9]. So, the SI cycle using
lP
the VHTR can be a good technical, political and economic candidates to produce hydrogen in mass, because the VHTR is very safe as compared with the existing nuclear power plant. The SI cycle consists of three reaction steps [10]: (1) sulfuric acid (SA) decomposition, (2) HI
na
decomposition and (3) Bunsen reaction. In the process of the SI cycle, SA decomposition should absorb heat from a wide temperature ranges from 550 oC to 850 oC. Therefore, the stability and activity of catalysts in very corrosive condition at a wide temperature of 550~850 o
ur
C becomes a main hurdle for the SA decomposition [11-13].
Jo
Metal oxides have been considered as catalysts for the SA decomposition. The mechanism of sulfuric acid decomposition on metal oxide proceeds as follows [14]: MxOy + ySO3 Mx(SO4)y
(1)
Mx(SO4)y MxOy +y SO2 + y/2O2
(2)
In this scheme, metal sulfate is an intermediate for the reaction, and oxygen and sulfur dioxide are produced by decomposition of sulfate. Although the catalytic activity of metal oxides are closely related to the decomposition temperature of metal sulfates, the
decomposition temperature of metal sulfates is not only one factor to choose an active catalysts [14-17]. The bimetallic oxide catalysts and supported bimetallic oxide catalysts from CuO, Fe2O3, Cr2O3 and V2O5 are reported to enhance catalytic activity [18-21]. If reaction temperature is lower than temperature of the metal sulfate decomposition, the metal oxides can be corroded so that the catalysts cannot be used in the wide temperature ranges. On the other hand, because noble metals such as Pt, Pd, Rh, Ir and Ru are mostly resistant to the sulfuric acid, platinum metals were examined for SA decomposition [22]. The experimental and theoretical estimation exhibits that Pt is the most active for the reaction based on the following
2SO3 + 2Pt 2PtO + 2SO2 2 PtO 2 Pt + 1/2 O2
ro of
mechanism [22-23]. (3) (4)
SO3 decomposition occurs via SO3 adsorption on the metal surface. The selection of
-p
supports is very important for dispersing active metal or metal oxides as rules. Although BaSO4 is resistant to SO3, BaSO4 can be applicable only in dry SO3 decomposition [24]. BaSO4 can
re
be dissolved by steam generated during SA decomposition. SiC support is also resistant to sulfuric acid, but it is suspicious as a stable support in a longer term [23]. Oxygen from SA
lP
decomposition oxidizes SiC to SiO2. Among SiO2, Al2O3 and TiO2, SiO2 is the most resistant to the sulfuric acid, but the binding of Pt to SiO2 is the weakest [25]. PtOx can be evaporated at high temperature [26-27], which is the cause of severe sintering at the high temperature in
na
O2 environment [28]. So, the weak binding of Pt to SiO2 results in severe deactivation by the Pt loss and Pt sintering at 850 oC [23]. The higher reaction temperature deactivates Pt catalysts the more, reversely from metal oxide catalysts. The structure and surface modification to
ur
enhance the binding of Pt to SiO2 can prevent the Pt loss and Pt sintering effectively [29, 30], but the schemes of the catalyst preparations are sophisticated. Rutile-TiO2 supported Pt and Fe
Jo
catalysts exhibited lower activity than Al2O3 supported catalysts at 850 oC, but exhibited much higher stability [31]. Decomposition temperatures of Ti-sulfate and Al-sulfate are observed at 580 oC and 800 oC, respectively [14], which can be a reason of the higher stability of rutileTiO2. Petkovic et al [31] reported that rutile TiO2 supported catalysts have the relatively high activity and selectivity at 700 oC-850 oC. On the other hand, Nur et. al [32] reported that the rutile-TiO2 supported Pt catalyst had unusually very low activity at 600 oC-700oC, emphasizing the importance of the high activity and stability of anatase-TiO2 supported catalysts at the relatively low reaction temperature lower than 650 oC.
Recently, we found that the commercial G5 support retained the anatase phase even at 850 oC. Therefore, we attempted to compare the catalytic activity of rutile and anatase supported Pt catalysts to clarify the structural effects on the SA decomposition in 650oC and 850 oC. For comparison, rutile phase can be obtained by the calcination of commercially available Degussa P25 at the temperature higher than 650 oC. Here, we scrutinized the effects of anatase and rutile phase in SA decomposition on the supported Pt catalysts using these supports. 2. Experiment
ro of
2.1 Preparation of Pt/TiO2 catalysts Ultrafine & Specialty Titanium Dioxide Products CristalACTiV™ G5 (anatase TiO2), Lot number (6450000615) was used as a support of Pt. Aqueous solution of 1.0 wt% H2PtCl6 was prepared and TiO2 was added into the aqueous H2PtCl6 solution under stirring. After the
-p
impregnation for 3 h, the suspension was dried at 80 oC with a rotary evaporator. The dried samples were ramped up to 120 oC for 4 h, followed by the calcination at 450 oC in air for 4 h
re
with ramping 4 oC/min in muffle furnace. Catalyst was pretreated at the reaction temperature for 2 h under a nitrogen flow in a tube furnace before sulfuric acid decomposition.
lP
G5 (Ultrafine & Specialty Titanium Dioxide Products CristalACTiV™) was used as an anatase support and P25 (TiO2, Degussa, Rutile: Anatase with ratio 85:15, 99.9%, 20 nm) was
na
used as a rutile support.
2.2 Characterizations of prepared catalysts
ur
The X-ray diffraction (XRD) patterns were recorded on a diffractometer (M/S, Shimadzu Instruments, Japan) operated at 40 kV voltage and 30 mA current using Ni-filtered Cu Kα (λ =
Jo
0.15418 nm). The surface areas, pore volumes, and pore size distributions of the samples were obtained by N2 physisorption analysis. The N2 adsorption–desorption measurements were performed at −196 °C using an automated gas sorption system (Belsorp II mini, BEL Japan, Inc.). The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used for calculating the surface areas and pore size, respectively. Each catalyst was degassed in vacuum 130 °C for 4 h before the analysis. TEM and STEM images were obtained with transmission electron microscopy (TEM-Talos; F 200X system). TEM-Talos was operated at an accelerating voltage of 80–200 kV using a LaB6 source.
ICP-OES (inductively coupled plasma-optical emission spectrometry) analysis was conducted to find the Pt metal content in the pristine as well as a spent catalyst on (ICP-OES, iCAP 6000 series, Thermo, USA). The chemisorption of CO was performed in a pulse mode (BELCAT,II BEL Japan, Inc.). Prior to the measurements, 0.05 g of the sample was thermally treated under a He stream at 450 ℃ for 50 min, to remove physically adsorbed water and other impurities. The sample was cooled down to room temperature, and heated to 500 ℃ with a heating rate of 10 ℃/min in pure H2 at a flow rate of 50 mL/min. The sample was then reduced in H2 at 450 ℃ for 2 h.
ro of
After the reduction, the sample was purged with He gas at the same temperature for 1 h. After cooling to 40 ℃. 1.0% CO/He gas was introduced into the probe cell for CO chemisorption. The CO loop gas was used for each pulse (a sample loop of 1 mL), and the pulse injections were repeated till saturation. The amount of CO was measured using a thermal conductivity
-p
detector. The metal dispersion in each catalyst was calculated from the amount of CO adsorbed, assuming the stoichiometry factor (SF) for Pt/CO to be 1.0:
100×V𝑠 ×S𝐹 ×M𝑊 S𝑊 ×F𝑛 ×22,414
re
Dispersion (%) =
where VS is the cumulative volume of adsorbed CO (cm3 at STP), MW is the molecular weight
lP
of Pt metal (g/mol), SW is the weight of the sample, and Fn is the Pt fraction in relation to the total weight of the catalyst sample. CO chemisorption with the spent catalysts was performed using above procedure after (1) purging the sample in N2 (50 mL/min) for 3h at the reaction
na
temperature and (2) cooling down the reactor temperature to room temperature in N2. The X-ray photoelectron spectra (XPS) of all calcined and reduced catalysts were obtained using an Ulvac-PHI spectrometer (PHI 5000 Versa Probe). Deconvolution of the C1s, Ti3s, and
ur
Pt peaks was performed using a sum of Lorentzian–Gaussian functions. The binding energies
Jo
were corrected by the C1s peak from carbon contamination to 284.6 eV. 2.3 Activity measurements for sulfuric acid decomposition The catalytic activity was evaluated in a bayonet type quartz reactor at the temperature of
650, 750, or 850 °C, and the stability of catalysts was tested for 100 h at 650 and 850 oC and a WHSV of 85 gH2SO4 gcat-1 h-1. A quartz reactor was made with a bayonet type (total length: 55 cm) with the diameter of 8.0 mm of an inner tube and the dia. of 28 mm of an outer tube for sulfuric acid evaporation. The feed with a 1:1 molar ratio of SA (85 wt%) to N2 was introduced
into the reactor through a quartz preheater. The flow rates of aqueous sulfuric acid and N2 were 0.25 g/min (liquid phase, 25 °C) and 95 mL min-1 (gas phase), respectively. The WHSV at the typical reaction condition is 85 gH2SO4 gcat-1 h-1. The prepared catalyst was pelletized and crushed into 120–180 mesh, and placed in the inner tube of the bayonet type quartz reactor. The amount of catalyst is 0.15 g and size of the catalyst bed is 8.0 mm in thickness and 10 mm to height. The catalysts were fixed with quartz wool. A thermocouple was placed in the catalyst bed. Sulfuric acid (SA, purity:85 wt%) was introduced into the reactor through an evaporator by a liquid micro-pump. The product O2 from the H2SO4 trap was analyzed by a gas
ro of
chromatography (YoungLin Inc. - M600D).
3. Results and discussion
3.1. Physico-chemical characterizations of pristine TiO2 supported Pt catalysts
-p
Table 1 shows physical properties for differently pretreated catalyst samples. BET surface area (SBET) of Pt/G5-450 was approximately two times larger than that of Pt/P25-450. SBET
re
decreased in an increase in temperature of thermal treatment as usual. SBET of Pt/G5-850 became approximately five times larger than that of Pt/P25-850. The measured average pore
lP
volume and average pore diameter is mainly from interparticles. Fig. 1 shows the XRD patterns of the prepared catalysts. A P25 support has both anatase (JCPDS #71-1166) and rutile (JCPDS # 73-1763) phases, while A G5 support has anatase phase
na
only. When pristine Pt/G5 and Pt/P25 catalysts were thermally treated at 650 oC and 850 oC in N2 for 2h, most of anatase phase in P25 changed to rutile even at 650 oC (Fig.1(C)), but anatase phase of G5 was retained even in the thermal treatment at 850 oC (Fig.1(H)). The appearance
ur
of Pt characteristic peaks in the XRD patterns indicates that PtOx is thermally reduced on the Pt-TiO2 catalysts treated in N2 at the temperature higher than 650 oC. The thermal reduction of
Jo
PtOx was also observed on Pt/Al2O3 [26] and Pt/TiO2 [25]. The reduction temperature of PtOx on Pt/Al2O3 was 637 oC from O2 TPD analysis. It is needed to note that the reduction of PtOx on Al2O3 with small Pt oxide particles occurs at a significantly higher temperature. So, the appearance of the metallic Pt on the Pt-TiO2 catalysts treated in N2 at the temperature higher than 650 oC should be due to the thermal reduction of PtOx. From XRD patterns, we can name the Pt/P25-650 and 850 catalysts to rutile-TiO2 supported Pt catalysts and we named the Pt/G5650 and 850 catalysts to anatase-TiO2 supported Pt catalysts.
CO chemisorption was attempted to measure metal surface area. Pt/P25-450 and Pt/G5450 (samples calcined at 450 oC) have the metal dispersions of 50.9% and 23.7 %, respectively. The metal sizes of Pt/P25-450 and Pt/G5-450 from CO chemisorption are corresponding to 2.2 and 4.8 nm, respectively. The Pt/P25-450 sample with the lower SBET has higher metal dispersion than the Pt/G5-450 with the larger SBET. However, it should be noted that no CO chemisorption was measured on the Pt/TiO2 catalysts treated in N2 at the temperature higher than 650 oC. No CO chemisorption of the catalysts treated at the temperature higher than 650 o
C should not mean no Pt presence, because XRF analysis and XRD patterns show the presence
of Pt clearly (Table 1 and Fig.1). This kind of phenomenon was also previously observed on
ro of
Pt/SiO2 catalytic system [30]. On the Pt/SiO2 system, the encapsulating Pt with SiO2 support on the surface was observed. Therefore, it is proposed that TiO2 encapsulates Pt at the high temperature treatment. Ono et al. [25] also showed that the encapsulation of Pt by TiOx on TiO2/Ti thin film was not only observed at the temperature above 425 oC in ultrahigh vacuum,
-p
but Pt-Ti-O was also observed on the surface. So, it is plausible that Pt is encapsulated by TiO2 and Pt-Ti-O phase can be formed on the thermally treated Pt/TiO2 at 850 oC, because the
re
reducibility of meal oxide is decided by thermodynamics.
Fig.2 shows TEM and STEM images. The morphology of both Pt/G5-850 and Pt/P25-850
lP
is not so much different. From the statistics of Pt particles, the average Pt particle sizes of Pt/G5-850 and Pt/P25-850 are calculated to be 4.7 nm and 3.0 nm, respectively. It should be noted that there are no CO chemisorbed on both Pt/G5-850 and Pt/P25-850. As observed with
na
CO chemisorption of Pt/G5-450 and Pt/P25-450, the CO chemisorption with the 1 wt% Pt catalysts with the size of 3.0-5.0 nm should be measured. It is clear there are little Pt loss from XRF analysis (Table 1) during the heat treatment up to 850 oC. Therefore, no CO chemisorption
ur
of Pt/G5-850 and Pt/P25-850 with the respective average Pt particle sizes of 4.7 nm and 3.0 nm does not means the severe Pt sintering or Pt loss. On the other hand, the selected area
Jo
electron diffraction (SAED) patterns of Pt particles on TiO2 show the mixed crystal structure of Pt and TiO2 on both Pt/P25 and Pt/G5 (Fig.2(a) and Fig.2(b)). The clear Pt and TiO2 Laue spots in SAED of a Pt particle suggest that TiO2 may cover Pt particles, supporting that Pt particles are encapsulated. It was reported that the suppression of CO chemisorption on Pt/TiO2 was ascribed to the formation of Pt-Ti-O via the strong metal support interaction (SMSI) [35]. Another phenomena from the SMSI is the Pt encapsulation [35]. Therefore, it can be concluded that no CO chemisorption on both Pt/G5-850 and Pt/P25-850 with the respective Pt particle size of 3.0 and 4.8 nm and the Laue spots of both Pt and TiO2 in SAED are the clear indication
of the Pt encapsulation via the SMSI. 3.2. Sulfuric acid decomposition on Pt-TiO2 catalysts Fig.3(a) shows initial SA conversions with respect to reaction temperature at a WHSV of 84 gH2SO4 gcat-1 h-1. The averaged SA conversion for 2 h was used as the initial SA conversion at the reaction temperature. Pt/P25 exhibits a little higher activity than Pt/G5 at the initial reaction time, which can be ascribed to the higher Pt dispersion of Pt/P25. Nur et.al [32] compared the catalytic activity at a WHSV of 110 gH2SO4 gcat-1 h-1 and the temperature lower than 800 oC on anatase-TiO2 (JRC-TIO-8, Japan) and rutile-TiO2 (JRC-TIO-6, Japan)
ro of
supported Pt catalysts. The SA conversion with rutile-TiO2 support Pt catalyst was one fourth of that of anatase-TiO2 supported Pt catalyst even at 800 oC. On the other hand, Petkovic et. al [31] reported the initial SA conversion of 65% on rutile (Johnson Matthey, West Deptford, NJ) supported Pt catalyst at ca. 840 oC and a WHSV of 49.5 gH2SO4 gcat-1 h-1, agreeing on our
-p
results on the rutile-TiO2 supported catalysts.
Fig.3(b) shows the stability of Pt/G5 and Pt/P25 catalysts. Pt/G5 catalyst, anatase-TiO2
re
supported Pt catalyst, exhibits very high stability on both 650 oC and 850 oC. On the other hand, Pt/P25, rutile-TiO2 supported catalyst, deactivated steadily up to 100 h, similarly to the result
lP
of Petkovic et. al. [31]. The main cause of deactivation of Pt catalyst has been assigned to the Pt loss as well as the Pt sintering. ICP analysis was performed to measure the Pt loss. The quantities of Pt with the pristine Pt/G5-850 and Pt/P25-850 catalysts were 1.06 wt% and 0.82
na
wt%, respectively, while those with the spent Pt/G5-850 and Pt/P25-850 catalysts were 1.02 and 0.54 wt%, respectively. It is clear that anatase phase can prevent the Pt loss via Pt oxidation. However, rutile phase cannot prevent the Pt loss, resulting in steady deactivation on the rutile
ur
supported Pt catalyst. Therefore, it can be concluded that the anatase supported Pt catalysts is highly stable in the wide temperature range of 650-850 oC.
Jo
In Pt/SiO2, the encapsulation of Pt by SiO2 at the inner wall was proposed to be a reason
for the Pt stabilization in SA decomposition at high temperature [30]. Reaction can proceed as the exposure of Pt by sulfuric acid SA decomposition re-encapsulation of Pt by SiO2. However, it was observed that Pt at the outer wall was lost due to the weak binding of Pt to SiO2. The Pt encapsulation cannot prevent the Pt loss on Pt/SiO2 at 850 oC, because the exposed PtOx is inevitably evaporated in the weak binding of Pt to SiO2. On Pt/TiO2, it is obvious from CO chemisorption and TEM analysis that Pt is also encapsulated on both anatase and rutile.
However, the calculation of the binding energies showed that the binding energy of Pt was much higher on stoichiometric anatase-TiO2 (2.80 eV) than on stoichiometric rutile-TiO2 (2.14 eV) [33]. Here, the stoichiometry means no generation of vacant sites or no reduction of TiO2. However, the binding energy of Pt to the reduced TiO2 increased a lot. It was revealed that the binding energy of Pt became 4.8 eV on the reduced anatase-TiO2 and 3.52 eV on the reduced rutile-TiO2. The binding of Pt to the reduced anatase-TiO2 increased much more than that to the reduced rutile-TiO2. The TiO2 reducibility even at the temperature higher than 450 oC was clearly observed on Pt on well fabricated TiO2/Ti thin film in thermal treated Pt-TiO2 [32]. Both Pt/P25 and Pt/G5 catalysts were thermally treated in N2 at 850 oC. Therefore, the
ro of
reduction of TiO2 near Pt can be probable at the temperature of 650~850 oC. Therefore, the thermal reduction of anatase-TiO2 can be an origin of the high thermal stability of Pt/G5 catalyst, increasing the relative binding energy of Pt to anatase-TiO2.
Fig. 4 shows XPS analysis of pristine and spent Pt/G5-850 and Pt/P25-850 catalysts. Both
-p
pristine Pt/G5-850 and Pt/P25-850 catalyst indicates that PtO is mostly reduced at high temperature. Ti(3s) at 75.9 eV is not negligible due to the Pt encapsulation by TiO2, which is
re
overlapped with Pt2+(4f5/2). After 100 h reaction, PtO is little present on the spent Pt/G5-850 sample in XPS analysis, while the low intensity of PtO is observed on the spent Pt/P25-850.
lP
The XPS spectra are qualitatively representative for the real Pt valence state during the reaction, because the samples are collected after cooling down to room temperature in N2 environment. Unfortunately, the low concentration of Pt-TiO2 interface cannot clarify the electron transfer in
na
XPS, but XPS analysis supports that Pt particles are covered by TiO2 from the high intensity of Ti3S.
ur
The experimental results show the remarkable two observations: (a) CO chemisorption was not detected with the catalysts pretreated at the temperature higher than 450 oC (b) Pt was
Jo
encapsulated. The SMSI effect has been pronounced on thermally treated Pt/TiO2 or H2reduced Pt/TiO2 [34]. The above observations are the peculiar phenomena of the SMSI effect [35]. The encapsulation came from the reduction of TiO2 to TiOx and the active metal was covered by the migration of the reduced TiO2. So, the encapsulation can be described as one of evidences for the SMSI effect on Pt/TiO2. Furthermore, it has been reported that SMSI (strong metal support interaction) between metals and TiO2 is predominant on anatase-TiO2 [36-39]. Rutile-TiO2 supported catalysts were more active than anatase-TiO2 supported catalysts in CO oxidation and methane combustion,
while anatase-TiO2 supported catalysts showed the higher thermal stability. The same behavior was also observed in sulfuric acid decomposition here. The higher initial activity of Pt/P25-850 in Fig.3 can be due to the higher metal dispersion or lower SMSI, as compared to Pt/G5. However, it is clear that the Pt/G5 is much more stable than Pt/P25. It was shown that the stronger binding of anatase-Pt than rutile-Pt could be more pronounced in the reduced TiO2 [33]. Because the origin of SMSI comes from the reduction of TiO2 in the vicinity of metal [34], it is proposed that the stronger binding of Pt to the reduced anatase-TiO2 can be the origin of the high stability of Pt/G5 in sulfuric acid decomposition.
ro of
Additionally, no Pt loss was observed on Pt/G5 at the reaction tempereature of 650 oC and 850 o
C for 100 h in sulfuric acid decomposition even for 100 h. The catalytic system with no Pt
loss for 100 h at both 650 oC and 850 oC is rarely reported in sulfuric acid decomposition. Therefore, it is suggested that the high stability of the anatase-TiO2 supported catalytic system in the wide temperature range of 650 oC-850 oC can afford the practical solution for the catalyst
-p
development in the decomposition of sulfuric acid: high stability, reasonable activity and easy
re
massive preparation of catalysts.
lP
4. Conclusion
We prepared anatase and rutile supported Pt catalysts using P25 and G5, respectively. Pt was dispersed better on P25 than on G5. The apparent initial activity of the Pt/P25 catalyst is
na
slightly higher than that of the Pt/G5 catalyst, while the stability of the Pt/P25 is much lower than that of Pt/G5. There is no Pt loss on Pt/G5 after the SA reaction at 650 oC and 850 oC for
ur
100 h, while there is 35% Pt loss on Pt/P25. The Pt encapsulation and anchoring on both anatase- and rutile-TiO2 are observed from CO chemisorption, TEM images and XPS analysis. The encapsulation and no CO chemisorption on Pt/TiO2 are typical peculiar phenomena of
Jo
SMSI interaction. It is shown on the well defined thin TiO2/Ti film that the surface of TiO2, thermally treated at the temperature higher than 450 oC, was reduced in the vicinity of Pt [25]. Therefore, the SMSI on both Pt/P25 and Pt/G5 is very probable, when the catalysts are thermally treated at 850 oC in N2. On the other hand, the Pt binding to anatase-TiO2 is stronger than that to rutile-TiO2 [33]. However, the Pt binding to anatase-TiO2 become much stronger than that rutile-TiO2, when TiO2 is reduced in the vicinity of Pt-SMSI. So, it is proposed that SA decomposition occurs on Pt anchored to anatase-TiO2 through Pt exposure of encapsulated
Pt in contact with sulfuric acid. The outstanding high stability without Pt loss can be ascribed to the strong binding of Pt to the anatase support.
Acknowledgment This work was financially supported by the Korea Institute of Science and Technology (KIST) and by Ministry of Science and ICT through the Korea CCS 2020 R&D program
ro of
References [1] S.S. Kumar, V. Himabindu, Materials Science for Energy Technologies 2 (2019) 442
[2] M. Reuβ, J. Reul, T. Grube, M. Langemann, S. Calnan, M. Robinius, R. Schlatmann, U. Rau, D. Stolten, Sustainable Energy & Fuel, 3 (2019) 801.
-p
[3] J.H. Kim, D. Hansora, P. Sharma, J.W. Jang, J.S. Lee, Chem. Soc. Rev. 48 (2019) 1855.
[4] A. Pandiyan, A. Uthayakumar, R. Subrayan, S.W. Cha, S.B. Moorthy, Nanomaterials and Energy 8 (2019) 1.
re
[5] M. Soser, M. Pecchi, T. Fend, energies 12 (2019) 352
lP
[6] F. Yilmaz, R. Selbas, Thermodynamic performance assessment of solar based Sulfur-Iodine thermochemical cycle for hydrogen generation, Energy 140 (2017) 520-529.
na
[7] J. Chang, Y.W. Kim, K.Y. Lee, Y.W. Lee, W.J. Lee, J.M. Noh, M.H. Kim, H.S. Lim, Y.J. Shin, K.K. Bae, K.D. Jung, A study of A nuclear hydrogen production demonstration plant. Nucl. Eng. Tech. 39 (2007) 111-122.
ur
[8] Y. Shin, T. Lee, K. Lee, M. Kim, Modeling and simulation of HI and H2SO4 thermal decomposers for a 50 NL/h sulfur-iodine hydrogen production facility. Appl. Energy 173 (2016) 460.
Jo
[9] K. Jedrzejewski, M. Hanuszkiewicz-Drapala, Analysies of the Efficiency of a HighTemperature Gas-Cooled Nuclear Reactor Cogeneratino System Generating Heat for the Sulfur-Iodine Cycle, J. Energy Resour. Technol, 140 (2018) 112001. [10] K. Onuki, S. Kubo, A. Terada, N. Sakaba, R. Hino, Thermochemical water-splitting cycle using iodine and sulfur, Energy & Environmental Science, 2 (2009) 491-497. [11] J. Park, J.H. Cho, H. Jung, S. kumar, IL. Moon, Simulation and experimental study on the sulfuric acid decomposition process of SI cycle for hydrogen production, Int. J. Hydrogen 38 (2013) 5507. [12] P. Zhang, C. Xhou, H. Guo, S. Chen, L. Wang, J. Xu, Design of integrated laboratoryscale iodine sulfur hydrogen production cycle at INET Int. J. Energy Res. 40 (2016) 1509-1517. [13] S. Kasahara, Y. Imai, K. Suzuki, J. Iwatsuki, A. Terada, X.L. Yan. Conceptual design of
the iodine-sulfur process flowsheet with more than 50% thermal efficiency for hydrogen production Nuclear Eng. Design 329 (2018) 213-222. [14] K.D. Jung, T.H. Kim, K.T. Gong, H.Y. Jeon, C.H. Shin, H. Kim, B.G. Lee, SO3 decomposition on Alumina and Titania Supported Catalysts in IS cycle to Produce Hydrogen. Proceedings of ICAPP’05 Seoul Korea Paper 5117 (2005). [15] Tagawa, T. Endo, Catalytic decomposition of sulfuric acid using metal oxides as the oxygen generating reaction in thermochemical water splitting process. Int. J. Hydrogen Energy 14 (1989) 11-17. [16] Neri G., Rizzo G., S. gavagno, Appl. Catal.A: General 274 (2004) 243-251.
ro of
[17] Rosen M.A., Scott D.S. “Comparative efficiency assessment for a range of hydrogen production process” Int. J. Hydrogen 23 (1998) 653. [18] T.H. Kim, G.T. Gong, B.G. Lee, K.Y. Lee, H.Y. Jeon, C.H. Shin, H. Kim, K.D. Jung, Catalytic decomposition of sulfur trioxide on the binary metal oxide catalysts of Fe/Al and Fe/Ti, Appl. Catal A: General 305 (2006) 39-45.
-p
[19] H. Abimanyu, K.D. Jung, K.W. Jun, J. Kim, K.S. Yoo, Preparation and characterization of Fe/Cu/Al2O3-composite granules for SO3 decomposition to assist hydrogen production Appl. Catal A: General 343 (2008) 134-141.
lP
re
[20] A.M. Banerjee, M.R. Pai, S.S. Meena, A.K. Tropathi, S.R. Bharadwaj, Catalytic acitivities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles. Int. J. Hydrogen 36 (2011) 47684780.
na
[21] M. Machida, T. Kawada, S. Hebishima, S. Hinokuma, S. takeshima Macroporous Supported Cu-V oxide as a promising substitute of the Pt catalyst for sulfuric acid decomposition in solar thermochemical hydrogen production. Chem. Mater. 24 (2012) 557561. [22] Rashkeev S.N., Ginosar D.M., Petkovic L.M., Farrel H.H. Catalytic activity of supported metal particles for sulfuric acid decomposition reaction. Catal Today 139 (2009) 291-298.
ur
[23] H.A. Khan, P.Natarajan, K.D. Jung Synthesis of Pt/mesoporous SiC-15 and its catalytic performance for sulfuric acid decomposition. Catal. Today 303 (2018) 25.
Jo
[24] B.M. Nagaraja, K.D. Jung, B.S. Ahn, H. Abimanyu, K.S. Yoo, Catalytic decomposition of SO3 over Pt/BaSO4 Materials in Sulfur-Iodine cycle for hydrogen production. Ind. Eng. Chem. Res. 48 (2009) 1451-1457. [25] L.K.Ono, B. Yuan, H. Heinrich, B.R. Cuenya, Formation and Thermal Stability of Platinum Oxides on Size-Selected Platinum Nanoparticles: Support effects. J. Phys. Chem. C 114 (2010) 22119. [26] E.S. Putna, J.M. Vohs, R.J. Gorte, Osygen desorption from α-AL2O3(0001) supported Rh, Pt and Pd particles. Surf. Sci. 391 (1997) L1178-L1182. [27] J.M. Bray, I.J. Skavdahl, J.-S. McEwen, W.F. Schneider, First-principles reaction site
model for coverage-sensitive surface reactions: Pt(111)-O temperature programmed desorption. Surf. Sci. 622 (2014) L1-L6. [28] P.N. Plessow, F.Abild-Pedersen, Sintering of Pt Nanoparticles via Volatile PtO2: Simulation and Comparison with Experiments, ACS catal. 6 (2016) 7098-7108. [29] H.A. Khan, M.I. Iqbal, A. Jaleel, I. Abbas, S.A. Abbas, K.D. Jung, Pt encapsulated hollow mesoprous SiO2 sphere catalyst for sulfuric acid decomposition reaction in SI cycle, Int. J. Hydrogen, 44 (2019) 2312-2322. [30] H.A. Khan, P. Natarajan, K.D. Jung, Stabilization of Pt at the inner wall of hollow spherical SiO2 generated from Pt/hollow spherical SiC for sulfuric acid decomposition, Appl.Catal.BEnviron. 231 (2018) 151-160.
ro of
[31] L.M. Petkovic, D.M. Ginosar, H.W. Rollins, K.C. Burch, P.J. Pinhero, H.H. Farell, Pt/TiO2(rutile) catalysts for sulfuric acid decomposition in sulfur-based thermochemical watersplitting cycles, Appl. Catal. A: General 338 (2008) 27-36.
-p
[32] A.S.M. Nur, T. Matsukawa, S. Hinokuma, and M. Machida, Catalytic SO3 Decomposition Activity and Stability of Pt Supported on Anatase TiO2 for Solar Thermochemical Water-Splitting Cycles, ACS Omega 2 (2017) 7057.
re
[33] H. Iddir, V. Skavysh, S. Ogut, H.D. Browning, M.M. Disko, Preferential growth of Pt on rutile TiO2, Physical Rev. B 73 (2006) 041403. [34] U. Diebold, The surface science of titanium dioxide, Surf, Sci. Reports, 48 (2003) 53-229.
lP
[35] C. Ocal, S. Ferrer, The strong metal-support interaction (SMSI) in Pt-TiO2 model catalysts, A new CO adsorption state on Pt-Ti atoms, J. Chem. Phys. 84 (1986) 6474-6478.
na
[36] Y. Li, Y. Fan, H. Yang, B. Xu, L. Feng, M. Yang, Y. Chen, Strong metal–support interaction and catalytic properties of anatase and rutile supported palladium catalyst Pd/TiO2, Chem. Phys. Lett. 372 (2003) 160–165.
ur
[37] A. Nobile, M.W. Davis, Importance of the Anatase-Rutile Phase Transition and Titania Grain Enlargement in the Strong Metal-Support Interaction Phenomenon in Fe/TiO2 Catalysts, J. Catal. 116 (1989) 383-398.
Jo
[38] H. Tang, Y. Su, B. Zhang, A.F. Lee, M.A. Isaacs, K. Wilson, L. Li, Y. Ren, J. Huang, M. Haruta, B. Qiao, X. Liu, C. Jin, D. Su, J. Wang, T. Zhang, Classical strong metal–support interactions between gold nanoparticles and titanium dioxide, Sci. Adv. 3 (2017) e1700231. [39] Z. Rui, S. Wu, C. Peng, H. Ji, Comparison of TiO2 Degussa P25 with anatase and rutile crystalline phases for methane combustion, Chem. Eng. J. 243 (2014) 254-264.
Table 1. Physical properties of differently pretreated Pt-TiO2 catalysts
Pt/P25-450 Pt/P25-650 Pt/P25-850 Pt/G5-450 Pt/G5-650
Calcined in air at 450 oC for 2 h Thermally treated in N2 at 650 oC for 2 h Thermally treated in N2 at 850 oC for 2 h Calcined in air at 450 oC for 2 h Thermally treated in N2 at 650 oC for 2 h Thermally treated in N2 at 850 oC for 2 h
SBET, m2 g-1
Av. pore vol. cm3 g-1
XRF Pt, wt%
Av. pore dia., nm
49.3
0.5
0.85
42.1
17.6
0.3
0.83
66.1
1.4
0.6
0.82
163.2
91.8
0.4
1.09
18.8
45.9
0.5
1.05
46.2
6.9
0.1
0.99
Jo
ur
na
lP
re
-p
Pt/G5-850
Pretreatment
ro of
Catalyst
65.1
ro of
Jo
ur
na
lP
re
-p
Fig. 1. XRD patterns of (A) P25 support, (B) Pt/P25-450, (C) Pt/P25-650, (D) Pt/P25-850, (E) G5 support, (F) Pt/G5-450, (G) P/G5-650, and (H) P/G5-850.
(a)
Pt (111) TiO2 A(004)
TiO2 A(004)
Pt (200)
lP
re
-p
(b)
ro of
4.7 nm
TiO2 R(002) 3.0 nm
na
Pt (200)
ur
Pt (200)
Jo
Fig. 2. TEM/STEM images and particle size distribution of (a) Pt/G5-850 and (b) Pt/P25850
100
(a)
SA converson (%)
80 60 40
0 600
650
700
750
800
ro of
20
850
900
o
Reaction temperature ( C) (b)
-p
80
re
60 40 20 0
10 20 30 40 50 60 70 80 90 100
na
0
lP
SA conversion (%)
100
Time (h)
Jo
ur
Fig. 3. Catalytic performance of sulfuric acid decomposition at WHSV of 85 gH2SO4 gcat-1 h-1: (a) reaction temperature effect of Pt/G5 () and Pt/P25 (), and (b) stability of Pt/G5 at 650 oC (), Pt/G5 at 850 oC (), Pt/P25 at 650 oC () and Pt/P25 at 850 oC ().
(b1)
(a1)
Ti 3s
Ti 3s Pt0
Pt0
Pt2+
(a2)
(b2)
Ti 3s
Ti 3s Pt0
ro of
Pt2+
Pt2+
lP
re
-p
Pt0
Jo
ur
na
Fig. 4. XPS analysis of pristine and spent Pt/TiO2 catalysts: (a1) pristine Pt/G5-850, (a2) spent Pt/G5-850, (b1) pristine Pt/P25-850, and (b2) spent Pt/P25-850.
ro of
-p
re
lP
na
ur
Jo