Subscriber access provided by Northern Illinois University
Article x
2
SO tolerant Pt/TiO catalysts for CO oxidation and the effect of TiO supports on catalytic activity 2
Kenji Taira, Kenji Nakao, Kimihito Suzuki, and Hisahiro Einaga Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01652 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 25
Environmental Science & Technology
1
SOx tolerant Pt/TiO2 catalysts for CO oxidation and the effect
2
of TiO2 supports on catalytic activity
3
Kenji Taira+§*, Kenji Nakao+, Kimihito Suzuki+, Hisahiro Einaga§ +
4 5 6 7 8 9
Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba, 293-8511 Japan
§
Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen Kasuga-city Fukuoka, Japan *
Corresponding Author
E-mail:
[email protected] Tel.: +81 70 3914 4689. Fax: +81 439 80 2745
10 11
ABSTRACT: We developed a new technique for mitigating catalyst deactivation caused by SO2 in ex-
12
haust gases. A series of 0.1 wt%-Pt/TiO2 catalysts with different surface, crystal and pore structures
13
were prepared and tested for CO oxidation activity in the presence of SO2 and H2O. The order of the CO
14
oxidation activity under the influence of SO2 was much different from that in the absence of SO2. Cata-
15
lysts with a high ratio of larger pores exhibited higher catalytic activity under the influence of SO2 and
16
H2O in the temperature range of 250-300oC, whereas other parameters, such as BET surface area and
17
crystal structure of the TiO2 support, had minor effects on the CO oxidation activity. The oxidation
18
state of Pt differed significantly depending on the kind of TiO2 support. Some catalysts were less active
19
without H2 reduction pretreatment due to the presence of oxidized Pt species.
20
KEYWORDS. CO oxidation, Pt, TiO2, catalyst, SO2
1
ACS Paragon Plus Environment
Environmental Science & Technology
21
Page 2 of 25
Introduction
22
We have long relied on fossil fuels as an energy source. Despite continuous improvements in the
23
fuel combustion processes, some of the fuel still remains in the exhaust gases after combustion, and the-
24
se fuels cause environmental issues. Exhaust gas from the sintering process of the steel industry includes
25
about 1vol% of CO. It is preferable to remove CO from the exhaust gas with end-of-pipe catalytic com-
26
bustion. However, toxic SOx is an unavoidable component of the exhaust gas from coal combustion be-
27
cause coal contains sulfur as an impurity. Catalysts are severely deactivated by SO2, regardless of their
28
composition 1,2. For example, non-noble metal catalysts, such as CeO2-based catalysts, lose their activity
29
3
30
the sulfate Ce(SO4)2, which alters the surface properties of the catalyst.
or selectivity 4 under the influence of SO2. In this reaction, SO2 oxidizes on the oxide surface to form
31
Much research has been conducted into the effects of SO2 on the activity of noble-metal cata5-16
32
lysts
. Pd/Al2O3, a widely-used combustion catalyst, has been reported to suffer from severe deacti-
33
vation because Pd reacts with SO2 to form PdSO4 5,6. While the catalytic activity of Pd/Al2O3 is partially
34
recovered by the elimination of SO3 from the Pd surface to the Al2O3 during the reaction, this recovery
35
process is inhibited by water vapor in the reaction gas 5,6. Thus, the co-existence of SO2 and H2O pro-
36
motes the deactivation of Pd/Al2O3.
37
The tolerance of noble metals against SO2 strongly depends on the kinds of metals. Pt-based
38
catalysts show higher catalytic activity for CO oxidation than Pd-based catalysts in the presence of SO2
39
7
40
based catalysts were deactivated in the presence of SO2
41
SO3, which migrated to the surface of the supports, giving rise to surface sulfates
42
sulfate formed on the supports depends on the reaction temperature, and it significantly increases as the
because the SO3 species formed on Pt are more easily removed than that on Pd 8. However, even Pt 9,10
, because SO2 was oxidized on Pt to form
2
ACS Paragon Plus Environment
10-13
. The amount of
Page 3 of 25
Environmental Science & Technology
43
reaction temperature decreases. Typically, the amount peaks at a reaction temperature around 250oC
13
44
and the removal of the sulfur species from the supports requires thermal treatment at temperatures above
45
350oC 13. Some studies have found that sulfates on acidic oxides, i.e. SiO2, TiO2 or ZrO2, are less stable
46
and can be removed from the supports more easily than Al2O3 14,15. TiO2 and ZrO2 also have advantages
47
over the other supports in that the oxidation of SO2 to SO3 can be inhibited: Pt catalysts supported on
48
TiO2 and ZrO2 are less susceptible to SO2 oxidation than those on SiO2 or Al2O3 16. On the basis of the-
49
se findings, TiO2-supported Pt catalysts are good candidates for the CO oxidation catalysts in the pres-
50
ence of SO2. However, there have been no reports on which physical or chemical properties of the sup-
51
ports strongly affect the SOx tolerance of Pt/TiO2 catalysts. Furthermore, only a few papers have been
52
published on the results of oxidation reactions under a mixture of SO2 and H2O at a reaction temperature
53
of around 250oC, which is a harsh reaction condition where the catalysts are severely poisoned by SO2.
54
In this report, we prepared Pt/TiO2 catalysts with a series of TiO2 supports having various surface areas
55
and pore structures and then compared their catalytic activity. The reaction gas was simulated exhaust
56
gas from the sintering process in the steel industry, which includes 40 ppm of SO2 and 20vol% of H2O.
57 58
Experimental Section
59
Catalyst preparation
60
All of the Pt/TiO2 catalysts were prepared by the impregnation method using H2PtCl6 as the
61
metal source. The TiO2 supports were supplied by Catalysis Society of Japan (CSJ) or a commercial
62
source. The supports used were JRC-TIO-2 (CSJ, BET area: 17 m2/g, pore volume (PV): 0.12 cm3/g),
63
JRC-TIO-4 (CSJ, BET area: 47 m2/g, PV: 0.33 cm3/g), JRC-TIO-6 (CSJ, BET area: 107 m2/g, PV: 0.54
64
cm3/g), JRC-TIO-7 (CSJ, BET area: 270 m2/g, PV: 0.38 cm3/g), ST-01 (Ishihara Sangyo, BET area: 285 3
ACS Paragon Plus Environment
Environmental Science & Technology
Page 4 of 25
65
m2/g, PV: 0.42 cm3/g), FTL-110 (Ishihara Sangyo, BET area: 15.8 m2/g, PV: 0.06 cm3/g) and FTL-200
66
(Ishihara Sangyo, BET area: 7.6 m2/g, PV: 0.03 cm3/g). The supports were dried at 110oC for 10 h and
67
then heated at 500oC in air for 1 h before use. The BET surface areas of the TiO2 decreased after these
68
treatments.
69
All samples were prepared by incipient wetness method with careful operation (see S2 for de-
70
tails). Precursor solutions of H2PtCl6 6H2O (Sigma Aldrich, > 99.995%) were added dropwise to 1.00 g
71
of each TiO2 support and mixed thoroughly at room temperature. Then, the samples were dried at 110oC
72
overnight and calcined at 500oC for 1 hour. The catalysts were denoted as follows corresponding to their
73
supports: Pt/TIO-2, Pt/TIO-4, Pt/TIO-6, Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200. For example,
74
Pt/TIO-2 was prepared using JRC-TIO-2. The amount of Pt was calculated to be 0.1wt% as metal Pt
75
based on the amount of the precursor used. For the X-ray photoelectron spectroscopic (XPS) experi-
76
ments, catalysts with 1wt% of Pt were also prepared with three supports: TIO-2, TIO-4 and TIO-6. All
77
of the catalysts were prepared following the same procedure as the catalysts with less Pt except that
78
concentrated H2PtCl6 solution was used. The prepared catalysts are hereinafter referred to as Pt1/TIO-2,
79
Pt1/TIO-4 and Pt1/TIO-6.
80 81
Catalyst characterization
82
The adsorption isotherm of N2 was measured at the temperature of liquid nitrogen (-196ºC) with
83
an adsorption measurement instrument (Japan BEL, BEL-max). The surface area of the catalysts was
84
determined by the Brunauer-Emmett-Teller (BET) method from the N2 adsorption isotherm. The pore
85
volume of the catalysts was estimated by the amount of N2 adsorbed on the catalysts at a relative pres-
4
ACS Paragon Plus Environment
Page 5 of 25
Environmental Science & Technology
86
sure of p/po = 0.990. The pore distribution was calculated by the Dollimore-Heal (DH) method using the
87
isotherm data 17.
88
X-ray diffraction (XRD) measurements were performed on all of the prepared catalysts using an
89
XRD instrument (Rigaku, RINT-TTR III). Scanning proceeded from 2θ = 20o to 60o with a 0.02o step
90
angle at a scanning rate of 1o/min. The X-ray tube voltage was 40 kV and the current was 150 mA. The
91
ratios between the rutile phase and anatase phase of the TiO2 in the catalysts were estimated using a
92
previously reported procedure18.
93
Dispersion of Pt was estimated by the CO pulse chemisorption method with a catalyst analyzer
94
(Japan BELL, BEL-CAT II). The pretreatment procedure was basically determined with reference to
95
other reports 19,20 and the dispersion was calculated under the assumption that CO:Pt = 1:1 21. However,
96
the calculated Pt dispersion values were unacceptably small when the CO pulse chemisorption was per-
97
formed directly after the H2 reduction step (Table S1) due to the strong-metal-support-interaction
98
(SMSI) like behavior
99
chemisorption to recover the Pt sites from the SMSI like state to the normal states 24. A detailed expla-
100
22,23
. Therefore, we added a re-oxidation step in diluted air flow before CO pulse
nation of the measurement procedures is included in the caption for Table S1.
101
TEM images of the Pt/TiO2 catalysts were taken with a transmission electron microscope (FEI,
102
Tecnai G2) to further assess the Pt dispersion. All of the images were measured as bright field images.
103
The accelerating voltage was adjusted to 150 kV, and the average Pt particle sizes were determined us-
104
ing the procedure described in the caption for Fig. S1.
105
Fourier Transform Infrared (FTIR) spectra were obtained using a Nicolet iS50 FT-IR (Thermo
106
scientific) in Attenuated Total Reflection (ATR) configuration with a diamond prism. The spectra were
5
ACS Paragon Plus Environment
Environmental Science & Technology
Page 6 of 25
107
recorded with a 2 cm-1 step and 64 times of the cumulated number. The differential spectra were calcu-
108
lated between the catalysts before and after the CO oxidation reaction test.
109
Thermal gravimetric analysis (TGA) profiles were recorded from room temperature to 850oC
110
with a thermal analysis instrument (Shimadzu, TG-50) for the catalysts before and after the CO oxida-
111
tion reaction tests. All of the measurements were performed in a N2 flow of 50 cm3/min.
112
The oxidation states of Pt in Pt1/TIO-2, Pt1/TIO-4 and Pt1/TIO-6 were estimated by X-ray pho-
113
toelectron spectroscopy (XPS). All of the spectra were taken in a XPS analyzer (ULVAC-Phi, Quan-
114
tum-2000) equipped with a monochromated Al X-ray source and a charge neutralizer. All of the meas-
115
urements were performed at a pass energy of 29.35 eV and recording step of 0.125 eV. The peak shift
116
derived from charge up of the catalysts was corrected by adjusting the binding energy of the C1s peak to
117
285.0 eV. Spectra of Ti2p and O1s were also recorded to assess the validity of the adjustment. The de-
118
viation of the peak-top positions between the catalysts was confirmed to be below 0.2 eV.
119 120
Catalytic CO oxidation studies
121
The catalytic activity of the Pt/TiO2 catalysts was evaluated by the CO oxidation reaction (CO +
122
1/2O2 → CO2) at atmospheric pressure. A schematic of the reactor is shown in Fig. S2. The gas compo-
123
sition was controlled with mass flow controllers, and water was introduced with a water pump (Nihon
124
Seimitsu Kagaku, NP-KX-510). The water pump nozzle was kept at 115oC, and all of the water was va-
125
porized on the upstream side of the reaction tube. The catalysts were packed into a quartz glass tube,
126
and their temperatures were monitored with a K-type thermocouple. In order to increase the thickness of
127
the catalyst layer, 10 mg of Pt/TiO2 catalyst was diluted with 20 mg of the same TiO2 support. This dilu-
128
tion operation was confirmed not to affect the catalytic activity but to ease the sample packing. A reduc6
ACS Paragon Plus Environment
Page 7 of 25
Environmental Science & Technology
129
tion pretreatment was performed at 500oC for 30 minutes in H2 before each reaction test. Some of the
130
catalysts were used without a reduction pretreatment. The gas composition was CO: 1%, O2: 10%, H2O:
131
20%, NO: 40 ppm and SO2: 40 ppm, with N2 making up the balance. The total amount of gas flow was
132
adjusted to 100 cm3/min, of which the Space Velocity (SV) was 600,000 cm3/h.gcat. Typically, the reac-
133
tions were performed at 250oC for 6 to 20 h. The gas composition of the outlet gas from the reactor was
134
determined with a GC-TCD (Shimadzu, GC-2014) or infrared gas analyzer (Yokogawa Electric, IR-
135
200) every 25 minutes, and the CO conversion was calculated using the CO and CO2 concentrations.
136
We confirmed that the values measured by both devices were consistent (Table S2). To determine the
137
dependence of the reaction rate on reaction temperature, we performed reaction tests with higher SV
138
(6,000,000 or 60,000,000 cm3/h.gcat). The other details of the experimental conditions are described in
139
the caption for Fig. S3.
140 141 142
Results and discussion
143
Effect of crystal structure and Pt dispersion on catalytic activity
144
Table 1 summarizes the textural properties of the Pt/TiO2 catalysts used in this study. TEM im-
145
ages showed that Pt particles with a size of 1.1-1.6 nm were dispersed on the TiO2 particles (Figure S1).
146
The Pt dispersion values estimated from the CO chemisorption were similar for each of the Pt/TiO2
147
catalysts and in the range of 42-55%. According to the Pt dispersion, the average Pt particle sizes were
148
estimated to be 2.1-2.7 nm, which were consistent with the sizes determined from the TEM observation.
149
It is worth noting that none of the Pt dispersion determined by the techniques correlates with the catalyst
150
surface area or the ratio of anatase/rutile in the TiO2 support. 7
ACS Paragon Plus Environment
Environmental Science & Technology
Page 8 of 25
151
Fig. 1 shows the time course profiles for CO oxidation of the Pt/TiO2 catalysts at 250oC in the
152
presence of SO2 and H2O. Although the initial conversions were comparable for all of the catalysts, the
153
stability varied depending on the catalyst support. Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 showed a stable CO
154
conversion around 60 % for as long as 19 hours, while the values of the other catalysts dropped to
155
around 10%. Thus, the catalytic activities of Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 for CO oxidation were
156
much higher than those of Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 in the presence of SO2. The
157
total amount of SO2 fed to the reactor was 9.8 × 10-6 mol after 1 h, which corresponds to 0.96 mg or 5.2
158
× 10-4 cm3 of H2SO4 assuming complete conversion of SO2 to H2SO4. This amount of H2SO4 is compa-
159
rable to a pore volume of 6 × 10-4 cm3, which is equivalent to 10-mg Pt/FTL-200 used in the activity test.
160
The total turnover number for CO conversion was 104 orders larger than that of the exposed Pt amounts
161
(Table S3), indicating that the CO oxidation catalytically proceeded for all the Pt/TiO2 catalysts.
162
As described above, the Pt/TiO2 catalysts were deactivated during CO oxidation in the presence
163
of SO2 and H2O. When the reaction was carried out in the absence of SO2 at 250°C, all of the catalysts
164
exhibited steady CO oxidation without any decrease in the catalytic activity. Under these conditions, the
165
CO oxidation activity decreased in the order of Pt/TIO-6 > Pt/TIO-7 > Pt/FTL-200 > Pt/ST-01 >
166
Pt/TIO-4 > Pt/TIO-2 > Pt/FTL-110 (Table S4). However, the order of the catalytic activity of Pt/TiO2
167
in the presence of SO2 and H2O varied significantly from that in the absence of SO2. Therefore, the fac-
168
tors controlling the catalytic activities of Pt/TiO2 in the presence of SO2 and H2O differed from those in
169
the absence of SO2.
170
To clarify the factors affecting the catalytic activity of Pt/TiO2 in the presence of SO2 and H2O,
171
we investigated the relationship between the catalytic activity and the textural catalyst properties listed
172
in Table 1. Assuming the adherence of SO2 on the Pt surface is a major cause of catalytic deactivation,
173
we first speculated that higher Pt dispersion may lead to higher tolerance against SO2. In the present 8
ACS Paragon Plus Environment
Page 9 of 25
Environmental Science & Technology
174
study, however, the difference in Pt dispersion among the catalysts was small and not reflected in the
175
catalytic activity for CO oxidation in the presence of SO2. Moreover, Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6,
176
which exhibited higher catalytic activity, did not have larger Pt dispersion values than the other cata-
177
lysts. The TiO2 supports used in this study had various anatase/rutile ratios in a range of 1/0 to 0/1 (Ta-
178
ble 1, Fig. S4). Both the catalysts with higher activities (Pt/TIO-2, Pt/TIO-4, Pt/TIO-6) and those with
179
lower activities (the rest of the catalyst group) had anatase and rutile phases. Therefore, the ana-
180
tase/rutile ratio in the TiO2 support was not an important factor for controlling the catalytic CO oxida-
181
tion activities of Pt/TiO2 in the presence of SO2 and H2O.
182 183
SO42- species after catalytic CO oxidation
184
SO42- species formed on the catalyst surface during CO oxidation in the presence of SO2 because
185
the reaction gas included not only SO2, but also O2 and H2O. The following side reactions proceeded
186
along with the CO oxidation reaction. Pt/TiO2 catalysts can catalyze the oxidation of SO2 to SO3 (eq(1)),
187
followed by the hydration of SO3 (eq(2)).
188
SO2 + 1/2 O2 → SO3
(1)
189
SO3 + H2O → H2SO4
(2)
190
The boiling point of sulfuric acid strongly depends on its concentration and can reach over 300oC at
191
around 98% 25,26. This indicates that by-product sulfuric acid could remain as a liquid on the surface of
192
the catalysts during CO oxidation at 250ºC.
193
Subsequently, we focused on the chemical state of S-containing species that formed on the cata-
194
lyst surface during CO oxidation. Figure S5 shows the differences in the FTIR spectra of the Pt/TiO2 9
ACS Paragon Plus Environment
Environmental Science & Technology
Page 10 of 25
195
catalysts, which were obtained by subtracting the spectra of Pt/TiO2 before reaction from those after re-
196
action. The bands were formed in the wavenumber range of 850-1300 cm-1 with the peaks at 1230,
197
1149, 1043, 982 cm-1, which are the characteristic bands of bidentate SO42- species on Ti4+ sites.27 This
198
indicates that SO2 was oxidized on the Pt/TiO2 catalysts to form SO42- species on the catalyst surface,
199
and this is considered to be the cause of catalyst deactivation.
200
The amount of the SO42- species on the catalysts after the reaction was estimated from the ther-
201
mogravimetric studies. Figure S6 shows the TGA profiles of the Pt/TiO2 catalysts that were used for
202
CO oxidation at 250ºC for 6 h. The catalyst weight decreased with increasing catalyst temperature in
203
the range of 20-400ºC due to desorption of water from the catalyst. The weight loss in the temperature
204
range of 400-800ºC was ascribed to the desorption of SO42- species from the catalyst surface
205
amount of SO42- species was estimated from the weight changes in this temperature range and listed in
206
Table S5. The amount of SO42- that formed on the Pt/TiO2 catalyst was calculated to be roughly propor-
207
tional to its BET surface area. In addition, Pt/ST-01, a catalyst with lower activity, had a lower amount
208
of SOx than the higher-activity catalysts, Pt/TIO-2 and Pt/TIO-4. The catalytic properties of Pt/TiO2 in
209
the presence of SO2 and H2O cannot be explained in terms of the amount of SO42- adsorbed on the cata-
210
lysts.
28
. The
211 212
Effect of pore distribution on catalytic activity
213 214
The vapor pressure of sulfuric acid on the porous catalyst surface obeys the Kelvin equation (3)
215
29
216
contact angle, rp is the pore radii, R is the gas constant, and T is the temperature.
, where p/po is the relative pressure, VL is the molar liquid volume, γ is the surface tension, θ is the
10
ACS Paragon Plus Environment
Page 11 of 25
217
Environmental Science & Technology
ln ቀ ቁ = −
ଶಽ ఊୡ୭ୱఏ
(3)
ோ்
218
The Kelvin equation states that the condensation of sulfuric acid occurs in smaller pores first and that
219
larger pores are less vulnerable to condensation. In fact, one study on activated carbon particles found
220
that sulfuric acid filled nanopores first before spilling over to the larger pores 30. Based on these results,
221
we postulated that pore blockage occurs due to by-product sulfuric acid on the Pt/TiO2 catalyst. A
222
schematic of this hypothesis is shown in Fig. 2. In our experiment, the formation of sulfuric acid was
223
accelerated on the Pt sites, and the nanopores and mesopores were filled or blocked by the sulfuric acid.
224
TEM and SEM images showed that the Pt/TiO2 catalysts were composed of agglomerated TiO2 particles,
225
and pores were formed as the spaces between the particles (Fig. S1, Fig. S7). Therefore, pore blockage
226
by sulfuric acid can retard the diffusion of reaction gases in the secondary particles for all of the Pt/TiO2
227
catalysts, and the reaction gases cannot access the Pt sites inside the pores, leading to the catalyst deac-
228
tivation.
229
According to the pore-blockage mechanism, catalysts with larger pores should be able to exhibit
230
stable catalytic activity in the presence of SO2. To verify this hypothesis, we calculated the pore distri-
231
bution of each catalyst using the Dollimore-Heal (DH) method 17. The pore size distribution is shown in
232
Figs. 3(a) and (b). Peaks appeared at rp > 10 nm for Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6, whereas Pt/TIO-
233
7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 had peaks at rp < 10 nm. These results indicate that the cata-
234
lysts with larger pore sizes exhibited higher CO oxidation activity than those having smaller pore sizes,
235
suggesting that pore-blockage by the sulfuric acid is a key factor controlling the catalytic properties of
236
Pt/TiO2.
237
The above results imply that Pt particles located in pores smaller than 10 nm cannot take part in
238
the oxidation reaction because CO and O2 cannot be adsorbed on the Pt sites. To further discuss the ef11
ACS Paragon Plus Environment
Environmental Science & Technology
Page 12 of 25
239
fect of the TiO2 support pore sizes, we defined a parameter to quantitatively evaluate the pore blockage
240
level. Based on the TEM images, the Pt particles were shown to be highly dispersed on the surface of
241
supports (Fig. S1). Therefore, it is possible to evaluate the tolerance against SO2 using the relative sur-
242
face area, specifically, the ratio of surface area attributable to small pores against the total surface area.
243
Pore distribution was recalculated in terms of surface area, and the cumulative sum of the surface area,
244
A0-rp, was calculated from the smaller rp. Every cumulative sum was normalized by the cumulative total
245
value of rp < 100 nm, A0-100, of each catalyst. Figure 4 shows the results of relative surface area calcula-
246
tions, A0-rp/A0-100, for all Pt/TiO2 catalysts. The catalysts can be divided into two categories in terms of
247
A0-rp/A0-100: Pt/FTL-110, Pt/FTL-200, Pt/ST-01 and Pt/TIO-7, which are in the lower activity group and
248
have a larger A0-rp/A0-100, and Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6, which belong to the higher activity
249
group and have a smaller A0-rp/A0-100. For example, Pt/TiO2 catalysts can be arranged in descending or-
250
der of A0-rp/A0-100 at rp = 10 nm as follows: Pt/FTL-110~Pt/TIO-7 > Pt/ST-01 > Pt/FTL-200 > Pt/TIO-4
251
> Pt/TIO-2 > Pt/TIO-6. This order is in agreement with the results of the catalytic activity test shown in
252
Fig. 1. The catalysts with larger A0-rp/A0-100 values exhibited deceased CO oxidation reaction activity.
253
Another piece of evidence for the pore blocking mechanism was obtained from the effect of the
254
reaction temperature on the catalytic activity of Pt/TiO2. The saturated vapor pressure of sulfuric acid,
255
po in equation (3), increases as the reaction temperature rises
256
tion is caused by pore blockage with sulfuric acid, it seems reasonable to expect that temperature in-
257
creases will lift the blockage at around the boiling point of sulfuric acid, which is estimated to be >
258
300oC, and lead to higher catalytic activity. In addition, the leap in catalytic activity due to the rise in
259
temperature is expected to be more evident for catalysts with small pores than for those with large pores.
260
Figure 5 shows the temperature dependence of catalytic activity for all catalysts. The catalysts Pt/TIO-2,
261
Pt/TIO-4 and Pt/TIO-6 exhibited higher catalytic activity as shown in Fig. 1, while the other catalysts
25 26
, . Under the assumption that deactiva-
12
ACS Paragon Plus Environment
Page 13 of 25
Environmental Science & Technology
262
Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 showed lower catalytic activity. Here, the SV of the
263
reaction is 10 times larger than that in Fig. 1. The CO conversion values of the catalysts in the former
264
group are larger than those of the latter group at temperatures lower than 300oC. The CO conversion of
265
the former group rose smoothly over the entire range of the experiment. However, the CO conversion of
266
the latter group soared at temperatures around 300oC to 315oC and reached a similar CO conversion to
267
that of the former group.
268
To further investigate the effect of SO2 on the CO oxidation activity of Pt/TiO2 catalysts, we car-
269
ried out kinetic studies of CO oxidation with Pt/ST-01, whose activity was greatly affected by SO2. Fig-
270
ure S3 and Table S6 show the Arrhenius plots for CO oxidation at lower temperatures of 250-270oC and
271
higher temperatures of 325-350oC in the presence of SO2 and H2O. In both cases, a linear relationship
272
was observed, and the apparent kinetic energy was calculated to be 151 kJ mol-1 for the higher tempera-
273
tures and 143 kJ mol-1 for the lower temperatures. The comparable values for both cases show that the
274
rate determining steps are identical for both temperature ranges but that the number of active sites are
275
different as a result of pore blockage by H2SO4 at the lower temperatures of 250-270oC. .
276
To add another confirmation for the validity of the pore blockage hypothesis, we carried out CO
277
oxidation using Pt/ZrO2 catalysts with various pore sizes. ZrO2 is also tolerant against SOx and H2SO4.
278
Therefore, H2SO4 can remain liquid and deactivate the Pt/ZrO2 catalysts by pore blocking. As a result,
279
the same tendency was observed for Pt/ZrO2 catalysts as the Pt/TiO2 catalysts. The Pt/ZrO2 catalyst
280
with pores larger than 10 nm, Pt/ZRO-4, exhibited much higher CO oxidation activity than the catalysts
281
with pores smaller than 10 nm, Pt/ZRO-3, 5, 6 (Fig. S8-10). The catalytic activity of the Pt/ZrO2 cata-
282
lysts also showed a similar dependence on the reaction temperature as the Pt/TiO2 catalysts (Fig. S11).
283
In addition, CO conversion activity of catalysts with a similar pore distribution showed an almost iden-
284
tical dependence on the reaction temperature regardless of whether TiO2 or ZrO2 supports were used. 13
ACS Paragon Plus Environment
Environmental Science & Technology
Page 14 of 25
285
Pt/TIO-2 and Pt/ZRO-4, which have peaks at around rp = 20 nm, exhibited a gradual increase in CO
286
conversion activity, while Pt/TIO-7 and Pt/ZRO-3, which have peaks at around rp = 6 nm, soared at
287
315-320oC (Fig. 5, Fig. S11). These results indicate that the pore-blockage hypothesis is correct and cat-
288
alysts with larger pores are more robust against SO2. Also, the theory has generality across a variety of
289
supports.
290 291
Oxidation state of Pt and its effect on catalytic activity without pre-reduction treatment
292
As discussed in the previous section, the catalysts Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 exhibited
293
higher catalytic activity than the other catalysts in the presence of SO2 and H2O. We subsequently inves-
294
tigated the effect of the catalyst pretreatment on the catalytic properties of Pt/TiO2 to shed light on the
295
impact of the oxidation state of the Pt. Figure 6 shows the time course for CO oxidation by Pt/TIO-2,
296
Pt/TIO-4 and Pt/TIO-6 without H2 reduction pretreatment. Pt/TIO-2 exhibited a similar CO conversion
297
rate regardless of whether the reduction treatment was performed. On the other hand, Pt/TIO-4 and
298
Pt/TIO-6 exhibited much lower CO oxidation activity compared with the reaction rate after H2 reduc-
299
tion.
300
We performed XPS measurements to study the oxidation states of the Pt particles before reduc31
301
tion treatment
. To obtain a sufficient peak intensity of Pt4f, we prepared Pt1/TIO-2, Pt1/TIO-4 and
302
Pt1/TIO-6 with a Pt loading level of 1.0wt%. The XPS measurement results are plotted in Fig. S12. The
303
Pt dispersion of Pt1/TIO-2, Pt1/TIO-4 and Pt1/TIO-6 were 46.9%, 44.9% and 53.4%, respectively, ac-
304
cording to the CO pulse chemisorption measurements. Furthermore, the Pt-particle sizes of Pt1/TIO-2,
305
Pt1/TIO-4 and Pt1/TIO-6 were 1.4 nm, 1.6 nm and 1.7 nm, respectively, based on the results of the
306
TEM measurements. These values are almost identical to those of the lower-Pt counterparts in Table 1. 14
ACS Paragon Plus Environment
Page 15 of 25
Environmental Science & Technology
307
Additionally, to evaluate the effect of Pt loading per unit surface area on the oxidative state of Pt, an
308
XPS measurement was also performed on Pt/TIO-2, which has a lower surface area than the rest of the
309
catalysts. A Pt4f spectrum of Pt/TIO-2 was obtained as a difference spectrum between the spectrum of
310
Pt/TIO-2 and that of TIO-2 (Fig. S13). The difference spectrum of Pt/TIO-2 had an almost identical
311
spectrum to that of Pt1TIO-2. Therefore, it is reasonable to estimate the oxidative state of Pt on the cata-
312
lysts with 0.1wt% of Pt using catalysts with 1.0wt% of Pt. In the spectrum for Pt1/TIO-2 in Fig. S12,
313
the peak for Pt4f7/2 appeared around the binding energy of 71.9 eV and a peak corresponding to Pt4f5/2
314
was observed around 75 eV. No other peaks were detected, even after peak separation of the spectrum.
315
In contrast, Pt1/TIO-4 and Pt1/TIO-6 had clear peaks around the binding energy of 78 eV, which is as-
316
signed to Pt4f5/2 of oxidized Pt 32. A peak corresponding to Pt4f/7/2 was detected over the binding energy
317
of 74 eV. Moreover, the Pt1/TIO-6 spectrum had no Pt4f7/2 peak around 72 eV, but it did have a peak
318
around 73 eV. These results indicate that the better part of Pt in Pt/TIO-6 is more highly oxidized than
319
the other catalysts. Oxidized Pt has lower catalytic activity than metallic Pt31. The existence of oxidized
320
Pt is assumed to be the prime cause of the lower catalytic activity of Pt/TIO-4 and Pt/TIO-6 without re-
321
duction pretreatment.
322
The above results show that Pt/TIO-2 is a good candidate for a CO oxidation catalyst in the
323
presence of SO2 because the catalyst exhibits high activity without H2 treatment. For the Pt/TIO-2 cata-
324
lyst, the Pt species is more resistant against oxidation compared with those on the other catalysts, and Pt
325
was present in the reduced form even after heat treatment in air. On the other hand, the other catalysts
326
needed H2 treatment to achieve high performance. One possible explanation for the difference between
327
these catalysts is ascribed to the difference in the crystal structures of TiO2. Pt/TIO-2 catalysts have ana-
328
tase phases, whereas Pt/TIO-6 catalysts have the rutile phase and Pt/TIO-4 catalysts have the both phas-
329
es33. Oxygen supplied through oxygen vacancies plays an important role in oxidizing noble metals on 15
ACS Paragon Plus Environment
Environmental Science & Technology
Page 16 of 25
330
the oxide supports 34. The number of TiO2 oxygen vacancies is reported to descend in order of: TIO-6,
331
TIO-4, TIO-2
332
phase
333
structure. Moreover, the unit cell has an almost identical size to that of rutile TiO2 (Table S7) 37,38. The
334
mismatch of a and b axes between rutile TiO2 and PtO2 remains 2.4%, which is small enough to ensure
335
lattice matching. Thus, oxygen supply from the vacancies and the epitaxy effects from the support are
336
considered to play important roles in oxidizing the Pt on rutile TiO2.
36
35
. Moreover, the rutile phase is more prone to create oxygen vacancies than the anatase
. Another possible cause of Pt oxidation is the epitaxy effect. PtO2 has a rutile or quasi rutile
337
We also considered the preparation method of the TiO2 supports. TIO-2 and TIO-6 were pre-
338
pared by the sulfuric acid decomposition method, and they include sulfur as an impurity (Table S8). Sul-
339
fur clearly remains in TIO-2 during the catalyst preparation process because TIO-2 was calcined at
340
900oC in the preparation process 39. On the other hand, TIO-4 was prepared by the chlorine method, and
341
it contained no detectable sulfur (Table S8)39. In addition, TiO2 supports prepared by the chlorine meth-
342
od are reported to have a tendency to oxidize the Pt deposited on them
343
prone to oxidization on TIO-4 than TIO-2 due to the difference in the preparation method as well as the
344
crystal structure.
40
. Pt is considered to be more
345
In this study, we investigated the relationship between the textural and catalytic properties of a
346
series of Pt/TiO2 catalysts for CO oxidation at relatively low temperature (250ºC) in the presence of SO2
347
and H2O. On the basis of the results described above, we concluded that the pore structure of Pt/TiO2
348
catalysts is one of the important factors controlling the activity because the adsorption-desorption of
349
SO2 is the key step for the reaction. The catalysts with larger pores than 10 nm are suitable for the reac-
350
tion under the influence of SO2 when the reaction was carried out at the temperatures than 500ºC.
351 16
ACS Paragon Plus Environment
Page 17 of 25
Environmental Science & Technology
352 353
Supporting Information Experimental details of the sample preparation method, Dependence of CO
354
pulse chemisorption results on the pretreatment method, TEM images of Pt/TiO2, schematics of the re-
355
actor, CO conversion calculated by GC-2014 (Shimadzu) and IR-200 (Yokogawa Electric), amount of
356
CO2 produced in the reaction test, CO conversion at 250oC in absence of SO2, Arrhenius plot of CO ox-
357
idation reaction, XRD spectra of Pt/TiO2, FT-IR spectra of catalysts, TGA results, SEM images of
358
Pt/TiO2 catalysts, data lists used for Arrhenius plot, experimental results relating to Pt/ZrO2 catalysts, ,
359
XPS Pt4f spectra of catalysts, structural data of PtO2, chemical composition of TiO2 supports.
360 361
REFERENCES
362 363
(1)
Bartholomew, C. H. Mechanisms of Catalyst Deactivation. Appl. Catal. A Gen. 2001, 212, 17– 60.
364 365
(2)
Kalantar Neyestanaki, A.; Klingstedt, F.; Salmi, T.; Murzin, D. Y. Deactivation of Postcombustion Catalysts, a Review. Fuel 2004, 83, 395–408.
366 367
(3)
Deshmukh, S. S.; Zhang, M.; Kovalchuk, V. I.; D’Itri, J. L. Effect of SO2 on CO and C3H6 Oxidation over CeO2 and Ce0.75Zr0.25O2. Appl. Catal. B Environ. 2003, 45, 135–145.
368 369 370
(4)
Ziolek, M.; Kujawa, J.; Saur, O.; Aboulayt, a.; Lavalley, J. C. Influence of Sulfur Dioxide Adsorption on the Surface Properties of Metal Oxides. J. Mol. Catal. A Chem. 1996, 112, 125– 132.
371 372
(5)
Lampert, J.; Kazi, M.; Farrauto, R. Palladium Catalyst Performance for Methane Emissions Abatement from Lean Burn Natural Gas Vehicles. Appl. Catal. B Environ. 1997, 14, 211–223.
373 374 375
(6)
Mowery, D. L.; McCormick, R. L. Deactivation of Alumina Supported and Unsupported Pdo Methane Oxidation Catalyst: The Effect of Water on Sulfate Poisoning. Appl. Catal. B Environ. 2001, 34, 287–297.
376 377 378
(7)
Kim, Y. S.; Lim, S. J.; Kim, Y. H.; Lee, J. H.; Lee, H. I. The Role of Doped Fe on the Activity of Alumina-Supported Pt and Pd Diesel Exhaust Catalysts. Res. Chem. Intermed. 2012, 38, 947– 955.
379 380
(8)
Dohmae, K. XPS Analysis for Sulfur on Noble Metals. R&D Rev. TOYOTA CRDL 2000, 35, 43– 50.
381 382 383
(9)
Ruth, K.; Hayes, M.; Burch, R.; Tsubota, S.; Haruta, M. The Effects of SO2 on the Oxidation of CO and Propane on Supported Pt and Au Catalysts. Appl. Catal. B Environ. 2000, 24, L133– L138.
384 385 386
(10)
Wakita, H.; Kani, Y.; Ukai, K.; Tomizawa, T.; Takeguchi, T.; Ueda, W. Effect of SO2 and H2S on CO Preferential Oxidation in H2-Rich Gas over Ru/Al2O3 and Pt/Al2O3 Catalysts. Appl. Catal. A Gen. 2005, 283, 53–61.
ACS Paragon Plus Environment
17
Environmental Science & Technology
Page 18 of 25
387 388 389
(11)
Melchor, A.; Garbowski, E.; Mathieu, M. V.; Primet, M. Sulfur Poisoning of Pt/Al2O3 Catalysts, I. Determination of Sulfur Coverage by Infrared Spectroscopy. React. Kinet. Catal. Lett. 1985, 29, 371–377.
390 391
(12)
Nam, S. W.; Gavalas, G. R. Adsorption and Oxidative Adsorption of Sulfur Dioxide on γAlumina. Appl. Catal. 1989, 55, 193–213.
392 393
(13)
Krocher, O.; Widmer, M.; Elsener, M.; Rothe, D.; Ag, M. a N. N. Adsorption and Desorption of SOx on Diesel Oxidation Catalysts. Ind. Eng. Chem. Res. 2009, 48, 9847–9857.
394 395 396
(14)
Matsumoto, S.; Ikeda, Y.; Suzuki, H.; Ogai, M.; Miyoshi, N. NOx Storage-Reduction Catalyst for Automotive Exhaust with Improved Tolerance against Sulfur Poisoning. Appl. Catal. B Environ. 2000, 25, 115–124.
397 398
(15)
Hirata, H.; Hachisuka, I.; Ikeda, Y.; Tsuji, S.; Matsumoto, S. NOx Storage-Reduction Three-Way Catalyst with Improved Sulfur Tolerance. Top. Catal. 16-17, 145–149.
399 400
(16)
Xue, E.; Seshan, K.; Ross, J. Roles of Supports, Pt Loading and Pt Dispersion in the Oxidation of NO to NO2 and of SO2 to SO3. Appl. Catal. B Environ. 1996, 11, 65–79.
401 402
(17)
Dollimore, D.; Heal, G. R. An Improved Method for the Calculation of Pore Size Distribution from Adsorption Data. J. Appl. Chem. 2007, 14, 109–114.
403 404 405
(18)
Depero, L. E.; Sangaletti, L.; Allieri, B.; Bontempi, E.; Salari, R.; Zocchi, M.; Casale, C.; Notaro, M. Niobium-Titanium Oxide Powders Obtained by Laser-Induced Synthesis: Microstructure and Structure Evolution from Diffraction Data. J. Mater. Res. 1998, 13, 1644–1649.
406 407 408
(19)
Teoh, W. Y.; Mädler, L.; Beydoun, D.; Pratsinis, S. E.; Amal, R. Direct (one-Step) Synthesis of TiO2 and Pt/TiO2 Nanoparticles for Photocatalytic Mineralisation of Sucrose. Chem. Eng. Sci. 2005, 60, 5852–5861.
409 410 411
(20)
Schulz, H.; Mädler, L.; Strobel, R.; Jossen, R.; Pratsinis, S. E.; Johannessen, T. Independent Control of Metal Cluster and Ceramic Particle Characteristics During One-Step Synthesis of Pt/TiO2. J. Mater. Res. 2005, 20, 2568–2577.
412 413 414
(21)
Perrichon, V.; Retailleau, L.; Bazin, P.; Daturi, M.; Lavalley, J. C. Metal Dispersion of CeO2ZrO2 Supported Platinum Catalysts Measured by H2 or CO Chemisorption. Appl. Catal. A Gen. 2004, 260, 1–8.
415 416 417
(22)
Ono, L. K.; Yuan, B.; Heinrich, H.; Cuenya, B. R. Formation and Thermal Stability of Platinum Oxides on Size-Selected Platinum Nanoparticles: Support Effects. J. Phys. Chem. C 2010, 114, 22119–22133.
418 419
(23)
Jochum, W.; Eder, D.; Kaltenhauser, G.; Kramer, R. Impedance measurements in catalysis: charge transfer in titania supported noble metal catalysts. Top. Catal. 2007, 46, 49-55.
420 421
(24)
Anderson, J. B. F.; Burch, R.; Cairns, J. The Reversibility of Strong Metal-Support Interactions. A Comparison of Pt/TiO2 and Rh/TiO2 Catalysts. Appl. Catal. 1986, 25, 173–180.
422 423
(25)
Gmitro, J. I.; Vermeulen, T. Vapor-Liquid Equilibria for Aqueous Sulfuric Acid. AIChE J. 1964, 10, 740–746.
424 425
(26)
Ayers, G. P.; Gillett, R. W.; Gras, J. L. On the Vapor Pressure of Sulfuric Acid. Geophys. Res. Lett. 1980, 7, 433–436.
426 427 428
(27)
Wang, X.; Yu, J. C.; Liu, P.; Wang, X.; Su, W.; Fu, X. Probing of Photocatalytic Surface Sites on SO42- / TiO2 Solid Acids by in Situ FT-IR Spectroscopy and Pyridine Adsorption. 2006, 179, 339–347.
429 430
(28)
Johnsson, M.; Pettersson, P.; Nygren, M. Thermal Decomposition of Fibrous TiOSO4·2H2O to TiO2. Thermochim. Acta 1997, 298, 47–54. ACS Paragon Plus Environment
18
Page 19 of 25
Environmental Science & Technology
431 432
(29)
Sing, K. S. W.; Gregg, S. J. Adsorption, Surface Area, & Porosity; Second Edi.; Academic Press, 1982.
433 434 435
(30)
LU, G. Q.; GRAY, P. G.; DO, D. D. A Study of H2SO4 Formation in the Process of Simultaneous Sorption of SO2, O2 and H2O onto a Single Carbon Particle. Chem. Eng. Commun. 1990, 96, 15– 30.
436 437 438
(31)
Hatanaka, M.; Takahashi, N.; Takahashi, N.; Tanabe, T.; Nagai, Y.; Suda, A.; Shinjoh, H. Reversible Changes in the Pt Oxidation State and Nanostructure on a Ceria-Based Supported Pt. J. Catal. 2009, 266, 182–190.
439 440
(32)
Kim, K. S.; Winograd, N.; Davis, R. E. Electron Spectroscopy of Platinum-Oxygen Surfaces and Application to Electrochemical Studies. J. Am. Chem. Soc. 1971, 93, 6296–6297.
441 442 443
(33)
Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. What Is Degussa (Evonic) P25? Crystalline Composition Analysis, Reconstruction from Isolated Pure Particles and Photocatalytic Activity Test. J. Photochem. Photobiol. A Chem. 2010, 216, 179–182.
444 445
(34)
Bruix, A.; Migani, A.; Vayssilov, G. N.; Neyman, K. M.; Libuda, J.; Illas, F. Effects of Deposited Pt Particles on the Reducibility of CeO2(111). Phys. Chem. Chem. Phys. 2011, 13, 11384–11392.
446 447 448
(35)
Murakami, N.; Mahaney, O. O. P.; Abe, R.; Torimoto, T.; Ohtani, B. Double-Beam Photoacoustic Spectroscopic Studies on Transient Absorption of titanium(IV) Oxide Photocatalyst Powders. J. Phys. Chem. C 2007, 111, 11927–11935.
449 450
(36)
Hanaor, D. a. H.; Sorrell, C. C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci. 2011, 46, 855–874.
451 452
(37)
Herrero Fernandez, M. P.; Chamberland, B. L. New High Pressure Form of PtO2. J. LessCommon Met. 1984, 99, 99–105.
453 454 455
(38)
Kim, D.-W.; Enomoto, N.; Nakagawa, Z.; Kawamura, K. Molecular Dynamic Simulation in Titanium Dioxide Polymorphs: Rutile, Brookite, and Anatase. J. Am. Ceram. Soc. 1996, 79, 1095–1099.
456
(39)
Manual for Reference Catalysts; 6th ed.; Japan catalyst society, 2014.
457 458 459
(40)
Jiang, Z.; Yang, Y.; Shangguan, W.; Jiang, Z. Influence of Support and Metal Precursor on the State and CO Catalytic Oxidation Activity of Platinum Supported on TiO2. J. Phys. Chem. C 2012, 116, 19396–19404.
460
ACS Paragon Plus Environment
19
Environmental Science & Technology
1 2
Page 20 of 25
Table 1. Physical properties and Pt dispersion and the particle diameter determined by CO pulse chemisorptions of prepared catalysts TEM Catalyst
Support
BET area (m2/g)
Pore Volume
CO pulse
Pt particle Pt dispersion Pt particle size (nm) (%) size (nm)
(cm3/g)
Pt/TIO-2
anatase
15.6
0.17
1.4
46.7
2.4
Pt/TIO-4
anatase 88%
50.2
0.55
1.5
46.6
2.4
rutile 12% Pt/TIO-6
rutile
44.1
0.44
1.5
52.5
2.2
Pt/TIO-7
anatase
82.2
0.39
1.4
43.8
2.6
Pt/ST-01
anatase
88.1
0.37
1.1
42.4
2.7
Pt/FTL-110
anatase 2%
13.6
0.08
1.6
48.7
2.3
9.6
0.06
1.6
54.3
2.1
rutile 98% Pt/FTL-200
anatase 2% rutile 98%
3 4 5
ACS Paragon Plus Environment
Page 21 of 25
Environmental Science & Technology
6 7 8 9 10
Figure 1. Time dependence of CO conversion Reaction temperature: 250oC, Pressure 1 atm, Gas composition: 1% CO, 10% O2, 20% H2O, 40 ppm SO2, 40 ppm NO and N2 balance, GHSV: 600,000 cm3STP g-1cat h-1
11
12 13
Figure 2. Schematics of pore blockage hypothesis
14
ACS Paragon Plus Environment
Environmental Science & Technology
Page 22 of 25
15 16
Figure 3. Pore distribution of each Pt/TiO2 catalyst determined by DH method
17 18
Results of Pt/TIO-6, Pt/TIO-7 and Pt/ST-01 are shown in (a), and those of Pt/TIO-2, Pt/TIO-4, Pt/FTL110 and Pt/FTL-200 are shown in (b).
19
ACS Paragon Plus Environment
Page 23 of 25
Environmental Science & Technology
20 21 22
Figure 4. Ratio between surface areas of pores smaller than each rp (A0-rp) against the surface area attributed to the pores of rp