Deactivation and regeneration of Pt/TiO2 nanosheet-type catalysts with exposed (0 0 1) facets for room temperature oxidation of formaldehyde

Article abstract of DOI:10.1016/j.molcata.2014.02.033

Formaldehyde (HCHO) is a major indoor air pollutant and long-term exposure to HCHO may cause health problems including nasal tumors and skin irritation. Room-temperature catalytic oxidation decomposition of HCHO is considered as the most promising strategy for the removal of HCHO due to its environmental- friendly reaction conditions and energy-saving consideration. In this work, surface-fluorinated anatase TiO₂ nanosheets with dominant {0 0 1} facets (FTiO₂-NS) were first prepared by a hydrothermal method using Ti(OC₄H₉)₄ and HF as precursors. Then, Pt/FTiO₂-NS catalyst with 0.5 wt% Pt loadings was obtained by a combined NaOH-assisted impregnation of titania with Pt precursor and NaBH ₄-reduction route. The catalytic activity was evaluated by catalytic oxidation decomposition of HCHO vapor at room temperature. Fluoride poisoning (F-poisoning) phenomenon of Pt/FTiO₂-NS was first observed for oxidative decomposition of HCHO. The mechanism of F-poisoning is mainly due to blocking of Pt catalytic active sites by strong interactions between Pt and highly electronegative F. In order to recover the catalytic performance, a simple regeneration method for the deactivated Pt/FTiO₂-NS catalysts was proposed by NaOH washing.

Full text of DOI:10.1016/j.molcata.2014.02.033

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Editor’s choice paper
Deactivation and regeneration of Pt/TiO nanosheet-type catalysts
2
with exposed (0 0 1) facets for room temperature oxidation of
formaldehyde
Longhui Niea,c, Peng Zhou , Jiaguo Yu
a
a,∗∗
, Mietek Jaroniec
b,∗
a
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070,
PR China
Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA
b
c
School of Chemistry and Chemical Engineering, Hubei University of Technology, Lijiadun Village 1#, Wuhan 430068, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Formaldehyde (HCHO) is a major indoor air pollutant and long-term exposure to HCHO may cause health
problems including nasal tumors and skin irritation. Room-temperature catalytic oxidation decom-
position of HCHO is considered as the most promising strategy for the removal of HCHO due to its
environmental-friendly reaction conditions and energy-saving consideration. In this work, surface-
fluorinated anatase TiO2 nanosheets with dominant {0 0 1} facets (FTiO2-NS) were first prepared by a
hydrothermal method using Ti(OC4H9)4 and HF as precursors. Then, Pt/FTiO2-NS catalyst with 0.5 wt%
Pt loadings was obtained by a combined NaOH-assisted impregnation of titania with Pt precursor and
NaBH4-reduction route. The catalytic activity was evaluated by catalytic oxidation decomposition of
HCHO vapor at room temperature. Fluoride poisoning (F-poisoning) phenomenon of Pt/FTiO2-NS was
first observed for oxidative decomposition of HCHO. The mechanism of F-poisoning is mainly due to
blocking of Pt catalytic active sites by strong interactions between Pt and highly electronegative F. In
order to recover the catalytic performance, a simple regeneration method for the deactivated Pt/FTiO2-NS
catalysts was proposed by NaOH washing.
Received 18 January 2014
Received in revised form 22 February 2014
Accepted 28 February 2014
Available online xxx
Keywords:
Heterogeneous catalysis
Fluorine poisoning
Regeneration
Pt/TiO2 nanosheets
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
oxidation. Among them, the Pt-supported catalysts exhibited an
excellent catalytic performance for decomposition of HCHO even
at room temperature. For instance, HCHO can be completely oxi-
Formaldehyde is one of major indoor air pollutants. A long-
term exposure to HCHO may cause health problems such as nasal
tumors and skin irritation [1]. Room temperature catalytic oxida-
tive decomposition of HCHO to CO and H O is considered to be the
most promising strategy for removal of HCHO because this process
is environmentally friendly and energy saving [2–6]. It overcomes
disadvantages of the relatively short lifetime of adsorbents [7–10]
and additional instrumentation and operating costs for high-
temperature catalytic [11] and photocatalytic oxidation [12,13]. In
the case of catalytic oxidation, a variety of supported noble metals
like Pt, Pd, and Au [2–6,14–16] were used as catalysts for HCHO
dized into CO2 and H O on Pt/TiO2 catalysts at room temperature
[2–6,15,17].
2
However, a major disadvantage of Pt/TiO2 and related cata-
lysts is their deactivation/poisoning. Therefore, there is a great
interest in the basic research devoted to the deactivation and regen-
eration of Pt/TiO2 catalysts. While chloride, sulfur, SO2 and CO
poisoning/deactivation of the Pt-supported catalysts were studied
in relation to ethane oxidation, fuel cells reactions and hydrogen
dissociation reactions [18–20]. Chlorine poisoning for the oxidation
activity of supported noble metal catalysts has been investigated by
several groups [18,21–24]. For example, Marceau and his cowork-
ers [21,22] reported that residual chlorine ions on Pt/Al O catalyst
2
2
2
3
would inhibit the oxidation of methane and the elimination of Cl
from catalyst led to higher activity. The poisoning effect of the resid-
ual chlorine ions was also observed on the supported Pd catalysts.
∗ Corresponding author at: Kent State University, Department of Chemistry and
Biochemistry, Kent, OH 44224, USA. Tel.: +1 3306723790.
∗
∗ Corresponding author at: Wuhan University of Technology, State Key Labora-
Simone et al. [23] found that the existence of Cl on the Pd/Al O3
2
tory of Advanced Technology for Material Synthesis and Processing, Wuhan 430070,
P. R. China. Tel.: +86-27-87871029.
reduced the activity of methane oxidation, and the removal of Cl
from catalyst could increase the catalyst’s activity. Roth et al. [24]
E-mail addresses: jiaguoyu@yahoo.com (J. Yu), jaroniec@kent.edu (M. Jaroniec).
http://dx.doi.org/10.1016/j.molcata.2014.02.033
1
381-1169/© 2014 Elsevier B.V. All rights reserved.
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Table 1
Experimental conditions for the as-synthesized catalysts and their physical properties.
2
3
Sample
Sample composition
Pt dispersion (%)
SBET (m /g)
Vpore (cm /g)
dpore (nm)
A
B
C
D
Pt/FTiO2-NS
A + NaOH
Pt/TiO2-NS
C + HF
6.3
53.7
66.9
9.0
92
87
91
93
0.33
0.27
0.27
0.24
14.3
12.1
12.1
9.4
also confirmed that the presence of Cl on Pd/Al O catalyst strongly
inhibited the conversion of methane and removal of Cl resulted
in the similar activity as Cl-free Pd catalysts. They explained that
6 h. To investigate F-poisoning deactivation mechanism, a F-free
Pt/TiO -NS catalyst (sample C) was prepared using F-free TiO -NS
(F was first removed by NaOH solution washing) as the support.
2
3
2
2
−
the observed reduction in the catalytic activity of Pd/Al O3 was
F-free TiO -NS was obtained by washing twice the sample with
2
2
caused by partial blocking of Pd surface active sites by the resid-
ual chlorine during the aforementioned reaction. Gracia et al. [18]
investigated the poisoning effect of Cl of supported Pt catalysts for
CO, methane and ethane oxidation in details by in situ IR and con-
trolled atmosphere EXAFS spectroscopy, and they also proposed
that site blocking is the mechanism of chloride poisoning. How-
ever, fluorine and chlorine are in the same group of periodic table
of elements, they have similar chemical properties. Would the flu-
orine poisoning be happened if supported Pt catalyst is prepared
0.5 M NaOH aqueous solution. Then, the sample C was prepared by
the above impregnation and NaBH -reduction method, similarly
4
to the preparation of A. In order to further confirm the effect of
fluoride poisoning, the sample C was fluorinated again with HF
solution. 0.5 g of sample C was dispersed in 10 mL deionized water,
and 0.5 mL HF solution (with a concentration ca. 40 wt%) was
added, then stirred for 60 min. Finally, the sample was separated
◦
from water and dried in an oven at 80 C for 6 h; it was denoted as
D. The experimental details are shown in Table 1.
−
from F-containing precursors or the supports contain F ion. To
the best of our knowledge, the F-poisoning/deactivation of Pt/TiO2
catalysts has not been reported yet.
2.2. Characterization
In this paper, F-poisoning/deactivation phenomenon is for the
first time reported for a Pt/TiO -NS catalyst during the HCHO
2
Pt/TiO -NS catalysts were analyzed using a D/Max-RB X-ray
2
oxidative decomposition at room temperature, and an efficient
regeneration method of the deactivated catalysts is proposed by
NaOH washing due to the residual fluorine easily removed from
TiO2 surface [25–27]. The mechanism of fluorine poisoning and
regenerated was also discussed.
diffractometer (Rigaku, Japan) with Cu K␣ radiation at a scan
◦
−1
rate (2ꢀ) of 0.05 s . Transmission electron microscopy (TEM)
and high-resolution transmission electron microscopy (HRTEM)
images were collected on a JEM-2100F microscope at an acceler-
ating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS)
measurements were performed on VG ESCALAB250xi with X-ray
monochromatisation. All binding energies (BE) were referenced
to the C 1s peak at 284.8 eV of the surface adventitious carbon.
The Brunauer-Emmett-Teller (BET) surface area (SBET) of powders
was evaluated from nitrogen adsorption data recorded by using
a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA).
2
. Experimental
2.1. Sample preparation
All reagents were of analytical grade and were used with-
◦
All the samples were degassed at 180 C prior to nitrogen adsorp-
out further purification. Anatase TiO2 nanosheets with exposed
0 0 1) facets (FTiO -NS, containing fluoride) were prepared by the
tion measurements. The BET surface area was determined by a
multipoint method using adsorption data in the relative pressure
(
2
hydrothermal method [28,29]. In the typical synthesis, 25 mL of
Ti(OC H ) and 3 mL of hydrofluoric acid solution (with a concen-
(
P/P ) range of 0.05–0.3. The pore size distributions were deter-
0
4
9 4
mined using desorption data by the Barrett–Joyner–Halenda (BJH)
method. The single-point pore volume was obtained from nitro-
gen adsorption volume at the relative pressure of 0.98. The relation
between the surface area, pore volume and pore width for cylin-
drical pore model was used to estimate the average value of the
latter. Platinum dispersion was measured by H2 chemisorption
on a Micrometrics AutoChem 2920 Pulse Chemisorption System,
using a thermal conductivity detector (TCD) to monitor H2 con-
tration ca. 40 wt %) were mixed in a dried Teflon-line autoclave
having capacity of 100 mL at ambient temperature, followed by
◦
hydrothermal treatment of the mixture at 180 C for 24 h. After
hydrothermal reaction, the white precipitate was collected by cen-
trifuge, washed three times with distilled water and ethanol, and
◦
then dried in an oven at 80 C for 6 h.
A sample of the F-poisoned/deactivated Pt/FTiO -NS catalyst
2
(
denoted as A) was prepared by impregnation of the as-prepared
sumption and assuming a H :Pt = 1:2 stoichiometric ratio. Prior to
2
FTiO -NS with Pt precursor followed by reduction with NaBH4
2
chemisorption, the catalyst was pretreated in flowing argon for 1 h
[
30,31]. In a typical preparation, 1 g of FTiO -NS was added into
2
◦
at 200 C and then it was cooled down to ambient temperature. The
an H PtCl solution (10 mL, 2.56 mmol/L) under magnetic stirring.
2
6
◦
chemisorption data were collected stepwise at 45 C. The platinum
After impregnation for 1 h, 2.5 mL of the mixed solution of NaBH4
solution (0.1 mol/L) and NaOH solution (0.5 mol/L) was quickly
added into the suspension under vigorous stirring for 30 min.
dispersion (D) was calculated from the H chemisorption data using
2
the following equation:
◦
After reduction, the suspension was evaporated at 100 C under
ꢀ
ꢁ
◦
Vs × Fs × M
Ws × Vm
stirring. Finally, the samples were dried at 80 C for 6 h. The
D =
× 100
nominal weight ratio of Pt to TiO2 was fixed to be 0.5 wt%. Then,
one sample of regenerated Pt/TiO -NS catalyst (denoted as B)
2
where Vs is the volume of adsorbed gas (STP, standard temperature
was obtained by washing the A sample with NaOH solution. In a
typical regeneration, 1 g of the sample A was dispersed in 10 mL of
3
and pressure) (cm ), Fs is the stoichiometric factor, which is equal
2
for Pt, Ws is the weight (g) of noble metal Pt, M is the molecular
0.5 M NaOH aqueous solution under vigorous shaking for 30 min in
weight of Pt (g/mol) and Vm is the ideal gas molar volume = 22414
an ultrasonic cleaner and then separated. The above process was
3
◦
(cm /mol).
repeated, and the resulting sample was dried in an oven at 80 C for
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Fig. 1. Changes in formaldehyde concentration (a) and ꢁCO2 (the difference between CO2 concentration at t reaction time and initial time, ppm) (b) as a function of reaction
time for the A–D samples.
−
2.3. Catalytic activity test
F
results in the deactivation of the catalyst, which is not beneficial
for room temperature oxidative decomposition of HCHO.
In order to investigate the difference in the catalytic per-
formance for HCHO oxidation, the A, B and C samples were
characterized by X-ray diffraction (XRD), transmission elec-
tron microscopy (TEM), high-resolution transmission electron
microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and
N2 adsorption-desorption isotherms. The Pt dispersion on the cat-
alysts was determined by pulse H2 chemisorption.
The room-temperature catalytic oxidation of HCHO was per-
formed in a dark organic glass box covered by a layer of aluminum
◦
foil on its inner wall at 25 C in the same way as it is reported in
our previous work [30,31]. An amount of 0.3 g catalyst was dis-
persed on the bottom of glass Petri dish with a diameter of 14 cm.
After placing the sample-coated dishes in the bottom of reactor
with a glass slide cover, 12 ␮L of condensed HCHO solution (38%)
was injected into reactor and a 5 W fan was placed on the bottom
of the reactor. After 2 h, the HCHO solution was volatilized com-
pletely and the concentration of HCHO was stabilized. The analysis
of HCHO and CO2 was on-line conducted by a Photoacoustic IR
Multigas Monitor (INNOVA air Tech Instruments Model 1412). The
HCHO vapor was allowed to reach adsorption equilibrium within
the reactor prior to catalytic activity experiments. The initial con-
centration of HCHO after adsorption equilibrium was controlled
at about 253 ppm, which remained constant until the glass slide
cover on the petri dish was removed to start the catalytic oxidation
reaction of HCHO. Each set of experiments was followed for about
3.2. Phase structures and morphology
The XRD patterns of the FTiO -NS, A–C samples are presented in
2
Fig. 2, indicating that the phase structure of the FTiO -NS samples
2
is anatase (JCPDS, No. 21-1272); also, there is no obvious change in
the position and height of TiO2 diffraction peaks before and after
Pt deposition and NaOH washing (for samples A–C). Further obser-
vation indicates that no diffraction peaks of Pt are observed in the
XRD patterns for the A–C samples due to its low loading (0.5 wt%),
small particle size, and good dispersion [30–32].
6
0 min. The CO concentration increase (ꢁCO , which is the differ-
2
2
TEM and HRTEM results are shown in Fig. 3, confirming the
ence between CO concentration at t reaction time and initial time,
2
presence of TiO nanosheets and Pt nanoparticles (NPs). TEM image
2
ppm) and HCHO concentration decrease was used to evaluate the
catalytic performance.
(
Fig. 3a) of A shows that a large amount of nanosheets with side
length of ca. 30–40 nm and the size of Pt NPs on their surface is ca.
–6 nm. NaOH washing did not affect the size of Pt NPs on B. Con-
4
3
. Results and discussions
trarily, Pt NPs appear to be aggregated (see Fig. 3b). Surprisingly,
3.1. Catalytic activity
The catalytic performance of F-poisoned/deactivated (A and D),
regenerated (B) and F-free (C) catalysts in HCHO oxidation is shown
in Fig. 1.
C
As can be seen from this figure, the HCHO concentration
decreased quickly during initial 6 min and then remained
almost unchanged in the subsequent 55 min for the F-
B
A
poisoned/deactivated A and D catalysts. Also, the CO concentration
2
was almost unchanged in the whole process, indicating that HCHO
was not decomposed and was mainly adsorbed on the surface of
the catalysts. When the F-removed or regenerated catalyst (B) was
used for HCHO oxidation, the HCHO concentration decreased with
increasing reaction time, and accordingly, the CO2 concentration
FTiO -NS
2
increased, indicating oxidation of HCHO into CO2 and H O; thus,
2
20
30
40
50
60
70
80
the catalytic performance was evaluated. Especially, for the F-free
catalyst (C), HCHO concentration decreased more rapidly and
CO2 concentration increased faster with increasing reaction time
than in the case of B. The above results imply that the presence of
2 theta (degree)
Fig. 2. XRD patterns of the FTiO2-NS, A–C samples.
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Fig. 3. TEM images (a–c) of the A (a), B (b) and C (c) samples and HRTEM image (d) of the C sample. The inset in the panel (d) shows that the lattice spacing parallel to the
top and bottom facets of a nanosheet is ca. 0.235 nm.
the size (around 1–3 nm) of Pt NPs on C is obviously smaller than
that on the A and B samples. Also the distribution of Pt NPs on the C
sample (Fig. 3c) is more uniform than that on the A and B samples.
HRTEM image (Fig. 3d) of black Pt NPs shows that the lattice
spacing in white circle is ca. 0.224 nm, which is consistent with
the lattice spacing of (1 1 1) plane of metallic Pt, confirming the
presence of Pt NPs [33]. The HRTEM image of TiO2 nanosheet (see
the inset of Fig. 3d) indicates that the lattice space parallel to the
top and bottom facets is ca. 0.235 nm, corresponding to the (0 0 1)
surprising because the A–C samples were mainly composed of TiO2-
NS. Table 1 shows also a comparison of the BET specific surface area,
pore volume and pore size values for the A–D samples, indicating
that there is no significant change in the aforementioned quantities.
3.4. XPS analysis
The chemical state of species present in the prepared samples
was investigated by XPS; the high-resolution XPS spectra of F 1s, Pt
f, Ti 2p and O 1s regions are shown in Fig. 5.
4
planes of anatase TiO , suggesting that the top and bottom facets of
2
the nanosheets are the (0 0 1) and (0 0 −1) planes of anatase TiO ,
2
respectively. Why smaller Pt NPs are observed in the case of the
C sample? One possible explanation is because TiO -NS support
2
was first washed with NaOH solution before Pt deposition, which
−
resulted in the removal of F and the creation of more hydroxyls
on this support [34]. The F-free TiO -NS support seems to enhance
2
adsorption of H PtCl precursor and deposition of Pt NPs. There-
2
6
fore, it is not surprising that the C sample showed smaller Pt NPs
and more uniformly distributed on the TiO -NS surface. It is notable
2
that the smaller Pt NPs are more beneficial for the oxidation of
HCHO because they have more active centers than the big Pt NPs.
3.3. BET surface areas and pore size distributions
Nitrogen adsorption-desorption isotherms and the correspond-
ing pore size distribution curves (inset) for the A–C samples are
shown in Fig. 4, indicating no obvious difference in the shape of
adsorption isotherms and pore size distributions. This result is not
Fig. 4. Nitrogen adsorption–desorption isotherms and the corresponding pore–size
distribution curves (inset) for the A–C samples.
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Fig. 5. High-resolution XPS spectra for F 1s (a), Pt 4f (b), Ti 2p (c) and O 1s (d) of the A–C samples.
−
The high-resolution F 1s spectra of the A–C samples (Fig. 5a)
Thus, it is not surprising that the produced free F can easily
adsorb the surface of the as-prepared Pt NPs, resulting in the for-
mation of Pt–F bond, and F-poisoning and deactivation of Pt NPs.
The above XPS result also suggests that the interaction between F
and Pt is stronger than the interaction between F and Ti in basic
solution. A positive shift in the binding energies of Ti 2p and O 1s is
also observed for the A sample (see Fig. 5c and d) because F induces
a consecutive electron transfer, first from Pt to F, then from O to Pt
and finally from Ti to O (see Fig. 6b). Thus, the XPS analysis shows
show one peak observed at 684.4 eV, which is a typical value for
−
physically absorbed F on the surface of TiO2
( Ti–F). No signal for
−
F
in the lattice of TiO (binding energy = 688.5 eV) was found. After
2
NaOH washing of A, the F 1s signal of the B sample decreased sig-
−
nificantly, implying that the most of F was removed from the TiO2
surface. Note that the lowest F concentration was in the case of C,
which exhibited the best catalytic activity. This result clearly indi-
−
cates that the presence of F results in the poisoning/deactivation
of Pt/TiO -NS catalysts.
that the F poisoning/deactivation of Pt/TiO -NS catalysts is due to
the adsorption of F on Pt NPs.
2
2
−
The high-resolution Pt 4f spectra of the B and C samples (Fig. 5b)
show two peaks at ca. 70.5 eV and 74.3 eV, which are assigned to
Pt 4f7/2 and Pt 4f5/2 of metallic Pt, respectively. It is known that the
Pt 4f7/2 binding energy of Pt0 is around 71.2 eV. A negative shift
for Pt 4f7/2 is observed as compared to the binding energy of Pt
0
4
f7/2 for bulk metallic Pt (71.2 eV) [35,36], which can be caused
by the electron transfer from TiO2 to Pt (see Fig. 6a) due to strong
metal-support interactions (SMSI) [5,30].
Further observation indicates that the Pt 4f7/2 peak of the A sam-
ple exhibits an obvious positive shift as compared to that of the B
and C samples. Also, the Pt 4f5/2 peak of the A sample (Fig. 5b) shows
one extra peak at ca. 76.8 eV, which can be due to the adsorption of
−
F
on the surface of Pt NPs and the formation of Pt–F bond, leading
to the transfer of electrons from Pt to highly electronegative F [36].
How the Pt–F bond is formed? One possible explanation is that dur-
−
ing the preparation of the A sample, partial F on the TiO2 surface
−
can be removed by a exchange reaction between Ti–F and OH due
to the addition of NaOH (see Eq. (1)) [37–39].
Fig. 6. Schematic diagram of electron transfer between Pt and TiO2 for the B and
C samples (a) and the transfer between F, Pt, O and Ti elements present in the A
sample (b).
Ti–F + OH−
→
Ti–OH + F−
(1)
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Fig. 7. Proposed F-poisoning/deactivation mechanism and regeneration by NaOH washing for the oxidative decomposition of HCHO on Pt/TiO2-NS.
3.5. Deactivation mechanism and regeneration analysis
4. Conclusions
The room temperature catalytic oxidative decomposition of
In summary, F-poisoning/deactivation mechanism is explained
HCHO on Pt/TiO2 takes place because HCHO and O2 are first
for Pt/TiO -NS catalysts used in the oxidative decomposition of
2
adsorbed onto TiO and Pt surface (see step II in Fig. 7), respectively.
HCHO at room temperature. Also, an effective and simple regen-
2
Next, O2 becomes an active oxygen (step III), which is very
important stage in the oxidation of HCHO. After that, HCHO is oxi-
dized into formate species on the surface (step IV) and these species
eration is proposed for F-poisoned Pt/FTiO -NS catalysts. This
2
study shows that the site blocking effect for F on Pt catalysts (F-
poisoning) is responsible for inhibition of O2 adsorption and for
production of active oxygen species. The F-poisoned/deactivated
are decomposed into adsorbed CO species and H O (step V); finally,
2
CO species react with O to generate gaseous CO (step VI). Because
Pt/TiO -NS catalysts can be effectively regenerated by NaOH wash-
ing, which removes the most of F and recovers the catalytic
2
2
2
−
of strong interactions between Pt and F on the surface of A, the for-
mation of active oxygen species is difficult due to the site blocking
effect (see the top part of Fig. 7), which is similar to the site block-
ing effect in the case of Cl on Pt NPs [18]; thus, HCHO oxidation is
activity of Pt/TiO -NS. Due to the importance and diversity of
2
noble-metal Pt catalysts, the proposed site-blocking mechanism of
F-poisoning/deactivation and effective regeneration of these cata-
lysts represent an important contribution to the development of
high-performance noble metal catalysts for indoor air purification.
−
inhibited. The site blocking effect for F was also verified by mea-
surement of Pt dispersion on the samples using H2 chemisorption.
After F-poisoning/deactivation, the Pt dispersion on the A and D
samples is 6.3% and 9.0% (see Table 1), respectively, which is much
smaller than that (53.7 and 66.9%) on the B and C samples. So, no
HCHO oxidation activity is observed on the A and D samples due to
the F-poisoning/deactivation of the Pt NPs (see Fig. 1).
Acknowledgments
This work was supported by the 863 Program (2012AA062701),
9
5
73 Program (2013CB632402), NSFC (21177100, 51272199,
1320105001 and 51372190), National Science Fund for Post-
In order to eliminate the effect of F-poisoning in the case of
Pt/TiO -NS catalysts, two regeneration methods were investigated
2
doctoral Scientists of China (2012M511292), Fundamental
Research Funds for the Central Universities (WUT: 2013-VII-030)
and Self-determined and Innovative Research Funds of SKLWUT
−
−
for the F removal. The previous investigations indicated that F
on TiO -NS can be almost completely removed by NaOH solution
2
washing in a strong basic environment [25–27]. Therefore, it is
(2013-ZD-1 and 2013-KF-10).
not surprising that the C sample exhibited the highest catalytic
−
activity because F was fully removed from TiO -NS by NaOH wash-
2
ing and the C sample has the smallest Pt NPs on its surface. Of
course, the smaller the Pt NPs, the higher their catalytic activity
is. Also, the NaOH washing was powerful for regeneration of the
F-poisoned/deactivated A sample to obtain B (see step I in Fig. 7).
So, the latter showed very good catalytic activity for HCHO decom-
position. Fig. 1 also indicates that the C sample exhibited higher
catalytic activity than the B sample, which is not surprising because
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