Active oxygen species adsorbed on the catalyst surface and its effect on formaldehyde oxidation over Pt/TiO2 catalysts at room temperature; Role of the Pt valence state on this reaction?

Article abstract of DOI:10.1039/c7ra11294g

Pt/TiO₂ catalysts, prepared by reduction pretreatment, showed enhanced catalytic activities in formaldehyde oxidation. X-ray photoelectron spectroscopy analysis confirmed that catalytic activity was affected by Pt valence states in the Pt/TiO₂ catalyst. Using O₂ re-oxidation tests, we showed that there was a correlation between the area of oxygen consumed and the ratio of metallic Pt species on the catalyst surface. The O₂ re-oxidation ability was involved in the production of the adsorbed formate intermediate from HCHO, confirmed through diffuse reflectance infrared Fourier transform spectroscopy analysis. Furthermore, metallic Pt species were involved in the oxidation of adsorbed CO to CO₂.

Full text of DOI:10.1039/c7ra11294g

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Active oxygen species adsorbed on the catalyst
surface and its effect on formaldehyde oxidation
over Pt/TiO2 catalysts at room temperature; role of
the Pt valence state on this reaction?
Cite this: RSC Adv., 2018, 8, 3626
*
*
Geo Jong Kim, Sang Moon Lee, Sung Chang Hong and Sung Su Kim
Pt/TiO2 catalysts, prepared by reduction pretreatment, showed enhanced catalytic activities in
formaldehyde oxidation. X-ray photoelectron spectroscopy analysis confirmed that catalytic activity was
affected by Pt valence states in the Pt/TiO2 catalyst. Using O2 re-oxidation tests, we showed that there
was a correlation between the area of oxygen consumed and the ratio of metallic Pt species on the
catalyst surface. The O2 re-oxidation ability was involved in the production of the adsorbed formate
intermediate from HCHO, confirmed through diffuse reflectance infrared Fourier transform spectroscopy
analysis. Furthermore, metallic Pt species were involved in the oxidation of adsorbed CO to CO2.
Received 13th October 2017
Accepted 15th December 2017
DOI: 10.1039/c7ra11294g
rsc.li/rsc-advances
specic intermediate products produced, including CO and
formate species. Nie et al. reported that the surface hydroxyl
1 Introduction
groups are indispensable and act synergistically of HCHO
reaction, and they propose a reaction model that hydroxyl group
participates in the HCHO reaction.13
Demand for comfortable and healthy living space has increased
due to the rise in living standards. Because a signicant portion
of the modern lifestyle is spent indoors, the necessity of
improving the air quality of living and work environments is
increasing. The removal of harmful substances from indoor air
has, therefore, emerged as a concern.
Indoor air quality is affected by various substances, such
as carbon monoxide (CO), formaldehyde (HCHO), nitrogen
dioxide (NO2), ozone (O3), particulates (PM10), asbestos,
volatile organic compounds (VOCs), microbial substances,
and radon (Rn).1–3 Among these substances, formaldehyde is
most harmful to human health, and is emitted from several
common sources including thermal insulation materials,
furniture, tobacco smoke, deodorant, cosmetics, and
plywood burning. Upon short-term exposure, formaldehyde
penetrates the body and enters mucous membranes in the
eyes and nose, while long-term exposure can cause diarrhea,
coughing, emotional instability, skin disease (atopy), and
amnesia.4,5
Tang et al. and Kim et al. reported the effect of catalyst
reduction as a pretreatment step on the feeding concentration
of HCHO, and showed that the reduced catalysts displayed
excellent activities, with an optimum reduction temperature of
873 K.14,15 A higher HCHO feeding concentration was also re-
ported, resulting in drastically decreased catalytic activity, with
metallic Pt species proving more active than PtOx species.
However, the report was limited to one catalyst, so further study
of various supports is required.
Huang and Leung reported that Pt/TiO2 and Pd/TiO2 cata-
lysts have strong potential for HCHO oxidation, with their
activities dependent on the magnitude of the strong metal-
support interaction (SMSI) phenomenon and the oxidation
states of the active metals. They also reported that catalytic
performance improved as the Pd or Pt oxidation states
decreased, caused by the SMSI effect between the active metal
and the TiO2 support.16–18 Only one TiO2 support was used,
however, so the effect of the type of TiO2 support on the SMSI
effect or the Pt valence was not reported. They also did not
determine why metallic Pt gave such a strong HCHO conversion
rate.
Extensive research on HCHO oxidation at room temperature
has been conducted recently, with principal technologies using
noble-metal-supported catalysts.6–11 Zhang et al. reported that
HCHO could be completely oxidized to CO2 over a Pt/TiO2
catalyst, and they performed diffuse reectance infrared Fourier
transform spectroscopy (DRIFTS) to propose a reaction model.12
In this model, differences in catalytic activity were caused by the
This study examined the activities of various Pt/TiO2
catalysts, using different types of supports, in HCHO oxida-
tion at room temperature. We also performed various anal-
yses to investigate the relationship between HCHO oxidation
catalytic activity and the physicochemical character of the
catalyst.
Department of Environmental Energy Engineering, Graduate School of Kyonggi
University, 94-6 San, Iui-dong, Youngtong-ku, Suwon-si, Gyeonggi-do 443-760,
Republic of Korea. E-mail: schong@kgu.ac.kr; Fax: +82-31-2544941
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Table 1 Physicochemical characteristics of TiO2 and Pt/TiO2 catalysts
and their activities. Reaction temperature ¼ 298 K, HCHO ¼ 25 ppm,
O2 ¼ 21%, N2 balance, total flow ¼ 500 ml minꢀ1, SV ¼ 360 000 hꢀ1
2 Results and discussion
2.1 Physicochemical characteristics of Pt/TiO2 catalysts and
catalytic activities
SSA of
SSA of
8 anatase and 2 rutile type TiO2 supports with varying surface
areas and particle sizes were chosen to investigate the inuence
of a catalytically active TiO2 support.
The structural properties of Pt/TiO2 catalysts were investi-
gated by XRD, shown in Fig. 1. All samples were found to exist as
anatase or rutile TiO2 structures. It was found that Pt highly PT-G
dispersed over the support, due to the absence of diffraction
lines on Pt of 1% Pt/TiO2 catalyst, as previously reported by
Zhang et al. and Kim et al.15,19,20
raw TiO2 Pt/TiO2
D
Reaction rate
TOF ꢁ
ꢀ1
Samples (m2 gꢀ1
)
(m2 gꢀ1
)
(%) (mmol gcat minꢀ1
)
100 (sꢀ1
)
PT-Aa
PT-Ar
PT-D
11
4
76
340
250
144
44
122
149
65
9
23
35
38
21
26
30
55
37
11
2.6 4.29 ꢁ 10ꢀ6
3.2 6.59 ꢁ 10ꢀ6
3.4 8.16 ꢁ 10ꢀ6
6.1 6.18 ꢁ 10ꢀ5
6.4 1.75 ꢁ 10ꢀ5
6.9 7.12 ꢁ 10ꢀ5
5.9 1.75 ꢁ 10ꢀ5
19.1 1.08 ꢁ 10ꢀ4
13.2 9.37 ꢁ 10ꢀ5
3.7 6.47 ꢁ 10ꢀ6
0.322
0.401
0.468
1.975
0.533
2.025
0.578
1.103
1.384
0.341
PT-Gs
PT-N
PT-P
PT-S
The physico-chemical properties of the various TiO2
PT-U
supports and Pt/TiO2 catalysts manufactured in this study are PT-W
showed in Table 1. Pt particle sizes in the Pt/TiO2 catalysts were
conrmed by TEM, shown in Fig. 2. The specic surface areas of
all catalysts were relatively low, compared with raw TiO2, while
phenomenon was thought to be caused by rich metallic Pt sites
on the catalyst, it will be discussed in more detail later.15
Differences in activities at a SV of 60 000 hꢀ1 were difficult to
nd. In a heterogeneous catalyst reaction, mass transfer does
not inuence the overall reaction rate if mass transfer is suffi-
ciently fast, or if the catalyst reaction rate is very slow. Under
these conditions, the overall reaction rate rely on the specic
reaction rate of the catalyst, occurring at the active site.
Pt dispersion and particle size varied among the types of TiO2
used as supports.
The Pt/TiO2 catalyst activities are shown in Fig. 3. The
reduced catalysts (Fig. 3(B)) showed greater levels of activity
than those that did not undergo the reduction pretreatment
under space velocity (SV) conditions of 60 000 hꢀ1. This
In contrast, if the mass transfer rate is relatively slower than
the catalyst reaction rate, the overall reaction rate is dependent
on the mass transfer rate. In this respect, the mass transfer rate
for the reduced catalyst was not sufficient, therefore, the activity
test under high SV conditions (Fig. 4) was necessary. When SV
was increased, the catalytic activities of all catalysts decreased,
however, the magnitude of this decrease varied with the type of
TiO2 support. The TiO2(S) catalyst showed the highest catalytic
activity, with a HCHO conversion of 90%. The order of activity
was as follows: TiO2(S) > TiO2(N) > TiO2(U) > TiO2(G) > TiO2(Gs)
> TiO2(P) > TiO2(W) ¼ TiO2(Ar) > TiO2(D) > TiO2(Aa).
The catalytic activity was also examined in terms of TOF and
reaction rate, shown in Table 1. Useful information can be ob-
tained by evaluating activity with TOF.15,21–23 If there was no
difference between the activities regarding the TOF and reaction
rate, catalyst activities would be determined by Pt dispersion,
however, if the activities differed, the magnitude of activity
would depend on the frequency of active sites. In this research,
a difference between the two reaction activities was observed
over each catalyst, suggesting that differences in catalytic
activity were not due to the number of active sites, but rather
differences in the intensity of activity at each catalyst active site.
2.2 Effect of Pt valence state and chemisorbed oxygen
Huang and Leung recently reported that the important factors
of HCHO oxidation at room temperature were the valance state
of the active metal and the mobility of activated chemisorbed
oxygen on the catalyst surface, rather than the noble-metal-
supported catalysts.16–18 In their reports, the mobility of chem-
isorbed oxygen was more important than that of lattice oxygen,
Fig. 1 X-ray diffraction patterns of the 1% Pt/TiO2 catalysts.
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Fig. 2 FE-TEM analysis of the 1% Pt/TiO2 catalysts.
and the formation of chemisorbed oxygen depended on a strong interfacial site.20 Gas-phase oxygen can dissociatively adsorb at
interaction between Pt and TiO2. This effect is known as this defect site, and the chemisorbed oxygen can then be acti-
a strong metal-support interaction (SMSI), and can be explained vated at the metal-support interface, forming the highly active
by changes in electron density with the movement of electrons oxygen species involved in oxidation.17 Based on this, XPS
from Ti to Pt sites by a strong interaction between a partially analysis was carried out to investigate Pt valence and chem-
reduced Ti site, and Pt. Pt–Ov–Ti3+ sites, which are formed isorbed oxygen species in the 10 types of Pt/TiO2 catalyst used in
between Pt and Ti, where Ov is an oxygen vacancy close to the Pt this study.
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Fig. 3 Catalytic activities of the Pt/TiO2 catalysts after (A) calcination
only, and (B) reduction treatment. Inlet HCHO ¼ 24 ppm, 21% O/N2,
temperature ¼ 298 K, SV ¼ 60 000 hꢀ1
.
Fig. 5 Pt 4f spectra of Pt/TiO2 catalysts from XPS analysis.
were reduced to metallic Pt (Pt0) or PtO (Pt2+). The magnitude of
Pt oxidation state changes between the various catalysts. A small
negative shi in binding energy was also found for metallic Pt
species, which can help conrm the SMSI effect. The SMSI effect
caused electrons to migrate from Ti3+ sites to adjacent Pt sites,
resulting in enhanced electron density at the Pt–TiO2 interfacial
site. A difference of approximately 1 eV in the binding energies
of metallic Pt sites adjacent to Ti3+ sites was reported for the
reduced Pt–TiO2 catalysts.18,26 In this study, the binding energy
shi was lower than that found in the literature. This difference
in the magnitude of the shi was believed to be caused by
differences in catalyst preparation methods, with the use of
a NaBH4 solution causing a more extreme SMSI effect than the
use of hydrogen gas.
Fig. 4 Catalytic activities of Pt/TiO2 catalysts after calcination and
reduction treatment. Inlet HCHO ¼ 24 ppm, 21% O/N2, temperature ¼
298 K, SV ¼ 60 000–360 000 hꢀ1
.
The positions of the Pt 4f peaks corresponding to Pt0, Pt2+
and Pt4+ were 70.9–71.1, 72.4–73.6 and 74.56–74.8 eV, respec-
tively.24,25 Fig. 5 conrms that almost all the Pt/TiO2 catalysts
A shi in the O 1s peak was also conrmed in the XPS spectra
(Fig. 6). An O 1s shoulder peak, corresponding to chemisorbed
oxygen species, was observed at 531.5–532 eV in all catalysts.27,28
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The peak positions for Pt 4f and O 1s are shown in Table 2.
According to Huang and Leung, metallic Pt sites and oxygen
vacancies can facilitate oxygen adsorption and activation, so the
amount of chemisorbed oxygen species depended on the
intensity of the SMSI effect (i.e. the population of metallic Pt).18
The correlation between metallic Pt atom population and
catalyst activity is shown in Fig. 7(A). A positive linear rela-
tionship between catalytic activity and the amount of reduced
metallic Pt formed on the catalyst surface was conrmed. It was
consistent with our previous report, which showed that reduced
Pt species were more active in HCHO oxidation at room
temperature than oxidized Pt species.15 The magnitude of
reduction of the Pt species in the Pt/TiO2 catalysts varied, even
though the catalysts were exposed to the same pretreatment
conditions. In this study, the properties of the TiO2 supports
varied substantially. Panagiotopoulou et al. mentioned that the
activity of Pt/TiO2 catalysts during the water-gas shi (WGS)
reaction was affected by the size of the TiO2 primary crystallite.29
Kim et al. also reported various activities for Pt/TiO2 catalysts in
the reverse WGS reaction, with their catalytic nature depending
on the properties of the TiO2 support. These studies suggested
that the SMSI effect depended on the properties of TiO2 in the
Pt/TiO2 catalyst.21–23 The results presented in Fig. 5 and 7
should, therefore, be related to TiO2 properties in the Pt/TiO2
Fig. 6 O 1s spectra of Pt/TiO2 catalysts from XPS analysis.
Table 2 XPS data of Pt/TiO2 catalysts
BE, eV
Catalysts
Pt 4f
O 1s, OII (OI)
PT-Aa
PT-Ar
PT-D
PT-G
PT-Gs
PT-N
PT-P
PT-S
PT-U
PT-W
70.9
70.9
70.9
70.5
70.7
70.6
70.7
70.7
70.4
70.9
531.8 (529.6)
531.8 (529.4)
531.4 (529.5)
531.7 (529.5)
531.4 (529.6)
531.8 (529.4)
531.8 (529.5)
531.7 (529.5)
531.8 (529.4)
531.4 (529.5)
Fig. 7 Correlation between the ratios of (A) metallic Pt/Pttotal and
catalyst activities, and (B) metallic Pt/Pttotal and TiO2 crystallite size in
the Pt/TiO2 catalysts.
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catalyst. The relationships between reduced Pt species and TiO2 chemisorbed oxygen, however, their relationship is still unclear.
crystallite size in the Pt/TiO2 catalysts aer pretreatment were To investigate the correlation between these two factors, an O2-
investigated (Fig. 7(B)). In this analysis, the TiO2 crystallite sizes reoxidation experiment was performed. When the TCD signal
aer catalyst preparation were evaluated, due to differences for all catalysts reached a steady state at room temperature, the
between the crystallite sizes in raw TiO2 and generated Pt/TiO2 consumed oxygen peaks were measured by feeding the catalyst
catalysts.22 The relationship between both factors is also with 1.5% O2/Ar gas.
strongly correlated (Fig. 7(B)). Based on this result, and litera-
Fig. 8 shows the consumed oxygen peaks over Pt/TiO2 cata-
ture on the SMSI effect, smaller TiO2 crystallites could result in lysts. Oxygen was rapidly consumed aer injection into the
larger Pt/TiO2 interfaces, leading to an increase in the amount catalyst, and this effect appeared aer oxygen adsorption onto
of oxygen vacancies, or Pt–Ov–Ti3+ sites (Pt–Ti interfacial the catalyst. Such oxygen consumption suggests that atmo-
sites).18,22,30 The origin of differences in the activation energy spheric oxygen can be adsorbed onto the Pt/TiO2 catalysts.
required for the formation of Pt–Ov–Ti3+ sites should be dis- Oxygen species adsorbed from the atmosphere can be used as
cussed. In general, noble metals, such as Ru, Pd, and Pt are not active oxygen, which can participate in the conversion of HCHO
expected to interact strongly with TiO2.31 Dispersed noble-metal into formate or CO. Interestingly, the amount of consumption
crystallites are well known to interact with partially reduced differed among the different TiO2 supports. To examine the
TiO2 supports, so smaller TiO2 supports exhibit better redox relationship between catalytic activity and O2 consumption rate,
properties, which can also affect SMSI.
the area of oxygen consumed by each catalyst was compared
However, Huang and Leung reported that the effect of with their catalytic activities.
support reducibility or type of support on HCHO oxidation at
Catalysts with excellent activity had good oxygen accepting
room temperature was not important.18 They used CeO2, Al2O3, abilities, while catalysts with poor activities had poor oxygen
ZrO2, MgO, and TiO2 as supports, but support reducibility only accepting abilities (Fig. 9(A)). In our previous study, we reported
weakly inuenced catalyst activity. This result differs slightly CO oxidation at room temperature over a reduced Pt/TiO2
from those presented in this study, which could be because the catalyst, which had a high activity level and was rich in Pt2+ and
catalyst TOFs were calculated from activity obtained under low Pt0 species, showing an excellent oxygen accepting ability, while
SV conditions. As discussed above, low SV resulted in a low
performance level, making it difficult to determine the effect of
support. In Fig. 4, the shi in the Pt 4f spectra was lower than
that previously reported.18 Similarly, the shi in O 1s in Fig. 5
was slight. Nevertheless, the results of this study suggested that
catalytic activity was dependent on support reducibility. All
supports used in this study were TiO2, and their pretreatment
conditions and methods were the same. The only differences
were in the crystallite size. The activity tests were performed
under extremely high SV conditions, therefore, support reduc-
ibility had a distinct effect on HCHO oxidation.
Zhang et al. reported a reaction model using in situ FTIR and
argued that differences in catalytic activity varies with certain
intermediate products, such as formate.12,20 Furthermore,
formate was converted to CO when inert helium gas was injec-
ted. Kim et al. also reported that HCHO was adsorbed into
formate and/or CO aer formaldehyde owed onto the catalyst
surface, even in the absence of oxygen.15 The literature shows
that formaldehyde was thought to convert into formate or CO
species by a catalyst with active oxygen; therefore a discussion of
the nature of this active oxygen would be worthwhile. If active
oxygen is limited to existing on the catalyst, catalytic activity will
decrease with time on stream. In contrast, if active oxygen can
be recovered by atmospheric oxygen, catalytic activity will be
stable with time on stream. Many previous studies on HCHO
oxidation at room temperature over Pt/TiO2 catalysts showed no
decrease in catalytic performance with time on stream during
activity tests, therefore, active oxygen in Pt/TiO2 catalysts was
thought to be recovered by atmospheric oxygen, with this
phenomenon proceeding at room temperature.16–18
Until now, key factors affecting the activity of Pt/TiO2 cata-
lysts in HCHO oxidation at room temperature were reported to
Fig.
8 Response profiles of Pt/TiO2 catalysts in O2-reoxidation
be the valence of the active metal, and the amount of experiments.
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Fig. 10 Dynamic changes in in situ FT-IR spectra of the Pt/TiO2 (G), Pt/
TiO2 (S), Pt/TiO2 (Aa) catalysts as a function of time in a gas flow of
HCHO mixed with N2 at room temperature. Inlet HCHO, 20 ppm.
into the reactor containing the Pt/TiO2 catalysts at room
temperature.
As shown in Fig. 10, aer HCHO adsorption on the Pt/TiO2
catalysts, positive bands were noticeable at 2861, 2842, 2048,
1770, 1577, 1359 cmꢀ1. The band at 2861 cmꢀ1 represented the
formate species (HCOO), while the bands at 2842 cmꢀ1 repre-
sented the formic acid species (HCOOH).20,32–36 The band at
2048 cmꢀ1 was assigned to linearly adsorbed CO on metallic Pt
(kink) sites, while the band at 1770 cmꢀ1 was assigned to CO
bridged between two Pt metal particles.20,37 Peaks at 1570 and
1360 cmꢀ1 indicated the presence of formate, while negative
bands were observed at 1635 cmꢀ1 and broad bands were
Fig. 9 Correlations between (A) catalytic activity and catalyst O2
consumption rate, and (B) area of oxygen consumed by each catalyst
and ratio of metallic Pt/Pttotal present in the catalyst.
observed in the range 3600–3200 cmꢀ1 36 Generally, the broader
.
peaks at 3200–3600 cmꢀ1 and strong peaks at 1635 cmꢀ1 were
assigned to the O–H stretching vibrations of water, –OH groups,
and hydrated species on the catalyst surface.34,36 Nie et al. and
Chen et al. reported that HCHO adsorbed on the hydroxyl group
over the catalyst surface.13,38 Therefore, the 1635 cmꢀ1 negative
peak was due to the adsorption of HCHO on –OH groups.
The same peaks were observed in all catalysts. These results
suggested that the same intermediate product was formed on
each catalyst surface. Peak intensities differed with each type of
TiO2, however. In this study, Pt/TiO2–G, which showed excellent
O2 consumption in Section 2.2, had large formate peak area at
1570 and 1360 cmꢀ1, conrming that a large amount of HCHO
was adsorbed and converted to formate by HCHO injection. The
peaks at 2048 and 1770 cmꢀ1 were also large, resulting from
formation of the CO intermediate product. In contrast, the Pt/
TiO2–Ar catalyst showed low O2 consumption aer HCHO
injection, with small peaks at 1570 and 1360 cmꢀ1, therefore,
the amount of formate was smaller than that of the Pt/TiO2–G
catalyst, and the CO adsorption peaks at 2048 and 1770 cmꢀ1
were also small. It was consistent with our previous study,
which reported that surface CO species were the main inter-
mediates in HCHO oxidation.15 Zhang and He also reported that
differences in catalytic activities were observed because formate
the oxygen accepting ability of the calcined catalyst, which had
a lower activity and was rich in Pt4+ species, was poor.31 This
suggested that the adsorption of oxygen depended on the
oxidation state of the Pt species. The strong correlation between
the amount of oxygen consumed for each catalyst and the ratio
of metallic Pt/Pttotal in each catalyst is shown in Fig. 9(B). As the
area of oxygen consumed increased, the ratio of metallic Pt/
Pttotal on the catalyst increased linearly. Catalysts with high
metallic Pt/Pttotal ratios were thought to have more sites for
accepting atmospheric oxygen due to them having more oxygen
vacancy sites on the catalyst surface. The reduced catalyst was,
therefore, thought to be able to adsorb atmospheric oxygen,
which could then play a role as active oxygen, resulting in
a continuous HCHO oxidation cycle at room temperature.
2.3 Effect of chemisorbed oxygen species on HCHO reaction
over Pt/TiO2 catalyst and HCHO reaction mechanism
DRIFTS studies of HCHO adsorption and the effect of chem-
isorbed oxygen species on adsorbed HCHO were investigated
using Pt/TiO2–S, Pt/TiO2–G, and Pt/TiO2–Ar catalysts. Fig. 10
shows changes in the peaks upon injection of HCHO (20 ppm)
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species on the catalyst surface did not easily convert into CO on
any of the active metals (Rh, Pd, and Au).20 Therefore, catalyst O2
consumption, as shown in Fig. 8, was involved in the formation
of the formate intermediate from HCHO adsorption. The reac-
tion mechanism will be explained in detail using DRIFTS
analysis.
Fig. 11 shows changes in the peaks upon injecting O2 and N2
gas into the Pt/TiO2–G catalysts for 30 min to investigate
changes in the adsorption characteristics of the HCHO species
adsorbed on the catalyst surface. As a result, under oxygen
injection conditions, the formate peaks at 2861, 2842, 1577 and
1359 cmꢀ1 showed no signicant uctuations, meaning that
formate species did not react with oxygen in the air and that no
oxygen in the air was used in the conversion of formate species
to CO. In contrast, the size of peaks at 2048 and 1770 cmꢀ1 for
CO adsorbed to metallic Pt were reduced, meaning that CO
adsorbed to metallic Pt reacted with oxygen in air, followed by
conversion to CO2 and desorption. This explains the results in
Fig. 7(A). Bourane et al. examined the adsorption and desorp-
tion of CO on the surface of a Pt/Al2O3 catalyst via FT-IR anal-
ysis, reporting that the linear CO species was adsorbed on either
metallic Pt or Pt2+, while the bridged CO species was adsorbed
between two metallic species.39 Representative CO adsorption
peaks were observed in this study at 2048 (linear CO on metallic
Pt) and 1770 cmꢀ1 (bridged CO on metallic Pt), however, the CO
adsorption peak corresponding to Pt2+ was not observed.
Therefore, conversion of HCHO and CO absorption occurred
only at metallic Pt sites.
Fig. 12 Dynamic changes in in situ FT-IR spectra of the H2O reaction
after HCHO adsorbance over Pt/TiO2 (G) catalyst as a function of time
in a gas flow of H2O/N2 at room temperature. Inlet H2O ¼ 55% relative
humidity.
4 min, while the size of the peak at 1577 cmꢀ1 reduced. In
contrast, the peak at 2048 cmꢀ1, originating from CO linearly
adsorbed on metallic Pt sites, continued to increase up to 3 min
but decreased aer 3 min. The bridged CO peak (1770 cmꢀ1
)
increased up to 4 min aer injecting moisture. HCOOH species
and formate species had formed on the catalyst surface aer the
injection of HCHO, which were converted to CO, which then
desorbed as CO2 aer reaction.
Using DRIFTS analysis, the room temperature HCHO
oxidation mechanism was identied, as shown in Scheme 1.
First, HCHO was converted and absorbed as HCOOH by
a reaction with oxygen at the catalyst surface (eqn (1))
Fig. 12 shows changes in peaks upon injecting H2O and N2
gas into Pt/TiO2–G catalysts for 30 min to investigate the
changes in adsorption characteristics of the HCHO species on
the catalyst surface. The peak at 2842 cmꢀ1, originating from
HCOOH species aer injecting moisture, decreased rapidly,
with no peak observed aer 3 min. Aer injecting moisture, the
formate peaks at 2661, 1577, and 1359 cmꢀ1 also decreased
sharply. No peaks were observed at 2861 and 1359 cmꢀ1 aer
Fig. 11 Dynamic changes in in situ FT-IR spectra of the O2 reaction
after HCHO adsorbance over Pt/TiO2 (G) catalyst as a function of time
in a gas flow of 21% O2/N2 at room temperature.
Scheme 1 Schematic model of HCHO oxidation over Pt/TiO2.
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HCHO + O*surf. / HCOOH
(1)
O2-consumption. The second factor was the Pt valence state and
Pt dispersion, which determined the response speed (eqn (4)).
The room temperature oxidation response of HCHO was
conrmed in Fig. 10, as it was converted and adsorbed to
metallic Pt as CO species and responded to O2. Therefore, an
increase in the metallic Pt ratio, which was the active site, and
high catalyst dispersion, increased the nal response speed
(eqn (4)) to show excellent activity.
This HCOOH was converted to adsorbed CO (eqn (2) and (3)).
HCOOH + OHꢀ / HCOO + H2O
HCOO + H+ / CO + H2O
(2)
(3)
Adsorbed CO species were converted into CO2 by the reaction
with oxygen in air, and desorbed (eqn (4)).
3 Experimental
3.1 Catalyst preparation
2CO + O2 gas / CO2
(4)
A TiO2-supported Pt catalyst was prepared with the wet
impregnation method, using various commercially available
TiO2 supports. Ten types of commercial TiO2 supports were
purchased from Millennium Chemical (types G, Gs, and D),
Ishihara (type S), Hombikat (type U), Sigma-Aldrich (types Aa
and Ar), Degussa (type P), Wako (type W), and Nano (type N).
The content of TiO2 platinum was calculated by weight ratio,
and then platinum chloride (PtCl4; Aldrich Co.) was dissolved in
distilled water at 333 K. The calculated amount of target TiO2
was added to the solution. The resultant slurry was stirred for
over 1 h, and then evaporated at 343 K using a evaporator (N-N
series, Eyela Co.). The product was dried at 376 K overnight. To
remove residual chlorine, the sample was reduced in a tubular
furnace at 673 K under 30% hydrogen gas. Finally, the sample
was calcined at 673 K under an air-pressure atmosphere, and
then reduced using hydrogen gas at 873 K for 1 h to create the
nal catalyst.14
Fig. 13 shows the changes in peaks upon injecting HCHO/
H2O/O2/N2 mixed gas into the Pt/TiO2–G catalysts for 30 min to
investigate the changes in adsorption characteristics of the
HCHO species on the catalyst surface. Formate peaks at 2861,
1577, and 1359 cmꢀ1 were observed aer gas injection and CO
species adsorbed on metallic Pt were observed at 2048 and
1770 cmꢀ1. The HCOOH adsorption peak at 2842 cmꢀ1 was not
observed, as shown in Fig. 10. This lack of adsorption peak for
HCOOH in the HCHO reaction by DRIFTS analysis suggested
that the conversion of adsorbed HCOOH to HCOO and CO
species was rapid (eqn (2) and (3)). The peak at 2048 cmꢀ1
,
generated by CO linearly adsorbed on metallic Pt sites, grew
slowly aer a certain response time. This result (eqn (4)) indi-
cated a slower response speed (eqn (2) and (3)), therefore, two
catalyst factors were identied that affected the response speed
in the room temperature oxidation of HCHO. First, adsorbed
surface oxygen in the catalyst formed HCOOH intermediate
species on the catalyst surface (eqn (1)). This result coincided
with excellent HCHO reaction activity for catalysts, for which
the HCOOH peak was large. Surface oxygen was replenished by
oxygen in the air, therefore, the same result of excellent
HCHO reaction activity was achieved by catalysts with excellent
3.2 Test of catalytic activity
The xed bed reactor had a part for gas injection part, an
oxidation column, a reactor, and a part for analyzing the reac-
tion gas. An impinger containing paraformaldehyde was placed
in a constant-temperature tank, maintained at 295 K, and
HCHO was put into the reactor by vaporization of the para-
formaldehyde, supplying a constant amount of carrier gas to the
impinger.14,15 The temperature of the impinger was kept so that
the HCHO concentration in the generated gas was 24 ppm.
Moisture was supplied by injecting moist air using a bubbler. To
maintain a constant supply of moist air, water was circulated at
a constant temperature (298 K) using a circulator, in the form of
a double jacket, outside the bubbler. The reactor used
a continuous ow xed bed reactor with a quartz tube (inner
diameter 8 mm). Reactor temperature was controlled by lling
the outer reactor with water, with a K-type thermocouple
installed to measure temperature.
To conrm initial and product concentrations, CO2 & CO
were analyzed using a non-dispersive infrared (NDIR) analyzer
(ZKJ-2, Fuji Electric Co.). To measure the concentration of CO2
converted from injected HCHO, a catalytic oxidation reactor was
used. 1 mole of CO2 was produced by 1 oxidized mole of HCHO.
ꢂ
The oxidation tower operated at 300 C, which can completely
Fig. 13 Dynamic changes in in situ FT-IR spectra of HCHO + O2 + H2O
reaction over Pt/TiO2 (G) catalyst as a function of time in a gas flow of
HCHO/O2/H2O/N2 at room temperature. Inlet HCHO, 20 ppm; inlet
H2O ¼ 55% relative humidity.
oxidize the injected HCHO to CO2, conrmed by measuring
unreacted HCHO at the tail end of the xed bed reactor using
a total hydrocarbon (THC) analyzer (Thermo 55 ꢂC) and detector
3634 | RSC Adv., 2018, 8, 3626–3636
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tube (CO, HCHO, MeOH, HCHO, and HCOO). Gases other than minutes at 573 K, was cooled down to 298 K and then saturated
CO2 were not detected.
with the injection of a CO pulse.
The inlet CO2 concentration was measured using a NDIR
analyzer aer the paraformaldehyde was vaporized by the
oxidation catalyst tower. The outlet concentration of CO2 was
measured aer passing through each lled Pt/TiO2 catalyst bed
reactor at 298 K. The outlet concentration was analyzed aer
gases were at a steady state.
Reaction rates were obtained using the below equation:
FHCHO
r ¼ ꢀ
lnð1 ꢀ XÞ
mcat
where r is the HCHO reaction rate (mol sꢀ1 gcatꢀ1), FHCHO is the
total HCHO ow rate (mol sꢀ1), mcat is the weight of the catalyst
(g), and X is the HCHO conversion.40,41 These results, along with
metal dispersion measurements, were used to calculate HCHO
turnover frequency (TOF), obtained as amount of moles of
HCHO converted per surface active metal atom per second (sꢀ1):
3.3 Catalyst characterizations
The surface areas of the Pt/TiO2 catalysts were measured using
an ASAP 2010C instrument (Micromeritics), and calculated by
using the Brunauer–Emmett–Teller (BET) equation. Each
sample was analyzed aer being degassed under vacuum at 383
K for 3 h.
rHCHO ꢁ ABM
TOF ¼
;
DXM
where ABM represents the atomic weight of active metal M, XM is
the active metal content (gmet gcatꢀ1), and D is the active metal
dispersion.8
To observe the crystal structure of the Pt/TiO2 catalysts, X-ray
diffraction (XRD) analysis was conducted. Cu Ka was used as
the radiation source (l ¼ 0.1506 nm). The catalyst measure- 4 Conclusions
ments were madeꢀi1n the 2theta range of 20–90ꢂ, at a scanning
In this study, different Pt/TiO2 catalysts prepared by various
TiO2 showed different catalytic activities. XPS analysis demon-
strated that the catalytic activities were related to Pt valence
states and chemisorbed oxygen species on the Pt/TiO2 catalysts,
and these characteristics were dependent on the type of TiO2. O2
reoxidation experiments conrmed that the areas of oxygen
consumed and metallic Pt species on the catalyst surface were
strongly correlated. DRIFTS analysis showed that the chem-
isorbed oxygen species were involved in the formation of
formate species. These formate species reacted with moisture,
converting to linear CO species on metallic Pt sites. Finally, the
linear CO species reacted with gaseous oxygen to desorb as CO2,
completing the mechanism.
velocity of 4ꢂ min
.
X-ray photoelectron spectroscopy (XPS) was performed using
an ESCALAB 210 spectrometer (VG Scientic), with AlKa mon-
ochromate (1486.6 eV) as an excitation source. Aer the catalyst
was dried for 24 hours at 373 K and the moisture was completely
removed, the catalyst was analyzed without surface sputtering
and etching to maintain the vacuum of the XPS apparatus at 10–
12 mmHg. The bond energies and intensities of Pt, O, Ti, and C
in the sample were analyzed with the wide scanning spectrum.
To evaluate the reoxidation abilities of the catalysts, we
conducted an O2-reoxidation experiment. The lled catalyst was
activated for 10 minutes at 393 K with 50 cm3 minꢀ1 Ar. The
catalyst was cooled to room temperature (306 K), the thermal
conduction detector (TCD) signal was stabilized, and a contin-
uous supply of 1.5% O2/N2 was provided to obtain O2 concen-
tration TCD.19
Field emission-transmission electron microscopy (FE-TEM)
were performed on a JEM-2100F microscope (JEOL Co.) oper-
ated at 200 kV. The FE-TEM sample was prepared by suspending
the sonicated catalyst powder in ethanol and this suspension
was dropped into a Cu grid.
The in situ DRIFTS analysis used in this study was carried out
with a FT-IR spectrometer (Nicolet IS 10, Thermo Fisher, USA),
and a diffuse reectance (DR) 400 accessory was used for solid
reectance analysis. The spectrum contained 30 cumulative
scans with a resolution (4 cmꢀ1) by a mercury-cadmium-
telluride (MCT) detector. To exclude the inuence of moisture
and impurities, samples were pre-processed under Ar at 423 K
for 1 h, and then maintained in a vacuum state using a vacuum
pump. HCHO was injected by using vaporized para-
formaldehyde, obtained by injecting N2-containing HCHO
vapor through the paraformaldehyde, and N2 gas to the
impinger.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (No. 2014R1A1A2059082).
This research was supported by Korea Ministry of Environment
as Advanced Technology Program for Environmental Industry
(2016000140004).
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CO chemisorption analysis was performed at 298 K to
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