Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate Nanotubes in Formaldehyde Oxidation at Room Temperature

Article abstract of DOI:10.1007/s10562-020-03254-4

Au, Pd and Rh nanoparticles were supported on Mn/Na₂Ti₃O₇ alkaline titanate nanotubes by the deposition–precipitation with NaOH method. The Pt-Mn/NT alkaline titanate showed outstanding catalytic performance accomplishing complete formaldehyde oxidation at 40?°C with apparent activation energy of 27?kJ?mol^?1. The following tendency was observed for catalytic activity Pt > Au > Rh. The characterizations revealed the importance of the noble metals in the formation of vacancies and OH groups and their ability to activate the alkaline titanate surface oxygen species, which had an impact on the formation of acid sites (Br?nsted and Lewis); another key factor for formaldehyde oxidation was metal dispersion. The presence of OH species facilitated the transformation of formaldehyde adsorbed on the M-Mn/alkaline titanate nanotubes, probably through the reaction with adsorbed O species, which promoted the decomposition of formaldehyde to CO₂ at room temperature. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03254-4

Catalysis Letters
https://doi.org/10.1007/s10562-020-03254-4
Comparison of the Performance of Au, Pt and Rh Nanoparticles
Supported on Mn/Alkali Titanate Nanotubes in Formaldehyde
Oxidation at Room Temperature
R. Camposeco · S. Castillo2,3 · N. Nava · V. Maturano · R. Zanella
1
2
1
1
Received: 10 March 2020 / Accepted: 4 May 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Au, Pd and Rh nanoparticles were supported on Mn/Na Ti O alkaline titanate nanotubes by the deposition–precipitation
2
3
7
with NaOH method. The Pt-Mn/NT alkaline titanate showed outstanding catalytic performance accomplishing complete
−
1
formaldehyde oxidation at 40 °C with apparent activation energy of 27 kJ mol . The following tendency was observed
for catalytic activity Pt>Au>Rh. The characterizations revealed the importance of the noble metals in the formation of
vacancies and OH groups and their ability to activate the alkaline titanate surface oxygen species, which had an impact on
the formation of acid sites (Brönsted and Lewis); another key factor for formaldehyde oxidation was metal dispersion. The
presence of OH species facilitated the transformation of formaldehyde adsorbed on the M-Mn/alkaline titanate nanotubes,
probably through the reaction with adsorbed O species, which promoted the decomposition of formaldehyde to CO at room
2
temperature.
Graphic Abstract
Keywords Formaldehyde · Nanoparticles · Alkali titanate · OH groups · Acidity
*
R. Zanella
1
Introduction
rodolfo.zanella@icat.unam.mx
1
Instituto de Ciencias Aplicadas y Tecnología, Universidad
Nacional Autónoma de México, Circuito Exterior S/N, C.
U., 04510 Mexico City, Mexico
Formaldehyde is an indoor air pollutant habitually emitted
from fully building and decorative materials; likewise, it
has been classiꢀed as a human carcinogen that causes naso-
pharyngeal cancer [1, 2]. Moreover, indoor air cleaning with
low energy demand and low formaldehyde concentration
necessarily requires a catalyst that exhibits high activity for
2
3
Product Technology, Mexican Institute of Petroleum,
0
7730 Mexico City, Mexico
Department of Chemical Engineering, ESIQIE-IPN,
5876 Mexico City, Mexico
7
Vol.:(0
1
123456789)
3

R. Camposeco et al.
complete formaldehyde oxidation at low temperatures [3].
On the other hand, indoor air is often enriched with water
vapor, which leads to catalytic deactivation because of the
strong adsorption of water on active catalyst sites, mainly at
low temperatures [4, 5]. Recently, studies related to formal-
dehyde removal have been carried out using supported noble
metals such as Pt, Rh, Au, and Pd [6, 7] supported on transi-
tion metal oxides like MnO , CO O , CeO [8, 9], in which
The solution was set under constant stirring, and then HNO
3
(Sigma-Aldrich ACS reagent, 70%) was added to adjust the
reaction medium at pH 3. Hydrolysis was accomplished by
adding bidistilled water (18 mL) and the proper manganese
amount by using a solution of manganese (II) nitrate hydrate
(3 wt%) [Mn(NO )2xH O] Sigma Aldrich (99.99%). After-
3
2
ward, the obtained sol–gel Mn/TiO material was dried at
2
80 °C for 12 h and annealed at 400 °C for 4 h under air ꢂow
2
3
4
2
−
1
the Pt/TiO catalyst is considered as the most active catalyst
(30 mL min ). Then, the Mn/TiO material was transformed
2
2
for formaldehyde oxidation at low temperatures [10]. In this
way, manganese oxides are employed as catalysts in envi-
ronmental reactions such as oxidation of carbon monoxide
and VOCs because of their low cost and high activity, their
catalytic properties are attributed to their ability to produce
labile lattice oxygen and store oxygen; also, it has been
shown that manganese oxide catalysts supported on various
into Mn/alkali titanate nanotubes prepared by the hydrother-
mal method. 1 g of Mn/TiO obtained by the sol–gel method
2
was dissolved in 80 mL of a 10 N NaOH aqueous solution;
after that, the solution was mixed in a closed cylindrical
Teꢂon-lined autoclave Parr reactor at 130 °C for 24 h, rotat-
ing the whole autoclave at 400 rpm. Thereafter, the solution
was ꢀltered and washed with abundant deionized water in
order to reach pH 9. The material was dried under vacuum
at 80 °C for 12 h.
TiO anatase and rutile mixed phases and titanate nanotubes
2
exhibit high catalytic activities in the SCR-NH reaction
3
[
11–13]. Regarding the use of transition metal oxides, the
Later, a series of M-Mn/NT catalysts, in which M=Au,
Pt, and Rh, with a load of (0.5 wt.%), were synthesized by
the deposition precipitation method. In detail, 0.5 g of Mn/
NT was dispersed in 50 mL of deionized water and stirred
at 80 °C in a water bath. Next, gold (III) chloride hydrate
(HAuCl ), dihydrogen hexachloroplatinate (H PtCl ) and
removal of formaldehyde has been achieved at high tem-
peratures over 150 °C. Likewise, in a previous study, it was
+
+
+
shown that the use of alkali ions such as Li , Na , and K
promote the catalytic eꢁciency of formaldehyde oxidation
14].
Furthermore, titanate nanotubes are good dispersants and
[
4
2
6
rhodium chloride (III) (RhCl ) solutions with required con-
3
catalytic promoters when used as supports since they have
large speciꢀc surface areas, leading to a higher potential
of applications in environmental puriꢀcation, carbonic acid
gas decomposition and generation of hydrogen gas [15, 16].
Additionally, it has been stated that during treatments using
NaOH or KOH on TiO , several Ti–O bonds in TiO are
centrations were added dropwise to the Mn/NT suspension
under stirring; the pH value was stabilized at around 9 with
an aqueous solution of 0.1 N NaOH. After stirring for 2 h
and aging for 1 h, the suspension was ꢀltered and washed
with abundant water to close pH 9. Then, it was dried under
vacuum at 80 °C and annealed at 400 °C for 4 h under hydro-
gen ꢂow, yielding the catalysts denoted as M-Mn/NT, where
M represents Au, Pt, and Rh, respectively.
2
2
broken, which leads to the formation of layered structures
17]. Na Ti O and K Ti O have been identiꢀed to pos-
[
2
3
7
2
4
9
sess layered structures with alkali metals present in their
interlayer regions. The aim of this work was to compare
the catalytic performance of Au, Pt and Rh deposited on
Mn/Na Ti O , prepared by a hydrothermal method, in the
2.2 Measurements of Formaldehyde Catalytic
Performance
2
3
7
oxidation of formaldehyde to CO at room temperature. It is
The catalytic oxidation of formaldehyde over M-Mn/NT
catalysts was performed in a continuous ꢂow ꢀxed bed
reactor, where 100 mg of catalyst was loaded in a U-type
quartz tubular reactor with quartz wool layers. The feed gas
2
shown that the bimetallic catalysts are more active than the
monometallic ones and that oxygen vacancies, OH groups,
and higher concentration ratios of Brönsted and Lewis acid
sites are the active sites for formaldehyde oxidation at low
temperatures.
contained 500 ppm of formaldehyde and 20 vol% of O , and
2
balanced N , under ambient conditions at a total ꢂow rate
2
−
1
of 100 mL min ; the M-Mn/NT catalysts were pretreated
−
1
with hydrogen in a ꢂow of 100 mL min at 400 °C for 3 h
2
Experimental Section
and cooled to room temperature in N for 10 min. Then, the
2
reactants were introduced in the reaction tube, where gase-
ous formaldehyde was generated through paraformaldehyde.
The gas compositions were monitored by an Agilent
2
.1 Deposition of Au, Pt and Rh Metal
Nanoparticles on Mn/Alkali Titanate Nanotubes
6
890 N gas chromatograph equipped with a hydrogen
Titanium dioxide was prepared by the sol–gel method as
follows: 18 mL of titanium isopropoxide (Sigma-Aldrich,
ꢂame ionization detector (FID). Therefore, the formalde-
hyde conversion was expressed as (CO out)/(CO ), where
2
2T
9
9.9%) was mixed with 90 mL of 2-propanol (Baker, 99%).
CO out was the concentration of CO produced at a certain
2
2
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
−
1
temperature, and CO2 T represented the concentration of CO2
in the outlet gas when formaldehyde was totally oxidized
operated at a resolution of 4 cm for 75 scans. Before each
measurement, the sample was reduced in situ by H /He at
2
into CO . The concentration of formaldehyde was calcu-
300 °C for 30 min and then purged with N for 30 min. The
2
2
lated by the external standardization through the calibrated
curve. In this study, the formaldehyde concentration was
controlled at around 500 ppm. For GHSV, the concentration
of formaldehyde was maintained at 500 ppm, and a ꢂow rate
gas consisting of formaldehyde and O or N was introduced
2
2
into the reaction chamber, and then N ꢂow was purged.
2
All the DRIFTS spectra were recorded by subtracting the
background.
−
1
of 100 mL min , only the catalyst mass was varied corre-
XPS experiments were performed with a Thermo VG
Scientiꢀc Escalab 250 spectrometer equipped with a hemi-
spherical electron analyzer and an Al Kα radiation source
(1486.6 eV) powered at 20 kV and 30 mA, respectively. The
binding energy was determined by using carbon C (1 s) as a
reference line (284.6 eV). The spectrometer was operated at
pass energy of 23.5 eV, and the base pressure in the analysis
−1
sponding to GHSV of 150,000, 100,000 and 70,000 h . For
the stability test, formaldehyde oxidation was carried out at
5
00 ppm and 30 °C for all catalysts. The speciꢀc reaction
rates were determined under conversions lower than 15%.
The reaction rate of formaldehyde was calculated as follows:
r
=X
*f
/m .n, where X is the conversion, f
HCHO
HCHO HCHO cat
−
8
is the molar gas ꢂow rate, m is the mass of catalyst in the
chamber was maintained in the order of 3×10 mbar. Peak
ꢀtting was done by using XPSPEAK 41.
cat
ꢀ
xed bed and n is the loading amount of metal. H O was
2
delivered by a syringe pump and it was vaporized in heated
lines at 120 °C before reaching the reactor.
3
Results and Discussion
2.3 Characterization Techniques
3.1 Catalytic Activity
X-ray diꢃraction (XRD) patterns were recorded at room
temperature with Cu Kα radiation in a Bruker Advance
D-8 diꢃractometer having a theta–theta conꢀguration and
a graphite secondary-beam monochromator. The data were
collected for scattering angles (2θ) ranging from 4° to 80°
with a step size of 0.02° for 2 s per point.
Formaldehyde conversions with similar loadings for the Au/
NT, Pt/NT, and Rh/NT catalysts and on the bare NT alkali
titanate nanotubes from 30 to 140 °C are shown in Fig. 1a;
likewise, on the NT support, it is observed that formalde-
hyde reached a conversion of 12% at 140 °C, while a pro-
motional eꢃect was observed when 0.5 wt% of Au, Pt, and
Rh were added. The formaldehyde conversion was outstand-
ingly increased to 38, 58, and 64% at 30 °C for Rh/NT, Au/
NT and Pt/NT respectively, suggesting that the noble metal
species play a major role in the removal of formaldehyde.
The remarkable activity was shown by Pt/NT, which reached
the complete oxidation of formaldehyde at 100 °C with a
notably high concentration of 500 ppm of formaldehyde and
High-resolution transmission electron microscopy
(
HRTEM) analyses of the samples were performed with a
JEOL 2200FS microscope operating at 200 kV and equipped
with a Schottky-type ꢀeld emission gun and an ultrahigh-
resolution pole piece (Cs=0.5 mm, point-to-point resolu-
tion, 0.190 nm). The textural properties were obtained by
means of an ASAP-2000 analyzer from Micromeritics.
The specific surface area was calculated from the
−1
Brunauer–Emmett–Teller (BET) equation from N phys-
a GHSV of 70,000 h . On the other hand, when the addition
of metal nanoparticles (Au, Pt, and Rh) to the Mn/NT alka-
line titanate nanotubes was carried out, a synergistic eꢃect
was observed increasing the conversion of Au, Pt and Rh,
which cannot be reached by themselves; the combination of
noble metals in the Mn/NT catalysts showed higher conver-
sion, reaching 96% for Pt-Mn/NT, 85% for Au-Mn/NT and
65% for Rh-Mn/NT at 30 °C, as can be seen in Fig. 1b, in
which Pt-Mn/NT resulted to be the best choice to remove
formaldehyde at low temperature. It is important to highlight
2
isorption at 77 K. The pore size distribution was obtained
by the Barrett–Joyner–Halenda (BJH) method from the des-
orption branch.
The hydrogen temperature-programmed reduction
2
(
H -TPR) study of the dried materials was performed
in a RIG-150 unit underꢂow of 10% H /Ar gas mixture
2
−1
−1
(
50 mL min ) with a heating rate of 10 °C min from room
temperature to 600 °C. H O produced while the reduction
2
process was trapped before the TCD detector.
Pyridine spectra were recorded in a Nicolet 8700 spec-
that the catalysts oxidize all formaldehyde towards CO , no
2
−
1
trophotometer with a resolution of 4 cm , accumulating 50
scans. In the cell, all the samples were treated in vacuum at
other compounds except CO were detected in the eꢄuent
2
for all the catalysts.
4
00 °C for 1 h; pyridine was then admitted and adsorbed.
For the most active catalyst in the oxidation of formal-
dehyde (Pt-Mn/NT), the gas hourly space velocity (GHSV),
shown in Fig. 1c, was studied over a wide range of GHSV
Evacuations were performed from room temperature to
1
00 °C.
−
1
Diffuse reflectance infrared Fourier transform spec-
(150, 000, 100, 000 and 70, 000 h ). It can be seen that at
−
1
troscopy (DRIFTS) was performed using a Nicolet 8700
30 °C with 70, 000 h , the Pt-Mn/NT catalysts began to
1
3

R. Camposeco et al.
Fig. 1 a Formaldehyde conversions as a function of the reaction tem-
of the reaction temperature on diꢃerent bimetallic catalysts, c formal-
dehyde conversions for Pt-Mn/NT at diꢃerent loads of catalyst and
(d) formaldehyde deactivation
perature on diꢃerent monometallic catalysts. Reaction conditions:
5
00 ppm formaldehyde, 20 vol% O , and balance N , (relative humid-
2 2
−1
ity) 0%, GHSV 70,000 h , b formaldehyde conversions as a function
be active at 30 °C (96% of formaldehyde conversion) while
and Rh-Mn/NT catalysts, respectively. An initial decrease
in conversion was observed in the three catalysts during the
ꢀrst 30 min in the presence of 500 ppm of formaldehyde at
−
1
at 100, 000 h , a small decrease was observed in the cata-
lytic activity, reaching around 84% of conversion at 30 °C
−1
−1
while at 150, 000 h only 58% of conversion was reached at
30 °C with GHSV of 70, 000 h . These catalysts exhibited
3
0 °C. These results indicate that the Pt-Mn/NT catalyst was
outstanding formaldehyde removal eꢁciency after 6 h.
Figure 2a shows that the activation of the Pt-Mn/NT cata-
lyst at 400 °C under hydrogen produces more active catalysts
than air activation at 300 and 200 °C. The catalyst pretreated
at 200 °C showed low activity while the catalysts pretreated
at 300 °C displayed intermediate catalytic activity, reaching
37% at 30 °C. However, it was found that 400 °C was the
optimal temperature for the Pt-Mn/NT catalyst. These diꢃer-
ences in catalytic activity could be ascribed to factors such
as the dispersion of platinum, where the activation tempera-
ture has an important consequence for the catalytic perfor-
mance because the conversion increases when the activation
temperature also increases. Similar results were observed for
the Pt/TiO catalysts pretreated by H reduction at diꢃerent
eꢃective in the oxidation of formaldehyde with a GHSV of
−
1
7
0, 000 h . In this sense, previous works have reported the
GHSV as a critical experimental parameter to reach 100%
of conversion, and it has been found that within the interval
−
1
ranging from 50, 000 to 120, 000 h , the formaldehyde
removal eꢁciency is close to 100% in a temperature range
from 80 to 140 °C [18, 19].
The stability of the Au-Mn/NT, Pt-Mn/NT, and Rh-Mn/
NT catalysts in the oxidation of formaldehyde was assessed
in dry conditions, and the results are shown in Fig. 1d. In
order to demonstrate the stability, 100% of formaldehyde
conversion was avoided in the test. Almost constant con-
versions of around 88, 80 and 67% were observed for 6 h
under reaction conditions with the Pt-Mn/NT, Au-Mn/NT,
2
2
temperatures (from 400 to 700 °C): the catalysts reduced at
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
Fig. 2 a Formaldehyde conversions at diꢃerent activation temperatures for Pt-Mn/NT, b Arrhenius plot for bimetallic catalysts and c formalde-
−1
hyde conversions for Pt-Mn/NT at 500 ppm formaldehyde, 20 vol% O , and balance N , (relative humidity 10 and 50%), GHSV 70,000 h
2
2
0
higher temperatures contained more Pt and exhibited higher
formaldehyde oxidation activity at room temperature [20].
On the other hand, the reaction rates of these catalysts
for formaldehyde oxidation were calculated and compared.
The results are displayed in Table 1 and Fig. 2b. It was found
that the reaction rate for Pt-Mn/NT was higher than those
of the Au-Mn/NT and Rh-Mn/NT catalysts at lower metal
loadings, which indicated the importance of metal species in
the conversion of formaldehyde. Also, the apparent activa-
tion energy Ea of the Pt-Mn/NT, Au-Mn/NT and Rh-Mn/
NT catalysts for the formaldehyde oxidation was measured.
As shown in Fig. 2b, without the presence of humidity, the
Table 1 Properties of
Property
Pt-Mn NT Au-Mn/NT Rh-Mn/NT Mn/NT
NT support
/
bimetallic/NT catalysts and
kinetic data during the oxidation
of formaldehyde reaction
Nominal Mn content, wt%
Nominal Au, Pt and Rh content, wt% 0.5
3
3
0.5
0.47
218
30
0.8
2.8×10
3.1
3
3
–
–
–
–
0.5
0.48
221
35
a
Actual Au, Pt and Rh content wt%
0.45
225
27
2.1
3.2×10
3.3
2.98
203
48
–
1.1×10
3.8
2
SBET, m /g
229
84
–
Activation energy Ea, kJ/mol
Au, Pt and Rh particle size, nm
Rate, mol/gcat s (at 25 °C)
Na content wt%
1
–
6
–6
–6
–7
–8
1.8×10
3.4
3.4×10
3.9
aMn actual content %
1
3

R. Camposeco et al.
−
1
Ea value for Pt-Mn/NT is around 27 kJ mol while the Ea
water with formaldehyde on the Pt-Mn/NT catalyst sur-
face, which conꢀrmed the fact that the water present in the
feed streams played an important role in the formaldehyde
removal process. It has been reported that water aꢃects the
active sites on the surface of these catalysts, where the mech-
anism involves a competitive adsorption process between
−
1
value for Au-Mn/NT is 30 kJ mol and the one for Rh-Mn/
−1
NT is 35 kJ mol . In previous reports, the activation energy
has been reported for diꢃerent catalyst tested in the formal-
dehyde oxidation, and the values have been found between
−
1
2
3 and 33 kJ mol when a noble metal is present [21, 22].
It is important to highlight that the amount of catalyst
H O and formaldehyde on the active catalyst sites. In this
2
used was lower (40, 80 and 100 mg), the concentration of
formaldehyde was higher (500 ppm) and the load of metallic
particles Au, Pt and Rh (0.5 wt%) was lower compared to
sense, it has been stated that adsorbed H O is dissociated
2
to form hydroxyls on the catalyst surface [23]. The surface
hydroxyls remain adsorbed on the active sites, hindering the
formaldehyde adsorption-activation process on the catalyst,
thus restricting the formaldehyde conversion; as for the pre-
sent work, the formaldehyde oxidation was kept above 90%
when 50% of humidity was used, and the Pt-Mn/NT catalyst
remained stable in the presence of humid air for 6 h under
stream. In previous studies, using diꢃerent approaches for
the formaldehyde oxidation reaction, supported noble met-
als have shown to play an important role in formaldehyde
oxidation at room temperature. Table 2 summarizes the
results previously reported using noble metals and manga-
previous works reporting Au, Pt or Rh supported on TiO .
2
Using concentrations of formaldehyde between 10 and
1
40 ppm the previous reported catalysts have shown con-
versions between 20 and 95% using reaction temperatures
between 30 and 140 °C [18, 19, 21]. In addition, it is note-
worthy that the alkaline titanium nanotube structure here
studied is preserved at 400 °C, as shown by the DRX and
TEM results (Figs. 3 and 4).
The eꢃect of relative humidity on formaldehyde oxida-
tion was studied by using a Pt-Mn/NT catalyst, which is
shown in Fig. 2c. For water vapor relative humidity of 0%,
the catalyst suꢃered a slight formaldehyde conversion drop
during the ꢀrst 30 min, keeping the conversion for 6 h. When
relative humidity of 10% was used, an increase in formal-
dehyde conversion was observed from 86 to 96%; likewise,
when the relative humidity was increased to 50%, a slight
increase in conversion during the ꢀrst 2 h was observed;
then, the catalyst underwent low deactivation after 6 h under
the established reaction conditions. It was elicited that the
presence of water vapor did not inhibit but rather enhanced
formaldehyde oxidation for relative humidity of 10% at room
temperature. Also, when the relative humidity was increased
further to 50%, the formaldehyde conversion fell slightly
from 96 to 93% because of the competitive adsorption of
nese on TiO for the complete oxidation of formaldehyde
2
and the conversion of formaldehyde to carbon dioxide at
room temperature (T ≤ 30 °C). The noble metal plays an
important role on the oxygen vacancies in the catalyst, for-
maldehyde oxidation follows trend Pt > Au > Rh. Pt is capa-
ble of achieving higher conversions of formaldehyde with
minimal metal loadings≤ 0.5. wt % because of its ability
to activate the surface oxygen species on Mn/NT. Another
important parameter for obtaining high formaldehyde oxi-
dation activities at low temperature is the dispersion of the
metal, in agreement with reference [17]. For Au and Rh,
higher metal loadings (> 1.0. wt%) are required to catalyze
the formaldehyde oxidation; moreover, Au activity depends
mainly on the support’s properties and on its dispersion and
oxidation state [9]. Regarding rhodium, its activity has been
attributed mainly to the presence of Rh small clusters [21].
The bimetallic catalysts exhibited outstanding formaldehyde
oxidation (Pt-Mn/NT > Au-Mn/NT> Rh-Mn/NT) regarding
monometallic ones (Pt/NT > Au/NT > Rh/NT) because the
interaction of metal nanoparticles with Mn promoted the
existence of oxygen vacancies and the formation of O–H
groups (see the characterization section below).
Na Ti O
7
JCPDS 31-1329
Na Ti O
2
3
(
001)
2
3
7
Na Ti O
7
201) (111)
2
3
Na Ti O
7
2
3
(
(020)
MnO
Mn/NT
Mn O
2
3
Au-Mn/NT
Pt-Mn/NT
3.2 Crystalline and Textural Properties
Rh-Mn/NT
Figure 3 displays the XRD pattern of the Mn/NT alka-
line titanate nanotubes annealed at 400 °C in air and with
the addition of Au, Pt, and Rh after annealing at 400 °C
in hydrogen. Figure 3 shows the presence of sodium tri-
NT
1
0
20
30
40
50
60
70
80
titanate (Na Ti O JCPDS 31-1329), which is in agree-
2
-Theta (Degrees)
2
3
7
ment with the literature, where A Ti O ·H O and A Ti O
2
4
9
2
2
8
17
(
A = H, Na, and K) structure types have been reported,
Fig. 3 XRD patterns of the NT, Au-Mn/NT, Rh-Mn/NT and Pt-Mn/
NT catalysts
which occurred as a consequence of a layer condensation
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
Fig. 4 HRTEM and HAADF-
STEM images for the a NT, b
Mn/NT, c Au-Mn/NT, d Pt-Mn/
NT and e Rh-Mn/NT catalysts
during the hydrothermal synthesis [17]. The XRD pattern
did not change signiꢀcantly with the addition of Au, Pt and
Rh nanoparticles, except that the diꢃraction peak located
at 2θ=9.8° shifted slightly to a smaller angle at 2θ=9.7°
as the annealing temperature was 400 °C. The peak located
at 2θ=9.8° was due to the reꢂection of the (2 0 0) crystal
plane and the shift toward a smaller diꢃraction angle was
attributed to the OH group in the alkaline titanate nanotube
supports, which is related to the interlayer distance in the
alkaline titanate nanotube wall [24]. On the other hand, Au,
Pt, and Rh peaks were not observed because of the low metal
load employed and because dispersed nanometric particles
1
3

R. Camposeco et al.
Table 2 Summary of works reporting on HCHO oxidation at room temperature (T≤30 °C) using supported noble metals and the removal con-
version obtained
Catalysts
Experimental details
Reaction tempera-
ture °C
HCHO conversion
(%)
References
1
1
1
1
1
0
1
1
0
2
0
.5% Pt/TiO2
% Au/TiO2
% Pd/TiO2
.5% Rh/TiO2
% Au-Pt/nest-like MnO2
.2% Pt/MnO /TiO2
HCHO=100 ppm, GHSV=50, 000 h−
HCHO=10 ppm, GHSV=120, 000 h−
20
25
25
20
30
30
30
25
99
100
37
68
95
96
86
65
[10]
[42]
[43]
[21]
[44]
[45]
This work
−
HCHO=140 ppm, GHSV=30, 000 h
HCHO=140 ppm, GHSV=95, 000 h−
−
HCHO=460 ppm, GHSV=20, 000 h
HCHO=50 ppm, GHSV=30, 000 h
−
1
2
1
Pt-Mn/NT
Au-Mn/NT
Rh-Mn/NT
HCHO=500 ppm, GHSV=70, 000 h−
NT titanate nanotubes
were found in the Mn/NT alkaline titanate nanotubes, see
micrographs in Fig. 4. Moreover, the Mn content revealed
7–8 nm; their lengths range from 500 nm up. HAADF-
STEM characterizations were examined to observe the
dispersion of Au, Pt, and Rh on the Mn/alkaline titanate
nanotube supports, as shown in Fig. 4c–e, where a tubu-
lar structure corresponding to Na Ti O can be observed,
slight diꢃraction peaks ascribed to MnO (JCPDS 44-0141)
2
and Mn O (JCPDS 41-1442), which were detected in all
2
3
the catalysts.
2
3
7
The BET surface areas for the Mn/NT alkaline titan-
ate nanotubes, and with the addition of Au, Pt, and Rh, are
shown in Table 3. The presence of Au, Pt, and Rh does not
imply a remarkable change in the speciꢀc surface area for
all the catalysts; in this way, it seems that the high surface
area improves the dispersion of Au, Pt, and Rh on alkaline
titanate nanotubes, producing more oxygen vacancies that
may exert a favorable impact on the catalytic activity of for-
maldehyde oxidation.
which is consistent with the XRD results. By analyzing the
images in Fig. 4c–e, it was found that the Au, Pt, and Rh
species were uniformly dispersed on the alkaline titanate
nanotubes in the order of sub-nano sizes of 0.6–1.2 nm. The
high dispersion and stability of monometallic Au, Pt, and
Rh nanoparticles on the alkaline titanate nanotubes seem
to be favorable during the formaldehyde oxidation process.
Alkaline titanate nanotubes seem to have an inꢂuence on
the particle size and dispersion of Au, Rh and Pt and, there-
fore, on the catalytic performance of the catalysts. Using
that alkaline support with pH 9, a high dispersion of Au,
Rh and Pt (0.6–1.5 nm) was observed, which produced an
outstanding catalytic activity promoted by the higher pres-
ence of hydroxyl groups, which is one of the characteristics
of alkaline titanate nanotubes. A previous study reported
that the basicity of the support had an inꢂuence on the metal
particle size, i.e., when the basicity increased, the particle
size decreased [25].
4
HR‑TEM
Figure 4a and b show the TEM image of the bare alkaline
titanate nanotubes and Mn/NT after washing in deionized
water and annealment at 400 °C; the sample contains many
nanotubes with narrow size distribution. The diameters
of the alkaline titanate nanotubes are uniform and around
Table 3 Physicochemical
Catalyst
S
C
C
C
C /C
B
Temperature
(°C)
BET
Brönsted
Lewis
Total
L
properties of NTs
m g−1)
2
(µ-mol m−2)
(µ-mol m−2)
(µ-mol m−2)
(
Ratio
NT
229
219
222
225
203
0.70
3.2
1.7
3.9
2.0
4.5
4.3
R.T
100
0
.39
0.83
.52
0.98
.86
1.34
.91
0.41
.32
Au-Mn/NT
Pt-Mn/NT
Rh-Mn/NT
Mn/NT
8.4
9.2
10.1
R.T
0
4.3
4.8
8.2
100
9.1
5.0
10.0
5.8
9.2
5.8
R.T
100
0
12.9
14.2
9.6
R.T
0
7.3
8.21
8.0
100
9.2
4.9
9.6
5.2
10.2
15.3
R.T
100
0
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
4
.1 Reducibility of Au, Pt, Rh and Mn Species
and 600 °C in which the presence of manganese is clearly
2
+
3+
4+
observed as Mn , Mn , and Mn in all catalysts, where
the presence of multiple oxidation states of Mn on alkaline
titanate nanotube supplied the electron transfer processes
during the formaldehyde oxidation; additionally, Au and Pt
can only display the main reduction peak, while rhodium
H -TPR experiments were carried out to investigate the
2
reducibility of Au, Pt, and Rh on the Mn/NT alkaline titanate
nanotubes. As shown in Fig. 5, there is a strong interaction
between Rh species and Mn/NT, which favors the high dis-
persion of Rh and the production of surface oxygen vacan-
cies on TiO . These results suggest that the O molecule can
3
+
0
presented two reduction peaks Rh and Rh , it is suggested
0
0
0
then that Au , Pt and Rh species exert an important role
on the formaldehyde oxidation at low temperature. Further-
more, it seems to be that the presence of Mn oxidized spe-
cies limits formaldehyde oxidation, as shown in Fig. 5b, as
is the case in Rh-Mn/NT reduced at 400 °C.
2
2
be adsorbed and dissociated on Rh sites to form O atoms
[
26]. On the other hand, the thermogram of the Au-Mn/NT
sample shows reduction peaks at around 130 °C. Since the
reduction peak of the supported Au is normally centered
between 100 and 150 °C, the peak located at 130 °C should
3
+
0
be mainly ascribed to the reduction of Au to Au species
4.2 Acidity by FTIR
[
27]. The peak at 450 °C can be due to the reduction of
surface oxygen in Mn/NT, indicating that the presence of
Pyridine adsorption was performed to characterize the sur-
face acidity of Mn/NT alkali titanate nanotube supports and
with the addition of Au, Pt, and Rh, employing pyridine as
model molecule. The Brönsted and Lewis acid sites of all
the samples are shown in Fig. 6. The bands located at 1640
Au facilitated the reduction of Mn and TiO surface oxygen
2
species.
For the Pt-Mn/NT sample, the reduction peak centered at
0
7
0 °C can be ascribed to the reduction of PtO to Pt , where it
−
1
seems that the strong interaction between Pt and MnO con-
and 1540 cm are characteristic of adsorbed pyridine on
the Brönsted acid sites [29, 30]; also, the bands from 1600
X
tributes to the lower Pt reduction temperature due to the tight
metal-support interaction in the catalysts [28]. The hydrogen
consumption peak at 350 °C is ascribed to the reduction of
−
1
−1
to 1630 cm and from 1440 to 1445 cm are attributed
to pyridine adsorbed on the Lewis acid sites, and the addi-
−
1
Mn species that interact strongly with the TiO support. The
tional band located at 1490 cm is associated with both
the Brönsted and Lewis acid sites [31]. In this way, all the
samples possess Brönsted and Lewis acid sites, as shown in
Fig. 6. This result conꢀrms that the lattice distortion due to
the scrolling of the alkali titanate nanotubes formed Brön-
sted acid sites [32]. The results show that all the catalysts
featured intense bands of Brönsted and Lewis acid sites; on
the other hand, formaldehyde increased gradually mainly
2
TPR results of all the catalysts suggest diꢃerent degrees of
strong metal-support interactions (SMSI) due to the varia-
tions in the reduction peaks from 25 to 400 °C, depending
on the studied metal; moreover, the reduction of the metal
was improved by the alkaline titanate nanotube supports
as follows: Pt-Mn/NT > Au-Mn/NT > Rh-Mn/NT. Also,
Fig. 5b shows the deconvolution of the peak between 200
Fig. 5 a H -TPR proꢀles for the NT, Mn/NT, Au-Mn/NT, Pt-Mn/NT, and Rh-Mn/NT catalysts and b deconvolution for the Mn/NT, Au-Mn/NT,
2
Pt-Mn/NT and Rh-Mn/NT catalysts
1
3

R. Camposeco et al.
L
NT
Au-Mn/NT
L
L
L
L + B
L + B
B
B
B
R.T
00°C
B
1
00°C
R.T
1
1
400 1450 1500 1550 1600 1650 1700 1400 1450 1500 1550 1600 1650 1700
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
L
Pt-Mn/NT
Rh-Mn/NT
L
L
L + B
L
B
L + B
B
B
B
R.T
R.T
1
00°C
100°C
1
400 1450 1500 1550 1600 -11650 1700
1400 1450 1500 1550 1600 11650 1700
-
Wavenumber (cm )
Wavenumber (cm )
L
Mn/NT
L
L + B
B
B
R.T
1
00°C
1
400 1450 1500 1550 1600- 1650 1700
1
Wavenumber (cm )
Fig. 6 Pyridine adsorption FTIR spectra for the NT, Mn/NT, Au-Mn/NT, Rh-Mn/NT and Pt-Mn/NT catalysts: (pyridine was evacuated from
room temperature [R.T to 100 °C])
on Pt-Mn/NT> Au-Mn/NT> Rh-Mn/NT, where the addi-
tion of 0.5 wt% of Pt, Au, and Rh to alkaline titanate nano-
tubes increased the concentration of Lewis acid sites almost
by three-fold, and the Brönsted acid sites were slightly
increased (see Fig. 6). This fact suggests that Pt, Au, and
Rh at 0.5 wt% act as promoters of the formaldehyde oxi-
dation, also improving the Brönsted and Lewis acidity in
the alkaline titanate nanotubes in comparison with the NT
support. Likewise, the presence of O–H groups seems to be
correlated with the presence of Brönsted acidity. Brönsted
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
acid sites seem to be generated on Ti–O surfaces because
the scrolling of Ti–O and Ti–O–H planes modiꢀes the Lewis
the fact that O–H species could improve the adsorption
and activate the oxygen molecules in close vacancy dis-
played by M-Mn/NT catalysts. The results featured in the
present work were compared with some reported catalytic
acid sites in comparison with other TiO phases (anatase,
2
rutile, and brookite) [33, 34]; both characteristics seem to
be correlated with higher formaldehyde oxidation due to
systems; from the comparison, it was found that α-MnO
2
Fig. 7 DRIFT spectra for a Pt-Mn/NT and b Pt/NT c Mn/NT, d NT in a ꢂow time HCHO+O and e DRIFT spectra of the Pt-Mn/NT catalyst as
2
a function of time in a ꢂow of O or a mixture of HCHO+O +H O at 30 °C (relative humidity: 10%)
2
2
2
1
3

R. Camposeco et al.
with a total acidity of 208.44 µmol g−1 showed complete
conversions of formaldehyde at about 160 °C [35] while,
when sepiolite was used as catalytic support in an Europium/
Na-Sepiolite catalyst, Brönsted and Lewis acid sites in the
interval of 100–220 °C were observed, with a total acid-
spectra for the Pt-Mn/NT and Pt/NT catalysts were obtained
at diꢃerent times of contact with the reactants at room tem-
perature as shown in Fig. 7a and b, where the peaks located
−
1
at 2069 and 1931 cm are attributed to CO intermediary
that comes from the dissociation of formaldehyde; as for the
–
1
−1
ity of 53 µmol g , reaching formaldehyde conversion from
50 °C [36]. According to our results and the reported ones
peaks located at 2868, 2927, 2907, 2952 and 2993 cm for
1
both catalysts, along with the strong peaks at 1577, 1639,
−
1
in the literature, it seems that the presence of Brönsted acids
sites promote formaldehyde oxidation at lower temperatures,
compared to the catalytic systems that do not present these
sites.
1556 and 1357 cm , they are ascribed to the stretching
mode of formats as (CH) and (COO-), respectively; addi-
−
1
tional peaks located at 1360 and 1239 cm are ascribed
to carbonate species [37]. Furthermore, the peaks located
−
1
The C
/C
ratio was calculated for Au-Mn/NT
at 1630 and 1633 cm in both catalysts are ascribed to the
water band with two protons in scissor-type vibration. In
addition, in the Pt-Mn/NT and Pt/NT catalysts, the gradual
Lewis Bronsted
(
8.0), Pt-Mn/NT (5.8), and Rh-Mn/NT (8.2). The results
suggest higher A
/A
, which can be ascribed to the
Lewis Bronsted
−
1
high dispersion of Au, Pt, Rh, and MnOx species on the
surface of the alkali titanate nanotubes. The three catalysts
decrease of the peak located at 3670 cm as time increases
is also observed, which is ascribed to isolated hydroxyl spe-
cies (O–H). The observed DRIFTS results suggest the fast
transformation of formaldehyde at ambient temperature
after its adsorption on both Pt-Mn/NT and Pt/NT catalysts
with the main participation of O–H species, allowing the
remarkable formaldehyde oxidation. The presence of for-
exhibited a C
/C
ratio, which seems to be corre-
Lewis Bronsted
lated with the potential to oxidize formaldehyde; however,
a lower C /C ratio was shown by the Pt-Mn/NT
Lewis Bronsted
(
5.8) catalyst, which generates the most active catalyst for
the oxidation of formaldehyde, see Table 3.
maldehyde+O on Pt-Mn/NT was also observed, producing
2
4.3 DRIFTS Formaldehyde Oxidation
more formate species during the adsorption of formalde-
hyde, with regard to monometallic Pt/NT by integrating the
−
1
DRIFTS experiments were carried out by adsorbing formal-
two times higher typical peak at 1571 cm . The previous
results indicate that the high catalytic activity in the Pt-Mn/
dehyde in the presence of O ; the formaldehyde adsorption
2
C
A
B
Mn 2p3/2
Ti 2p3/2
Mn/NT
Ti 2p
O 1s
Mn/NT
Mn/NT
Mn 2p
Mn 2p1/2
Ti 2p1/2
4+
satellite
Ti
Ti 2p
468
Binding Energy (eV)
456
460
464
528 530 532 534
Binding Energy (eV)
640 644 648 652 656
Binding Energy (eV)
E
F
D
Ti 2p3/2
O 1s
NT
NT
Ti 2p
NT
Na 1s
Na
Ti 2p1/2
4+
Ti
5
26 528 530 532 534
1068 1071 1074 1077
Binding Energy (eV)
456
460
464
468
Binding Energy (eV)
Binding Energy (eV)
Fig. 8 XPS and deconvolution spectra of titanium Ti 2p (a, d), oxygen O 1s (b, e), manganese Mn 2p (c) and (O) sodium Na 1s (f) for the NT
and Mn/NT catalysts
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
A
E
I
Ti 2p3/2
Au-MnNT
Pt-Mn/NT
Rh-Mn/NT
Ti 2p1/2
Ti 2p3/2
Ti 2p3/2
Ti 2p1/2
Ti 2p1/2
3
+
Ti
3+
4
+
4+
Ti
4+
Ti
3
+
Ti
Ti
Ti
Ti 2p
Ti 2p
Ti 2p
4
56
460
464
468
456
460
464
468
4
56
460
464
468
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
B
F
J
Au-MnNT
O 1s
Pt-Mn/NT
O 1s
Rh-Mn/NT
O 1s
5
26 528 530 532 534
526 528 530 532 534
526 528 530 532 534
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
C
G
K
4
f 7/2
Au-MnNT
Pt-Mn/NT
Rh-Mn/NT
*
5
*5
*5
Rh 7/2
4
f 5/2
4
f
Rh 5/2
5
/2
4
f 7/2
Au 4f
Pt 4f
Rh 3d
8
0
82 84 86 88 90
70 72 74 76 78 80
303 306 309 312
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
H
L
D
Mn 2p3/2
Pt-Mn/NT
Mn 2p1/2
Mn 2p3/2
Mn 2p3/2
Rh-Mn/NT
Mn 2p1/2
Au-MnNT
Mn 2p1/2
4
+
4
+
4+
2
+
2
+
2
+
3+
3+
3
+
Mn 2p
Mn 2p
Mn 2p
6
35 640 645 650 655 660 635 640 645 650 655 660
Binding Energy (eV)
Binding Energy (eV)
635 640 645 650 655 660
Binding Energy (eV)
Fig. 9 XPS and deconvolution spectra of titanium Ti 2p (a, e, i), oxygen O 1s (b, f, j), manganese Mn 2p (d, h, l), gold Au 4f (c), platinum Pt 4f
g) and rhodium Rh 3d (k) for the Au-Mn/NT, Pt-Mn/NT and Rh-Mn/NT catalysts
(
NT catalysts for formaldehyde oxidation stems mainly from
the reaction between formate intermediates and OH species
rather than from the reaction between intermediates of CO
and O species as shown by the Pt/NT catalyst. Additionally,
in the Mn/NT and NT catalysts, the presence of formate
and carbonate species was observed only at 50 and 90 °C,
respectively (see Fig. 7c and d), which is in agreement with
the light oꢃ curves shown by both catalysts (Fig. 1a and b).
Figure 7e shows the changes in the DRIFT spectra of the
Pt-Mn/TiO catalyst before and after the admission of O or
2
2
a HCHO+O +H O mixture to the cell. In this ꢀgure, the
2
2
−
1
main diꢃerence is found in the band located at 3670 cm ,
1
3

R. Camposeco et al.
which is ascribed to O–H groups that increased their number
[39]. On the other hand, the presence of Au, Pt, and Rh
3
+
with respect to the catalyst treated with HCHO+O . Moreo-
generates the presence of Ti , which is related to oxygen
vacancies that usually play an important role in catalytic
2
−
1
ver, a broad water band located between 3000–3500 cm
3
+
was observed, in which formate species were identiꢀed at
oxidation reactions; then, the presence of Ti species in
−
1
4+
1
580, 1363 and 1237 cm . With the completion of this
Au-Mn/NT, Pt-Mn/NT and Rh-Mn/NT implies that Ti
−
1
experiment, O–H groups (located at 3670 cm ) on the
ions occupy electrons from nearby oxygen vacancies and
3+
Pt-Mn/TiO catalyst surface increased their number by the
are transformed into Ti ions [39]. Thereby, the presence of
2
3
+
addition of H O (10% of relative humidity). These results
Ti species, associated with the presence of oxygen vacan-
cies (Table 5) may be due mainly to the presence of Pt and
Rh sub-nanometric particles in the lattice of the alkaline
titanate nanotubes.
2
conꢀrm that the presence of water does not completely deac-
tivate the catalyst for the oxidation of formaldehyde.
4.4 Surface Evolution by XPS Studies
The Au 4f spectra show the presence of Au 4f and Au
7
/2
4
f
peaks at binding energies of 82.7 and 86.4 eV, respec-
5/2
The surface elemental composition of Mn/NT, Au-Mn/NT,
Pt-Mn/NT and Rh-Mn/NT is shown in Figs. 8 and 9. The O
tively, with the interval of the Au 4f doublet at 3.7 eV, where
0
the peak was deconvoluted into two components, Au and
1
+
1
s spectrum could be deconvoluted into three and four com-
Au , located at 82.4 and 84.1 eV, respectively [40], (see
Fig. 9 and Table 6).
ponents. The ꢀrst one located at around 529 eV is ascribed to
the surface lattice oxygen (Ti–O), the second one located at
In the Pt 4f spectra, corresponding to two peaks related to
−
5
30.2 eV is related to oxygen species adsorbed as O , O ,
the spin-orbital doublet, components of Pt, 4f (72.1 eV),
ads
7/2
2
−
2−
O
or O and the third one located at 532.5 eV is related
and 4f (75.3 eV), can be observed in the Pt-Mn/NT cata-
2
3/2
to H O or organic carbon species [38]. Also, with the addi-
lyst (Fig. 9), where the deconvolution corresponds to two
2
0
2+
tion of Au, Pt, and Rh, a fourth component is observed at
around 529.5 eV, which is related to the lattice oxygen pre-
sent as M–O species; Figs. 8 and 9. It can also be observed
that there is a split in O 1s (529–530 eV) with the addition
of Au, Pt, and Rh which clearly indicates the formation of
large amounts of oxygen vacancies (see Table 4).
peaks related to the presence of Pt and Pt located at 71.6
0
and 73.2 eV [41]; the presence of Pt exerts an eꢃect on
2
+
Ti(OH) , facilitating Pt reduction and hydroxyl group
x
loss, which leads to the production of vacancies acting as
oxygen sources (see Table 6). In the case of rhodium spe-
cies, the deconvoluted spectrum displays two peaks that are
characteristic of rhodium 3d for 3d and 3d . The ꢀrst one
The Ti 2p spectrum of Mn/NT, Au-Mn/NT, Pt-Mn/NT
and Rh-Mn/NT is formed by two peaks located at 458.6
and 464.3 eV, corresponding to the splitting of the 2p and
5
/2
3/2
0
is located at 306 eV for Rh . The second one is located at
+
3
308 eV for RhO that is characteristic of Rh species (see
3
/2
x
2
p
peaks with a separation of 5.7 eV, in which only the
Table 6).
1
/2
4
+
contribution of Ti species is detected, which is in agree-
In addition, the presence of nanoparticles exerts an
influence on manganese species, in which two states
ment with previous reports on alkaline titanate nanotubes
Table 4 Deconvolution results
Catalyst
O
BE
O
BE
O
BE
O
IV(M–O)
BE
I(Ti–O)
II(O–H)
III(H2O)
of O 1s binding energies of
catalysts
(
% atom)
(eV)
(% atom)
(eV)
(% atom)
(eV)
(% atom)
(eV)
NT
Mn/NT
Au-Mn/NT
Pt-Mn/NT
Rh-Mn/NT
68
51
44
40
44
530.1
530.3
528
528
528.2
28
34
36
34
28
531.2
531.1
530
530
531.0
4
5
7
6
532.5
532.5
532.0
532.0
532.1
–
–
10
12
18
17
528.2
528.4
528.5
529.1
11
Table 5 Deconvolution results
of Ti 2p and Mn 2p binding
energies of catalysts
4+
3+
2+
3+
4+
Catalyst
Ti
BE
Ti
BE
Mn
BE
Mn
BE
Mn
BE
(
% atom) (eV) (% atom) (eV) (% atom) (eV) (% atom) (eV) (% atom) (eV)
NT
Mn/NT
Au-Mn/NT 84
Pt-Mn/NT 79
Rh-Mn/NT 91
100
100
458.5
458.6
460.8 16
460.8 21
0
0
–
–
–
40
–
–
–
–
–
642.4
643.4
644.4
643.9
639.4 44
639.4 52
639.4 60
639.4 48
640.5 16
641.2 26
641.3 18
641.0 22
458.0 21
458.2 22
457.3 20
458.3
9
1
3

Comparison of the Performance of Au, Pt and Rh Nanoparticles Supported on Mn/Alkali Titanate…
for Mn 2p were observed; the main peak located at
3
/2
3
+
6
41.4 eV is attributed to Mn , which is in good agree-
ment with literature data, from 641.3 to 641.9 eV [22];
and the minor peak with higher binding energy of
4
+
6
43.6 eV was ascribed to a higher oxidation state: Mn .
The experimental data of Mn 2p could also be fitted into
three Gaussian peaks with core levels at 639.4, 641.5 and
2
+
3+
4+
6
44.4 eV, corresponding to Mn Mn and Mn , respec-
tively, as displayed in Fig. 9.
The XPS spectra of the Na 1 s, illustrated in Fig. 8f,
display the only peak at 1072 eV, which indicates the pres-
ence of sodium ions; that peak is ascribed to the sodium
ions in Ti–O-Na groups originating from hydrothermal
synthesis to obtain alkaline titanate nanotubes. Further-
more, the little peak located at 1071 eV represents the Na
intercalated into the carbon C-Na.
3
+
4+
The relative atomic percentage value of Mn /Mn was
characterized by deconvoluted XPS spectra for Au-Mn/
NT (2.0), Pt-Mn/NT (3.2), and Rh-Mn/NT (2.18). The
results suggest that the support surface was covered by
Mn species, which can be ascribed to the high disper-
sion of MnOx species on the surface of the alkali titanate
nanotubes. The three catalysts exhibited relatively lower
4
+
3+
Mn surface concentration than Mn . In this sense, it is
very interesting to note that the relative atomic concentra-
3
+/
4+
tions of Mn Mn seem to be correlated with the poten-
tial to oxidize formaldehyde in the Pt-Mn/NT > Au-Mn/
NT > Rh-Mn/NT catalysts (see Table 5).
5
Conclusions
Formaldehyde oxidation with low Au, Pt, and Rh content
displayed high activity at low temperature by using Mn/alka-
line titanate nanotubes as supports. The results revealed the
complete conversion of formaldehyde to CO and reliable
2
stability at a low temperature. The experimental analyses
showed that the presence of Au, Pt, and Rh on the sub-
nanometer scale aꢃects the metal/support structure of the
Mn/NT catalysts, in which the Pt-Mn/NT catalyst exhibited
a remarkably high capacity for formaldehyde oxidation of
almost 100% of conversion at 30 °C. In addition, it seems
that Au, Pt, and Rh interact strongly with the Mn/NT alkali
titanate nanotubes, resulting in smaller particle sizes and
uniform dispersion, which allowed the important presence of
vacancies and O–H groups and higher concentration ratios
of the Brönsted and Lewis acid sites; these three parameters
could be responsible for activating the surface lattice and
oxygen vacancies, which increased the formaldehyde oxida-
tion at low temperature, and that was observed by the XPS
technique. The results show the design of eꢁcient catalysts
that are highly active for formaldehyde oxidation at low
temperatures.
1
3

R. Camposeco et al.
Acknowledgments The authors would like to thank the financial
16. Cui WY, Xue D, Yuan XL, Zheng B, Jia MJ, Zhang WX (2017)
support provided by the Consejo Nacional de Ciencia y Tecnología
Acid-treated TiO nanobelt supported platinum nanoparticles for
2
(
CONACYT), through the PDNPN1216 and CB-18269 grants; Direc-
the catalytic oxidation of formaldehyde at ambient conditions.
ción General de Asuntos del Personal Académico-UNAM, through the
PAPIIT IN103719 grant; and the Mexican Petroleum Institute via the
Molecular Engineering Program (Project D.00477).
Appl Surf Sci 411:105–112
17. Bavykin DV, Parmon VN, Lapkin AA, Walsh FC (2004) The
eꢃect of hydrothermal conditions on the mesoporous structure of
TiO nanotubes. J Mater Chem 14:3370–3377
2
1
8. Zhang C, He H (2007) A comparative study of TiO supported
2
noble metal catalysts for the oxidation of formaldehyde at room
temperature. Catal Today 126:345–350
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