Alkali titanate nanobelts-supported Pd catalysts for room temperature formaldehyde oxidation

Article abstract of DOI:10.1016/j.catcom.2020.106034

Pd clusters supported on alkali titanate (Na- and K-titanate) nanobelts were synthesized and investigated for the catalytic HCHO oxidation at room temperature. The presence of interlayer alkali metal promoted the dispersion of Pd species and simultaneously made the Pd clusters more negatively charged, thus enhancing the catalytic performance for HCHO oxidation reaction at room temperature. Especially, Pd clusters with the smaller size and larger quantity of Pd⁰ supported on K-titanate nanobelts contributed to higher catalytic activity and stability, where nearly complete HCHO oxidation was achieved with a Pd loading of 1 wt% at ambient temperature.

Full text of DOI:10.1016/j.catcom.2020.106034

CatalysisCommunications142(2020)106034
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Catalysis Communications
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Short communication
Alkali titanate nanobelts-supported Pd catalysts for room temperature
formaldehyde oxidation
Li Zhoua, Shuren Hea, Yuanhua Sangb, |Xiaomei Zhanga, Hong Liub, Chuancheng Jiac,⁎
,
Xiaohong Xua,⁎
a School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China
c International Frontier Center of Single-Molecule Science, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Heterogeneous catalysis
Palladium
Alkali metal
Titanate nanobelt
HCHO oxidation
Pd clusters supported on alkali titanate (Na- and K-titanate) nanobelts were synthesized and investigated for the
catalytic HCHO oxidation at room temperature. The presence of interlayer alkali metal promoted the dispersion
of Pd species and simultaneously made the Pd clusters more negatively charged, thus enhancing the catalytic
performance for HCHO oxidation reaction at room temperature. Especially, Pd clusters with the smaller size and
larger quantity of Pd0 supported on K-titanate nanobelts contributed to higher catalytic activity and stability,
where nearly complete HCHO oxidation was achieved with a Pd loading of 1 wt% at ambient temperature.
1. Introduction
the addition of Na+ species, the HCHO oxidation activity of catalyst is
highly improved as a result of the formation of highly dispersed and
Formaldehyde (HCHO) as a primary indoor air pollutant with high
carcinogenicity and mutagenicity [1,2] has received considerable at-
tention for its removal in reducing public health risk. The character-
istics of an ideal process for indoor HCHO purification consist of non-
secondary pollutant production, convenience, and low energy con-
sumption. The catalytic oxidation was shown to meet this requirement,
in which HCHO can be completely oxidized over well-designed cata-
lysts at room temperature producing only H2O and CO2 as clean pro-
ducts [3,4]. Efficient catalysts for room-temperature HCHO oxidation
are mainly supported noble metal catalysts, including Pt, Au, Pd and Ag
[5–9]. Their catalytic performances on HCHO oxidation are strongly
correlated with the use of reducible supports because of the remarkable
influence of strong metal-support interaction (SMSI) on intrinsic prop-
erties of metal species, such as chemical state and particle sizes [10,11].
Regarding catalytic oxidation of HCHO over supported noble metal
catalysts, titanium dioxide (TiO2) is one of the most widely investigated
supports [5,7]. It has been found that the addition of alkali metal ions
(Na+ and K+) can significantly enhance the catalytic performances of
TiO2 supported noble metal catalysts, such as Ag, Pt and Pd [12–15]. He
et al. [16] reported that the addition of Na+ ions to Pt/TiO2 catalyst can
markedly improve the dispersion of Pt species, thus intensifying the
catalytic performance. Similarly, such enhancement effect of alkali
metal ions has also been reported for Pd-based catalysts [17,18]. With
negatively charged Pd species [14,18]. However, there is no yet a
general mechanism to explain satisfactorily the promoting effect of
alkali metal species on HCHO catalytic oxidation reaction. In most
cases, it is postulated that the presence of alkali metal salts can provide
OH– ions which contribute to the enhanced activity of catalysts, instead
of additional alkali metal ions. Moreover, the alkali metal additions
usually lead to more negatively charged and dispersed metal species
[11,17]. The negatively charged Pd species is beneficial to H2O acti-
vation and oxygen chemisorption, and highly dispersed Pd clusters/
nanoparticles would bring more active sites [18,19].
At the present time, most studies have focused on the external ad-
dition of alkali metals, while few reports involved using layered alkali
metal titanates directly as supports in noble metal catalysts. Sodium
and potassium titanate are layered alkali metal titanates, where TiO6
octahedrons are joined together to form two-dimensional planes
stacking layer by layer, between which the alkali metal ions are located
in the crystal (Fig. S1, ESI) [20]. Consequently, Na-titanate and K-ti-
tanate nanobelts (NTO-NB and KTO-NB) can be easily synthesized by
hydrothermal treatment of TiO2 precursors in concentrated alkaline
solution due to their layered structure [21,22]. These one-dimensional
nanostructures also intrinsically possess Na+ or K+ ions. More sig-
nificantly, the hydrothermally synthesized NTO-NB and KTO-NB have
high specific surface areas and clean surface with uniform defects, on
⁎ Corresponding authors.
E-mail addresses: jiacc@chem.ucla.edu (C. Jia), xhxu@sdu.edu.cn (X. Xu).
https://doi.org/10.1016/j.catcom.2020.106034
Received 16 December 2019; Received in revised form 30 April 2020; Accepted 1 May 2020
Availableonline06May2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

L. Zhou, et al.
CatalysisCommunications142(2020)106034
Fig. 1. The SEM images of Pd/NTO-NB and Pd/KTO-NB samples.
which the deposition of uniform and well dispersed noble metal na-
noparticles and clusters can be obtained [23,24].
Thermo ESCALAB 250 X-ray photoelectron spectrometer.
In this work, Na-titanate and K-titanate were synthesized and used
to prepare supported palladium catalysts (code: Pd/NTO-NB, Pd/KTO-
NB) by a facile deposition-precipitation method. The results showed
that Pd/NTO-NB and Pd/KTO-NB catalysts are much more efficient for
HCHO oxidation at room temperature compared to TiO2 nanobelts
(TiO2-NB) supported palladium catalyst (Pd/TiO2-NB), where Pd/KTO-
NB catalyst provided the best catalytic performance.
2.3. Catalytic activity tests
Before catalytic testing and characterization of a given solid, the as-
synthesized solid samples were treated in pure H2 gas flow at 400 °C for
2 h, and marked as Pd/KTO-NB, Pd/NTO-NB and Pd/TiO2-NB, re-
spectively. The samples pretreated in simulated air (O2/N2 = 21:79)
are denoted as PdO/KTO-NB, PdO/NTO-NB and PdO/TiO2-NB, re-
spectively. The fresh catalyst samples before any gas/thermal treatment
are marked as Pd/KTO-NB-F, Pd/NTO-NB-F and Pd/TiO2-NB-F, re-
spectively. The catalytic oxidation of HCHO was carried out in a quartz
tubular (i.d. = 6 mm) fixed-bed reactor under 1 atm pressure at room
temperature (25 °C). Gaseous HCHO was produced by flowing N2 gas
via pyrolysis of paraformaldehyde in a water bath thermostatically held
at 28 °C. Water was added in a vaporization furnace by an injection
pump with the flow of 0.30 mL min−1, and evaporated at 120 °C. Thus,
the feed gas contained 140 ppm HCHO, simulated air (O2/N2 = 21:79)
and water vapor (25% relative humidity). The total flow rate was
50 mL min−1 and the gas hourly space velocity (GHSV) was
20,000 h−1. The gas flow rate was regulated by a mass flow controller.
The HCHO concentration in the feed gas and exit gas streams was de-
2. Experimental
2.1. Synthesis of Pdx/M (x = 0.5, 1 wt%, M = KTO-NB, NTO-NB and
TiO2-NB) catalysts
Firstly, NTO-NB was synthesized by a typical hydrothermal process
[22]. KTO-NB and H2Ti3O7 nanobelt (HTO-NB) were then synthesized
through hydrothermal treating of NTO-NB in KOH solution and acid-
treating of NTO-NB, respectively [22,25]. The detailed experimental
procedure is provided in the Electronic Supporting Information (ESI).
Pdx/NTO-NB catalysts were prepared by the deposition-precipita-
tion method, using PdCl2 as the precursor compound. NTO-NB was
dispersed evenly in PdCl2 solution under stirring, and appropriate
amount of urea was then dropped to adjust the pH of precursor solution
(urea/Pd = 200 M ratio, 100 mL). Next, the suspension was thermo-
statically kept at 80 °C for 4 h under vigorous magnetic stirring in the
dark. The resulting product was filtered and washed with ultrapure
water for several times, then dried in air at 80 °C for 12 h, and then
treated at 400 °C for additional 2 h with a 5 °C min−1 heating rate
under pure H2 gas flow.
termined
by
the
phenol
reagent
colorimetric
method
(GBT18204.2–2014, china).
3. Results and discussion
As expected, typical Na-titanate nanobelts with
a
width of
50–200 nm and length of up to dozens of micrometers were synthesized
by the hydrothermal method. A very similar one-dimensional mor-
phology is seen for acidified H2Ti3O7 nanobelts, Pd/NTO-NB and Pd/
TiO2-NB samples (Fig. 1, Fig. S2 in ESI). Powder XRD characterization
shows that the diffraction peaks of Na2Ti6O13 and Na2Ti3O7 appear in
the pattern of Pd/NTO-NB sample after H2-treatment at 400 °C (Fig. S3,
ESI). For the Pd/TiO2-NB sample, the observed diffraction peaks are all
attributed to TiO2 (B) (Fig. S3, ESI), a dehydrated product of H2Ti3O7 at
lower calcination temperature. Pd/KTO-NB sample displays a different
morphology, composed of narrow nanobelts mixed with incompletely
splitting wide nanobelts (Fig. 1). This indicates that the applied hy-
drothermal treatment in KOH solution resulted in Na-titanate nanobelts
splitting into narrow K-titanate nanobelts due to bulk phase transfor-
mation [21]. The transformation process is incomplete. For Pd1/KTO-
NB sample, K2Ti6O13 peaks appear in the XRD pattern but no Na2Ti6O13
peaks appear, and weaker Na2Ti3O7 reflections still can be observed
(Fig. S3, ESI). For all samples, no diffraction peaks of metallic Pd or PdO
were detected, due to the low metal loading used and the high dis-
persion of Pd species formed. ICP-AES analysis gave a 0.34–0.75 wt%
Pd-loading for the different samples (Table S1, ESI).
Pdx/KTO-NB and Pdx/TiO2-NB catalysts were synthesized by the
same process as the synthesis of Pdx/NTO-NB, except using KTO-NB and
HTO-NB as support, respectively.
2.2. Catalysts characterization
The metal loading in the as-prepared catalysts was analysed using
inductively coupled plasma atomic emission spectroscopy (ICP-AES).
Powder X-ray diffraction (XRD) analysis of the samples was conducted
on a PANalytical X'pert3 powder diffractometer using Cu Kα radiation.
Scanning electron microscope (SEM) and energy dispersion spectra
(EDS) were acquired on a Zeiss Gemini300 scanning electron micro-
scope. Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM) images were obtained with
a JOEL JEM 2100 microscope. Brunauer-Emmett-Teller (BET) surface
areas of the samples were measured by nitrogen adsorption-desorption
isotherms at −196 °C using a Builder SSA-4200 apparatus. Diffuse re-
flectance infrared Fourier transform spectroscopy (DRIFTS) measure-
ments were carried out on a BRUKER VERTEX-70 FTIR. X-ray photo-
electron spectroscopy (XPS) measurements were carried out with a
As shown in SEM images of Pd/NTO-NB, Pd/TiO2-NB and Pd/KTO-
NB samples (Fig.
1 and Fig. S2 in ESI), their one-dimensional
2

L. Zhou, et al.
CatalysisCommunications142(2020)106034
Fig. 2. Typical TEM images and the corresponding cluster size distributions of as-prepared Pd1/KTO-NB, Pd1/NTO-NB, Pd1/TiO2-NB and Pd1/KTO-NB after reaction
(Pd1/KTO-NB-A).
nanostructures with large aspect ratio enable the formation of a high
porosity structure by overlapping with each other. Such uniform and
highly porous structures are favorable for improving mass transfer rates
in a heterogeneous catalytic reaction process. The HRTEM images of
Pd1/KTO-NB, Pd1/NTO-NB and Pd1/TiO2-NB all show that a large
amount of tiny Pd clusters is uniformly distributed on nanobelts, and
their corresponding average diameters are ~1.2, ~1.5 and ~1.7 nm,
respectively (Fig. 2). In particular, the Pd clusters with size less than
1 nm are more numerous in Pd1/KTO-NB and Pd1/NTO-NB catalysts
compared to Pd1/TiO2-NB catalyst. It demonstrates that the existence of
interlayer Na+ and K+ ions prompt the dispersion of Pd species on the
KTO-NB and NTO-NB supports. In addition, DRIFTS spectra of CO ad-
sorption presented a larger proportion of linear CO IR band for the Pd1/
NTO-NB than Pd1/TiO2-NB catalyst, also suggesting for a better dis-
persion of Pd species on the NTO-NB support (Fig. S4, ESI) [26]. Such
result agrees with previous findings that the addition of Na+ ions into
Pd/TiO2 can markedly increase the dispersion of Pd nanoparticles
[17,18]. Pd1/KTO-NB catalyst gave the smallest Pd clusters in size,
which is related to the much smaller diameter of KTO-NB. The mea-
of reactive Pd clusters when exposed to the air during sample handling.
In more detail, for Pd1/TiO2-NB sample, Pd 3d5/2 spectra can be fitted
to two peaks of 335.5 and 337.0 eV, identified as Pd0 and PdO, re-
spectively [28,29]. Similarly, the binding energies of Pd 3d5/2 in Pd1/
NTO-NB and Pd1/KTO-NB can also be deconvoluted into two peaks, ca.
335.1 and 337.6 eV, as well as 335.0 and 336.3 eV, corresponding to
Pd0 and PdO respectively [14,17]. The lower Pd0 binding energies and
higher Pd0/PdO ratio (Table S2, ESI) appeared in Pd1/NTO-NB and
Pd1/KTO-NB indicate that Pd clusters on NTO-NB and KTO-NB are
more negatively charged than those on TiO2-NB. This result might be
related to the electron donating effect of interlayer alkali metals
[18,30]. As regards Pd1/NTO-NB and Pd1/KTO-NB, there is a charge
transfer from alkali metal to the TiO6 octahedron planes which leads to
an increase in the electron density at Ti atoms [17,31]. Indeed, the
binding energies of Ti 2p in both Pd1/NTO-NB and Pd1/KTO-NB are
negatively shifted to 458.3 eV in comparison to that in Pd1/TiO2-NB
(458.6 eV). This electron donating effect of interlayer alkali metals
could then be transferred to Pd species, making Pd clusters more ne-
gatively charged.
The as-synthesized one-dimensional KTO-NB, NTO-NB and TiO2-NB
supported palladium clusters were applied in the room temperature
HCHO oxidation, and results are shown in Fig. 4. Evidently, all three
catalysts can effectively catalyze HCHO oxidation at ambient tem-
perature (Fig. 4a). The catalytic activities are attributed to the presence
of Pd species, because pure KTO-NB, NTO-NB and TiO2-NB showed no
activity in this reaction. Pd1/KTO-NB exhibited the best catalytic per-
formance, and a HCHO conversion larger than 97% was achieved at
room temperature. Pd1/NTO-NB, giving ~90% HCHO conversion after
7 h of reaction, also is more active than Pd1/TiO2-NB, showing ~48%
conversion. The same dependence of activity on support was observed
on the 0.5 wt% Pd loading samples. A conversion of ~85% and ~ 70%
of HCHO was respectively achieved over Pd0.5/KTO-NB and Pd0.5/NTO-
NB after 7 h of reaction, while a lower conversion of ~25% was
sured BET surface area of Pd1/KTO-NB (~55 m2
larger than those of Pd1/NTO-NB (~36 m2 −1) and Pd1/TiO2-NB
(~23 m2 −1). Such KTO-NB surface has more active sites for Pd
g
−1) is significantly
g
g
clusters nucleation, and is in favor of the formation of smaller clusters
in size. On the other hand, the different surface electron structure of the
three nanobelts may also influence the dispersion of Pd species due to
strong interactions between Pd species and the support.
The impact of support on the electron structure of Pd/KTO-NB, Pd/
NTO-NB and Pd/TiO2-NB catalysts was investigated by XPS analysis,
and results are presented in Fig. 3. For all three as-prepared samples, Pd
3d XPS peak can be deconvoluted into two parts, metallic Pd and PdO,
demonstrating that Pd clusters are comprising of metallic Pd as main
component along with PdO. The latter comes from either the in-
complete hydrogen reduction of Pd2+ [27] or the inevitable oxidation
3

L. Zhou, et al.
CatalysisCommunications142(2020)106034
Fig. 3. XPS spectra of Pd 3d and Ti 2p in as-prepared Pd1/NTO-NB, Pd1/TiO2-NB, Pd1/KTO-NB and Pd1/KTO-NB after 30 h reaction (Pd1/KTO-NB-A).
obtained on Pd0.5/TiO2-NB. The higher activity observed in Pd/KTO-NB
and Pd/NTO-NB than in Pd/TiO2-NB can be attributed to the interlayer
Na+ and K+ ions in NTO-NB and KTO-NB. As discussed above, for Pd
species in Pd/KTO-NB and Pd/NTO-NB catalysts, the existence of Na+
and K+ ions increases their dispersion and makes them more negatively
charged. The best catalytic performance of Pd/KTO-NB catalysts largely
originates from their smallest Pd clusters size and the largest Pd0/Pd
ratio among the series of palladium catalysts tested. In general, smaller
Pd nanoparticles have more abundant active sites which favor the
dissociative adsorption of oxygen on the catalyst surface, and lead to
higher HCHO oxidation rates [19,32]. On the other hand, the nega-
tively charged Pd species could enhance the electron transfer from Pd to
the π* orbital of O2, further promoting O2 adsorption and HCHO oxi-
dation [18,30].
In order to further clarify the nature of catalytic active Pd species,
the catalytic performances of O2-pretreated PdO1/KTO-NB, PdO1/NTO-
NB and PdO1/TiO2-NB samples were tested. Much lower HCHO con-
versions of ~30%, ~ 20% and ~ 8% were achieved, respectively (Fig.
S5, ESI). Moreover, all the fresh Pd1/TiO2-NB-F, Pd1/NTO-NB-F and
Pd1/KTO-NB-F samples without hydrogen reduction treatment also
gave poor HCHO conversion, below ~20% (Fig. S5, ESI). This result
agrees with the previous findings that metallic Pd is the catalytically
active species in palladium catalysts for room temperature HCHO oxi-
dation, and not Pd2+ ions or PdO species [18,33].
Pd1/KTO-NB catalyst kept this excellent catalytic activity in a second
reaction cycle, where almost complete HCHO conversion was still ob-
served (Fig. S6, ESI). Such high catalytic stability may relate to the
capability of KTO-NB to stabilize small sized Pd clusters and metallic Pd
species. As shown in Fig. 2b, for the Pd1/KTO-NB catalyst underwent
30 h of reaction, the mean diameter of Pd clusters increased slightly
from ~1.2 to ~1.4 nm, suggesting a weaker agglomeration of Pd
clusters during reaction. The almost unchanged Pd cluster size in Pd1/
KTO-NB catalyst before and after reaction can be ascribed to the strong
interaction between Pd species and KTO-NB. This strong interactions
makes the Pd clusters in Pd1/KTO-NB catalyst most negatively charged
and smallest-sized. Also, the latter catalyst showed best sintering re-
sistance among the three catalysts.
In comparison, Pd1/NTO-NB and Pd1/TiO2-NB catalysts showed a
more significant increase in Pd clusters size during reaction, and the
mean diameter of Pd clusters increased from 1.5 and 1.7 nm to 2.1 and
2.3 nm after reaction for 30 h, respectively (Fig. S7, ESI). The catalytic
activity of these two solids presented a distinct decrease with reaction
time (Fig. 4b). On the other hand, the XPS analysis revealed that the
binding energy of Pd0 in the used Pd1/KTO-NB catalyst (Pd/KTO-NB-A)
presents a little positive shift, and the PdO content slightly increased
from 39.54 to 39.64% (Table S2, ESI). This result indicates that the Pd
clusters on K-titanate nanobelts could keep their electronic structure
almost unchanged upon 30 h of continuous reaction, demonstrating a
high stability of metallic Pd species during reaction.
Pd/KTO-NB catalyst presented the highest catalytic stability among
the three prepared palladium catalysts. Also, an almost complete HCHO
conversion was achieved with a GHSV of 20,000 h−1 and a HCHO inlet
concentration of 140 ppm at ambient temperature, and this was
maintained over 30 h time-on-stream (Fig. 4b). Furthermore, the used
4. Conclusions
In summary, supported palladium catalysts were prepared by the
Fig. 4. (a) The comparison of HCHO oxidation over pure S, Pd0.5/S and Pd1/S catalysts (S = TiO2-NB, NTO-NB and KTO-NB). (b) Catalytic stability test of Pd1/TiO2-
NB, Pd1/NTO-NB and Pd1/KTO-NB catalysts. Reaction conditions: HCHO concentration 140 ppm, 21/79 of O2/N2, total flow rate 50 mL min−1, catalysts of 50 mg,
reaction temperature 25 °C, GHSV = 20,000 h−1
.
4

L. Zhou, et al.
CatalysisCommunications142(2020)106034
deposition-precipitation method using K-titanate, Na-titanate and TiO2
nanobelts as supports. The presence of interlayer alkaline metal could
significantly promoted the dispersion of Pd species on NTO-NB and
KTO-NB carriers and made Pd clusters negatively charged simulta-
neously in comparison with the TiO2-NB support, which contributed to
the enhancement of catalytic performance for the room temperature
HCHO oxidation reaction. Especially, when KTO-NB is formed by in-
troducing K+ ions substituting the Na+ in NTO-titanate, the smallest
sized and more negatively charged Pd clusters were obtained, which
gave the highest catalytic activity and stability. This suggests that
metallic Pd should be the active species in these alkali titanate sup-
ported palladium catalysts. The use of one-dimensional alkali titanate
nanostructures as supports may provide a new approach to construct
high-efficiency noble metal catalysts.
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Acknowledgements
We acknowledge primary financial support from the National
Natural Science Foundation of China (21176144, 21977064,
51732007), the National Key Research and Development Program of
China (2017YFE0102700), and the Science Fund for Distinguished
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Declaration of Competing Interest
There are no conflicts to declare.
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