Catalytic oxidation of formaldehyde in water by calcium phosphate-based Pt composites

Article abstract of DOI:10.1039/c5ra00353a

Platinum nanoparticles (PtNPs) protected by dendrimer were incorporated in calcium phosphate particles, and the degradation of a pollutant, formaldehyde (HCHO), in water was investigated by using the resultant composite powders as a catalyst. The reaction was performed with dissolved oxygen from air as an oxidant. The analytical results revealed that high PtNP and HCHO concentrations and a high temperature effectively accelerated the oxidation of HCHO. The results were also kinetically analyzed using the Elovich equation. Additionally, the complete oxidation process of HCHO could be concluded to depend on the adsorption process of HCHO on the catalyst. The present system is a valuable catalyst for use with atmospheric oxygen at atmospheric pressure and at mild temperatures, and more importantly this catalytic system can easily be removed from the reaction solution.

Full text of DOI:10.1039/c5ra00353a

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PAPER
Catalytic oxidation of formaldehyde in water by
calcium phosphate-based Pt composites†
Cite this: RSC Adv., 2015, 5, 15944
a
ab
*
Yakub Fam and Toyoko Imae
Platinum nanoparticles (PtNPs) protected by dendrimer were incorporated in calcium phosphate particles,
and the degradation of a pollutant, formaldehyde (HCHO), in water was investigated by using the resultant
composite powders as a catalyst. The reaction was performed with dissolved oxygen from air as an oxidant.
The analytical results revealed that high PtNP and HCHO concentrations and a high temperature effectively
accelerated the oxidation of HCHO. The results were also kinetically analyzed using the Elovich equation.
Additionally, the complete oxidation process of HCHO could be concluded to depend on the adsorption
process of HCHO on the catalyst. The present system is a valuable catalyst for use with atmospheric
oxygen at atmospheric pressure and at mild temperatures, and more importantly this catalytic system
can easily be removed from the reaction solution.
Received 8th January 2015
Accepted 21st January 2015
DOI: 10.1039/c5ra00353a
www.rsc.org/advances
use of additives and the subsequent generation of by-products.
Nevertheless, there is only a limited number of reports con-
cerning the catalytic oxidative degradation of HCHO in an
aqueous solution,13,16,26–29 that is, oxidation using metal
Introduction
Since the discovery of formaldehyde (HCHO) in 1855,1 HCHO
has been regarded as one of the most versatile basic chemicals
in various industries2–5 and in day-to-day products.6–9 Although
it displays such versatility, HCHO is a well-known toxic chem-
ical10 and an allergen in allergic contact dermatitis11 and is
certied as a carcinogen.12 Owing to its global applications,
HCHO is always released into the environment, particularly via
industrial wastewater.13 In order to prevent such ecological
issues, it is imperative that the effective and environmentally-
friendly maintenance of wastewater is performed before its
discharge. One of the most common methods is the biological
process14,15 but it always needs pretreatment by another
method.16 Air oxidation17 usually requires high temperatures
(180–315 ꢀC) and pressures (20–150 bar).18 The advanced
oxidation methods, which involve ozonation, UV irradiation,
the Fenton process, or a combination of these,19,20 have some
disadvantages, such as the requirement of a very low pH,20–23 the
interference from particulate matter during UV irradiation,24
their propensity to produce by-products,21 and the usage of a
dangerous and unstable oxidant.25
compound or composite catalysts at high temperatures (150–
13,26,27
200 ꢀC) and in the presence of excess air or O2
and
degradation in an O2-saturated solution or under O3 ow at
25 ꢀC by utilizing a metal oxide catalyst.16,29 However, it should
be noted that in these procedures demanding excess or enough
oxidizing agents, the low solubility of O2 at atmospheric pres-
sure might reduce the oxidation rate.28
It is necessary for catalysts such as those described above to
be recovered/removed from the effluent aer the reaction. On
the other hand, the catalysts can be created as a system, which
can be easily separated from the effluent using a catalyst-
supporting matrix; the mass transfer effects in the oxidation
of HCHO at a mild temperature (75 ꢀC) have been examined
using a Pt catalyst mounted on a porous polymer and under air
ow, although the degradation of HCHO was low (41%).28
Therefore, there is a demand for the development of Pt-
mounting catalytic systems.
Pt nanoparticles protected by dendrimers (DENPtNPs) have
been loaded on carbon nanotubes30,31 and used as a catalyst for
the methanol oxidation reaction.32 DENPtNPs have also been
embedded in mesoporous particles consisting of calcium
phosphate (CaP)33 as an analog of a dendrimer porogen.34
Therefore, CaP plays a role in supporting the Pt catalyst in a
novel composite consisting of CaP and DENPtNPs (CaP–
DENPtNP composite). The aim of the present study was to
investigate the catalytic performance of the CaP–DENPtNP
composites. Moreover, other variables, such as the PtNPs
content, HCHO concentration, reaction temperature, and
preparation pH were examined so as to explore the kinetics of
The use of a catalyst is an alternative and promising solution
for all the foregoing issues, since catalytic reactions can be
carried out under mild operating conditions, by eliminating the
aDepartment of Chemical Engineering, National Taiwan University of Science and
Technology, 43 Section 4, Keelung Road, Taipei 10607, Taiwan, Republic of China.
E-mail: imae@mail.ntust.edu.tw
bGraduate Institute of Applied Science and Technology, National Taiwan University of
Science and Technology, 43 Section 4, Keelung Road, Taipei 10607, Taiwan, Republic
of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra00353a
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these catalytic systems. The as-prepared catalytic system ach- spectrometer at a scan speed of 200 nm minꢂ1 with a 1 cm
ieves the effective catalytic decomposition of HCHO in water at quartz cell.
a low temperature and under atmospheric pressure.
The solution (with an initial HCHO concentration of 4.7
ppm) aer the reaction (of ESI Scheme S1†) exhibited an
absorption band at 412 nm (see Fig. S1†), and this band was
hereaer utilized as an indicator band to determine the HCHO
concentration. Incidentally, the CA solution had no absorption
band in the measured wavelength region, and this solution was
hereaer used as a reference solution. When the absorbance at
412 nm was plotted against the HCHO concentration, a linear
relationship with an adequate correlation coefficient of 0.996
was obtained between them (see Fig. S2†).
Experimental
Materials
Calcium nitrate (Ca(NO3)2$4H2O, 99+%), diammonium
hydrogen phosphate ((NH4)2HPO4, 99+%), fourth generation
amine-terminated poly(amide amine) dendrimer (DEN, 10 wt%
solution in methanol), sodium hexachloroplatinate hexahy-
drate (Na2PtCl6$6H2O, 98%), an aqueous ammonia solution
(NH4OH, 35%), an aqueous hydrochloric acid solution (HCl,
35%), sodium borohydride (NaBH4, 98%), sodium hydroxide
(NaOH, 98%), an aqueous HCHO solution (37%), acetic acid
(CH3COOH, glacial), acetylacetone (pentane-2,4-dione, 99%),
and ammonium acetate (CH3COONH4, anhydrous) were of
analytical grade and commercially available. Ultrapure water
(18.2 MU cm resistivity) was utilized throughout all of the
experiments.
DENPtNPs were prepared according to the previously
reported procedure.31,32 Na2PtCl6 was added to an aqueous DEN
solution. Aer stirring for 3 days, a reducing agent, NaBH4, in a
NaOH solution was added and stirred for 1 day. CaP–DENPtNP
composite powders were synthesized using the hydrothermal
method.33 Typically, an aqueous DENPtNP solution was mixed
with an aqueous (NH4)2HPO4 solution and stirred for 3 h.
Subsequently, an aqueous Ca(NO3)2 solution was added with
vigorous stirring, and the mixed solutions were adjusted to pH
12, 8, and 4. The resultant precipitates were then annealed in an
autoclave at 150 ꢀC for 15 h, and the powders were separated by
centrifugation, rinsed and dried. These composites are
described as catalyst-pHx (x ¼ 12, 8, 4) hereaer. The optimi-
zation of the formation of the CaP–DENPtNP composites and
their characterization have been investigated by means of
transmission electron microscopy, dynamic light scattering,
laser Doppler electrophoresis, Fourier transform infrared
absorption spectroscopy, energy dispersive X-ray spectroscopy,
thermogravimetric analysis, wide-angle X-ray diffraction, small
angle X-ray scattering, and surface area analysis.33
Catalytic oxidation of formaldehyde
A limited volume (25 mL) of the HCHO solution was put into a
250 mL volumetric ask as a reactor. Successively, a catalyst-
pHx was added with a specied mass ratio between the catalyst
and HCHO (0.5 : 1, 2 : 1, or 4 : 1). Then the ask was capped to
avoid the evaporation of HCHO and water. Aer a certain
reaction time, up to 48 h, the reaction solution was used for the
spectrophotometric determination of the decomposed HCHO
concentration aer treatment with the CA solution, as
described above, and using the calibration curve prepared from
the absorbance at 412 nm (Fig. S2†).
Results and discussion
Oxidation of formaldehyde by a catalyst prepared at pH 8
When HCHO reacts chemically with acetylacetate and the
ammonium ion in the CA solution (see the reaction in Scheme
S1†), the reaction solution is able to take on a visible color.
From this phenomenon, the quantitative HCHO concentration
can be spectrophotometrically measured. Then, the degrada-
tion of HCHO by the catalytic oxidation reaction was evaluated,
based on the HCHO concentration remaining in the system by
means of the calibration curve in Fig. S2.† The oxidation reac-
tion HCHO + O2 / CO2 + H2O was carried out at 25 ꢀC, using O2
in air as an oxidizing agent for a 10 ppm initial concentration of
HCHO and by adding catalyst-pH8 prepared at a PO43ꢂ : NH2
molar ratio of 1 : 1.
Standard calibration curve for formaldehyde concentration
Spectrophotometric determination of the HCHO concentration
began with the preparation of a standard calibration curve. An
aqueous solution of the colorimetric agent (CA) was prepared by
dissolving solid CH3COONH4 (15 g) in water, followed by add-
ing 0.3 mL of CH3COOH and 0.2 mL of acetylacetone. Then, the
mixture was diluted with water to a total of 100 mL CA solution.
Stock solutions of HCHO (10, 100 or 1000 ppm) were prepared
by diluting an aqueous HCHO solution (37 wt%) with water.
Then, portions of these HCHO solutions were mixed with 5 mL
of the CA solution, and subsequently immersed for 30 min in a
thermostatic water-bath at 40 ꢁ 2 ꢀC, followed by cooling for 30
min at room temperature. Absorption spectrum measurements
Fig. 1 Effects of stirring and crushing in a mortar on HCHO decom-
were carried out on a Jasco V-670 series UV absorption position at a catalyst-pH8 : HCHO mass ratio of 2 : 1.
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In order to verify the effectiveness of a CaP–DENPtNP cata-
Using the optimum amount of catalyst is also of high
lyst, the catalytic performance was compared to that of the importance in order to get a highly effective result. For this
control (without the catalyst) and the CaP (without PtNPs) purpose, the amount of catalyst-pH8 was varied against the
particle. The results (Fig. 1) showed that while the control and concentration of HCHO. The results shown in Fig. 2(A) indi-
the CaP particle did not exhibit any catalytic activity in the cated that increasing the amount of the catalyst gave an
degradation of HCHO, the catalyst-pH8 demonstrated activity. increase in HCHO decomposition during the oxidation process.
Since the control showed a slight decrease in HCHO weight (by The amounts of HCHO decomposed over the 48 h reaction were
3.8 wt%) over the 48 h reaction, possibly due to the evaporation 23.5, 45.6 and 51.5 wt% for catalyst-pH8 : HCHO ¼ 0.5 : 1, 2 : 1,
of HCHO, this value then needed to be subtracted from all of the and 4 : 1, respectively. This tendency is reasonable, because a
other samples. Therefore, the decomposition of HCHO was 3.0 larger amount of catalyst provides a greater number of active
wt% for a CaP particle and 41.8 wt% for catalyst-pH8 aer the 48 sites, where the reaction takes place easily, and thus the reac-
h reaction. These results suggest that the PtNPs play a key role tion is faster. However, the tendency is almost saturated above a
in oxidative HCHO degradation.
molar ratio of 2 : 1, as seen in Fig. 2(B).
The investigation of the oxidation of HCHO was also per-
formed to study the effects of stirring on the HCHO solution
and of crushing the catalyst in a mortar. The results shown in
Fig. 1 signied that stirring considerably accelerated the
oxidation of HCHO. The decomposition of HCHO over the 48 h
reaction under stirring reached 34.1 wt%, while it was only 11.3
wt% without stirring. This is because HCHO and O2 are
frequently brought into close proximity on the surface of the
catalyst under stirring and the dissolution of oxygen from the
air in water is also promoted. However, the speed of stirring and
the crushing of the catalyst in a mortar were not inuential.
Hereaer, catalysts crushed in a mortar were used for the
oxidation reaction under stirring at 750 rpm.
Comparison of the oxidation performance by catalysts
prepared at pH 12, 8 and 4
The preparation of each catalyst-pHx, namely, of CaP–DENPtNP
composites at different pH values (12, 8, and 4) brought about
the differences in their structural characteristics.33 That is, the
CaP–DENPtNP composites possessed rod shapes consisting of
hydroxyapatite crystalline CaP at pH 12 and 8, but the rod
composites at pH 4 were constructed of monetite CaP crystals.
With decreasing pH, the particle sizes of the composites and the
number of PtNPs in the composites increased. Therefore, the
catalytic performance should vary among the catalysts. The
reaction was performed using an oxidant, O2, in air dissolved in
a reaction solution, where different species of catalyst-pHx
(x ¼ 12, 8 and 4) were added at a catalyst : HCHO mass ratio of
2 : 1. The variable conditions were the PO43ꢂ : NH2 molar ratio
in the catalyst-pHx samples and the initial HCHO concentration
in the reaction solutions.
The HCHO decomposition experiments in Fig. 3 were per-
ꢀ
formed at 25 C in an HCHO solution (with an initial concen-
tration of 10 ppm) using catalyst-pHx at a PO43ꢂ : NH2 molar
ratio of 1 : 1. Catalyst-pH4 presented the highest catalytic
activity (93.7 wt% aer 48 h) in the decomposition of HCHO,
whereas the activity of catalyst-pH12 (69.4 wt% aer 48 h) was
lower than that of catalyst-pH4 but unexpectedly higher than
that of catalyst-pH8 (45.6 wt% aer 48 h). The PtNPs should play
a key role as a catalyst in the decomposition of HCHO.
Fig. 2 (A) Effect of the catalyst concentration on HCHO decompo- Fig. 3 Effect of the catalyst species on HCHO decomposition using
sition. (B) A plot of HCHO decomposition against the molar ratio of catalyst-pHx for reaction solutions at a PO43ꢂ : NH2 molar ratio of 1 : 1,
catalyst-pH8 : HCHO.
Co ¼ 10 ppm and at 25 ꢀC.
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Therefore, catalyst-pH4 is able to decompose the most HCHO increasing the HCHO concentration resulted in the fast
molecules in the system due to its very high PtNPs content (60.2 decomposition of HCHO. It has been reported that an increase
wt%).33 However, the trend in increasing HCHO decomposition in HCHO concentration lowers the surface tension of the reac-
from catalyst-pH8 to catalyst-pH12 was inconsistent with the tion liquid, which in turn brings favorable effects in terms of the
trend in PtNPs content. Namely, catalyst-pH8 possessed a mass transfer of O2 to the catalyst surface and thus promotes
higher PtNPs content (17.4 wt%) than catalyst-pH12 (15.2 the oxidation reaction.28 In the present case, equally, the
wt%).33 Consequently, it is obvious that other factors, besides decrease in the surface tension in the solution of high HCHO
PtNPs content, affect HCHO oxidation. According to a previous concentration promotes the dissolution of a large amount of O2
report,33 the surface area (105.0 m2 gꢂ1) of catalyst-pH12 is gas and causes a large number of O2 molecules to bind with
higher than that of catalyst-pH8 (83.4 m2 gꢂ1), which is HCHO on the catalyst, allowing the total oxidation process
consistent with the trend in the activity performance for HCHO producing CO2 and H2O. Besides, Fig. 3 and 5 also substantiate
oxidation. Therefore, the surface area can be another key factor the effects of the catalysts by showing that the highest decom-
controlling the oxidation reaction, since the most HCHO and O2 position of HCHO was by catalyst-pH4, while catalyst-pH8
can adsorb on catalyst-pH12 but the adsorbed amount exhibited the lowest activity, similar to the effect of the PtNPs
decreases from catalyst-pH12 and catalyst-pH8 to catalyst-pH4 described above. Table 1 indicates that the catalytic reaction by
(surface area ¼ 62.2 m2 gꢂ1).33 Therefore, the catalytic perfor- catalyst-pH4 achieved 100 wt% decomposition within 24 h at a
mance should reect the competing effects of the PtNPs content 1000 ppm HCHO concentration and 99 wt% decomposition
and the surface area. Thus the catalytic performance was in the aer 48 h at a 100 ppm HCHO concentration.
order of catalyst-pH4 > catalyst-pH12 > catalyst-pH8.
Since the temperature is also one of the key factors in the
In order to conrm the effect of the PtNPs on the degrada- chemical reaction, a study of the activity of the system in HCHO
3ꢂ
tion of HCHO, the molar ratio between PO4 from the CaP degradation was performed at different temperatures. Fig. 3
precursors and the NH2 groups from the dendrimers in the and 6 and Table 1 point out that the elevated temperature of the
DENPtNPs nanoparticles was varied between 0.3 : 1 and 3 : 1, system sped up the decomposition of HCHO. This is due to the
for comparison with Fig. 3 at 1 : 1 catayst-pHx. As the concen- high frequency of collisions between reactants on the catalyst
3ꢂ
tration of the PO4 precursors decreased, namely, the relative and the increasing kinetic energy as the temperature rises,
concentration of NH2 groups in DENPtNPs increased, the rela- resulting in accelerated degradation. Table 1 also shows that
tive number of PtNPs consequently increased (for example, 5.1, catalyst-pH4 decomposed the most HCHO and that catalyst-
17.1 and 32.0 wt% in 3 : 1, 1 : 1, and 0.3 : 1, respectively, for pH8 decomposed the least, which is consistent with the cases
catalyst-pH8).33 Therefore, Fig. 3 and 4 indicate that the HCHO reported above. It should also be noted that the fastest
decomposition was accelerated as a result of the quantitative accomplishment of the decomposition reaction at 75 ꢀC was
increase in PtNPs, although the acceleration was always in the reached aer 16, 24, and 4 h by catalyst-pH12, catalyst-pH8 and
order of catalyst-pH4 > catalyst-pH12 > catalyst-pH8, for the catalyst-pH4, respectively, while the decomposition at 40 ꢀC was
reasons described above. In addition, Table 1 points out that achieved aer 16 h by catalyst-pH4 but not even aer 48 h by
catalyst-pHx achieved 96 wt% decomposition of HCHO under catalyst-pH12 and catalyst-pH8.
the experimental conditions of a 0.3 : 1 molar ratio, and that
such high degradation was achieved at 24, 48 and 8 h by cata-
Mechanism of the catalytic oxidation of formaldehyde
lysts of x ¼ 12, 8 and 4, respectively.
The effect of different HCHO concentrations on HCHO Several kinetic mechanisms for the oxidative degradation of
oxidation was also studied. Fig. 3 and 5 and Table 1 showed that HCHO have been presented in previous reports,13,29,35–37 ranging
from a simple and straightforward model to a very complicated
one. Christoskova et al.29 have suggested that the oxidation of
HCHO in an aqueous medium generates, at rst, an interme-
diate of formic acid (HCOOH), which is rapidly converted into
CO2. Sodhi, et al.35 have proposed that the gaseous HCHO binds
with O2 to produce CO2 and H2O at a molar ratio of O2 : HCHO
# 0.5, but that production at a molar ratio of > 0.5 produces a
signicant amount of the intermediate product CO, which
could proceed to form CO2 and H2O. Chuang et al.36 have stated
that gaseous HCHO on a hydrophobic catalyst is converted into
HCOOH and CO2 at 63 ꢀC, 100% CO2 is selectively obtained at a
temperature over 125 ꢀC, and 100% conversion is achieved at a
temperature over 150 ꢀC. Zhang, et al.37 have averred that
gaseous HCHO is completely oxidized into CO2 through inter-
mediate products (dioxymethylene and formate species). Silva,
et al.13 have claimed a modied generalized kinetic model,
including a non-oxidized compound (methanol as a stabilizer in
using catalyst-pHx in reaction solutions with Co ¼ 10 ppm and at 25 ꢀC. an HCHO solution) and an intermediate species (HCOOH),
Fig. 4 Effect of the PO43ꢂ : NH2 molar ratio on HCHO decomposition
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Table 1 Effects of the PO43ꢂ : NH2 molar ratio, initial HCHO concentration, temperature (T), and catalyst species on HCHO decomposition by
catalyst-pHx
Decomposed HCHO (wt%) at 48 h
Saturation time (h)
PO43ꢂ : NH2 molar
ratio
T
(ꢀC)
Co (ppm)
Catalyst-pH12
Catalyst-pH8
Catalyst-pH4
Catalyst-pH12
Catalyst-pH8
Catalyst-pH4
3 : 1
1 : 1
0.3 : 1
1 : 1
1 : 1
1 : 1
1 : 1
10
10
10
100
1000
10
25
25
25
25
25
40
75
42.2
69.9
95.6
85.6
97.0
85.4
96.8
31.5
45.6
95.9
67.9
89.2
65.1
95.5
52.8
91.8
95.3
98.7
99.9
97.5
98.6
24
48
16
48
24
8
48
24
16
4
10
Scheme 1 An illustration of the process of the catalytic oxidative
degradation of HCHO.
the HCHO solution. Both molecules of dissolved O2 and HCHO
in the solution are then adsorbed onto the catalyst active sites of
the PtNP surface, where the complete oxidation of HCHO by O2
happens to generate CO2 and H2O. The intermediate product,
HCOOH, was neglected in this case, because the consumption
rate of HCOOH was very high in comparison with the rate of its
formation.29 Although the presence of a stabilizer (methanol) in
an aqueous HCHO solution cannot be ignored, it was consid-
ered as a refractory species throughout the reaction because of
its non-oxidized characteristic.13 The high activation energy of
methanol between 395.0 and 478.6 kJ molꢂ1 in the temperature
range of 450–550 ꢀC and at a pressure of 246 bar38 makes it hard
for this compound to be oxidized, especially under the mild
conditions of the present experiment.
Fig. 5 Effect of the initial HCHO concentration on HCHO decom-
position using catalyst-pHx for reaction solutions at a PO43ꢂ : NH2
molar ratio of 1 : 1 and 25 ꢀC.
Kinetic analysis of formaldehyde oxidation
The degradation of HCHO was investigated as a function of the
reaction time. The plots in Fig. 1–6 show the gradual degrada-
tion of HCHO with time but with different degradation rates
depending on the systems. Therefore, quantitative analysis was
performed to comprehend the degradation kinetics of HCHO in
the present case. Thus, based on the mechanism described
above, the possible kinetic steps that occurred during the
degradation were assumed to be:
Fig. 6 Effect of the temperature on HCHO decomposition using
catalyst-pHx for reaction solutions (with an initial concentration of 10
ppm) at a PO43ꢂ : NH2 molar ratio of 1 : 1 and Co ¼ 10 ppm.
which is decomposed slowly to CO2 as the by-product. Overall,
those reports have this in common: an oxidized HCHO turns
into CO2 and H2O in the end. Based on those reports, the
reaction mechanism in this experiment was generally assumed.
The detailed mechanism is illustrated in Scheme 1, begin-
ning with the dissolution of O2 gas from the atmosphere into
Diffusion: O2 in bulk gas / dissolved O2 in liquid
Adsorption: HCHO + catalyst / HCHO–catalyst
Surface reaction: HCHO–catalyst + dissolved O2 / CO2[
+ H2O + catalyst
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Fig. 7 Replotting of Fig. 3 and 4 based on eqn (5).
In order to form kinetic equations, it is crucial to initially oxidation performance by catalysts prepared at different pH
determine the rate-determining step in the above reactions, values, as described above.
starting from the diffusion process of O2. The dissolution of a
It was rst assumed that the surface reaction is the rate-
gas into a liquid through a gas–liquid interfacial area (a determining step, and quantitative analysis was carried out
diffusion process) depends on the stirring of the liquid, and using the procedure that follows.13 Since the O2 molecule used
the mass transfer of the dissolved gas to the external surface of in this experiment comes from the gas phase atmosphere, the
the catalyst depends on the size of the catalyst particle.28 Since order of the reaction with respect to excess O2 can be assumed
the variation in the stirring speed did not have any signicant to be zero and the complete oxidation of HCHO is analyzed
effect on the reaction rate (Fig. 1), the diffusion was a very fast using a rst-order reaction equation, as follows:
process, so it was not taken as the rate-determining step.
dC
r ¼ ꢂ rHCHO ¼ ꢂ
¼ kC
(1)
Therefore, the rate-determining step will be either the surface
reaction, where the complete oxidation process takes place, or
the adsorption of HCHO onto the active sites of the catalyst.
These steps are related to the PtNPs content in the catalysts
and the surface area of the catalysts, and these competitive
contributions are apparent from the comparison of the
dt
where r is the overall reaction rate, ꢂrHCHO is the oxidative
degradation rate with respect to HCHO, C is the remaining
concentration of HCHO in the system at a degradation time t,
and k is the degradation rate constant. The integration of eqn
(1) was then executed to obtain the following equation:
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Fig. 8 Replotting of Fig. 3 and 5 based on eqn (5).
ꢀ
ꢁ
where r is the overall reaction rate, rH0 CHO is the adsorption rate
with respect to HCHO, C0 is the concentration of HCHO
adsorbed at time t, b is the initial adsorption rate, and g is the
Elovich constant.39 The integration of eqn (3) was then per-
formed to obtain the following equation:
C
ꢂln
¼ kt
(2)
Co
where Co is an initial concentration of HCHO.
The experimental results of HCHO concentration versus time
were replotted based on eqn (2) and each trend line was
subsequently drawn. Although eqn (2) had to be a linear curve
coming across the origin (0,0), the accuracies (R2 values) of the
plots were rather low (see Fig. S3†). Hence, the rst assumption
was regarded to be invalid.
lnð1 þ gbt Þ
C0 ¼
(4)
g
To simplify the equation, it was assumed that gbt [ 1 and
Next, the adsorption of HCHO on the surface of the catalyst
was considered to be the rate-determining step, and quantitative
analysis was carried out based on the procedure that follows.39
The adsorption of HCHO on the catalyst surface was analyzed
using an Elovich equation, which is generally expressed as:
eqn (4) was linearized, resulting in:
1
1
C0 ¼ lnðgbÞ þ
ln t
(5)
g
g
The experimental results of the HCHO concentration versus
time were replotted based on eqn (5) and each trend line was
dC0
0
r ¼ r0HCHO
¼
¼ beꢂgC
(3)
dt
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Fig. 9 Replotting of Fig. 3 and 6 based on eqn (5).
subsequently drawn (Fig. 7–9). The linearity was approved with
Table 2 shows that the b value increased by between 2.5 times
high accuracies (R2 values). Then the g value was calculated as an and two orders of magnitude in the order of catalyst-pH8 <
inverse of the tilt of the resulting lines and the b value was catalyst-pH12 < catalyst-pH4, reecting the variation of reaction
obtained from their intercept, as listed in Table 2.
as seen in Fig. 3–6, whereas the g value decreased in this order
Table 2 Effects of the PO43ꢂ : NH2 molar ratio, initial HCHO concentration, temperature (T), and catalyst species on the kinetic parameters of
HCHO decomposition by catalyst-pHx
b (ppm hꢂ1
)
g
PO43ꢂ : NH2 molar
ratio
T
(ꢀC)
Co (ppm)
Catalyst-pH12
Catalyst-pH8
Catalyst-pH4
Catalyst-pH12
Catalyst-pH8
Catalyst-pH4
3 : 1
1 : 1
0.3 : 1
1 : 1
1 : 1
1 : 1
1 : 1
10
10
10
100
1000
10
25
25
25
25
25
40
75
1.49
3.46
14.3
32.8
299
0.889
1.82
3.79
22.6
198
3.79
4.53
50.8
86.1
1670
13.6
1.02
1.44
1.12
1.03
0.683
0.532
0.0544
0.00425
0.406
0.793
0.470
0.527
0.0499
0.00491
0.394
0.837
0.433
0.0754
0.00477
0.542
0.361
69.9
421
1.05
2.83
10
2150
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Table 3 Comparison of the catalytic oxidative degradation of HCHO in an aqueous solution
Temperature
Reaction time
(h)
Catalyst
Removal (%)
Oxidant
(ꢀC)
Year
1988
Reference
26
Ru/Ce
Cu(NO3)2
CuO–ZnO/Al2O3
CeO2, Ag/Ce
Co/Ce
Mn/Ce
Nix(OH)yOz$mH2O
MgO
Pt on porous polymer
CaP–DENPtNPs
96
24
80
55
70
99
90
79
41
95
99
1 MPa (air)
150
1
1.5 MPa (O2)
1.5 MPa (O2)
200
200
3
3
2003
2003
13
27
Continuous ow (air)
0.153 g Lꢂ1 minꢂ1 (O3)
0.24 L minꢂ1 (air)
Atmospheric air
25
25
75
25
75
2.5
2.5
2.5
8
2002
2009
2001
Present
29
16
28
4
but the decrease was at most 1/2.4. Table 2 also revealed the the catalytic activation order of catalyst-pH4 > catalyst-pH12 >
variation of the b and g values along with some elevated vari- catalyst-pH8, which was kinetically substantiated by the trends
ables, such as the molar ratio of PO43ꢂ : NH2 (that is, the PtNPs in the Elovich parameters, namely the increasing b value and
content), initial HCHO concentration, and reaction temperature: decreasing g value. The reaction rate and the b and g values
the b values increased by more than two orders of magnitude were also most strongly inuenced by the HCHO concentration
within the present variation of these variables, especially of and by the reaction temperature.
HCHO concentration and temperature, although the remarkable
(two orders of magnitude) decrease in the g values was obtained the catalyst in the degradation of HCHO were the surface area
by the variation of the initial HCHO concentration. and the PtNPs content. In particular, for the composites
The key factors in utilizing the CaP–DENPtNP composite as
The trends, described above, in the b and g values for each prepared under acidic conditions, the predominant PtNPs
variable can be compared with those in previous reports. The content (60 wt%)33 inuenced the catalytic reaction, even
Elovich equation has been used to analyze the sorption kinetics though having the lowest surface area of the composites33 may
of copper(II) as the catalyst onto peat at various concentrations of have depressed the reaction. On the other hand, the surface
peat40 and to elucidate the sorption of cadmium ions onto bone area was advantageous to the composite prepared under alka-
char.41 The outcomes related to the Elovich parameters indicated line conditions, although its lower PtNPs content34 was the
that the increase in concentration of the peat dose40 or cadmium negative factor.
solution41 caused the b value to increase but the g value to
Compared to the previous reports concerning to the catalytic
decrease. It has been also reported that a rise in temperature oxidative degradation of HCHO in an aqueous solution (see
commonly induces the b value to increase and g value to Table 3), the present investigation proposes an advantageous
decrease.42 Although the present results are along the lines of the procedure, where the reaction is operated at atmospheric air
previous analyses, it should especially be noted that the b value pressure and mild temperatures and almost 100% decomposi-
(initial adsorption rate) was most signicantly inuenced by the tion can be achieved.
catalyst species, initial HCHO concentration and reaction
temperature, whereas the g value (Elovich constant) was most
notably affected by the initial HCHO concentration.
Acknowledgements
This work was nancially supported by the National Taiwan
University of Science and Technology, Taiwan, under a grant
100H451201. Y.F. gratefully acknowledges the National Taiwan
Conclusions
In this investigation, the oxidative degradation of HCHO was
University of Science and Technology, Taiwan, for the scholar-
performed by using air as the source of the oxidant and catalyst-
ship of master degree. We give thanks to Prof. Masaki Ujihara,
pHx (CaP–DENPtNP composites) as the catalyst. The stirring
process, catalyst species, catalyst content, HCHO concentration,
Taiwan, for his kind discussion.
and reaction temperature were the effective and adjustable
parameters that could be used to accelerate the decomposition
of HCHO.
Notes and references
The complete oxidation process of HCHO to produce CO2
and H2O in the current research depended on the adsorption
process of HCHO onto the active sites of PtNPs, which was
successfully described by the Elovich equation with high
correlation coefficients. Differences in the reaction rate were
remarkably characterized by the catalyst-production pH, with
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