ACS Catalysis
Research Article
purification, not only because of its high surface area and
where [HCHO]in and [HCHO]out are the inlet and outlet
HCHO concentrations, respectively. [CO ] is the outlet CO
suitable pore size distribution but also because of its low
2
2
3
0−32
35
cost.
Unfortunately, fast adsorption saturation is the
concentration.
−
1
hidden defect of its application. In view of its advantages, AC, a
promising candidate as a support of noble metal catalysts,
could offer a huge surface area to disperse metals and adsorb
reactants. Meanwhile, the catalyst service life would be
lengthened remarkably. Therefore, it is worthwhile exploring
AC-supported noble metal catalysts for indoor HCHO
purification.
36
the conversion of HCHO was kept below 20% by varying the
inlet HCHO concentration and GHSV for TOF calculations.
3. RESULTS AND DISCUSSION
3.1. Structural Features of Catalysts. The surface areas
The HCHO oxidation mechanism has been studied for
many years. Usually, the intermediate products of HCHO
and pore size distributions of samples were determined by
nitrogen physisorption analysis, as shown in Figure S1. N2
adsorption−desorption isotherms of these samples were typical
of type I according to the IUPAC classification and there were
no hysteresis loops observed, indicating the presence of
33,34
oxidation are dioxymethylene (DOM), formate, and CO.
HCHO could be easily oxidized to DOM by oxygen species,
followed by DOM transformation into formate groups by
losing a proton. In a low humidity or a hydroxyl-deficient
environment, the formate could decompose to CO while the
formate could be transformed into H O and CO in a sufficient
37
microporosity. Among the four samples, there were no
observable differences in nitrogen saturation adsorption at
lower relative pressure, revealing that the four samples
possessed similar pore structure properties. In terms of the
specific surface area (as shown in Table 1), it was shown that,
2
2
4
,7,33
hydroxyl environment.
CO could be easily oxidized to
CO by oxygen groups as soon as it was produced. However,
33
2
these mechanisms have been studied based on metal oxide-
supported noble metal catalysts, which are more favorable for
catalytic oxidation reactions, as mentioned above. The
mechanism of HCHO oxidation on non-oxide supported
noble mental catalysts is still worthy of investigation.
In the present study, we utilized Pt/AC as a model catalyst
system for HCHO oxidation, to investigate the promotion
effect of alkali metals (taking Na as a representative) on Pt
species, in terms of microstructure, activity, and stability. It was
verified that Na addition had a dramatic promotion effect on
the Pt/AC catalyst in contrast to the Na-free catalyst, achieving
Table 1. Specific Surface Area (BET), Total Pore Volume,
of AC−R, Na/AC−R, Pt/AC−R, and Na−Pt/AC−R
Catalysts, and Pt Dispersion (D) and TOF on Pt/AC−R
and Na−Pt/AC−R Catalysts
pore
a
−2
b
c
volume
Dco
TOF × 10
d
M
3
3
−1
samples
AC-R
Na/AC−R
Pt/AC−R
SBET (m /g) (cm /g)
(%)
(s
)
(nm)
1812.8
1810.7
1759.6
1715.8
0.87
0.82
0.79
0.77
59
65
2.6
6.6
2.4
1.1
1
00% HCHO conversion at 25 °C. The promotion effect of
Na−Pt/AC−R
alkali metals on noble catalysts for catalytic oxidation of
HCHO was further extended to a catalyst system with a non-
oxide support and compared to the previously reported catalyst
systems using metal oxide supports.
a
b
Pt dispersion measured with CO pulse chemisorption. Turnover
c
frequencies (TOF) calculated from D . Averaged Pt particle size
co
determined by TEM.
2
. EXPERIMENTAL SECTION
.1. Materials Preparation and Catalyst Character-
compared to bare AC−R, the specific surface area decreased to
patterns of the samples are shown in Figure S2. Besides the
two broad peaks observed at 24.0 and 43.5°, which were
2
38
2
.2. Activity Test for HCHO Oxidation. The activity tests
ascribed to activated carbon, there were no diffraction peaks
of Pt species observed in any of the profiles. This indicated that
for the catalytic oxidation of HCHO on the catalysts (30 mg)
were performed in a fixed-bed quartz flow reactor (d = 4 mm)
in an incubator kept at 25 °C. Gaseous HCHO was generated
by flowing He gas through a paraformaldehyde container.
Water vapor was generated by flowing He through a water
bubbler. The feed gas composition was 150 ppm of HCHO,
39
the Pt species were well dispersed on the AC support. The
ICP−MS and the results are listed in Table S1. The Pt loading
on AC was 0.19 and 0.20 wt % within Pt/AC and Na−Pt/AC
catalysts, respectively, which were almost identical to the
theoretical value.
2
0% O , and 35% relative humidity (RH) balanced by He. The
2
total flow rate was 100 mL/min, corresponding to a gas hourly
The EDS elemental mapping was carried out to investigate
displayed in Figure S3. It can be clearly seen that Na, O, and Pt
were uniformly dispersed at a scale of 100 nm. The Pt particle
size was measured by HAADF-STEM and the results are
shown in Figure 1. According to Figure 1a,d, the Pt mean
particle size on the Pt/AC−R and Na−Pt/AC−R catalysts was
2.4 and 1.1 nm, respectively, based on the arithmetic mean of
around 300 particles (the particle size distribution details of Pt
species are displayed in Table S2). In addition, the Pt particle
sizes within catalysts based on volume average and CO
chemisorption were also calculated, which were 2.9 and 1.9 nm
for the Pt/AC-R catalyst and 2.4 and 1.7 nm for the Na−Pt/
AC−R catalyst (as shown in Table S1). The results indicate
−
1
space velocity (GHSV) of 80,000 h . The inlet and outlet
gases were monitored by Fourier transform infrared (FTIR)
conversion was calculated using the following equation
i
[HCOH]
y
j
out z
j
z
HCOH conversion (%) = j1 −
z × 100%
j
j
z
z
[
HCOH]
k
in {
The CO yield was defined as follows
2
[
CO ]
2
CO Yield(%) =
× 100%
2
[
HCOH]
in
4
57
ACS Catal. 2021, 11, 456−465
Wang, Chunying
