A Nonoxide Catalyst System Study: Alkali Metal-Promoted Pt/AC Catalyst for Formaldehyde Oxidation at Ambient Temperature

Catalyst for Formaldehyde Oxidation at Ambient Temperature

Research Article

A Nonoxide Catalyst System Study: Alkali Metal-Promoted Pt/AC

Chunying Wang, Yaobin Li,* Lirong Zheng, Changbin Zhang, Yin Wang, Wenpo Shan, Fudong Liu,*

and Hong He*

Cite This: ACS Catal. 2021, 11, 456 −465

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sı Supporting Information

ABSTRACT: Metal oxides have been extensively used as supports

in noble metal-based catalysts for formaldehyde (HCHO)

oxidation, utilizing the beneﬁt of the strong metal−support

interaction (SMSI). Non-oxide supported noble metal catalysts

are still rarely investigated in the ﬁeld of HCHO oxidation. Herein,

a series of Pt catalysts supported on a nonoxide, that is, activated

carbon (AC), were studied in detail. Pt/AC catalysts with or

without addition of the alkali metal Na were prepared and tested

for HCHO oxidation. HCHO could be completely oxidized to

form H O and CO at 25 °C on the Na−Pt/AC−R (reduced Na−

Pt/AC) catalyst, but in contrast, the Pt/AC−R catalyst only

achieved 40% HCHO conversion under the same reaction

conditions. The characterization results indicated that Na addition

to the Na−Pt/AC−R catalyst could induce the formation of Pt−O −Na species, which improves the stability of Pt species, increases

Pt dispersion, and enhances the activation of chemisorbed oxygen and water to form abundant surface-active hydroxyl groups.

Meanwhile, HCHO−TPD conﬁrmed that HCHO could be transformed into CO and H O directly because of the presence of

abundant OH groups on the Na−Pt/AC−R catalyst. In this study, the promotion eﬀect of alkali metals on supported Pt catalysts for

HCHO oxidation was further extended to a catalyst system with a nonoxide carrier.

KEYWORDS: formaldehyde, alkali metal, activated carbon, surface hydroxyl, catalytic oxidation

. INTRODUCTION

eﬀects by alkali metals for noble metal catalysts on nonoxide

supports is rare.

Noble metal catalysts have been widely used in various

chemical reactions because of their excellent catalytic activity.

In many cases, promoters are essential, to either further

improve the intrinsic activity or enhance the stability. Alkali

metals (i.e., Li, Na, K, Rb, and Cs) have been found to be

HCHO is mostly derived from wooden building products,

25−27

decorative materials, furniture, and so forth.

When

exposed to indoor air containing HCHO for a long time,

people may suﬀer ophthalmic diseases, headaches, respiratory

28,29

diseases, pneumonia, and even cancer.

Taking HCHO

eﬀective promoters for certain reactions, such as Na−Pt/TiO

oxidation as an example, which is a very important catalytic

reaction for indoor air pollution control, no one has reported a

promotion eﬀect of alkali metals on noble metals on nonoxide

supports, such as cost-eﬀective carbon-based supports. This

highly important research topic, which is also of great

importance to the environmental catalysis industry, is still

poorly studied fundamentally.

−3

and K−Pt/Al O for the water gas shift (WGS) reaction

and Na−Pd/TiO , Na−Ir/TiO , Na−Pt/TiO , and K−Pd/

−7

TiO for HCHO oxidation.

For most of the noble metal catalysts promoted by alkali

metals, the noble metals are typically supported on metal

oxides (TiO , SiO , Al O , ZrO , MnO , and so forth.)

zeolites (ZSM-5, β-zeolites).

besides the promoting eﬀect from alkali metals or alkali earth

metals for the noble metal active components, the strong

metal−support interaction (SMSI) between a metal and a

support was also found to aﬀect the catalytic performance

signiﬁcantly.

catalytic reactions included the stabilization of metal clusters

,8−14

3,15,16

In the referenced works,

Activated carbon (AC) is widely used as an adsorbent in

most waste treatment processes, especially in indoor air

Received: July 22, 2020

Revised: December 7, 2020

7−22

Demonstrated eﬀects of SMSI on the

or the improvement of charge transfer. However, no SMSI

exists on nonoxide supports, so the study of such promotion

XXXX American Chemical Society

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Research Article

puriﬁcation, not only because of its high surface area and

where [HCHO]_i_nand [HCHO]_o_u_tare the inlet and outlet

HCHO concentrations, respectively. [CO ] is the outlet CO

suitable pore size distribution but also because of its low

0−32

cost.

Unfortunately, fast adsorption saturation is the

concentration.

The turnover frequency (TOF, s ) was calculated

−

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.

(equation displayed in Supporting Information ). Typically,

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

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,

,7,33

hydroxyl environment.

CO could be easily oxidized to

CO by oxygen groups as soon as it was produced. However,

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

−2

volume

D_c_o

TOF × 10

−1

samples

AC-R

Na/AC−R

Pt/AC−R

S_B_E_T(m /g) (cm /g)

(%)

)

(nm)

1812.8

1810.7

1759.6

1715.8

0.87

0.82

0.79

0.77

2.6

6.6

2.4

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.

Pt dispersion measured with CO pulse chemisorption. Turnover

frequencies (TOF) calculated from D . Averaged Pt particle size

determined by TEM.

. EXPERIMENTAL SECTION

.1. Materials Preparation and Catalyst Character-

compared to bare AC−R, the speciﬁc surface area decreased to

diﬀerent extents after loading with precious metals. The XRD

patterns of the samples are shown in Figure S2. Besides the

two broad peaks observed at 24.0 and 43.5°, which were

ization. Materials preparation and catalyst characterization are

described in Supporting Information.

.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,

the Pt species were well dispersed on the AC support. The

actual Pt loadings within the catalysts were determined by

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.

0% O , and 35% relative humidity (RH) balanced by He. The

total ﬂow rate was 100 mL/min, corresponding to a gas hourly

The EDS elemental mapping was carried out to investigate

the distribution of diﬀerent elements and the results are

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

−

space velocity (GHSV) of 80,000 h . The inlet and outlet

gases were monitored by Fourier transform infrared (FTIR)

spectroscopy (the speci ﬁ c experimental conditions are shown

in Supporting Information ). In all the experiments, HCHO

conversion was calculated using the following equation

[HCOH]

out z

HCOH conversion (%) = j1 −

z × 100%

[

HCOH]

in {

The CO yield was deﬁned as follows

[

CO ]

CO Yield(%) =

× 100%

[

HCOH]

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Research Article

Figure 2. XPS spectra of the Pt/AC−R and Na−Pt/AC−R samples:

Pt 4f [inset: Distribution ratio of Pt /(Pt +Pt +Pt )].

ratios were 22.2 and 18.5%, respectively. This indicates that

there are more metallic Pt species on the Pt/AC−R catalyst

and more Pt on the Na−Pt/AC−R catalyst, which were

stabilized by Pt−O −Na or Pt−(OH) species. In addition,

this could be con ﬁ rmed by the analysis of the O 1s spectra

(

shown in Figure S4). Compared to the Pt/AC−R catalyst, the

peak of the Na−Pt/AC−R catalyst was shifted to a higher

binding energy, which indicates there are more surface-active

oxygen (such as Pt−O −Na and OH species) rather than

lattice oxygen (such as PtO bulk).

.3. X-Ray Absorption Fine Structure Spectroscopy

Analysis. X-ray absorption ﬁne structure spectroscopy

XAFS) of the Pt L edge was further utilized to explore the

(

Figure 1. HAADF-STEM images and particle size distribution of (a−

c) Pt/AC−R and (d−f) Na−Pt/AC−R samples.

atomic structures of Pt species within the samples. X-ray

absorption near edge structure (XANES) was employed to

examine the oxidation state of Pt species and extended XAFS

(EXAFS) was used to study the coordination environment.

The normalized XANES spectra with the ﬁrst-order derivative

and the EXAFS data in R space as well as the curve ﬁtting

results are displayed in Figure 3 and Table 2. According to the

results in Figure 3a, the initial white-line intensity of the Pt L₃

edge for the two as-prepared samples (Pt/AC and Na−Pt/AC)

that Na addition could eﬃciently decrease the Pt particle size.

Furthermore, from Figure 1b,e, it can be observed that there

were more atomically dispersed Pt atoms on the Na−Pt/AC−

R catalyst, which have been considered as the active sites for

,7,40

HCHO oxidation and the WGS reaction.

Recently, it was reported that nanosized and oxidized Pt or

Au species, which inﬂuenced the neighboring OH groups, were

also the major catalytic sites for the WGS reaction. In this

model, the key question was how to stabilize the active noble

metals in a nanosized and oxidized state. Flytzani-Stephano-

poulos group conﬁrmed that the alkali metals could eﬀectively

stabilize the noble metals on metal oxide supports (such as

was close to that of PtO , indicating that the Pt species in these

two samples were in a highly oxidized state, which was

conﬁrmed to be Pt species from the ﬁrst-order derivative of

the XANES spectra (Figure 3a). As shown in Figure 3b and

Table 2, there were similar coordination numbers and bond

lengths between Pt and O atoms on the Pt/AC and Na−Pt/

AC samples, indicating that the as-prepared Pt/AC catalysts

possessed similar microstructures regarding Pt species.

titania, zirconia, and ceria) through the formation of Pt−O −

Na species. This type of active site was conducive to

preventing agglomeration of Pt into large nanoparticles, which

was consistent with our HAADF-STEM results. According to

the results in Figure 1c,f, some low-density species could be

observed around the Pt particles on the Na−Pt/AC−R

After reduction, as shown in Figure 3a, the Pt L white lines

δ+

of Pt species (marked as Pt ) on Pt/AC−R and Na−Pt/AC−

R catalysts were close to those of Pt foil, indicating that PtO

species were reduced to metallic Pt species by H . However, it

catalyst, which may be attributed to NaO species. Such a

was noted that the Pt L white lines on Na−Pt/AC−R showed

structure was never found on the Pt/AC−R catalyst,

conﬁrming that there was a strong interaction between Pt

a slight shift to higher energy, indicating that there was a

greater proportion of high-valence Pt species, which was in line

with the Pt XPS results. According to Figure 3c and Table 2,

compared with the Pt/AC−R catalyst, the Na−Pt/AC−R

catalyst had lower Pt−Pt coordination, indicating that Pt

particles dispersed better on the Na−Pt/AC−R catalyst than

on the Pt/AC−R catalyst, which is in accordance with the

results of HAADF-STEM (Figure 1). Though the Pt particle

size decreased, the Na−Pt/AC−R catalyst exhibited a higher

Pt−O coordination number than the Pt/AC−R catalyst,

indicating again there were more high-valence Pt species on

the Na−Pt/AC−R catalyst, which may be because of the

and NaO species on the Na−Pt/AC−R catalyst.

.2. XPS Measurements. XPS measurements were carried

out to identify the states of Pt atoms on the catalyst surface,

and three types of Pt species were revealed, as shown in Figure

. The peak at 74.00 eV was assigned to Pt , which may be

attributed to the reoxidation of metal Pt particles by O and/or

H O in air during transfer of the sample to the XPS chamber.

The peak at 72.45 eV was assigned to Pt and the peak at

1.60 eV was ascribed to metallic Pt.

Pt +Pt +Pt ) ratios on the Pt/AC−R and Na−Pt/AC−R

catalysts were 21.9 and 27.1%, and the Pt /(Pt +Pt +Pt )

(

The Pt /

existence of rich Pt−O −Na or Pt−(OH) species.

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Research Article

Figure 3. (a) Normalized Pt L edge XANES spectra (inset: ﬁrst order derivative of normalized XANES) and Fourier transformed Pt L edge

EXAFS of (b) samples before H reduction and (c) samples after H reduction.

.4. HCHO Oxidation Test. The data for HCHO

AC−R and Na- 0.1 wt % Pt/AC−R catalysts, respectively, at

RT, which were much worse than on 0.2 wt % Pt loading

catalysts. A suﬃcient amount of Pt species on AC is important

for maintaining excellent HCHO oxidation performance.

Based on the Pt dispersion data, TOF values for the two

catalysts were calculated at 25 °C, and the results are

summarized in Table 1. The Pt/AC−R and Na−Pt/AC−R

conversion and CO yield on AC, Na/AC, Pt/AC, and Na−

Pt/AC samples without and with reduction are shown in

Figures S6 and 4. As shown in Figure S6a, the HCHO removal

rate on the as-prepared samples gradually decreased, and there

was no CO produced from the beginning of the reaction (as

shown in Figure S6b), implying that adsorption rather than

catalytic oxidation occurred on these samples. A similar

phenomenon was observed on AC−R and Na/AC−R samples

−

−2 −1

catalysts showed a TOF of 2.6 × 10 and 6.6 × 10 s ,

respectively. It is clear that Na doping signiﬁcantly enhanced

the intrinsic activity of the Na−Pt/AC−R catalyst, with a TOF

2.5 times higher than that of the Pt/AC−R catalyst. In

addition, the kinetic experiments for HCHO oxidation on the

two catalysts were also carried out (Figure S10) and the

apparent activation energies (E ) were obtained. Clearly, E of

as shown in Figure 4a,b. However, after reduction in H ,

HCHO conversion on the Pt/AC−R and Na−Pt/AC−R

catalysts was greatly enhanced. About 40% HCHO conversion

was stabilized on Pt/AC−R with the corresponding CO yield

at ambient temperature and the carbon balance data displayed

in Table S3, which illustrated that the catalytic oxidation of

HCHO indeed took place without secondary products.

Interestingly, with Na addition, HCHO could be completely

oxidized to form H O and CO (at a concentration ≤150

−

HCHO oxidation on the Pt/AC−R catalyst was 83.4 kJ mol ,

while the Na−Pt/AC−R catalyst possessed a lower E of 25.2

−

kJ mol for HCHO oxidation, which indicated that there

might be two diﬀerent reaction pathways of HCHO oxidation

on these two catalysts.

ppm) under the same evaluation conditions as the Pt/AC−R

catalyst. Even at a low temperature (5 °C, as displayed in

Figure S7 ) or low HCHO concentration (10 ppm, as showed

in Figure S8 ) condition, the Na−Pt/AC−R catalyst still

exhibited excellent HCHO oxidation performance (HCHO

adsorption was excluded based on the carbon balance results

shown in Figures S7 and S8 ). The results suggest that the

alkali−metal promotion eﬀect also applies to non-oxide

supported noble metal catalysts, at least for this novel Na−

Pt/AC−R system, similar to other metal oxide-supported

3.5. Surface-Active Oxygen. In order to clarify the O₂

activation capacity of the catalysts, H −TPR of the reduced

catalysts was carried out and the results are shown in Figure 5a.

No H consumption was observed on AC−R and Na/AC−R

samples, implying that there were no reducible oxygen species.

By contrast, the catalysts with supported Pt exhibited broad H₂

consumption peaks in the temperature range of 10−140 °C. It

was evident that the amount of H consumption on the Na−

Pt/AC−R catalyst was much higher than that on the Pt/AC−R

catalyst, indicating that the amount of active oxygen was

signiﬁcantly enhanced by Na addition. This was further

catalyst systems (Pt/TiO , Na−Pt/TiO and Pt/MnO , and

Na −Pt/MnO ) for HCHO oxidation. As shown in Figure

S6c , the Na−Pt/AC−R catalyst exhibited excellent stability

and eﬃciency under the same reaction conditions, with

approximately 100% HCHO conversion for a 150 h long

test. From the perspective of application cost, we further

studied the HCHO oxidation performance on 0.1 wt % Pt

loading catalyst and the results are shown in Figure S9. The

stable HCHO conversion was 10 and 25% on the 0.1 wt % Pt/

conﬁrmed by O −TPD. As shown in Figure 5b, only one

broad desorption peak for active oxygen was observed in the

range of 250−550 °C over both Pt/AC−R and Na−Pt/AC−R

samples. It is clear that the Na−Pt/AC−R catalyst possessed

more active oxygen than the Pt/AC−R catalyst.

3.6. Surface Hydroxyl Species. The surface OH species

on the reduced catalysts were further measured by CO−TPR,

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Research Article

according to the reaction mechanisms 2CO + 2OH → 2CO +

H , CO + O* → CO , and the CO and H peaks are shown in

Figure 6. Only small amounts of CO and H were produced

on the Pt/AC−R catalyst without Na doping, which indicated

that there were few OH groups on the Pt/AC−R catalyst

because of the limited ability of the platinum surface to

transform H O and O into active hydroxyl at relatively low

temperature. By contrast, large amounts of CO and H were

clearly found on the Na−Pt/AC−R catalyst in the range of

0−220 °C, suggesting that abundant OH groups existed on

the Na−Pt/AC−R surface. Combining the O −TPD and

CO−TPR analyses, we found that the metallic Pt sites have

excellent O activation ability and limited ability for the

simultaneous activation of H O and O to form surface-active

OH groups, according to the equation O + H O → O* +

10,45−48

OH.

As we know, surface OH groups could be

produced via the oxygen vacancies on “active” oxide supports,

such as TiO and CeO .

,49,50

However, in this study, the

carriers were inert with no ability to activate H O or/and O to

form surface OH groups. Some previous works have reported

that the active OH sites are associated with Pt−Na −O −

(

OH) species that are structural and independent of the type

40,51

of support.

Therefore, the abundant surface OH groups on

the Na−Pt/AC−R surface may be attributed to H O and O

activation on the Pt species associated with NaO species.

To further conﬁrm the vital role of active OH groups formed

through H O and O activation on the surface, we also

investigated the catalytic oxidation of HCHO with H O or O

startup−shutdown cycles. As shown in Figure 7a,b, a similar

trend was found on Pt/AC−R and Na−Pt/AC−R catalysts.

When the catalysts were exposed to a gas ﬂow of 230 ppm

HCHO/He/H O (RH 35%)/O , about 25 and 90% HCHO

conversion was achieved on Pt/AC−R and Na−Pt/AC−R

catalysts, respectively. After several hours without any

deactivation, H O was shut oﬀ, and then HCHO conversion

drastically dropped to 10 and 15% on the catalysts,

respectively. When H O was reintroduced into the gas ﬂow,

the activity of the catalysts completely recovered, and sharply

decreased again following the removal of H O. The

phenomenon conﬁrmed that H O was important for both of

the catalysts, especially Na−Pt/AC−R, on which more H O

was dissociated to form surface-active OH groups. As shown in

Figure 7c,d, the catalysts were exposed to the same conditions

of HCHO/He/H O/O (20%), and HCHO conversion rates

on the Pt/AC−R and Na−Pt/AC−R catalysts were similar to

those observed in the H O startup−shutdown experiments.

After O was shut oﬀ, the Pt/AC−R catalyst showed almost no

HCHO conversion, while 10% HCHO conversion remained

on the Na−Pt/AC−R catalyst, which means that O was also

important for HCHO oxidation.

The above phenomenon indicated that both H O and O

played an important role in the HCHO oxidation reaction on

Pt/AC−R and Na−Pt/AC−R catalysts at room temperature,

which was also conﬁrmed in the Ir/TiO catalyst system.

However, diﬀerent from the Pt/AC−R catalyst, the synergistic

eﬀect was more conspicuous on the Na−Pt/AC−R catalyst,

which was clearly because of the presence of Pt−O −Na

groups on this catalyst with higher capability to activate H O

and O to form active OH species.

.7. Reaction Pathways. HCHO−TPD was further

carried out to investigate the mechanism of HCHO oxidation

on the catalysts, and the results are shown in Figure 8. Distinct

desorption peaks of CO, CO , and H were observed on the

ACS Catal. 2021, 11, 456−465

reaction conditions: 150 ppm of HCHO, 20% O , 35% RH, He balance, and a GHSV of 80,000 h .

Research Article

Figure 4. (a) HCHO conversion and (b) CO yield at room temperature over the AC−R, Na/AC−R, Pt/AC−R, and Na−Pt/AC−R samples;

−

Figure 5. (a) H −TPR and (b) O −TPD proﬁles of AC −R, Na/AC−R, Pt/AC−R, and Na−Pt/AC−R samples.

As previously reported, formate is an important intermedi-

ate, and its decomposition is the rate-limiting step in HCHO

,47

oxidation.

Based on the desorption of CO in the HCHO−

TPD results, the formate decomposition may occur, as shown

in eq 5. Many studies investigated the HCHO oxidation

mechanism by in situ DRIFTS and it was found that CO was

−

the main intermediate in the absence of O (HCOO + H →

CO + H O). Moreover, another reaction path for HCHO

oxidation showed no CO formation because of the presence of

active hydroxyl groups, which could directly convert formate

into CO and H O. Herein, the CO formation was because of

the lack of surface-active oxygen groups on the Pt/AC−R

catalyst. A portion of generated [CO]_sreacted with

chemisorbed oxygen to form CO , and [OH] reacted with

Figure 6. CO−TPR proﬁle (a) CO and (b) H on Pt/AC−R and

Na−Pt/AC−R samples.

chemisorbed hydrogen in eqs 3 and 4 to form H O, as shown

in eqs 6 and 7. Meanwhile, direct oxidation of formate by OH

groups might also have taken place on the Pt/AC−R catalyst

according to eq 8, but this was not signiﬁcant because of the

lack of suﬃcient OH groups.

Pt/AC−R catalyst at 74 °C, which were because of the

reaction between the adsorbed HCHO and surface OH

groups. Based on our previous research and experimental

phenomena, we can reasonably conclude the reaction pathways

for HCHO oxidation on the Pt/AC−R catalyst. First, gaseous

[HCOO] → [CO] + [OH]

(5)

(6)

(7)

O adsorbs on the catalyst surface and then is activated into

[CO] + [O] → [CO ] → CO

2 s

34,52

active oxygen on metallic Pt sites.

At the same time, a small

amount of active hydroxyl and oxygen species is produced

[

OH] + [H] → [H O] → H O

because of the synergy between H O and O . Then, HCHO

can be rapidly oxidized into formate species by the active

HCOO] + [OH] → [CO ] + [H O] → CO + H O

2 s

hydroxyl and oxygen groups.

These processes can be described as shown in eqs 1−4

(

In terms of the intermediate H in the HCHO−TPD proﬁle,

[

O ] → 2[O]

(1)

(2)

(3)

(4)

there may be two pathways for its generation, as shown in eqs

and 10. The most probable one shown in eq 9 is the

H O] + [O ] → 2[OH] + [O]

2 s

synthetic reaction of two [H] generated in the reaction of eqs

HCHO] + [O] → [HCOO] + [H]

3 and 4 to form H . The other one is the water gas shift

reaction, that is, CO reacting with surface OH groups to form

HCHO] + [OH] → [HCOO] + 2[H]

CO and H , as presented in eq 10. In addition, other [H]_s

2 2

ACS Catal. 2021, 11, 456−465

RH, 20% O , He balance, and a GHSV of 150,000 h .

Research Article

Figure 7. H O (a,b) and O (c,d) eﬀects on the activity of the Pt/AC−R and Na−Pt/AC−R catalysts. Reaction conditions: 230 ppm HCHO, 35%

−

4. CONCLUSIONS

In summary, the alkali metal-promotion eﬀect on HCHO

oxidation was extended to non-oxide supported Pt catalysts. It

was found that Na−Pt/AC−R showed more eﬃcient catalytic

oxidation of HCHO than a Pt/AC−R catalyst at room

temperature, and complete conversion of 150 ppm HCHO

−

could be achieved in a GHSV of 80,000 h at 25 °C. Na

addition could increase the Pt dispersion on AC and activate

surface-chemisorbed H O and O to produce more active OH

groups, which can directly oxidize HCHO into the ﬁnal

products CO and H O. By contrast, the Pt/AC−R catalyst

displayed lower capacity for oxygen species activation and thus

much lower HCHO oxidation activity.

Figure 8. HCHO−TPD proﬁles of Pt/AC−R and Na−Pt/AC−R

samples.

ASSOCIATED CONTENT

sı Supporting Information

■

species react with [OH] or [O] to form H O directly,

according to eqs 7 and 11.

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acscatal.0c03196.

[

H] + [H] → [H ] → H

(9)

2 s

Related experimental methods and ﬁgures of the CO₂

selectivity proﬁles, BET results, XRD patterns, and O 1s

XPS and EXAFS ﬁtting spectra (PDF)

[CO] + 2[OH] → 2[CO ] + [H ] → 2CO + H

2 s

(

10)

[H] + [O] → [H O] → H O

(11)

AUTHOR INFORMATION

In comparison, only a peak of CO was observed for the

■

Na−Pt/AC−R catalyst, with no desorption peak for CO or H

Corresponding Authors

detected in the studied temperature range. This phenomenon

indicated that a more eﬀective pathway for HCHO oxidation

occurred, because there were abundant surface-active OH

groups on the Na−Pt/AC−R catalyst. As reported in our

previous work, the formate species could be directly oxidized

Yaobin Li − Center for Excellence in Regional Atmospheric

Environment, Key Laboratory of Urban Pollutant

Conversion, Institute of Urban Environment, Chinese

Academy of Sciences, Xiamen 361021, China; Zhejiang Key

Laboratory of Urban Environmental Processes and Pollution

Control, Ningbo Urban Environment Observation and

Research Station, Chinese Academy of Sciences, Ningbo

315800, China; Phone: 86-574-86085855; Email: ybli@

iue.ac.cn; Fax: 86-574-86085855

,34,47,55,56

by surface-active OH species to form CO and H O.

The reaction pathways could be described as shown in eqs 1−4

and 7 and 8 and 11. Therefore, Na−Pt/AC−R exhibited

excellent catalytic activity compared to the Pt/AC−R catalyst.

ACS Catal. 2021, 11, 456−465

ACS Catalysis

Flytzani-Stephanopoulos, M. Alkali-Stabilized Pt-OH

Research Article

Fudong Liu − Department of Civil, Environmental, and

Construction Engineering (CECE), Catalysis Cluster for

Renewable Energy and Chemical Transformations (REACT),

NanoScience Technology Center (NSTC), University of

Central Florida, Orlando, Florida 32816, United States;

orcid.org/0000-0001-8771-5938; Phone: 1-407-823-

performed at 1W1B XAFS beam line at the Beijing

Synchrotron Radiation Facility (BSRF), China.

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■

Y. L. and H. H. acknowledge the support from the National

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the Science and Technology Planning Project of Xiamen City

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