The behaviors of ultra-low-gold loaded catalysts (Au/CeO ...DOI The Behaviors of Ultra-Low-Gold...

Nano Res

1

The behaviors of ultra-low-gold loaded catalysts

(Au/CeO2) for CO oxidation in the presence of water on

the catalysts

Qiaoling Li1 (), Wei Xie1, Guoqin Chen1, Yuefeng Li1, Yanjun Huang1, and Xiaodong Chen2 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0813-z

http://www.thenanoresearch.com on May 13, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research

DOI 10.1007/s12274-015-0813-z

TABLE OF CONTENTS (TOC)

The Behaviors of Ultra-Low-Gold Loaded Catalysts

(Au/CeO2) for CO Oxidation in the Presence of Water

on the Catalysts

Qiaoling Li1*, Wei Xie1, Guoqin Chen1, Yuefeng Li1, Yan

jun Huang1, and Xiaodong Chen2*

1 Technology Center of China Tobacco Fujian Industrial

Co., Ltd, China

2 Xiamen University, China

1, Water possessed by catalyst itself could promote O2 adsorption and

activation.

2, Oact on the interface is the key factor to obtain excellent catalytic

activity.

The Behaviors of Ultra-Low-Gold Loaded Catalysts

(Au/CeO2) for CO Oxidation in the Presence of Water on

the Catalysts

Qiaoling Li1 (), Wei Xie1, Guoqin Chen1, Yuefeng Li1, Yanjun Huang1, and Xiaodong Chen2 ()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

ultra-low-gold catalysts,

water addition on the

catalysts, hydroxyl and

carbonate-like species,

reaction pathways

ABSTRACT

The catalytic behaviors of ultra-low-gold loaded catalysts Au/CeO2 during CO

oxidation have been studied using HRTEM, TGA, in-situ FTIR, CO-TPD,

O2-TPD and O2-TPO. Herein, water is present on the catalysts rather than from

the reactant gas to facilitate CO oxidation. Experimental results give direct

evidence that the presence of water on catalysts enhances the adsorption and

activation of O2 to produce Oact on the catalyst surface, resulting in excellent

catalytic activities. Different reaction pathways for CO oxidation over Au/CeO2

catalysts may be at work, but which one is dominant is influenced by the

species on the catalyst surface.

1 Instruction

Gold-based catalysts are commonly used in CO

oxidation due to their superior activity [1-11], but the

high price of gold and low availability limit their

application. Previously, we have reported our

ultra-low-gold loaded catalysts are quite reactive,

some of which could oxidize 70% CO at ambient

temperature even though the gold loaded were only

0.06 wt% [12]. The main reason can be attributed to

the contribution of water. Herein, water is present on

the catalysts rather than from the humid reactant gas

to facilitate CO oxidation. As we known, many

reported studies focused on the catalytic roles of

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Q. L. Li, [email protected]; X. D. Chen, [email protected]

Research Article

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

water from the humid reactant gas [13-15], while few

studies mentioned the situation of the water

contained in catalyst. Haruta et al. have reported that

the amount of water absorbed on the catalyst

influences the activity rather than the water content

in the gas phase [13]. Consequently, we investigated

the cases where the catalysts possess water by

themselves, which omits the necessary step of water

adsorption on the catalysts from the humid gas

phase.

Even though extensive experimental studies have

also shown that the presence of water is crucial to

obtain excellent catalytic activity for CO oxidation

reaction, but the intrinsic role of water in this process

is still unclear. Haruta et al. considered the water not

only activate the O2 molecular but also benefit the

decomposition of carbonate-like species via the

transformation of stable species to unstable species

[16]. In contrast, Goodman et al. proposed that the

promotional effect of water can only be rationalized

by its effects on O2 adsorption and activation, and the

appearance of carbonate-like species inhibit CO

oxidation [17]. From the active debate, the consensus

of the adsorption and activation of O2 by water has

been reached. Liu et al. provided the calculation

results by DFT that -OH groups play a key role in O2

adsorption on TiO2(110) [18]. Bongiorno et al. used

the first-principles calculations to investigate and

found that an adsorbed H2O molecular serves as an

“attractor” of O2 to its vicinity [19]. Based on DFT

and kinetic isotope effects study, a conclusion was

obtained that H2O mediated O2 activation via the

formation of hydroperoxy intermediates (-OOH),

which readily reacts with CO and account for the

remarkable reactivity [20, 21]. However, to date no

direct experimental evidence could be obtained to

validate the persistent speculation, which was just

obtained by theoretical studies [18-21] and exclusive

method [17]. How the water change during the

activation process, and how to directly prove the real

existence of the proposed active -OH or -OOH

groups? Furthermore, the biggest difference between

the two opinions is the role of carbonate-like species

resulting from the reaction between CO and -OH or

-OOH groups. Do the carbonate-like species

contribute to the activity or inhibit? What is the

rate-determining step for CO oxidation, the

appearance of carbonate species or the activation of

O2?

In-situ FTIR (fourier transform infrared

spectroscopy) is the most widely used technique for

studying the reaction pathways. In order to directly

shed light on how the water improve the catalytic

activity, the reaction pathways for CO oxidation in

the presence of water on the catalysts were

monitored by in-situ FTIR. It is important to note

that the choice of the water on the catalyst rather

than the water from the reactant gas has played an

important role in detecting the delicately changed

signals of -OH groups by in-situ FTIR. In the case of

the water coming from the reactant gas, it is very

difficult to apply in-situ FTIR to investigate the

changes occurring in the -OH (hydroxyl) region,

because the water content in the environment is

much higher than that adsorbing on the catalyst,

which would cover the delicately changed signals

of -OH groups on the catalyst surface. For this

reason, our study about the water possessed by the

catalyst itself avoids the above problem.

Furthermore, we used CO-TPD

(temperature-programmed desorption), O2-TPD

and O2-TPO (temperature-programmed oxidation)

to characterize the catalysts to investigate the role of

carbonate species and the correlation between water

and O2.

2 Experimental

2.1 Catalyst preparation

In this work, different support structures of

CeO2 were prepared by direct thermal

decomposition of Ce(NO3)3.6H2O at 873 and 1073 K

under atmosphere.

0.1 wt% gold was then introduced to above

CeO2 supports by deposition precipitation method.

Appropriate quantity of CeO2 support was

suspended in de-ionized water to produce a

suspension. The pH value of the suspension was

raised to 9 by NaOH and then the base suspension

was heated to 343 K, followed by introducing

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

HAuCl4 aqueous solution into it and reacted for 60

min. The resulted suspension was cooled and

filtered. The solid obtained was washed thoroughly

and just dried at 333 K for 16 h without further

calcinations to produce the catalysts. Such a low

heat-treating temperature could result in some

water residual on/in the catalysts. By using different

CeO2 supports, different catalysts can be obtained,

named Au/CeO2-873 and Au/CeO2-1073

respectively.

For comparison, similar pretreatment were

performed on support CeO2-873 and CeO2-1073,

which was stirred in alkaline water (pH=9) at 343 K

for 60 min, but without the addition of HAuCl4, and

also just dried at 333 K for 16 h with some water

presenting on/in the support, named h-CeO2-873

and h- CeO2-1073.

2.2 Catalytic activity measurement

Activity measurements were performed in a

quartz tubular fixed–bed reactor with 0.2 g of the

catalyst per pass under the reaction conditions of

atmosphere pressure, 298 K and GHSV=18000 h-1;

the composition of the reactant gas includes 1% CO,

1% O2 and 98% N2.

2.3 Catalyst characterization

The HRTEM images of the samples were obtained

using a Phillips Analytical FEI Tecnai 30 electron

microscope operating at 300 kV. The samples were

ground to fine powder, which was then mixed with

acetone to make a suspension. A drop of the

suspension was placed on a lacey carbon nickel grid,

subsequently dried at room temperature for

measurement. In order to examine the size of Au

particles, the same method was used to prepare the

above two catalysts but the gold content was

increased to 1 wt% rather than 0.1 wt%.

The water contents in studied catalysts were

evaluated by using SDT-Q600 instrument. Samples of

approximately 6.5 mg were used in TGA tests. The

temperature was increased to 850 K at a heating rate

of 10 K/min. Weight and temperature data were

recorded using TGA software.

In-situ FTIR investigations were performed in a

Nicolet Nexus. This setup allows measurement in a

continuous flow of gas mixtures, equaling to those

used in the activity measurements (1% CO, 1% O2

and 98% N2), and at 298 K. The samples were in

self-supported pellets without KBr and putted in the

in-situ reaction cell unit. The samples surfaces were

cleaned by vacuum pumping and then exposed to

the reactant gas. The FTIR data were reported as in

Kubelka-Munk unit, and 400 scans (nominal

resolution 8 cm-1) were co-added for one spectrum. It

is worth mentioning that the background spectra

were measured in the 3rd minute after the reactant

gas had flowed through the catalyst rather than in

the vacuum system, therefore any peak or valley

were related to the amount of corresponding species

increasing or decreasing during the reaction period.

If the surface species don’t change, the FTIR spectra

will be a straight line.

CO-TPD and O2-TPD: Temperature-programmed

desorption of CO and O2 experiments were

performed in a quartz microreactor connected to a

MS (Hiden QIC-20). The fresh catalyst was first

pretreated under CO or O2 at room temperature for

1 h (30 mL/min), and purged with Ar for 1 h. The

temperature was then ramped from room

temperature to 1073 K linearly (10 K/min) in a

carrier Ar gas introduced at a rate of 10 mL/min.

O2-TPO: Temperature-programmed oxidation of

O2 experiments was performed in a quartz

microreactor connected to a MS (Hiden QIC-20).

The catalyst after reacting 1 hour was treated under

O2, and the temperature was ramped from room

temperature to 1073 K linearly (10 K/min) in a

carrier Ar gas introduced at a rate of 10 mL/min.

3 Results and discussion

3.1 Physicochemical properties of ultra-low-gold

loaded catalysts

We have reported that the introduced Au (0.1

wt%) remained 0.06 wt% on the surface or the

subsurface layers of Au/CeO2-873 and

Au/CeO2-1073; the gold nano-particle size and the

specific surface areas are: 4.8 nm and 66 m2/g for

Au/CeO2-873, and 5.1 nm and 6 m2/g for

Au/CeO2-1073 [12]. The respective gold particle size


4 Nano Res.

is close to each other but the specific surface areas

dropped significantly with increasing the support

calcination temperature (Fig. 1). Since the catalysts

were prepared by depositing gold on an already

preformed ceria support and just dried at 333 K

without any further heat-treatment, the effect of

particle size on the catalytic activity is negligible.

The water contents in studied catalysts were

evaluated by using TGA. The results show that the

catalysts Au/CeO2-873 and Au/CeO2-1073 dried at

333 K contain 2.39 and 0.53 wt% water (Fig. 2). The

relative higher water content in Au/CeO2-873 can be

attributed to the higher specific surface areas.

3.2 Reaction pathway of ultra-low-gold loaded

catalyst Au/CeO2-873

Herein, the special reaction pathways of the

ultra-low-gold loaded catalysts for CO oxidation in

the presence of water on the catalysts were

monitored by in-situ FTIR. In the OCO- (carbonyl)

region, 1588 cm-1 is assigned to bidentate formate,

1369 and 1292 cm-1 are related to carbonate-like

species in WGS (water-gas shift) reaction [22]. For

the pure h-CeO2-873 support, the peaks 1588, 1369

and 1292 cm-1 were observed and remained about

constant (Fig. 3b), indicating that CO could adsorb

on the hydroxylated support surface and form

formate and carbonate-like species even though

without gold. In contrast, for the catalyst

Au/CeO2-873, the intensity of 1588, 1369 and 1292

cm-1 increased rapidly with time during the first

35min reaction, but leveled off after that (Fig. 4b).

Such formation rates are identical to CO conversion

rate during the activity test (Fig. 4a), inferring that

they are probably the reaction intermediates during

CO oxidation. If these formate and carbonate-like

species were formed by reaction product CO2

reabsorbing on the surface [23], the corresponding

peaks of them could not be detected on the pure

h-CeO2-873 support, because which shows no

activity for CO oxidation at 298 K (Fig. 3a).

The adsorption behavior of CO on h-CeO2-873

support and Au/CeO2-873 catalyst can be further

investigated with CO-TPD experiments (Fig. 5). The

main signal was CO2 rather than CO, suggesting

CO can be oxidized to CO2 during the desorption

process. The results show that h-CeO2-873 has the

ability to absorb CO, but the adsorption capacity is

much lower than Au/CeO2-873. In addition, Au

could weaken Ce-O bond to increase lattice oxygen

mobility [24], so CO2 desorption peak at 663 K on

h-CeO2-873 migrated to 613 K on Au/CeO2-873.

Moreover, even though the hydroxylated support

h-CeO2-873 could adsorb CO to form formate and

carbonate-like species as Au/CeO2-873 did, it shows

no activity for CO oxidation at 298 K. Therefore we

speculate that the excellent activity of the

Au/CeO2-873 catalyst was governed by other key

factors, and less concerns with the formate and

carbonate-like species.

In the -OH (hydroxyl) region, the peaks of

Au/CeO2-873 at 3590, 3630, 3710 and 3728 cm-1 (Fig.

4c), representing active -OH groups [22,25], also

had similar appearance rates with formate and

carbonate-like species, but not much changes could

be observed for h-CeO2-873 in the -OH region (Fig.

3c). Meantime, the water bending mode at 3650 cm-1

decreased and formed a valley for Au/CeO2-873. By

comparing the FTIR and CO-TPD results between

h-CeO2-873 and Au/CeO2-873, it can be found that a

small amount of gold loaded on the surface not

only enhanced the adsorption of CO on the catalyst,

but also activate H2O to produce some new -OH

groups locating at different position.

How H2O can produce the new -OH species in

the conditions of the system being studied, reacting

with CO or O2? Although CO and H2O were present

simultaneously, water gas shift reaction required

significantly higher temperature to convert CO to

CO2, thereby excluding the possibility of the WGSR

during CO oxidation in the presence of H2O at

room temperature. The only way to produce the

active -OH group just left the reaction between H2O

and O2.There are two kinds of way for H2O to

activate O2. One was proposed by Haruta et al [16]:

O2 + H2O → 2-OH + Oact, another was reported by

Ojeda et al [20]: O2 + H2O → -OOH + -OH. O2 could

be activated by H2O to produce active -OH groups,

hydroperoxy groups -OOH and Oact at the

perimeter interfaces between gold and support.


5 Nano Res.

More facile oxygen adsorption in the presence of

H2O is also corroborated by DFT calculations [18].

Oact could directly react with the Au bonded CO

(Au-CO) to produce CO2, while the active -OH

groups and the hydroperoxy groups -OOH could

react with Au-CO to form the intermediates, such as

formate (Au-COOH)interface and carbonate-like

(Au-CO3H)interface species. These intermediates

forming at the perimeter interfaces are easily

converted into CO2 with the attack of Oact. Boccuzzi

et al, performed CO oxidation over Au/TiO2 with

C16O and 18O2 in the presence of H216O and observed

only the formation of C16O18O. This excluded the

participation of oxygen from the support or H2O

but indicates that the promotional effect of H2O

may be associated with the dissociation of O2 [26].

Despite Oact did not be detected in this work, our

experimental results from the water side to shed

light on the relationship between water and O2.

However, the formate (-COOH)support and

carbonate-like (-CO3H)support species forming on the

support, which were observed in the OCO- region

of h-CeO2-873, kept stable. This is due to the lack

of Au to produce Oact. In WGS reaction, the

decomposition of the formate and carbonate-like

species needs a much higher temperature, which is

also due to the lack of the source of Oact in the

reactant gas. In addition, because without the

addition of O2 in the CO-TPD experiment system,

the desorption peak of CO2 at 403 K also did not

shift to room temperature as the activity test shown

over the catalyst Au/CeO2-873.

Consequently, there are two kinds of formate and

carbonate-like species on the surface of the catalyst

Au/CeO2-873: (1) forming on the interfaces between

gold and support; and (2) forming solely on the

support. The former could act as reaction

intermediates to produce CO2 by the attack of Oact.

The latter would keep stable on the support and

reduce O2 adsorption capacity of CeO2, finally

inhibit the catalytic activity. In order to prove our

argument, a comparative experiment was designed.

CO-TPD and CO2-TPD were used for the fresh

catalyst Au/CeO2-873, and O2-TPO was applied for

the catalyst Au/CeO2-873 after reacting 1 hour (Fig.

6). CO2 signal are detected in the above three cases.

The peak around at 400 K is detected in all cases,

but there are a small shoulder peak at 335 K and

two peaks at 527 and 605 K for O2-TPO, which are

not detected in CO2-TPD and CO-TPD. These three

special CO2 desorption peaks can be attributed to

the decomposition of formate (-COOH) and

carbonate-like (-CO3H) species, which produced by

the co-adsorption of the active -OH, -OOH groups

and CO on the catalyst surface. In the cases of

CO-TPD and CO2-TPD without O2 addition, these

three CO2 desorption peaks cannot be found,

because H2O in the catalyst has no chance to react

with O2 to produce active -OH groups and

hydroperoxy groups -OOH. However, the peak at

335 K represents the decomposition of

intermediates (Au-COOH)interface and

(Au-CO3H)interface species by the attack of O2, and the

peaks at 527 and 605 K represent the thermal

decompositions of the formate (-COOH)support and

carbonate-like (-CO3H)support species at high

temperature.

This comparative experiment supply the

information that during the reaction process, some

formate and carbonate-like species can be produced

on the catalyst surface because of co-adsorption of

the active -OH, -OOH groups and CO, therefore the

formation rates of the species in the OCO- region

keep synchronization with those in the -OH region.

Meantime, the excellent CO activity is decided by

the promotion of O2 activation by H2O (O2 + H2O →

2-OH + Oact and O2 + H2O → -OOH + -OH), since the

formation rates of the species in the OCO- region

and in the -OH region are also identical to CO

conversion rate during the activity test. But just the


species (Au-CO3H)interface forming on the interfaces

are reaction intermediates, and the others would

keep stable on the support, such as (-COOH)support

and (-CO3H)support.

3.3 Reaction pathway of ultra-low-gold loaded

catalyst Au/CeO2-1073

Figure 7 and 8 show the in-situ FTIR results of

h-CeO2-1073 and Au/CeO2-1073. It can be found


6 Nano Res.

that no significant changes happened on

h-CeO2-1073 in the OCO- region (Fig. 7b) and -OH

region (Fig. 7c), therefore showed no activity for CO

oxidation (Fig. 7a). Figure 2 shows the catalysts

Au/CeO2-873 and Au/CeO2-1073 dried at 333 K

contain 2.39 and 0.53 wt% water, speculating the

water content in h-CeO2-1073 would also be much

lower than h-CeO2-873, therefore the FTIR intensity

of formate (-COOH)support and carbonate-like

(-CO3H)support species forming on the support

h-CeO2-1073 was also weaker than h-CeO2-873.

Meanwhile, without the addition of gold in

h-CeO2-1073, H2O and O2 lacked the interface to

produce some new -OH group and Oact.

Interestingly, no obvious peaks were detected for

Au/CeO2-1073 in the OCO- region (Fig. 8b), while

the intensities of peaks in the -OH region slightly

increased and kept stable after 15 min (Fig. 8c),

suggesting this catalyst experienced different

reaction pathway, in which formate or

carbonate-like species forming on the interface are

not necessary and Au-CO directly react with Oact to

produce CO2. Based on the XPS results [12], only

Au0 could be detected on Au/CeO2-1073 but 16%

Auδ+ was found on Au/CeO2-873. An inference can

be drawn that Auδ+ are the requisite species to form

the formate and carbonate-like species, which is

consistent with the WGS reaction mechanism. No

matter Au0 species act as spectators or contribute to

the WGS activity, Auδ+ species have been accepted

as active species to form reaction intermediates [22,

27].

We also reported the catalyst Au/CeO2-1073

showed poor activity to oxidize CO after drying at

373 K (CO conversion decreased from 35% to 10%)

[12]. TGA results show that the water content

decreased from 0.53 wt% to 0.35 wt% (Fig. 2). Lack

of the water-derived species makes it difficult to

exhibit the promotional effects on molecular oxygen

adsorption and activation. Au/CeO2-873 is definitely

more active than Au/CeO2-1073, however based on

the XPS results, it can be inferred that a large

amount of Auδ+-OH still remained on the surface of

Au/CeO2-873, and drying these catalysts at 373 K

will result in the thermal decomposition of Auδ+-OH

species or the recombination of -OH groups, in such

a way that Auδ+ species transformed to Au0, thus the

changes of the activity could not only be attributed

to the effect of water or the Au0/Auδ+ ratio. On the

contrary, Au0 was the main gold species on the

surface of Au/CeO2-1073, and the sintering of Au

particle does not occur at so low temperatures,

therefore the change of the activity can be chiefly

attributed to the amount of water absorbed on the

surface [12]. In order to understand the specific

effect of O2 and H2O on CO conversion, O2-TPD

experiments were performed over the catalyst

Au/CeO2-1073 dried at 333 K and 373 K. The mass

spectra of O2 and H2O in process O2-TPD are

illustrated in Figure 9 respectively.

When the catalysts were dried from 333 to 373 K,

the amount of H2O on/in catalysts decreased

significantly. Especially for the H2O peak at 673 K

nearly disappeared in the case of Au/CeO2-1073

dried at 373 K, which represents the chemisorbed

water or the recombination of -OH groups. During

the O2 adsorption process, the following reactions

happened: O2 + H2O → 2-OH + Oact and O2 + H2O →

-OOH + -OH. Oact could be stored by CeO2 and then

desorbed as the form O2 when the temperature

increased. With the decrease of H2O content, the

amount of the adsorbed O2 also reduced. In

addition, when the catalyst was dried at 333 K, most

of O2 tend to desorb at lower temperature,

attributing to the adsorption of O2 at metal support

interfacial sites [28]. On the contrary, most of O2

desorbed at higher temperature when the catalyst

was dried at 373 K, which is due to the desorption

of chemisorbed O2 molecule in the pores of catalyst

[26]. These results further proved the promotional

effect of water on the adsorption and activation of

O2.

3.4 Effect of Oact，H2O and -OH groups on the

catalyst activity

Most of reported catalysts applied in low

temperature CO oxidation had higher gold loadings,

nearly above 1 wt%, but the reaction activity in this

study could be improved when the catalyst with

own water on the surface, even though the gold


7 Nano Res.

content was decreased to less than 0.1 wt%. These

findings lead to the conclusion that Oact greatly

contribute to the activity, and the interfaces between

gold and support supply a platform to produce Oact.

The addition of H2O accelerated the adsorption and

activation of O2 and -OH groups are the byproducts

during the process. Even though -OH groups can

react with Au-CO to form intermediates, such as


(Au-CO3H)interface species, which will keep stable

rather than decompose to CO2 without the attack of

Oact. From these studies, they just suggest the

participation of -OH groups in CO oxidation, but

Oact is the most important active species. Saavedra

et al. evaluated the reaction kinetics and also found

that weakly adsorbed water increased the effective

number of active sites rather than changing their

inherent reactivity [21]. It is worth to note that these

studies just show us that the product CO2 may be

obtained by two reaction pathways, but which

reaction pathway is more effective still keeps

unclear. Because of the difference of surface areas,

water amounts and Au0/Auδ+ ratio between

Au/CeO2-873K and Au/CeO2-1073K, the activities of

them were difficult to compare. Therefore, it is

difficult to analyze that Oact attack the formate and

carbonate-like species to produce CO2 is more easier

or Oact directly react with Au-CO to produce CO2 is

more easier. In addition, a suitable amount of water

on the surface is crucial to obtain excellent activity,

because too much water also could cause the

coverage of active sites and the accumulation of

stable carbonate-like species on the support, which

all negatively effect on the activity.

4 Conclusions

In summary, in-situ FTIR combined with HRTEM,

TGA, CO-TPD, O2-TPO and O2-TPD experiments

presented herein provide a more direct way to

understand the catalytic performance of

ultra-low-gold loaded catalysts with own water for

CO oxidation at ambient temperature. It is the first

time to detect the delicately changed signals of -OH

groups on the catalyst surface by using in-situ FTIR,

which supply the experimental evidence of the

changes of water during the reaction process. The

superior catalytic activity could be rationally

explained by that the appropriate water content in

catalysts themselves could promote O2 adsorption

and activation to produce Oact species. The product

CO2 may be obtained by two reaction pathways: (1)

the decomposition of reaction intermediates on the

perimeter interface by the attack of Oact, such as


(Au-CO3H)interface species; (2) and the direct reaction

between Au-CO and Oact. Auδ+ species are requisite

to form the formate and carbonate-like species on

the interface, therefore the contribution of each is

dependent on the catalyst surface composition.

Au/oxide catalysts also have been attracted much

attention in PEM fuel cells field duo to their

excellent catalytic activities for the selective

oxidation of CO. In the presence of H2, what will

happen on the catalyst surfaces which possess

water by itself? In future, an interesting research

about the behavior of Au/oxide catalysts with water

for selective oxidation of CO will be conducted.

Acknowledgements

The authors are grateful for the financial support of

Technology Center of China Tobacco Fujian

Industrial Co., Ltd.

References

[1] Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold,

London, 2006.

[2] Carabineiro, S. A. C.; Bogdanchikova, N.; Avalos-Borja, M.;

Pestryakov, A.; Tavares, P. B.; Figueiredo, J. L. Gold

supported on metal oxides for carbon monoxide oxidation.

Nano. Res. 2011, 4, 180–193.

[3] Wu, B. H.; Zhang, H.; Chen, C.; Lin, S. C.; Zheng, N. F.

Interfacial activation of catalytically inert Au (6.7

nm)-Fe3O4 dumbbell nanoparticles for CO oxidation. Nano.

Res. 2009, 2, 975–983.

[4] Peng, S.; Lee, Y. M.; Wang, C.; Yin, H. F.; Dai, S.; Sun, S.

H. A facile synthesis of monodisperse Au nanoparticles and

their catalysis of CO oxidation. Nano. Res. 2008, 1,

229–234.

[5] Kast, P.; Kučerová, G.; Behm, R. J. On the nature of the

active Au species: CO oxidation on cyanide leached


8 Nano Res.

Au/TiO2 catalysts. Catal. Today. 2015, 244, 146–160.

[6] Gavril, D.; Georgakab, A.; Loukopoulosb, V.; Karaiskakisb,

G.; Nieuwenhuysa, B. E. On the mechanism of selective

CO oxidation on nanosized Au/γ-Al2O3 catalysts. Gold.

Bull. 2006, 39, 192–199.

[7] Gavril, D. CO oxidation on nanosized Au/Al2O3 by surface

hydroxyl groups and in the absence of O2, studied by

inverse gas chromatography. Catal. Today. 2015, 244,

36–46.

[8] Hutchings, G. J. Catalysis by gold. Catal. Today. 2005, 100,

55–61.

[9] Chen, M. S.; Goodman, D. W. The structure of catalytically

active gold on titania. Science. 2004, 306, 252–255.

[10] Vijay, A.; Mills, G.; Metiu, H. Adsorption of gold on

stoichiometric and reduced rutile TiO2 (110) surfaces. J.

Chem. Phys. 2003, 118, 6536–6551.

[11] Varganov, S. A.; Olson, R. M.; Gordon, M. S.; Metiu, H.

The interaction of oxygen with small gold clusters. J.

Chem. Phys. 2003, 119, 2531–2537.

[12] Li, Q. L.; Zhang, Y. H.; Chen, G. X.; Fan, J. Q.; Lan, H. Q.;

Yang, Y. Q. Ultra-low-gold loading Au/CeO2 catalysts for

ambient temperature CO oxidation: Effect of preparation

conditions on surface composition and activity. J. Catal.

2010, 273, 167–176.

[13] Date, M.; Haruta, M. Moisture effect on CO oxidation

over Au/TiO2 catalyst. J. Catal. 2001, 201, 221–224.

[14] Kung, H. H.; Kung, M. C.; Costello, C. K. Supported Au

catalysts for low temperature CO oxidation. J. Catal. 2003,

216, 425–432.

[15] Calla, J. T.; Davis, R. J. Oxygen-exchange reactions during

CO oxidation over titania- and alumina-supported Au

nanoparticles. J. Catal. 2006, 241, 407–416.

[16] Date, M.; Okumura, M.; Tsubota, S.; Haruta, M. Vital Role

of Moisture in the Catalytic Activity of Supported Gold

Nanoparticles. Angew. Chem. Int. Ed. 2004, 43,

2129–2132.

[17] Gao, F.; Wood, T. E.; Goodman, D. W. The effects of water

on CO oxidation over TiO2 supported Au catalysts. Catal.

Lett. 2010, 134, 9–12.

[18] Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. Identifying an

O2 supply pathway in CO oxidation on Au/TiO2(110): A

density functional theory study on the intrinsic role of

water. J. Am. Chem. Soc. 2006, 128, 4017–4022.

[19] Bongiorno, A. Water-enhanced catalysis of CO oxidation

on free and supported gold nanoclusters. Phys. Rev. Lett.

2005, 95, 106102-106105.

[20] Ojeda, M.; Zhan, B. Z.; Lglesia, E. Mechanistic

interpretation of CO oxidation turnover rates on supported

Au clusters. J. Catal. 2012, 285, 92–102.

[21] Saavedra, J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.;

Chandler, B. D. The critical role of water at the

gold-titania interface in catalytic CO oxidation. Science.

2014, 345, 1599–1602.

[22] Karpenko, A.; Leppelt, R.; Plzak, V.; Behm, R. J. The role

of cationic Au3+ and nonionic Au0 species in the low

temperature water-gas shift reaction on Au/CeO2 catalysts.

J. Catal. 2007, 252, 231–242.

[23] Ntho, T. A.; Anderson, J. A.; Scurrell, M. S. CO oxidation

over titanate nanotube supported Au: Deactivation due to

bicarbonate. J. Catal. 2009, 261, 94-100.

[24] Fu, Q.; Saltsburg, H. Flytzani-Stephanopoulos, M. Active

nonmetallic Au and Pt species on ceria-based water-gas

shift catalysts. Science. 2003, 301, 935–938.

[25] Karpenko, A.; Denkwitz, Y.; Plzak, V.; Cai, J.; Leppelt, R.;

Schumacher, B.; Behm, R. J. Low-temperature water-gas

shift reaction on Au/CeO2 catalysts-the influence of

catalyst pre-treatment on the activity and deactivation in

idealized reformate. Catal. Lett. 2007, 116, 105–115.

[26] Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.;

Ichikawa, S.; Haruta, M. Au/TiO2 nanosized samples: A

catalytic, TEM, and FTIR study of the effect of

calcination temperature on the CO oxidation. J. Catal.

2001, 202, 256–267.

[27] Kim, C. H.; Thompson, L T. On the importance of

nanocrystalline gold for Au/CeO2 water–gas shift catalysts.

J. Catal. 2006, 244, 248–250.

[28] Parida, K. M.; Sahu, N.; Mohapatra, P.; Scurrell, M. S.

Low temperature CO oxidation over gold supported

mesoporous Fe-TiO2. J. Molecular. Catal. A: Chem. 2010,

319, 92–97.


9 Nano Res.

FIGURES.

Figure 1 High-resolution transmission electron microscopic

images of the catalysts with 1 wt% gold content: (a)

Au/CeO2-873, (b) Au/CeO2-1073.

Figure 2 TGA test for the catalysts Au/CeO2-873, Au/CeO2-1073

dried at 333 K, and the catalyst Au/CeO2-1073 dried at 373 K.

Figure 3 Activity (a) and series of in-situ FTIR spectra (b and c)

obtained during the CO oxidation at 298 K over 55 min on

h-CeO2-873.



Au/CeO2-873.

Figure 5 Mass spectra of CO2 during CO-TPD on Au/CeO2-873

and h-CeO2-873.

Figure 6 Mass spectra of CO2 during CO-TPD, CO2-TPD for the

fresh catalyst Au/CeO2-873 and O2-TPO for the catalyst

Au/CeO2-873 after reacting 1 hour.


10 Nano Res.



h-CeO2-1073.



Au/CeO2-1073.

Figure 9 Mass spectra of O2 and H2O during O2-TPD on the

catalyst Au/CeO2-1073 dried at 333 K and 373 K.


Nano Res.

Date post:	08-Jul-2020
Category:	Documents
Upload:	others
View:	7 times
Download:	0 times

The behaviors of ultra-low-gold loaded catalysts (Au/CeO ...DOI The Behaviors of Ultra-Low-Gold...

Documents