Enhanced liquid phase catalytic hydrogenation reduction of bromate over Pd-on-Au bimetallic catalysts

Article abstract of DOI:10.1016/j.apcata.2018.06.009

Pd-Au/TiO₂ bimetallic catalysts with varied Au contents were prepared by the sequential photocatalytic deposition method and the liquid phase catalytic hydrogenation reduction of bromate over these catalysts was investigated. The catalysts were characterized using X-ray diffraction, transmission electron microscope, UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, H₂ chemisorption and energy dispersive spectroscopy. Characterization results showed that Pd atoms were site-deposited on the surface of varied size Au cores and formed Pd-on-Au core-shell like bimetallic nanoparticles on TiO₂. The bimetallic catalysts showed higher Pd dispersions and more exposed active sites than that of Pd/TiO₂, and the amount of exposed active sites first increased then decreased with Au content. For a similar Pd loading, the bimetallic catalyst exhibited volcano-shape activity as a function of Au loading and the highest activity was identified on Pd-Au(1.0)/TiO₂ with Au core size around 8.4 nm. In addition, the catalytic reduction of bromate could be well-fitted by the Langmuir-Hinshelwood model, reflecting an adsorption controlled mechanism.

Full text of DOI:10.1016/j.apcata.2018.06.009

Accepted Manuscript
Title: Enhanced liquid phase catalytic hydrogenation
reduction of bromate over Pd-on-Au bimetallic catalysts
Authors: Juan Zhou, Xin Zhou, Liyuan Li, Quanyuan Chen
PII:
S0926-860X(18)30279-5
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.06.009
APCATA 16697
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Applied Catalysis A: General
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Revised date:
Accepted date:
29-3-2018
4-6-2018
5-6-2018
Please cite this article as: Zhou J, Zhou X, Li L, Chen Q, Enhanced liquid phase
catalytic hydrogenation reduction of bromate over Pd-on-Au bimetallic catalysts,
Applied Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.06.009
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Enhanced liquid phase catalytic hydrogenation reduction of bromate over Pd-on-Au bimetallic
catalysts
Juan Zhoua, Xin Zhoua, Liyuan Lib, Quanyuan Chena,*
a College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
b Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai , 201208, China
*Corresponding author. Tel: +86-25-67792540; Fax: +86-25-67792522.
E-mail: qychen@dhu.edu.cn
Graphical abstract
1
Pd
0.8
TiO2
0.6
Pd
Pd
Pd
Pd
0.4
0.2
0
Pd
Au
TiO2
5
6
7
8
9
10
11
12
Au core size (nm)
The catalytic bromate reduction TOF first increased and then decreased with Au core size.
Highlights
 Pd-on-Au core-shell like bimetallic nanoparticles were prepared by the sequential
photocatalytic deposition method.
1

 The bimetallic catalyst exhibited a substantially higher catalytic activity for the liquid phase
catalytic reduction of bromate than the monometallic ones.
 The catalytic activity of bimetallic catalyst first increased then decreased with Au core size.
 Both the H2 activation and reactant adsorption play important roles on the catalytic
hydrogenation reduction of bromate.
Abstract
Pd-Au/TiO2 bimetallic catalysts with varied Au contents were prepared by the sequential
photocatalytic deposition method and the liquid phase catalytic hydrogenation reduction of bromate
over these catalysts was investigated. The catalysts were characterized using X-ray diffraction,
transmission electron microscope, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron
spectroscopy, H2 chemisorption and energy dispersive spectroscopy. Characterization results showed
that Pd atoms were site-deposited on the surface of varied size Au cores and formed Pd-on-Au core-
shell like bimetallic nanoparticles on TiO2. The bimetallic catalysts showed higher Pd dispersions and
more exposed active sites than that of Pd/TiO2, and the amount of exposed active sites first increased
then decreased with Au content. For a similar Pd loading, the bimetallic catalyst exhibited volcano-
shape activity as a function of Au loading and the highest activity was identified on Pd-Au(1.0)/TiO2
with Au core size around 8.4 nm. In addition, the catalytic reduction of bromate could be well-fitted
by the Langmuir-Hinshelwood model, reflecting an adsorption controlled mechanism.
Keywords: Liquid phase catalytic hydrogenation; Bromate; Pd-Au/TiO2; Photocatalytic deposition;
core-shell structure
1. Introduction
2

-
Bromate (BrO3 ) is a common disinfection byproduct generated from bromide-containing source
waters undergo ozonation or chlorination [1]. Bromate is considered as a potential carcinogen to
human beings [2,3], the maximum concentration in the drinking water regulated by the U.S.
Environmental Protection Agency (EPA) and World Health Organization (WHO) is 0.01 mg l-1 [4,5].
Nowadays varies treatment methods have been developed to eliminate bromate from drinking
water, such as biological treatment [6,7], activated carbon adsorption/reduction [8], photocatalytic
reduction [9,10], electrochemistry reduction [11,12] etc. Alternatively, catalytic hydrogenation
reduction is one of the clean and efficient methods to deal with the halogen-containing compounds
and high valence ions [13-15].
Supported noble metal catalysts usually applied in the hydrogenation reduction reaction involved
the activation of H2 [16,17]. Pd-based catalysts were the most frequently used materials due to its
superior hydrogen dissolving capacity [18]. Our previous studies showed that the catalyst performance
were intimately related to the supports and preparation methods which could adjusted Pd particle size,
electronic state and so on [19-21]. Significantly, the properties of the monometallic catalyst could be
further improved by decorating with another metal to form bimetallic systems and metal Au as the
modifier has received great interest. For example, Kittisakmontree et al. [22] found that the liquid
phase hydrogenation of 1-heptyne could be greatly enhanced over Au-Pd bimetallic catalyst compared
to the pure metal catalyst. Zhou et al. [23] suggested that the activities of Pd-Au bimetallic catalysts
were much higher than the monometallic ones for the catalytic hydrodechlorination of 2,4-
dichlorophenol. Pongthawornsakun et al. [24] improved the selective hydrogenation of acetylene using
Au-Pd-TiO2 bimetallic catalysts.
To deeper understand the catalyst properties or catalytic mechanism, many studies concentrated
on better control of the bimetallic nanostructure to modulate the configuration of bimetallic particles
3

and/or valence electron density of the combined metals [25-27]. The Core-shell Pd-Au bimetallic
catalysts have received great interest because of the controllable structure and special catalytic
performance, which frequently applied in the oxidation [28,29], hydrogen peroxide synthesis [30,31],
hydrogenation [32,33], and electrochemistry reactions [34]. The catalytic activity and selectivity of
the core-shell structure bimetallic catalyst mainly improved by regulating the core size or metal
coverage. Accordingly, the improvement of the bimetallic catalyst performance is generally due to the
geometric and electronic effects. For example, Zhang et al. [35] prepared Au-Pd/SiO2 catalysts using
the electroless deposition method and found that higher coverages of Au on Pd resulted in smaller
ensembles of Pd sites, which favored the hydrogenation of acetylene to ethylene by adsorbing as a π-
bonded species of acetylene on Pd sites. Pretzer et al. [36] prepared Pd-on-Au nanoparticles and
adjusted the Pd surface coverages using the magic cluster model on Au core with varied diameters and
concluded that both the Au particle size and Pd surface coverage affected the catalytic activity
remarkably. Nutt et al. [37] synthesized Pd/Au nanoparticles with different Pd loadings and suggested
that the electronic structure of metal Pd could be modified by Au only when the Pd-Au metal were in
intimate contact, and the intimate interface supplied additional active sites for the hydrodechlorination
of trichloroethene.
The photocatalytic deposition method with semiconductor material as the support is one of the
alternative methods to simply obtain uniform bimetallic nanostructure [38,39]. For the preparation of
TiO2 supported Pd-on-Au bimetallic catalyst via the two-step photocatalytic deposition method, TiO2
acted as the catalyst to generate photoelectrons for the reduction of metal ions. During the second
deposition process metal Au acted as the sink of photo-induced electrons and Pd atoms were site
deposited on the surface of Au particles, forming Pd-on-Au core-shell like bimetallic nanoparticles on
TiO2 [40,41], which may exhibit interesting catalytic performance. However, thus far few studies have
4

been conducted on the behavior of Pd-Au bimetallic catalysts for the catalytic hydrogenation reduction
of bromate. Hence, it is of interest to examine the catalytic reduction of bromate by Pd-Au/TiO2
prepared using the photocatalytic deposition method.
In the present study, we prepared monometallic and Pd-on-Au core-shell like bimetallic catalysts
with varied Au contents supported on TiO2 by the two-step photocatalytic deposition method, and
investigated the liquid phase catalytic hydrogenation reduction of bromate.
2. Experimental
2.1. Catalyst preparation
The monometallic and bimetallic Pd-Au catalysts with varied Au deposition amounts were
prepared using the photocatalytic deposition method. For the preparation of Pd/TiO2 with Pd
deposition amount at 1.0 wt%, 2 g of TiO2 (P25, Degussa) was suspended in 480 ml of distilled water
and added a desired amount of PdCl2 solution and 10 ml of methanol. After purging with a N2 flow
(50 ml min-1) under stirring for 1 h in the dark, the mixture was exposed to ultraviolet (UV) irradiation
with a high pressure Hg lamp (500W) for 4 h. The Pd/TiO2 sample was obtained by filtration, followed
by washing thoroughly with distilled water, drying at 110 oC for 8 h under vacuum. For the bimetallic
Pd-Au/TiO2 catalysts, Pd deposition amount was kept around 1.0 wt% and Au loading amount was
varied. Typically, metal Au was first deposited onto TiO2 with HAuCl4·4H2O as the precursor and
followed the same procedure as the preparation of Pd/TiO2, then, the sample experienced a second
deposition with PdCl2 as the precursor, and monometallic Au/TiO2 were synthesized for comparison.
The collected bimetallic catalyst was referred to as Pd-Au(x)/TiO2, where x is the Au actual deposition
amount (wt. %) detected by ICP-AES.
2.2. Catalyst characterization
Brunauer-Emmett-Teller (BET) surface areas of the samples were collected using a Micromeritics
5

o
ASAP 2020 instrument (Micromeritics Instrument Co., Norcross, GA, USA) at -196 C. The X-ray
diffraction (XRD) patterns of the catalysts were recorded on a Rigaku D/max-RA powder diffraction-
meter with a Cu Kα radiation (Rigaku, Tokyo, Japan). The Pd content in the catalyst was measured on
an inductive coupled plasma emission spectrometer (ICP) (J-A1100, Jarrell-Ash, USA). The X-ray
photoelectron spectroscopy (XPS) was tested by a PHI5000 VersaProbe equipped with a
monochromatized Al Ka excitation source (hv= 1486.6 eV) (ULVAC-PHI, Japan). The C 1s peak
(284.6 eV) was used as the calibration of binding energy. The UV-vis spectra were analyzed using
SHIMADZU UV-2450 UV/vis spectrophotometer (Shimadzu Co., Kyoto, Japan) with BaSO4 as a
reference. Energy dispersive X-ray spectroscopy (EDS) elemental mapping was collected from an X-
MAX energy dispersive spectrometer (Oxford, UK) equipped with an ARM-200F transmission
electron microscope (JEOL Co., Japan.).
The TEM images of the catalysts were obtained on a JEM-200CX electron microscope (JEOL
Co., Tokyo, Japan). The average metal particle sizes of the catalysts were obtained using Eq (1) [42]:
d = n d3/ n d2
(1)
 
s
i
i
i i
Where ni is the number of counted metal particles with diameter of di, and the total number of
counted particles (Σ ni) is larger than 300.
The zeta potentials of the materials were tested on a Zeta Potential Analyzer (Zeta PALS,
Brookhaven Instruments Co.). Typically, 40 mg of the materials were ultrasonic dispersed in 400 mL
distilled water, then the solution pH was adjusted using 0.1 M HCl and 0.1 M NaOH, the measurement
was conducted after the suspension standing at room temperature for 12 h.
Pd dispersion of the samples was performed using the H2 chemisorption method. Simply, 200 mg
of catalyst was pressed into pieces and loaded in a U-shaped quartz tube, and heated at 120℃ in a H2
flow (40 ml min-1) for 1h. Then, the tube was purging with a Ar flow (40 ml min-1) for 1h. After
6

cooling to room temperature, the H2 chemisorption was started using the pulse titration model with H2
partial pressure below 0.011 atm. H2 contents were tested using an online gas chromatograph (GC)
with a thermal conductivity detector (TCD).
2.3. Liquid phase catalytic reduction reaction
The liquid phase catalytic reduction of bromate was carried out under atmospheric pressure in a
250 ml of four-necked flask equipped with a sample port, pH-stat, H2 inlet and outlet. The reaction
temperature was stabilized at 20±0.5 °C by a water-bath (SDC-6, scientz Co., China). Typically, 20
mg of catalyst was suspended in 200 ml of 0.4 mol l-1 potassium bromate solution. The reactor was
purging with a N2 flow (50 ml min-1) under stirring for 30 min, and then the N2 flow was switched into
a H2 flow (150 ml min-1) during the reaction process. Samples were collected at selected time intervals
and the catalyst particles were removed using 0.45 μm filter. The concentrations of reactant and
products in the filtrate were analyzed using Ion Chromatography (ICS1000, Dionex) with a mobile
phase of 18 mM KOH solution. The initial activities were calculated using Eq (2), and the turnover
frequency (TOF), defined as the reduction rate of bromate over per exposed active site, was determined
by Eq (3):
C0 Ct
Wcat t
(2)
Initial Activity 
Initial Activity
NPd
(3)
TOF=
-
-
In which, C0 is the initial concentration of BrO3 ; Ct is the concentration of BrO3 at reaction
time t; Wcat is catalyst dosage; NPd is the number of exposed Pd atom per gram catalyst, which
determined by the H2 chemisorption data [43].
3. Results and discussion
3.1. Catalyst characterization
7

The zeta potentials of TiO2, Pd/TiO2 and Pd-Au(1.0)/TiO2 as a function of pH are presented in
Fig. 1. Clearly, the zeta potentials decreased with pH, and all the isoelectric points (IEPs) of the
materials were around 6.0, implying that the catalyst surface was positively charged when pH was
lower than 6.0, while negatively charged when solution pH was higher than 6.0.
The XRD patterns of Pd/TiO2 and Pd-Au bimetallic catalysts with varied Au loading amounts are
presented in Fig. 1S, supporting information. All the samples displayed diffraction peaks at 25.2o,
27.6o, 37.9o, 48.6o, 54.1o, 55.3o and 62.7o, respectively characterize the anatase and rutile phases of
TiO2 [44]. However, characteristic peaks of Pd were not identified, likely due to the small metal
particle size and/or the low metal loading amounts. Additionally, only Pd-Au(4.3)/TiO2 showed the
diffraction peak of metal Au around 44.4o [45], reflecting more exposed Au particles which may
ascribe to the lower Pd surface coverage on Au particle.
The Pd-Au bimetallic nanostructure was tested by UV-vis and the UV-vis spectra of monometallic
and Pd-Au bimetallic catalysts are shown in Fig. 2 and Fig. 2S, supporting information. All samples
presented strong absorbance below 400nm, attributed to anatase and rutile phases of support TiO2 [46].
Additionally, there was another peak around 550 nm for Au(1.0)/TiO2 while no peak was observed for
Pd/TiO2, reflecting that only metal Au particles exhibited surface plasmon resonance. Comparing with
Au(1.0)/TiO2, Pd-Au(1.0)/TiO2 displayed a much weaker absorbance peak, which may due to the
surface of Au particles covered by Pd atoms and resulted in the disappearance of Au plasmon band,
conforming that metal Pd tended to site grow on Au ensemble and formed Pd shell-Au core like
bimetallic nanostructure. For bimetallic catalysts, the absorbance peak become stronger with Au
loading, reflecting decreased Pd surface coverage and increased exposed Au species with Au loading.
TEM images and histograms of metal particle size distributions of monometallic Au catalysts with
varied Au loadings are shown in Fig. 2S, supporting information. Increasing Au amount from 0.5 to
8

4.3%, the metal particle size increased from 5.8 to 10.1 nm, implying that Au particles aggregated and
formed larger Au ensembles with the loading amount gradually. TEM images and histograms of metal
particle size distributions of Pd/TiO2, Au(1.0)/TiO2 and the bimetallic catalysts are shown in Fig. 3.
The calculating metal particle sizes are listed in Table 1. For all samples, TiO2 particles with irregular
shape and sizes around 30-40 nm were clearly observed. The average metal particle was 4.7 nm for
Pd/TiO2, much smaller than that of Au(1.0)/TiO2 (8.4 nm). In addition, the monometallic and
bimetallic catalysts with the same Au loadings exhibited similar metal particle size. For example, the
metal particle sizes were 5.8, 8.4, 9.3 and 10.1 nm for Au(0.5)/TiO2, Au(1.0)/TiO2, Au(2.1)/TiO2 and
Au(4.3)/TiO2, respectively, while the metal particle sizes were 6.0, 8.2, 9.4 and 10.1 nm for Pd-
Au(0.5)/TiO2, Pd-Au(1.0)/TiO2, Pd-Au(2.1)/TiO2 and Pd-Au(4.3)/TiO2, respectively, suggesting that
Pd mainly coated on metal Au instead of forming discrete Pd particles on TiO2 directly.
To further study the morphology of the bimetallic particles in the catalysts, EDS elemental
mapping was performed on Pd-Au(1.0)/TiO2, and the images are compared in Fig. 4. The high
intensity signal of Ti and O elements can be clearly tested and the Pd and Au intensity profiles were
collected from a spherical particle of 8.5 nm (Fig. 4a). The elemental mapping of Au showed a sphere
with higher atomic density in the center region, and a similar distribution of Pd element to that of Au
element was observed. Besides, the signal of Pd and Au elements displayed in the same region, the
excellent overlapping of element Pd on Au suggested that Pd and Au were in close contact, confirming
the site-specific deposition of Pd on Au rather than on TiO2. In addition, Pd element showed uniform
and much lower atomic density than that of Au element, implying a high Pd dispersion on the surface
of Au particle.
Pd dispersions were evaluated by H2 chemisorption and the results are compiled in Table 1.
Because metallic Au is incapable of adsorbing H2, the H2 adsorption amounts only related to the
9

exposed Pd sites in the catalysts. All the bimetallic catalysts showed higher Pd dispersions than that
of Pd/TiO2 at the same Pd loading amounts, suggesting that metal Au facilitated the dispersion of Pd
particles. Increasing Au content from 0.5 to 1.0%, Pd dispersion increased from 20.5 to 25.8%, while
further increase Au content to 4.3%, Pd dispersion decreased to 17.2%. Notably, for Pd-Au(4.3)/TiO2,
Pd dispersion was similar to the monometallic Pd catalyst, reflecting that Pd dispersion could not be
improved over large Au aggregates. In parallel, the H2 adsorption amount first increased and then
decreased with Au content and the maximum value was observed on Pd-Au(1.0)/TiO2.
The structural properties of metal Pd and Au are further verified by XPS analysis and the results
are compared in Fig. 5 and Fig. 4S, supporting information, the relative parameters are shown in Table
2. Because the Pd 3d5/2 peak was overlapped by Au 4d5/2 peak, the surface properties of the Pd-Au
bimetallic particles were identified using Pd 3d3/2 and Au 4 f7/2. For Pd/TiO2 and Au(1.0)/TiO2, the
binding energies were observed at 340.7eV and 83.7eV, respectively assigned to Pd 3d3/2 and Au 4f7/2
[47]. For the bimetallic catalysts, the binding energies of metal Pd had a negative shift with the increase
of Au loading amount. Similarly, the Au binding energies also had a slight blue shift, resulted in
enhanced electronegativity of the nanoparticles, reflecting electron transfer between Pd and Au. Lee
et al. [48] and Yi et al. [49] studied the properties of Pd-Au bimetallic catalysts and also found blue
shifts of binding energies in the bimetallic Pd-Au particles and proposed a charge compensation model
that Pd loses sp-electrons and gains d-electrons while Au gains sp-electrons and loses d-electrons.
Here, the electron transfer between Pd and Au could be described by the same mechanism. Notably,
Pd-Au(0.5)/TiO2 and Pd/TiO2 showed the same Pd binding energy (340.7 eV), which can be explained
by no intimate contact between surface Pd atoms and inner Au particles in Pd-Au(0.5)/TiO2. In
addition, the Pd/Au surface ratios are much higher than the Pd/Au bulk ratios, further implying that
Pd particles mainly site deposited on the surface of Au and formed Pd-on-Au core-shell like structure.
10

3.2. Liquid phase catalytic reduction of bromate
3.2.1 Bromate reduction over mono- and bi-metallic catalysts
The liquid phase catalytic reduction of bromate over Pd/TiO2, Au(1.0)/TiO2 and Pd-Au(1.0)/TiO2
are shown in Fig. 6. Notably, the catalytic activities of the catalysts differed with metals at a similar
metal loading level. For example, after reaction for 50 min, negligible bromate conversion was
observed on Au(1.0)/TiO2, while approximately 49.5 and 94.6% of bromate was reduced on Pd/TiO2
and Pd-Au(1.0)/TiO2, respectively, suggesting that the bimetallic core-shell like catalyst had much
higher catalytic activity than the monometallic ones. During the reaction, H2 was adsorbed and
dissociated into active H by the Pd active sites and formed Pd-H and/or β-PdH phase [35,50], which
-
then converted BrO3 into Br- [51]. The hydrogenation reduction of bromate over Pd-based catalyst are
illustrated in Scheme 1. As indicated by H2 chemisorption, Pd-Au(1.0)/TiO2 had much higher H2
adsorption amount than that of Pd/TiO2 and Au(1.0)/TiO2, providing more active sites in the reduction
reaction and resulted in much higher bromate conversion.
3.2.2 Effect of Au deposition amount
To further research the activities of the core-shell like structure catalysts with varied Au core
sizes, the catalytic hydrogenation reduction of bromate over bimetallic catalysts with different Au
amounts are compiled in Fig. 7 (a). Increasing Au deposition amount from 0.5 to 4.3%, the catalyst
activities first increased and then decreased and Pd-Au(1.0)/TiO2 showed the highest activity. More
insight into the active dependence of exposed active sites on Au core size can be obtained by
comparing the TOF. The results are shown in Fig. 7 (b) and the TOF value displayed a volcano-like
dependence on Au core size. For the liquid phase catalytic hydrogenation reactions over supported Pd
catalyst, the affinity of Pd particles with the reactant and H2 are considered to be the crucial steps [19].
Generally, lower Pd dispersion with larger Pd particle size facilitates H2 solubility and formation of β-
11

PdH phase, thus exhibited stronger catalytic ability for H2 activation [50]. For the bimetallic Pd-Au
catalysts, Pd dispersion first increased and then decreased and resulted in negative peak like H2
activation ability with Au core size. On the other hand, the affinity between Pd active sites and bromate
varied
with
valence
electron
density
of
Pd
particles.
As evidenced by XPS analysis, Pd surface became electron-rich with Au content, which inhibited the
hydrogenation reduction of bromate as a result of the electrostatic repulsion between the anionic
-
reactant (BrO3 ) and Pd surface. Hence, for the bimetallic catalysts with Au core size larger than 8.4
nm, the activation of H2 was enhanced while the adsorption of bromate over Pd particles was attenuate
with the increase of Au core size, and finally resulted in reduced TOF, indicating that the surface
adsorption was more important than the activation of H2. However, Pd-Au(0.5)/TiO2 had larger Pd
particle size and attenuate electronegativity than that of Pd-Au(1.0)/TiO2, but showed lower TOF value.
For the Pd-on-Au bimetallic nanoparticles, different Pd islands formed on Au core with varied
diameters, and more highly active Pd ensembles (as small as 2-3 Pd atoms) may form on larger Au
core as a result of Pd-Au interaction [37]. In this study, Pd-Au(1.0)/TiO2 had larger Au core size than
that of Pd-Au(0.5)/TiO2 and may show more active Pd ensembles as well as higher TOF value because
of stronger Pd-Au interaction. Undoubtedly, more research should be warranted in the future to
determine the Pd-Au coordination effect on the liquid phase catalytic activity.
3.2.3 Effect of bromate adsorption
For a heterogeneous catalytic reaction, the reaction was conducted on the surface of the catalyst,
thus the role of bromate adsorption is extremely important. The catalytic reduction of bromate with
different initial concentrations over Pd-Au(1.0)/TiO2 was performed and the results are shown in Fig.
8 (a). The bromate reduction efficiency increased with the initial concentration, suggesting that the
adsorption of bromate on the catalyst surface facilitated the hydrogenation reduction reaction. The
12

results can be described using the Langmuir-Hinshelwood model [52]:
bC0
r  ks  k
(4)
(5)
0
1bC0
1
1
1


r
kbC0
k
0
where r0 is the initial reduction rate at bromate concentration of C0, θs is the coverage of bromate
adsorption on the catalyst surface, k is the reaction rate constant, and b is the adsorption equilibrium
constant for bromate.
The linear plot of 1/r0 with 1/C0 is shown in Fig. 8 (b), the good liner relationship (R2>0.98)
indicated that the reduction of bromate could be well fitted by Langmuir-Hinshelwood model and
bromate pre-adsorption on the catalyst surface was the rate-controlling step [53].
More insight into the role of bromate adsorption, the catalytic reduction of bromate was
conducted with varied solution pH and the results are compiled in Fig. 9. Increasing the solution pH
from 2.1 to 8.1, the initial reduction rate decreased from 105.5 to 3.18 mM gcat-1 h-1, implying that the
reduction reaction was inhibited by increasing the solution pH. As suggested by IEP results, the
catalyst surface was positively charged when pH was lower than 6.0 while negatively charged when
pH>6.0. Therefore, the interaction between the catalyst surface and bromate changed from
electrostatic absorption to electrostatic repulsion with solution pH, resulted in weaker adsorption and
declined reduction rate of bromate.
4. Conclusions
In the present study, bimetallic Pd-Au/TiO2 catalysts with different Au loading amounts were
prepared by the stepwise photocatalytic deposition method and the liquid phase catalytic
hydrogenation reduction of bromate over the catalysts was investigated. Characterization results
showed that the bimetallic particles formed Pd-on-Au core-shell like nanostructure over TiO2, and the
13

Pd dispersion first increased and then decreased with Au amount. At a similar metal deposition amount,
the bimetallic catalyst presented much higher activity than the monometallic ones due to more
accessible active sites. The catalytic activity first increased and then decreased with Au content and
Pd-Au(1.0)/TiO2 had the highest activity. In addition, TOF of bromate reduction exhibited a volcano-
type activity with Au content, which can be explained by the changeable of H2 activation ability and
the adsorption interaction between the reactant and the surface of Pd nanoparticles. The electronic
repulsion between the catalyst surface and bromate was enhanced with the reaction solution pH and
the catalytic reduction of bromate can be well described by the Langmuir-Hinshelwood model,
reflecting an adsorption controlled mechanism.
Acknowledgements
The financial support from the Natural Science Foundation of China (No. 51508080) and the
Fundamental Research Funds for the Central Universities (Nos. 2232015DZ1-01 and 2232018D3-43)
are gratefully acknowledged.
14

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17

Tables and Figures
Figure captions:
Figure 1: Surface charge densities (ζ) of TiO2, Pd(1.0)/TiO2 and Pd-Au(1.0)/TiO2 as a function of
solution pH.
Figure 2: UV-vis diffuse reflectance spectra of Pd/TiO2, Au(1.0)/TiO2 and Pd-Au(1.0)/TiO2.
Figure 3: (a) TEM images and (b) histograms of metal particle size distributions of the catalysts.
Figure 4: (a) HR-TEM image of a Pd-Au particle on Pd-Au(1.0)/TiO2 and the corresponding EDS
elemental mapping of (b) Ti element (red), (c)O element (green), (d) Au element (pink), (e)
Pd element (blue), (f) the overlapping of Pd and Au elements.
Figure 5: XPS profiles of Pd/TiO2 and Pd-Au/TiO2 with varied Au contents in Pd 3d region.
Figure 6: Catalytic hydrogenation reduction of bromate over Pd/TiO2, Au(1.0)/TiO2, and Pd-
Au(1.0)/TiO2. Reaction conditions: pH 5.6. Catalyst dosage: 0.1 g l-1, bromate
concentration: 0.4 mM.
Figure 7: (a) The catalytic hydrogenation reduction of bromate over Pd-Au/TiO2 with varied Au
contents. (b) TOF dependence on Au core size. Reaction conditions: pH 5.6. Catalyst
dosage: 0.1 g l-1.
Figure 8: (a) The catalytic hydrogenation reduction of bromate at different initial bromate
concentrations and (b) linear plot of 1/r0 versus 1/C0. Reaction conditions: pH 5.6. Catalyst:
Pd-Au(1.0)/TiO2. Catalyst dosage: 0.1 g l-1. Line represents the fitting curve using the
Langmuir–Hinshelwood model.
Figure 9: Initial activity of Pd-Au(1.0)/TiO2 catalyst versus pH. Reaction condition: initial bromate
concentration 0.4 mM, catalyst dosage: 0.1 g l-1.
18

-
Scheme 1. Reaction pathway of the catalytic hydrogenation reduction of BrO3
H·
Pd
H·
Pd
-
BrO3
H2
H·
H·
Pd
Pd
Au
TiO2
Br- + H2O
catalyst
1 atm, 25 οC
-
BrO3 + 3H2
Br- + 3H2O
19

Figure 1:
60
◆: TiO2
▲: Pd(1.0)/TiO2
■: Pd-Au(1.0)/TiO2
40
20
0
-20
-40
-60
0
3
6
9
12
pH
Figure 2:
20

Pd-Au(1.0)/TiO2
Au(1.0)/TiO2
Pd/TiO2
700
300
400
500
600
800
Wavelength (nm)
21

Figure 3:
(a)
Pd-Au(1.0)/TiO2
Pd-Au(4.3)/TiO2
Au(1.0)/TiO2
Pd-Au(0.5)/TiO2
Pd-Au(2.1)/TiO2
Pd/TiO2
22

(b)
50
40
30
20
10
0
Pd-Au(0.5)/TiO2
Pd-Au(1.0)/TiO2
Pd-Au(2.1)/TiO2
Pd-Au(4.3)/TiO2
<4
4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 >12
Particle size (nm)
50
40
30
20
10
0
Pd/TiO2
Au(1.0)/TiO2
<3
3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 >11
Particle size (nm)
23

Figure 4:
a
b
e
c
d
f
24

Figure 5:
Pd 3d3/2
Au 4d5/2
Pd/TiO2
Pd-Au(0.5)/TiO2
Pd-Au(1.0)/TiO2
Pd-Au(2.1)/TiO2
Pd-Au(4.3)/TiO2
330 332 334 336 338 340 342 344 346
Binding energy (eV)
25

Figure 6:
▲: Au(1.0)/TiO2
◆: Pd/TiO2
■: Pd-Au(1.0)/TiO2
Time (min)
26

Figure 7:
0.5
(a)
●: Pd-Au(0.5)/TiO2
■: Pd-Au(1.0)/TiO2
◆: Pd-Au(2.1)/TiO2
◇: Pd-Au(4.3)/TiO2
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
120
Time (min)
(b)
Au core size (nm)
27

Figure 8:
(a)
▲: 0.39 mM
◆: 0.31 mM
■: 0.19 mM
●: 0.096 mM
Time (min)
(b)
1/C0
Figure 9:
28

pH
29