Deactivation and Regeneration of NaTaO3 Photocatalyst in Cooperating Dehydrogenation Coupling of Isopropanol and Hydrogenation Coupling of Acetone Reaction System

Article abstract of DOI:10.1111/php.12856

Photocatalyst activity is influenced by many factors, such as adsorption of by-products, runoff of surface hydroxyl groups and carriers. In this study, a simple and efficient photocatalyst regeneration method was developed. Results indicated that NaTaO₃ photocatalyst lost its photoactivity after three cycles of reaction that involves coupling of isopropanol and hydrogenation coupling of acetone reaction system. Runoff of Na on the surface was the main reason for the deactivation of NaTaO₃ photocatalyst. After hydrothermal treatment of the deactivated NaTaO₃ with 10 m NaOH at 180°C for 12 h, its photocatalytic activity was restored to the original level. The photocatalytic activity remained stable even after 10 cycles.

Full text of DOI:10.1111/php.12856

Article type : Research Article
Corresponding author email id: 502783577@qq.com
Deactivation and Regeneration of NaTaO3 Photocatalyst in Co-operating
Dehydrogenation Coupling of Isopropanol and Hydrogenation Coupling of Acetone
Reaction System
Baoyue Cao1, Yan Yu1, Shan Xu1, Xiangting Wang1, Li Zhang2, Zhijian Wang2, Shumin
Wang1*, and Guochun Zhang1*
1Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources, College of
Chemical Engineering and Modern Materials, Shangluo University, Shangluo 726000, China.
2State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan, 030001, China.
*Corresponding authors’ emails: (Shumin Wang) and
(Guochun Zhang)
ABSTRACT
Photocatalyst activity is influenced by many factors, such as adsorption of by-products,
runoff of surface hydroxyl groups, and carriers. In this study, a simple and efficient
photocatalyst regeneration method was developed. Results indicated that NaTaO3
photocatalyst lost its photo-activity after three cycles of reaction that involves coupling of
isopropanol and hydrogenation coupling of acetone reaction system. Runoff of Na on the
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1111/php.12856
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surface was the main reason for the deactivation of NaTaO3 photocatalyst. After
hydrothermal treatment of the deactivated NaTaO3 with 10 M NaOH at 180 °C for 12 h, its
photocatalytic activity was restored to the original level. The photocatalytic activity remained
stable even after 10 cycles
INTRODUCTION
Given that photocatalysis is environmentally friendly and can effectively solve energy
and environmental problems brought by traditional chemical engineering (1-2), it has been
extensively adopted in various applications, such as photocatalytic hydrogen production (3),
photocatalytic organic synthesis (4), and pollutant disposal (5). Numerous research and
reports on the development of new photocatalytic materials, modification of photocatalysts,
and design of high-efficiency photocatalytic reaction have been conducted.
Catalytic hydrogenation and dehydrogenation reactions play a key role in the chemical
industry, particularly in the synthesis of various pharmaceuticals and fine chemicals, in
addition to bulk chemicals (6). This knowledge compelled us to combine hydrogenation
reactions and dehydrogenation reactions in one catalytic process to achieve excellent thermal
balances, efficient hydrogen use, and consequently high overall efficiency (7). Our group
previously reported co-operating the dehydrogenation coupling of isopropanol and
hydrogenation coupling of acetone reaction system. The photogenerated holes and electrons
were efficiently used, which dramatically promoted the total efficiency and cleanliness of
photocatalytic systems and photocatalytic organic syntheses (8). However, the instability of
photocatalysts has been the key restriction in its industrialization, the same with other
photocatalytic reaction systems (9). Therefore, developing photocatalysts with favorable
stability is essential for the development of photocatalysis technology. Photo-activity and the
stability of photocatalysts are related to multiple factors, mainly including its preparation,
pretreatment technology, reaction conditions, and operation (10-11). For different structures
and reaction systems, their deactivation mechanisms are different. In a photocatalytic reaction
system, studying the deactivation mechanism and maintaining stability of photocatalyst
constitute a research topic of great practical significance (12,13). Currently, methods such as
photo-irradiation (14), heat treatment (15), and ultrasonic wave regeneration (16) are
generally used. However, different reaction systems have different photocatalyst deactivation
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mechanisms. Thus, the deactivation mechanism should be considered to obtain a
photocatalyst with high catalytic activity and good stability. Based on the above research, a
simple and effective photocatalytic regeneration method was developed by studying the
photocatalytic deactivation mechanism of coupling the reaction systems of isopropanol
dehydrogenation and acetone hydrogenation through NaTaO3 photocatalysis.
MATERIALS AND METHODS
Preparation of NaTaO3 photocatalysts. The synthesis of NaTaO3 photocatalysts was similar
to the method reported before (17). Firstly, 4.42 g of Ta2O5 was mixed with 30 ml of NaOH
with desired concentration and stirred at room temperature for 2 h. Then the mixture was
filled into a Teflon reactor (50 mL) and was heated up to 180oC for a 24 h reaction. The
NaTaO3 photocatalyst was soaked in 100 mL of 1 M HCl solution for 12 h to replace the Na+
on the surface of NaTaO3.
Regeneration of deactivated NaTaO3 photocatalysts. Ultrasonic cleaning to remove the by-
products on the surface of the NaTaO3, then hydro-thermal treated with 10 M NaOH at 180oC
for 12 h. After the reaction, the NaTaO3 products were collected by centrifugation and
washed with deionized water and ethanol to remove the redundant NaOH, and finally dried at
80oC for 12 h.
Characterization of NaTaO3 photocatalysts. The surface functional groups of the NaTaO3
photocatalysts were observed by Fourier Transform infrared spectroscopy (FTIR, Nicolet
380). The crystal structures were characterized by an X-ray diffraction (XRD) (Bruker D8
Advance) with a Cu Kα (λ=0.15406 nm), operated at 40 kV. The element contents and their
binding state on NaTaO3 surface were measured by X-ray photoelectron spectroscopy (XPS,
Kratos Axis Ultra, Al KR source), the C 1s reference of 284.5 eV as the calibration of binding
energy.
Photocatalysis tests. All photocatalytic tests were operated in a 250 mL inner-irradiation-type
Pyrex reactor under 300 W UV light and atmosphere pressure (Figure S1). The photocatalytic
experiments were performed in 200 mL of aqueous solution, using 1.0 g of NaTaO3
photocatalyst. Liquid samples were analyzed by a gas chromatograph (Haixin; GC-950)
equipped with a flame ionization detector (FID).

RESULTS AND DISCUSSION
Stability evaluation of NaTaO3 photocatalyst in coupling reaction system
Our previous results indicated that NaTaO3 photocatalyst presented excellent photocatalytic
activity in co-operating isopropanol dehydrogenation coupling and acetone hydrogenation
coupling reaction systems (acetone and isopropyl alcohol conversion rates were 61.25% and
53.12%, respectively, and pinacol selectivity was greater than 90%) (8). After one cycle of
the reaction, NaTaO3 was directly used for photocatalytic coupling reaction without any
treatment. Its photocatalytic activity evidently dropped, isopropyl alcohol conversion rate was
only 19.18%, and pinacol selectivity was about 82.67%. When the reaction entered its third
cycle, isopropyl alcohol conversion rate was only 7.60% and pinacol selectivity was reduced
to 77.31%. Results are presented in Table 1.
Analysis of deactivated NaTaO3 photocatalyst morphology and structure
Figure 1 shows the XRD patterns of NaTaO3 photocatalyst before and after reaction. It
showed that deactivated photocatalyst still had characteristic peak of NaTaO3 with complete
crystal structure (17). This observation indicated that the crystal structure was not damaged.
FTIR can effectively analyze surface conditions of the photocatalyst and species
adsorbed on its surface. As shown in Figure 2, after the reaction of NaTaO3 photocatalyst for
12 h, its infrared spectrogram had apparent characteristic absorption peaks of C=C, C=O, -
CH3, and -CH and C-OH when compared with NaTaO3 photocatalyst before the reaction. This
result indicated that after the photocatalytic reaction, the by-products or intermediate products
might have been adsorbed or accumulated on the NaTaO3 surface.
UV-vis spectra were obtained to effectively analyze the light absorption capacity of the
photocatalyst. As shown in Figure 3, the absorption capacity of NaTaO3 photocatalyst was
basically unchanged even after 12 h of reaction.
Element contents and their binding state on the NaTaO3 surface were analyzed by X-ray
photoelectron spectroscopy (XPS). XPS data showed that the binding energies of Ta 4f
(Figure 4b) and Na 1s (Figure 4d) on NaTaO3 surface had obvious displacement after the
reaction (Figure 4). Through the hydrothermal treatment of 10 M NaOH with deactivated
NaTaO3 photocatalyst, binding energies of the photocatalyst’s Ta 4f (Figure 4c) and Na 1s
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(Figure 4f) returned to their original positions after the treatment, and the photocatalytic
activity of NaTaO3 was recovered. Na, O, and C atom contents on the catalyst’s surface were
analyzed, and results indicated that the Na content on NaTaO3 photocatalyst’s surface
decreased from 21.41% to 17.58% after coupling reaction. On the other hand, through
hydrothermal treatment with 10 M NaOH at 180oC for 12 h, the Na content approximately
returned to its original value (Table 2).
NaTaO3 photocatalyst deactivation mechanism in coupling reaction system
The intermediate products, by-products, and surface-active species were detected to
analysis the relationship between the photocatalytic activity and intrinsic of NaTaO3. Then,
the main reason for the deactivation of the NaTaO3 was analyzed. Figure 1 shows that the
crystal structure of the catalyst before and after the reaction did not change, which meant that
NaTaO3 was not damaged. Moreover, the UV-vis spectra were basically unchanged. These
results showed that NaTaO3 deactivation might be caused by the adsorbed impurities
occupation of active sites or the loss of surface active constituents.
Studies showed that one of the main reasons for a photocatalyst’s deactivation was that
adsorption and accumulation of intermediate products or by-products on the photocatalyst’s
surface (18). The infrared data in Figure 2 shows that after NaTaO3 photocatalyst was
subjected to illumination in a coupling reaction system for 12 h, its infrared spectrogram had
obvious C=C, C=O, -CH3, and -CH2 characteristic absorption peaks when compared with
NaTaO3 before the photocatalytic reaction. This result indicated that after the photocatalytic
reaction lasted for a period, the reactants (isopropanol and acetone) and products (pinacol) or
intermediate products (2,5-hexanedione, and others by GC/Mass analysis, in Figure S3)
might been adsorbed or accumulated on the NaTaO3 surface.
Hence, deactivated NaTaO3 was treated by ultrasonic washing with acetone, and then it
was roasted in a muffle furnace under 500°C (Table 3). Then, NaTaO3 photocatalytic activity
was evaluated after the adsorbed species on the surface were removed (Table 3).
Experimental results indicated that ultrasonic washing and high-temperature roasting
treatment could remove adsorbed species occupying the catalyst’s surface. However,
photocatalytic activity efficiency remained low. This result indicated that NaTaO3
photocatalyst deactivation was not caused by the adsorption of by-products or intermediate
products on NaTaO3 surface. Also, studies have reported that over-consumption of hydroxyl
on the photocatalyst’s surface would result in photocatalyst deactivation. Photogenerated OH

with strong oxidizability is a key factor for promoting a series of oxidative reactions (19).
Hydrothermal treatment of deactivated photocatalyst could remove some intermediate
products adsorbed on its surface, could supplement hydroxyl consumed in the reaction,
promote OH generation, and regenerate deactivated catalyst. However, results showed that
the hydrothermal treatment of NaTaO3 photocatalyst at 180°C did not improve the conversion
rates of reactants. On the contrary, the treatment inhibited the conversion rate of acetone
(Table 3). This result also indicated that the over-consumption of OH on the catalyst’s
surface was not the main reason for the deactivation of NaTaO3 photocatalyst. The XPS data
showed that the displacement of binding energies of Ta and Na atoms on NaTaO3
photocatalyst’s surface after reacting for 12 h changed (Figure 4), which meant the
microstructure of NaTaO3 changed after illumination. As shown in Figure 4, the partial
surface NaTaO3 (E Ta4f7/2 = 27.2 eV and E Ta4f5/2 = 25.7 eV) changed into (E Ta4f7/2 =29.0
eV and E Ta4f5/2 =27.0 eV), which were consistent with previous reference data. Ta has an
energy difference in the range 4.1–5.4 eV, depending on the surface preparation procedure
(20). And also, the partial surface NaTaO3 (E Na1s = 1069.5 eV) changed into (E Na1s =
1074.1 eV). The results showed that Na content on NaTaO3 surface decreased after reacting
for 12 h (decreased from 21.41% to 17.58%, Table 2). Literature has reported that in NaTaO3
photocatalytic hydrogen production experiment, Na loss on the catalyst’s surface is one of the
key factors of photocatalyst deactivation (21). Excess Na enhanced the population of excited
electrons and suppressed recombination. Moreover, surface Na+ acted as H+ transport channel
in the coupling reaction system (8). However, the sodium ions dissolved in aqueous solution,
in a phenomenon called photocorrosion, as the photocatalytic reaction occurred, which led to
the loss of Na+ on the NaTaO3 surface. Thus, in this coupling reaction system, surface Na can
be assumed critical for photocatalytic coupling reaction. Hence, photocatalytic activity
evaluation of deactivated NaTaO3 after photocatalysis was implemented after its
hydrothermal treatment with 10 M NaOH at 180°C for 12 h. After the treatment, the photo-
activity of NaTaO3 returned to its original level before the reaction. The XRD data indicated
that the crystal structure of NaTaO3 photocatalyst after NaOH hydrothermal treatment
basically remained unchanged (Figure 1). XPS data indicated that coordination and contents
of Na and Ta on NaTaO3 surface after NaOH hydrothermal treatment were consistent with
that before the reaction. Deactivation of NaTaO3 in the coupling reaction system can be
assumed to be mainly caused by the loss of surface Na.
Furthermore, as shown in our previous report (8), when NaTaO3 photocatalyst was
soaked in 4 M HCl solution for 12 h, surface Na+ was exchanged with H+, which led to the
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evident decrease in pinacol generation rate, as shown in Figure S4. An expected pleasure, the
photocatalytic activity of NaTaO3 returned to its original level after hydrothermal treatment
with 10 M NaOH. This further emphasized the significant role of surface Na+ on the
photocatalytic activity of NaTaO3 (Figure 5).
Regeneration and stability of deactivated NaTaO3 in coupling reaction system
The NaTaO3 photocatalyst regenerated with NaOH hydrothermal treatment was used in
photocatalytic coupling reaction system. Repetitive regenerated 10-cycle (T) operation test,
the regenerated NaTaO3 consistently showed excellent reactant conversion rate and target
product pinacol selectivity (Table 4). This result further verified that in the coupling reaction
system, loss of surface Na on NaTaO3 was the main reason for its deactivation. NaOH
hydrothermal treatment could regenerate deactivated NaTaO3 photocatalyst. Thus, a simple
and efficient photocatalyst regeneration method was successfully demonstrated.
CONCLUSION
In NaTaO3 photocatalytic isopropanol dehydrogenation coupling and acetone
hydrogenation coupling reaction systems, NaTaO3 photocatalyst lost its photo-activity after
three cycles. Results indicated that runoff of Na on the surface was the main reason for the
deactivation of NaTaO3 photocatalyst. After hydrothermal treatment of the deactivated
NaTaO3 with 10 M NaOH at 180°C for 12 h, the photocatalytic activity returned to the
original level. The photocatalytic activity of NaTaO3 remained stable even after 10 cycles of
regenerated by 10 M NaOH. This study developed a simple and efficient photocatalyst
regeneration method.
ACKNOWLEDGEMENTS
This work was supported by National Natural Science Foundation of China (no.
21703134) and Project of the Key Laboratory of Scientific Research in Shaanxi Province
Department of Education (no.17JS035).
SUPPORTING INFORMATION
Additional Supporting Information is available in the online version of this article:
Figure S1. A schematic of the employed photocatalysis reactor.
Figure S2. The XPS full spectra of the NaTaO3 photocatalyst at different conditions.

Figure S3. GC-MS spectra of the coupling reaction system (a) 2,5-hexanedione, (b) pinacol.
Figure S4. Response of DMB formation to an exchange of Na+ ions on NaTaO3 surfaces with
HCl and to a subsequent re-exchange with NaOH.
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FIGURE CAPTIONS
Figure1. XRD pattern of NaTaO3 photocatalyst before and after reaction.
Figure 2. FTIR spectra of NaTaO3 photocatalyst before and after reaction.
Figure 3. UV-vis spectra of NaTaO3 photocatalyst before and after reaction.
Figure 4. XPS spectra of the samples: (a) Ta 4f of NaTaO3 before photocatlytic reaction; (b)
Ta 4f of NaTaO3 after photocatlytic reaction; (c) Ta 4 f of NaTaO3 and Ta2O5 after
photocatlytic reaction, then hydrothermal treated in 10 M NaOH solution at 180oC for 12 h;
(d) Na 1s of NaTaO3 before photocatlytic reaction; (e) Na 1s of NaTaO3 after photocatlytic
reaction; (f) Na 1s of NaTaO3 after photocatlytic reaction, then treated in 10 M NaOH
solution at 180oC for 12 h.
Figure 5. The deactivated NaTaO3 photocatlyst was regenerated by hydro-thermal treatment
of 10 M NaOH at 180oC for 12 h.

Table 1. The evaluation of stability of NaTaO3 photocatalyst[a].
Conversion (%)
Cycle
Selectivity (%)
Acetone
61.25
Isopropanol
53.12
DMB
HXD
3.32
1T
2T
3T
91.25
82.67
77.31
58.02
55.01
19.18
7.60
15.60
19.02
[a]Reaction conditions: 1.0 g NaTaO3 photocatalyst, 200 mL of reactant aqueous solution (0.25 mol
acetone and 0.25 mol isopropanol); light, 300 W high pressure Hg lamp (λ=365 nm); temperature, ~25°C;
reaction duration, 12 h. (2,3-dimethyl-2,3-butanediol abbreviated as DMB, 2, 5-hexanedione abbreviated
as HXD).
Table 2. Surface atom content of NaTaO3 photocatalyst under different conditions.
Surface atom content (100%)
Conditions
Na
Ta
O
Before reaction
After reaction
Treated with NaOH
21.41
17.58
20.58
12.64
12.95
13.01
48.43
52.11
47.49
Table 3. The reaction activity of regenerated NaTaO3 photocatalyst[a].
Conversion (%)
Selectivity (%)
Catalyst
Regeneration
[b]Ultrasonic and
roasting
Acetone
Isopropanol
DMB
HXD
61.25
58.02
55.01
53.12
91.25
82.67
77.31
3.32
[c]Hydrothermal
treatment
19.18
7.60
15.60
19.02
[d]Treated with
10 M NaOH
[a]Reaction conditions: 1.0 g NaTaO3 photocatalyst, 200 mL of reactant aqueous solution
(0.25 mol acetone and 0.25 mol isopropanol); light, 300 W high pressure Hg lamp (λ=365
nm); temperature, ~25°C; reaction duration, 12 h. [b]The deactivated NaTaO3 was treated by
ultrasonic washing with acetone, and then it was roasted in a muffle furnace under 500oC;
The deactivated NaTaO3 was treated by hydrothermal methods at 180°C. [d]The deactivated
NaTaO3 was hydrotherma treated by 10 M NaOH at 180oC for 12 h.
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Table 4. The stability evaluation of regenerated NaTaO3 photocatalyst.
Conversion (%)
Selectivity (%)
Cycles (T)
Acetone
Isopropanol
53.12
DMB
91.25
90.91
90.83
90.97
90.61
91.23
90.99
90.76
91.25
90.99
HXD
3.21
3.71
4.01
3.44
3.55
3.15
3.30
3.89
3.00
3.45
1T
2T
3T
4T
5T
6T
7T
8T
9T
10T
61.25
60.69
61.36
66.72
62.60
63.30
66.72
68.71
64.02
64.58
53.36
53.87
51.66
54.37
56.18
56.82
59.12
61.28
61.28
[a]Reaction conditions: 1.0 g NaTaO3 photocatalyst, 200 mL of reactant aqueous solution
(0.25 mol acetone and 0.25 mol isopropanol); light, 300 W high pressure Hg lamp (λ=365
nm); temperature, ~25°C; reaction duration, 12 h.

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