Enhance the activity of the photocatalyst TiO2-P25 degussa by doping it with N and Mg on degradation of phenol under visible light

Catalyst TiO2 with N-doped, Mg-doped and N-Mg co-doped were created by impregnation method with Mg(NO3)2 (add Mg in) and mix method with urea (add N in). The XRD, SEM, DR, BET, FT-IR measurements were carried out for structural characterization of the TiO2 samples. The result performed, compared with the sample of initial TiO2-P25 particle size and specific surface area, samples of doped-catalyst changed insignificantly.

ENHANCE THE ACTIVITY OF THE PHOTOCATALYST TiO 2 -P25 DEGUSSA

BY DOPING IT WITH N AND Mg ON DEGRADATION OF PHENOL

UNDER VISIBLE LIGHT

Pham Phat Tan1

1 An Giang University

Information:

Received: 31/05/2018

Accepted: 29/09/2018

Published: 02/2019

Keywords:

Photocatalyst, N-Mg co-doped

TiO 2 , phenol, degradation

ABSTRACT

Catalyst TiO 2 with N-doped, Mg-doped and N-Mg co-doped were created by impregnation method with Mg(NO 3 ) 2 (add Mg in) and mix method with urea (add N in) The XRD, SEM, DR, BET, FT-IR measurements were carried out for structural characterization of the TiO 2 samples The result performed, compared with the sample of initial TiO 2 -P25 particle size and specific surface area, samples of doped-catalyst changed insignificantly The average crystalline size was 30-32 nm, specific surface area about 50 - 60 m 2 /g The crystalline phase components did not change, including only anatase and rutile Doped-catalyst samples narrowed the band gap of TiO 2 (values of Eg decreased to under 3.0 eV) In there, N-doped sample possessed the lowest value of Eg (Eg = 2.53 eV), it meant this sample responded to the visible light area obviously In the degradation of phenol, N-Mg co-doped TiO 2 had better catalytic performance than N-doped and Mg-doped alone The order

of doping elements (N and Mg) shown to be an important factor on the catalytic activity of the doping catalysts The catalysts TiO 2 were doped by N first then Mg demonstrated synergic effect and activity of the catalysts increased 1.5- 1.6 times compared with pure TiO 2 By contrast, in the reverse order (first Mg then N) the inhibited effect was observed and the catalytic activity would decrease

1 INTRODUCTION

TiO2-based photocatalysis is one of the advanced

oxidation processes due to the high catalytic

activity of TiO2 in the presence of UV light It still

has some limitations, however, needed to be

resolved For instance, (i) narrowing Eg (the band

gap) will allow the use of extensible optical

performance beyond the ultraviolet to the visible

range (UV-VIS) of solar energy patterns; (ii)

preventing the recombination of e

-CB and h+

VB of the TiO2 after the influence of the photon The

study with the aim of seeking for photocatalyst of TiO2, which is highly active when using solar energy in two directions, is indispensable, which contributes in applying the treatment of biodegradable organic pollutants with "green" technology

If non-metallic elements are implanted such as N,

S, F, I, C into TiO2 so that they replace partially the position of O in TiO2 crystal lattice, this will narrow the band gap of TiO2 to lower level, for example, Eg = 2.54 – 2.66 eV then TiO2

photocatalyst activity will be promoted

throughout the UV-VIS range of solar energy

(Asahi, 2001; Ihara, 2003; Mozia, 2005)

The second direction is to decelerate

recombination of pairs of photo-generated

electron on conductor band and photo-generated

hole on the valence band of TiO2 to create many

free radicals *OH – an extremely strong oxidizing

agent plays a pivotal role in the oxidation of

organic contaminants (Colmenares, 2006) This

direction is conducted by doping TiO2 with metal

elements: Cu, Ag, Fe, Ni, Pt, Pd, Zn, Zr, Cr, W,

Ru (Bandara, 2004; Barakat, 2005; Iliev, 2006;

Vaidya, 2004; Wang, 2004; Xu, 2004; Zhang,

2004) to create electron capturing centers on

conductor band to prevent photo-generated

electrons return valence band, then reduce

recombination e

-CB và h+

VB Besides, the tendency of research metal-nonmetal

co-doped TiO2 has been especially notified, and

there were studies on metal-nonmetal co-doped

TiO2 (Huang, 2007; Morikawa, 2006; Pan, 2006;

Wei, 2004) The result increased the catalyst

activity but some studies reflected reverse results

Nowadays, very few studies in using magnesium

and nitrogen concurrently to dop TiO2 are

published and this considered as an interesting

and promising study On the basis of science,

doping TiO2 with a specific element with certain

content to increase activity, however if co-doping

with both elements happens, their activity may

increase due to synergic effect among the roles of

nitrogen and magnesium, but may decrease their

activity due to inhibition among these factors

This article, the photocatalysts N, Mg – co-doped

TiO2 were prepared to degrade phenol under

visible light

2 EXPERIMENTS

2.1 TiO 2 -P25 modification with N, Mg

TiO2-P25 Degussa powder (anatase ≈ 80%, rutile

≈ 20%, BET surface area ≈ 50 m2/g, particle size

≈ 30 nm) was impregnated by Mg(NO ) solution

of given concentration in such a way as to Mg content in the catalyst reached 1.0% The mixture was stirred for two hours, then stabilized for 24 hours Drying a sample at 110 oC for three hours and heated at 450 oC for three hours (Denoted as TiO2-1Mg) The magnesium content above is optimal, and this was published before In that study, TiO2 Degussa P25 catalyst containing Mg with different content (0.5 -10%) was prepared by

an impregnating method and showed photocatalytic activity increased when Mg content was in 0.5-1% and peaked when Mg content reached 1%, then gradually decreased when Mg content was higher than 1% in the decomposition reaction of phenol with UV-VIS light (P.P Tân, 2017)

TiO2-P25 Degussa powder were mixed with urea with molar ratios 1:1, calcinated at 4000C, for two hours in the air with gradient 50C/minute (denoted as TiO2-1U) The ratio above is optimal and was published before (P.P Tân, 2008) The catalysts TiO2 contained both N and Mg (TiO2-N-Mg) were prepared in different order TiO2-1Mg powders were mixed with urea with molar ratios 1:1, calcinated at 4000C for two hours (denoted as TiO2-1Mg-1U) TiO2-1U powders were impregnated in Mg(NO3)2 solution with magnesium content at 1%, calcinated at 4500C for three hours (denoted as TiO2-1U-1Mg)

The catalysts were finely pulverized before surveying physical-chemical characteristics and their activity Compared samples were carried out similarly but without the Mg(NO3)2

2.2 Reactor and light source

The activity of doped TiO2 catalysts was surveyed

on the reaction system discontinuously pyrex glass, with 150 ml volume, 160 mm high, 42 mm

of diameter A 150W halogen light (OSRAM HLX) possesses wavelengths from 360 nm to 830

nm was placed in a cylinder of quartz and was cooled by the surrounding water

Reactant is phenol concentration of 50mg/l

2.3 Process and analytical methods

The catalyst samples were analyzed on their

structure and physical-chemical characteristics by

methods such as X-ray diffraction (XRD) - the

sample was measured on XRD instrument

(SIEMENS - Germany) with CuKα anode

electrode (1,5406 A0), 2θ scanning angle from 15o

to 70o; method of scanning electron microscopy

(SEM) was done on SEM instrument (JOEL-JSM-

5500-Japan) The BET surface area measurement

was performed on the CHEMBET 3000

The diffuse reflection spectra was measured on

DRS (JASCO V-550-Japan) The Eg values were

calculated by Eg = hC/λ = 1239,8/λ (eV)

The activity of catalysts was assessed by the

conversion and mineralization of phenol The

concentration of phenol in the reaction time was

determined in the characteristic absorption peaks:

211 and 270 nm (measured on UV-VIS Jasco

V530 machine, Japan, mineralization was

determined on ANATOC II, Australia) The

calculation formula was as follows:

( ) % o t.100

phenol

o

C C C

In there, C0: initial concentration of phenol, Ct: concentration of phenol at time t, α: the conversion of phenol at time t

Total Organic Carbon (TOC) measurements were performed using ANATOC II by direct injection

of the samples Results of the total initial organic carbon and sample of corresponding reactants at time t were analyzed automatically The mineralization of the samples were calculated:

TOC

o

TOC TOC TOC

In there, TOC0, TOCt: total initial organic carbon and sample of corresponding reactants at time t, β: the mineralization

3 RESULTS AND DISCUSSION 3.1 Research on the structure of the prepared catalysts

Physical-chemical characteristics of the catalysts are shown in Table 1

Table 1 Physical-chemical features of TiO2-P25 and doped-TiO2 catalysts

The result from Table 1 showed that with N, Mg

alone or concurrently at TiO2-P25 did not change

significantly the specific surface compared with

TiO2-P25 pure, the specific surface fluctuates in

the range of 50-60 m2/g This indicated, the

doping TiO2-P25 pure with two elements above

with 1% Mg content or catalyst and urea with

molar ratios 1:1 or doping with both N and Mg did not change significantly their specific surface area though N was added first then Mg or in reverse order The calculated results from XRD patterns (Figure 1) performed the samples above had stable particle sizes within the range of 30 –

32 nm

Figure 1 XRD patterns of TiO 2 -P25 catalyst samples: (1) TiO 2 -P25, (2) TiO 2 -1Mg, (3) TiO 2 -1U, (4) TiO 2 -1U-1Mg và (5) TiO 2 -1Mg-1U

The XRD patterns showed that catalyst which

were doped with N or Mg or two elements above

had crystalline phase components and they were

similar to the original TiO2-P25 All samples

appeared specific peaks of anatase phase (2θ = 25.3; 37.8; 48.1) and rutile phase (2θ = 27.5; 36.1; 54.4), to doped samples, there was no any new peak

6 5

4 3

2

5

4

3

2

1

-1U-TiO 2

-1Mg-Figure 2: The SEM image of the catalysts:

TiO2-P25 and doped-TiO2

Figure 3 The SEM image of the catalyst Mg doped TiO2-P25

Figure 4 The TEM image of the catalysts: TiO2-1U và TiO2-1U-1Mg

The SEM images (Figure 2) indicated that the

catalysts have the same shape and distribution

This demonstrated that there was no clumped

phenomenon of the crystal lattice in TiO2 catalyst

samples despite doping with element alone or

co-doping with N and Mg when being calcinated at

450 0C In Figure 3, the elements were distributed equally on the surface of the catalyst TEM images (Figure 4) of the catalysts showed the particle size of doped-catalysts changed insignificantly (about 30-32 nm)

Therefore, doping TiO2-P25 with qualitative and

quantitative components and doping condition as

mentioned above, particularly, when co-doping

with N first then Mg or in reverse order happened,

physical properties and catalyst structure had same results This is an advantageous point to study their activity later

3.2 Order of adding N and Mg in TiO 2 -P25 photocatalyst

Figure 5 The DRS partterns of the catalysts: TiO2-P25 and doped-TiO2

The DRS patterns (Figure 5) proved that the TiO2

-doped catalysts had widening ability towards

visible light absorbtion If N was added first then

Mg (TiO2-1U-1Mg), the ability of visible light

absorption will be lower than the opposite case

(TiO2-1Mg-1U) However, both co-doped

situations had weaker ability of visible light

absorbtion than N-doped sample alone (TiO2-1U)

DRS patterns reflected that N-doped sample

(TiO2-1U) widened to visible light area most

obviously, Eg decreased to just 2.53 eV The

reason of increasing the ability of visible light

absorption of doped sample was due to the role of

N on TiO2 crystal lattice, this sample only doped

with N, N went into TiO2 crystal lattice so that it

performed as widening role in visible light

absorption To co-doped sample, if N was added

in TiO2 which was doped with Mg previously,

MgO layer will prevent N from entering the

lattice, thus reducing the role of N, led to reducing

in visible light absorbtion (Bandara, Hadapangoda

& Jayasekera, 2004) If the doping process was conducted in reverse order, added N first then Mg, the MgO layer will partly cover the TiO2 surface which led to decrease the ability of visible light absorbtion; as a result, this ability was not much preferable than the two cases above Ascending order of visible light absorption: TiO2-P25 < TiO2-1U-1Mg < TiO2-1Mg-1U < TiO2-1U The

Eg values of the corresponding samples gradually decreased (Table 2) In terms of photosensitivity, the samples TiO2-P25 and TiO2-1Mg were white, while the N-containing samples were yellow However, if doped with N first then Mg, the catalyst color would be lighter, changed from light yellow to ivory white The N-doped sample alone was slightly lighter in color than the rest

Table 2 The Eg value of TiO2-P25 and doped-TiO2 catalysts

Depending on previous researches, the catalysts of

N-doped alone and N-Cr-co-doped TiO2

simultaneously (Pan & Wu, 2006),

N-Cu-co-doped TiO2 (Morikawa, Irokawa & Ohwaki,

2006), N-Fe-co-doped TiO2 (Rane al et, 2006)

indicated that co-doped samples had better ability

of extending the light absorption compared with

N-doped samples alone This is partly due to the

color of oxides of crom, copper, and iron

However, the result in Table 2 showed that the

ability of visible light absorption of

Mg-N-co-doped TiO2 was lower than that of N-doped alone

This demonstrated that MgO might play a better

role in preventing recombination e-CB-h+VB not

because of its color (Bandara, Hadapangoda &

Jayasekera, 2004)

Recently, there are not many studies in

co-doping two elements on TiO2, particularly the

elements N and Mg The results of this study and

the works of the authors mentioned above

indicated that the ability of widening the light

absorption to the visible light had a close-knit

relationship to doping order and the color of

doped metal or oxide of the metal This is an

extremely important factor in choosing the suitable doping element, doping order to create a new highly photo-activity catalyst generation

3.3 Characteristic catalytic activity of TiO 2

-N-Mg catalyst sample

The results above demonstrated the photocatalytic co-doped samples with the two elements above in order of N first then Mg had a narrowed surface and increased particle size compared to un-doped catalyst Their photosynthetic activity should be reduced, but the opposite was true The two co-doped samples had crystalline compositions close

to TiO2-P25, but their activity was still higher The above results proved that in co-doped photocatalyst sample with order of adding N first then Mg, synergy of N and MgO role, meant it had active ability in visible light, and prevented recombination e-CB-h+VB as well, has greatly improved the photocatalytic activity of the TiO2 -N-Mg catalyst samples

The TiO2 photocatalytic activity at TiO2-P25 surface was affected by order of N and Mg shown

in Table 3 and Figure 6

Table 3 The activity of TiO 2 -P25 and doped-TiO 2 catalysts

Catalytic sample Phenol degradation levels (%) Mineralization levels (%)

Figure 6 Comparison of the activity of TiO2-P25 and doped-TiO2 catalysts

From the results above, if TiO2-P25 catalyst was

doped with N or Mg alone, the phenol conversion

after 180 minutes under UV-VIS light increased

from 1.2 to 1.5 times compared with TiO2-P25

Degussa However, if co-doping TiO2-P25 with

two elements in order of N first then Mg (TiO 2

-1U-1Mg), the activity rose 1.6 times compared

with original sample and increased around 1.2

times compared with N-doped sample alone

(TiO2-1U)

Optical response spectra of samples (Figure 4)

cho thấy, TiO 2 -1U-1Mg sample did not absorb

visible light as good as TiO2-1U, its band gap

energy was higher (Eg=2.88 eV), meanwhile Eg

of TiO2-1U was only 2.36 eV When the Mg was

added to the sample TiO2-1U, the Eg value

increased leading to the activity should have

decreased but in turn its activity increased This

illustrated that e

-CB capturing ability on conductor band of MgO with narrowing the band gap of N

made a synergy effect of two seperated role of N

and MgO

However, if TiO2-P25 were co-doped with two

elements in reverse order, Mg was added first then

N (TiO2-1Mg-1U) then the result reflected quite

opposition, photoactivity reduced about 10%

be accounted for MgO possessed no photoactivity due to high band gap energy (Eg = 9 eV) but when crystallines MgO located at TiO2 surface with certain contain, it played a role in capturing

e

-CB of semiconductors when receiving suitable light illuminated, then increased the photoactivity

of catalyst Then when N was doped, maybe because N contacted to MgO layer directly, most

of N could accessed inside crystal lattice TiO2 to occupy space of oxygen in lattice but perhaps it replaced partial oxygen in crystalline MgO, this is also the reason why this catalyst did not absorb visible light as good as TiO2-1U On the other side, because inside crystalline MgO the element

N had valene III which was higher than the valence of oxygen element, it filled up partial defects on crystalline MgO Maybe this is one of the reasons for reducing the role of electron capture of MgO

Therefore, this indicated two elements N and

Mg were co-doped on TiO2 P25 Degussa catalyst confirmed synergic effect of N and MgO role Particularly, photoactivity was optimized when doping with N first then Mg

However, studies in co-doped TiO2 catalyst were published with different results: Huang L studied

44.7

54.2

66.3

41

71.3

39

48.1

61.3

40.1

65.8

0 10 20 30 40 50 60 70 80

Phenol metabolism levels Mineralization levels

degradation of RB was the best under visible light

(Huang, Sun, & Liu, 2007) Morikawa T doped

TiO2 with N and Cu, the co-doped sample had

higher acitivity than N-doped sample alone in the

oxidation of acetaldehyde under visible light

(Morikawa, Irokawa & Ohwaki, 2006) On the

contrary, Pan C C claimed the photoactivity in

the methylene green decomposition or co-doped

with Cr first then N on TiO2 was lower than that

of the seperated doped-samples (Pan & Wu,

2006)

Therefore, depends on the condition of doping

and characteristic of doping elements, the activity

of co-doped catalyst has not performed consistent

result yet This direction will be studied more in

the future

The above statements and research results in this

study show that the co-doped TiO2 catalyst will

open up a highly effective catalyst prospect in

processing of organic pollutants treatment in

water with solar energy source

4 CONCLUSION

Producing and doping the photocatalysts TiO2 by

adding N into TiO2-P25 in the stage of forming

solid crystalline TiO2; doping TiO2 with Mg by

the impregnated method with the content of 1.0%

Mg; similarly N and Mg going into crystalline

TiO2 change insignificantly particle size, specific

surface area and crystalline phase components

compared with the original TiO2 However, the

ability of extending the visible light area

absorption may be changed virtually, in there

N-doped sample alone increases this ability, the Eg

value decreases to 2.53 eV Compared with the

TiO2-P25, catalytic activity rises 1.2 and 1.5 times

with the samples of corresponding TiO2-1U and

TiO2-1Mg

Co-doping TiO2 with N and Mg can increase or

decrease the photocatalytic activity of TiO2

catalyst on the degradation of phenol Detection

of this characteristic depends on the order and

condition of adding doped-elements The synergic

effect increases the activity of catalyst under

UV-VIS light area when N is added in TiO2 first then

Mg Compared with TiO2-P25, activity of photocatalyst that doped in this order increases about 1.6 times On the contrary, adding Mg first then N is noticed an inhibited effect and reduced photocatalytic activity in degradation of phenol

In this situation, MgO may play a better role in preventing recombination e

-CB-h+

VB than N in the role of narrowing the band gap energy This is such an interesting content; the following research will investigate further the role in preventing recombination e-CB-h+VB and the role of narrowing the band gap energy by doping metal and non-metal elements on the photocatalysts in processing of organic pollutant treatment in water with the solar energy source

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