recovery properties of hydrogen gas sensor with pdtitanate

Recovery properties of hydrogen gas sensor with Pd/titanateand Pt/titanate nanotubes photo-catalyst by UV radiation Dae Ung Honga,b, Chi-Hwan Hanb,*, Sang Hyun Parkb, Il-Jin Kimb, Jihye

Recovery properties of hydrogen gas sensor with Pd/titanate

and Pt/titanate nanotubes photo-catalyst by UV radiation

Dae Ung Honga,b, Chi-Hwan Hanb,*, Sang Hyun Parkb, Il-Jin Kimb, Jihye Gwakb,

a Department of Electrical and Electronic Engineering, Yonsei University, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, Republic of Korea

b Electrical and Electronic Materials Research Center, Korea Institute of Energy Research, 71-2, Jangdong, Yuseong, Daejeon 305-343, Republic of Korea

Received 15 October 2007; received in revised form 17 January 2008; accepted 17 January 2008

Available online 1 February 2008

Abstract

Recovery properties after H2S catalytic poisoning of catalytic-type gas sensor with photo-catalysts and UV radiation have been exam-ined Each sensing material of the sensor consists of Pd, Pt supported on c-Al2O3and Pd/titanate, Pt/titanate nanotubes or TiO2particles Pd/titanate and Pt/titanate nanotubes photo-catalyst were synthesized by hydrothermal synthesis method All the sensors were deactivated after 500 ppm H2S exposure for 20 h The sensors with Pd/titanate or Pt/titanate nanotubes showed regenerated voltage response under UV radiation However the sensor with TiO2particles showed negligible regenerated voltage response Regenerated voltage response with Pd/ titanate or Pt/titanate nanotubes may stem from location of Pd or Pt catalyst on the titanate nanotube photo-catalyst

Ó 2008 Elsevier B.V All rights reserved

PACS: 07.07.Df

Keywords: Catalytic poisoning; H 2 S; H 2 sensor; Photo-catalyst; Titanate nanotubes

1 Introduction

The research interest on hydrogen as a clean energy

resource or a fuel gas has been increased remarkably

because it is renewable, abundant and efficient with zero

emissions It is extensively used to make ammonia,

metha-nol, gasoline, heating oil, and rocket fuel, etc The amount

of energy produced by hydrogen is three times bigger than

the energy contained in equal weight of gasoline and about

seven times that of coal Hydrogen can replace natural gas

Like any other gas type fuel, hydrogen is flammable and

potentially dangerous Safety is the first priority in using

hydrogen gas as fuel Sensing hydrogen leakage from

stor-age and transportation equipment is essential Hydrogen also demands a careful handling, because a 4% (v/v)

mon-itoring of the concentration of this gas close to its production and consumption plants is necessary to avoid accidents due to hydrogen explosions

of catalytic combustion sensors Catalytic type gas sensors

Com-bustible gas mixtures do not burn until they reach an igni-tion temperature However, in the presence of certain chemical media, the gas can ignite and burn at lower tem-perature This phenomenon is known as a catalytic com-bustion A gas molecule oxidizes on the catalyzed surface

of the sensor at a much lower temperature than its normal ignition temperature Every conductive material has its

1567-1739/$ - see front matter Ó 2008 Elsevier B.V All rights reserved.

doi:10.1016/j.cap.2008.01.010

*

Corresponding author Tel.: +82 42 860 3449; fax: +82 42 860 3307.

E-mail address: hanchi@kier.re.kr (C.-H Han).

www.elsevier.com/locate/cap

www.kps.or.kr Current Applied Physics 9 (2009) 172–178

which has large Ct in comparison with other metals is a

good candidate for the catalytic combustible sensor

because it can detect flammable gases by measuring

range at which the sensor needs to operate The catalytic

surface is generally prepared by sintering noble metal

[4–7] However, there are still certain limitations associated

with the catalytic sensors to be applied These sensors show

low sensitivity due to the lack of adsorption sites for

hydro-gen and are affected by a small amount of poisonous gases

Therefore, removing catalyst poisoning is extremely

impor-tant The poisoning has been reported in many classes of

chemical products such as molecules containing sulfur,

hexamethyldisiloxane (HMDS), nitrogen, silicon, nitric

semiconductor type titania nanotubes hydrogen sensor

from sensor poisoning through UV photo-catalytic

The recovery properties of catalytic type hydrogen

species because it is one of the worst and most commonly

encountered catalyst deactivating compounds among

sul-fur containing compounds Many kinds of catalysts were

and recovery properties by UV radiation was tested

2 Experimental

The synthesis of Pd/titanate and Pt/titanate nanotubes

was processed with several steps All the chemicals were

purchased from Aldrich Anatase-type titanate powder

(4 g) was dispersed into an aqueous solution of NaOH

into the solution, which charged into a Teflon-lined

the hydrothermal treatment, the precipitate was washed

with deionized water and separated by filtration Final

[17,18] Morphology of the samples was observed by a field

emission scanning electron microscopy (FE-SEM) using

Hitachi S-4300 and by field emission transmission electron

microscopy (FE-TEM) using JEOL JEM-2100 F The

ele-ments ratio of the sensor surface was observed by disper-sive X-ray spectroscopy (EDS) using Horiba 7200-H BET surface area of each sensor material was measured

by the nitrogen sorption method at the liquid nitrogen tem-perature using Micromeritics ASAP 2010 Before each

until constant pressure (3 lm Hg) was obtained

Table 2 The reference material was an inactive c-Al2O3 The metal oxide powder material was mixed with an organic and inorganic vehicle at a concentration of

15 wt.% followed by ball-milling for 24 h, to prepare the pastes suitable for drop coating

Fig 1shows the structure and size of the present sensor device The sensor device was fabricated in the following pro-cedure First, a platinum micro-heater was formed on an alu-mina plate by a screen-printing method with platinum paste (METECH, Platinum conductor PCC 11417) followed by

ele-ment was formed by drop coating of a catalytic layer on the

the sensing and compensating elements were linked to signal pins of the sensor body by spot welding (WITH Corpora-tion, WMH-V1) with platinum wire (ø 50 lm)

The compensating element forms one arm of the

is connected in series with the bridge The surface temper-ature of the sensor at each applied voltage was measured

by a radiation thermo tracer (NEC TH9100MLN) Measurements were carried out using an environmental

Fresh air was introduced then inlet and outlet of the cham-ber were closed The device was exposed to a hydrogen gas sample for around 30 s for gas response test and device was

Table 1

Compositions of the fabricated sensor materials in wt.% ratios and voltage response of each sensor

Sample number Composition of the used materials (wt.%) DV at 310 °C (mV)

c-Al 2 O 3 Pt/titanate nanotube Pd/titanate nanotube TiO 2 Pd Pt

Table 2 EDS results and BET surface area of each sensor Sample number EDS results (wt.%) BET surface area (m 2 /g)

O Al Ti Pd Pt S1 49.2 40.5 5.2 5.1 18.0 S2 54.8 25.0 13.1 3.1 4.0 17.5 S3 49.5 20.0 16.8 5.3 7.5 43.4 S4 45.7 22.4 15.6 7.8 6.2 25.9 S5 48.7 24.3 16.2 5.8 5.0 32.5

recovered by exposing to purified air again A mass flow

controller (MFC) was employed to fix the gas flow rate

The gas concentration was controlled by selecting

appro-priate values of the flow rates For practical poisoning test

before the simulation gas was fed Then sensitivity of the

by blacklight blue lamps which efficiently emit near

ultravi-olet rays at 315–400 nm Gas sensitivity (DV) was defined

as the difference between the output voltage in a sample

3 Results and discussion 3.1 Characterization of photo-catalyst Fig 3 shows typical TEM images demonstrating uni-form sized titanate nanotubes over which Pt or Pd nano-particles are randomly distributed The outer diameter of nanotubes in TEM images is approximately 100 nm Detailed characteristics of Pd/titanate and Pt/titanate

3.2 Sensor performance with different catalyst Compositions of the fabricated sensor materials and the

Fig 4shows the response of sensors to 1% hydrogen gas

Fig 4 that sensing performance of the sensors using Pd/ titanate or Pt/titanate nanotubes catalyst supported on

sites and surface area of Pd/titanate and Pt/titanate nano-tube catalysts BET surfaces areas of the sensor materials

surface area increased as titanate nanotube increased S3

Fig 1 Schematic diagram of (a) the sensor structure and (b) the

fabricated sensor Dimensions are in lm.

Fig 2 (a) Bridge circuit for the output voltage and (b) schematic view of test system.

Fig 3 TEM images of (a) Pd/titanate and (b) Pt/titanate nanotubes.

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

S1

S2

S3 S4 S5

Heater temperature (ºC)

Operating temperature (310 o C)

Fig 4 The voltage responses of sensors using various catalysts to 1%

hydrogen.

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

0.075

H 2 S concentration (ppm)

Fig 5 The voltage response property of S1 sensor for 1% H 2 with

different H S exposure conditions at 100 °C.

-5 0 5 10 15 20 25 30 35 40 45 50 55 0.02

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

UV radiation time (h)

S1 S2

S3

S4

S5

Fig 7 Response changes of the sensors by poisoning of 500 ppm H 2 S and reactivation treatment with UV radiation.

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Time (h)

S1

S2

S3 S4 S5

Fig 6 The voltage response properties of gas sensors for 1% H 2 after H 2 S

500 ppm poisoning at 100 °C.

sensor including highest weight ratio of titanate nanotube

cause for the enhanced response may due to the better

adsorption of hydrogen on the titanate nanotube surface

which facilitates the oxidation of hydrogen reaction by

the Pd and Pt catalysts on the titanate nanotube surface

[19]

Among S3, S4, and S5 sensors, S5 sensor showed highest

inFig 4, although it had smaller surface area than S3 The

response of S5 sensor is 11.2 mV higher than those of S3

these results, it may be thought that catalytic property

for the hydrogen gas combustion of Pd/titanate nanotube

is considerably better than that of Pt/titanate nanotube

Fig 5shows the voltage response property of S1 sensor

and it worked as catalytic poisoning species

The relationship between the voltage response of the

Fig 8 SEM images of fabricated sensing layer for (a) S1, (b) S2, (c) S3, (d) S4 and (e) S5.

and then the device was recovered by exposing to purified

Fig 6that the voltage response of various sensors for 1%

3.4 The recovery properties of sensor response after UV

radiation

Fig 7shows the recovery properties of sensors with UV

radiation for 50 h When UV light was illuminated on the

catalytic sensor, a remarkable difference in the recovery

properties of the sensors was observed The S1 and S2

sen-sors showed negligible regenerated voltage responses

How-ever, the voltage responses of S3, S4 and S5 sensors for 1%

radia-tion A common point of S3, S4, S5 sensors is using Pd or

To elucidate the different recovery properties between

S1, S2 sensors and S3, S4, S5 sensors, the sensor surface

nanotubes of S3, S4, S5 sensors were found to be entangled

to other particles and forming net structure on the

photo catalyst is irradiated with UV light, electrons and

holes are generated in it The photogenerated holes in the

valence band can oxidize water to produce highly reactive

OH), and the photogenerated electrons

in the conduction band can reduce oxygen to form highly

3

[15,16]

Maxted has reported that the poisoning effect of sulfate

Sulfide compounds are coordinated directly with Pd using

two anti-bonding lone pairs The activity of Pd catalyst

was drastically reduced by sulfide In contrast, the sulfur

atom of sulfate is surrounded by oxygen atoms The

struc-ture of sulfate satisfies with the octet rule and the sulfur

atom of sulfate does not bind directly with Pd The

interac-tion between Pd and S atom of sulfate is smaller than that

of sulfide, and thus the poisoning effect of sulfate is smaller

From our experimental results Pd or Pt dispersed titanate

nanotubes catalysts were recovered by UV radiation from

radia-tion The regenerated voltage response with titanate

nano-tubes may stem from location of Pd or Pt catalyst on the

titanate nanotube photo-catalyst The life time of hydroxyl

on the photo-catalyst surface is very short and only can

poi-soned Pd or Pt can not be recovered because the (Pd, Pt)

catalysts existed apart from the photo-catalyst

Fig 9 shows a relationship between change of output voltage and the hydrogen concentration after UV radiation

the voltage difference was proportional to the hydrogen

in the concentration range of 0.5–4% (v/v), and the catalyst after UV radiation was still efficiently active However the catalyst without UV radiation became deactivated at the same reaction time

4 Conclusion

We have clearly shown the recovery properties of hydro-gen sensor with titanate nanotube catalysts by UV

hydrogen sensor with Pd or Pt/titanate nanotubes having

by UV radiation can be a good candidate for future hydro-gen sensor The rehydro-generated voltage response with titanate nanotubes may stem from location of Pd or Pt catalyst on the titanate nanotube photo-catalyst, and can be explained

difference of sensors was proportional to the hydrogen in the concentration range of 0.5–4% (v/v)

Acknowledgement This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Pro-gram, funded by the Ministry of Science and Technology of Korea

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