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Applied Catalysis B: Environmental 72 (2007) 187–195

Sulfur durability of NOx storage and reduction catalyst with

supports of TiO2, ZrO2 and ZrO2-TiO2 mixed oxides

Naoki Takahashi a,*, Akihiko Suda a, Ichiro Hachisuka b, Masahiro Sugiura a,1,Hideo Sobukawa a, Hirofumi Shinjoh a

a TOYOTA Central Research and Development Labs., Inc., Nagakute, Aichi 480-1192, Japanb Toyota Motor Corp., Toyota, Aichi 471-8571, Japan

Received 27 December 2005; received in revised form 16 October 2006; accepted 21 October 2006

Available online 28 November 2006

Abstract

This research focuses on investigating the influence of the various compositions of TiO2 and ZrO2 on the NOx removal ability over a sulfur-

treated NSR catalyst. On the NSR catalyst, potassium was loaded as the NOx storage material and platinum as the precious metal, and the ZrO2

content was varied from 0 to 100 wt%. A relatively high NOx removal ability above 500 8C was obtained with the 60 to 80 wt% ZrO2 content,

and the maximum value was 70 wt%, while the TiO2-rich supports were superior below 400 8C when compared to the ZrO2-rich supports. K/Pt/

ZrO2 had a poor NOx removal activity over the entire temperature range. The analysis of the sulfur-aged catalysts with the supports of

70 wt%ZrO2-30 wt%TiO2, pure TiO2, and ZrO2 indicated that the TiO2 support presented a higher resistance to potassium sulfate-formation,

while the ZrO2 support suppressed the solid phase reaction with potassium. The catalyst with 70 wt%ZrO2-30 wt%TiO2 retained the highest

amount of remaining potassium, which was neither the formed-sulfate nor the solid-phase-reacted potassium. The sulfur-deactivation of the

potassium sites could increase the activity of the metallic platinum, and a suitable combination of metallic platinum with the adequate potassium

sites lead to a higher NOx removal activity for the TiO2-rich catalysts at low temperatures. In the case of the K/Pt/ZrO2 catalyst, almost all the

potassium changed into sulfate, which caused a poor de-NOx ability over the entire temperature range. The support’s acidity is an important

factor regarding the sulfur tolerance of the NSR catalyst. The ZrO2-TiO2 catalyst containing 70 wt% ZrO2 was verified to have the highest acid

amount among the sample supports, and was supposed to be the best support against sulfur-poisoning. In addition, this support contained

60 mol% ZrO2, and favorably suppressed the solid phase reaction with potassium. These properties of the ZrO2-TiO2 support containing

60 mol% ZrO2 balanced the sulfur tolerance and thermal resistance, and led to its highest NOx purification ability at high temperatures following

the sulfur-aging treatment.

# 2006 Elsevier B.V. All rights reserved.

Keywords: NOx storage and reduction catalyst; ZrO2-TiO2; Potassium; Sulfur-poisoning; Thermal deterioration

1. Introduction

The reduction of CO2 emissions from automobiles is a very

important effort for global environmental protection. Mean-

while, the development of more fuel-efficient engines is also

regarded as an additional benefit for the automobile industry.

As one of the fuel-efficient engines, the lean-burn gasoline

engine has attracted much attention for its remarkable potential

* Corresponding author at: 41-1 Yokimichi, Nagakute, Nagakute-cho, Aichi

480-1192, Japan. Tel.: +81 561 63 6293; fax: +81 561 63 6150.

E-mail address: e0790@mosk.tytlabs.co.jp (N. Takahashi).1 Present address: Japan Synchrotron Radiation Research Institute, Sayo,

Hyogo 679-5198, Japan.

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.10.014

to improve the fuel economy compared to the conventional

stoichiometrically operating engine systems. However, the

wide use of the lean-burn engine system is somewhat restricted

by environmental regulations. This is mainly because the

conventional three-way catalysts are not able to detoxify NOx in

the lean-burn engine exhaust, in which oxygen is in excess. A

NOx storage and reduction (NSR) catalyst system has been

proposed and is currently being developed as one of the most

feasible and attractive solutions to this technical challenge

[1,2].

The typical NSR catalyst consists of precious metals (mainly

platinum), alkaline and alkaline earth metal oxides as the NOx

storage materials (usually barium compounds), and a metal

oxide as the support (alumina). Fig. 1 shows the NOx storage

Fig. 1. Scheme of the NOx storage and reduction reaction.

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195188

and reduction reaction on the NSR catalyst. During the NOx

storage stage in an oxidative or lean-burn atmosphere, NOx

(NO) is first oxidized to NO2 over the precious metals, then

combined with the NOx storage materials, and finally stored as a

nitrate ion. During the following reduction stage, under a

stoichiometric or reductive atmosphere (rich), the stored nitrate

ion is released as NOx (NO or NO2) from the NOx storage

materials, and then reduced to nitrogen [1,3]. The engine

control system periodically provides a fuel-rich spike (RS)

containing reducing agents such as hydrogen (H2), carbon

monoxide (CO) and hydrocarbons (HC) to convert the stored

nitrate ions on the NSR catalyst [4].

The NSR catalysts deactivate due to sulfur-poisoning and/

or thermal deterioration [3–6]. The sulfur poisons the

precious metals [7,8], supports [9] and NOx storage materials

[3,7]. It has been confirmed that the adsorbed sulfur

transforms the NOx storage materials into sulfates [3,7–10].

The sulfates and the desorbed sulfur compound decompose

into sulfur oxides or hydrogen sulfide when the temperature

goes above 600 8C under rich conditions, hence its NOx

storage ability against sulfur-poisoning consequently get

partially restored [4,7].

There are several reports about improving the sulfur

tolerance of the NSR catalyst. One strategy is to facilitate

the decomposition of sulfates by adding some certain materials

to the catalyst. For instance, iron addition suppresses the grain

growth of the barium sulfate (BaSO4) [11]. H2 is the most

effective reducing agent to regenerate the NOx storage

compound [9,12], and the zirconia (ZrO2) supported rhodium

(Rh) catalyst promotes the H2 generation via a steam reforming

reaction [9,13].

Another effective strategy to improve the sulfur tolerance

was the use of titanium dioxide (TiO2) [14]. Matsumoto et al.

found that the decomposition temperature of sulfates on a TiO2

support was lower than that on an Al2O3 support under

reducing conditions. By blending TiO2 with Al2O3 [9], they

simultaneously suppressed the sulfur deposit and enhanced the

NOx storage of the sulfur-aged catalyst, and then found that the

blend of the non sulfur-poisoned Pt/TiO2 catalyst with the

sulfur-poisoned Ba/Pt/Al2O3 catalyst improved the sulfur

desorption from the Ba/Pt/Al2O3 catalyst under rich condi-

tions. They postulated that the sulfur compound desorption

was promoted by the interface between the non sulfur-

poisoned Pt/TiO2 and sulfur-poisoned Ba/Pt/Al2O3. This work

led them to successfully improving the tolerance of the NSR

catalyst against sulfur-poisoning by the use of fine TiO2

particles [15,16].

Our previous study revealed that the NOx storage amount on

the potassium containing NSR catalyst turned two or five times

higher than that on the barium containing NSR catalyst at 450

and 500 8C, respectively [17]. This was related to the higher

thermal stability of potassium nitrate (KNO3) than barium

nitrate (Ba(NO3)2), which was estimated from the thermo-

dynamic equilibrium constants between their nitrates and

carbonates [18]. The potassium compound is apt to interact

with TiO2 during the durability testing, thus the thermal

stability of potassium nitrate decreased, and then loosing its

NOx storage function [17].

As an acid-base catalyst, ZrO2-TiO2 [19–21] has a higher

thermal stability [22] than the pure TiO2 and ZrO2. Our

previous study showed that the use of the ZrO2-TiO2 (or TiO2-

ZrO2) mixed oxide rather than TiO2 improved the NOx removal

activity of the NSR catalyst [17].

Except for our above research, no other publication has yet

been found on loading an alkaline metal compound as a NOx

storage material on ZrO2-TiO2 and further investigating the

involved interactions. It is clear that the NSR catalyst with a

ZrO2-TiO2 support achieves a higher durability of NOx removal

than that with only the TiO2 or ZrO2 support [17]. However, the

influence of the ZrO2-TiO2 composition on the durability, the

dependence of the NOx removal activity against the reaction

temperature, if there is an optimized ZrO2-TiO2 composition,

and how exactly the interactions among the components occur

upon sulfur-aging treatment, have still not yet been clarified,

although they are truly meaningful for the actual formulation

and development of more efficient NSR catalysts. To elucidate

these issues, we first synthesized several kinds of ZrO2-TiO2

supports with different compositions and also pure ZrO2 and

TiO2 supports, and then prepared the NSR catalysts by loading

platinum and potassium on these supports. The catalytic

activity of the sulfur-aged catalysts was evaluated, and a

subsequent analysis was conducted to reveal the reactions and

interactions that occurred among the components on the

catalysts.

Table 1

Composition of samples

Sample name ZrO2/TiO2

Weight ratio Molar ratio

TiO2 0/100 0/100

ZT10 10/90 7/93

ZT20 20/80 14/86

ZT30 30/70 22/78

ZT40 40/60 30/70

ZT50 50/50 39/61

ZT60 60/40 49/51

ZT70 70/30 60/40

ZT80 80/20 72/28

ZT90 90/10 85/15

ZrO2 100/0 100/0

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195 189

2. Experimental

2.1. Supports synthesis and the characterizations

The ZrO2-TiO2 supports were synthesized as follows [23]. A

certain amount of zirconyl nitrate dehydrate (ZrO(-

NO3)2�2H2O, Wako Pure Chemical Industries) and titaniu-

m(IV) chloride (TiCl4) solution (Wako Pure Chemical

Industries) were dissolved in ion-exchanged water, and then

an ammonia solution was added for precipitation. The

precipitated material was dried at 150 8C, heated to 500 8Cat the rate of 50 8C/h, and calcined at 500 8C for 5 h in air. As

listed in Table 1, the ZrO2 content of the ZrO2-TiO2 was varied

from 10 to 90 wt% of ZrO2 in 10 wt% increments. The ZrO2

weight ratio, a, is sometimes used to denote the supports in this

study, for example, ZT70 indicates a 70 wt% (60 mol%) ZrO2

containing ZrO2-TiO2. Pure TiO2 and ZrO2 were also prepared

using the same procedure for ZrO2-TiO2, except that only a

ZrO(NO3)2�2H2O or TiCl4 solution was used.

The specific surface area of the supports was obtained using

the BET one-point method with a Microdata Microsorp 4232II.

The crystalline structure of the supports was analyzed by

powder X-ray diffraction using Cu Ka radiation with a Rigaku

RINT-2100 diffractometer. The temperature programmed

desorption (TPD) of NH3 and CO2 for characterizing the

acid-base property of the supports were measured using a Best

Sokki CATA5000-4 conventional flow-type fixed bed reactor at

atmospheric pressure. A 1.0 g support was packed in a quartz

tube with a 10 mm inner diameter, and then in situ pretreated at

600 8C for 30 min under N2. The support was exposed to NH3

or CO2 at 100 8C until the outlet concentration reached the

same level as the inlet gas. Thereafter, the inlet gas was

switched to flowing N2 until no NH3 or CO2 was detected in the

outlet gas, then the support was heated to 600 8C at the rate of

Table 2

Simulated exhaust gas composition for sulfur-aging test

Atmosphere CO2 (%) O2 (%) CO (%) H2 (%)

Lean 9.6 7.7 1.43 0.47

Rich 10 0 4.5 1.5

20 8C/min for the TPD spectrum. The outlet NH3 and CO2

concentrations were monitored using a nondispersive infrared

(NDIR) -type analyzer. The distribution of Zr and Ti in the

support was analyzed using a Hitachi HF-2000 field-emission

transmission electron microscope (FE-TEM) equipped with an

energy dispersive X-ray analyzer (EDX).

2.2. Catalyst preparation

The synthesized supports were suspended in a nitric acid

solution of dinitro-diamino platinum (Pt(NH3)2(NO2)2, Tanaka

Precious Metals) with stirring for 3 h, then subsequently heated

to vaporize the solution, dried at 120 8C overnight and finally

calcined at 250 8C for 1 h in air. The potassium was then

impregnated in an aqueous solution of potassium acetate

(CH3COOK, Wako Pure Chemical Industries) the same as the

platinum loading, and then calcined at 500 8C for 3 h. The

loading amounts of platinum and potassium as K2O were 1.5

and 7.2 wt%, respectively. The obtained platinum and

potassium supported powder was pressed, crushed and formed

into pellets (diameter: 300–700 mm) for testing.

2.3. Sulfur-aging treatment of catalysts

A 1.0 g catalyst sample was used for the sulfur-aging

experiment. The catalyst was exposed to a cyclic feedstream,

which simulated the actual engine exhaust gas under lean and

rich conditions, heated from ambient temperature to 600 8C in

30 min and then held at this temperature for 100 min. The lean

and rich atmospheres listed in Table 2 were switched every 30 s.

SO2 was introduced to the catalyst for 130 min during heating

and maintained at 600 8C, then switched off during the cooling

process. The gas flow rate was 500 cm3/min, and the molar ratio

of the added sulfur to potassium was 1.1.

2.4. NOx storage measurement and analysis of sulfur-aged

catalysts

The NOx storage was measured using a conventional fixed-

bed reactor [1,3] with simulated exhaust gases at atmospheric

pressure. A 0.5 g sulfur-aged catalyst was used for the NOx

storage and reduction test. Table 3 shows the composition of the

feedstream of the simulated exhaust gas used for the activity

test. The aged catalyst was heated to 300 8C under a rich

feedstream. When the temperature and the outlet gas

concentration reached constant values, the inlet gas was

switched to lean until the outlet NOx concentration reached a

constant value. A 3-s rich spike was then introduced to the

catalyst. Subsequently, the lean gas was again provided to the

catalyst until the outlet NOx concentration reached a constant.

C3H6 (%) SO2 (ppm) H2O (%) N2

0.15 480 3 Balance

0.16 500 3 Balance

Table 3

Simulated exhaust gas compositions for measurement of NOx storage amount

Atmosphere CO2 (%) O2 (%) CO (%) H2 (%) C3H6 (%) NO (ppm) H2O (%) He

Rich 11 0.3 5.6 1.9 0.11 0 3 Balance

Lean 11 6.6 0.025 0.008 0.02 800 3 Balance

Rich Spike 11 0 5.6 1.9 0.11 50 3 Balance

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195190

The NOx storage amount following the rich spike is the

difference in the NOx amount between the inlet and the outlet

gases, and it was used to indicate the catalytic activity. The NOx

storage amounts at 400, 500 and 600 8C were measured similar

to that at 300 8C. The gas flow rate was 3300 cm3/min,

corresponding to a gas hourly space velocity (GHSV) of

200 000/h. The NOx concentration in this study is defined as the

integration of NO and NO2 in the gases measured by a

chemiluminescent NOx meter attached to a Horiba MEXA-

7100 evaluation system.

Potassium sulfate (K2SO4), potassium carbonate (K2CO3)

and potassium nitrate (KNO3) are soluble in hot water, while the

mixed oxide compounds of potassium with titanium and/or

zirconium are insoluble. The state of the potassium on the aged

catalysts was analyzed as follows: soaking the aged catalyst in

distilled water at 60 8C for 2 h, and then removing the catalyst by

filtration. The sulfur content in the filtered liquid was quantified

using inductively-coupled plasma emission spectroscopy (ICP).

Rhor et al. reported that when only the barium compound was

used as the NOx storage material and exposed to an oxidative or

reductive atmosphere, barium, the most basic element on this

type of NSR catalyst, preferentially formed barium sulfate

(BaSO4) or barium sulfide (BaS) [24,25]. Compared with

barium, potassium is a more basic element, and potassium sulfide

(K2S) is apt to react with water to form K2SO4. Therefore, the

sulfur-poisoned potassium only exists as K2SO4 on the sulfur-

aged catalysts. We termed this sulfur-poisoned potassium

(K2SO4) as the ‘‘sulfate-formed potassium’’. It could be

quantified from the sulfur amount in hot water.

As for the remaining potassium, such as carbonate (K2CO3)

and potassium nitrate (KNO3), it potentially functions as NOx

storage and reduction sites, while soluble in hot water. This part

of the potassium is the difference in the potassium amount

between that was soluble in hot water and the sulfate-formed

potassium. Hereafter, we term this difference as the ‘‘remaining

active potassium’’. The catalyst after the filtration was

dissolved in hydrofluoric acid solution, and the amount of

potassium in the solution was quantified using ICP. This part of

the potassium was assumed to have reacted with the supports,

and thus we defined it as the ‘‘solid-phase-reacted potassium’’.

The total deposited amount of sulfur on the catalyst was

measured using a Horiba EMIA-1200 combustion infrared

absorption analyzer.

The platinum dispersion was measured by the CO pulse

adsorption method using an Ohkura Riken R6015 based on the

standard measurement procedure established by the Catalysis

Society of Japan [26,27]. The platinum dispersion was

calculated assuming that one CO molecule adsorbs on one

surface Pt atom.

2.5. Analysis of the desorption of the sulfur compound

The catalyst was pretreated under the same conditions as the

sulfur-aging treatment except that the atmosphere was the lean

condition listed in Table 2 during the entire sulfur-poisoning

treatment. The desorption performance of the sulfur compound

from the aged catalysts was examined using the temperature

programmed reduction (TPR) method. The experimental system

consisted of a Rigaku TG8120 thermogravimeter (TG) and a

Hewlett-Packard 6890 gas chromatograph–mass spectrometer

(GC–MS). A 50 mg sample of the catalyst was placed in the TG

chamber, a 5% H2 gas balanced with He at a 120 cm3/min flow

rate was provided to the catalyst, then heated from ambient

temperature to 800 8C at the rate of 20 8C/min for the TPR

spectrum. Masses from 10 to 100 were scanned every 2 s.

3. Results and discussion

3.1. Influence of composition of supports on the catalytic

activity

The NOx storage versus the ZrO2 content after the sulfur-

aging treatment is shown in Fig. 2, and therein the 0 and

100 wt% ZrO2 contents indicated pure TiO2 or pure ZrO2,

respectively. At 300 8C (Fig. 2a), the NOx storage amount

decreased versus the ZrO2 content up to 60 wt%, while it

became low and constant for the ZrO2 above 60 wt%. At 400 8C(Fig. 2b), it was almost the same for the ZrO2 up to 70 wt%;

while it apparently decreased versus the ZrO2 content above

70 wt%. At 500 8C (Fig. 2c), it gradually increased versus the

ZrO2 content up to 70 wt%, while in contrast, it decreased when

the ZrO2 was greater than 70 wt%. At 600 8C (Fig. 2d), it

reached a maximum value when the ZrO2 was 70 and 80 wt%,

and decreased even if the ZrO2 content was higher or lower than

these values. In a recent review [22] on ZrO2-TiO2 supports and

related catalysts, Reddy and Khan mentioned that almost all the

reports stated that the ZrO2-TiO2 containing 50 mol% of ZrO2

showed the highest catalytic activity. Our result was somehow

different from their results. The peak value of the NOx storage

amount versus the ZrO2 content depended on the reaction

temperature, that is, the TiO2-rich ZrO2-TiO2 supports were

superior to the ZrO2-rich supports at low temperatures, while

the NOx storage with the ZT70 support (ZrO2 content is

60 mol%) was the highest at high temperatures.

3.2. Characterization of the sulfur-aged catalysts

Why are the TiO2-rich supports superior to the ZrO2-rich

supports at low temperature? Why did the ZrO2-TiO2 supports

Fig. 2. NOx removal performance of the sulfur-aged catalysts versus ZrO2 content: (a) 300 8C; (b) 400 8C; (c) 500 8C; (d) 600 8C.

Fig. 3. The state of potassium over the sulfur-aged catalysts. ( ): sulfate-

formed potassium; ( ): solid-phase-reacted potassium; ( ) : remaining active

potassium (neither sulfate-formed nor solid-phase-reacted).

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195 191

show a higher NOx storage ability than the pure TiO2 or ZrO2

support at high temperatures? Furthermore, why did the K/Pt/

ZrO2 catalyst present a poor NOx removal over the entire

temperature range? To answer these questions, following the

sulfur-aging treatment, the three catalysts of K/Pt/TiO2, K/Pt/

ZT70 and K/Pt/ZrO2 were analyzed using the combustion

infrared absorption technique. The total sulfur depositions on

the K/Pt/TiO2, K/Pt/ZT70 and K/Pt/ZrO2 catalysts were

0.78 wt%, 1.00 wt% and 2.46 wt%, respectively. This means

that the total sulfur deposition on K/Pt/ZT70 was approxi-

mately 30% higher than that on K/Pt/TiO2, while about 60%

lower than that on K/Pt/ZrO2. If the sulfur only reacted with

potassium to form K2SO4, thus, 32%, 41% and 100% of the

potassium loaded on the K/Pt/TiO2, K/Pt/ZT70 and K/Pt/ZrO2

catalysts, respectively, would have been transformed into

sulfate.

The main reasons for the potassium deactivation upon the

sulfur-aging treatment are thought to be the sulfate-formation

and solid-phase-reaction with the supports. The ratios of the

sulfate-formed (black boxes) and the solid-phase-reacted

(white boxes) potassium to the originally loaded potassium

are shown in Fig. 3. The former was derived from the sulfur

amount dissolved in hot water, while the latter was obtained

from the nondissolved potassium amount. As shown in this

figure, the ratio of the sulfate-formed potassium on the K/Pt/

ZT70 catalyst was nearly 25% greater than that on the K/Pt/

TiO2 catalyst, while the ratio of the solid-phase-reacted

potassium on K/Pt/ZT70 was approximately 40% of that on

K/Pt/TiO2. Consequently, the ratio of the remaining active

(neither the sulfate-formed nor the solid-phase-reacted)

potassium (gray boxes) on the K/Pt/ZT70 catalyst was 1.3

times higher than that on the K/Pt/TiO2 catalyst. The potassium

compound on the K/Pt/ZrO2 catalyst entirely formed a sulfate,

corresponding to the combustion infrared absorption analysis

and with its poor NOx removal activity over the entire

temperature range in Fig. 2.

The platinum dispersions from the CO adsorption for these

three catalysts were all about 5%. Therefore, if there are any

differences between the catalytic activities of platinum on these

catalysts, they should be derived from the difference in the

chemical property of platinum. Yoshida et al. reported that the

addition of an acidic material stabilized the platinum in the

metallic state [28]. In contrast to this research, we added basic

material to our NSR catalysts. The metallic-state platinum

should then be more stabilized if the remaining active

potassium amount decreased. This metallic-state platinum

would be more active than the oxidized platinum for the NO2

formation under the lean condition as well as for the reduction

of the stored NOx to N2 upon the rich spike. Therefore, a

suitable combination of the adequate metallic platinum and

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195192

potassium NOx storage compound increased the NOx storage

with the TiO2-rich catalysts at 300 8C, as illustrated in Fig. 2a,

although there was no significant difference versus the platinum

dispersion. The thermodynamic equilibrium constant from

potassium carbonate to nitrates decreased with the temperature

[18]; thus the NOx storage amounts on the TiO2-rich catalysts

varied along with the reaction temperature from 400 to 600 8C,

as shown in Fig. 2b–d.

The relatively large amount of the remaining active

potassium on the K/Pt/ZT70 catalyst leads to the higher NOx

storage amount at 500 and 600 8C compared to that on the K/Pt/

TiO2 catalyst. However, such a potassium type makes the

platinum in the oxidized state and suppresses its catalytic

activity; and this leads to the relatively small NOx storage

ability on the K/Pt/ZT70 catalyst at 300 8C, as illustrated in

Fig. 2a.

Moreover, since no solid-phase-reacted potassium on the K/

Pt/ZrO2 catalyst was detected, thus the zirconium-rich supports

preferably suppress the solid-phase reaction between the

potassium and support. In addition, the K/Pt/ZrO2 catalyst had

no remaining active potassium, thus platinum on this catalyst

might have a higher activity for the NO oxidation and stored

NOx reduction. No NOx storage site remained on the K/Pt/ZrO2

catalyst, which led to its poor NOx removal activity over the

entire temperature range.

3.3. Sulfur compound desorption

In this section, to clarify the property of the supports that

limits the sulfur-poisoning, following the sulfur-aging treat-

ment under the continuous oxidative condition, the desorption

of the sulfur compound under the reductive condition from the

K/Pt/TiO2, K/Pt/ZT70 and K/Pt/ZrO2 catalysts was conducted.

H2S (m/e = 34) was the only detected sulfur-containing

compound and no SO2 (m/e = 64) was observed in the outlet

gas. Fig. 4 shows the spectra of the H2S evolution. In agreement

with the previous experiment [17], the H2S concentration on the

K/Pt/ZrO2 catalyst (heavy gray line) was lower than that on the

K/Pt/TiO2 catalyst (thin black line). The H2S concentration on

the K/Pt/ZT70 catalyst (heavy black line) was similar to that on

Fig. 4. H2S desorption spectra with the support oxides: K/Pt/TiO2 ( ), K/

Pt/ZT70 ( ), K/Pt/ZrO2 ( ).

the K/Pt/TiO2 catalyst, rather than that on the K/Pt/ZrO2

catalyst. It was noticed that the H2S peaks on the K/Pt/ZT70

catalyst slightly shifted to the higher temperature region

compared to those on the K/Pt/TiO2 catalyst. These results

corresponded to the sulfur-poisoning amount on the sulfur-aged

catalysts.

3.4. Property of the supports

Among the NSR catalysts using the ZrO2-TiO2 oxide as a

support, why did the K/Pt/ZT70 catalyst show the maximum

NOx storage performance at high temperatures? To clarify the

answer to this question, the obtained NOx storage performance

is discussed and correlated with the acid-base properties and

structures of the supports, and the dispersion of titanium and

zirconium in this section.

3.4.1. Acid-base property

It has been proposed that the acidity of the TiO2 enhanced

the sulfur tolerance of the NSR catalyst [9,14–16]. The acidic

property of the supports was then analyzed by NH3 or CO2-

TPD. The NH3 desorption spectra for TiO2 (thin black line),

ZT70 (heavy black line) and ZrO2 (heavy gray line) are shown

in Fig. 5. Each spectrum had a peak around 200 8C, indicating

that the acid strength of each support were almost the same,

while the NH3 desorption amounts decreased as

ZT70 > TiO2 > ZrO2. Fig. 6 shows the CO2-TPD spectra for

the same supports in Fig. 5. The CO2 desorption on ZrO2 (heavy

gray line) was significant, while it was nearly negligible on the

TiO2 (thin black line) and ZT70 (heavy black line). Fig. 7 shows

the total desorption amount of NH3 (black circles) and CO2

(white circles) from 1 g of each support. The NH3 desorption

amounts on ZT60, ZT70 and ZT80 were obviously greater than

those on the other supports, and reached the maximum value on

ZT70. The CO2 desorptions on ZrO2 and ZT90 were two times

greater than that on the other supports, while the NH3

desorption amount significantly dropped. These results

indicated that, except for ZT90, the ZrO2-TiO2 supports had

a higher acid amount than ZrO2, and ZT70 had the highest acid

amount among the supports.

Fig. 5. NH3 desorption spectra with the support oxides: TiO2 ( ), ZT70

( ), ZrO2 ( ).

Fig. 6. CO2 desorption spectra with the support oxides: TiO2 ( ), ZT70

( ), ZrO2 ( ).

Fig. 7. NH3 and CO2 desorption amounts with the support oxides: (*), NH3

desorption amount; (*), CO2 desorption amount.

Fig. 9. NH3 desorption amounts of supports based on the specific surface area.

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195 193

Fig. 8 illustrates the specific surface areas of the supports.

The ZrO2-TiO2 supports had higher specific surface areas than

TiO2 (ZrO2 content is 0 wt%) and ZrO2 (ZrO2 content is

100 wt%), and the ZrO2-TiO2 support containing 70 wt% of

ZrO2 had the specific surface area of approximately 200 m2/g,

the highest among all the supports. The NH3 desorption amount

Fig. 8. Specific surface area of supports calcined at 500 8C.

divided by the surface area in Fig. 9 produced no significant

peak for ZT70, indicating that the highest acid amount with

ZT70 resulted from its high specific surface area. In his review,

Reddy [22] stated that almost all publications reported that the

50 mol% ZrO2 containing ZrO2-TiO2 showed the highest acid

amount and highest specific surface area. Our materials were

somewhat different from the published result, that is, the

60 mol% ZrO2 had the maximum values. The reason for the

difference is not clear at this point, but might result from a

material difference.

3.4.2. Supports structures

The XRD spectra of TiO2, ZT10 and ZT20 in Fig. 10 only

had the characteristic peaks of the anatase-type TiO2, and these

peaks shifted to a lower diffraction angle along with the

increasing ZrO2 content. The ionic radii of Ti4+ and Zr4+ are

0.064 nm and 0.087 nm, respectively. When the larger Zr4+ ion

partially substitutes for the smaller Ti4+ ion, the lattice of the

anatase crystal consequently expands. As for the spectra of

ZT30, ZT40 and ZT50, the observed peaks could be assigned to

the anatase TiO2 or cubic (or tetragonal making it distinguished

from the cubic and tetragonal, hereafter, denoted as cubic)

ZrO2. In the ZT60, ZT70 and ZT80 spectra, no characteristic

peaks were visible, indicating that they were amorphous

materials. Several studies have reported that ZrO2-TiO2 of

Fig. 10. XRD spectra of support oxides calcined at 500 8C: A, anatase; C,

cubic; M, monoclinic.

Table 4

Zr/Ti composition of ZT70 support analyzed by EDX

Composition (at%)

Zr Ti

Overall field average of Fig. 11 63.06 36.94

Local spot 1 63.15 36.85

Local spot 2 61.05 38.95

Local spot 3 64.79 35.21

Local spot 4 67.50 32.50

Local spot 5 67.44 32.56

Local spot 6 61.45 38.55

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195194

some certain compositions after calcining at 500 8C was an

amorphous material [19,22]. The peaks in the spectra of ZT90

and ZrO2 could be assigned to the cubic or monoclinic ZrO2.

The peaks of ZT90 shifted to a higher diffraction angle

compared with those of ZrO2. It was postulated that the smaller

Ti4+ ion was partially substituted by the larger Zr4+ ion, and the

lattice of the cubic or monoclinic crystal contracted. The same

phenomenon was observed with the cubic ZrO2 peaks in the

ZT50, ZT40 and ZT30 spectra. From the correlation between

the specific surface area in Fig. 8 and the XRD spectra in

Fig. 10, it could be concluded that a higher specific surface area

could be achieved when the supports consisted of the mixture of

anatase TiO2 and cubic ZrO2 or the amorphous materials.

3.4.3. Dispersion of titanium and zirconium

The XRD patterns of ZT60, ZT70 and ZT80 hardly

explained the mixing state of titanium and zirconium.

Therefore, the distributions of zirconium and titanium were

further analyzed by the TEM equipped with an EDX. The

micrograph of ZT70 in Fig. 11 obviously showed that the

secondary particle around 1 mm consisted of agglomerated

primary particles (ca 1 nm). The compositions in the average

view field and in six local spots (approximately 1 nm diameter)

with a random selection in Fig. 11 were analyzed and the results

are listed in Table 4. The overall field average composition was

about 60 at% ZrO2 (ca. 70 wt% ZrO2) and well matched with

the target value. The compositions of all the local spots were

almost the same and consistent with the field average. These

results suggested that zirconium and titanium are uniformly

dispersed in the ZT70 support. For ZT60 and ZT80, the

dispersion of zirconium and titanium should be the same as that

of ZT70. It is well known that an acid site is formed when one

kind of cation substitutes for another kind of cation in binary

composite oxides [22]. In our research, the acid sites are

supposed to be formed in this way.

Based on the above analysis, we can summarize that the

acidity of the support oxide improves the sulfur tolerance, and

Fig. 11. FE-TEM microgram of ZT70 support.

the zirconium rich composition enhances the resistance against

thermal deterioration. ZT70 has the highest BET surface area,

similar acid site concentration as others, but due to its higher

area, a greater number of acid sites are exposed compared to the

others. Its properties facilitate the balance between the sulfur

tolerance and thermal resistance, thus leading to the highest

NOx purification ability of the K/Pt/ZT70 catalyst at high

temperatures following the sulfur-aging treatment.

4. Conclusions

NSR catalysts were synthesized from ZrO2-TiO2 with nine

different compositions, pure TiO2 and ZrO2 supports, and with

platinum and potassium loadings on these supports. Following

the sulfur-aging treatment at 600 8C, the relatively higher NOx

removal ability above 500 8C was obtained on the ZrO2-TiO2

catalyst containing from 60 to 80 wt% ZrO2, and the maximum

value was obtained with the ZT70 catalyst containing

70 wt%ZrO2-30 wt%TiO2, while the TiO2-rich supports were

superior to the others below 400 8C. K/Pt/ZrO2 had a poor NOx

removal activity over the entire temperature range. The analysis

of the sulfur-aged catalysts with the supports of ZT70, TiO2 and

ZrO2 indicated that the TiO2 support had a higher resistance to

potassium sulfate-formation, while the ZrO2 support sup-

pressed the solid-phase-reaction with potassium. The deactiva-

tion of potassium by the sulfate-formation or solid-phase-

reaction with supports should increase the activity of the

metallic platinum, and lead to a higher NOx removal activity of

the TiO2-rich catalysts at low temperatures. In the case of the K/

Pt/ZrO2 catalyst, the potassium totally changed into the sulfate,

and it caused the poor NOx removal ability of this catalyst over

the entire temperature range.

On the K/Pt/ZT70 catalyst, the ratio of the sulfate-formed

potassium to that on the K/Pt/ZrO2 catalyst, and the ratio of the

solid-phase-reacted potassium to that on the K/Pt/TiO2 catalyst

were both approximately 40%. ZT70 had the maximum acid

amount among the investigated supports, which resulted from

the uniform dispersion of zirconium and titanium. The acid

sites promoted the desorption of the sulfur compound, thus the

highest acidity with ZT70 certainly matched its highest sulfur

tolerance.

No solid-phase-reacted potassium was detected on the

sulfur-aged K/Pt/ZrO2 catalyst. This verified that the use of the

zirconium-rich support was effectively suppressed the solid-

N. Takahashi et al. / Applied Catalysis B: Environmental 72 (2007) 187–195 195

phase reaction between potassium and the support. The ZrO2-

rich composition of ZT70 was another reason for the highest

NOx purification ability of the K/Pt/ZT70 catalyst at high

temperatures.

The results and the analysis in this study clarified the roles

played by the TiO2 and ZrO2 oxides, revealed the reasons why

the ZrO2-TiO2 oxide supports can apparently enhance the NOx

storage ability of the sulfur-aged NSR catalyst, and elucidated

what happened over catalysts during the NOx storage and

sulfur-poisoning. Meanwhile, the results suggested a very

promising way to balance the sulfur tolerance and the resistance

against thermal deterioration of the NSR catalysts with the

appropriate acidity and zirconium-rich composition.

Acknowledgements

The authors gratefully acknowledge the combustion infrared

absorption analysis by Mr. Yasuhito Kondou, the ICP analysis

by Mr. Yuzo Kawai and the significant help with this

manuscript preparation by Dr. Fei Dong.

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