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Applied Surface Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

nsight into the effects of different ageing protocols on Rh/Al2O3

atalyst

aohuai Zhaoa, Rui Rana,∗, Yidan Caoa, Xiaodong Wua, Duan Wenga,un Fanb, Xueyuan Wua

Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, 100084 Beijing, ChinaThe Administrative Center for China’s Agenda 21, 100038 Beijing, China

r t i c l e i n f o

rticle history:eceived 31 August 2013eceived in revised form 7 March 2014ccepted 18 April 2014vailable online xxx

eywords:h/Al2O3

geing protocols

a b s t r a c t

In this work, a catalyst of Rh loaded on Al2O3 was prepared by impregnating method with rhodium nitrateaqueous solution as the Rh precursor. The catalyst was aged under different protocols (lean, rich, inert andcyclic) to obtain several aged samples. All the Rh/Al2O3 samples were characterized by X-ray diffraction(XRD), Brunauer-Emmett-Teller (BET) method, CO-chemisorption, H2-temperature programmed reduc-tion (H2-TPR), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Itwas found that a specific ageing treatment could strongly affect the catalytic activity. The N2 aged andthe H2 aged samples had a better catalytic activity for CO + NO reaction than the fresh sample while theair aged and the cyclic aged samples exhibited much worse activity. More surface Rh content and bet-

iffusionnteraction

ter reducibility were obtained in the N2 and the H2 aged samples and the Rh particles existed with anappropriate size, which were all favorable to the catalytic reaction. However, the air and the cyclic ageingprotocols induced a strong interaction between Rh species and the Al2O3 support, which resulted in asevere sintering of particles of Rh species and the loss of active sites. The structure evolution scheme ofthe catalysts aged in different protocols was also established in this paper.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Noble metals such as Pt, Pd, and Rh are used as catalysts for thebatement of pollutants in the automotive exhaust. Among them,h is the most effective in reducing nitrogen oxides (NOx) because

t can rapidly dissociate NO, and no alternative material has beenound so far that can serve as a substitute for Rh [1–7]. The catalyticeduction of NO by CO makes a good model reaction to simulate theeducing NOx in real automotive emissions purification, especiallyor Rh catalysts [8–11].

Generally, the stoichiometric air-to-fuel ratio (A/F = 14.63) can-ot be precisely controlled when the motor is running. The catalystsre always submitted to periodic changes of gas compositionith alternate periods in reducing and in oxidizing atmosphere

9,12–14]. Thermal ageing under different atmosphere is there-

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ore always used to simulate the actual working environment forhe catalysts, so that the researchers can rapidly get the informa-ion of the durability and other properties of a catalyst under a

∗ Corresponding author. Tel.: +86 10 62772510; fax: +86 10 62772510.E-mail address: ranr@tsinghua.edu.cn (R. Ran).

ttp://dx.doi.org/10.1016/j.apsusc.2014.04.140169-4332/© 2014 Elsevier B.V. All rights reserved.

certain working condition. Lassi and co-workers [15] reported thatthe thermal treatment under the reducing and oxidizing ageingatmospheres strongly affected the catalysts’ stability in terms ofsurface areas and chemical states of active metals. Stoyanovskiiet al. [16] found that the thermal treatment in oxidizing atmosphereat temperatures exceeding 800 ◦C resulted in the active Rh speciesmigrating from surface to bulk of the Al2O3 support, eventuallyleading to relatively uniform Rh distribution in the alumina phase.From a practical point of view, how to prevent the catalyst deac-tivation is an important issue in reducing the usage of Rh. Thus, itis important to understand the deactivation mechanism caused bydifferent ageing protocols under high temperature.

For the thermal deactivation of Rh/Al2O3 catalyst, there havebeen several approved reasons: the sintering of the support andthe sintering of noble metals. The former results in a loss of spe-cific surface area which is clearly detrimental for the activity ofcatalysts, and the later reduces the catalytic activity because of thedecrease of the number of active surface sites [9,17–19]. Besides,

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

the Rh species would evolve to change their physical and chemicalstate during the high temperature thermal ageing and the inter-action between noble metals and support materials is consideredas another important factor to influence the performance of the

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atalysts [7,9,20,21]. Yates et al. researched the Rh/Al2O3 modelatalyst calcined in vacuum and in static air, respectively. Theyound Rh calcined in static air existed in platelets or rafts in theurface of the support [22], while Rh calcined in vacuum couldiffuse into the A12O3 near surface region or the A12O3 film diff-sed onto the Rh film surface [7]. Hwang et al. [3] also pointed outhat a catalytically inactive binary compound between Rh oxidesnd alumina forming in the process of Rh diffusing in to the Al2O3>500 ◦C, oxidizing atmosphere). Zimowska’s study confirmed such

diffusion phenomenon systematically [23]. They investigated thathermal treatment at 500 ◦C in air caused the incorporation of Rhnto the alumina structure in the form of Rh4+/ı-Al2O3. The Rh4+/ı-l2O3 would decrease after the annealing in air at 840 ◦C or inydrogen at 500 ◦C or in methane at 600 ◦C, along with the Rh2Al9lloy formation. For catalysts operated higher temperatures (up to000–1050 ◦C), the diffusion of the supported metal is accompa-ied by the phase transformations of the support leading to theorundum (�-Al2O3) formation. Moreover, Burch et al. [24] foundhat calcination in static air at 500–700 ◦C caused strong interac-ion between Rh and the alumina support since EXAFS data showedhat most of the Rh presented as the isolated ions with high co-rdination to both oxygen and alumina during the calcination.ccording to their point, the Rh species during ageing spread over

he support and diffused into defect sites in the alumina surface sohat they were extremely difficult to reduce. But when the calciningemperature is up to 1000 ◦C, the Rh3+ ions would be captured byhe corundum phase growing during the thermal treatment, andannot go back on the surface [16].

Based on the literature review, it was found that most of theelated studies were focused on the case in air ageing or vacuumgeing. How does the Rh evolution occur in reduced atmosphere oryclic atmosphere is also a key point on the lifetime of a real catalyst,ut still lack of exploration so far. In this work, we will study thetructure evolution and catalytic performance of Rh/Al2O3 catalystn different feed-streams including O2, H2, N2 and stationary cyclicas composition to simulate different working conditions the actualutomotive exhaust. The characterization including NO + CO light-ff test, TPR, CO chemisorption, XRD as well as XPS will help uso understand the effects of the different ageing protocols on theh/Al2O3 catalyst.

. Experimental

.1. Preparation of samples

Rh/Al2O3 catalysts were prepared via the incipient-wetnessmpregnation on Al2O3 (SBET = 150 m2 g−1, BASF) materials withh(NO3)3 (8.39 wt% Rh, Heraeus) as precursor. First, Rh(NO3)3 waseposited on high-surface-area Al2O3 with the Rh loading ratio of.5 wt%. Then the obtained slurries were pretreated sequentiallyith an overnight drying at 110 ◦C for 12 h before being calcined

t 550 ◦C for 3 h in static air. The as-obtained sample was aged inifferent ageing protocols as follows: (1) N2 (inert), (2) H2 (rich, 7%2 in N2), (3) air (lean) and (4) cyclic air/H2 (lean/rich cycle: 10 minir, 8 min N2, 10 min 4% H2 in N2, 8 min N2; the last cycle ended initrogen and the sample was cooled down to room temperature inir), with 10% steam at 1050 ◦C for 12 h to obtain the aged series.he catalysts were donated as fresh, N2 aged, H2 aged, air aged andyclic aged samples and RA was short for the Rh/Al2O3 catalysts.

.2. Characterizations

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.2.1. Structure and texture propertiesThe X-ray diffraction analysis (XRD) was performed on a

ermany Bruker D8 Advance diffractometer employing Cu K�

PRESSience xxx (2014) xxx–xxx

radiation (� = 0.15418 nm) and the X-ray tube was operated at40 kV and 300 mA. The continuous X-ray powder diffractogramwas recorded at 0.02◦ intervals in the range 20◦ ≤ 2� ≤ 80◦ with ascanning velocity of 6◦ min−1.

The specific surface area was determined by Brunauer–Emmett–Teller (BET) method with an automatic surface analyzer(F-Sorb 3400, Gold APP Instrument) using N2 as adsorbent. Beforethe measurements, the samples were degassed in vacuum at 200 ◦Cfor 2 h.

CO chemisorption uptake of the catalysts was carried out on theMicromeritics Auto Chem II 2920 apparatus with a thermal con-ductivity detector (TCD). For each experiment, 0.1 g of the catalystwas placed in an U-shaped quartz tube and reduced in a flow of 10%H2/Ar (50 ml min−1) while ramping the temperature up to 350 ◦C atthe rate of 10 ◦C min−1, and then held at 350 ◦C for 30 min in flow-ing He (50 ml min−1) for proper degassing. After that the samplewas cooled down to 25 ◦C, the loop gas of 10% CO/He (20 ml min−1)was pulsed over the sample and the TCD signal was recorded untilthe peak area became constant and the total uptake of CO wascalculated.

H2-TPR was performed on the Micromeritics Auto Chem II 2920apparatus. For each experiment, approximately 50 mg of the cata-lyst was placed in a U-shaped quartz tube and pretreated in flowingHe at 300 ◦C for 30 min or 10% O2/Ar at 500 ◦C for 30 min. After thatthe sample was cooled down to 0 ◦C. Then the sample was reducedin a flow of 10% H2/Ar (50 ml min−1) while ramping the tempera-ture from 0 ◦C up to 800 ◦C while the TCD signal was recorded bythe detector.

Transmission electron microscope (TEM) images were taken ona FEI Tecnai G20 instrument with an acceleration voltage of 200 kV.All the catalysts were reduced in 10% H2/Ar at 400 ◦C for 30 minbefore the TEM observation. Mean Rh particle size was calculatedbased on TEM images by averaging 50 points of Rh particles.

The X-ray photoelectron spectroscopy (XPS) experiments werecarried out on a PHI-5300 ESCA system with Al K� radiation underUHV (1.33 × 10−8 Pa), calibrated internally by the carbon deposit C1s binding energy (B.E.) at 284.8 eV.

2.2.2. Activity measurementThe mixture of 100 mg catalyst and 300 mg quartz sands was

prepared for the activity test which was carried out using a flowreactor system by passing a stoichiometric reaction gas mixturecontaining NO (5000 ppm) and CO (5000 ppm) diluted in N2 ata rate of 100 ml min−1 over the catalyst. The activity was mea-sured while raising the temperature from 100 to 600 ◦C at a rateof 10 ◦C min−1. Then the effluent gas was analyzed with an FTIR gasanalyzer (Thermo fisher Nicolet IS10). Each of the activity test isoperated twice using the same catalyst and the data of the secondrun is used to get the NO + CO light off curves.

3. Results and discussions

3.1. Catalytic activity for CO + NO

In this study temperature-programmed reduction of NO by COhas been carried out on the catalysts, to investigate the effects of thedifferent ageing protocols on the catalytic activity. The results areshown in Fig. 1. It can be seen that the catalysts show different cat-alytic activities after different ageing protocols. T50 (temperaturesof 50% NO conversion) of the five samples follows the sequenceof N2 aged (195 ◦C) < H2 aged (225 ◦C) < fresh (241 ◦C) < air aged

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

(349 ◦C) < cyclic aged (366 ◦C). It is interesting that the activity ofthe N2 aged and the H2 aged samples are even better than thefresh one. And the air aged and the cyclic aged samples behavemuch worse in terms of the NO reduction than the fresh sample.

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Fig. 1. Light-off curves of CO + NO reaction over the RA catalysts.

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Table 1BET area, CO uptake and Rh particle size of the RA catalysts.

Sample Specific surfacearea (m2 g−1)

CO uptake(�mol g−1)

Particle size byTEM (nm)

Fresh 136 79.68 0.5–0.6N2 aged 45 14.23 3.5–3.6

employed to measure the oxidation state of Rh before and after

Fig. 2. X-ray diffraction patterns of the RA catalysts.

enerally, both the structure and the chemical properties of theatalysts may be responsible for the catalytic activities. For exam-le, Oh et al. [25] found the specific rate of the CO + NO reaction onh catalysts increased with increasing Rh particle size in a certainange. Djéga-Mariadassou et al. [10,26] confirmed that both Rh0

nd Rhx+ participated in the reaction but they behaved differently.lso, surface area, microstructure of the catalysts can influence theatalytic activity indirectly. Thus, it is necessary to get more infor-ation about the above properties of the samples in this work.

.2. Structure properties

The X-ray diffraction patterns of the catalysts are shown in Fig. 2.he results reveal that the fresh sample is in a poorly crystallinend there is only �-Al2O3 in it. After ageing at 1050 ◦C, the �-Al2O3nd the transient phase �-Al2O3 form in all of the aged sampleshatever the ageing atmosphere is. No Rh related phase is detected

y XRD for the catalysts, due to the low Rh loading ratio and theell dispersion of Rh species.

The results of BET specific surface area are displayed in Table 1.he fresh sample has the highest specific surface area of 136 m2 g−1

hich is 2–3 times higher than those of the aged samples becausef the sintering and the Al2O3 phase transition after ageing. The

Please cite this article in press as: B. Zhao, et al., Insight into the effecSci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.140

2 aged and the cyclic aged samples exhibit a similar specific sur-ace area of 45 and 41 m2 g−1, respectively. The H2 aged sample hashe lowest specific surface area of 26 m2 g−1. Combining with the

H2 aged 26 12.18 3.4–3.5Air aged 60 <3 ∼35Cyclic aged 41 5.12 ∼44

results of catalytic activities, it can be considered that the surfacearea is not the crucial factor for the activities.

3.3. TEM and CO chemisorption

Fig. 3 shows the TEM images of the catalysts. All the catalysts arereduced in H2 at 400 ◦C before the TEM observation so that the RhOx

can be reduced to metallic Rh for size investigation. In Fig. 3a–c,which are corresponding to the fresh, the N2 aged and the H2 agedsamples, respectively, the particles labeled by arrows are identifiedto be metallic Rh particles and the Rh particles are finely dispersed.In Fig. 3d and e related to the air aged and the cyclic aged sam-ples, respectively, large Rh particles corresponding to the partiallyreduced RhOx are observed and they are tightly bound to the sup-ports. During the air and the cyclic ageing protocols, metallic Rhis oxidized to RhOx. Then the RhOx particles grow and diffuse intothe Al2O3 supports simultaneously. They are difficult to be reducedin the pretreatment and have low contrast in comparison to Al2O3since they are combined well with the supports. TEM observationsin Fig. 3d and e also reveal a significant sintering of Al2O3 in the airaged and the cyclic aged samples.

The particle size of Rh species estimated by TEM is displayed inTable 1. The particle size data shows that the Rh particles are largerin the N2 and the H2 aged samples than those in the fresh sample,but much smaller than those in the air aged and cyclic aged ones. Itsuggests that a moderate particle size of Rh species on the surfaceof catalysts should play an important role in NO + CO reaction. Thisis in agreement with the results in some literatures [2,25,27]. It wasfound that large Rh particle size in a certain range was good for thecatalytic activity of Rh/Al2O3 catalyst, and the turn over numberfor the NO + CO reaction increases drastically (45-fold increase inspecific rate) with increasing Rh particle size in a certain range inRh/Al2O3 catalyst [25].

The CO uptakes estimated by chemisorption method are alsodisplayed in Table 1. All of the samples have the same total contentof Rh but the CO uptake is very different. The CO uptake of the freshsample is extremely higher than that of the other samples due to theCO spill-over effect [28]. The CO uptake of the air aged (<3 �mol g−1)and the cyclic aged (<5.12 �mol g−1) samples is very low althoughthe specific surface area of the two samples are not the smallest. Onthe contrast, the N2 and the H2 aged samples with the best catalyticactivities have CO uptake of 14.23 and 12.18 �mol g−1, respectively,demonstrating that more CO can be adsorbed on the active sites toparticipate in the reaction in the N2 aged and the H2 aged samplesthan in the air aged and the cyclic aged samples. Without regard tothe fresh sample, this suggests that there may be more active Rhsites exposing on the surface of the N2 and the H2 aged samplesthan the other two.

3.4. Properties of surface Rh species

The X-ray photoelectron spectra (XPS) measurement is

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

different ageing protocols. As Fig. 4 shows, the spectra of the fivecatalysts are presented in the binding energy (B.E.) range includingthe Rh (3d5/2, 3d3/2) peaks. The XPS spectrums of the fresh, the N2

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4 B. Zhao et al. / Applied Surface Science xxx (2014) xxx–xxx

) H2 a

aasaa3

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Fig. 3. TEM images of (a) fresh, (b) N2 aged, (c

ged and the H2 aged samples are fitted using 6 peaks while the airged and the cyclic aged samples could only be fitted using 4 peaksince there is no peaks of metallic Rh in the two samples. The peaksre marked as u and u’ for Rh0 3d5/2 (307.1–307.3 eV) and 3d3/2, vnd v’ for Rh3+ 3d5/2 (308.3–308.6 eV) and 3d3/2, w and w’ for Rh4+

d5/2 (310.3–310.5 eV) and 3d3/2, respectively [15].The quantitative data obtained from fitted XPS spectra of the

articular catalysts are presented in Table 2. The N2 aged and the2 aged samples have the surface Rh content of 0.53%, 0.63% respec-

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ively, which is much more than that of the air aged (0.14%) and theyclic aged (0.13%) samples. By contrast, the surface Rh content ofhe fresh sample is 0.27% since the percentage of Rh atoms in theeveral atomic layers which the soft X-ray employed in XPS can

ged, (d) air aged and (e) cyclic aged samples.

penetrate is low due to the highest specific surface area of the freshsample. In addition, the Rh0/Rh ratios of the N2 aged and theH2 aged samples are 59.17% and 54.35% respectively, which aretwo times higher than that of the fresh sample. None metallicRh content is detected in the air aged and the cyclic aged sam-ples and the Rh species present mainly as Rh3+ and Rh4+ of RhOx

in the two samples, demonstrating that Rh is easily to be oxi-dized in the air and cyclic ageing protocols. Taking the bindingenergy of the Rh species into account: the binding energy of metal-

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

lic Rh decreases as the sequence of fresh > H2 aged > N2 aged dueto the particle growth during ageing since there is a less effec-tive screening of the core holes created during photoemissionin small particles [29,30]. However, the binding energy of Rh3+

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ged, (

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Fig. 4. XPS spectra of Rh 3d on the (a) fresh, (b) N2 a

ncreases follows the sequence of N2 aged < H2 aged < fresh < air

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ged < cyclic aged. The increasing binding energy of RhOx indicatesartial Rh species in form of RhOx diffuse into alumina [29,31].oreover, the diffused RhOx might interact with the support to

able 2uantification of Rh (3d5/2) signals of the RA catalysts.

Sample Rh (atom%) Rh0/Rh (%) Rh3+/Rh (%)

Fresh 0.27 25.97 42.60

N2 aged 0.53 59.17 24.26

H2 aged 0.63 54.35 28.80

Air aged 0.14 0 64.10

Cyclic aged 0.11 0 44.84

c) H2 aged, (d) air aged and (e) cyclic aged samples.

form the metal–support interphase such as Rh(AlO2)y which was

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

verified to be catalytically inactive [31,34]. Although both Rh0

and Rhx+ are active for the NO + CO reaction [10,26], the oxida-tive Rh species in the air aged and cyclic aged samples show little

Rh4+/Rh (%) B.E. of Rh 3d5/2 (eV)

Rh0 Rh3+ Rh4+

31.43 307.76 309.01 310.0116.57 306.81 308.26 310.0816.85 307.26 308.61 310.0135.90 – 309.36 310.0655.16 – 309.51 310.21

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F(

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Fig. 6. Scheme of the RA catalysts aged under different protocols.

ig. 5. H2-TPR profiles of RA catalysts (a) pretreated with He at 300 ◦C for 30 min,b) pretreated with O2 at 500 ◦C for 30 min.

ctivity due to the strong interaction between Rh species and theupports.

.5. Redox properties of the catalysts

To confirm the redox properties of different Rh species, H2-TPRxperiment is performed on each sample. The catalysts are pre-ared in flowing He at 300 ◦C for 30 min before the TPR tests toeep the Rh species and the supports in the as-prepared states, andhe profiles of the catalysts are shown in Fig. 5a. Each TPR profilef the fresh sample as well as the N2 aged and the H2 aged sam-le is composed of two broad reduction peaks with temperaturerom ambient temperature to 280 ◦C and from 280 ◦C to 500 ◦C,espectively. According to the results of the literatures [3,32,33],he former reduction peak with the temperature below 280 ◦C cor-esponds to the reduction of surface RhOx species and the latterne can be attributed to the reduction of RhOx species which inter-ct with the Al2O3 support. There is a more intensive reductioneak above 280 ◦C appearing in each of the air aged and the cyclicged samples but shifting to much higher temperature than in the2 aged sample. It is considered as the strong Rh-support interac-

ion in the form of the formation of the metal–support interphaseh(AlO2)y in the air and the cyclic ageing protocols due to theiffusion of Rh oxides into sub-layers of the Al2O3 structure atigh oxidation temperatures [3,34]. The formation of Rh(AlO2)y has

Please cite this article in press as: B. Zhao, et al., Insight into the effecSci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.140

een regarded as one of the major reasons for the deactivation ofh/Al2O3 catalysts towards many oxidation reactions [31,35–38].

For comparing, the TPR profiles of the catalysts pretreated withowing 10% O2/Ar at 500 ◦C for 30 min are shown in Fig. 5b. For

the fresh sample, there is a similar broad peak corresponding tothe reduction of surface RhOx species. The N2 aged and the H2aged samples show two main reduction peaks centered at 40 ◦Cand 80 ◦C. The peaks located at 40 ◦C are attributed to uniformlydistributed particles which are easily reduced, while the peaks at80 ◦C are attributed to a bulk-like crystalline RhOx on the surface[39,40]. It suggests that the metallic Rh particles in the N2 aged andthe H2 aged samples are re-oxidized to RhOx on the surface of thesupports during the O2 pretreating process and such particles areeasily reduced. Meanwhile, no obvious reduction peak is observedfor the air aged and cyclic aged samples in the temperature range(0–300 ◦C), though the surface of the samples are oxidized. Thisresult confirms the chemical inertness of the surface in the air agedand cyclic aged samples leading by the strong interaction betweenRh species and the supports.

Overall, it is considered that the evolution of Rh species andthe interaction between Rh species and the support during diverseageing atmospheres play the most important role in the catalyticactivity of the Rh/Al2O3 catalysts. The structure scheme of the fivesamples can be established, which is presented in Fig. 6. The Rhspecies mainly exist in metallic Rh in the N2 and the H2 aged sam-ples. Such two samples have larger Rh particles than the freshsample, but little interaction between the Rh and the Al2O3 sup-port. It is beneficial for the NO + CO activity. On the contrary, theair and the cyclic ageing protocols result in even larger RhOx par-ticles, and most of the RhOx diffuse into the sublayer of the Al2O3support to form the inactivate Rh(AlO2)y, leading the deactivationof the catalysts.

4. Conclusion

In this study, the effects of different ageing protocols (air, N2, H2and cyclic ageing) on the catalytic activity for NO + CO reaction andthe structure evolution of the supported Rh/Al2O3 catalysts havebeen investigated. It is found that the catalytic activity in terms ofthe NO reduction of the N2 aged and the H2 aged Rh/Al2O3 catalystsare even better than the fresh sample, while the air aged and thecyclic aged samples behave much worse than the fresh one. Thereare more Rh remaining on the surface of the N2 and the H2 agedsamples to provide more active sites for catalytic reactions. Andtheir appropriate Rh particle size is good for the catalytic activities.The air and the cyclic ageing protocols induce a strong interactionbetween Rh species and the Al2O3 support. Rh is oxidized to RhOx inthe presence of O2 and the RhOx grows to very large particles andmost of them diffuse into the support to form the inactive phaseRh(AlO2)y in the interface of Rh species and Al2O3. Such a strong

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

interaction resulting in a loss of surface active sites is the majorreasons for the deactivation of Rh/Al2O3 catalysts.

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cknowledgments

The authors would like to acknowledge Project 51202116 byhe Nature Science Foundation of China, and Project 2013AA065302upported by the Ministry of Science and Technology, PR China. Weould also thank the Key Laboratory of Advanced Materials (MOE)

or performing materials characterizations.

eferences

[1] T. Tanabe, A. Morikawa, M. Hatanaka, N. Takahashi, Y. Nagai, A. Sato, O. Kuno, H.Suzuki, H. Shinjoh, The interaction between supported Rh- and Nd2O3-enrichedsurface layer on ZrO2 for Rh sintering suppression, Catalysis Today 184 (2012)219–226.

[2] D. Martin, D. Duprez, The effects of support and of particle size on the redoxproperties of rhodium, Applied Catalysis A: General 131 (1995) 297–307.

[3] C. Hwang, C. Yeh, Q. Zhu, Rhodium-oxide species formed on progressive oxi-dation of rhodium clusters dispersed on alumina, Catalysis today 51 (1999)93–101.

[4] Z. Weng-Sieh, R. Gronsky, A. Bell, Microstructural evolution of �-alumina-supported Rh upon aging in air, Journal of Catalysis 170 (1997) 62–74.

[5] I. Heo, D. Yoon, B. Cho, I. Nam, J. Choung, S. Yoo, Activity and thermal stabilityof Rh-based catalytic system for an advanced modern TWC, Applied CatalysisB: Environmental 121 (2012) 75–87.

[6] M. Bowker, Automotive catalysis studied by surface science, Chemical SocietyReviews 37 (2008) 2204–2211.

[7] J. Chen, M. Colaianni, P. Chen, J. Yates, G. Fisher, Thermal behavior of arhodium/alumina model catalyst: disappearance of surface rhodium uponheating, Journal of Physical Chemistry 94 (1990) 5059–5062.

[8] J. Kim, W. Myeong, S. Ihm, Characteristics of CeO2–ZrO2 mixed oxide preparedby continuous hydrothermal synthesis in supercritical water as support of Rhcatalyst for catalytic reduction of NO by CO, Journal of Catalysis 263 (2009)123–133.

[9] J. González-Velasco, J. Botas, R. Ferret, M. Pilar González-Marcos, J. Marc, M.Gutiérrez-Ortiz, Thermal aging of Pd/Pt/Rh automotive catalysts under a cycledoxidizing–reducing environment, Catalysis Today 59 (2000) 395–402.

10] G. Djéga-Mariadassou, From three-way to deNOx catalysis: a general model,Catalysis Today 90 (2004) 27–34.

11] W. Hecker, A. Bell, Infrared observations of Rh NCO and Si NCO species formedduring the reduction of NO by CO over silica-supported rhodium, Journal ofCatalysis 85 (1984) 389–397.

12] J. González-Velasco, J. Botas, R. Ferret, et al., Comparative three-way behaviourof Pt, Pd and Rh single and combined phases in a full gas mixture withoscillating feedstream, Studies in Surface Science and Catalysis 116 (1998)73–82.

13] J. Schlatter, R. Sinkevitch, P. Mitchell, Laboratory reactor system for three-wayautomotive catalyst evaluation, Industrial & Engineering Chemistry ProductResearch and Development 22 (1983) 51–56.

14] S. Suhonen, M. Valden, M. Hietikko, R. Laitinen, A. Savimäki, M. Härkönen, Effectof Ce–Zr mixed oxides on the chemical state of Rh in alumina supported auto-motive exhaust catalysts studied by XPS and XRD, Applied Catalysis A: General218 (2001) 151–160.

15] U. Lassi, R. Polvinen, S. Suhonen, K. Kallinen, A. Savimäki, M. Härkönen, M.Valden, R. Keiski, Effect of ageing atmosphere on the deactivation of Pd/Rhautomotive exhaust gas catalysts: catalytic activity and XPS studies, AppliedCatalysis A: General 263 (2004) 241–248.

16] V. Stoyanovskii, A. Vedyagin, G. Aleshina, A. Volodin, A. Noskov, Character-ization of Rh/Al2O3 catalysts after calcination at high temperatures underoxidizing conditions by luminescence spectroscopy and catalytic hydrogenol-

Please cite this article in press as: B. Zhao, et al., Insight into the effecSci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.140

ysis, Applied Catalysis B: Environmental 90 (2009) 141–146.17] M. Hietikko, U. Lassi, K. Kallinen, A. Savimäki, M. Härkönen, J. Pursiainen, R.S.

Laitinen, R.L. Keiski, Effect of the ageing atmosphere on catalytic activity andtextural properties of Pd/Rh exhaust gas catalysts studied by XRD, AppliedCatalysis A: General 277 (2004) 107–117.

[

PRESSience xxx (2014) xxx–xxx 7

18] C. Larese, F. Galisteo, M. Granados, R. Mariscal, J. Fierro, M. Furió, R. Ruiz, Deac-tivation of real three way catalysts by CePO4 formation, Applied Catalysis B:Environmental 40 (2003) 305–317.

19] S. Lakhapatri, M. Abraham, Analysis of catalyst deactivation during steamreforming of jet fuel on Ni-(PdRh)/�-Al2O3 catalyst, Applied Catalysis A: Gen-eral 405 (2011) 149–159.

20] J. Van Zon, D. Koningsberger, H. Van’t Blik, R. Prins, D. Sayers, On the detectionwith EXAFS of metal-support oxygen bonds in a highly dispersed rhodium onalumina catalyst, The Journal of Chemical Physics 80 (1984) 3914–3915.

21] C. Linsmeier, E. Taglauer, Strong metal–support interactions on rhodium modelcatalysts, Applied Catalysis A: General 391 (2011) 175–186.

22] D. Yates, E. Prestridge, Electron diffraction and anomalous X-ray diffraction ofrhodium oxide platelets on alumina, Journal of Catalysis 106 (1987) 549–552.

23] M. Zimowska, J. Wagner, J. Dziedzic, J. Camra, B. Borzecka-Prokop, M. Najbar,Some aspects of metal-support strong interactions in Rh/Al2O3 catalyst underoxidising and reducing conditions, Chemical Physics Letters 417 (2006)137–142.

24] R. Burch, P. Loader, N. Cruise, An investigation of the deactivation of Rh/aluminacatalysts under strong oxidising conditions, Applied Catalysis A: General 147(1996) 375–394.

25] S. Oh, C. Eickel, Influence of metal particle size and support on the catalyticproperties of supported rhodium: CO + O2 and CO + NO reactions, Journal ofCatalysis 128 (1991) 526–536.

26] G. Djéga-Mariadassou, F. Fajardie, J. Tempère, et al., A general model for boththree-way and deNOx catalysis: dissociative or associative nitric oxide adsorp-tion, and its assisted decomposition in the presence of a reductant: part I. Nitricoxide decomposition assisted by CO over reduced or oxidized rhodium speciessupported on ceria, Journal of Molecular Catalysis A: Chemical 161 (2000)179–189.

27] P. Araya, F. Gracia, J.n. Cortés, E.E. Wolf, FTIR study of the reduction reaction ofNO by CO over Rh/SiO2 catalysts with different crystallite size, Applied CatalysisB: Environmental 38 (2002) 77–90.

28] S. Royer, D. Duprez, Catalytic oxidation of carbon monoxide over transitionmetal oxides, ChemCatChem 3 (2011) 24–65.

29] A. Talo, J. Lahtinen, P. Hautojärvi, An XPS study of metallic three-way catalysts:the effect of additives on platinum, rhodium, and cerium, Applied Catalysis B:Environmental 5 (1995) 221–231.

30] T. Huizinga, H. Van’t Blik, J. Vis, R. Prins, XPS investigations of Pt and Rh sup-ported on �-Al2O3 and TiO2, Surface Science 135 (1983) 580–596.

31] D. Duprez, G. Delahay, H. Abderrahim, J. Grimblot, Characterization of Rh/Al2O3

catalysts by gas adsorption and X-ray photoelectron spectroscopy, Journal deChimie Physique et de Physico-Chimie Biologique 83 (1986) 465–477.

32] R. McCabe, R. Usmen, K. Ober, H. Gandhi, The effect of alumina phase-structureon the dispersion of rhodium/alumina catalysts, Journal of Catalysis 151 (1995)385–393.

33] I. Cuauhtemoc, G. Del Angel, G. Torres, V. Bertin, Catalytic wet air oxidation ofgasoline oxygenates using Rh/�- Al2O3 and Rh/�- Al2O3-CeO2 catalysts, Catal-ysis Today 133 (2008) 588–593.

34] S. Lakhapatri, M. Abraham, Deactivation due to sulfur poisoning and carbondeposition on Rh-Ni/Al2O3 catalyst during steam reforming of sulfur-dopedn-hexadecane, Applied Catalysis A: General 364 (2009) 113–121.

35] C. Wong, R. McCabe, Effects of high-temperature oxidation and reduction onthe structure and activity of Rh/Al2O3andRh/SiO2 catalysts, Journal of Catalysis119 (1989) 47–64.

36] H. Yao, S. Japar, M. Shelef, Surface interactions in the system Rh/Al2O3, Journalof Catalysis 50 (1977) 407–418.

37] H. Yao, H. Stepien, H. Gandhi, Metal-support interaction in automotive exhaustcatalysts: Rh-washcoat interaction, Journal of Catalysis 61 (1980) 547–550.

38] J. Li, F. Huang, W. Weng, X. Pei, C. Luo, H. Lin, C. Huang, H. Wan, Effect of Rh load-ing on the performance of Rh/Al2O3 for methane partial oxidation to synthesisgas, Catalysis Today 131 (2008) 179–187.

39] P. Fornasiero, R. Dimonte, G. Rao, J. Kaspar, S. Meriani, A. Trovarelli, M. Graziani,

ts of different ageing protocols on Rh/Al2O3 catalyst, Appl. Surf.

Rh-loaded CeO2-ZrO2 solid-solutions as highly efficient oxygen exchangers:dependence of the reduction behavior and the oxygen storage capacity on thestructural-properties, Journal of Catalysis 151 (1995) 168–177.

40] C. Diagne, H. Idriss, A. Kiennemann, Hydrogen production by ethanol reformingover Rh/CeO2–ZrO2 catalysts, Catalysis Communications 3 (2002) 565–571.