TiO2

TiO2

Chemical Physics Letters 372 (2003) 160–165 www.elsevier.com/locate/cplett Strong metal-support interaction and catalytic properties of anatase and r...

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Chemical Physics Letters 372 (2003) 160–165 www.elsevier.com/locate/cplett

Strong metal-support interaction and catalytic properties of anatase and rutile supported palladium catalyst Pd=TiO2 Yuanzhi Li a

a,b

, Yining Fan a,*, Hanpei Yang a, Bolian Xu a, Lingyun Feng a, Mingfeng Yang a, Yi Chen a

Department of Chemistry, Laboratory of Mesoscopic Materials and Molecular Engineering, Nanjing University, Nanjing, Jiangsu 210093, China b Department of Chemistry, Three Gorges University, Hubei, Yichang 443000, China Received 28 January 2003; in final form 1 March 2003

Abstract In situ EPR investigation by using CO as probe molecules shows that even pre-reduced by H2 at lower temperature results in SMSI for anatase titania supported palladium catalyst, but not for rutile titania supported palladium catalyst. The reason of the different behavior between rutile and anatase titania supported palladium catalyst is discussed. The very different catalytic properties between anatase and rutile titania supported palladium catalyst pre-reduced at lower temperature, and the rapid change of conversion and selectivity of titania supported palladium catalyst with the elevation of pre-reduction temperature further confirm the above-mentioned results. Ó 2003 Elsevier Science B.V. All rights reserved.

1. Introduction Strong metal-support interaction (SMSI) has attracted much attention since its first report because noble metal supported on reduced oxides (with SMSI) show important differences in the catalytic activity and selectivity of hydrogenation reaction when reduced at high temperature, compared with one reduced at lower temperature or the corresponding noble metal supported on unreducible supports (without SMSI) [1–8]. Most of all studies of SMSI concentrated on titania sup-

*

Corresponding author. Fax: +86-25-3317761. E-mail address: [email protected] (Y. Fan).

ported noble metal catalyst. Traditional criterion for SMSI is the rapid reduction of H2 or CO adsorption capacity without a significant enlargement of metal particles with the elevation of prereduction temperature. Usually, SMSI is observed when the reduction temperature is above 300 °C. It is well known that titania exists in three main crystalline form e.g., anatase, rutile, and brookite, and each structure exhibits different physical properties. It is inferred that noble metal supported on titania with different crystalline forms might exhibit different physic-chemical properties and catalytic properties. However, to our best knowledge, there have been no reports of such difference up to now. Herein, we make comparative investigation into SMSI for anatase and rutile

0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00383-X

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titania supported palladium catalyst by EPR using CO as probe molecules, and their catalytic properties for selective hydrogenation of alkenes. It was discovered that anatase titania supported palladium catalyst shows very different properties from rutile titania supported palladium catalyst, and even pre-reduced by H2 at lower temperature results in SMSI for anatase titania supported palladium catalyst, but not for rutile titania supported palladium catalyst. TiCl4 (0.25 mol) was added drop wise into 1000 ml distilled water. Then 2 mol l1 dilute NH4 OH was added to the above aqueous TiCl4 solution ð0:25 mol l1 Þ until the pH value was above 9. Subsequently, the precipitated hydrous titania was separated from the solution by filtering, and repeatedly washed with distilled water to make the precipitant free of chloride. The hydrous titania was dried at 120 °C for 24 h, the anatase titania was obtained by calcining the dried hydrous titania titania at 500 °C for 3 h. TiCl4 (0.25 mol) was added drop wise into 1000 ml distilled water. Then the above aqueous solutions were kept at 40 °C for hydrolysis of TiCl4 in a temperature-controlled bath. In order to promote crystallization of titania, a small amount of nano rutile TiO2 (ca. 70 mg) with average crystal size of 6.9 nm and a specific area of 141:0 m2 =g1 were added into the above solutions. After hydrolysis and crystallization for several days, the precipitates formed in the solutions were filtered and washed thoroughly with distilled water, and then dried at 150 °C in air for 24 h. The rutile titania was obtained by calcining the dried titania at 500 °C for 3 h. The details of the preparation of rutile titania was described in our previous work [9]. The Pd=TiO2 (A), Pd=TiO2 (R), and Pd=Al2 O3 catalyst were prepared by impregnating TiO2 (A), TiO2 (R), and c  Al2 O3 with a known PdCl2 solution, respectively, the catalyst samples were dried at 120 °C overnight. The catalyst samples were oxidized and de-chlorinated by flowing air saturated by water (80 °C) at a rate of 80 ml(STP)/min into the catalysts in a quartz tube at 500 °C for 12 h. Powder X-ray diffraction analysis was performed with Ni-filtered CuKa radiation with a

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Shimadzu XD-3A X-ray diffractometer. The working voltage of 35 kV and the electronic current of 25 mA were employed. The EPR spectra were taken at 123 K with EMXEPR spectrometer (Bruker). The quartz reactor with valves was equipped with side arm EPR tube where the prereduced catalyst was transferred. The catalyst was reduced by H2 for 1 h at lower temperature (300 °C) or higher temperature (450 °C). After the reduction the valves were turn off, and the catalysts in quartz reactor was transferred to side arm EPR tube for EPR measurement. Then the catalyst in the quartz reactor was evacuated to remove H2 , and CO (P ¼ 200 Torr) (99.95%) deoxygenated by MnO=SiO2 was introduced into the quartz reactor to make catalyst absorbed CO for second EPR measurement in order to know the effect of CO on the stability of Ti3þ in catalyst. To evaluate the catalytic properties of the assynthesized Pd=TiO2 catalyst, we tested its catalytic activity for liquid selective hydrogenation of longer chain alkadienes ðC10 –C13 Þ, which is an important petrochemical industrial reaction for production of alkyl benzene. The details of the reaction conditions and method of product analysis were reported in our previous work [10]. Fig. 1 shows the XRD patterns of as-synthesized TiO2 . As can be seen from Fig. 1, the

Fig. 1. XRD patterns of the as-synthesized anatase and rutile TiO2 .

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as-synthesized titania has pure anatase or rutile crystalline structure, respectively. The EPR spectra of 0:075%Pd=TiO2 (R) and 0:075%Pd=TiO2 (A) catalyst pretreated under different conditions were shown in Figs. 2 and 3, respectively. The signals of g values less than 2 were assigned to Ti3þ ð3d1 Þ [11,12]. Both the 0:075%Pd=TiO2 (R) and 0:075%Pd=TiO2 (A) catalysts reduced at 200 °C all have strong Ti3þ EPR signal. The results show that Ti4þ can be reduced to Ti3þ in the presence of Pd even at lower temperature (200 °C), which is caused by the dissociatively chemsorbed hydrogen on palladium diffusing from Pd to TiO2 and reducing Ti4þ to Ti3þ [13,14]. The elevation of reduction temperature from 200 to 450 °C leads to widening and weakening rapidly decrease of Ti3þ EPR signal both for 0:075%Pd=TiO2 (R) and 0:075%Pd=TiO2 (A) catalyst, which is contributed to the formation of diamagnetically coupled Ti3þ pairs and the coupling of no-diamagnetic Ti3þ ions whose EPR signal is broadened with the increasing of the concentration of Ti3þ ions [15,16] under higher reduction temperature. For the rutile titania supported Pd catalyst (0:075%Pd=TiO2 (R)) reduced at lower temperature (200 °C), the signal at g? ¼ 1:969, gk ¼ 1:946

is assigned to assigned to surface Ti3þ ions which are not in contact with palladium [17]. The absorption of CO on this catalyst leads to the rapidly decreasing of intensity of Ti3þ signal. The disappearance of most Ti3þ ions is caused by the oxidation of Ti3þ to Ti4þ ions by O atoms produced at the metal interface during CO dissociation [18,19]. When pre-reduction temperature increases to 450 °C, A new signal is observed at g? ¼ 1:973, gk ¼ 1:934 which is assigned to Ti3þ ion located at the contact of Pd-surface [17]. The absorption of CO on this catalyst pre-reduced at higher temperature (450 °C) only leads to very little weakening of Ti3þ EPR signal, which shows that such Ti3þ ion located at the contact of Pd-surface is stable in the presence of CO, and could not be oxidized to Ti4þ by CO. In such case, we assume that there exists SMSI between Pd and Ti3þ , which could stabilize Ti3þ . For the anatase titania supported Pd catalyst (0:075%Pd=TiO2 (A)) reduced at lower temperature (200 °C), compared with 0:075%Pd=TiO2 (R), the profile of Ti3þ EPR signal of 0:075%Pd=TiO2 (A) is widened, and its g values are very different from the former, there are two shoulder peaks at g ¼ 1.969, g ¼ 1.952, which is obviously caused by overlapping of different Ti3þ EPR signals. This

Fig. 2. EPR spectra of Pd=TiO2 (R) catalysts pre-reduced at different temperature (in bracket) with or without CO absorbed on them: a-0:075%Pd=TiO2 (R) (200 °C), b-075%Pd/TiO2 (R)(200 °C)–CO, c-0:075%Pd=TiO2 (R)(450 °C), d-0:075%Pd= TiO2 (R)(450 °C)–CO.

Fig. 3. EPR spectra of Pd=TiO2 (A) catalysts pre-reduced at different temperature (in bracket) with or without CO absorbed on them: a-0:075%Pd=TiO2 (A) (200 °C), b-075%Pd=TiO2 (A) (200 °C)–CO, c-0:075%Pd=TiO2 (A) (450 °C), d-0:075%Pd= TiO2 (A) (450 °C)–CO.

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EPR signal at g  1:91 or so (including 1.918 and 1.906) was ascribed to vacancy-stabilized Ti3þ in the lattice sites or even to Ti7þ 2 , or similar center in the subsurface layer of TiO2 [20,21]. The absorption of CO on this catalyst leads to decreasing of the intensity of EPR signal at g ¼ 1:983, and the appearance of two peaks at g ¼ 1:969 and g ¼ 1:952. Based on the analysis of EPR signal for 0:075%Pd=TiO2 (R), we know that the Ti3þ ions which are not in contact with palladium are instable in the presence of CO as they are easily oxidized by CO to Ti4þ ions. Therefore the EPR signal at g ¼ 1:983 is reasonably ascribed to the Ti3þ ions which are not in contact with palladium, and the two peaks at g ¼ 1:969 and g ¼ 1:952 are assigned to the two kind of Ti3þ ions located at the contact of Pd-surface as they are stable in the presence of CO because of existence of SMSI between Ti3þ and Pd. In contrast to the instability of most Ti3þ in 0:075%Pd=TiO2 (R) catalyst prereduced at 200 °C, the stability of most Ti3þ in 0:075%Pd=TiO2 (A) catalyst reduced at 200 °C in the presence of CO shows that there is SMSI which could stabilize Ti3þ in anatase titania supported palladium catalyst even reduced by H2 at lower temperature, which has not observed so far by means of generally accepted SMSI criterion of suppression of H2 or CO chemsorption capacity. We assume that the Ti3þ ions produced by reduction of Ti4þ by the dissociatively chemsorbed hydrogen on palladium diffusing from Pd to TiO2 are fixed in the surface lattice of TiO2 , as rutile titania is more thermodynamically and structurally stable than anatase titania [12] so that the Ti3þ ions fixed in the surface lattice of anatase TiO2 is easier to diffuse to surface of palladium particle than one in the surface lattice of rutile TiO2 .

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After introduction of CO into reduced 0:075% Pd=TiO2 (A) catalyst reduced at 450 °C in the EPR tube the intensity of Ti3þ signal keep unchanged. The stability of Ti3þ in 0:075%Pd=TiO2 (A) catalyst reduced at 450 °C in the presence of CO suggests that reduction of anatase titania supported palladium catalyst at higher temperature gives rise to SMSI between Ti3þ and Pd which could stabilize Ti3þ . The reason why the pre-reduction of both anatase and rutile supported palladium catalyst at higher temperature results in SMSI between Ti3þ and Pd is attributed that the thermal diffusion of produced Ti3þ ion at higher temperature is much easier than at lower temperature so that it could overcome the binding of surface lattice of both anatase and rutile titania to move to the surface or surrounding of palladium particle, and give rise to SMSI between Ti3þ and Pd. The conversion and selectivity of as-synthesized catalysts were summarized in Table 1 (the data was an average value of the results after hydrogenation reached steady state). From Table 1, it can be seen that under lower pre-reduction temperature, the rutile titania supported palladium catalyst (0:075%Pd=TiO2 (R)) has similar catalytic properties to 0:075%Pd=Al2 O3 catalyst, especially, in selectivity, but the anatase titania supported pallidium catalyst (0:075%Pd=TiO2 (A)) has very different catalytic properties to the rutile titania supported palladium catalyst (0:075%Pd=TiO2 (R)). With the elevation of reduction temperature of catalyst from 200 to 450 °C, there is almost no change of catalytic properties for 0:075%Pd=Al2 O3 catalyst, but for titania (rutile or anatase) supported palladium catalysts, the elevation of reduction temperature gives rise to sharp change of

Table 1 Activity and selectivity of catalysts at 100 °C, 14 atm, 8:33 ml g1 –catalyst h1 and H2 =alkadienesðmolar ratioÞ ¼ 1:28 Catalyst

Temperature of reduction by H2 (°C)

Conversion of alkadienes (%)

Selectivity of alkenes (%)

Yield of alkenes (%)

0:075%Pd=Al2 O3 0:075%Pd=TiO2 ðRÞ 0:075%Pd=TiO2 ðRÞ 0:075%Pd=TiO2 ðAÞ 0:075%Pd=TiO2 ðAÞ

200 200 450 200 450

50.5 62.6 69.5 72.6 60.5

30.2 34.7 86.4 72.4 89.5

15.3 21.7 55.3 52.7 54.1

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catalytic properties, especially for selectivity of alkenes. The consideration of the surface and catalytic properties of titania supported noble metals is associated with strong metal support interaction (SMSI), which is one of the most interesting and most studied effects in catalysis. There have been many interpretations for the SMSI effect. The main interpretations are the morphological and the electronic effects. The morphological effects could reasonably explain our results: titania from catalyst is partly reduced and suboxide phase migrates onto the metal particle. Thus, the part of metal surface partially covered by TiOx is blocked [22,23], which results in the moderating of dissociative processes leading to the formation of carbonaceous deposits, ethylidine species or selfhydrogenation reaction that are claimed to be responsible for the decrease of selectivity [24,25], due to the limited availability of contiguous adsorption of Pd sites. The similar catalytic properties between 0:075%Pd=TiO2 (R) and 0:075%Pd=Al2 O3 catalyst and the much higher conversion and selectivity of 0:075%Pd=TiO2 (A) than 0:075%Pd= TiO2 (R) under the condition of lower pre-reduction temperature further confirms the presence of SMSI for 0:075%Pd=TiO2 (A) pre-reduced at lower temperature. The much higher selectivity of 0:075%Pd=TiO2 (A) and 0:075%Pd=TiO2 (R) under the condition of higher pre-reduction temperature than one under the condition of lower pre-reduction temperature further confirms the presence of SMSI both for 0:075%Pd=TiO2 (A) and 0:075%Pd=TiO2 (R) pre-reduced at higher temperature. In conclusion, In situ EPR investigation by using CO as probe molecules shows that even prereduced by H2 at lower temperature results in SMSI for anatase titania supported palladium catalyst, but not for rutile titania supported palladium catalyst, which is attributed that the Ti3þ ions produced by reduction of Ti4þ by the dissociatively chemsorbed hydrogen on palladium diffusing from Pd to TiO2 are fixed in the surface lattice of TiO2 , as rutile titania is more thermodynamically and structurally stable than anatase titania so that the Ti3þ ions fixed in the surface lattice of anatase TiO2 is easier to diffuse to surface of palladium particle

than one in the surface lattice of rutile TiO2 . The reason why the pre-reduction of both anatase and rutile supported palladium catalyst at higher temperature results in SMSI between Ti3þ and Pd is attributed that the thermal diffusion of produced Ti3þ ion at higher temperature is much easier than at lower temperature so that it could overcome the binding of surface lattice of both anatase and rutile titania to move to the surface or surrounding of palladium particle. The very different catalytic properties between 0:075%Pd=TiO2 (R) and 0:075%Pd=TiO2 (A) catalyst pre-reduced at lower temperature, and the rapid change of conversion and selectivity of 0:075%Pd=TiO2 (A) and 0:075% Pd=TiO2 (R) with the elevation of pre-reduction temperature further confirm the presence of SMSI both for anatase titania supported palladium catalyst pre-reduced at lower temperature, and titania (rutile and anatase) supported palladium catalyst pre-reduced at higher temperature. Acknowledgements The authors are grateful to Ministry of Science and Technology of China (Grant No.1999022400), Institute of Jinling Petrochemical Co. Ltd., and Jiangsu Province Hi-Tech Program (Grant No. BG 2002016) for their financial supports. References [1] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [2] J.C. Lavalley, J. Saussey, J. Lamotte, R. Breault, J.P. Hindermann, A. Kiennemann, J. Phys. Chem. 94 (1990) 5941. [3] J. Van de Loosdrecht, A.M. Van de Kraan, A.J. Dillen, J.W. Geus, J. Catal. 170 (1997) 217. [4] K.R. Krishna, A.T. Bell, J. Catal. 130 (1997) 597. [5] M. Englisch, A. Jentys, J.A. Lercher, J. Catal. 166 (1997) 25. [6] A. Dandekar, M.A. Vannice, J. Catal. 183 (1999) 344. [7] P. Reyes, M.C. Aguirre, I. Melian-Cabrera, M.L. Granados, J.L.G. Fierro, J. Catal. 208 (1) (2002) 229. [8] J.H. Kang, E.W. Shin, W.J. Kim, J.D. Park, S.H. Moon, J. Catal. 208 (2002) 310. [9] Y.Z. Li, Y. Fan, Y. Chen, J. Mater. Chem. 12 (5) (2002) 1387. [10] Y.Z. Li, Y. Fan, Y. Chen, Catal. Lett. 82 (1–2) (2002) 111.

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