SiO2 catalysts: reduction and regeneration of RhVO4

Applied Catalysis A: General 236 (2002) 113–120

CO hydrogenation over RhVO4 /SiO2 , Rh/V2 O3 and Rh/SiO2 catalysts: reduction and regeneration of RhVO4 Shin-ichi Ito, Chikashi Chibana, Ken Nagashima, Satoshi Kameoka, Keiichi Tomishige, Kimio Kunimori∗ Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received 26 January 2002; received in revised form 27 April 2002; accepted 8 May 2002

Abstract The CO hydrogenation over a RhVO4 catalyst on an SiO2 support (RhVO4 /SiO2 ) has been investigated after H2 reduction at 300 ◦ C, and the results are compared with those of Rh/V2 O3 and unpromoted Rh/SiO2 catalysts. The RhVO4 compound was formed on the SiO2 support by calcination in air at 700 ◦ C with the atomic ratio of V/Rh = 1. The RhVO4 was decomposed to Rh metal particles (3.0 nm) and V2 O3 particles (highly dispersed) by the H2 reduction, where a strong metal–oxide (Rh–vanadia) interaction (SMOI) was induced. The RhVO4 /SiO2 catalyst after the H2 reduction exhibited higher activity in CO hydrogenation, compared with the other Rh catalysts. The selectivity to C2 oxygenates of the RhVO4 /SiO2 and Rh/V2 O3 catalysts was higher than that of the unpromoted Rh/SiO2 catalyst, although the selectivity decreased with increasing CO conversion. The RhVO4 was also formed on the SiO2 surface by calcination in air even at 500 ◦ C, although the XRD study suggested that the formation was less perfect. After H2 reduction at 300 ◦ C, the catalytic activity of the catalyst calcined at 700 ◦ C was higher than that of the one calcined at 500 ◦ C, indicating a stronger Rh–vanadia interaction in the former catalyst. The RhVO4 can be regenerated by calcination in O2 at 700 ◦ C after an intentional sintering treatment (at 700 ◦ C in He) of the RhVO4 /SiO2 catalyst, and hence, the activity and the selectivity to C2 oxygenates were restored. © 2002 Elsevier Science B.V. All rights reserved. Keywords: CO hydrogenation; RhVO4 ; C2 oxygenates; SMSI; SMOI; Sintering; Regeneration

1. Introduction There has been much interest in the strong metal-support interaction (SMSI) not only for metal (Rh, Pt, Pd, etc.) catalysts supported on SMSI oxide (TiO2 , Nb2 O5 , V2 O3 , MnO) but also for metal/nonSMSI oxide (SiO2 ) catalysts promoted with SMSI oxides [1–4]. V2 O5 -, Nb2 O5 - or MnO-promoted Rh catalysts have been reported to have a high activity ∗ Corresponding author. Tel.: +81-298-53-5026; fax: +81-298-55-7440. E-mail address: [email protected] (K. Kunimori).

and selectivity in CO hydrogenation for the production of C2 oxygenates such as ethanol and acetic acid [5–13]. For instance, in both vanadia-supported and vanadia-promoted systems, reports suggest that the Rh particles are partially covered by vanadium oxide after H2 reduction (so-called SMSI [1,2,14–18]) and that the role of the vanadium oxide promoter is to enhance the CO dissociation, which leads to a high activity, and/or the CO insertion into the metal-carbon bond, which leads to the formation of C2 oxygenates [7–9]. We have found a phenomenon of calcinationinduced metal-promoter interaction: a mixed oxide

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 2 8 3 - 1

114

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

such as RhVO4 , RhNbO4 and Rh2 MnO4 can be formed on an SiO2 support by mutual interaction between Rh and the oxide (vanadia, etc.) during calcination in air or O2 at high temperature (700–900 ◦ C) [10–13,19–24]. RhVO4 is decomposed to highly dispersed Rh metal particles covered with vanadium oxide (VOx ) by H2 reduction above 200 ◦ C, and a strong metal–oxide (Rh–VOx ) interaction (SMOI) is induced on SiO2 support [10–12,20,22]. One of the characteristic features in this system is that regeneration of RhVO4 is possible by the calcination treatment. Therefore, redispersion of Rh metal can be achieved by the calcination and reduction treatments of a spent (coked and/or sintered) Rh catalyst [11,12]. The RhVO4 /SiO2 (JRC-SIO-7) catalyst after H2 reduction showed higher activity in CO hydrogenation than vanadia-promoted Rh catalysts [10,11]. For the formation of RhVO4 on the SiO2 support (JRC-SIO-7; BET surface area: 81 m2 /g), however, a higher-temperature calcination (
800 ◦ C) with an excess amount of vanadium (V/Rh
2) was needed [10,11]. Beutel et al. [25] reported the formation of RhVO4 on an SiO2 support (BET surface area: 340 m2 /g) with the atomic ratio of V/Rh = 4.2 by a higher-temperature calcination (900 ◦ C). It is suggested that the stronger the vanadia–SiO2 interaction is, the greater excess amount of V2 O5 is needed for the formation of RhVO4 , because V2 O5 on an SiO2 support with higher surface area may be less mobile during the calcination because of the stronger interaction [11]. V2 O5 on an SiO2 support with lower surface area may be more mobile to form RhVO4 because of a weaker vanadia–SiO2 interaction. In this work, we have reinvestigated the formation and regeneration of RhVO4 on an SiO2 support which has a lower BET surface area. The results of the activity and selectivity to C2 oxygenates in CO hydrogenation are compared with those of vanadia-supported Rh (Rh/V2 O3 ) and unpromoted Rh/SiO2 catalysts.

2. Experimental An SiO2 support (Fuji Silysia Chemical Ltd., Q-100) had been precalcined in air at 900 ◦ C (BET surface area: 30 m2 /g) to avoid structural change during the following high-temperature calcination. It was

first impregnated with an aqueous solution of RhCl3 (Rh: 4 wt.%), then dried in air at 110 ◦ C overnight. RhVO4 /SiO2 catalyst was prepared by impregnating this sample with an aqueous solution of NH4 VO3 (the atomic ratio of V/Rh = 1), followed by calcination in air at 500 or 700 ◦ C for 3 h. The calcination temperature (◦ C) was denoted in square brackets, for example, RhVO4 /SiO2 [700]. V2 O3 support was prepared by H2 reduction of V2 O5 (Wako Pure Chemical Industries Ltd.) at 600 ◦ C for 7 h. Rh/V2 O3 catalyst was prepared by impregnating the V2 O3 with an aqueous solution of RhCl3 (Rh: 4 wt.%), then drying in air at 110 ◦ C overnight. For a comparison, unpromoted 4 wt.% Rh/SiO2 (Q-100) catalyst was also prepared by the same impregnation method, followed by calcination in air at 500 ◦ C. All catalysts were treated in a stream of H2 (30 cm3 /min) at 300 ◦ C for 1 h before CO hydrogenation. To investigate regeneration of the catalyst, we first treated RhVO4 /SiO2 [700] catalyst in a stream of He (30 cm3 /min) at 700 ◦ C for 1 h, and then we recalcined it in a stream of O2 (30 cm3 /min) at 700 ◦ C for 1 h. CO hydrogenation over the Rh catalysts after H2 reduction at 300 ◦ C was carried out in a flow reactor system at atmospheric pressure using a 1:3 mixture of CO and H2 (3.0 cm3 /g cat. min). Analysis of the products was performed by an on-line gas chromatograph system equipped with a TCD detector (Shimadzu, GC-8A) using a Porapak Q column (2 m) in He carrier gas (30 cm3 /min) [10–13]. X-ray diffraction (XRD) measurements were carried out on an X-ray diffractometer (Rigaku) equipped with a graphite monochromator for Cu K␣ (40 kV, 20 mA) radiation. The mean Rh particle size (metal dispersion) was calculated from the XRD line broadening measurement using the Scherrer equation [20,23,24,26]. The H2 and CO chemisorption measurements were carried out on a conventional volumetric adsorption apparatus; detailed procedures were described elsewhere [24,27]. The amounts of the total H2 chemisorption (H/Rh) and the irreversible CO chemisorption (CO/Rh) were measured at room temperature after O2 treatment at 500 ◦ C, followed by H2 reduction at 300 ◦ C. Transmission electron microscope (TEM, JEOL 100CX) measurements were carried out for calcined and reduced RhVO4 /SiO2 catalysts. The mean Rh particle size was calculated by using the volume–area mean diameter [27].

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

3. Results and discussion Fig. 1 shows XRD patterns of the catalysts after air calcination, reduction and intentional sintering treatment (in a stream of He at 700 ◦ C) of the Rh catalysts. As shown in Fig. 1(a), RhVO4 was formed exclusively on the SiO2 support (Q-100) after air calcination at 700 ◦ C (RhVO4 /SiO2 [700]). RhVO4 was also formed on the SiO2 support by air calcination even at 500 ◦ C with the atomic ratio of V/Rh = 1 (Fig. 1(b)), although weaker XRD peaks suggest that the formation was less perfect. It is suggested that after the calcination at 500 ◦ C some amounts of vanadium oxide are present separately from Rh oxide particles, which may lead to weaker Rh–vanadia interaction. As mentioned in Section 1, higher-temperature calcination with an excess amount of vanadium was needed for the formation of RhVO4 on SiO2 , which has a higher surface area [11,25]. Since, RhVO4 is formed easily on the SiO2 support (Q-100), the mobility of V2 O5 during the air calcination may be

Fig. 1. XRD patterns of RhVO4 (Rh: 4 wt.%, V/Rh = 1) on SiO2 support. (a) RhVO4 /SiO2 [700]: after air calcination at 700 ◦ C for 3 h; (b) RhVO4 /SiO2 [500]: after air calcination at 500 ◦ C for 3 h; (c) RhVO4 /SiO2 [700]: after air calcination at 700 ◦ C for 3 h followed by H2 reduction at 300 ◦ C for 1 h; (d) after (c) followed by He treatment at 700 ◦ C for 1 h.

115

higher on the SiO2 surface, which has lower surface area. Inumaru et al. [28] have reported that vanadium oxides are present as isolated vanadium oxide species (VOx ), which has stronger interaction with SiO2 (i.e. less mobile), with lower V2 O5 loadings, and in higher loading V2 O5 crystallites are present on VOx /SiO2 surface. In the same loading the V2 O5 concentration is higher on the SiO2 surface which has lower surface area. Therefore, the mobility of V2 O5 would be increased with decreasing the surface area and/or with increasing the V2 O5 loading. RhVO4 was reduced to Rh metal by H2 reduction at 300 ◦ C (Fig. 1(c)). Formation of vanadium oxide was not detected by the XRD measurement after the H2 reduction. However, V2 O3 was observed after the H2 reduction, followed by He treatment at 700 ◦ C (Fig. 1(d)). These results suggest that highly dispersed V2 O3 particles are formed by the decomposition of RhVO4 in H2 and those particles were sintered by the He treatment at 700 ◦ C. The particle sizes from XRD and the H2 or CO chemisorption values of RhVO4 /SiO2 [700], as well as those of the other catalysts, are summarized in Table 1. In the RhVO4 /SiO2 system, the large RhVO4 particles (23.5 or 10.2 nm) split into smaller Rh particles (3.7 or 2.8 nm) after the H2 reduction. The mean particle sizes of RhVO4 [700] and Rh from TEM were 33.3 and 3.0 nm, respectively, which are in fair agreement with those from XRD. The Rh metal was highly dispersed after the reduction of RhVO4 , which is in good agreement with the previous results using different SiO2 supports [10–12,20]. As shown in Table 1, the H/Rh and CO/Rh values were substantially lower than the metal dispersion (D) from XRD, suggesting that the Rh metal was partly covered with V2 O3 (SMOI). For the Rh/V2 O3 catalyst, no peak of Rh particles was observed in the XRD measurement, indicating that the Rh metal is highly dispersed. However, the H2 and CO chemisorption was severely suppressed (a classical SMSI catalyst). Examples of the TEM micrographs of RhVO4 /SiO2 [700] are shown in Fig. 2. After the calcination, RhVO4 particles are seen (Fig. 2(a)). After the H2 reduction, a lot of highly dispersed Rh particles, which are uniform in size (3.0 nm), are concentrated like “fish eggs” (Fig. 2(b)). This morphology may be typical for a system of noble metal and SMSI oxide such as vanadium oxide. For instance, Tesche et al. [29] and Foger [30] reported that Rh–V2 O3 and Ir–TiO2

116

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

Fig. 2. Transmission electron micrograph of the RhVO4 /SiO2 [700] catalyst. (a) After calcination in air at 700 ◦ C; (b) after calcination in air at 700 ◦ C followed by H2 reduction at 300 ◦ C.

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

Fig. 3. A model for regeneration of RhVO4 and redispersion of Rh on SiO2 support.

117

118

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

Table 1 The results of the characterization of the Rh catalysts Catalysta

RhVO4 /SiO2 [700] RhVO4 /SiO2 [500] Rh/V2 O3 Rh/SiO2 [500]

Particle size (nm)b

Dd

RhVO4

Rhc

23.5 10.2 – –

3.7 2.8 n.d. 11.8

0.30 0.39 1.00 0.09

H2 or CO chemisorptione H/Rh

CO/Rh

0.101 0.282 0.035f 0.050

0.016 0.075 0.043f 0.024

a

Calcination temperature in brackets. Calculated from XRD measurement using the Scherrer equation. c After H reduction at 300 ◦ C. 2 d Metal dispersion based on the Rh particle size from the XRD measurement. e Calculated from H or CO uptake at room temperature after O treatment at 500 ◦ C, followed by H treatment at 300 ◦ C. 2 2 2 f After H treatment at 300 ◦ C (without O treatment). 2 2 b

systems both exhibit similar morphology (fish eggs). This unique morphology may be due to the interaction between Rh and V2 O3 after the decomposition of RhVO4 by the H2 reduction. From these results, the structure change of the RhVO4 /SiO2 catalyst during the H2 treatment can be described in a model shown in Fig. 3. Fig. 4 shows the activity of CO hydrogenation over the Rh catalysts after H2 reduction at 300 ◦ C. The activity of the RhVO4 /SiO2 [700] was much higher than those of the other Rh catalysts. The order of the activity of CO hydrogenation was as follows: RhVO4 /SiO2

Fig. 4. The activity of CO hydrogenation over the RhVO4 /SiO2 , Rh/V2 O3 and unpromoted Rh/SiO2 catalysts after H2 reduction at 300 ◦ C. (䊏): RhVO4 /SiO2 [700]; (䉱): RhVO4 /SiO2 [500]; (䊉): Rh/V2 O3 ; (䊊): Rh/SiO2 [500].

[700] > RhVO4 /SiO2 [500] > Rh/V2 O3  Rh/SiO2 [500]. As shown in Table 2, the turnover frequency (TOF, based on CO/Rh) of the RhVO4 /SiO2 [700] catalyst was also much higher than those for the other Rh catalysts. It should be noted that the CO chemisorption of the RhVO4 /SiO2 [700] was more severely suppressed (Table 1). Similar behaviors have been reported in niobia- and vanadia-promoted Rh catalyst systems after high-temperature calcination followed by H2 reduction [13,19,29,31]. In spite of the very small value (CO/Rh = 0.016), the CO conversion was higher than those of the other Rh catalysts. A strong Rh–vanadia interaction (SMOI) after the decomposition of RhVO4 by H2 reduction may facilitate the enhancement of the CO dissociation, which leads to the higher activity. The activity of the RhVO4 /SiO2 [500] catalyst was lower than that of the RhVO4 /SiO2 [700] catalyst, although it was higher than those of the Rh/V2 O3 and the unpromoted Rh/SiO2 catalysts. Because the formation of RhVO4 was less perfect, the interaction between Rh and vanadia was not so strong as that in the RhVO4 /SiO2 [700] catalyst. The products were CO2 , CH4 , C2 + hydrocarbons, CH3 OH and C2 oxygenates (C2 H5 OH, CH3 COOH, CH3 CHO and HOCH2 CH2 OH). The order of the selectivity to C2 oxygenates was as follows: Rh/V2 O3 > RhVO4 /SiO2 [500] > RhVO4 /SiO2 [700] > Rh/SiO2 (see Table 2). However, it should be noted that the selectivity depends strongly on the CO conversion [10]. Fig. 5 shows the relationship between the CO conversion and the selectivity to C2 oxygenates. The

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

119

Table 2 The results of CO hydrogenation over the Rh catalysts at 180 ◦ C after H2 reduction at 300 ◦ C Catalysta Rh/SiO2 [500]b

RhVO4 /SiO2 [700]

RhVO4 /SiO2 [500]

Rh/V2 O3

CO conversion (%) TOF(×10−3 s−1 )c

32.9 29.5

17.4 3.3

11.4 3.8

1.3 0.8

Selectivity (%)d CH4 CO2 C 2 +e CH3 OH C2 oxygenatesf

19.3 25.3 30.6 7.1 17.7

36.7 17.5 16.7 3.8 25.3

49.2 1.9 14.5 3.4 31.0

12.3 11.9 40.8 24.2 10.8

Yield (%) C2 oxygenatesf

5.8

4.4

3.5

0.1

a

(◦ C)

The number in brackets represents the calcination temperature before the H2 reduction. The reaction temperature was 200 ◦ C for the unpromoted Rh/SiO2 [500] catalyst. c The turnover frequency based on the CO/Rh values in Table 1. d Expressed as carbon efficiency (%). e Amounts of ethane, ethylene and C +. 3 f Amounts of ethanol, acetic acid, acetaldehyde and ethylene glycol. b

selectivity to C2 oxygenates decreased as the CO conversion increased in the cases of the RhVO4 /SiO2 and Rh/V2 O3 catalysts. At the same conversion level, the selectivity of the RhVO4 /SiO2 catalysts was similar to that of the Rh/V2 O3 catalyst. The selectivity of the unpromoted Rh/SiO2 catalyst was significantly lower in spite of the low conversion. Kowalski et al. [7] and van der Lee et al. [8] have reported that Rh–VOx interaction enhances CO insertion, which leads to the formation of C2 oxygenates. In this work, the

Fig. 5. The selectivity to C2 oxygenates vs. CO conversion. (䊏): RhVO4 /SiO2 [700]; (䉱): RhVO4 /SiO2 [500]; (䊉): Rh/V2 O3 ; (䊊): Rh/SiO2 [500].

selectivity to C2 oxygenates of the RhVO4 /SiO2 catalysts was much higher than that of the unpromoted Rh/SiO2 catalyst. These results suggest that strong metal–oxide interaction (SMOI) enhances not only CO dissociation but also CO insertion. Therefore, the yield of C2 oxygenates of the RhVO4 /SiO2 [700] and the RhVO4 /SiO2 [500] catalysts was much higher than that of the unpromoted Rh/SiO2 catalyst. In order to demonstrate the catalytic performance of the RhVO4 /SiO2 system, an experiment of catalyst regeneration was carried out, as shown in Table 3. The activity of the RhVO4 /SiO2 [700] catalyst decreased drastically after an intentional heat treatment in a stream of He at 700 ◦ C. The selectivity to C2 oxygenates also decreased to 1.1% after the treatment. The mean particle size of the Rh was 13.4 nm from XRD and 16.0 nm from TEM. These results suggest that the severe sintering of the Rh particles and the V2 O3 particles resulted in the deactivation of CO hydrogenation. However, the activity of the RhVO4 /SiO2 [700] catalyst was almost recovered after the retreatment at 700 ◦ C in a stream of O2 followed by H2 reduction at 300 ◦ C (Table 3). The selectivity to C2 oxygenates was also increased to the initial level of the fresh RhVO4 /SiO2 [700] catalyst. Therefore, regeneration of RhVO4 on the SiO2 support was possible after the retreatment (at 700 ◦ C in

120

S.-i. Ito et al. / Applied Catalysis A: General 236 (2002) 113–120

Table 3 The changes in the turnover frequency (TOF) of CO hydrogenation over the RhVO4 /SiO2 [700] catalyst at 180 ◦ C after the sequential treatments Treatments

300 ◦ C

H2 reduction at He treatment at 700 ◦ C after H2 reduction at 300 ◦ C H2 reduction at 300 ◦ C after recalcination at 700 ◦ C a

TOF (×10−3 s−1 )a

Rh particle size (nm) From TEM

From XRD

From TEM

From XRD

3.0 16.0 3.3

3.7 13.4 4.7

1.3 0.4 1.1

1.6 0.3 1.6

Based on the Rh particle size from TEM or XRD measurements.

O2 ), and redispersion of Rh particles was achieved after H2 reduction at 300 ◦ C (see Fig. 3). 4. Conclusions RhVO4 was formed on the SiO2 support which has low BET surface area even at low-temperature calcination (500–700 ◦ C) with the atomic ratio of V/Rh = 1. The RhVO4 on the SiO2 support was decomposed to highly dispersed Rh particles and V2 O3 particles after the H2 reduction at 300 ◦ C, and strong metal–oxide (Rh–V2 O3 ) interaction (SMOI) was induced. The SMOI enhanced not only CO dissociation (CO conversion) but also CO insertion (selectivity to C2 oxygenates) in CO hydrogenation. Moreover, RhVO4 was regenerated by retreatment in O2 at 700 ◦ C even after the severe sintering of Rh by the heat treatment in He at 700 ◦ C. Therefore, redispersion of the Rh particles was achieved after H2 reduction. Acknowledgements We are grateful to Fuji Silysia Chemical Ltd. for providing the SiO2 support (Q-100). References [1] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1) (1978) 170. [2] D.E. Resasco, G.L. Haller, J. Catal. 82 (1983) 279. [3] K. Kunimori, Y. Doi, K. Ito, T. Uchijima, J. Chem. Soc., Chem. Commun. (1986) 965. [4] E.I. Ko, R. Bafrali, N.T. Nuhfer, N.J. Wagner, J. Catal. 95 (1985) 260. [5] T. Beutel, O.S. Alekseev, Yu.A. Ryndin, V.A. Likholobov, H. Knözinger, J. Catal. 169 (1997) 132.

[6] B.J. Kip, P.A.T. Smeets, J. van Grondelle, R. Prins, Appl. Catal. 33 (1987) 181. [7] J. Kowalski, G. van der Lee, V. Ponec, Appl. Catal. 19 (1985) 423. [8] G. van der Lee, A.G.T.M. Bastein, J. van den Boogert, B. Schuller, H.-Y. Luo, V. Ponec, J. Chem. Soc., Faraday Trans. 1 83 (1987) 2103. [9] W.H.M. Sachtler, M. Ichikawa, J. Phys. Chem. 90 (1986) 4752. [10] S. Ito, S. Ishiguro, K. Nagashima, K. Kunimori, Catal. Lett. 55 (1998) 197. [11] S. Ito, S. Ishiguro, K. Kunimori, Catal. Today 44 (1998) 145. [12] S. Ishiguro, S. Ito, K. Kunimori, Catal. Today 45 (1998) 197. [13] S. Ito, T. Fujimori, K. Nagashima, K. Yuzaki, K. Kunimori, Catal. Today 57 (2000) 247. [14] S.J. Tauster, S.C. Fung, J. Catal. 55 (1978) 29. [15] G. van der Lee, B. Schuller, H. Post, T.L.F. Favre, V. Ponec, J. Catal. 98 (1986) 522. [16] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173. [17] T. Beutel, V. Siborov, B. Tesche, H. Knözinger, J. Catal. 167 (1997) 379. [18] F.B. Noronha, C.A. Perez, M. Schmal, R. Fréty, Phys. Chem. Chem. Phys. 1 (1999) 2861. [19] Z. Hu, H. Nakamura, K. Kunimori, T. Uchijima, Catal. Lett. 1 (1988) 271. [20] Z. Hu, T. Wakasugi, A. Maeda, K. Kunimori, T. Uchijima, J. Catal. 127 (1991) 276. [21] K. Kunimori, Z. Hu, T. Uchijima, K. Asakura, Y. Iwasawa, M. Soma, Catal. Today 8 (1990) 85. [22] K. Kunimori, K. Yuzaki, T. Yarimizu, M. Seino, S. Ito, Stud. Surf. Sci. Catal. 105 (1997) 2083. [23] Z. Hu, H. Nakamura, K. Kunimori, H. Asano, T. Uchijima, J. Catal. 112 (1988) 478. [24] Z. Hu, H. Nakamura, K. Kunimori, Y. Yokoyama, H. Asano, M. Soma, T. Uchijima, J. Catal. 119 (1989) 33. [25] T. Beutel, H. Knözinger, A.V. Siborov, V.I. Zaikovskii, J. Chem. Soc., Faraday Trans. 88 (18) (1992) 2775. [26] T.A. Dorling, R.L. Moss, J. Catal. 7 (1967) 378. [27] K. Kunimori, T. Uchijima, M. Yamada, H. Matsumoto, T. Hattori, Y. Murakami, Appl. Catal. 4 (1982) 67. [28] K. Inumaru, M. Misono, T. Okuhara, Appl. Catal. 149 (1997) 133. [29] B. Tesche, T. Beutel, H. Knözinger, J. Catal. 149 (1994) 100. [30] K. Foger, J. Catal. 78 (1982) 406. [31] K. Kunimori, H. Nakamura, Z. Hu, T. Uchijima, Appl. Catal. 53 (1989) L11.