Infrared Study of ZrO2 Surface Sites Using Adsorbed Probe Molecules

Nov 4, 2000 - The properties of two types of surface OH species, terminal (t) and bridged (b), as well as corresponding vacant sites on ZrO2 at 573 K ...
0 downloads 7 Views 72KB Size
Download PDF
J. Phys. Chem. B 2000, 104, 11253-11257

11253

Infrared Study of ZrO2 Surface Sites Using Adsorbed Probe Molecules. 2. Dimethyl Ether Adsorption Feng Ouyang* and Shuiliang Yao Research Institute of InnoVatiVe Technology for the Earth, 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0929, Japan ReceiVed: July 12, 2000; In Final Form: September 7, 2000

The properties of two types of surface OH species, terminal (t) and bridged (b), as well as corresponding vacant sites on ZrO2 at 573 K have been studied using dimethyl ether (DME) adsorption as an IR molecular probe. On the dehydroxylated ZrO2 surface, the molecularly adsorbed CH3OCH3 species was dissociated moderately. On the other hand, the molecularly adsorbed CH3OCH3 species was not observed on the fully hydroxylated ZrO2 surface because of the rapid dissociation of CH3OCH3. Kinetic analysis revealed that the activity of the dissociative adsorption of DME on surface sites is in the order t-OH sites > vacant sites > b-OH sites on ZrO2, which parallels that of the adsorption of pyridine, CO2, and HCOOH at the same sites, as reported previously. Furthermore, the surface t-18OH species has been found to provide active 18O atoms for the dissociation of CH3OCH3 to OCH3 and 18OCH3 species at 573 K, which shows that the 18OH species is involved in the dissociation of DME. We suppose that the nucleophilic adsorption of DME to replace t-OH species occurs at t-HO-Zr sites and that the release of t-OH species is favorable for the dissociation of DME. This is consistent with the results of the adsorption of pyridine, CO2, and HCOOH, demonstrating a characteristic of acidic and basic bifunctions of t-HO-Zr sites.

Introduction Terminal (t) and bridged (b) OH species have been found to be present on ZrO2,1 and corresponding t- and b-OCH3 species have been formed by CH3OH adsorption1 and during CO hydrogenation.2 We have been investigating the acidic and basic properties of t- and b-OH species and corresponding vacant sites using pyridine, CO2, HCOOH, and CH3OH as molecular probes. The preferential reaction of acidic CO2 with the t-OH group demonstrated that the t-OH group acts as a base, in which t-OH oxygen lone pairs are favorable in the electrophilic reaction. Furthermore, the basicity of t-OH species has been examined by the dissociative adsorption of electrophilic HCOOH on ZrO2 and OCH3 preadsorbed ZrO2 surfaces. The selective reactions of HCOOH with t-OH and t-OCH3 also suggested that the reaction occurred by an electrophilic reaction mechanism. On the other hand, we found that pyridine adsorption occurs more actively at t-HO-Zr sites than at vacant sites of the ZrO2 surface and suggested that the Zr ion covered by the t-OH group is a potential Lewis site and that pyridine is adsorbed on the site by replacement of the t-OH group.3 In this study, we further examined the reactivity of surface OH species in the dissociative adsorption using nucleophilic DME as a molecular probe. Using a method similar to that used in the previous study, we investigated the changes in the concentrations of two types of OH species with increasing coverage of OCH3 species by successive adsorption of DME on ZrO2. The sequential dehydroxylation in the adsorption is examined to demonstrate not only the activity of the two types of OH species on the hydroxylated ZrO2 surface but also the relative activity at bare Zr4+O2- sites on the dehydroxylated ZrO2. The reactions of * To whom correspondence should be addressed. Present address: National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki, Japan. Fax: +81-298-614487. E-mail: [email protected].

DME with surface 18OH have been used to elucidate the roles of hydroxyl species in the dissociation of DME. Several types of OH species have also been observed on other metal oxides. Those surface OH species were formed on dehydroxylated TiO24 or eliminated on hydroxylated TiO2,4 ThO2,5 and Al2O36 in the formation of OCH3 from CH3OH adsorption in some relative studies. To avoid the complexity resulting from the formation and elimination of surface OH species, DME has been used to form various types of OCH3 species on a dehydroxylated CeO2 surface in the study of OCH3 decomposition.7 In addition, the protonated DME species has been regarded as the precursor of DME dissociation on the acidic OH group surface of zeolite.8 However, a few reports have been made on weakly acidic metal oxides,9,10 and the roles of various types of surface OH species in DME dissociation have not been fully elucidated. Experimental Section The preparation of the ZrO2 catalyst and the system of in situ IR measurement have already been described.3 To obtain ZrO2 with different concentrations of OH groups, zirconium hydroxide was calcinated at 803 and 983 K in the air for 3 h. Brunauer-Emmett-Teller (BET) surface areas were measured to be 70 m2 g-1 for ZrO2 calcinated at 803 K and 58 m2 g-1 for ZrO2 calcinated at 983 K. The disk of ZrO2 in the IR cell was pretreated in oxygen at 803 K for 2 h, followed by outgassing at 773 and 983 K (referred to as 773- and 983-ZrO2). IR spectra were recorded on a Jasco 7300 FT-IR with an MCT detector at 4 cm-1 resolution with 64 scans. The pressure was measured using a Baratron meter 127A (MKS) with an accuracy of 0.1 Torr (1 Torr ) 133.32 Pa). CH3OH (99.8%), DME (99.9%), and H218O (97.4 atom % 18O) were used as received. Two types of t- and b-OH species have been found to decrease on ZrO2 with the formation of two types of corresponding OCH3

10.1021/jp002509m CCC: $19.00 © 2000 American Chemical Society Published on Web 11/04/2000

11254 J. Phys. Chem. B, Vol. 104, No. 47, 2000

Ouyang and Yao

Figure 2. Dependence of the number of OCH3 groups on the amount of CH3OH introduced on 773-ZrO2 at 573 K under the experimental conditions in Figure 1.

Figure 1. IR spectra of 773-ZrO2 exposed to CH3OH for 3 min followed by evacuation at 573 K: (a) background spectrum; introduction of (b) 20 µmol and (c) 200 µmol of CH3OH.

species by methanol.3 To compare the reactivity of these species, their coverage is needed. Because a relatively high pressure of DME is required to reach the saturation adsorption of OCH3 species, we used CH3OH to measure the amounts of the saturation adsorption. The IR spectra of 773-ZrO2 in the adsorption of methanol at 573 K are shown in Figure 1. The νOH bands at 3763 and 3665 cm-1 have been assigned to t- and b-OH groups, respectively.1 Considerable amounts of two types of OH groups were observed on 773-ZrO2 (Figure 1a). After the formation of t- and b-OCH3 groups from CH3OH adsorption, t- and b-OH groups decrease in amount. The coverage of tand b-OH groups is defined as θt-OH and θb-OH, respectively:

θt-OH or θb-OH ) A/A0

(1)

where A is the integrated absorbance of t- or b-OH and A0 corresponds to the integrated absorbance of t- or b-OH species in the spectrum of 773-ZrO2 (Figure 1a). The fraction of coverage of t-OCH3 groups (θtm) and total coverage of OCH3 groups (θm) are defined as follows:

Atm Atm + 1.7Abm

(2)

Atm + 1.7Abm Atm0 + 1.7Abm0

(3)

θtm ) θm )

where A is the integrated absorbance of νCO bands due to OCH3 species and the subscripts tm and bm represent t- and b-OCH3 species, respectively. Atm0 and Abm0 correspond to the values of Atm and Abm, respectively, at t- or b-OCH3 saturation on 773ZrO2. The ratio of relative absorption coefficients of νCO bands due to t-OH species to those due to b-OH species is known to be 1.7.11 The amount of OCH3 groups at saturation adsorption was measured on 773-ZrO2 at 573 K by the following procedure. The catalyst was exposed successively to CH3OH for 3 min to reach stable adsorption, then IR spectra were recorded after

evacuation. Two typical IR spectra are shown in Figure 1b,c; t- and b-OCH3 species νCO bands at 1155 and 1051 cm-1, respectively, and νCH bands at 2800-3000 cm-1 were present on ZrO2.1,12 The volumetric method for measuring the amount of OCH3 species formed from CH3OH adsorption was as described previously.3 The error is estimated to be 0.6 cannot be explained only by the heterogeneity. In general, more active species should react at the first stage, which is contradictory to the fact that the b-OH species were removed markedly at θm > 0.6. Because t-OH groups have disappeared at θm > 0.6, both t- and b-OCH3 groups were formed at this adsorption stage, possibly at the expense of b-OH groups. Thus, we supposed that t-OCH3 species were formed on vacant t-sites, whereas b-OCH3 groups were formed on b-OH sites. This is reasonable for the analysis of the number of remaining surface Zr ions. We have calculated that ca. 70% of surface Zr ions were covered at saturation adsorption. We may consider vacant b-sites to be present in a small amount after θm ) 0.6; hence, subsequent b-OCH3 groups must be formed at b-OH sites. The b-OH groups are suggested be less active in respect to the other sites in the DME adsorption, so they are involved in the reaction only for high values of θm. On the other hand, the preferential decrease of t-OH species at the first stage indicates that the formed t-OCH3 groups replaced t-OH groups because the dissociation of DME at Zr4+O2- sites does not result in surface dehydroxylation. Adsorption of DME on 983-ZrO2. Figure 5 shows the spectra of 983-ZrO2 after adsorption of CH3OCH3 (6 µmol) for 3 min and subsequent evacuation at 573 K. Spectra b and c were recorded after evacuation for 1 and 15 min (the ZrO2 background spectrum was subtracted), and spectrum d is a ratio of (c)/(b). Both t- and b-OCH3 groups (νCO bands at 1142 and 1032 cm-1 and νCH bands at 2831 and 2933 cm-1) are present in Figure 5b. The t-OH groups disappeared with the formation of OCH3 groups at first (noting that the intensity of the reverse band at 3763 cm-1 was the same as that in the ZrO2 background spectrum in Figure 5a), whereas b-OH groups (3673 cm-1) increased slightly, possibly because some H atoms removed from t-OH groups converted to bridged surface O2- ions. On the other hand, bands at 875 and 2957 cm-1 were also observed in Figure 5b. The molecularly adsorbed CH3OCH3 as an electron donor can be held on surface Lewis sites; consequently, the νOCO vibration shifted downward with the increase in strength of the Lewis sites.7 On 983-ZrO2, ca. 75 and 55% of t- and b-OH species were removed (see Figure 1 in ref 3) and yielded Lewis sites. In comparison with gas-phase IR data of CH3OCH3 and CH3OCH3 molecularly adsorbed on CeO2,7 the bands at 2957 and 875 cm-1 are assigned to νCH and νOCO vibrations of molecularly adsorbed CH3OCH3 species. The downshift (50 cm-1) indicated that the CH3OCH3 species was coordinated to a Lewis acid site. In Figure 5c, the intensity of the band at 1142 cm-1 increased with the decrease of the band at 875 cm-1, whereas the bands at 3763 cm-1 did not vary in the process because the active t-OH groups must be used after the dissociative adsorption of CH3OCH3 (Figure 5). This clearly shows that the molecularly adsorbed CH3OCH3 species was dissociated slowly to t-OCH3 species without involvement of t-OH species. The slow dissociation was probably ascribed to the lack of active surface O atoms on the dehydroxylated ZrO2, forming some “strong acidic and weak basic sites”, in which CH3OCH3 was not polarized easily and dissociated. The dependence of θt-OH, θb-OH, and θtm on θm has also been examined in the adsorption of DME, and the results are shown in Figure 6. At θm < 0.2, t-OH was almost completely removed. At θm ) 0.2-0.4, θtm was constant (ca. 0.58) and θt-OH and θb-OH were hardly altered, indicating the formation of OCH3

11256 J. Phys. Chem. B, Vol. 104, No. 47, 2000

Ouyang and Yao

Figure 5. IR spectra of 983-ZrO2 at 573 K: (a) background spectrum; the introduction of DME (6 µmol) and outgassing for (b) 1 min and (c) 15 min; (d) the ratio of (c)/(b). Spectra b and c show subtracted spectra.

Figure 6. Changes of θt-OH, θb-OH, and θtm as functions of θm in adsorption of DME on 983-ZrO2 under the same experimental conditions as those in Figure 5.

Figure 7. IR spectra of 983-ZrO2 hydroxylated by H218O: (a) background and (b) exposed to 29 µmol of CH3OCH3 followed by evacuation at 573 K (subtracted spectrum).

species at vacant t- and b-sites. Above θm ) 0.4, the b-OH groups decreased with the increase in the amount of t- and b-OCH3 groups. Thus, the b-OCH3 could be formed at the b-OH site and the t-OCH3 species was produced at vacant t-sites. These results clearly showed that the dissociation of CH3OCH3 at t-OH sites occurred more easily than on vacant sites, for example, bare Zr4+O2- sites. To examine further the involvement of OH species in the dissociation of DME, adsorption of DME was conducted on hydroxylated 983-ZrO2 after H218O pretreatment. Adsorption of DME on 983-ZrO2 Hydroxylated by H218O. Lavalley and co-workers have found that surface OH species on ZrO2 are exchanged with H218O at 573 K.1 For complete exchange, the following procedure was employed. 983-ZrO2 was exposed to H218O (29 µmol) for 30 min, followed by evacuation at the same temperature; the pretreatment was repeated three times. IR spectra of the ZrO2 exposed to 41 µmol of CH3OCH3 followed by outgassing at 573 K are shown in Figure 7 (the background spectra are given in the region of 4000-3400 cm-1 in Figure 7a). As shown in Figure 7a, hydroxyl bands after H218O treatment were observed to shift toward low frequencies at 3718 and 3643 cm-1, indicating that t- and b-OH groups were exchanged to t- and b-18OH groups; moreover, hydroxyl species also increased largely.11 After adsorption of CH3OCH3, t-16OCH3 (νt-C16O at 1140 cm-1), t-18OCH3 (νt-C18O at 1107 cm-1), b-16OCH3 (νb-C16O at 1040 cm-1), and b-18OCH3 (νb-C18O at 1012 cm-1) were also formed. Under the present conditions, the

oxygen isotope exchange reaction of formed methoxy species with surface ones is not carried out, because we have confirmed that 18OCH3 species migrate on ZrO2 at 523-573 K rather than methyl species.15 The oxygen atom in methoxy species must come from DME and surface oxygen obtained during DME dissociation. The decreases in the amounts of t- and b-18OH were estimated to be ca. 25 and 7%, respectively. This confirmed that CH3OCH3 could react with the surface 18OH through the following reaction:

CH3OCH3 + 18OH- f t(or b)-OCH3- + t(or b)-18OCH3- + H+ where H+ perhaps reacted with surface O2- to form OH- and finally was removed by the elimination of H218O.9 Here, more t-18OH groups should be involved in the reaction, because they decreased much more than b-18OH groups. In addition, the molecularly adsorbed CH3OCH3 species was not observed, showing rapid dissociation of CH3OCH3 on the surface. This means that surface hydroxyl groups are favorable to the dissociation. On the basis of the above results, the dissociative adsorption of DME on hydroxylated and dehydroxylated ZrO2 may occur via at least three pathways: (i) reaction at surface t-OH sites; (ii) dissociation at vacant sites, such as bare Zr4+O2- ions; (iii)

Infrared Study of ZrO2 Surface Sites a reaction involving removal of surface b-OH. Moreover, the activity of surface sites was different in the three reactions. As shown in Figures 4 and 6, the activity for the dissociation of DME is in the order t-OH sites > vacant sites > b-OH sites. Like the dissociation of alcohol on metal oxides, DME dissociation is interpreted by a mechanism of surface acidbase reaction.7-10 Alkyl ether having oxygen lone pairs is first coordinated to the Lewis site and then dissociated to alkoxy species on the Lewis acid-base pair site (Mx+O2-) of metal oxides, such as CeO27 and Al2O3.10 On completely dehydroxylated ZrO2 by evacuation at 1003 K, the dissociation of molecularly adsorbed DME to OCH3 species at 573 K is supposed to occur on Zr4+O2- ions.16 On the other hand, Graham and co-workers have found that propyl ether was dissociated to isopropoxide species, leading to a decrease in surface OH species on hydroxylated TiO2. They suggested that propyl ether first coordinated to Lewis acid sites and then formed isopropoxide species, replacing surface OH species.9 In Figures 4 and 6, different types of surface reactions occurred at various types of surface sites at different adsorption stages. Because t-18OH groups have been confirmed to be involved in the dissociation of CH3OCH3 molecules (Figure 7), the rapid elimination of t-OH groups at the first adsorption stage can be ascribed to the direct reaction of CH3OCH3 with t-OH groups followed by dehydration. Lewis acidic sites connected to t-OH groups or adjacent to t-OH groups are required to realize the reaction. We found that pyridine can replace t-OH groups on ZrO2 and confirmed that surface Zr4+ ions covered by t-OH groups are potential Lewis sites. Therefore, we also supposed that the first step in CH3OCH3 dissociation is substitution adsorption on t-HO-Zr sites because CH3OCH3 with oxygen lone pairs acts as an electronic donor to show nucleophilic attraction like that of pyridine with nitrogen lone pairs. Furthermore, CH3OCH3 was dissociated more easily on the fully hydroxylated ZrO2, whereas molecularly adsorbed CH3OCH3 was dissociated moderately on dehydroxylated 983-ZrO2 having more bare Zr4+ ions and less active t-OH species (Figures 5 and 7), indicating that the presence of bare Zr4+ ions is not a prerequisite for the dissociation of CH3OCH3. Hence, we suggest that the nucleophilic substitution reaction of CH3OCH3 on t-HO-Zr sites causes the release of the t-OH groups. The released t-OH groups are more active than stable b-OH species and even more active than bare Lewis acid-base pairs, and they promote the dissociation of CH3OCH3. This viewpoint has been presented in our previous report.3 Because pyridine is adsorbed on t-HO-Zr sites to replace the t-OH groups before adsorption on vacant sites and because the adsorbed pyridine species has a relatively higher heat stability than that in coordination to some vacant sites, we have supposed that the surface Zr ion that releases a t-OH group possesses stronger Lewis acidity than that at a vacant site. Hence, we can consider the released t-OH group to be Lewis basic. Tanabe has also proposed a similar Lewis acid-base model when OH species depart from the catalyst surface.17 On the other hand, Nagao and Morimoto reported that the occupation of Zn2+O2- sites by stable OH groups inhibited the dissociative adsorption of CH3OH at room temperature.18 In the present study, b-OH sites of ZrO2 had lower activity for the dissociative adsorption of the DME molecule than bare Zr4+O2- sites (Figure 7). The difference in activity is probably related to the structures of the t- and b-OH species.3 The detailed mechanism of the CH3OCH3 dissociation on b-OH sites is not clear yet. In addition, the ratio of the number of t-OCH3 species to that of b-OCH3 species in CH3OCH3 adsorption is independent of

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11257 the amount of OH species in the same process (Figures 4 and 6). Some of the OCH3 species can be proposed to move to b-sites after dissociation at highly active t-OH sites. This is similar to the case of the conversion of OCH3 species from bto t-sites on 773-ZrO2 at 573 K when the vacant t-sites were produced from the decomposition of coadsorbed formate species. The conversion has been supposed as the migration of OCH3 groups.11 Indeed, the migration can result in a stable distribution value of the ratio of the number of t-OCH3 groups to that of b-OCH3 groups, which is not dependent on the original adsorption state or the content of surface OH groups. For example, adsorbed CH3OCH3 can obviously also dissociate to t-OCH3 groups; then, the ratio becomes similar to that in the absence of the molecularly adsorbed species (Figures 5). Thus, t- and b-OCH3 species are distributed in a different step from the dissociation. Conclusion A quantitative examination of the change of t- and b-OH with an increase in OCH3 coverage showed that t-OH sites are more active than bare Zr4+O2- sites for the dissociation of DME and b-OH sites are the least active. Hydroxyl species providing active oxygen atoms in the reaction of DME with isotope-labeled 18OH groups confirmed that t-OH groups are involved in the dissociation of DME. We suggest that DME is dissociated through a nucleophilic substitution reaction at t-HO-Zr sites. Abbreviations θ A A0 m t-OH b-OH m tm bm

coverage integrated absorbance initial integrated absorbance of OH species total OH groups t-OH groups b-OH groups total OCH3 groups t-OCH3 groups b-OCH3 groups

References and Notes (1) Bensitel, M.; Moravek, V.; Lamotte, J.; Sauer, O.; Lavalley, J.-C. Spectrochim. Acta 1987, 43A, 1487. (2) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Domen, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1 1988, 84, 511. (3) Ouyang, F.; Nakayama, A.; Tabada, K.; Suzuki E. J. Phys. Chem. B 2000, 104, 2012. (4) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2655. (5) Montagne, X.; Lynch, J.; Freund, E.; Lamotte, J.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1417. (6) Busca, G.; Rossi, P. F.; Lorenzelli, V.; Banaissa, M.; Lavalley, J.C. J. Phys. Chem. 1985, 89, 5433. (7) Binet, C.; Lavalley, J.-C. J. Phys. Chem. B 1997, 101, 1484. (8) Forester, T. R.; Howe, R. F. J. Am. Chem. Soc. 1987, 93, 5076. (9) Graham, J.; Rudham, R.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 895. (10) Arai, H.; Saiti, Y.; Yoneda, Y. J. Catal. 1968, 10, 128. (11) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. J. Phys. Chem. B 1997, 101, 4867. (12) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. J. Chem. Soc., Faraday Trans. 1997, 93, 169. (13) Adam, J.; Rogers, M. D. Acta Crystallogr. 1959, 12, 951. (14) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. J. Chem. Soc., Faraday Trans. 1996, 92, 4491. (15) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. Catal. Lett. 1998, 50, 179. (16) Kondo, J. Doctoral Thesis, Tokyo Institute of Technology, Tokyo, Japan, 1991; p. 64. (17) Tanabe, K. Catalysis-Science and Technology; Springer-Verlag: Berlin, 1981; Vol. 2, Chapter 5, p 271. (18) Nagao, M.; Morimoto, T. J. Phys. Chem. 1980, 84, 2054.