2012
J. Phys. Chem. B 2000, 104, 2012-2018
Infrared Study of a Novel Acid-Base Site on ZrO2 by Adsorbed Probe Molecules. I. Pyridine, Carbon Dioxide, and Formic Acid Adsorption Feng Ouyang,* Akira Nakayama, Kenji Tabada, and Eiji Suzuki Research Institute of InnoVatiVe Technology for the Earth, 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0929, Japan ReceiVed: August 23, 1999; In Final Form: December 13, 1999
The adsorption of pyridine, carbon dioxide (CO2), and formic acid (HCOOH) on various types of surface sites of ZrO2, vacant sites (for example: bare surface Zr4+O2- ions), and those modified by terminal (t-) and bridged (b-) OH as well as t- and b-methoxy (OCH3) groups, has been studied by infrared (IR) spectroscopy. The selective reaction of CO2 with the t-OH group at 213 K and the adsorption of pyridine to replace the t-OH group on ZrO2 at 373-573 K confirms that t-HO-Zr site possesses acidic and basic properties. Moreover, the nucleophilic substitution adsorption of pyridine occurs more actively at the t-HO-Zr site than that at the vacant t-site on ZrO2. At 523 K, HCOOH reacts with basic t-OH or t-OCH3 species before the b-OH or b-OCH3 species. The selective reaction of HCOOH with t- and b-OCH3 groups on ZrO2 disappears at 573 K. The high activity of t-HO-Zr sites is attributed to relatively stronger Lewis acidic sites generated by releasing t-OH groups in the adsorption, and is also related to the high reactivity of t-OH groups. The Zr ions or t-OH groups at t-HO-Zr sites can play different roles in the adsorption of HCOOH, CO2, and pyridine, leading to its distinguishable features from those on vacant and b-OH sites. On the other hand, the OCH3 species at 573 K is supposed to be movable during the adsorption, resulting in the disappearance of the difference in reactivity of t- and b-OCH3 groups in the adsorption of HCOOH to replace OCH3 on ZrO2.
Introduction catalyst.1-5
The ZrO2 has been known to be a acid-base acidity and basicity of various kinds of zirconia catalysts have been widely investigated using several probe molecules, such as pyridine, ammonia, carbon monoxide, and carbon dioxide.1-9 The obtained acid-base properties are thought to depend greatly on the synthetic procedure and calcination temperature of ZrO2,1-4 giving rise to various types of acid-base sites, such as Lewis acidic-basic Zr4+O2- pair, and basic or Brønsted acidic OH groups on ZrO2. An early study by Tanabe and coworkers showed that Lewis acid-base sites were predominant on ZrO2 surface, while a small amount of OH groups on ZrO2 became Brønsted acidic, protonating pyridine above 473 K. They proposed that Lewis basic O2- ions play an important role for the activation of a proton of 1-butene in 1-butene isomerization reaction.1 Recently, Aramendia et al. prepared a ZrO2 catalyst with Brønsted acidic OH groups, and supposed that simultaneous interaction of an acid-base pair having adjacent acidic OH group with the proton of the carbon and OH group in propane-2-ol would cause both groups to be released, producing propene.4 Lavally et al. have found that terminal (t-) and bridged (b-) OH species exist on ZrO2, and t-OH group as a base reacts with CO2 above 373 K but b-OH groups do not.6 Previously,10 we observed t-OH species to be removed preferentially in CH3OH adsorption on ZrO2. On the other hand, the two types of OH species exhibit similar properties depending on the type of reaction. Similar activation energies (35 and 41 ( 4 kJ mol-1, respectively) have been obtained for H/D isotope exchange reactions of t- and b-OH species with gaseous D2, and the kinetic orders with respect to D2 pressure for both t- and b-OH species were found to be the * Corresponding author.
same (0.5).11 Although the results of the isotope exchange reactions of OH species with gaseous D2 have been explained from the mobility of dissociated D species,11 the special properties of t-OH species on ZrO2 have not been fully elucidated. The reactivity of OH groups is an important issue in catalysis and surface chemistry, and is related to structures of surface sites. Several types of OH species on Al2O3 have been investigated. The OH species at 3775 cm-1 on Al2O3 is the most typical example among these OH species, and shows special features.12 Kno¨zinger has found that pyridine was oxidized to pyridone ions on Al2O3, releasing H2 at above 623 K. Therefore, he suggested that the OH group adjacent to the pyridine coordinated to the Lewis site is responsible for the reaction.13 Morterra et al. considered that the Al-OH site is located in crystallographic defect on Al2O3 on the basis of carbon monoxide adsorption on the sites; moreover, the OH species can react with CO2.12 Nevertheless, as Morterra et al. noted,12 none of the models indicated a special accessibility of the 3775 cm-1 OH species for acidic and basic probe molecules. Therefore, to elucidate the role of this type of surface site, more detailed experimental investigation is needed. In this study, we examined the acid-base properties of tand b-OH groups as well as relevant sites in the adsorption of pyridine and CO2 by IR. We found that the replacement of t-OH groups by pyridine takes place on t-HO-Zr sites and that CO2 can also react with these t-OH groups. Thus, t-HO-Zr sites can act as an acid-base site under some reaction conditions. Moreover, the kinetic analysis for the dependence of coverage of OH species on the amount of the adsorbed species is used to identify the activity of relevant sites on ZrO2. Furthermore, we studied the coadsorption of HCOOH and CH3OH and correlated with the acid-base properties.
10.1021/jp992970i CCC: $19.00 © 2000 American Chemical Society Published on Web 02/11/2000
Infrared Study of a Novel Acid-Base Site on ZrO2
J. Phys. Chem. B, Vol. 104, No. 9, 2000 2013
Figure 1. IR spectra of ZrO2 that was pretreated with O2 at 803 K for 2 h and followed by evacuation for 20 min (a) at 773 K, and (b) 983 K.
Experimental Section The ZrO2 catalyst was prepared by precipitation from an aqueous zirconium oxynitrate solution with NH4OH and by calcination at 803 or 983 K in the air for 3 h.14 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, respectively. CH3OH (99.8%), HCOOH (98%), pyridine (99.5%), and CO2 (99%) were used as received. ZrO2 (ca. 50 mg) was pressed into a self-supporting disk of 20 mm in diameter. The ZrO2 disk was placed in an IR cell that was connected to a closed gas circulation system. 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 0.1 Torr of accuracy (1 Torr ) 133.32 Pa). The disk of ZrO2 was pretreated in O2 at 803 K for 2 h and followed by outgassing at 773 and 983 K (referred to as 773- and 983-ZrO2, respectively). The IR spectra of 773- and 983-ZrO2 after pretreatment are shown in Figure 1. The νOH bands at ca. 3760 and 3660 cm-1 have been assigned to t- and b-OH groups, respectively.6 Considerable amounts of two types of OH groups were observed on 773-ZrO2 (Figure 1a), while the 983-ZrO2 surface was partly dehydoxylated (75% of t-OH groups and 50% of b-OH groups were removed relative to these of 773-ZrO2, respectively). Thus, various types of surface groups or sites exist on 773- and 983-ZrO2, for example t- and b-OH groups or bare Zr4+O2- sites. To discuss the properties of these surface groups or ions conveniently, we refer to the sites that include the t- and b-OH groups as “t- and b-OH sites”, respectively, and the bare sites not covered by OH groups as “vacant sites”. Obviously, the vacant sites include bare Zr4+O2- pairs. The coverage of t- and b-OH is defined as θt-OH and θb-OH, respectively:
θt-OH (θb-OH) )
A A0
Figure 2. IR spectra of pyridine adsorbed on 773-ZrO2 at various temperatures: (a) 773-ZrO2 at 373 K before adsorption; (b) after 170 µmol of pyridine was introduced and followed by evacuation at 373 K; (c) after subsequent elevation of temperature (5 K/min) to 473 K; (d) 573 K; (e) the ratio spectrum of (d)/(a). The spectra b, c, and d at 1700-1400 cm-1 were obtained by subtracting a relevant spectrum of 773-ZrO2 before adsorption (referred as subtracted spectra).
the IR cell at 573 K and the integrated intensities of νCO bands due to t- and b-OCH3 species were recorded. The absorption coefficient was calculated by Beer’s law. We assume that the absorption coefficients of t- and b-OCH3 are independent of the amount of the adsorption (in calculating the amounts of pyridine and HCOOH, their absorption coefficients are also considered to be independent of the amount of relevant adsorption). Thus, the amounts of adsorbed OCH3 species could be calculated from the absorption coefficients and integrated intensities of corresponding νCO bands measured. The error was estimated to be less than 20%. Using an analogous method, the amount of adsorbed pyridine species was measured. The extinction coefficient was obtained from the adsorption of pyridine on dehydroxylated 983-ZrO2 at 473 K by Beer’s law. The taken quantitative band is the 19b vibration mode of pyridine and the absorption coefficient was also measured after the introduction of a low amount of pyridine. The amount of adsorbed pyridine species after evacuation can be calculated by Beer’s law, and the error was estimated to be less than 20%. The absorption coefficient of bidentate formate species was also measured on 773-ZrO2 at 473 K. The amount of adsorbed formate species under evacuation can be also determined by the absorption coefficient and the integrated intensities of νaOCO band due to adsorbed formate species.
(1)
where A is the integrated absorbance of t- or b-OH groups, and A0 corresponds to the integrated absorbance of t- or b-OH groups of 773-ZrO2. The amount of formed OCH3 species was measured by the volumetric method from CH3OH adsorption. To obtain the extinction coefficients of the desired absorption bands, 10 µmol of CH3OH (more than 90% was adsorbed) was introduced to
Results and Discussion Adsorption of Pyridine on 773-ZrO2. Pyridine (170 µmol) was exposed to 773-ZrO2 at 373 K, evacuated, and then subsequently heated to 573 K. IR spectra of pyridine adsorbed on 773-ZrO2 at various temperatures are shown Figure 2. The adsorption of pyridine at 373 K led to appearance of strong bands at 1603, 1573, 1486, and 1443 as well as a weak band at 1637 cm-1. Evacuation at elevating temperature to 473 K
2014 J. Phys. Chem. B, Vol. 104, No. 9, 2000 decreased the intensities of these bands; nevertheless, the peak positions hardly shifted from 373 to 473 K (Figure 2b,c). The bands further decreased in intensity after the increase of the temperature to 573 K except for the band at 1486 cm-1, which shifted to 1477 cm-1 and became broader; meanwhile, a weak band at 1540 cm-1 appeared. On the other hand, after pyridine adsorption at 373 K, the t-OH groups (3763 cm-1) disappeared and ca. 25% of b-OH groups (3665 cm-1) decreased (Figure 2b). The intensity of the t-OH band increased with the decrease of adsorbed pyridine by an increase in temperature from 473 to 573 K, and the intensity of b-OH band was also restored. The bands at 1603, 1573, 1486, and 1443 cm-1 have been assigned to 8a, 8b, 19a, and 19b vibration modes of pyridine coordinated to the Lewis acid site on ZrO2,1 respectively. This was basically consistent with the reports by Nagano et al 1 and Hertl2 that pyridine exhibited mainly its Lewis acid coordinated form on monoclinic zirconia above 373 K. However, the decrease of OH groups in the adsorption of pyridine, may occur as follows: (1) involvement in hydrogen-bond with pyridine, (2) protonating pyridine and (3) removal by production of H2O. When Lewis acid coordinated pyridine on the Al2O3 surface was heated to 623 K, its 8a vibration was observed to shift toward a high frequency, which was attributed to formation of pyridone. The hydrogen-bonded pyridine through its ring electron with an OH group was assumed to be the precursor of pyridone.13 In our study, when a weak band at 1540 cm-1 appeared at 573 K, the band at 1486 cm-1 was broad and shifted to 1477 cm-1 (a band at 1347 cm-1 was also present, which is not shown in Figure 2). However, shifting of 8a vibration of pyridine was not observed. Hence, these bands at 1540 and 1477 cm-1 were involved in a product from the decomposition of pyridine.16 In addition, we have not observed any obviously broadened hydrogen-bond band. Therefore, hydrogen-bonded pyridine is not considered to exist in our adsorption experiment. On the other hand, the pyridinium ion was characterized by the bands at 1632-1640 or 1530-1550 cm-1.1,9 However, since the band at 1637 cm-1 decreased in intensity with the decrease in the amount of adsorbed pyridine, we assign the band to the profiles of (1+6a) overtone due to pyridine12 and adsorbed H2O (δHOH).6 In our adsorption condition, the decrease of intensities of bands at 3600-3800 cm-1 were, hence, regarded as arising primarily from the removal of surface OH groups when pyridine is adsorbed, rather than protonated pyridine or hydrogen-bonded pyridine. Since the t-OH group is bounded to a single surface Zr ion and the removal of the OH group can give rise to a Lewis acidic site, the elimination of t-OH groups in the adsorption of pyridine indicates the substitution adsorption of pyridine for t-OH groups. The released t-OH groups subsequently yielded H2O (involving band 1637 cm-1). After pyridine was desorbed by increasing temperature to 473 K, the adsorbed H2O or decomposed pyridine can provide some hydrogen atoms for the regeneration of surface OH species. The amount of adsorbed pyridine at various temperatures in Figure 2 was determined, and then changes of θt-OH and θb-OH with increasing number of adsorbed pyridine molecules are shown in Figure 3. The curve of θt-OH was lowered sharply in a low amount of covered pyridine, whereas θb-OH hardly changed (Figure 3). When the amount of adsorbed pyridine was 1.5 molecules/nm2 at 373 K, t-OH groups diminished. The amount of adsorbed pyridine became 0.76 molecule/nm2, corresponding to the replacement of 80% of t-OH groups at 473 K. Apparently, the amount of desorbed pyridine of 0.74 molecule/nm2 with the increase in temperature form 373 to 473 K corresponds to the removal of the other 20% of the t-OH
Ouyang et al.
Figure 3. Changes of θt-OH and θb-OH as a function of the number of pyridine molecules adsorbed on 773-ZrO2 in Figure 2.
groups. This means that part of pyridine was indeed adsorbed on vacant sites at 373 K. Moreover, elevation of temperature from 373 to 473 K caused predominant desorption of pyridine coordinated to the vacant sites, but the main part of pyridine that had replaced 80% of the t-OH groups still remained. This shows that pyridine substituted for t-OH groups has higher thermal stability than these adsorbed on vacant sites, which may be due to the fact that the Zr ions connected to t-OH groups possess stronger Lewis acidity than bare Zr ions do. Adsorption of Pyridine on 983-ZrO2. Evacuation at 773 K gives rise to hydroxylated ZrO2 (Figure 1). The unsaturated Zr ion has been found to be yielded after dehydration of ZrO2 surface by evacuation at 823 K,17,18 and is evident by evacuation at 1003 K.19 Surface of ZrO2 has been dehydroxylated at 973 K (t-OH groups on 983-ZrO2 were estimated to be 25% of those on 773-ZrO2), consequently, the concentration of bare Zr ions increases largely on the surface. To compare clearly the relative adsorption activity between t-OH sites and vacant sites, we used 983-ZrO2 for study of the adsorption of pyridine. Figure 4 shows the spectra of 983-ZrO2 after different amounts of pyridine were introduced and followed by evacuation for 2 min at 473 K. Lewis-coordinated pyridine (1606, 1489, and 1444 cm-1) was observed in all spectra in Figure 4, and the amount of adsorbed pyridine increased with the amount of pyridine added. The changes in the number of t- and b-OH groups with the number of adsorbed pyridine molecule were examined. The adsorption of a small amount of pyridine (0.46 pyridine molecule/nm2) caused t-OH groups to diminish (Figure 4a), while b-OH (3665 cm-1) did not change, even when the amount of adsorbed pyridine was increased to 1.2 molecule/nm2 (Figure 4b) by the addition of a supplementary dose of pyridine. Obviously, at the first dosing pyridine replaced the t-OH groups, whereas at the second dosing pyridine was adsorbed on vacant t-sites because the t-OH groups were absent on the 983-ZrO2 surface after the first dosing. The preferential substitution of pyridine for t-OH groups although more vacant t-sites existed on the surface at 473 K, demonstrates the Lewis acidity of t-HO-Zr site, even though its acidic strength after t-OH group released is stronger than that of the bare Zr ion. The conclusion from the selective coordination of pyridine obtained at 473 K is consistent with the result obtained from 773-ZrO2.
Infrared Study of a Novel Acid-Base Site on ZrO2
J. Phys. Chem. B, Vol. 104, No. 9, 2000 2015
Figure 6. IR spectra of formate species on 773-ZrO2 after different amounts of formic acid were introduced successively: (a) 15 µmol; (b) 65 µmol and (c) 140 µmol and followed by evacuation at 573 K for 5 min (subtracted spectra). Figure 4. IR spectra of adsorbed pyridine species on 983-ZrO2 after various amounts of pyridine were introduced and followed by evacuation at 473 K: (a) background of 983-ZrO2 before adsorption; (b) 12 µmol of pyridine was dosed, and (c) 25.7 µmol was further introduced (showing subtracted spectra at 1700-1400 cm-1).
Figure 5. IR spectrum of CO2 adsorbed on 773-ZrO2 after 262 µmol of CO2 was introduced and followed by evacuation at 213 K (showing subtracted spectrum).
Adsorption of CO2 on 773-ZrO2 at 213 K. CO2 is usually used as a probe molecule to identify the basic sites of the catalyst surface. The behavior of adsorbed CO2 species on ZrO2 has been described previously.6,20 Bidentate carbonate20 and bicarbonate species,6 both coordinated to single surface Zr ion, were found on ZrO2 after CO2 adsorption at 373-573 K. After 773-ZrO2 was exposed to gaseous CO2 (262 µmol) at 213 K and then the cell was evacuated at the same temperature, the IR spectrum of the adsorbed species on 773-ZrO2 is shown Figure 5. The formation of bidentate carbonate species having bands at 1559 (νCdO), 1310 (νaCO), and 1065 (νsCO) cm-1 was observed after the adsorption of CO2.20 Bicarbonate species giving bands at 3607 (νOH), 1610 (νaCO), 1453 (νsCO), and 1224 (γOH) cm-1 were also present on ZrO2 (Figure 5).6 The formation
of bicarbonate species from the reaction of acidic CO2 with t-OH species having oxygen lone pairs6 resulted in t-OH groups diminishing (the spectrum of 773-ZrO2 has been subtracted from that of adsorbed CO2, giving reverse bands at 3767 cm-1). The low reaction temperature shows the high activity of t-OH groups in the reaction. The Adsorption of Formate Species on ZrO2. Figure 6 shows IR spectra of adsorbed species on ZrO2 after exposure of a different dosing amount of HCOOH followed by evacuation for 3 min at 573 K. The adsorption of a small amount of HCOOH yielded bidentate formate species coordinated to the single surface Zr ion (2874, 1568, 1385, and 1367 cm-1 in Figure 6a).11 As formate species increased, t-OH groups (reverse band at 3770 cm-1) were decreased before b-OH groups (3668 cm-1) decreased. When t-OH groups had disappeared completely, only ca. 10% of b-OH groups had decreased (Figure 6c, noting that t- and b-OH groups have approximate intensities on 773-ZrO2 in Figure 1). The change of t-OH coverage as the increase in the number of formate species is shown in Figure 7. T-OH groups disappeared rapidly at the first stage of the adsorption, suggesting that HCOOH replaced t-OH to form bidentate formate species and that dehydration occurred. When ca. 70% of t-OH species had disappeared, the number of formate species was 0.6 molecule/nm2. This shows that not only basic pyridine but also acidic adsorbates can react selectively on the sites. CO2 has been found to react selectively with t-OH and t-OCH3 species, which has been attributed to the fact that an interaction of CO2 with the oxygen lone pairs of t-OH or t-OCH3 groups is favored at the reaction.6 HCOOH was also introduced to OCH3-preadsorbed ZrO2 to examine if HCOOH reacted selectively with t-OCH3 species. Coadsorption of CH3OH and HCOOH on ZrO2. The OCH3-preadsorbed ZrO2 was prepared by the adsorption of methanol (175 µmol) on 773-ZrO2 at 523 K. IR spectrum of adsorbed species after evacuation at the same temperature is shown in Figure 8a. The formed t- and b-OCH3 species gave νCO bands at 1155 and 1051 cm-1 as well as νCH bands at 2924 and 2817 cm-1.11 On the surface, almost all of the t-OH species and ca. 70% of b-OH species were removed, respectively
2016 J. Phys. Chem. B, Vol. 104, No. 9, 2000
Figure 7. Change of θt-OH as function of the number of formate species on 773-ZrO2 in Figure 6.
(reverse band at 3765 and 3667 cm-1). A small amount of bidentate formate species (1568 cm-1) was formed by the oxidation of OCH3 species.11 HCOOH (70 µmol) was further introduced to the OCH3-preadsorbed ZrO2 at 523 K, and the spectrum recorded after evacuation is shown in Figure 8b. When bidentate formate species (1568, 1385, and 1367 cm-1) increased, 50% of t-OCH3 and 27% of b-OCH3 species were decreased. The preferential substitution of HCOOH for t-OCH3 groups as t-OH groups shows that both substitutions occur via a similar mechanism, in which oxygen lone pairs of t-OCH3 and t-OH species may be involved in the reaction with HCOOH. On the other hand, t- and b-OH groups did not show any obvious changes in Figure 8. The same coadsorption experiment was also conducted at 573 K. To show clearly OH species, we did not subtract background spectrum at 4000-2700 cm-1 in Figure 9. As shown in Figure 9a, t- and b-OH (3765 and 3665 cm-1) were present on 773ZrO2, respectively. The OCH3-preadsorbed surface was prepared by the adsorption of methanol on 773-ZrO2 at 573 K. The IR spectrum of the adsorbed species after evacuation is shown in
Ouyang et al. Figure 9b. On the surface t- and b-OCH3 species (1155 and 1051 cm-1) were present, and t-OH species diminished but ca. 20% of b-OH species were remained. After HCOOH was successively introduced to the OCH3-preadsorbed ZrO2 at 573 K, the cell was evacuated, and the IR spectra were recorded. A typical spectrum is shown in Figure 9c. As increasing bidentate formate species (1568 cm-1) formed from the adsorption of HCOOH, t- and b-OCH3 species decreased, whereas b-OH groups were regenerated (3665 cm-1 in Figure 9c). Finally, ca. 90% coverage of b-OH groups was restored on 773-ZrO2, while t-OH groups hardly increased. This means that bidentate formate species replaced t-OCH3 species principally in the process. The changes in the numbers of t- and b-OCH3 species are shown as the function of number of formate species in Figure 10. The increase of formed formate species resulted in the decrease of both types of OCH3 species in about equal molecule number. This can be interpreted by the migration of methoxy spices. The reaction of HCOOH on t-OCH3-preadsorbed sites resulted in preferential removal of t-OCH3 groups as shown in Figure 8. However, the decomposition or desorption of a part of unstable formate species at 573 K generated vacant t-sites, leading to the subsequent conversion of b-OCH3 species toward these vacant sites.15 Consequently, the activities of t- and b-OCH3 species cannot be distinguished. The existence of vacant t-sites on the surface is confirmed by comparing the amount of increased formate species with the amount of decreased t-OCH3 species in Figure 10. When 1.3 molecule per nm-1 of the t-OCH3 species was removed, the amount of increased formate species was estimated to be ca.0.7 molecule/nm2 (Figure 10), indicating that formate species did not take up completely the sites where t-OCH3 groups had been removed. Unsaturated cation-anion pairs on most of the dehydroxylated metal oxides have been known as active sites for heterolytic dissociation of alcohol21,22 and carboxylic acids.23,24 The occupancy of other stable molecules or groups on these sites, such as H2O or OH at Lewis acidic sites, may inhibit the dissociation of alcohol at these sites.25 Silica is regarded as a special example that has been well investigated.21,26 On silica, the dissociation of methanol has been proposed to be involved in an electron donor-acceptor interaction between CH3OH and HO-Si. On partly dehydroxylated Al2O3, acetic acid was
Figure 8. IR spectra of CH3OH and HCOOH coadsorbed on 773-ZrO2 at 523 K: (a) 175 µmol of CH3OH was dosed followed by evacuation; (b) subsequently 30 µmol of HCOOH was introduced followed by evacuation (subtracted spectra).
Infrared Study of a Novel Acid-Base Site on ZrO2
Figure 9. IR spectra of CH3OH and HCOOH coadsorbed on 773ZrO2 at 573 K: (a) 773-ZrO2 before adsorption (b) after 200 µmol of CH3OH was introduced followed by evacuation; (c) 150 µmol of HCOOH was introduced followed by evacuation (showing subtracted spectra at 1700-900 cm-1).
Figure 10. Dependence of numbers of OCH3 groups on the number of formate species adsorbed on 773-ZrO2 in Figure 9.
supposed to be adsorbed on bare Al ions at the first dosing. After the Al ions were filled, the acetic acid would react with OH species.23,24 The replacement of t-OH group by pyridine confirms that the Zr ion joined to the t-OH group acts as Lewis acidic site at the reaction temperature. Moreover, the acidic strength is relative stronger than vacant sites (Figures 2, 3, and 4). Thus, we believe the interaction of lone pairs of pyridine with the surface Zr ions is a predominant process in the reactions because the lone pairs are most reactive in the pyridine
J. Phys. Chem. B, Vol. 104, No. 9, 2000 2017 molecules. On the other hand, the reaction of t-OH with CO2 shows the nucleophilicity (Lewis basicity) of the sites. Lavally et. al have found that CO2 reacts selectively with t-OH or t-OCH3 groups but not with b-OH or b-OCH3 groups. An important character of the t-OH or t-OCH3 groups is its oxygen lone pairs. Therefore, they suppose that the first step of both reactions is likely an interaction of electrophilic CO2 and the lone pair of t-OH or t-OCH3 groups on ZrO2.6 In addition, as acidic alcohol (CF3)3COH,21 HCOOH exhibits predominant electrophilicity in its adsorption. Apparently, the proton in HCOOH is strongly electrophilic, and the dissociation of HCOOH is likely involved in an electrophilic interaction with oxygen lone pairs of t-OH groups. The selective reactions of t-OH groups with HCOOH and CO2 and replacement of t-OH groups by pyridine, regardless of the difference in acid-base properties of these adsorbates, demonstrate the acid-base properties of the sites, where t-OH group or Zr ion can play different roles depending on the electronic features of these adsorbates. As t-OH groups were spent, bare Zr4+O2- sites were responsible for subsequent dissociative adsorption of formic acid or the adsorption of pyridine at a high coverage of formate species and pyridine (Figures 3, 4, and 7). Several types of OH species have been found on alumina. A number of models of the coordinated OH groups have been proposed to explain the features of these OH species on alumina. Nevertheless, the early models do not explain the special activity of terminal OH species at 3775 cm-1.12 Morterra et al. suggested that the higher activity of the OH species reflects a higher accessibility for all types of surface probes and this should reflect the presence of the OH group in the particularly exposed zone of the surface. They attributed the activity to Al-OH groups present in the portion of the surface belonging to crystallographically defective configuration.12 For ZrO2, Morterra et al. detected different Lewis acidic sites by CO adsorption after the elimination of OH groups on the surface and the site formed by the elimination of the t-OH species shows higher CO adsorption heat. Thus, Zr ion at the site was suggested to be in highly unsaturated coordination.8 These findings are in agreement with our finding that Lewis acidic sites can be generated by removal of t-OH groups. We considered that the release of t-OH groups is enhanced by basic groups of adsorbates. Tanabe indicated that the bond in HO-M is ruptured easily to liberate OH- ion when M (a metal ion) electronegativity is low.27 The low-temperature (213 K) reaction of t-OH group with CO2 suggests that the t-HO-Zr bond is easily broken. When basic adsorbates attack the sites, the lone pairs of the basic adsorbates promote the rupture of t-HO-Zr bond. Moreover, the dissociation of t-OH group will lead to a strongly electrophilic state of Zr ions, which gives rise to a relatively stronger acidity (Figures 2-4). Thus, facile dissociation of t-OH groups is one of the origins of high activity of t-HO-Zr sites. The property of a surface under a reaction condition may differ from that under a nonreaction condition, and the formation of surface sites in a reaction should be taken into consideration. On the other hand, when HCOOH was used to replace the preadsorbed OCH3, the ratio of the numbers of t-OCH3 to b-OCH3 was different at 523 and 573 K. The selective reaction of bidentate formate species for t-OCH3 species was observed at 523 K, whereas the difference in reactivity of t- and b-OCH3 species with HCOOH disappeared at 573 K because of the migration of b-OCH3 to the vacant t-site.15 Thus, the distinguishable features among t-OH-Zr, vacant or b-OH sites in pyridine adsorption and HCOOH dissociation are originated from the different structure of the surface sites, whereas features
2018 J. Phys. Chem. B, Vol. 104, No. 9, 2000 of t- and b-OCH3 groups disappear because of the mobility of OCH3 species on surface. Two different factors control different reaction steps. Previously, we observed a similarity between tand b-OH species in H/D isotope exchange reaction on ZrO2 because migration of activated D atoms is a rate-determining step.11
Ouyang et al. Subscripts OH t-OH b-OH
total OH groups t-OH groups b-OH groups
References and Notes Conclusion The adsorption of pyridine and CO2 on t-HO-Zr sites showed that the sites have an acid-base property. Pyridine has been found to ligand to Lewis acidic Zr ion by replacing the t-OH group. Moreover, by comparison of the kinetic behavior of the adsorption of pyridine on t-HO-Zr sites with that on vacant sites we found the preferential replacement of t-OH groups by pyridine demonstrating that the adsorption of pyridine occurs more actively at t-HO-Zr sites than at vacant sites on ZrO2. The high activity at the t-HO-Zr site is attributed to the promotion of the release of highly reactive t-OH groups by pyridine forming stronger Lewis acidic sites. In addition, the basicity of t-OH groups is demonstrated though its selective reactions with HCOOH, and the selective reaction of t-OCH3 groups with HCOOH is also observed. These suggest that the reactions are involved in the interaction of the proton of HCOOH with lone electron pairs of t-OH and t-OCH3 groups. At 573 K, OCH3 groups are proposed to be movable on ZrO2, which leads to the disappearance of difference of t- and b-OCH3 species in reactivity.
Acknowledgment. The financial support from the New Energy and Industrial Technology Development Organization (NEDO, Japan) is gratefully acknowledged.
Abbreviations θ A A0
coverage integrated absorbance initial integrated absorbance of OH groups
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