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 SuzukiResearch 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 surfacesites of ZrO2, vacant sites (for example: bare surface Zr4+O2- ions), and those modified by terminal (t-) andbridged (b-) OH as well ast- andb-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 thet-OH group on ZrO2 at 373-573 K confirms thatt-HO-Zr site possesses acidic and basic properties. Moreover,the nucleophilic substitution adsorption of pyridine occurs more actively at thet-HO-Zr site than that at thevacantt-site on ZrO2. At 523 K, HCOOH reacts with basict-OH or t-OCH3 species before theb-OH orb-OCH3 species. The selective reaction of HCOOH witht- andb-OCH3 groups on ZrO2 disappears at 573 K.The high activity oft-HO-Zr sites is attributed to relatively stronger Lewis acidic sites generated by releasingt-OH groups in the adsorption, and is also related to the high reactivity oft-OH groups. The Zr ions ort-OHgroups att-HO-Zr sites can play different roles in the adsorption of HCOOH, CO2, and pyridine, leading toits distinguishable features from those on vacant andb-OH sites. On the other hand, the OCH3 species at 573K is supposed to be movable during the adsorption, resulting in the disappearance of the difference in reactivityof t- andb-OCH3 groups in the adsorption of HCOOH to replace OCH3 on ZrO2.

Introduction

ZrO2 has been known to be a acid-base catalyst.1-5 Theacidity and basicity of various kinds of zirconia catalysts havebeen widely investigated using several probe molecules, suchas pyridine, ammonia, carbon monoxide, and carbon dioxide.1-9

The obtained acid-base properties are thought to depend greatlyon the synthetic procedure and calcination temperature ofZrO2,1-4 giving rise to various types of acid-base sites, suchas Lewis acidic-basic Zr4+O2- pair, and basic or Brønstedacidic OH groups on ZrO2. An early study by Tanabe and co-workers showed that Lewis acid-base sites were predominanton 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 importantrole for the activation of a proton of 1-butene in 1-buteneisomerization reaction.1 Recently, Aramendia et al. prepared aZrO2 catalyst with Brønsted acidic OH groups, and supposedthat simultaneous interaction of an acid-base pair havingadjacent acidic OH group with the proton of the carbon andOH group in propane-2-ol would cause both groups to bereleased, producing propene.4 Lavally et al. have found thatterminal (t-) and bridged (b-) OH species exist on ZrO2, andt-OH group as a base reacts with CO2 above 373 K butb-OHgroups do not.6 Previously,10 we observedt-OH species to beremoved preferentially in CH3OH adsorption on ZrO2. On theother hand, the two types of OH species exhibit similarproperties depending on the type of reaction. Similar activationenergies (35 and 41( 4 kJ mol-1, respectively) have beenobtained for H/D isotope exchange reactions oft- and b-OHspecies with gaseous D2, and the kinetic orders with respect toD2 pressure for botht- andb-OH species were found to be the

same (0.5).11 Although the results of the isotope exchangereactions of OH species with gaseous D2 have been explainedfrom the mobility of dissociated D species,11 the specialproperties of t-OH species on ZrO2 have not been fullyelucidated.

The reactivity of OH groups is an important issue in catalysisand surface chemistry, and is related to structures of surfacesites. Several types of OH species on Al2O3 have beeninvestigated. The OH species at 3775 cm-1 on Al2O3 is the mosttypical example among these OH species, and shows specialfeatures.12 Knozinger has found that pyridine was oxidized topyridone ions on Al2O3, releasing H2 at above 623 K. Therefore,he suggested that the OH group adjacent to the pyridinecoordinated to the Lewis site is responsible for the reaction.13

Morterra et al. considered that the Al-OH site is located incrystallographic defect on Al2O3 on the basis of carbonmonoxide adsorption on the sites; moreover, the OH speciescan react with CO2.12 Nevertheless, as Morterra et al. noted,12

none of the models indicated a special accessibility of the 3775cm-1 OH species for acidic and basic probe molecules.Therefore, to elucidate the role of this type of surface site, moredetailed experimental investigation is needed.

In this study, we examined the acid-base properties oft-andb-OH groups as well as relevant sites in the adsorption ofpyridine and CO2 by IR. We found that the replacement oft-OHgroups by pyridine takes place ont-HO-Zr sites and that CO2can also react with theset-OH groups. Thus,t-HO-Zr sitescan act as an acid-base site under some reaction conditions.Moreover, the kinetic analysis for the dependence of coverageof OH species on the amount of the adsorbed species is used toidentify the activity of relevant sites on ZrO2. Furthermore, westudied the coadsorption of HCOOH and CH3OH and correlatedwith the acid-base properties.* Corresponding author.

2012 J. Phys. Chem. B2000,104,2012-2018

10.1021/jp992970i CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 02/11/2000

Experimental Section

The ZrO2 catalyst was prepared by precipitation from anaqueous zirconium oxynitrate solution with NH4OH and bycalcination at 803 or 983 K in the air for 3 h.14 BET surfaceareas were measured to be 70 m2 g-1 for ZrO2 calcinated at803 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 cellthat was connected to a closed gas circulation system. IR spectrawere recorded on a Jasco 7300 FT-IR with an MCT detector at4 cm-1 resolution with 64 scans. The pressure was measuredusing a Baratron meter-127A (MKS) with 0.1 Torr of accuracy(1 Torr ) 133.32 Pa). The disk of ZrO2 was pretreated in O2 at803 K for 2 h and followed by outgassing at 773 and 983 K(referred to as 773- and 983-ZrO2, respectively). The IR spectraof 773- and 983-ZrO2 after pretreatment are shown in Figure1. TheνOH bands at ca. 3760 and 3660 cm-1 have been assignedto t- andb-OH groups, respectively.6 Considerable amounts oftwo types of OH groups were observed on 773-ZrO2 (Figure1a), while the 983-ZrO2 surface was partly dehydoxylated (75%of t-OH groups and 50% ofb-OH groups were removed relativeto these of 773-ZrO2, respectively). Thus, various types ofsurface groups or sites exist on 773- and 983-ZrO2, for examplet- and b-OH groups or bare Zr4+O2- sites. To discuss theproperties of these surface groups or ions conveniently, we referto the sites that include thet- andb-OH groups as “t- andb-OHsites”, respectively, and the bare sites not covered by OH groupsas “vacant sites”. Obviously, the vacant sites include bareZr4+O2- pairs. The coverage oft- andb-OH is defined asθt-OH

andθb-OH, respectively:

whereA is the integrated absorbance oft- or b-OH groups, andA0 corresponds to the integrated absorbance oft- or b-OH groupsof 773-ZrO2.

The amount of formed OCH3 species was measured by thevolumetric method from CH3OH adsorption. To obtain theextinction coefficients of the desired absorption bands, 10µmolof CH3OH (more than 90% was adsorbed) was introduced to

the IR cell at 573 K and the integrated intensities ofνCO bandsdue to t- and b-OCH3 species were recorded. The absorptioncoefficient was calculated by Beer’s law. We assume that theabsorption coefficients oft- and b-OCH3 are independent ofthe amount of the adsorption (in calculating the amounts ofpyridine and HCOOH, their absorption coefficients are alsoconsidered to be independent of the amount of relevantadsorption). Thus, the amounts of adsorbed OCH3 species couldbe calculated from the absorption coefficients and integratedintensities of correspondingνCO bands measured. The error wasestimated to be less than 20%. Using an analogous method, theamount of adsorbed pyridine species was measured. Theextinction coefficient was obtained from the adsorption ofpyridine on dehydroxylated 983-ZrO2 at 473 K by Beer’s law.The taken quantitative band is the 19b vibration mode ofpyridine and the absorption coefficient was also measured afterthe introduction of a low amount of pyridine. The amount ofadsorbed pyridine species after evacuation can be calculatedby Beer’s law, and the error was estimated to be less than 20%.The absorption coefficient of bidentate formate species was alsomeasured on 773-ZrO2 at 473 K. The amount of adsorbedformate species under evacuation can be also determined bythe absorption coefficient and the integrated intensities ofνaOCO

band due to adsorbed formate species.

Results and Discussion

Adsorption of Pyridine on 773-ZrO2. Pyridine (170µmol)was exposed to 773-ZrO2 at 373 K, evacuated, and thensubsequently heated to 573 K. IR spectra of pyridine adsorbedon 773-ZrO2 at various temperatures are shown Figure 2. Theadsorption of pyridine at 373 K led to appearance of strongbands at 1603, 1573, 1486, and 1443 as well as a weak band at1637 cm-1. Evacuation at elevating temperature to 473 K

Figure 1. IR spectra of ZrO2 that was pretreated with O2 at 803 K for2 h and followed by evacuation for 20 min (a) at 773 K, and (b) 983K.

θt-OH (θb-OH) ) AA0

(1)

Figure 2. IR spectra of pyridine adsorbed on 773-ZrO2 at varioustemperatures: (a) 773-ZrO2 at 373 K before adsorption; (b) after 170µmol of pyridine was introduced and followed by evacuation at 373K; (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 at1700-1400 cm-1 were obtained by subtracting a relevant spectrum of773-ZrO2 before adsorption (referred as subtracted spectra).

Infrared Study of a Novel Acid-Base Site on ZrO2 J. Phys. Chem. B, Vol. 104, No. 9, 20002013

decreased the intensities of these bands; nevertheless, the peakpositions hardly shifted from 373 to 473 K (Figure 2b,c). Thebands further decreased in intensity after the increase of thetemperature to 573 K except for the band at 1486 cm-1, whichshifted to 1477 cm-1 and became broader; meanwhile, a weakband at 1540 cm-1 appeared. On the other hand, after pyridineadsorption at 373 K, thet-OH groups (3763 cm-1) disappearedand ca. 25% ofb-OH groups (3665 cm-1) decreased (Figure2b). The intensity of thet-OH band increased with the decreaseof adsorbed pyridine by an increase in temperature from 473to 573 K, and the intensity ofb-OH band was also restored.

The bands at 1603, 1573, 1486, and 1443 cm-1 have beenassigned to 8a, 8b, 19a, and 19b vibration modes of pyridinecoordinated to the Lewis acid site on ZrO2,1 respectively. Thiswas basically consistent with the reports by Nagano et al1 andHertl2 that pyridine exhibited mainly its Lewis acid coordinatedform on monoclinic zirconia above 373 K. However, thedecrease of OH groups in the adsorption of pyridine, may occuras 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 surfacewas heated to 623 K, its 8a vibration was observed to shifttoward a high frequency, which was attributed to formation ofpyridone. The hydrogen-bonded pyridine through its ringelectron with an OH group was assumed to be the precursor ofpyridone.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 shiftedto 1477 cm-1 (a band at 1347 cm-1 was also present, which isnot shown in Figure 2). However, shifting of 8a vibration ofpyridine was not observed. Hence, these bands at 1540 and 1477cm-1 were involved in a product from the decomposition ofpyridine.16 In addition, we have not observed any obviouslybroadened hydrogen-bond band. Therefore, hydrogen-bondedpyridine is not considered to exist in our adsorption experiment.On the other hand, the pyridinium ion was characterized by thebands at 1632-1640 or 1530-1550 cm-1.1,9 However, sincethe band at 1637 cm-1 decreased in intensity with the decreasein the amount of adsorbed pyridine, we assign the band to theprofiles of (1+6a) overtone due to pyridine12 and adsorbed H2O(δHOH).6 In our adsorption condition, the decrease of intensitiesof bands at 3600-3800 cm-1 were, hence, regarded as arisingprimarily from the removal of surface OH groups when pyridineis adsorbed, rather than protonated pyridine or hydrogen-bondedpyridine. Since thet-OH group is bounded to a single surfaceZr ion and the removal of the OH group can give rise to a Lewisacidic site, the elimination oft-OH groups in the adsorption ofpyridine indicates the substitution adsorption of pyridine fort-OH groups. The releasedt-OH groups subsequently yieldedH2O (involving band 1637 cm-1). After pyridine was desorbedby increasing temperature to 473 K, the adsorbed H2O ordecomposed pyridine can provide some hydrogen atoms for theregeneration of surface OH species.

The amount of adsorbed pyridine at various temperatures inFigure 2 was determined, and then changes ofθt-OH andθb-OH

with increasing number of adsorbed pyridine molecules areshown in Figure 3. The curve ofθt-OH was lowered sharply ina low amount of covered pyridine, whereasθb-OH hardlychanged (Figure 3). When the amount of adsorbed pyridine was1.5 molecules/nm2 at 373 K, t-OH groups diminished. Theamount of adsorbed pyridine became 0.76 molecule/nm2,corresponding to the replacement of 80% oft-OH groups at473 K. Apparently, the amount of desorbed pyridine of 0.74molecule/nm2 with the increase in temperature form 373 to 473K corresponds to the removal of the other 20% of thet-OH

groups. This means that part of pyridine was indeed adsorbedon vacant sites at 373 K. Moreover, elevation of temperaturefrom 373 to 473 K caused predominant desorption of pyridinecoordinated to the vacant sites, but the main part of pyridinethat had replaced 80% of thet-OH groups still remained. Thisshows that pyridine substituted fort-OH groups has higherthermal stability than these adsorbed on vacant sites, which maybe due to the fact that the Zr ions connected tot-OH groupspossess stronger Lewis acidity than bare Zr ions do.

Adsorption of Pyridine on 983-ZrO2. Evacuation at 773 Kgives rise to hydroxylated ZrO2 (Figure 1). The unsaturated Zrion has been found to be yielded after dehydration of ZrO2

surface by evacuation at 823 K,17,18and is evident by evacuationat 1003 K.19 Surface of ZrO2 has been dehydroxylated at 973K (t-OH groups on 983-ZrO2 were estimated to be 25% of thoseon 773-ZrO2), consequently, the concentration of bare Zr ionsincreases largely on the surface. To compare clearly the relativeadsorption activity betweent-OH sites and vacant sites, we used983-ZrO2 for study of the adsorption of pyridine. Figure 4 showsthe spectra of 983-ZrO2 after different amounts of pyridine wereintroduced and followed by evacuation for 2 min at 473 K.Lewis-coordinated pyridine (1606, 1489, and 1444 cm-1) wasobserved in all spectra in Figure 4, and the amount of adsorbedpyridine increased with the amount of pyridine added. Thechanges in the number oft- andb-OH groups with the numberof adsorbed pyridine molecule were examined. The adsorptionof a small amount of pyridine (0.46 pyridine molecule/nm2)causedt-OH groups to diminish (Figure 4a), whileb-OH (3665cm-1) did not change, even when the amount of adsorbedpyridine was increased to 1.2 molecule/nm2 (Figure 4b) by theaddition of a supplementary dose of pyridine. Obviously, at thefirst dosing pyridine replaced thet-OH groups, whereas at thesecond dosing pyridine was adsorbed on vacantt-sites becausethe t-OH groups were absent on the 983-ZrO2 surface after thefirst dosing. The preferential substitution of pyridine fort-OHgroups although more vacantt-sites existed on the surface at473 K, demonstrates the Lewis acidity oft-HO-Zr site, eventhough its acidic strength aftert-OH group released is strongerthan that of the bare Zr ion. The conclusion from the selectivecoordination of pyridine obtained at 473 K is consistent withthe result obtained from 773-ZrO2.

Figure 3. Changes ofθt-OH andθb-OH as a function of the number ofpyridine molecules adsorbed on 773-ZrO2 in Figure 2.

2014 J. Phys. Chem. B, Vol. 104, No. 9, 2000 Ouyang et al.

Adsorption of CO2 on 773-ZrO2 at 213 K. CO2 is usuallyused as a probe molecule to identify the basic sites of the catalystsurface. The behavior of adsorbed CO2 species on ZrO2 hasbeen described previously.6,20 Bidentate carbonate20 and bicar-bonate species,6 both coordinated to single surface Zr ion, werefound on ZrO2 after CO2 adsorption at 373-573 K.

After 773-ZrO2 was exposed to gaseous CO2 (262 µmol) at213 K and then the cell was evacuated at the same temperature,the IR spectrum of the adsorbed species on 773-ZrO2 is shownFigure 5. The formation of bidentate carbonate species havingbands at 1559 (νCdO), 1310 (νaCO), and 1065 (νsCO) cm-1 wasobserved after the adsorption of CO2.20 Bicarbonate speciesgiving 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-OHspecies having oxygen lone pairs6 resulted in t-OH groupsdiminishing (the spectrum of 773-ZrO2 has been subtracted fromthat of adsorbed CO2, giving reverse bands at 3767 cm-1). Thelow reaction temperature shows the high activity oft-OH groupsin the reaction.

The Adsorption of Formate Species on ZrO2. Figure 6shows IR spectra of adsorbed species on ZrO2 after exposureof a different dosing amount of HCOOH followed by evacuationfor 3 min at 573 K. The adsorption of a small amount ofHCOOH yielded bidentate formate species coordinated to thesingle surface Zr ion (2874, 1568, 1385, and 1367 cm-1 inFigure 6a).11 As formate species increased,t-OH groups (reverseband at 3770 cm-1) were decreased beforeb-OH groups (3668cm-1) decreased. Whent-OH groups had disappeared com-pletely, only ca. 10% ofb-OH groups had decreased (Figure6c, noting thatt- andb-OH groups have approximate intensitieson 773-ZrO2 in Figure 1). The change oft-OH coverage as theincrease in the number of formate species is shown in Figure7. T-OH groups disappeared rapidly at the first stage of theadsorption, suggesting that HCOOH replacedt-OH to formbidentate formate species and that dehydration occurred. Whenca. 70% oft-OH species had disappeared, the number of formatespecies was 0.6 molecule/nm2. This shows that not only basicpyridine but also acidic adsorbates can react selectively on thesites.

CO2 has been found to react selectively witht-OH andt-OCH3 species, which has been attributed to the fact that aninteraction of CO2 with the oxygen lone pairs oft-OH or t-OCH3

groups is favored at the reaction.6 HCOOH was also introducedto OCH3-preadsorbed ZrO2 to examine if HCOOH reactedselectively witht-OCH3 species.

Coadsorption of CH3OH and HCOOH on ZrO 2. TheOCH3-preadsorbed ZrO2 was prepared by the adsorption ofmethanol (175µmol) on 773-ZrO2 at 523 K. IR spectrum ofadsorbed species after evacuation at the same temperature isshown in Figure 8a. The formedt- andb-OCH3 species gaveνCO bands at 1155 and 1051 cm-1 as well asνCH bands at 2924and 2817 cm-1.11 On the surface, almost all of thet-OH speciesand ca. 70% ofb-OH species were removed, respectively

Figure 4. IR spectra of adsorbed pyridine species on 983-ZrO2 aftervarious amounts of pyridine were introduced and followed by evacu-ation at 473 K: (a) background of 983-ZrO2 before adsorption; (b) 12µmol of pyridine was dosed, and (c) 25.7µmol was further introduced(showing subtracted spectra at 1700-1400 cm-1).

Figure 5. IR spectrum of CO2 adsorbed on 773-ZrO2 after 262µmolof CO2 was introduced and followed by evacuation at 213 K (showingsubtracted spectrum).

Figure 6. IR spectra of formate species on 773-ZrO2 after differentamounts of formic acid were introduced successively: (a) 15µmol;(b) 65 µmol and (c) 140µmol and followed by evacuation at 573 Kfor 5 min (subtracted spectra).

Infrared Study of a Novel Acid-Base Site on ZrO2 J. Phys. Chem. B, Vol. 104, No. 9, 20002015

(reverse band at 3765 and 3667 cm-1). A small amount ofbidentate formate species (1568 cm-1) was formed by theoxidation of OCH3 species.11 HCOOH (70µmol) was furtherintroduced to the OCH3-preadsorbed ZrO2 at 523 K, and thespectrum recorded after evacuation is shown in Figure 8b. Whenbidentate formate species (1568, 1385, and 1367 cm-1) in-creased, 50% oft-OCH3 and 27% ofb-OCH3 species weredecreased. The preferential substitution of HCOOH fort-OCH3

groups ast-OH groups shows that both substitutions occur viaa similar mechanism, in which oxygen lone pairs oft-OCH3

andt-OH species may be involved in the reaction with HCOOH.On the other hand,t- andb-OH groups did not show any obviouschanges in Figure 8.

The same coadsorption experiment was also conducted at 573K. To show clearly OH species, we did not subtract backgroundspectrum at 4000-2700 cm-1 in Figure 9. As shown in Figure9a, t- andb-OH (3765 and 3665 cm-1) were present on 773-ZrO2, respectively. The OCH3-preadsorbed surface was preparedby the adsorption of methanol on 773-ZrO2 at 573 K. The IRspectrum of the adsorbed species after evacuation is shown in

Figure 9b. On the surfacet- and b-OCH3 species (1155 and1051 cm-1) were present, andt-OH species diminished but ca.20% of b-OH species were remained. After HCOOH wassuccessively introduced to the OCH3-preadsorbed ZrO2 at 573K, the cell was evacuated, and the IR spectra were recorded. Atypical spectrum is shown in Figure 9c. As increasing bidentateformate species (1568 cm-1) formed from the adsorption ofHCOOH, t- and b-OCH3 species decreased, whereasb-OHgroups were regenerated (3665 cm-1 in Figure 9c). Finally, ca.90% coverage ofb-OH groups was restored on 773-ZrO2, whilet-OH groups hardly increased. This means that bidentate formatespecies replacedt-OCH3 species principally in the process.

The changes in the numbers oft- and b-OCH3 species areshown as the function of number of formate species in Figure10. The increase of formed formate species resulted in thedecrease of both types of OCH3 species in about equal moleculenumber. This can be interpreted by the migration of methoxyspices. The reaction of HCOOH ont-OCH3-preadsorbed sitesresulted in preferential removal oft-OCH3 groups as shown inFigure 8. However, the decomposition or desorption of a partof unstable formate species at 573 K generated vacantt-sites,leading to the subsequent conversion ofb-OCH3 species towardthese vacant sites.15 Consequently, the activities oft- andb-OCH3 species cannot be distinguished. The existence of vacantt-sites on the surface is confirmed by comparing the amount ofincreased formate species with the amount of decreasedt-OCH3

species in Figure 10. When 1.3 molecule per nm-1 of thet-OCH3 species was removed, the amount of increased formatespecies was estimated to be ca.0.7 molecule/nm2 (Figure 10),indicating that formate species did not take up completely thesites wheret-OCH3 groups had been removed.

Unsaturated cation-anion pairs on most of the dehydroxy-lated metal oxides have been known as active sites forheterolytic dissociation of alcohol21,22and carboxylic acids.23,24

The occupancy of other stable molecules or groups on thesesites, such as H2O or OH at Lewis acidic sites, may inhibit thedissociation of alcohol at these sites.25 Silica is regarded as aspecial example that has been well investigated.21,26 On silica,the dissociation of methanol has been proposed to be involvedin an electron donor-acceptor interaction between CH3OH andHO-Si. On partly dehydroxylated Al2O3, acetic acid was

Figure 7. Change ofθt-OH as function of the number of formate specieson 773-ZrO2 in Figure 6.

Figure 8. IR spectra of CH3OH and HCOOH coadsorbed on 773-ZrO2 at 523 K: (a) 175µmol of CH3OH was dosed followed by evacuation; (b)subsequently 30µmol of HCOOH was introduced followed by evacuation (subtracted spectra).

2016 J. Phys. Chem. B, Vol. 104, No. 9, 2000 Ouyang et al.

supposed to be adsorbed on bare Al ions at the first dosing.After the Al ions were filled, the acetic acid would react withOH species.23,24 The replacement oft-OH group by pyridineconfirms that the Zr ion joined to thet-OH group acts as Lewisacidic site at the reaction temperature. Moreover, the acidicstrength is relative stronger than vacant sites (Figures 2, 3, and4). Thus, we believe the interaction of lone pairs of pyridinewith the surface Zr ions is a predominant process in the reactionsbecause the lone pairs are most reactive in the pyridine

molecules. On the other hand, the reaction oft-OH with CO2

shows the nucleophilicity (Lewis basicity) of the sites. Lavallyet. al have found that CO2 reacts selectively witht-OH ort-OCH3 groups but not withb-OH or b-OCH3 groups. Animportant character of thet-OH or t-OCH3 groups is its oxygenlone pairs. Therefore, they suppose that the first step of bothreactions is likely an interaction of electrophilic CO2 and thelone pair oft-OH or t-OCH3 groups on ZrO2.6 In addition, asacidic alcohol (CF3)3COH,21 HCOOH exhibits predominantelectrophilicity in its adsorption. Apparently, the proton inHCOOH is strongly electrophilic, and the dissociation ofHCOOH is likely involved in an electrophilic interaction withoxygen lone pairs oft-OH groups. The selective reactions oft-OH groups with HCOOH and CO2 and replacement oft-OHgroups by pyridine, regardless of the difference in acid-baseproperties of these adsorbates, demonstrate the acid-baseproperties of the sites, wheret-OH group or Zr ion can playdifferent roles depending on the electronic features of theseadsorbates. Ast-OH groups were spent, bare Zr4+O2- sites wereresponsible for subsequent dissociative adsorption of formic acidor the adsorption of pyridine at a high coverage of formatespecies and pyridine (Figures 3, 4, and 7).

Several types of OH species have been found on alumina. Anumber of models of the coordinated OH groups have beenproposed to explain the features of these OH species on alumina.Nevertheless, the early models do not explain the special activityof terminal OH species at 3775 cm-1.12 Morterra et al. suggestedthat the higher activity of the OH species reflects a higheraccessibility for all types of surface probes and this should reflectthe presence of the OH group in the particularly exposed zoneof the surface. They attributed the activity to Al-OH groupspresent in the portion of the surface belonging to crystallo-graphically defective configuration.12 For ZrO2, Morterra et al.detected different Lewis acidic sites by CO adsorption after theelimination of OH groups on the surface and the site formedby the elimination of thet-OH species shows higher COadsorption heat. Thus, Zr ion at the site was suggested to be inhighly unsaturated coordination.8 These findings are in agree-ment with our finding that Lewis acidic sites can be generatedby removal oft-OH groups. We considered that the release oft-OH groups is enhanced by basic groups of adsorbates. Tanabeindicated that the bond in HO-M is ruptured easily to liberateOH- ion when M (a metal ion) electronegativity is low.27 Thelow-temperature (213 K) reaction oft-OH group with CO2

suggests that thet-HO-Zr bond is easily broken. When basicadsorbates attack the sites, the lone pairs of the basic adsorbatespromote the rupture oft-HO-Zr bond. Moreover, the dissocia-tion of t-OH group will lead to a strongly electrophilic state ofZr ions, which gives rise to a relatively stronger acidity (Figures2-4). Thus, facile dissociation oft-OH groups is one of theorigins of high activity oft-HO-Zr sites. The property of asurface under a reaction condition may differ from that undera nonreaction condition, and the formation of surface sites in areaction should be taken into consideration.

On the other hand, when HCOOH was used to replace thepreadsorbed OCH3, the ratio of the numbers oft-OCH3 tob-OCH3 was different at 523 and 573 K. The selective reactionof bidentate formate species fort-OCH3 species was observedat 523 K, whereas the difference in reactivity oft- andb-OCH3

species with HCOOH disappeared at 573 K because of themigration of b-OCH3 to the vacantt-site.15 Thus, the distin-guishable features amongt-OH-Zr, vacant orb-OH sites inpyridine adsorption and HCOOH dissociation are originatedfrom the different structure of the surface sites, whereas features

Figure 9. IR spectra of CH3OH and HCOOH coadsorbed on 773-ZrO2 at 573 K: (a) 773-ZrO2 before adsorption (b) after 200µmol ofCH3OH was introduced followed by evacuation; (c) 150µmol ofHCOOH was introduced followed by evacuation (showing subtractedspectra at 1700-900 cm-1).

Figure 10. Dependence of numbers of OCH3 groups on the numberof formate species adsorbed on 773-ZrO2 in Figure 9.

Infrared Study of a Novel Acid-Base Site on ZrO2 J. Phys. Chem. B, Vol. 104, No. 9, 20002017

of t- andb-OCH3 groups disappear because of the mobility ofOCH3 species on surface. Two different factors control differentreaction steps. Previously, we observed a similarity betweent-andb-OH species in H/D isotope exchange reaction on ZrO2

because migration of activated D atoms is a rate-determiningstep.11

Conclusion

The adsorption of pyridine and CO2 on t-HO-Zr sites showedthat the sites have an acid-base property. Pyridine has beenfound to ligand to Lewis acidic Zr ion by replacing thet-OHgroup. Moreover, by comparison of the kinetic behavior of theadsorption of pyridine ont-HO-Zr sites with that on vacantsites we found the preferential replacement oft-OH groups bypyridine demonstrating that the adsorption of pyridine occursmore actively att-HO-Zr sites than at vacant sites on ZrO2.The high activity at thet-HO-Zr site is attributed to thepromotion of the release of highly reactivet-OH groups bypyridine forming stronger Lewis acidic sites. In addition, thebasicity of t-OH groups is demonstrated though its selectivereactions with HCOOH, and the selective reaction oft-OCH3

groups with HCOOH is also observed. These suggest that thereactions are involved in the interaction of the proton ofHCOOH with lone electron pairs oft-OH andt-OCH3 groups.At 573 K, OCH3 groups are proposed to be movable on ZrO2,which leads to the disappearance of difference oft- andb-OCH3

species in reactivity.

Acknowledgment. The financial support from the NewEnergy and Industrial Technology Development Organization(NEDO, Japan) is gratefully acknowledged.

Abbreviations

θ coverage

A integrated absorbance

A0 initial integrated absorbance of OH groups

Subscripts

OH total OH groups

t-OH t-OH groups

b-OH b-OH groups

References and Notes

(1) Nakano, Y.; Iizuka, T.; HaTorri, H.; Tanabe, K.J. Catal. 1979,57, 1.

(2) Hertl, W. Langmuir1989, 5, 96.(3) Auroux, A.; Artizzu, P.; Ferino, I.; Solinas, V.; Padovan, M.;

Messina, G.; Mansani, R.J. Chem. Soc., Faraday Trans. 1995, 91, 3263.(4) Aramendı´a, M. A.; Borau, V.; Jimenez, C.; Marinas, J. M.; Porras,

A.; Urbano, F. J.J. Chem. Soc., Faraday Trans. 1997, 93, 1431.(5) Yamaguchi, T.; Nakano, Y.; Tanabe, K.Bull. Chem. Soc. Jpn. 1978,

51, 2482.(6) Bensitel, M.; Moravek, V.; Lamotte, J.; Sauer, O.; Lavalley, J.-C.

Spectrochim. Acta1987, 43A, 1487.(7) Morterra, C.; Cerrato, G.; Bolis, V.; Lamberti, C.; Ferroni, L.;

Montanaro, L.J. Chem. Soc., Faraday Trans. 1995, 91, 113.(8) Bogillo, V. I.; Morterra, C.; Volante, M.; Orio, L.; Fubini, B.

Langmuir1996, 6, 695.(9) Larsen, G.; Rahavan, S.; Ma´rquez, M.; Lotero, E.Catal. Lett. 1996,

37, 57.(10) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K.J. Chem. Soc.,

Faraday Trans. 1997, 93, 169.(11) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K.J. Chem. Soc.,

Faraday Trans. 1996, 92, 4491.(12) Morterra, C.; Magnacca, G.Catal. Today1996, 27, 497.(13) Knozinger, H.; Krietenbrink, H.; Mu¨ller, H. D.; Schulz, W.Proc.

VIth Int. Congr. Catal.; The Chemical Society, London, 1977; Vol. 1, p183.

(14) Maehashi, T.; Maruya, K.; Domen, K.; Aika, K.; Onishi, T.Chem.Lett. 1984, 747.

(15) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K.J. Phys. Chem.1997, 101, 4867.

(16) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E.J. Chem. Soc.,Faraday Trans. 1979, 75, 271.

(17) He, M.-Y.; Ekerdt, J. G.J. Catal. 1984, 98, 238.(18) Morterra, C.; Giamello, E.; Orio, L.; Volante, M.J. Phys. Chem.

1990, 94, 3111.(19) Kondo, J.; Sakata, Y.; Domen, K.; Maruya, K.; Onishi, T.J. Chem.

Soc., Faraday Trans. 11990, 86, 397.(20) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Domen, K.; Onishi,

T. J. Chem. Soc., Faraday Trans. 11988, 84, 511.(21) Lavally, J. C.Catal. Today1996, 27, 377.(22) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B.J. Chem.

Soc., Faraday Trans. 1991, 87, 2655.(23) Cheveigne´, S. De; Gauthier, S.; Klein, J.; Le´ger, A.; Guinet, C.;

Belin, M.; Defourneau, D.Surf. Sci. 1981, 105, 377.(24) Knozinger, H.; Ratnasamy, P.Catal. ReV. Sci. Eng. 1978, 17, 31.(25) Nagao, M.; Morimoto, T.J. Phys. Chem. 1980, 84, 2054.(26) Bogillo, V. I.; Gunko, V. M.Langmuir1996, 12, 115.(27) Tanabe, K.Catalysis-Science and Technology, Vol. 2; Springer-

Verlag: Berlin, 1981; Chapter 5, p 271.

2018 J. Phys. Chem. B, Vol. 104, No. 9, 2000 Ouyang et al.