Langmuir 1994,10, 4534-4541
4534
Surface Chemistry of Oxidized and Reduced Chromia: A Fourier Transform Infrared Spectroscopy Study Konstantin Hadjiivanov*>? and Guido Buscat Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Istituto di Chimica, Facolta di Ingegneria, 16129 Genova, Italy Received March 15, 1994. I n Final Form: July 18, 1994@ CO adsorption of reduced chromia leads to formation of Cr3+-CO carbonyls with an absorption band at 2167 cm-l. Surface carbonates and formates appear with time and, simultaneously, the 2167-cm-l band is shifted to 2186 cm-l (after passing through an intermediate position at ca. 2179 cm-'). This shift is interpreted as due t o the inductive effect of the carbonate structures on the v(C0) position. With a decrease of the equilibrium pressure, the maximum of the 2186-cm-l band is gradually shifted to 2196 cm-l due to the decrease ofthe adsorbate-adsorbate interaction. When chromia is deeply reduced, a band at 2101 cm-l, interpreted as due t o Cr2+-CO carbonyls, also appears after CO adsorption. Adsorption of ammonia on oxidized chromia shows the presence of Lewis and Br~nstedacidity on the sample surface. On the contrary, CO does not form any Crfl+-CO carbonyls due to its weak basicity. However, reactive adsorption of CO (180 "C and above) leads to formation of carbonates, bicarbonates, and formates. Simultaneously,Cr3+-CO carbonyls (v(C0) at 2196 cm-1) are detected as a result of the surface reduction. C02 adsorption on a reduced sample gives rise to carbonates, formates, and bicarbonates that are almost absent after adsorption on an oxidized sample. However, the same kinds of species are observed after reactive CO adsorptionon oxidized chromia. In addition, two kinds of Lewis acid sites are observed, where C02 is linearly adsorbed. However, on oxidized chromia, a very low concentration of one of these sites is detected.
relatively well studied €or the case o€"reduced"~ u r f a c e . ~ l - ~ ~ One band a t ca. 2187 cm-l, shifted to 2176-2170 cm-l Among transition-metal oxides, chromia has probably with increasing equilibrium pressure, is detected. Such the most complicated surface chemistry. Thus, a t least a shift is often reported in the literature for CO adsorbed six kinds of surface oxygen species have been detected on on different ~ x i d e s l l - ~ ~ and J ~isJ ~thought to be due to its surface after adsorption of oxygen.'-7 Reactive adadsorbate-adsorbate interaction. This means that the sorption of CO and COz leads to formation of several types respective active sites are situated in close vicinity and of (bi)carbonates, carboxylates, and f o r m a t e ~ . ~ Dif~ ~ - l ~ the plane of adsorption is flat. On the basis of the shift ferent carbonyls have been observed during nonreactive value it is concluded that CO is adsorbed on chromia CO a d s o r p t i ~ n . ~ ~This ~ J -variety ~~ of surface sites on predominantly via a a-bond.12J3 chromia is due to two main reasons: (i)the easy oxidation/ When chromia is treated in oxygen atmosphere at, e.g., reduction of the chromium ions, which provides the 400 "C, its surface is oxidized and, as a result, new bands possibility ofexistence of Cr"+ionson the surface, changing appear in the 1020-800 cm-I region that are assigned to their valency from 2 to 61-3,11and (ii) the pronounced the stretching modes of Cr=O and Cr-0-Cr groups with varying in the state of the coordinatively unsaturated a chromium ion in a high oxidation state, i.e. Cr5+and/or surface (cus)Cr"+ions exposed on different faces and edges Cr6+.1333However, this oxidation appears to occur in one of the corundum-type crystallites of a-chromia, their step, partially oxidized surfaces being constituted by the coordination number being supposed t o range from 3 to same species as is the case of totally oxidized surfaces, 5.3J1-13 The nature of the ligands (02-,02-, OH) may however in a lower concentration. No evidence is found also be different.1,4J4 of Cr"+ ions with an intermediate oxidation state. The state of oxide surfaces is usually tested by adsorpAccording to Davydov4 the adsorption of oxygen on tion of probe molecules, the most used one being carbon chromia is competitive with the adsorption of CO a t room monoxide. The reversible CO adsorption on chromia is temperature, while a t higher temperatures CO reacts with the oxidized chromia surface giving rise to carbonates, Bulgarian Academy of Sciences. bicarbonates, formates, and carbonyls. t Istituto di Chimica. The present paper summarizes the results of a study Abstract Dublished inAduanceACSAbstruct, October 15,1994. of the surface properties of oxidized and partially reduced (1)Zecchina, A,; Collucia, S.; Guglielminotti, E.; Ghiotti, G. J. Phys. Chem. 1971,75,2774,2783,and 2790. chromia and of the effect of reduction on its surface acidity (2)Davydov, A. J . Chem. SOC., Faraday Trans. 1991,87,913. and basicity. Two samples of very different origin but (3)Klissurski, D.; Hadjiivanov, K.; Davydov, A.J . Catal. 1988,111, with the same BET specific surface area have been used 421. (4)Davydov, A. ZR Spectroscopy Applied to Surface Chemistry of to check any effect of morphology or of the preparation Oxides; Nauka: Novosibirsk, 1984. method. (5) Carrott, P. J. M.; Sheppard, N. J . Chem. Soc., Faraday Trans. 1
Introduction
@
1983,79,2425. (6)Davydov, A.;Schekochikhin, Yu. M.; Keier, N. P. Kinet. Katal. 1972,13,1088. (7)Shopov, D., Palasov, A. Kinet. Katal. 1966,6, 864. (8)Davydov, A,; Schekochikhin, Yu. M.; Keier, N. P. Kinet. Katal. 1969,10,1337. (9)Chu, C. C.;Sheppard, N. XV EUCMOS, Norwich, 1981. (10)Hadjiivanov, K.; Klissurski, D. Proc., I1 Symposium Solid State Chemistry, Czechoslovakia, June 26-30, 1989;p 71. (11)Zaki, M.; Knoezinger, H. J . Catal. 1989,119, 311. (12)Scarano, D.; Zecchina, A.; Reller, A.Surf. Sei. 1988,198,11. (13)Scarano, D.; Zecchina, A. Spectrochim. Acta 1987,43A,1441. (14)Schraml-Marth, M.; Wokaun, A,; Curry-Hyde, H. E.; Baiker, A. J . Catal. 1992,133,415.
0743-7463/94/2410-4534$04.50/0
Experimental Section 1. Samples. Two a-chromia samples were used for the experiments: (i) chromia from Degussa, prepared by flame hydrolysis of chromium chlorideand having a BET specific surface area of 44 m2g-l, and (ii) a home prepared sample synthesized by thermal decompositionof ammonium dichromate followed by 1 h calcination in air at 500 "C. The BET surface area of this (15)Morterra, C.J . Chem. Soc., Faraday Trans. 1 1988,84,1617. (16)Zverev, S.;Denisenko, L.; Tsyganenko, A. Usp. Fotoniki 1987, 9,96.
0 1994 American Chemical Society
Surface Chemistry of Chromia
Langmuir, Vol. 10,No.12, 1994 4535
I
L
' 0.1 A
1
w
1800 1400 1000
wavenumberlcm-l Figure 2. FT-IR spectra of an oxidized CR2 sample (a),after heating the sample in an atmosphere of 30 Torr CO for 2 min at 250 "C (b), at 365 "C (c), and after 5 min of evacuation at ambient temperature (d). 1800
1400 1000
wovenumber 1cm-l Figure 1. FT-IR spectra of an oxidized CR1 sample (a),after heating the sample in an atmosphere of 30 Torr CO for 10min at 180 "C (b), at 250 "C (c), and after 5 min of evacuation at ambient temperature (d). sample was also 44 m2 g-l. Further on, the Degussa and the home prepared sample will be denoted by CR1 and CR2, respectively. The adsorbates used were commercial products and were additionally dried bypassing through a cartridge filled with PzOs. The IR spectra were registered using a NICOLET 5ZDX FTIR spectrometer with a resolution of 2 cm-l. Self-supporting disks were prepared from the samples and treated in an IR cell connected directly to a vacuudadsorption apparatus. In order to obtain an "oxidized"surface, chromia was activated by 1h treatments at 500 "C under air, in vacuum, and under 100 Torr oxygen. After cooling to room temperature, the oxygen was evacuated. The "reduced"chromia was obtained by reduction of an oxidized sample in 30 Torr CO (1 h, 385 "C) followed by 1h of evacuation at 500 "C that ensures the decompositionof the surface species formed with the participation of CO.
Results 1. Adsorption of CO on an Oxidized Sample. The IR spectrum of the oxidized CR1 sample contained several bands (Figure 1). The three intense bands with maxima a t 1015,995, and 980 cm-l can be assigned, according to data from the l i t e r a t ~ r e , to ~ ?the ~ , CP+==O ~ stretchings of three different kinds of chromyl groups. The weak bands below 945 cm-I (at 945,930,890, and 810 cm-') probably characterize oxygen species ofthe Cr-0-Cr type. Several low-intensity bands (1235,1145,and 1045cm-l) are typical for the chromia background spectrum and arise from harmonics of the skeletal bands.17 The broad band centered a t ca. 3400 cm-l and the weak band at 1600 cm-l (corresponding to v(OH) and 6(H-0-H), respectively) evidence the existence of some amount of water left on the surface after our treatment.18 The two weak bands a t 1680 and 1540 cm-' are probably due to C-0 vibrations and indicate the presence of some amount of residual carbonate-type species.l8 (17) Marshall,R.; Mitra, S. S.; Cielisse,P. J.; Prendl,J. N.; Mansur,
L. C.J. Chem. Phys. 1966,43, 2893. (18)Nakamoto, K. IR spectra ofhorganic and Coordination Com-
pounds; Mir: Moscow, 1966.
Table 1. IR Frequencies (1800-1200 cm-l Region) of the Species, Arising after Reactive CO and COSAdsorption on Oxidized and Reduced Chromia IR frequenciesof the species observed after adsorption on oxidized surface reduced surface species ofCO ofCO2 ofCO ofCOz formates 1550-47 1549-45 1543
bicarbonates
1358-52 1605 1440-17 1234-22 1508 1312 1715a
1352-50 1605 1419-17 1222
1347 1605 1425 1222
mono(po1y)dentate carbonates bridged or 173P bidentate 1680-40 1679 1285 1280 carbonates a Observed only at low coverage as intermediate species.
A similar situation is observed with the CR2 sample (Figure 2). The only differences are that in this case a C r O - C r band is clearlyvisible a t 890 cm-l and the water/ hydroxyl bands are less intense. Introduction of CO (30 Torr) into the IR cell does not lead to any change in spectrum of the CR1 sample. However, heating the pellet in the same atmosphere (10 min, 180 "C) leads to the appearance of a series of new bands which significantly rise in intensity after heating a t 250 "C(see Figure 1). Simultaneously, the intensities of the oxygen- and water-due bands decrease. The same is observed with the CR2 sample (Figure 2). The new bands appearing after reactive CO adsorption on oxidized chromia can be grouped and interpreted as follows (see also Table 1): (i) Bandsat3619,1605,1417,and 1222cm-I.They are assigned to v(OH)(3619 cm-'), vas(CO)(1605 cm-9, vs(CO) (1417 cm-l), and 6(0-H-0) (1222 cm-l) modes of bicarbonate species.lg It is interesting that part of these species are easily decomposed upon evacuation (see Figure l), most probably according to the following reaction: Cr-CO,H
-
Cr-OH
+ CO,
(1)
However, prolonged evacuation does not lead to any further decrease in intensity of the corresponding bands. (19)Busca, G.; Lorenzelli,V. Mater. Chem. 1982, 7, 89.
Hadjiivanov and Busca
4536 Langmuir, Vo2. 10, No. 12, 1994
a, 0
C 0 n L
0 v)
n
0
800 1400 1000 wovenumber Icm-'
2200
2100 wavenumbericm-1
I
Figure 3. FT-IR spectra of an oxidized CR2 sample after 2 min of heating in 8 Torr CO at 365 "C and cooling the sample (a), and 1 h evacuation (b), subsequent adsorption of 5 Torr COz (c), and 2 min outgasing at 130 "C (d).
This indicates that, regardless of the same spectral parameters, the surface bicarbonates are a t least two types differing in their stability. (iij Bands at 1548,1352,2964, and 2881 cm-'. They characterize the v,,(COO) (1548 cm-'1, v,(COO) (1352 cm-l), and v(C-H) (2881 cm-'1 vibrations, as well as the (v,(COO) vas(COO))combination mode (2964 cm-l) of surface formate s p e c i e ~ . ~ JThe ~ , ~spectral ~ splitting between the v(C00) vibrations is below 200 cm-l, which agrees with the relatively low thermal stability of the surface complexes4(see Figure 3). (iii) Bands at 1508 and 1312 cm-'. They correspond to the two modes of the v3 split vibration of surface carbona t e ~ The . ~ ~ value of the splitting and the band positions indicate that these carbonates are mono- or polydentate. These bands are more pronounced in the spectrum of the CR2 sample (see Figure 2), probably due to its higher dehydroxylation degree. (ivj A Band at 2200 cm-'. This band is very weak and can be assigned to v(C0) of Cr"+-CO surface carb~nyls.~J' Its appearance means that a small part of the sites where oxygen has been adsorbed is set free after the reduction, but most cationic sites remain blocked by some (or all) of the species described above. To establish whether reaction 1is reversible or not, the following experiment was performed. An oxidized CR2 sample was heated in CO atmosphere a t 365 "C, which resulted in formation of surface bicarbonates (bands a t 1605,1417, and 1222 cm-l), formates (bands a t 1547 and 1352 cm-'1, and carbonates (bands at 1508 and 1312 cm-l) (see Figure 3). As already observed with the CR1 sample, evacuation a t room temperature leads t o a decrease in intensity of the bands due to bicarbonate species. However, subsequent COz adsorption leads to the restoring of the initial intensity of these bands, which shows that reaction 1is reversible. Evacuation of the sample a t 130 "C leads to the disappearance of the bicarbonates from the surface of the CR2 sample; i.e. they are less stable than on the CR1 sample. This could be due to the lower hydroxyl coverage of the CR2 sample. When the interaction of chromia with CO (30 Torr) is performed a t 365 "C, and then the spectrum is registered after rapid cooling, a complex band with a maximum a t 2186 cm-l is observed in addition to the different bands
+
(20)Baraldi, P.Spectrochim. Acta 1979, 35A, 1003.
1800
' 1600 ' 1400 wavenumberlcm-'
'
1200
Figure 4. FT-IR spectra of oxidized CR1 heated for 2 min in 30 Torr CO and cooled t o room temperature (a) and the same spectra after 3, 6, 9, and 18 min (b, c, d, and e, respectively): A (top), v(C0) region; B (bottom), carbonate region.
due to carbonate, bicarbonate, and formate species (Figure 4). This band decreases in intensity with increasing time of contact with the gaseous CO and is shifted to 2196 cm-l. Another broad and weak band is observed at about 2122 cm-l. Its intensity slightly rises with time and its maximum shifts to 2118 cm-l. It should be noted that when the CO band has a maximum a t 2186 cm-', the bands due to bicarbonate species have a low intensity. In this case two bands, a t 1440 and 1420 cm-', are observed for v,(COO), and two, a t 1222 and 1234 cm-', for d(OH0). A weak band a t 1715 cm-', disappearing with time, has also been observed. This band can be assigned to one of the v3 components of another type of carbonate. 2. Adsorption of Ammonia and Subsequent CO Adsorption on Oxidized Samples. Adsorption of ammonia (5 Torr, 5 min, followed by evacuation) on a n oxidized CR1 sample (Figure 5) leads to the disappearance from the spectrum of the band a t 1015 cm-l which characterizes one type of the adsorbed oxygen species. This effect has been reported earlier3 and has been attributed to replacement of oxygen by ammonia. The other Cr-0 bands have also changed their shape. Simultaneously, bands due to both, protonated and coordinated ammonia, appear in the spectrum: 6,,(NH4+) a t 1420 cm-l, d,(NH4+)a t 1660 cm-' and 6,(NH3) at 1200 cm-l (weak), the d,,(NH3) band being probably masked by the other bands around 1600 cm-l. The asymmetric and symmetric N-H stretchings and the (2d,, - v,) combination mode of ammonia were detected as a broad absorbance in the 3500-3000-cm-' region. These results show the existence ofboth Lewis and Br~nstedacidity on the surface
Surface Chemistry of Chromia
Langmuir, Vol. 10, No. 12, 1994 4537
0)
0
C O
n
2\50
L
0 v)
n
2iOO 2350 2300 wavenumber Icm-'
22kO
Figure 6. FT-IR spectra of COz (10 Torr) adsorbed on an oxidized (a) and reduced (b) CR1 sample.
0
18'00 1400 1000 wave number I cm-I
Figure 5. FT-IR spectra of an oxidized CR1 sample (a) and aRer adsorption of ammonia (5 Torr, 10 min, followed by evacuation) (b), introduction of 30 Torr CO into the cell and heating at this atmosphere for 10 min at 180 "C (c), at 250 "C (d), and 10 min evacuation at 250 "C (e) and at 365 "C (0.
of our sample. The adsorption of ammonia on the CR2 sample is already ~ t u d i e d . In ~ this case no Bronsted acidity is reported, due to the almost full dehydroxylation of the sample. Introduction of CO (30 Torr) to the CR1 sample with adsorbed ammonia does not change the spectrum. However, heating the sample in this atmosphere (10 min, 180 "C) leads to the following changes (see Figure 5): (i)The residual bands in the 1100-800 cm-l region due to adsorbed oxygen species decrease in intensity. (ii) The bands due to NH4+ and coordinated ammonia disappear from the spectrum. The band at 1430 cm-l, which has a maximum almost coinciding with the das(NH4+)band position, cannot be attributed to protonated ammonia for two reasons: the respective d,(NH4+) and v(N-H) vibrations are absent from the spectrum and this band is sharper than the da,(NH4+) band. (iii) Intense bands (3619, 1605, 1430, and 1224 cm-l) due to surface bicarbonates and weak bands (2964,2881, 1547, and 1352 cm-') due to formates appear in the spectrum. (iv)One weak band a t 2196 cm-I due to surface CP+-CO carbonyls has also appeared and is evidence of the existence of free cus Crn+ions; i.e. some of these ions have been liberated after the thermal treatment. Further heating of the sample (10 min, 250 "C) leads to the following: (i)additional strong decrease of the C r O band's intensity; (ii) a strong increase in intensity of the formate-due bands; (iii) appearance of two bands a t ca. 1508 and 1312 cm-l (both detected as shoulders), characterizing surface carbonates. The bands due to bicarbonates remain almost the same, while the intensity of the Cr-CO bands decreases. It is interesting that thermal desorption during evacuation leads to destruction, first of all, of the carbonates and part of the bicarbonates, then of the formates, and, finally, of the remaining part of the bicarbonates.
30 wavenumber Icm-l
'
1800
1600 ' 1 i o o w a v e nu mb e r I c m-l '
1200
Figure 7. FT-IR spectra of CO adsorbed on an reduced CR1 sample. Equilibrium pressure of 50 (a),32 (b), 16 (c), and 1(d) Torr CO, background spectrum (e): A (top), v(C0) region; B (bottom), carbonate region.
3. Adsorption of COZon an Oxidized Surface. COZ adsorption (10 Torr) on a n oxidized CR1 sample leads to formation of a weak band at 2347 cm-l (Figure 6) which, in agreement with data from the literature,21 may be ascribed to the v3 vibration of COZcoordinatively bonded to chromium ions. No bands due to carbonate species can be detected. 4. CO Adsorption on a Reduced Sample. Adsorption of CO (50 Torr) on a reduced CR1 sample results in formation of one band a t 2167 cm-' having high-frequency shoulders (Figure 7). In the low-frequency region a set of bands with very low intensities appear, the respective maxima being a t about 1640,1545,1352, and 1285 cm-'. The bands a t 1545 and 1352 cm-' may be assigned to surface formates, whereas the bands at 1640 and 1290 (21)Busca, G.; Saussey, H.; Saur, 0.; Lavalley, L. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245.
Hadjiivanov and Busca
4538 Langmuir, Vol. 10, No. 12, 2994
I
21 00
wavenumber I c m-' Figure 8. FT-IR spectra of CO (30 Torr)adsorbed on a reduced CR1 sample: taken immediately after the coolingofthe sample (a) and after 3 min (b). cm-' belong to the v3 split vibration of bidentate or bridged carbonates. A decrease of the equilibrium pressure (32 Torr) leads to a drop in intensity of the carbonyl band. However, in this spectrum the CO band has a contour which shows the overlapping of three bands, with maxima a t about 2167,2179, and 2186 cm-', respectively. In the region of 1800-1000 cm-' the bands due to carbonates and formates have significantlyrisen in intensity. A weak band a t about 1730 cm-', already assigned to one of the v3 components of surface carbonates, has also been detected. Loweringofthe pressure to 16 Torr leads to an additional slight increase in intensity of the carbonatelformatebands and disappearance of the band a t 1730 cm-'. Simultaneously, bands a t 1605,1417,and 1222,due to bicarbonate species appear. In the carbonyl-stretching region, the Cr-CO band is shifted to 2193 cm-', its shape becoming symmetric. Additional decrease of the pressure leads to a decrease ofthe intensity ofthis band and gradual shifting of its maximum to 2196 cm-l. The above results allow the supposition that the change of the band maximum of adsorbed CO is affected not only by the pressure but also by the time, probably due to the carbonatelformate formation. To check this supposition, we have recorded time-dependent spectra (see Figure 8). Taking the spectrum after 3 min, it is possible to see two bands, a t 2196 cm-l and a t about 2167 cm-l. After some time the latter band disappears and only the band a t 2196 cm-' remains in the spectrum, its maximum being set a t 2197 cm-'. A sample was reduced in CO a t a higher temperature (500 "C) and 32 Torr CO was introduced into the cell (Figure 9). The above phenomena were observed again, i.e., formation of carbonates, formates, and bicarbonates and appearance of two bands in the carbonyl stretching region, the band a t 2167 cm-l disappearing with time. However, in this case another band of low intensity a t 2101 cm-l was observed. This band slightly changed its position and intensity with time and after decreasing the equilibrium pressure to 1 Torr. The stability of these carbonyls as well as the band position suggested a high n-back-bonding character of the metal-carbon bond. 5. COZAdsorption on a Reduced Sample. Adsorption of COZ (10 Torr) leads to formation of surface bicarbonates (1605,1425,and 1224 cm-l), formates (1543 and 1347 cm-l), and carbonates (1679 and 1280 cm-') (see Figure 10). Simultaneously, two overlapping bands
Io., A
I
-'
w a v en u m b e r I c m Figure 9. FT-IR spectra of 32 Torr CO adsorbed on the CR1 sample, prereduced at 500 "C in the same atmosphere (a),after 10 min (b), and after decreasing the equilibrium pressure to 1 Torr (c).
wovenumber Icm-l
I
/ -
1800
'
1600
'
1400
'
1200
w a venu m be r / c m-l Figure 10. FT-IR spectra of COZ (10 Torr) adsorbed on a reduced CR1 sample (a), the same spectrum after 3 min (b), and 6 min (c), after decreasing the equilibrium pressure to 1 Torr(d),and after 3 min of evacuation (e),background spectrum (0:A (top), v ( C 0 ~ region; ) B (bottom),carbonate region.
a t 2360 and 2347 cm-l are detected. They correspond to the v3 vibrations oftwo kinds of COz coordinatively bonded to cus Cr"+ ions. The respective v1 band is observed a t 1372 cm-'. This band is IR inactive in the gas phase but is observed with adsorbed molecules due to the change of their symmetry. Two weak bands a t 2291 and 2279 cm-' can be attributed to the vibrations of 13C02molecules, the observed shift coinciding well with the calculated one. The intensity of these bands slightly decreases with time, whereas the intensity of the bands due to bicarbonates slightly increases, which suggests the same location of both species. Prolonged evacuation leads to a drop of the intensities of the bands a t 2360 and 2347 cm-I, the maxima being in this case well resolved. Simultaneously, the
Surface Chemistry of Chromia intensity of the carbonate bands is also reduced. The intensity of the band a t 2347 cm-l is much stronger than the intensity of the same band when C02 is adsorbed on a n oxidized surface (see Figure 6).
Discussion
Langmuir, Vol. 10, No. 12, 1994 4539 the coordination sphere) but are not sufficiently ionic to "polarize" CO. This behavior is definitely different with respect to that of C 9 + sites on reduced chromia, where both CO and ammonia can be adsorbed. Another significant difference between oxidized and reduced chromia is the appearance, in the former case, of Bransted acidity. As discussed e l s e ~ h e r e ,the ~ ~B, r~~~n sted acidity of M-0-H groups increases with the electronegativity of the metal center, and, consequently, with the metal oxidation state. Accordingly, while low oxidation metal oxides are basic and produce hydroxides by reaction with water, the corresponding compounds of high-oxidation cations are anhydrides and acids. It has recently been shown that high-oxidation metal oxides like Vz0523 and W03,30 but also M003~lare acidic anhydrides and possess surface Bransted acidity, in contrast to lower oxidation metal oxides like MgO, CaO, TiO2, and Zr02,as well as Cr2O3. The same occurs when the above oxides are supported on metal oxides.32 Thus, it is not surprising that oxidized chromia, that can be viewed a s CrOB supported on Cr2O3, also possesses a B r ~ n s t e dacidity, due to the high electronegativity of Cr6+ or Cfl+ ions present on the surface. 2. Reactivity of the SurfceOxygen and Hydroxyls on Oxidized Chromia. Our results also confirm the high reactivity of oxygen species and hydroxyl groups on oxidized chromia. The following reactions take place during interaction with CO:
1. Surface Acidity of Oxidized Chromia. Our experiments on CO adsorption have shown that a fully oxidized surface of chromia does not adsorb CO a t room temperature and absorbs only a very small amount of C02, in contrast to what happens on reduced chromia. This indicates that chromium cations on oxidized chromia form Cr=O groups and are "blocked" by oxygen ions and, as a result, they cannot interact with CO. In other words, one can propose that adsorption of CO occurs on C 9 +ions on reduced chromia but does not occur on more oxidized Crn+(n = 5 or 6). A similar conclusion has been made for Vn+ions. Different authors agree that CO does not interact with V5+ ions22-25but can interact with less oxidized cations such as V4+22 or V3+.23 The results obtained by using CO and ammonia as probe molecules appear contradictory at first sight, since ammonia shows the presence of Lewis acidity on oxidized samples. However, recent calculations have shown that the interaction of CO with cationic centers on metal oxides can be almost entirely electrostatic, without any significant a-donation.26 This means that the interaction of cations with a base as weak as CO is different from the molecular adsorption of a true (and rather strong) base such as Cr-OH CO Cr-HCOO (2) ammonia, where a coordination bond really occurs. It is well-known that the electronegativity of a metal center increases with its oxidation state2' and this results in less [O=Crnf-OH](n-3)+ + [Cr'"-2'+- HC03]'"-3'f ionic bonds with the oxygen. Hence, the Cr6+-0 or (3) Cr5+-0 bonds are more covalent in nature than are the Cr3+-0 bonds. Thus, the real charge on Crn+centers of oxidized chromia is probably smaller than that on Crn+ centers ofreduced chromia. This results in a much weaker The first reaction does not imply any reduction of Cr electrostatic polarization of CO, if any, on oxidized than centers and is typical ofmetal ~ x i d e s . ' ~Carbonate J ~ ~ ~ ~ and on reduced chromia. bicarbonate species can also be formed by adsorption of In the case of molecular ammonia adsorption, a true COZwithout any redox effect.lg The formates observed coordination bond is formed and this likely occurs together during C02 adsorption are possibly due to interaction of with a significant change in the coordination sphere of Cr3+ with CO produced during COz dissociation. the cation, with some kind of rearrangement of the site. 3. Surface Acidity of Reduced Chromia. It is It is known28that Cr6+and, mostly, CF+,assume in general difficult to establish the valency of C P + by the IR spectra a tetrahedral coordination (chromate ions (Cr04I2- and of adsorbed CO only. Davydov,2making analogy between (Cr04)3-) but can enlarge their coordination sphere. bulk and silica-supported chromia, has concluded that a Square pyramidal and distorted octahedral complexes of band a t 2195 cm-l characterizes Cr3+-CO carbonyls, the chromyl cation (CI-=O)~+are among the most stable whereas carbonyls of the Cr2+-CO type have IR bands a t compounds of Cr5+,but similar compounds are also known 2170 cm-'. However, Zaki and Knoezinger,ll Scarano et for Cr6+, e.g. F4Cr=0.28 In the anhydride Cr03 the real a ~of~the opinion that all bands in the coordination of Cr ions approaches a trigonal b i ~ y r a m i d . ~ ~ al.,12J3and B u ~ c are 2190-2160-cm-' region arise from carbonyls of the In this view, the changes in the Cr=O stretching region Cr3+-CO type. We agree with this interpretation. after ammonia adsorption can be interpreted as a lower To our knowledge, no one has reported the band a t 2101 frequency shift due to the increase of the overall coordicm-' that we observed only after reduction with CO a t a nation of the chromium centers giving rise to a smaller high temperature. The respective carbonyls are stable, Cr=O bond order. which suggests a pronounced n-character of the metalWe can propose that oxidized Crn+sites ( n = 5 or 6) are carbon bond. This is also in agreement with the band able to interact with ammonia (with a n enlargement of position. It is known that back n-electron donation leads
+
(22)Dawdov, A,; Budneva, A,: Maksimiov, N. React. Kinet. Catal. Lett. 1982,20,93. (23)Busca, G.; Ramis, G.; Lorenzelli, V. J . Mol. Catal. 1989,50,231. (24)Busca, G.;Centi, G.; Marchetti, L.; Trifiro, F. Langmuir 1986, 2,568. (25)Jonson, B.; Rebenstorf,B.; Larson, R.; Anderson, S. L. T.; Lundin, S. T. J . Chem. SOC.,Faraday. Trans. 1 1986,82,767. (26) Pacchioni, G.; Cogliandro, G.; Bagnus, P. S. Surf. Sci. 1991,255, 344. (27)Tanaka, K.;Osaki, A. J . Catal. 1967,8, 1. (28) Cotton, F.A.; Wilkinson, G.Advanced Inorganic Chemistry, 4th ed.: Wiley: New York, 1980. (29)Hyde, B. G.; Andersson, S. Inorganic Crystal Structures; Wiley: New York, 1989;p 372.
co -
(30)Ramis, G.;Cristiani, C.; Elmi, A. S.; Villa, P. L.; Busca, G. J . Mol. Catal. 1990,61, 319. (31)Bianchi, D.; Bernard, J . L.; Camelot, M.; Benaili-Chaoui, R.; Teichner, S. J . Bull. SOC. Chim. Fr. 1980,1-275. (32)Del Arco, M.; Martin, C.; Rives, V.; Sanches-Escribano,V.;Ramis, G.; Busca, G.; Lorenzelli, V.; Malet, P. J . Chem. SOC.,Faraday Trans. 1 1993,89,1071. (33)Gopal, P. G.; Schneider, R. L.; Watters, K. L. J . Catal. 1987,105, 366. (34)Criado, J. M.; Domingues, J.; Munuera, G.; Gonzalez, F.; Trillo, J. M. Proceedings, IV International Congress Catalysis, Moscow, 1968, Part 2,1969;p 676. (35) Busca, G. J . Catal. 1989,120,303.
4540 Langmuir, Vol. 10, No. 12, 1994 to a decrease of the C-0 bond order and the stretching C-0 frequency, respectively. However, the position of the band is very high to be due to CrO-CO specie^.^ Taking into account the reduction conditions, we can interpret this band as due to Cr2+-CO carbonyls. The other carbonyls formed are more easily destructed during evacuation, which indicates a relatively low back n-bonding. The bands above 2150 cm-' have to be attributed to Cr"+-CO type carbonyls where the chromium ion valency, n L 3. Obviously, the bands with the lowest frequencies in the 2196-2150-~m-~region can be interpreted as due to Cr3+-CO species, in which the chromium ions have different surroundings. 4. Effect of Surface Species on the Acidity of Reduced Chromia. Our experiments have shown that the appearance of a band a t 2167 cm-l (shifted to 2196 cm-l with decreasing equilibrium pressure) is related to the appearance of carbonates and formates. Indeed, CO adsorption on hydrogen-reduced chromia leads to the formation of a band with a maximum a t 2170 cm-' a t high ~ o v e r a g e s . ~Hence, ~ J ~ we can assume that one of these anions, a carbonate or formate, affects the chromium ions situated in the vicinity, increasing their electroacceptor properties and their acidity, respectively. As a result, the stretching frequencies of adsorbed CO increase. This is not the first observation of the phenomena for a-bonded carbonyls. Thus, it is r e p ~ r t e dthat ~ ~ adsorption ?~~ of COz on ZnO leads to formation of carbonates which increase the Lewis acidity ofthe adjacent Zn2+ions. Sulfate groups can also change the CO stretchings when adsorbed in the vicinity of the metal ions.38 The resulting increase of the adsorbed CO frequency is about 20 cm-'. In our case there is another interesting phenomenon. The shift proceeds in two stages: 2167-2179-2186 cm-'. This may be explained in different ways: (1)There is formation of some type of surface species, which affects the acidity of the adjacent chromium ion (and the CO stretching modes). However, after increasing the population of these species they change their symmetry, e.g. bridged carbonates a t low coverages pass to carbonates bonded to one Crn+each a t higher coverages. Of course, the latter species would affect the chromium ion acidity even more strongly. However, this is not confirmed by our spectral data, since no change of the surface complex symmetry is detected. (2) The acidity of the chromium ion is affected by both surface formate and surface carbonate anions. Formation of one of these species affects the acidity of the chromium ion but to a moderate extent. We cannot support this supposition, since it requires a very high population of both anions and cannot explain the observed effect a t different degrees of surface hydroxylation. (3) The carbonates (or the formates) form a row with the Cr ions. We can propose the following scheme: 0
0
0
C
C
C
I
I
I
I
I
Cr-0-Cr-0-Cr
I
v (CO) = 2167 cm-I
I
0 I
0
I
I
I
0 C
C
Cr-COS-Cr-0-Cr
I
C
v (CO) = 2179 cm-I
1 b-CO3-Cr-CO3-Cr
v (CO) = 2186 cm-1
Hadjiivanov and Busca It is known that not all of the cus chromium ions are active toward CO adsorption a t room temperature.lZ This accounts for the fact that the shift of the v(C0) frequency occurs a t carbonate/formate coverages lower than the uptake coverage. Thus, it may be also concluded that the formation of surface carbonates and formates occurs first of all in the vicinity of the Cr ions, active toward CO adsorption. The above scheme also explains the CO adsorption on an oxidized surface. When the whole surface is occupied by oxygen, no Cr"+-CO carbonyls are observed. After a weak reduction with CO, some carbonates and formates appear and affect all of the Cr3+ sites formed. Since a t low concentration the respective active sites are not close to one another, no adsorbate-adsorbate interaction occurs. In this case the v(C0) is observed at ca. 2200 cm-l. This is in agreement with the reported fact that this band position does not depend on the coverage." In contrast, with a reduced surface all of the Cr3+sites are free, which explains the coverage dependence of the CO band. In this case a n adsorbate-adsorbate interaction is possible. 5. (Bilearbonate Species on Oxidized and Reduced Chromia. The formation of surface carbonates on the reduced surface can be explained by the reducibility of this metal oxide. It is, in fact, known that CrzO3 tends to become a p-type semiconductor in a n oxidizing atmosphere and a n-type semiconductor in a reducing atmosphere. Accordingly, we have observedthat CO can reduce chromia a t higher temperatures and Cr2+-CO carbonyls appear. It is reasonable to propose that stoichiometric Crz03 can be reduced by CO and the excess electrons are in part delocalized in the bulk. This possibility has been proposed in order to justify the ability of reduced chromia to absorb hydrogen in the form of hydride species.35 The result of chromia reduction by CO is the formation of surface carbonates and bicarbonates. On the other hand, these species are relatively strongly adsorbed on reduced chromia as a result of the high ionicity of the C++-O bonds (and the consequent relatively high basicity of oxide anions). On the contrary, these species, if any, are weakly adsorbed on oxidized chromia due to the low ionicity of surface Cr6+-0 or Cr5+-O bonds and the consequent low basicity of the oxide anions. When the 02-ions are not basic, carbonates are not formed from COZor, when the carbonates originate from CO oxidation, they very easily decompose giving gas phase or molecularly adsorbed COZ. Table 1 shows that bridged carbonates are formed on a reduced surface, where the carbonates produced after CO interaction with oxidized chromia are most probably monodentate. This is likely due to the lack of oxygen on a reduced surface, which results in a lower carbonate population (as also suggested by the lower intensity ofthe bands). When the carbonates are produced on an oxidized surface they are monodentate and possess a higher population. The comparative study of both samples under identical conditions shows that different chromias have close surface properties: however some differences, probably due to the crystal morphology, are observed. This is in agreement with differencesconcerning chromia surface chemistry in the studies of different authors.
Conclusions The main conclusions from the present study can be summarized as follows: (36) Saussey, J.; Lavalley, J. C.; Bovet, C. J. Chem. SOC.,Faraday Trans. 1 1982, 78,1457. (37) Lavalley, J. C.; Saussey, J.;Re, T. Souiet-French Sem. Catal., 6th 1983,97. (38)Hadjiivanov, K.; Davydov, A. Kinet. Katal. 1988,29,460.
Surface Chemistry of Chromia 1. The surface chemistry ofchromia is deeply influenced by the oxidation state ofthe surface chromium ions. This is due to the high covalency of the C P - 0 or Cr6+-0 bonds in contrast to the high ionicity of the C13+-0 bonds. 2. The high covalency of C P - 0 (or Cr6+-O) bonds on oxidized chromia results in the lack of detectable adsorption of CO at ambient temperature and in the detection of Bransted acidity, in contrast to the strong adsorption of CO and lack of B r ~ n s t e dacidity on reduced chromia. 3. The same reasons allow explaining the relatively strong adsorption of carbonate and bicarbonate species on reduced chromia, in contrast to oxidized chromia. 4. Cationic centers on reduced chromia are predominantly C$+ ions which produce carbonyls having IR bands in the 2200-2165 cm-' region. 5. The reduction of surface sites on oxidized chromia by CO to Cr3+, which is typical of reduced chromias, appears to occur in one step, as does their oxidation (without the appearance of any intermediate oxidation state).
Langmuir, Vol. 10, No. 12, 1994 4541 6. Reduction of chromia with CO at higher temperatures produces deeply reduced chromia with the appearance of Cr2+-CO carbonyls (v(C0) = 2101 cm-'). 7. The CO stretching band of surface carbonyls on clean reduced chromia is found a t 2167 cm-' a t relatively high coverages. However,this band is sensitive to the presence of near carbonate, bicarbonate, and formate ions that are also formed upon CO adsorption on reduced chromia. These species, if present, cause a shift toward higher frequencies up to about 2196 cm-'. This effect must be taken into account when CO is used as a probe molecule in the characterization of Cr-containing oxide surfaces. 8. The surface properties offumed chromia and chromia produced by ammonium dichromate decomposition are quite similar in both, the oxidized and the reduced state, with very small differences attributed to morphological factors.
