J. Phys. Chem. 1994,98, 8067-8073
8067
Microcalorimetric and Infrared Spectroscopic Studies of ?-A1203 Modified by Basic Metal Oxides Jianyi Shen$# R. D. Cortright,? Yi Chen,* and J. A. Dumesic'9t Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706, and Chemistry Department, Nanjing University, Nanjing 21 0008, P. R . China Received: January 4, 1994; In Final Form: May 4, 1994"
Microcalorimetric and infrared spectroscopic studies of ammonia and carbon dioxide adsorption were used to study the surface acid/base properties of y-Al203 following addition of K20, MgO, and La203 Microcalorimetric measurements of NH3 adsorption at 423 K showed that addition of basic metal oxide cations at loadings less than 200 pmol/g converted the stronger acid sites (heats from 140-1 60 kJ/mol) to sites of intermediate strength (100-140 kJ/mol). Increasing the loadings of the basic oxides further eliminated the stronger acid sites and eliminated sites of intermediate strength. Infrared spectra of adsorbed NH3 showed that y-Al203 contained both Brolnsted and Lewis acid sites after calcination at 723 K, and addition of potassium removed Brolnsted acid sites. The addition of basic metal oxides at loadings less than 500 pmol/g increased the number of weak basic sites (heats from 60-100 kJ/mol) and sites of intermediate strength (100-140 kJ/mol). Strong basic sites (140-160 kJ/mol) were formed at higher loadings of basic oxides on alumina. The effectiveness of basic metal oxides to neutralize acid sites and to generate basic sites on alumina can be related to the electronegativities of these oxides. 160
Introduction
The acidfbase properties of metal oxides are important for determining the catalytic and adsorptive properties of these materials.1-l3 For example, y-A1203has been widely used as an acidic catalyst and catalyst support. Furthermore, the addition of basic metal oxides to alumina leads to a support that is effective for dehydrogenation c a t a l y s t ~ . ~However, ~J~ questions remain regarding how the nature and strength of the acidfbase sites of alumina may be controlled by the addition of basic metal oxides to the surface. Accordingly, we have studied the acidfbase properties of y-A1203containing various amounts of potassium, magnesium, and lanthanum oxides. These basic oxides were chosen as representatives of univalent, bivalent, and trivalent metal oxides, respectively. In this investigation, we have employed ammonia and carbon dioxide adsorption to probe surface acidity and basicity, respectively. These two molecules have been widely utilized for this purpose.699J6-19 We have used microcalorimetryfor measurement of the number and strength of acid and base sites,4,6,20-22and infrared spectroscopy was utilized to identify the types of acid and base ~ites.23-2~In this latter respect, infrared spectra of adsorbed ammonia can be used to identify Lewis and Bransted acid sites on alumina, and carbon dioxide adsorption can be used to titrate basic oxygen anions and hydroxyl species on the surface.9,l1,12.29,30 Experimental Section
The y-A1203used in this investigation was supplied by Davison. The reported impurity levels by weight are 1160 ppm Ti, 475 ppm Si, 356 ppm S, and 6.4 ppm Mg. Samples containing basic metal oxides were prepared by impregnating y-A1203 powder with aqueous solutions of KOH (Johnson-Mathey, 99.999%), Mg(N03)2.6H20 (Johnson-Matthey, 99.999%), and La(NO&. 6 H 2 0 (Aldrich, 99%). After impregnation, samples were dried overnight at 393 K. Samples containing potassium and magnesium were calcinedin Ozat 723 K for 6 h, and samplescontaining lanthanum were calcined in 02 at 833 K for 24 h to achieve
* To whom correspondence should be addressed.
University of Wisconsin. Nanjing University. 0 Abstract published in Aduance ACS Abstracts. July 15, 1994. t
0022-365419412098-8067$04.50/0
140
-
3
5 120
g!
-.-m E
100
80
2 60 0
40
t....l....l....l....l....l....,....,... 0
50
100 150 200 250 300 350 400 Coverage (pmollg)
Figure 1. Differential heat versus adsorbate coverage for adsorption of NH3 (e) and C02 (0) on y-AlzO3 at 423 K.
TABLE 1: BET Surface Areas catalyst sample
-
Y A 1203 200 pmol K/g ?-A1203 500 pmol K/g y-Al203 3000 pmol K/g 7-AlzO3
surface area (mZ/g) 213 289 28 1 163
decomposition of lanthanum nitrate.31 Table 1 shows the BET surface areas for y-A1203and samples impregnated with KOH. All samples exhibited the X-ray diffraction pattern of the initial y-A1203. Microcalorimetric studies of the adsorption of NH3 and C02 were carried out using a Tian-Calvet heat-flux apparatus, which has been described elsewhere.32 The microcalorimeter was connected to a gas-handling and volumetric adsorption system, equipped with a Baratron capacitance manometer for precision pressure measurements. The differentialheat of adsorption versus adsorbate coverage was obtained by measuring the heats evolved when doses of gas (2-5 pmol) were admitted sequentially onto the catalyst until the surface was saturated by adsorbed species. Ammonia was purified by successive freeze/pump/thaw cycles. Carbon dioxide with a purity of 99.99% (Anaerobe, AGA SpeciaN. Gases & Equipment) was used without further purification. Before "calorimetric measurements,the samples were typically dried under vacuum at 573 K for 1 h, calcined in 500 Torr 0 2 at 723 K for 6 h, and evacuated at 723 K for 1-2 0 1994 American Chemical Society
Shen et al.
8068 The Journal of Physical Chemistry, Vol. 98, No. 33, I994 160,
0
50 100 150 200 250 300 350 400 Ammonia Coverage (Fmol/g)
Figure 2. Differential heat versus adsorbate coverage for adsorption of NH3 at 423 K on yAl2O3 ( 0 )and samples with potassium loadings of 110 pmol/g (O),200 pmollg ( O ) , 500 pmol/g (D), and 3000 pmollg
(4.
=
50
0
100
150
200
250
Carbon Dioxide Coverage (pmoi/g)
Figure 6. Differential heat versus adsorbate coverage for adsorption of CO2 at 423 K on -pAl2O3 ( 0 )and samples with magnesium loadings of and for adsorption on MgO (D). 110 pmol/g (0)and 3000 pmollg (0)
140
-
5
c 120 r"- 100 .-E
0 0
3c
80
e,
8
*a
60
B
a
40 0
100
200
300
400
500
0
Ammonia Coverage (pmol/g)
Figure 3. Differential heat versus adsorbate coverage for adsorption of NH3at 423 K on y-AlzO3 ( 0 )and samples with magnesium loadings of 110 pmol/g (0)and 3000 pmol/g (0)and for adsorption on MgO (D).
50
I
I
Brgnsted Acid Site
g
100
p
80
8E
60
s
250
Lewis Acid Site
140 . 2 120
-
200
Figure 7. Differential heat versus adsorbate coverage for adsorption of CO2 at 423 K on y-Al2Op ( 0 )and samples with lanthanum loadings of 110 pmol/g (0)and 3000 pmol/g (0).
8 E m
150
- f1245
160,
-
100
Carbon Dioxide Coverage (pmoi/g)
t
a
40
0
100
200
300
400
500
Ammonia Coverage (pmoi/g)
Figure 4. Differential heat versus adsorbate coverage for adsorption of NH3at 423 K on y-Al2O3 ( 0 )and samples with lanthanum loadings of 110 pmol/g (0)and 3000 pmol/g (D).
L
2600
80 n -
1j90
1580 1370 WAVENUMBER
1160
450
Figure 8. Infrared spectra for ammonia adsorption at 423 K and subsequent evacuation at 298 K on y-Al203 (a) and samples with potassium loadings of 110 pmol/g (b), 200 pmol/g (c), and 5 0 0 pmol/g
60
(4. 0
100
200
300
400
500
600
Carbon Dioxide Coverage (pmollg)
Figure 5. Differential heat versus adsorbate coverage for adsorption of C02at 423 K on y-Al2O3 ( 0 )and samplea with potassium loadings of 110 pmol/g (O),200 pmollg (a), 500 pmol/g (D), and 3000 pmol/g (A).
h. Studies of the adsorption of ammonia and carbon dioxide were performed at 423 K. Infrared spectra were collected with a Nicolet FX 7000 FTIR system equipped with a liquid-nitrogen-cooled MCT detector. Each spectrum was recorded at 2 cm-1 resolution with 32 coadded scans. Sample pellets were formed with a thickness of 20-30 mg/cm2. The samples were loaded into a quartz cell
equipped with CaFz windows, followed by the same sample treatments used for microcalorimetric adsorption studies. Ammonia and carbon dioxide were then dosed onto the sample a t 423 K for 0.5 h. The cell was then isolated, cooled to room temperature, and evacuated sequentially at 298,373, and 473 K. Infrared spectra were collected after each evacuation. Results
Figure 1 shows the differential heats of adsorption versus coverage of ammonia and carbon dioxide on y-Al203. The initial heats of ammonia and carbon dioxide adsorption were 155 f 5 and 145 f 5 kJ/mol, respectively. For both compounds, the
?A1203 Modified by Basic Metal Oxides differential heats decrease with increasing coveragedue probably to adsorption on weaker sites and/or possible lateral interactions between adsorbed species. Figure 1 shows that the y-A1203 surface has more acidic sites (-400 pmol/g) than basic sites (-80 pmol/g). Figure 2 shows the effects on the adsorption of ammonia of adding potassium to y-Al203. The addition of potassium cations at a loading of 110 pmol/g decreases the heat of ammonia adsorption on the stronger sites. Increasing the loadings of potassium to 200 and 500 pmol K/g decreases further the heat of ammonia adsorption on the stronger sites. The acidic sites of 7-A1203were completely neutralized for the sample containing 3000 pmol K/g, and only weak adsorption of NH3 took place. The addition of magnesium and lanthanum also influenced the stronger sites for ammonia adsorption. Figure 3 shows that the addition of magnesium cations at levels of 110 and 3000 pmol/g decreased the initial heat of ammonia adsorption to 145 f 5 and 135 f 5 kJ/mol, respectively. Unlike the case for the sample containing 3000 pmol K/g, the addition of 3000 pmol Mg/g did not completely neutralize the acid sites. However, a sample of pure MgO possessed only weak acidic sites for the adsorption of ammonia. Figure 4 shows that the addition of lanthanum cations at loadingsof 1 10 and 3000pmol/g decreased the initial heat of ammonia adsorption to 147 f 5 and 121 f 5 kJ/mol, respectively. There is little difference in the acidities of the samples containing either magnesium or lanthanum at levels of 110 pmol/g. However, the addition of lanthanum cations appears to have a larger effect on the heat of ammonia adsorption compared to the addition of magnesium at loadings of 3000 pmol/ g. The results of carbon dioxide adsorption on thevarious samples are shown in Figures 5-7. Figure 5 shows that the addition of potassium at levels of 110-500pmol/g increasesslightlytheextent of carbon dioxide adsorption; however, the addition of 3000 pmol K/g leads to a large increase in the amount of carbon dioxide adsorbed. Figures 6 and 7 show similar results for the effects of magnesium and lanthanum cations on the extent and heat of carbon dioxide adsorption. The magnitudes of the changescaused by magnesium and lanthanum, however, do not appear to be as large as the changes caused by addition of potassium to alumina. Figure 8 shows IR spectra collected after exposure of y-A1203 to ammonia at 423 K. The bands at 1620 and 1245 cm-1 originate from the asymmetric and symmetric vibrations, respectively, of NH3 molecules coordinated to aluminum cations, revealing Lewis acid sites on y-A1203.33.34The bands at 1700, 1485, and 1395 cm-l are due to deformation modes of NH4+ formed by the interaction of NH3 with Brtansted acid sites on y-A1203.33934 Figure 8 also shows IR spectra collected following ammonia adsorption on y-A1203samples containing potassium. It can be seen that the relative intensities of the bands near 1700, 1485, and 1395 cm-1 for ammonia on Brtansted acid sites decrease with increasing loading of potassium. Accordingly, Brsnsted acid sites on y-Alz03 are neutralized upon addition of potassium. The bands near 1485 and 1395 cm-l appear to shift to higher frequencies,suggesting that the Br~nstedacid sites have become weaker.34 The band of symmetric vibration of adsorbed NH3 also shifts from 1245 to 1235 cm-I, suggesting that the Lewis acid sites also become weaker upon addition of potassium.34 Figure 9 shows infrared spectra collected following exposure of y-A1203and samples containing potassium to carbon dioxide, following the same procedure used to collect IR spectra of adsorbed ammonia. The bands near 1650, 1480, and 1235 cm-I can be assigned to the three features normally seen for bicarbonate species,35which are formed on y-A1203by adsorption of COa on surface hydroxyl gr0ups.~0Addition of potassium decreases the intensity of the bicarbonate band near 1480 cm-1, indicating that potassium decreases the concentrationof surface hydroxyl groups. In contrast, the intensities of the bands near 1650 and 1235 cm-I decrease less significantly with addition of potassium, suggesting that the bands near 1650 and 1235 cm-1 also contain contribu-
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8069 Bidentate/A@+
I Bidentat&+
I Bicarbonate
m
'"ofi460
Free Carbonate 1660
Free Carbonate
1238
"
1580 1370 lis0 450 WAVENUMBER Figure 9. Infrared spectra for carbon dioxide adsorption at 423 K and subsequent evacuation at 298 K on y-AlzO3 (a) and samples with potassium loadings of 110 amol/g (b), 200 fimol/g (c), and 500 pmol/g (d). Following the collection of spectrum d, the sample was evacuated at 473 K and spectrum e was recorded.
2600
lf90
tions from bidentate carbonate species associated with alumina30 that are not strongly affected by potassium. Finally, addition of potassium leads to an increase in the intensity of a band near 1305 cm-l. The IR spectrum of the sample containing potassium at a loading of 3000 pmol/g also shows a shift to lower frequency of the band near 1650 cm-1. This shift is especially pronounced after evacuation of C02 at higher temperatures (spectrum e). Thus, we assign bands at 1640 and 1305 cm-1 to bidentate carbonate species associated with potassium. The bands near 1460and 109Ocm-l may beattributed tofreecarbonateion~.30,35,3~
Discussion Table 1 shows that the surface area is not significantly altered by the addition of smaller amounts of KOH and calcination at 723 K. However, Table 1 shows a decrease in surface area with the addition of 3000 pmol K/g, followed by a calcination at 723 K. It should be noted that the samples modified with lanthanum were calcined at 833 K, and Davison indicates a 12%decrease in surface area at this temperature. Figures 3-5 show the effects of adding basic oxides on the initial differential heats as well as the total number of sites for ammonia adsorption. Further information is obtained from histograms of the differential heat distribution which show how the addition of basic oxides influences the apparent distribution of site strengths. Figure 10 shows histograms of the apparent distribution of site strengths for ammonia adsorption on the modified samples of y-A1203. These histograms were generated by first smoothing the data of differential heat versus coverage with a least squares fitted polynomial and then using this polynomial to determine the amount of adsorbate adsorbed within a given range of differential heats. It can be seen in Figure 10that y-AI2O3exhibits a heterogeneous site distribution; i.e., each interval of differential heat contains
8070 The Journal of Physical Chemistry, Vol. 98. No. 33, 1994
Shen et al.
60
60
40
40
20
20
0
0
110 pmo1 K l g
80
80
60
60
40
40
-
20
20
-
100 200 pmol K l g
80
1 1 0 mmol M g l g
-
loo
3000 wnol M g l g
60
40 20
20
0
loo j 5 0 0 "01 80
40
r
0 ; Klg
t
80
t
60
-
1 1 0 mmol Lalg
40 20
20
0
:
0 -
100
80
3000 pmol W g
I
3000 pmol Lalg
60 40
20 0 50
Differential Heat (kJlmo1)
130
110
90
70
50
DifferentialHeat (kJlmol)
Flyre 10. Histograms of the apparent distribution of site strengths for NHI adsorption on y-AhOl samples wntaining different loadings of potassium.
magnesium, and lanthanum.
a significant number of adsorption sites. This result is in agreement with thethermogravimetric resultsofDeebaand Hall' and the microcalorimetric results of Spiewak et a/?' Addition of 110 pmol K/g decreases the number of strong sites (>I40 kJ/mol) and increases the number of sites with intermediate strength (100-140 kJ/mol). Increasing the potassium loading toZOOpmol/gleads toafurtherdecreasein thenumberofstrong sites,and increasing thepotassiumloadingto500pmol/greduces the number of sites having intermediate strength. Finally, the addition of 3000 pmol K/g eliminated all acid sites with heats greater than 60 kJ/mol. Table I shows only a small change of surface area with the addition of 500 pmol K/g. Accordingly, changes in the site strength distribution for the smaller additions of potassium can be attributed to the chemical interaction of potassium with the acidic sites and not to structural changes of the alumina. Addition of 110 pmol Mg/g leads to a shift from strong acid sites to sites of intermediate strength. A further shift to weaker acidity is Observed upon addition of 3000 pmol Mg/g. The addition of lanthanum to y-AI201 causes shifts in the acid site distributions that are similar to those caused by addition of magnesium. For example, Figure IO shows a decrease in the
number of strong acid sites and an increase in the number of sites with intermediate strength upon addition of 110 pmol La/& and the addition of 3000 pmol La/g eliminated the strong sites and reduced the number of sites with intermediate strength. Figure I 1 summarizes the above changes of the apparent site strength distributions for ammonia adsorption of y-A1201 containing various amounts of potassium, magnesium, and lanthanum. The addition of K,Mg,and La at loadings of 110 pmol/gcausesadecreasein thenumber ofstrongacidsites,with approximately I strong acid site being altered for addition of 2 basic metal cations. The number of sites with intermediate strength increases with a similar stoichiometry with the addition of K, Mg. and La. At loadings higher than 200 pmollg, the number of sites having intermediate strength decreases with increasingamounts of K,Mg,and La. Therefore, it appears that the addition of basic metal oxides first converts the strong acid sites on y-Al2O3to sites of intermediate strength, followed by neutralization of the sites having intermediate strength and formation of weak acid sites. The number of weak acid sites decreases gradually with potassium loading, while the number of weak acid sites increases slightly with increased loadings of magnesium and lanthanum.
The Journal of Physical Chemistry, Vol. 98. No. 33, 1994 8071
y-A1203Modified by Basic Metal Oxides
interactionof basicoxideswith acidsitesanda decreasein surface area. In addition, the higher calcination temperature of 833 K employed for the lanthanum-modified alumina further reduces the surface area of this sample. The plots in Figure 1 I allow comparison of the effectiveness of potassium, magnesium, and lanthanum oxides to neutralize the acidity of alumina. Low loadings of all three oxides are effective for elimination of strong acid sites. At higher loadings, potassium and lanthanum oxides also neutralize acid sites of intermediatestrength, while only potassium oxide eliminates acid sitesof weakstrength. Thus, we may rank theseoxidesaccording to effectiveness in neutralizing the acidity of alumina in the following order:
K,O (1.24) > La,O, (2.04) > MgO (2.21) The Sanderson electronegativitiesoftheseoxidesare shown above in parentheses,'* and it is interesting to note that the order of effectivenessfor acidity neutralization is consistent with theorder of decreasine electroneeativitv. , The oriein of this correlation may bethatoxygenanionshavea higher negativechargeinoxides with lower electronegativities.
Fipre 11. Variation of the number of strong (140-160 kJ/mol), intermediate (IW140kJ/mol), and weak acid sites (60-100 kJ/mol) versus loadings of basic metal oxide Cations on y-AlzO3.
-
At the highest loading of basic oxides (Le., 3000 rmol/g), the decrease in the number of acid sites can be attributed to both the
CO, Adsorptlon Sltes (pmollg)
CO- Adsorptlon Sltes (wmollg)
I 20
-
:
0
~
1
: 2
I.
100
80 60
40 20 0 100
3000 pmol Mglg
80 60
40 20
0 500 pmol K l g 60
60
40
40
I i
J
20 0 100
80
I
3000 pmol K l g
100
so:
3000
pmol Lalg
60
40 20 150
130
110
90
70
50
150
130
110
90
70
50
Differential Heat (kJ/mol) Differential Heat (kJ/mol) Histograms of the apparent distribution of Site strengths for COz adsorption on y-Al2O) samples containing differentloadingsofpotassium, magnesium, and lanthanum. Figure 12.
Shen et al.
8072 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 70 60 50 40
number of sites with intermediate strength and weak basic sites, but it does not generate strong basic sites. However, the addition of 3000 pmol K/g increases the number of carbon dioxide adsorption sites, including strong basic sites. Figure 12 shows similar results for the addition of magnesium and lanthanum to y-Al2O3. Figure 13 summarizes the above changes of the apparent site strength distributions for carbon dioxide adsorption of yA1203 containing various amounts of potassium, magnesium, and lanthanum. This figure shows that strong basic sites are not produced a t low loadings of these basic cations on y-A1203.For samples containing potassium, the number of strong basic sites did not increase significantly even a t loadings of 500 pmol K/g. Thenumber of sites with intermediate strength appears toincrease at all cation loadings. Significant increases in the numbers of strong sites, sites with intermediate strength, and weak basic sites were observed with the addition of all basic metal oxides at levels of 3000 pmol/g. The plots in Figure 13 allow comparison of the effectiveness of potassium, magnesium, and lanthanum oxides to generate basicity on alumina. Low loadings of all three oxides are not effective for generation of strong basic sites, because the role of these oxides at low loadings is to neutralize strong acid sites. At higher loadings, however, potassium is the most effective oxide for generation of strong sites, sites with intermediate strength, and weak basic sites. Lanthanum oxide is less effective for generation of surface basic sites, and magnesium oxide has only a minor effect on surface basicity. Thus, we may rank these oxides according to effectiveness in generation of basicity in the following order:
20
,300 3 250 200 150 d 100 50 f
t
3
;
O
B
,200
%8
50
Mg/AI,O, 0
f
500 1000 1500 2000 2500 3000 3500 Dopant Concentratlon (F moligram)
Figure 13. Variation of the number of strong (140-160 kJ/mol), intermediate (100-140 kJ/mol), and weak basic sites (60-100 kJ/mol) versus loadings of basic metal oxide cations on y-AlzO3.
Figure 12 shows histograms of the apparent distribution of site strengths for carbon dioxide adsorption on the various y-A1203 samples. It can be seen that y-A1203exhibited few strong basic sites (140-160 kJ/mol) and a relatively larger number of sites with intermediate strength (100-140 kJ/mol) and weak sites (60-100 kJ/mol) for carbon dioxide adsorption. The addition of potassium a t levels of 110,200, and 500 pmol/g increases the (1 6 2 0 , l 245cm-1 )
(1700,1485,1395cm-1) 0-NH4+ I AI-0-AI-0-AI
iH3 -
A 1-0-A I 0-A I
I
AI-0-AI-0-AI I
AI-0-AI-0-AII
\:g
AI-O-AI-C03'-AI Ob) J m 2 I (1 650,l 235cm-1 )
(111) N
> La20, (2.04) > MgO (2.21)
K 2 0 (1.24)
J K I
"2\
(Ilb)
HCO3' I
P
AI-0-AI-0-AI 1 (1 6 5 0 , 1 4 8 0 , 1 2 3 5 ~ m - ~ )
K
p?=
py2
AI-0-AI-0-AI y I
K TH3 I AI-0-AI-0-A I I
(Illb)
(Ills)
K
(W
A I- 0 - A I - 0 - A I I
I I AI-0-AI-0-AI I
NH3 K I
(W
+ p
KI
oI
A I-0-AI-0-A I I
K-CO3'- K I I 0 (IVb) I AI-0-AI - 0 A I I
-
7
(1 640,1305cm-l) Figure 14. Schematic representation of variations in surface acid/base sites caused by addition of potassium to yAI2O3.
y-A1203 Modified by Basic Metal Oxides Again, it is noteworthy that the order of effectiveness for basicity generation is consistent with the order of decreasing electronegativity. The presence of Brernsted acid sites on the alumina sample of this study may be related to impurities and to the low calcination temperatures used. Elemental analysis indicated that the alumina contained 24 pmol Ti/g and 17 pmol Si/g, and Brernsted acid sites have been observed both on silica-alumina supports as well as on Ti02.2 In addition, Brernsted acid sites on A1203may be related to hydroxyl groups or coordinated water present as H30+ cations.34J5-39 The infrared spectra in Figure 8 of ammonia adsorbed on 7-Alz03 samples containing potassium show that potassium neutralizes Brernsted acid sites. The spectra in Figure 9 of adsorbed carbon dioxide show that the band near 1480cm-1 (due to bicarbonate species) decreases upon addition of potassium at levels of 110 pmol/g, and this band essentially disappears at 200 pmol/g. Accordingly, we conclude from the IR spectra of ammonia and carbon dioxide that potassium decreases the concentration of hydroxyl groups that serve as both acid and base sites. The suppression of surface hydroxyl groups on 7-A1203by the addition of alkali metal cations was also reported by Scokart, et al.4O-41
The bands near 1650 and 1235 cm-I due to bidenate carbonate species and the bands near 1460 and 1090 cm-l due to free carbonate ions are not significantly altered by addition of potassium to alumina. Thus, it appears that the interaction of potassium with alumina is rather specific, involving neutralization of amphoteric surface hydroxyl groups. When the loading of potassium increased to 500 pmol/g, the bands due to ammonium ions associated with Brransted acid sites and the band near 1480 cm-1 due to bicarbonate speciesessentially disappeared, indicating that the amphoteric surface hydroxyl groups had been completely eliminated by potassium. In this case, the intensities of the bands a t 1640 and 1305 cm-l were fully developed, due to formation of bidentate carbonate species associated with potassium. As shown in Figure 9, evacuation at 473 K decreased the intensities of the bands near 1650 and 1235 cm-1 due to carbon dioxide adsorption on basic sites associated with 7-A1203,while evacuation had a smaller effect on the band near 1305 cm-I resulting from basic sites associated with potassium. This behavior is consistent with the microcalorimetric results that addition of potassium to 7-A1203generates stronger basic sites than those originally present on alumina. Summary The effectiveness of basic metal oxides to neutralize acid sites and to generate basic sites on alumina can be related to the electronegativities of these oxides. Accordingly, potassium was the most effective additive for this purpose in the present study. Figure 14 presents a schematic summary of the effects of potassium addition to alumina. Lewis acid sites on alumina are associated with unsaturated aluminum cations, and Lewis base sites exist associated with unsaturated oxygen anions. The adsorption states of ammonia and carbon dioxide on these sites are shown in species Iaand Ib, respectively. In species Ia, ammonia is coordinatively adsorbed on the aluminum cation, showing IR bands near 1620 and 1245 cm-1. In species Ib, COz forms bidentate carbonate anions, forming IR bands near 1650 and 1235 cm-I. Species I1 represents a hydroxyl group on 7-A1203, which can be a Brernsted acid site and a base site. Species IIa and IIb are states of ammonia and C02 adsorbed on the hydroxyl group, respectively. The adsorbed ammonia forms NH4+,showing IR bands at 1700, 1485, and 1395 cm-1, while adsorbed C02 forms bicarbonate anions, with IR bands at 1650,1480, and 1235 cm-1. Upon addition of potassium, sites I and I1 can be converted into site 111. The Lewis acidity of site III is weaker than site I, due to the presence of potassium cations near the aluminum cation forming the Lewis acid site. The oxygen anion located between
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8073 aluminum and potassium cations should show basic character. The corresponding adsorption states of ammonia and C02 on site III are represented by species IIIa and W.Further addition of potassium leads to weaker acidity and stronger basicity, such as the conversion of site 111to form site IV. These latter sites involve a larger number of potassium cations and fewer aluminum cations. Accordingly, the Lewis acid sites are weaker, and the oxygen anions are more basic. For example, species IVa shows ammonia adsorption on a weak Lewis acid site, and species IVb shows C02 adsorbed on an oxygen anion with two adjacent potassium cations. The IR bands corresponding to bidentate carbonate species associated with species IVb are near 1640 and 1305 cm-I. Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the U S . Department of Energy and through a Joint China-U.S. Cooperative Research Grant administered by the National Science Foundation. We acknowledge G. Yaluris, B. Spiewak, B. Handy, R. Casper, and B. Grady for their help during experiments performed for this project. References and Notes (1) Mross, W. D. Catal. Rea-Sei. Eng. 1983, 25, 581. (2) Tanabe, K. Solid Acids and Bases and Their Catalytic Properties; Kodansha: Tokyo, Japan, 1970. (3) Tanabe, K., Hattori, H., Yamaguchi, T., Tanaka, T., Eds. Acid-Base Catalysis; Kodansha: Tokyo, Japan, 1988. (4) Cardona-Martinez, N.; Dumesic, J. A. J . Caral. 1990, 125, 427. (5) Cardona-Martinez, N.; Dumesic, J. A. J . C a r d 1991, 128, 23. (6) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (7) Gervasini, A.; Auroux, A. J . Catal. 1991, 131, 190. (8) Deeba, M.; Hall, W. K. 2.Phys. Chem. 1985, 144, 85. (9) Mulcahy,F. M.;Kozminski,K.D.;Slike,J.M.;Cicwne,F.;Scierka, S.J.; Eberhardt, M. A.; Houalla, M.; Hercules, D. M. J . Catal. 1993, 139, 688. (10) Ai, M. J . Card 1986, 100, 336. (1 1 ) Rossi, P. F.; Busca, G.; Lorenzelli, V.; Lion, M.; Lavalley, J. C. J. Caral. 1988, 126, 208. (12) Kurokawa, H.; Kato, T.; Kuwabara, T.; Ueda, W.; Morikawa, Y.; Moro-Oka; Ikawa, T. J . Catal. 1990, 126, 208. (13) Bakshi, K. R.; Gavalas, G. R. J . Catal. 1975, 38, 312. (14) Bloch, H. S.US Patent 3391218, 1968. (15) Davis, R. J.; Derouane, E. G. Nature 1991, 349, 313. (16) Breysse, M.; Claudel, B.; Prettre, M.; Veron, J. J . Catal. 1972, 24, 106. (17) Malinowski, S.;Szczepanska, S.J . Catal. 1967, 7,67. (18) Ai, M. J . Catal. 1975, 40, 318. (19) Ai, M. J . Catal. 1978, 52, 16. (20) Benesi, H. A.; Winquist, B. H. C. Adu. Caral. 1978, 27, 98. (21) Cardona-Martinez, N.; Dumesic, J. A. Ado. Catal. 1992, 38, 149. (22) Gervasini, A.; Auroux, A. J . Thermal Anal. 1991. 37, 1737. (23) Eberly, P. E. J . Phys. Chem. 1968, 72, 1043. (24) Hughes, T. R.; Hughes, H. M. J. Phys. Chem. 1967, 71, 2192. (25) Parry, E. P. J . Caral. 1963, 2, 371. (26) Utterhoven, J. B.;Christner, L. G.; Hall, W. K. J . Phys. Chem. 1967, 69, 2117. (27) Ward, J. W. J . Catal. 1967, 9, 225. (28) Ward, J. W. J . Catal. 1968, 10, 34. (29) Tanaka, T.; Kumagai, H.; Hattori, H.; Kudo, M.; Hasegawa, S.J . Catal. 1991, 127, 221. (30) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (31) Xie, Y. C.; Qian, M. X.; Tang, Y. Q. Sci. Sin. Ser. B 1984, 27, 549. (32) Handy, B. E.; Sharma, S.B.; Spiewak, B. E.; Dumesic, J. A. Meas. Sci. Technol. 1993, 4, 1350. (33) Tsyganenko, A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J. Mol. Struct. 1975, 29, 299. (34) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface ojTransition Metal Oxides; John Wiley & Sons: New York, 1990. (35) Parkyns, N. D. J . Phys. Chem. 1971, 75, 526. (36) Kantschewa, M.; Albano, E. V.; Ertl, G.; KrBzinger, H. Appl. Catal. 1983, 8, 71. (37) Spiewak, B. E.; Handy, B. E.; Sharma, S.B.; Dumesic, J. A. C a r d Lett. 1994, 23, 207. (38) Huheey, J. E. Inorganic Chemistry: Principles of Structure and Reactiuity; 3rd 4.;Harper & Row, Publisher, Inc.: New York, 1983; p 936. (39) KnSzinger, H.;Ratnasamy, P. Coral. Reu.-Sci. Eng. 1978,17(1), 31. (40) Scokart, P. 0.;Rouxhet, P. G. J . Colloid Interface Sci. 1982,86,96. (41) Scokart, P. 0.; Amin, A.; Defosse, C.;Rouxhg, P. G. J . Phys. Chem. 1981, 85, 1406.
