Langmuir 1988, 4 , 329-337 primarily to the Ti02 support. It should be noted that Orita et al. have detected adsorbed acetate ions during the formation of acetaldehyde from the hydrogenation of CO over Rh/Si02.21 Prior work in these laboratories suggested that acetaldehyde and acetone were produced from the bridged dirhodium carbonyl species during the hydrogenation of CO over 2.2% Rh/TiOz, while methane was produced from the other adsorbed CO speciesa2Spectrum 3c shows that the only adsorbed CO/Rh species produced during the decomposition of acetaldehyde was the bridged carbonyl species. Spectrum 3d indicates that the presence of potassium ions poisoned, to some extent, the decomposition of acetaldehyde (reduced to 54%) and eliminated the formation of CHI and acetone. Most likely the potassium ions cause the formation of inactive carbon at the expense of the active carbon surface species, which normally yield hydrocarbon and oxygenated products. Figure 4 shows that the decomposition of acetaldehyde over 2.2% Rh/Al2O3again produced CHI, although very little COz or CO, surface acetate, and acetone. Spectrum 4e indicates that the acetate species was primarily confined to the A1203 support. Again potassium ions (spectrum 4d) poisoned the production of CHI and acetone. The most interesting feature appears in spectrum 4b. Upon initial heating at 440 K, twin adsorbed CO bands occurred at 2012 and 2082 cm-'. It is well-known16that the presence of sharp twin bands that grow and decline in intensity together in this region of the spectrum are indicative of the presence of a rhodium gem-dicarbonyl species. However, it is also a fact that these twin bands generally appear at ca. 2030 and 2100 cm-' and refer to a highly dispersed Rh+ gem-dicarbonyl.16 The gem-dicarbonyl bands in spectrum 4b that do grow proportionately appear 18 cm-' lower, which could very well mean that acetaldehyde decomposes over Rh/Al2O3 to yield a rhodium gem-dicarbonyl species with Rh in a zero oxidation state. If so, to our knowledge, this is the first infrared data that have been interpreted as evidence for the existence of metallic
329
rhodium gem-dicarbonyl species. Yates and Cavanagh also observed abnormally low frequency infrared bands (2021 and 2088 cm-l) for a rhodium gem-dicarbonyl species during the decomposition of formaldehyde over 2.2% Rh/A1203,but they attributed the low frequency of the bands as being due to support perturbation of the CO modes by the presence of oxide-bound species derived from HzC0.22 It is possible that the rhodium gem-dicarbonyl species produced from HzCO also involved metallic rhodium rather than the usual Rh+. Finally, a study of acetone decomposition over 2.2% Rh/TiOz and 2.2% Rh/AlZO3was also performed here. However, the results were not inspiring. The infrared bands for gas-phase acetone disappeared, and GC indicated complete decomposition, upon heating at 440 K, but no adsorbed CO species or any other products could be detected by infrared. It would thus appear that acetone decomposes to inactive carbon over supported rhodium.
Conclusions From this study it may be concluded that (1)methanol decomposes over supported Rh at 440 K to produce methane if Ti02 is the support or dimethyl ether if Alz03 is the support; (2) acetaldehyde decomposes over supported Rh at 440 K to produce methane and acetone; (3) acetone decomposes over supported Rh at 440 K to produce inactive carbon only; (4) potassium ions poison the formation of all products by presumably causing the production of large amounts of inactive carbon on the surface; and ( 5 ) the new rhodium gem-decarbonyl state produced during the decomposition of acetaldehyde over 2.2 70 Rh/Alz03probably refers to metallic rhodium rather than the usual Rh+ species.
Acknowledgment. We gratefully acknowledge the support of the Office of Naval Research for this work. Registry No. MeOH, 67-56-1;MeCHO, 75-07-0; MezCO,6764-1; Rh, 7440-16-6. -
(21) Orita,H.; Shuichi, N.; Tamani, K. J. Catal. 1984, 90,183.
(22) Yates, J. T.;Cavanagh, R. R.
J. Catal. 1982, 74, 97.
Nitrogen Adsorption Isotherms on M02-TypeOxide Surfaces David Amati and Ervin sz. Kovits* Laboratoire de Chimie technique de 1'Ecole Polytechnique F6d6rale de Lausanne, CH-1015 Lausanne, Switzerland Received July 7, 1987. I n Final Form: September 3, 1987 Surface area of SiOz (amorphous),TiOz (anatase), and ZrOz (baddeleyite) powders was determined by three methods. First, surface areas were obtained by the BET evaluation of the Nzisotherm. Second, surface areas were calculated from the monolayer capacity of a dense chemically bonded trimethylsiloxy (TMS)layer. Third, surface areas of the nontreated samples were calculated from the Nzadsorption isotherm of powders covered with a dense (3,&dimethylbutyl)dimethylsiloxy (DMB) monolayer. On the basis of the results it was concluded that for the BET evaluation of the untreated powders the same space requirement of the N2molecule has to be applied. In the multilayer adsorption model of Brunauer, Emmett, and Teller,' one of the parameters is identified as the space requirement of the adsorbate molecule in the first adsorbed layer. The proposal of the authors of ref (1)Brunauer, S.; Emmett, P. H.; Teller, E. J.Am. Chem. SOC.1938, 60,309.
1was to use u(N2)= 16.2 Azfor this parameter for nitrogen adsorption independently of the nature of the surface. In the present paper we do not question the model and the resulting equation, which must be wrong on thermodynamical reason~.~f I t will rather be considered as a (2) Camel, H. M. J. Chem. Phys. 1944, 12, 115.
0743-7463/88/2404-0329$01.50/00 1988 American Chemical Society
Amati and sz. Koudts
330 Langmuir, Val. 4, No. 2, 1988
Langmuir equation corrected for multilayer formation regardless of the correction being right or wrong, but it is hoped that the error introduced is systematic if the BET evaluation is systematically performed in the same domain of relative pressure4 ( p , / p , = 0.05 - 0.23).5 Thus the parameter u(N2) will be considered as physical reality. The first question to be put forward concerns the correctness of the value of 16.2 A2. The question of the absolute value of u(N2)was studied experimentally in three principal manners. In one group of papers it is advanced that there exists a "universal isothermne13(or two universal isotherm^'^) in the multilayer region assuming that the potential field attracting N2 molecules is independent of the solid matrix at large enough distances. Pierce8i9proposes an isotherm of the Frenkel-Halsey-Hil11"18 type with an exponent of 2.75. I t was already shown1g that this isotherm is not generally valid further arguments against this hypothesis will be given in this work. A second approach is based on similar arguments, but, in our opinion, it has a more sound experimental basis. Rouqubol et aL20 showed, by extending the method of Harkins and Jura,21 that the heat of immersion in water of an oxide adsorbent with preadsorbed water on its surface approaches a constant at not too high water coverages. It can now be supposed with Harkins and Jura that at these conditions the surface energy (and not the free energy as in the proposals in ref 6-14) is equal to that of pure water. Consequently,the surface area can be calculated from the heat of immersion by using the areal surface energy of water. It is interesting to note that in the excellent experimental work of ref 20 the authors compare specific surface areas determined by this method with those calculated with the BET evaluation of the N2 isotherm. However, they hesitate to advance explicitly the absolute value of ci(N2)= 17.6 A2 on silicon dioxide, though easily calculable from the data given if it is assumed that by the modified Harkins-Jura procedure the true surface area was measured. The third possible determination of the absolute value of the space requirement of the N2molecule in the first adsorbed layer is to compare surface areas calculated from the electron microscope micrograph of a powder having individual grains of regular shape. Pickering et a1.22found d(N2)= 16.2 A2 on titania (anatase); however, we believe that the experimental error was considerable. The determination of the relative cross section of the N2 molecule on a series of model surfaces was described r e ~ e n t 1 y . l ~Silicon ~ ~ ~ dioxide surfaces were chemically (3)Cassel, H. M.J. Phys. Chem. 1944,48,195. (4)Everett, D.H.;Parfitt, G. D.; Sing, K. S. W.; Wilson, R. J.Appl. Chem. Biotechnol. 1974,199,24. (5)Gobet, J.; sz. Kovlts, E. Adsorpt. Sci. Technol. 1984,1, 77. (6)Shull, C. G.J. Am. Chem. SOC.1948,70,1405. (7)Cranston, R. W.;Inkley, F. A. Adu. Catal. 1957,9,143. (8)Pierce, C.J. Chem. Phys. 1959,63,1076. (9)Pierce, C.J. Phys. Chem. 1968,72,3673. (10)Lippens, B. C.; Linsen, B. G.; de Boer, J. H. J.Catal. 1964,3,32. (11)De Boer, J. H.; Linsen, B. G.; Oringa, Th. J. J.Catal. 1965,4,643. (12)Lecloux, A.;Pirard, J. P. J. Colloid Interface Sci. 1979,70,265. (13)Karnaukhov, A. P. J. Colloid Interface Sci. 1985,103,311. (14)Zettlemoyer, A. C.J. Colloid Interface Sci. 1968,28, 343. (15)Frenkel, J. Kinetic Theory o f Liquids; Oxford University Press: Oxford, 1946. (16)Halsey, G. J. Chem. Phys. 1948,16,931. (17)Hill, T. L. J. Chem. Phys. 1949,17,590,668. (18)Hill, T.L. Adu. Catal. 1952,4,211. (19)Amati, D.;sz. Kovlts, E. Langmuir 1987,3,687. (20)Partyka, S.;RouquBrol, J. J . Colloid Interface Sci. 1979,68,21. Jura, G . J. Am. Chem. SOC.1944,66,1362, 1366. (21)Harkins, W. D.; (22)Pickering, H. L.; Ecstrom, H. C. J.Am. Chem. SOC.1952,74,4775. (23)Gobet, J.; sz. Kovlts, E. Adsorpt. Sci. Technol. 1984, 1 , 285.
..... DMB
TMS
Figure 1. The trimethylsiloxy, TMS, and the (3,3-dimethylbutyl)dimethylsiloxy, DMB, group on the surface of M0,-type oxides. M = Si, Ti, and Zr.
modified by a dense monolayer of substituents thick enough to efficiently shield the underlying matrix. With acceptance of the globular uniform model2*for the structure of the nonporous starting material (surface-hydrated Aerosil OX50 from Degussa, FRG),the specific surface area of the modified product could be ~ a l c u l a t e d . ~The ~,~~ relative space requirement, iu,,, was then calculated with eq 1:
,gF$)~p) S:FT)S:P) "a(N2)
=
@Fl':BT)Si$y)
- ,:($W),:$ye)
(1)
where iu,(N2) is the relative cross section of N2and u , ( ~ ~ ) is the monolayer capacity calculated with the BET evaluation of the isotherm of the standard (std) and of the sample (sa). The ratio of the monolayer capacities can, of course, be replaced by that of the specific surface areas, dBET), if they were determined with the same u(N2). The true surface area of the standard (the starting material) was considered to be that originating from the BET evaluation with d(NJ = 16.2 A2 (in this case sg(&ue) = s?:"), and the true surface area of the sample was taken as calculated with the globular uniform model. This method is only applicable for model surfaces prepared on the same starting material. A first method applicable to the comparison of surface areas of different powders is based on irreversible chemisorption. It was proposed25to cover the surface by a chemically bonded monolayer. After treatment the surface was considered to support the densest arrangement of substituents so that the surface density of the layer was determined by the van der Waals diameter of the substituent and not by the number of anchoring points on the surface of the starting material. The trimethylsiloxy substituent (see Figure 1) was shown to be a good compromise in applying the method for adsorbents with hydroxylated surfaces. Its space requirement is large enough, so its surface concentration is considerably less than that of the surface hydroxyls on the starting material which served as anchoring points. On the other hand it is small enough, so that there will be no need for fractal correction on a nonporous sample. This method is applicable for comparison of different powders having the necessary hydroxyl population at the surface. In eq 1the true surface area will be that measured by this chemical method. A second chemical/physical method is proposed in the present paper. It involves a modification of the surface by a dense chemically bonded monolayer of substituents thick enough that the nature of the surface of the products (24)Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (25) Gobet, J.; sz. Kovlts, E. Adsorpt. Sci. Technol. 1984,I , 111.
Nitrogen Adsorption Isotherms on MOz Surfaces Table I. Properties of the Titanium and Zirconium Dioxide SamDlesO s(16),
sample Titandioxid P25 Ti02/Hy Dynazirkon F ZrOz/HY
d,P, treatment m2 g-1 C g cm-3 e as received 56.1 f 0.7 105 0.29 H2O/95 "C/72 h 54.9f 0.7 117 0.63 0.61 as received 42.3f 0.6 80 0.72 HzO/95 OC/72 h 42.4 f 0.6 107 1.56 0.67
The symbol s(16)represents the specific surface area, where the N2 isotherm was evaluated with the BET.method with ci(N2) = 16.2A2 (average of three determinations). C is the constant of the BET equation, dsppis the apparent density of the sample, and e is the porosity.
obtained with different starting materials can be considered the same. In this case it can be supposed that the adsorption properties of the modified powders are the same; consequently, their surface areas can be compared by the a,-method26or by comparison of BET areas evaluated with the same ci(N2).From the results, the surface area of the underlying matrix is calculated by applying the globular uniform model. This result is then used as the true surface area in eq 1. Actually, this procedure was already invented by Rouqu6rol et ale2' The surface area of titania powders was compared with that of the same powder covered by a thin silicon dioxide layer deposited by the method of Iler.% For the present purpose the bulky (3,3-dimethylbutyl)dimethylsiloxylayer is proposed (see Figure 1: DMB). It was shown by wetting experiments that a dense DMB monolayer masks very efficiently the underlying matrix.29 In the present project the space requirement of the N2 molecule is studied on titania and zirconia relative to that on silicon dioxide as a reference by applying the methods exposed in this introduction.
Experimental Section General. Titanium and zirconium dioxide samples were stored and handled in a drybox from Mecaplex AG (Grenchen, Switzerland; Model GB-80) in an argon atmosphere containing
