Langmuir 1995,11, 2592-2599
2592
Active Sites on SiOa: Role in CH30H Decomposition Edward A. Wovchko, John C. Camp, John A. Glass, Jr., and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received November 3, 1994. In Final Form: April 17, 1995@ The dehydroxylation of Si02 and adsorption and thermal decomposition of CH30H on dehydroxylated Si02 prepared at extreme temperatures up to 1475 K has been studied using transmission infrared spectroscopy. Formation of Si-0-Si linkages (as evidenced by 888 and 908 cm-l bands) is directly related to the loss of the isolated SiO-H band at 3748 cm-l during Si02 dehydroxylation to produce H2O. Highresolution (0.75 cm-l) studies of the isolated SiO-H band show sharpening and a slight 4 cm-' shift to higher frequency with decreasing -OH coverage and may be explained by site inhomogeneity effects. Adsorption of CH30H on dehydroxylated Si02 occurs first by reaction with SiOSi sites, followed by exchange with SiOH groups at elevated temperatures leading to SiOCH3 surfaces species. CH3180H adsorption studies demonstrated that adsorption on SiOSi sites occurs by cleavage of the methanol 0-H bond forming SiOH and SilsOCH3. Thermal decomposition of SiOCH3groups creates SiH, ( x = 1,2)species and prevents formation ofthe SiOSi sites indicative ofthe production of free radical sites at the surface. At temperatures above 1300 K, SiH, decomposition leads to SiOSi site formation. 1. Introduction Much of the early work on amorphous silica surfaces was devoted to the characterization of surface silanols using infrared spectroscopy.l It is widely accepted2that several types of SiOH species exist: isolated, geminal, and hydrogen-bonded silanols as schematically shown below. OH I
/%
000 isolated
OH OH \\ ,/ Si
o"0 geminal
OH- - - - - O H I
I
/R /?,
0 0 0 0 0 0 hydrogen - bonded
However, after 30 years of investigation there is still controversy as to the exact assignment of the species observedby infrared spectroscopy.2-6The sharp 3748 cm-l SiO-H stretching band has been generally attributed to a free isolated SiOH surface species. Many believe this band also contains a spectral contribution from the geminal species based on silicon-29 CPMAS NMR studies.'-1° Silica has also been subjected to various thermal activation processes and chemical modifications in a n attempt to create surfaces of greater than usual activity.2J1 The most simple method of activation is dehydroxylation a t elevated temperatures. If silica is treated a t extremely high temperatures (> 1000 K) under evacuation, active sites develop accordingto eq 1.2J1 Evidence for this process SiOH
+ SiOH
-
SiOSi
+ H,O(g)
(1)
can be seen in the infrared spectrum a t 888 and 908 cm-', Abstract published in Advance A C S Abstracts, June 15,1995. (1)Hair, M. L. Infrared Spectroscopy in Surface Chemistry;Dekker: New York, 1967. (2)Boehm, H.-P.;Knozinger, H. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1983;Vol. 4, Chapter 2. (3)Hoffmann, P.; Knozinger, E. Surf. Sci. 1987,188,181. (4)Ryason, P. R.; Russell, B. G. J . Phys. Chem. 1975,79,1276. ( 5 ) Morrow, B.A.; Cody, I. A. J . Phys. Chem. 1973,77, 1465. (6)Morrow, B. A.; McFarlan, A. J. J . Non-Cryst. Solids 1990,120, 61. (7)Morrow, B. A.;Gay, I. D. J . Phys. Chem. 1988,92,5569. (8) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. SOC.1983,105,1487. (9)Haukka, S. Ph.D. Thesis, University of Helsinki, 1993. (10)Haukka, S.;Lakomaa, E.-L.; Root, A. J . Phys. Chem. 1993,97, @
5085.
(11)Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
located in a spectral window region between the two completely absorbing low frequency Si02 bulk modes. Morrow has described this site as an asymmetrically strained siloxane bridge with electron deficient Lewis acid centers.12 The electron-deficient Si atoms are situated in a position to accept a pair of electrons and achieve greater than 4-fold bonding geometryto oxygen atoms. Later work by Bunker et al.13suggested that two distinct edge-shared tetrahedral defects exist, giving large Si-0 bond strain. One silica site is described as being attached to the surface by four bridging oxygens, the other being a silicon atom bonded with three bridging oxygens and a terminal hydroxyl. The former site is classified as being more reactive. The activated sites involving bridging oxygen atoms are capable ofvarious dissociative chemisorptionreactions. It has been reported that the sites provide both acidic and basic character. For example, in the reaction with BF3,14 the bridged oxygen atom is susceptible to attack by the electron-deficient boron atom causing OSi-0 bond scission. The basic nature of the bridging oxygen atom is apparent since the surface becomes terminated with SiOBF2 species. The sites are consumed completely (as evidenced by a loss of the 888 and 908 cm-l features) in the reaction with BF3 before any BF3 reaction with SiOH groups takes place. In other cases the presence of surface silanols completely prevents chemisorption of other molecules. Dubois and Zegarski found that surface silanol groups strongly inhibit the adsorption of CH3Si(OCH& a t 330 K.15 With fully hydroxylated Si02 surfaces, essentially no adsorption occurs. By increasing the degree of dehydroxylation, the extent of the reaction is found to increase. Methanol also reacts readily with Si02 and has motivated much research to identify and characterize the resultant SiOCH3 surface species by infrared spectros c ~ p y . ' ~Low J ~ and co-workers studied this with a strong focus on high-temperature pyrolysis of the methoxylated (12)Morrow, B. A.;Cody, I. A. J . Phys. Chem. 1976,BO, 1995. (13)Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Surf Sci. 1989,222,95. (14)Morrow, B. A.;Devi, A. J . Chem. Soc., Faraday Trans. 1 1972, 68,403. (15)Dubois, L. H.; Zegarski, B. R. J . Phys. Chem. 1993,97,1665. (16)Borello, E.; Zecchina, A,; Morterra, C. J . Phys. Chem. 1967,71, 2938. (17)Morrow, B. A.J . Chem. Soc., Faraday Trans. 1 1974,70,1527.
0743-746319512411-2592$09.00/0 0 1995 American Chemical Society
Langmuir, Vol.11, No. 7,1995 2593
Active Sites on Si02 surface. 18s9 They discovered that the resulting “reactive silica” was capable of adsorbing molecules that were not reactive on conventionally dehydroxylated silicas. The presence of thermally stable silicon hydrides following this process provided evidence for a surface containing silicon atoms with radical-like behavior associated with a reactive bridging 0-0 linkage.18-20The “reactivesilica” was also found to adsorb and polymerize olefins via a silicon radical-initiated chain propagation reaction.21 We have chosen to examine, in detail, these two hightemperature activation processes on SiOz, namely dehydroxylation of the silica and pyrolysis of methoxylated silica. First, a step-by-step dehydroxylation of Si02using small temperature intervals was carried out, focusing on the isolated SiO-H spectral region and the Si-0-Si spectral region. The reaction of CH30H on highly dehydroxylated Si02 was also studied with the intention of following the two adsorption processes-reaction with SiOSi sites, and exchange with SiOH groups. Second, a detailed thermal decomposition study of the chemisorbed methoxy species was conducted in an effort to observe surface hydride and silanol formation as well as spectral developments in the Si-0-Si region. These studies illustrate the power of FTIR spectroscopy, coupled with modern vacuum technology, for enhancing our understanding of the complexities of Si02 surface chemistry.
2. Experimental Section All experiments were conductedin a special infrared cell. The design and use are similar to those described previously.22The cell consists of a stainlesssteel cube with six-conflatflange porta. Two ports are equipped with differentially pumped KBr optical windows sealed with double Viton O-rings. Si02 samples are spray deposited onto a tungsten grid (optical transparency = 70%)mounted on an electrical feedthrough and positioned in the center of the cell. The sample can be rotated in vacuum 360” about the vertical axis by means of a Thermionics differentially pumped rotary seal. In addition, the cell may by translated laterally in the infrared spectrometer. This facilitates appropriate positioning for sample and gas phase scans. Heating is achieved electronically with a digital temperature programmed controller. The sample can be cooled by filling the Dewar containingthe feedthroughwith an ethanol-liquid nitrogen bath. The sample temperature can be held constant to fl K over a temperature range of 150-1500 K and is measured by a K-type thermocouple spot welded on the top center of the support grid. The samples were heated and cooled linearly to specified temperatures using the electronic temperature programmed controller at rates of approximately 1.6 Ws.Both pumping and gas delivery is achieved through a single port flexibly connected with a stainless steel bellows to a bakeable stainless steelvacuum Torr system. The vacuum system can be pumped to 1 x using a liquid nitrogen cooled zeolite sorption pump, and further evacuated by 50 Us turbomolecular and 20 Us ion pumps. Base pressures 1 x 10-8 Torr are obtained after approximately 16 h of evacuation. The system is equipped with a programmable UTI quadrupole mass spectrometer for gas and thermal desorption product analysis and for leak checking. Gas pressures are measured by 0-10 Torr and 0-1000 Torr Baratron capacitance manometers. Silica samples are prepared by ultrasonically dispersing 1 g of Si02 (Degussa Aerosil, 200 m2/g)in 20 mL of distilled HzO for 30 min and mixing with 160 mL of acetone (Mallinckrodt, A.R.). This slurry is sprayed onto the warmed (-335 K) tungsten grid (0.0254mm thick, 5.2 cm2 total area) using a nitrogen gas pressurized atomizer. Spraying is interrupted intermittentlyto allow for solvent evaporation. The amount of sample deposited can be regulated by adjusting the total time of spraying to give loadings ranging from 14 to 22 mg (2.7-4.2 mg/cm2). The grid (18) Morterra, C.;Low, M. J. D. J. Phys. Chem. 1969,73, 321. (19)Morterra, C.;Low, M. J. D. J. Phys. Chem. 1969,73, 327. (20) Low, M. J. D. J. Catal. 1974, 32, 103. (21) Low, M. J. D.; Mark, H. J. Catal. 1977, 50, 373. (22) Basu, P.; Ballinger, T. H.; Yates, J. T.,Jr. Rev. Sci. Znstrum. 1988,59, 1321.
P=,.bTorr
R = o 75 cm”
2984
2980
2976
2848
2840
Wavenumber (cm-’) Figure 1. Infrared spectra of selected regions of gas phase CHs160Hand CH3180H. (Spectra have been normalized and baseline corrected.) containing the silica sample is then transferred to the infrared cell and evacuated at 300 K(or elevated temperatures if a higher degreeof degassing is desired) for 16 h. In experiments involving methanol adsorption, the Si02 was heated in vacuum at 900 K for 16 h and heated to 1300-1340 K for 15 min in order to maximize the 888 and 908 cm-l intensity. All infrared spectra were recordedusing a nitrogen gas purged Mattson Research Series I Fourier transform infrared spectrometer. Data were processed and manipulated using Mattson Fourier infrared software tools (FIRST). The spectrometer was equipped with a liquid nitrogen cooled HgCdTe detector and is capable of obtaining spectra from 5000 to 400 cm-l at 0.25 cm-l resolution. In this study, spectra were measured by averaging 1024 or 1500 scans at spectral resolutions of either 0.75 or 2 cm-l. Single beam spectra due to the KBr windows were measured by translating the cell and rotating the Si02 sample 90”out of beam. Absorbance spectra were obtained by ratioing single beam spectra of the Si02 sample to the window single beam spectra. All spectra were recorded at 300 K under evacuation at pressures less than 4 x 10-6 Torr (with exception to Figure 1 where gas pressures were 1 Torr). Anhydrous CH3OH (Baker, 99.95%), and isotopically labeled CHPOH (95% l80)were vacuum transferred to preevacuated and baked glass bulbs. Five freeze-pump-thaw cycles were performed on each prior to use. The vacuum system was pretreated with two 1 Torr, 1 min cycles of the desired methanol adsorbate, followed by evacuation before a third gas dose was used to achieve exposure to the cell and the sample. Selected regions of the gas phase (1Torr pressure) of CH316OHand CHPOH are shown at 0.75 cm-l resolution in Figure 1. These spectra clearly show that vaIm(CH3)for CH30H(g)and vs,(CH3) for CH3OH(g) display band contours which are isotopically (l80substitution) shifted by -0.4 cm-l (va,,(CH3)) and -1 cm-’ (vs,(CH3)); the rotational fine structure is not resolved. This shift differs from lower resolution results reported in the literat~re.~~,2* This information will be needed for spectral interpretation of our results for SiOCH3surface speciesproduced by methanol adsorption on SiOz.
3. Results 3.1. Dehydroxylation of SiOz: Analysis of the SiO-H and Si-0-Si Regions. Figure 2shows infrared spectra of the SiO-H and Si-0-Si regions during dehydroxylation of Si02 from 300 to 1050 K. This temperature range does not produce major spectral developments in the Si-0-Si spectral region. Broad absorptions centered around 3500 cm-I are attributed to various hydrogen-bonded or associated hydroxyls and persist up to temperatures below 1050 K. The sharp feature around 3748 cm-I is assigned to isolated andor geminal hydroxyl specie^.^ This band will be designated (23) Ioli, N.;Moretti, A.; Pereira, D.; Strumia, F.; Garelli, G. Appl. Phys. B 1989,48,299. (24) Barnes, A. J.; Hallam, H. E. J. Chem. SOC.,Faraday Trans. 1 1970,66, 1920.
Wovchko et al.
2594 Langmuir, Vol. 11,No. 7,1995
20
0.8 0'9 a,
50
e $ 9
L
, , ,P ,,,T,
