J. Phys. Chem. 1981, 85, 2543-2546
2543
Determination of I R Extinction Coefficients for Linear- and Bridged-Bonded CO on Supported Palladium M. Albert Vannice‘ and S-Y. Wang Department of Chemlcal Englneerlng, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: March 3 1, 198 1; In Final Form: June 2, 198 1)
Infrared spectra were obtained of CO adsorbed on Pd dispersed on four supports: 7-A1203,SO2,Si02-A1203, and Ti02. All catalysts were characterized by both H2 and CO chemisorption which allowed the calculation of the surface coverages of both linearly adsorbed CO, which gives a band above 2000 m-l,and the bridged-bonded form, which produces a band below 2000 cm-’. From these values, integrated absorbances, A, and extinction coefficients, e, have been determined for each adsorbed species on Pd for the first time. For individual catalysts, absorbances and extinction coefficients were always much higher for the bridged-bonded CO than the linearly adsorbed co, i.e., average values were Ab = 85 X lo’ cm mol-’, A, = 3.3 X 10’ cm mol-’, €b = 4.1 X lo6 cm2 mol-’, and €1 = 0.36 X lo6 cm2mol-’. Because a specially designed IR reactor cell was utilized, spectra were also taken under steady-state CO hydrogenation reaction conditions to form CH4. The A1203-, SO2-,and Si02-A1203-supportedPd catalysts still exhibited strong CO bands; however, Pd/Ti02 wafers exhibiting SMSI (strongmetal-support interactions)showed no detectable CO bands although they were the most active catalyst. Using the E values obtained in this study, we estimated the maximum surface coverage on Pd to be less than 1%for the linear species and 0.1% for the bridged species.
Since the first application of IR spectroscopy by Eischens and co-workers to study supported catalysts,’ this technique has been in the forefront of approaches to study adsorbed molecules on metal surfaces. Although numerous papers have dealt with CO adsorption on group VI11 metals, few studies have quantitatively related the intensities of the observed spectra for adsorbed CO with surface Only Pt and Rh have been examined until now, and no such studies have been reported for palladium. Undoubtedly this is a consequence of the requirement that accurate CO concentrations on the metal must be known at the time the spectrum is recorded, and the design of most IR cells has not allowed these uptake results to be obtained. In addition, many CO spectra have been obtained after evacuation, which can alter surface coverage and complicate the determination of CO surface concentrations. More than one IR band for adsorbed CO frequently occurs on metal surfaces, and typically a high-frequency (HF) band exists between 2000 and 2100 cm-l along with a low-frequency (LF) band between 1800 and 2000 cm-’ which is apt to be much broader and may appear to contain several p e a k ~ . ~ pThe ~ J ~HF band is usually associated with linearly adsorbed (single site) CO whereas multiply coordinated CO is assumed to give rise to bands in the LF region.’J’ It would be especially informative if extinction (1)Eischens, R. P.; Francis, S. A.; Pliskin, W. A. J. Phys. Chem. 1956, 60,194. (2) Eischens, R. P.; Pliskin, W. A. Adu. Catal. 1958, 10,1. (3) Darensbourg, D. J.; Eischens, R. P. “Catalysis”; Hightower, J. W., Ed.; North Holland: Amsterdam, 1973; p 371. (4) Heyne, H.; Tompkins, F. C. Trans. Faraday SOC.1967,63, 1274. (5) Shigeishi, A.; King, D. A. Surf. Sci. 1976, 58, 379. (6) Seanor, D. A.; Amberg, C. H. J. Chem. Phys. 1965, 42, 2967. (7) Vannice, M.A.; Twu, C. C.; Moon, S. H. Submitted for publication. (8)Duncan, T. M.; Yates, J. T., Jr.; Vaughn, R. W. J. Chem. Phys. 1980,73,975. (9) Little, L. H.“Infrared Spectra of Adsorbed Species”; Academic Press: New York, 1966. (10)Hair, M. L. “Infrared Spectroscopy in Surface Chemistry”;Marcel Dekker: New York, 1967. 0022-3654/81/2085-2543$01.25/0
coefficients, associated with band maxima, and integrated absorbances, which are related to the area under each band, could be obtained for each of these two major bands. Such individual values have been published only recently for CO adsorbed on Rh and Pt,‘ls and values for other group VI11 metals have not yet been reported, principally because of the difficulty in determining the representative fractions of linearly adsorbed CO and of multiply bonded CO, which is most likely to be the bridged-bonded (2-fold coordination) species. In a recent study, we obtained IR spectra for CO chemisorbed on Pd dispersed on four different support materials (7-A1203,Si02, Si02-A1203and TiOJ and each catalyst was carefully characterized by both CO chemisorption and hydrogen chemisorption measurements.12J3 This allowed the calculation of fractional coverages of linear- and bridged-bonded CO and the estimation of extinction coefficients and integrated absorbances for each CO band. The influence of Ti02 on CO adsorption on Pd was of special interest because of SMSI behavior which can be induced in this catalyst system.14 This paper reports these results.
Experimental Section The Catalysts were prepared with aqueous impregnation techniques by using PdClz (Ventron Corp.), and after impregnation the samples were dried in air at 393 K for 16 h, bottled, and stored in a desiccator. The support materials used were Grade 57 SiOzand Grade 979 Si02-A1203 (Davison Chemical Co.), P-25 TiOz (Degussa Co.), and 7-A1203donated by Exxon Research and Engineering Co. The H2 (99.999%), CO (99.99%), and He (99.9999%), all from the Linde Co., were further purified before use. A Deoxo unit (Engelhard) and an Oxy-trap (Alltech Assoc.) were used for hydrogen, an Oxy-trap was employed for the helium, and the CO was passed through a molecular sieve (11)Bradshaw, A. M.;Hoffmann, F. M. Surf. Sci. 1978, 72,513. (12) Wang, S-Y., Ph.D. Thesis, Pennsylvania State University, 1980. (13) Vannice, M. A.; Moon, S. H.; Wang, S-Y. J. Catal. In press. (14) Tauster, S. J.; Fung, S. C.; Garten, R. L. J.Am. Chem. SOC.1978, 100,170.
0 1981 American Chemical Society
2544
The Journal of Physical Chemistry, Vol. 85, No. 17, 1981
trap. Weight loadings were determined by using neutron activation analysis. Details on the materials and preparative technique are provided elsewhere.13J5 Adsorption measurements were performed in a conventional, mercury-free, glass adsorption system capable of achieving an ultimate vacuum near 4 X torr (5.3 X Pa). Pressures during adsorption runs were measured with a Texas Instruments precision pressure gage. This system has been described previo~s1y.l~The IR spectra were obtained in a specially designed cell which could be heated above 700 K to allow in situ pretreatment (reduction) in Hz and could also be operated as a differential, plug-flow reactor, if desired, to obtain spectra of CO adsorbed on a catalyst surface under reaction conditions.l3Js All spectra were recorded on a Perkin-Elmer 580 dual-beam spectrophotometer in which a pure support wafer was placed in the reference beam and the supported catalyst wafer was placed in the sample beam. The recorded spectra represent the difference in transmission between the beams passing through these two wafers, and this technique very efficiently minimized or eliminated contributions of gas-phase CO and CO physically adsorbed on the support surface because identical gas mixtures were flowed through each cell. As a result, spectra under CO pressures as high as 500 torr could easily be obtained; however, all results reported here were at a constant CO pressure of 190 torr (0.25 atm). The total pressure in the cell was 1atm (101 kPa) with the balance composed of He. The wafers normally weighed 50-60 mg and were 0.2 mm thick after being pressed into the stainless steel cylinder which comprised the cell. The cell design forced gas flow through the wafer, and high space velocities with low pressure drops could be attained. Precise details of the design of this cell and the IR system have been previously published. l6 The Pd/A1203,Pd/Si02, and Pd/SiOZ-Al2O3catalysts were reduced 1 h at 673 K in flowing H2in the adsorption system following a specified pr0~edure.l~ Two different Pd/Ti02 catalysts were prepared by using the two reduction procedures of Tauster et al.;14 i.e., reduction at 448 K provided a Pd/Ti02 catalyst with normal adsorption behavior while reduction at 773 K produced an SMSI state which markedly suppressed Hz and CO ads0rpti0n.l~ Hydrogen and CO uptakes were measured by using the dual isotherm technique of Benson, Hwang, and Boudart17 to determine chemisorbed hydrogen at 300 K, and the method of Yates and Sinfelt'* was employed to determine irreversibly adsorbed CO at 300 K. With the exception of Pd/Ti02, these pretreated catalyst powders were used, after passivation in air, to press the wafers for the IR study, and the wafers, including the reference wafer, were then reduced in situ at 548 K. After this reduction step, adsorbed hydrogen was removed by flowing He through the cell a t 548 K. The absence of H2 in the gas phase was determined by using a gas chromatograph to analyze the exit stream from the IR cell. The wafers were then cooled under flowing He to 300 K and a baseline was obtained in pure He before CO was introduced into the gas stream. The Pd/Ti02 wafer was pressed from fresh, unreduced catalyst powder and IR spectra were obtained after an in situ reduction at 448 K. The wafer was then further reduced at 773 K to induce SMSI behavior and cooled in He to 300 K, and the spectra were obtained. (16)Palmer, M. B., Jr.; Vannice, M. A. J. Chem. Tech. Biotechnol, 1980, 30, 205. (16)Vannice, M. A.; Moon, S. H.; Twu, C. C.; Wang, S-Y. J. Phys. E. 1979, 12, 849. (17) Benson,J. E.; Hwang, H. S.;Boudart, M. J.Catal. 1973,30,146. (18) Yates, D. J. C.; Sinfelt, J. H. J. Catal. 1967, 8, 348.
Vannice and Wang I
I
I
I
I
I
T
I 22011
I
L
I
21100
I
I
1800
w (an-')
Figure 1. Transmittance spectra of CO adsorbed on Pd at 300 K, P, = 190 torr, P, = 570 torr: (A) 1.98% Pd/A1901, (B)1.86% PdITIO. (448 K), (C) 1.53% Pd/SI02; (0) 2.12% Pd/&Ai20~, (E) i.86./0' Pd/TI02 (SMSI).
TABLE I: CO Bands in He at 300 K HF (A) 1.98% Pd/s-A1,0, 2085 (B) 1.86% Pd/TiO, 2096 (C)1.93% Pd/SiO, 2085 ( D ) 2.12% Pd/SiO,-Al,O, 2098 (E) 1.86% Pd/TiO, (SMSI) 2058
LF 1970 (s), 1930 1926,1890 (s) 1985,1938 1965 1925
Results and Discussion The spectra obtained at 300 K (and higher temperatures), along with baseline behavior in the absence of CO, are reported elsewhere,13but the transmittance spectra are shown again in Figure 1 for comparison purposes. The narrower HF band and the broader, more dominant LF band typically found for supported Pd catalysts are clearly evident. The baseline intensity, To,for each support was superimposed on the CO spectra in the region between 2100 and 2600 cm-', and relative intensities, TIT,, were calculated on this basis. The CO peak positions in Figure 1 are listed in Table I. Small shifts (5-10% T )were sometimes observed in the baseline obtained in pure He and that measured during the spectrum for adsorbed CO, which introduced some uncertainty in the absorbance values. The integrated form of the Beer-Lambert law is a(v) = (Cl)-' In T o / T ,where a(v)is the absorption coefficient, which can be dependent on the radiation frequency, v, C is the molar concentration of CO in the sample, I is the distance through the sample through which the IR beam penetrates, and T and Toare the transmitted radiation intensities in the presence and
The Journal of Physical Chemistty, Vol. 85, No. 17, 198 1 2545
Linear- and Bridged-Bonded GO on Supported Palladium
TABLE 11: Extinction Coefficientsa and Integrated Absorbancesb for CO Adsorbed on Pd at 300 K chemisorption on used sample, 10-7x 10-7x io-’X 10-6 x pmol g-’ A,
H*
catalyst 1.98% Pd/q-Al,O, 1.93% Pd/SiO, 2.12% Pd/SiO,-Al,O, 1.86% Pd/TiO, (448 K)
19 6 16 16d
1.86% Pd/TiO, (SMSI) 1.86% Pd/TiO, (SMSI)
CO
col/cob
cm/mol
23 11 27 41d
0.53 10 4.4
25 19 6.9 6.8 7.6 9.2 14 14 0.042
2.0 1.5e
7.0 5.0e
average gas-phase CO (ref 21) E = log (T,/Tmax)/(CZ). takes at 548 K.
A =J
u
dv/(CZ).
Al,
cm/mol cm/mol
At band maxima in Figure 1.
absence of the adsorbate, respectively. From a plot of In (To/T)vs. v, such as those shown in Figure 2, the total intensity of a given band was obtained by graphical integration using a planimeter. For each catalyst, an overall integrated absorption coefficient, A = ~,,‘a(v)dv, was obtained based on all adsorbed CO species under both bands using CO uptakes per gram determined in the adsorption experiments and the diameter of the wafer (diameter = 15.9 mm). The sample thickness, 1, is not needed because C1= Um/A where U is uptake in mole of CO g-l, m is the wafer mass, and A is the cross-sectional area of the wafer. Integrated absorption coefficients for the HF band, A,, and the LF band, Ab, and extinction coefficients determined at the frequencies corresponding to these two band maxima, el and q, where E = 1/C1log (To/T-), cannot be determined unless the fractions present as linearly adsorbed CO, COI,and bridged-bonded CO, cob, are known. The assumption that these two forms of adsorbed CO predominate on Pd is supported by the model of Bradshaw and Hoffmann, which proposes that CO in threefold or higher coordination should have frequencies below 1880 cm-l.ll Since intensities are quite low below 1880 cm-l (Figure I), this assumption seems justified; however, this is the only assumption which allows an estimation of these four parameters. Because hydrogen adsorption was also measured on these catalysts and its adsorption stoichiometry is established, i.e., H,d/Pd, is near unity,” where Pd, is a surface Pd atom. This technique provides a value for the total number of surface Pd atoms (Xp,J. It is assumed that a surface Pd atom interacts with no more than one CO molecule, either in the bridged form (COb/Pd, = 1/2) or in the linear form (COl/Pd, = 1). The assumption that only one- and two-site CO adsorption occurs produces two straightforward algebraic equations involving CO and H2 uptakes from which surface fractions of linear and bridge-bonded CO can be determined, i.e.
2(Hz uptake) = Xpd,
[cob]+ [CO,] = CO uptake The values of AI, Ab, E,, and tb based on these fractions are listed in Table 11. Because some decrease in the Pd surface area occurred during the course of these studies, adsorption values on used samples, listed in Table 11, were employed in these calculations. The altered state of adsorption on Pd/Ti02 samples, especially H2 adsorption on the SMSI sample, prevented calculation of values for individual bands and only an overall A value could be determined for these two catalysts.
Ab,
6.7 1.4 1.9
35 190 29
3.3
85
x
fbC
EbrC
cm2/mol
cm*/mol
0.66 0.16 0.26
2.1 9.3 0.87
0.36 0.00018
4.1
Based on fresh, reduced catalyst.
e
Up-
el >> study. The usefulness of these values for A and 6 is demonstrated by in situ studies in which simultaneous kinetic data and IR spectra were obtained for Pd methanation catalysts under well-defined, steady-state reaction conditions after H2was substituted for flowing He at 548 K.13 High band intensities similar to those at 300 K were observed for typical Pd catalysts under reaction conditions at 548 K, indicating high surface coverages of CO; however, the Pd/Ti02 (SMSI) catalyst exhibited markedly different behavior, no IR bands were detectable under reaction conditions, as shown in Figure 3, despite its high activity. The results in Table I1 show that A values are very similar for CO adsorbed at 300 and 548 K and are not strongly dependent on temperature. Therefore, if the average q and €b values in Table I1 are also assumed not to be strongly dependent on temperature or on adsorbed hydrogen, maximum surface coverages on Pd under reaction conditions can be estimated. The first assumption is supported by the spectrum of CO in He obtained 548 K for the Pd/Ti02 (SMSI) catalyst. The CO uptake was 5.0 pmol g-l at 548 K resulting in an A value of 14 X lo-’, as shown in Table 11, which is very close to values obtained a t 300 K. The second assumption is supported by the study Conrad, Ertl, and Latta, which showed that essentially no interaction occurs between adsorbed CO and hydrogen on Pd single-crystal surfaces.24 With a minimum sensitivity of 1%transmittance for each peak, the minimum detectable coverage of linearly adsorbed CO is 0.39 pmol g-l and the minimum coverage of bridged-bonded CO is 0.033 pmol g-l. Assuming the total Pd surface area measured after the 448 K reduction is not changed by the 773 K reduction, less than 1% of the Pd surface is covered by the HF CO species and less than 0.1% is covered by the LF CO species. The evidence that Pd surfaces of very active methanation catalysts, such as Pd/Ti02 in the SMSI state, can have very low CO coverage has led us to conclude that only a small fraction of Pd surface atoms constitute “active sites” for methane formation,26and illustrates the usefulness of in situ IR characterization of catalysts under well-defined reaction conditions. Acknowledgment. Equipment used in this study was purchased under Contract No. EG 778-02-4463, Department of Energy, Division of Basic Energy Sciences. Additional funds were provided by a grant from the Research Corporation. Support for S-Y. Wang was provided by the government of the Republic of China in Taiwan. (21) Twu, C. C., Ph.D. Thesis, Pennsylvanic State University, in progress. (22) Blyholder, G.; Allen, M. C. J. Am. Chem. SOC.1969, 91, 3168. (23) Norton, P. R.;Goodale, J. W.; Selkirk, E. B. Surf. Sci. 1979,83, 189. (24) Conrad, H.; Ertl, G.; Latta, E. E. J. Catal. 1974, 35, 363. (25) Wang,S-Y.; Moon, S. H.; Vannice, M. A. J . Catal. In press.
