8879
J. Phys. Chem. 1991, 95, 8879-8881
Infrared Spectroscopy at High Pressure. Adsorption of Dinitrogen on Supported Rhodium in the Absence of Carbon Monoxide J. P. Wey, W. C. Neely,and S. D. Worley* Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: January 29, 1991)
A primary limitation in using FTIR to study adsorbed species on supported catalysts in the presence of high pressures of reactant gases is the possible presence of small amounts of impurity gases that bind strongly to the active metal sites and/or that have high extinction coefficients for their IR-active vibrational modes. A catalytic converter containing Rh/A1203has been employed to remove traces of CO from high-purity N2 so as to produce the IR spectrum of N2/Rh/AI2O3 without interference from CO surface species. The spectra obtained for 8000 Torr of pure N2 at 298 K verified the assignments of Wang and Yates for their low-temperature, low-pressure studies of the same system and showed that impurity CO does influence the nature of the N2/Rh/A1203surface species.
Introduction A novel infrared cell reactor has been developed in these laboratories, which can be used for in situ spectroscopic investigations of chemical reactions in the presence of catalytic films at variable temperature ( 100-600 K) and under moderately high-pressure conditions (106-1@ Torr) such as those that might be encountered in industrial reactors. This new IR cell reactor was employed initially to study the interaction of high-pressure N 2 (8000 Torr) with Rh/AI2O3 films at ambient temperature.' It was found that spectra very similar to those obtained by Wang and Yates2 at low pressures (1218 Torr) and low temperatures (1200 K) were obtained, which contained bands resolved for N2 adsorbed on Rho and RhH, dependent upon the sample pretreatment procedures.' A band corresponding to N2 physisorbed on the A1203support was also observed unexpectedly at ambient temperature when high pressures of N2 were employed.' A summary of the infrared bands detected for the N2/Rh/AI2O3system at high pressure and ambient temperature' and for low pressure and low temperature,2 along with the assignments proposed by the authors, is given in Table I. Generally, alkali-metal promotion for Rh/A12033*4or the SMSl Ti02supports*6is required for chemisorption of lowpressure N 2 on Rh at ambient temperature. Thus far, the greatest limitation noted in these laboratories pertinent to the TR investigation of supported catalysts at high pressures is the presence of small amounts of impurity gases that bind strongly the supported transition metal and/or possess high inherent extinction coefficients. Indeed, this was the case for the earlier N,/Rh/AI2O3 experiments in these laboratories,' for a small amount of C O impurity caused the usual CO/Rh/AI2O3 surface species' when 4OOO Torr of N 2 was employed, and the band intensities for the adsorbed CO species were comparable to those for the adsorbed N2 species at 8000 Torr of N2. Wang and Yates also observed a small CO/Rh/AI2O3 band in their work even at only 218 Torr of N2.2 Although the presence of adsorbed CO species was actually advantageous to us in making band assignments in the N2/Rh/AI2O3work,' it would be obviously desirable to eliminate such impurities under normal experimental circumstances. Historically, an effective method of removal of trace impurities from gas streams has been to pass the gas through a catalytic converter containing the catalyst of interest before entrance into the catalytic reactor. This work will report our efforts along these lines. A spectrum free of bands corresponding to adsorbed CO species for high-pressure N2/Rh/AI2O3 will be presented, and prior N2 band assignments will be verified.
TABLE I: Infrared Bands for Nz/Rb/A120, Species 9" WAb (ref I): (ref 2). cm-l assignment" cm-I assignmentb 2330 N,/AI,O, 233 1 NZ/A1203 ph ysisorbed physisorbed
Experimental Section Supported Rh/AI2O3 films (2.2 wt % Rh) were prepared by spraying a slurry of RhC13.3H20 (Johnson Matthey), alumina spectroscopic grade (Aluminumoxid C from Degussa, 100 m2 g'),
(1) Wey, J. P.; Burkett, H. D.; Neely, SOC.1991, 113, 2919.
'Author to whom correspondence should be addressed.
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2301 2276 2256
N2/Rh*+ N2/Rh6+ N2/Rho or N2/Rh+C
2303 2270 2257
N2/Rh*+ N2/Rh*+ N2/Rho
'298 K, 8000 Torr of N,, 2.2% Rh/A120,, impurity CO present. b L o temperatures, ~ low pressures of N2, 2.2% Rh/AI2O3. 'This paper will show that the assignment should be N2/Rho.
acetone, and distilled, deionized water onto a 25-mm CaF2 IR window held at 80 OC. Evaporation of the solvents left a thin IR-transparent film of RhCI3.3H20/Al2O3(4.4 mg cm-2) attached to the window. The window containing the catalyst film was mounted in a new high-pressure infrared cell reactor, which has been described recently.' The sample was then exposed to evacuation (lo4 Torr) at 373 K for 1 h and then reduced to 2.2% Rh/AI2O3 by exposure to 100 Torr of H2 (Air Products, 99.999% trapped at 77 K) in exposure/evacuation cycles of IO, 5, 10, and 20 min at 473 K. The reactor was evacuated to l o d Torr at 298 K for 1 h and then exposed to successively higher pressures of N2 (Air Products, 99.999%) that had been passed through a trap held at 158 K. The purity of the N2 employed as described above proved inadequate. A catalytic converter was constructed from stainless steel tubing (20-cm length, 1.10-cm inside diameter), which was installed in the high-pressure N2 manifold. The converter was filled with glass wool and 2 g of dry 5% RhCl3.3H2O/Al2O3 powder, prepared by evaporating the water and acetone from a suspension of RhC13.3H20 and A1203. The powder in the converter was heated under vacuum (10" Torr) a t 373 K for 1 h, reduced in 400 Torr of H2 at 423 K for IO h, and evacuated at 10" Torr at 298 K before introduction of N2. The converter was held at 373 K during the introduction of high-pressure N, in the cell. The purified N 2 was also subjected to a trap held a t 158 K during some experiments. For experiments involving a preoxidized Rh/AI2O3 film, the prereduced sample film was exposed to 100 Torr of O2 (Air Products, 99.993%, trapped at 77 K) for 6 min at 298 K. W.C.; Worley, S. D.J. Am. Chem.
(2) Wang, H.P.;Yates, J. T. J . Phys. Chem. 1984,88, 852. ( 3 ) Oh-Kita, M.; Aika, K. I.; Urabe, K.;Ozaki, A. J. Chem. Soc., Chem. Commun. 1975, 147. (4) Oh-Kita, M.; Aika, K. I.; Urabe, K.; Ozaki. A. J . Caral. 1976,44,460. ( 5 ) R y s c o , D.; Haller, G. L. J. Chem. Soc., Chem. Commun. 1980, 1 1 50. (6) Vishwanathan, V. J . Chem. Soc., Chem. Commun. 1989, 848.
0 1991 American Chemical Society
Wey et al.
8880 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
ZZDO
2300 235d.o
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2100
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Figure 1. Infrared spectra for N2 and impurity CO adsorbed on a prereduced 2.2% Rh/AI2O3film at 298 K. (a) 30 Torr; (b) 244 Torr; (c) 202 Torr; (d) 790 Torr; (e) 3890 Torr; (f) 8000 Torr.
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Figure 2. Infrared spectra for N2 and impurity C02(g) adsorbed on a prereduced 2.2%Rh/AI20, film at 298 K. (a) 2987 Torr; (b) 5260 Torr; (c) 7728 Torr.
All IR spectra were obtained by using an IBM 32 Fouriertransform spectrometer operated at 2-cm-' resolution. Generally 1000 scans accumulated over a period of 15 min were generated for each spectrum.
Results and Discussion The spectra in Figure I , which have been discussed previously,' correspond to exposure of the 2.2% Rh/AI2O3 film to increasing pressures of N, that was not purified in the catalytic converter described in the Experimental Section. For the highest pressure of N 2 employed (8000Torr) spectrum f exhibits bands due to N 2 chemisorbed on probable Rho (2256 cm-I), N, chemisorbed on Rh6+ (2301 cm-I), and Nz physisorbed on A1203(2330 cm-I). In addition bands due to adsorbed impurity CO at 2096 cm-' and centered at 2038 cm-' are comparable in intensity to the bands due to adsorbed N 2 for spectrum f. These CO bands plus a broad weak band in the 1800-1900-cm-' region (not shown in Figure 1) correspond to the well-known "gem-dicarbonyl" (ca. 2100 and 2030 cm-I), "linear CO" (2040-2070 cm-I), and "bridged carbonyl" ( I 800-2000 cm-I) species for very low coverage.' At high pressures of CO the linear CO species band is normally resolved from the 2030-cm-' gem-dicarbonyl band because it shifts to higher wavenumbers as surface coverage increases. The fact that the 2301-cm-' band appeared only at 8000 Torr of N 2 when appreciable adsorbed CO was present led us to postulate that it corresponded to N2/Rh6+for rhodium sites oxidized by CO for the prereduced sample. Wang and Yates observed this band also, but only for a preoxidized surface;2 their impurity CO level at 218 Torr was obviously much lower than ours at 8000 Torr. When N2 was subjected to the catalytic converter, the spectra shown in Figure 2 were produced. It is clear from these spectra that the CO impurity was no longer present, for all structure in (7) Yang, A. C.; Garland, C. W.J . Phys. Chem. 1957,61, 1504.
ieoo
Figure 3. Infrared spectra for pure N2 adsorbed on a prereduced 2.2% Rh/AI2O3film at 298 K. (a) 3000 Torr; (b) 5493 Torr; (c) 8000 Torr.
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Figure 4. Infrared spectra for pure N2 adsorbed on a preoxidized 2.2% Rh/AI2O3film at 298 K. (a) 2603 Torr; (b) 5430 Torr; (c) 8003 Torr.
the 2100-1800-cm-' region can be attributed to noise, even for spectrum c, corresponding to 7728 Torr of N2 The band at 2256 cm-' in Figure If now seems to be partially resolved into components at 2247 and 2264 cm-' in Figure 2c, which probably indicates the presence of N 2 adsorbed to Rho sites and Rh sites that are partially oxidized, respectively. The band components at 2329 and 2360 cm-' indicate the presence of gas-phase C 0 2 (P and R branches) as well as physisorbed N2 (2329 cm-I). Evidently the high-pressure 99.999% N2 contained appreciable O2as an impurity as well as CO. The catalytic converter thus oxidized CO to CO,;the O2and/or C 0 2 impurities then led to partial oxidation of Rh to give rise to the N2/Rh* species causing the band shoulder at 2264 cm-' and the small band near 2300 cm-I, which declines in intensity as the N: pressure increases. Figure 3 shows the spectra obtained for high-pressure N2 that was purified by the catalytic converter and trapped at 158 K. As can be seen from the spectra, CO surface species were eliminated, and very little, if any, C 0 2gas was present. Now the only two surface species detected were N2/Rho (band at 2250 cm-I) and physisorbed Nz/AI2O3(band at 2329 cm-I). The band component at 2264 cm-' was no longer evident, indicating a well-reduced surface. Most important, Figure 3 indicates that high-pressure gases can be purified, albeit with difficulty, of those impurities binding strongly to supported metals and/or possessing high extinction coefficients. Figure 4 shows spectra obtained for high-pressure N2 purified by the catalytic converter and a 158 K trap and then exposed to a preoxidized 2.2% Rh/AI2O3 surface. No bands corresponding to adsorbed CO species or C02gas were observed. The four bands resolved may be assigned as follows: 2250 cm-' (N2/Rho), 2271 cm-' (N2/Rhd+), 2305 cm-' (N2/Rh6+),and 2329 cm-' (N2/ A1203),in complete accord with the assignments of Wang and Yates2 and our prior work in which CO was a contaminant.' The preoxidized surface contained some Rho sites as evidenced by the broad band shoulder near 2250 cm-' in Figure 4a. This can be attributed to the mild preoxidation conditions (100 Torr of O2
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J. Phys. Chem. 1991, 95, 8881-8886
for 6 min a t only 298 K). A preoxidized surface containing appreciable adsorbed CO did not provide a band a t 2250 cm-l,] which indicates that CO displaces N2 from Rho sites on a partially oxidized Rh/AI2O3 surface. The other three bands noted above were observed on the preoxidized surface even in the presence of heavy CO contamination. It should be noted that the 2305-cm-l band in Figure 4 decreased in intensity with a concomitant increase in intensity of the 2250-cm-I band as the pressure of N2 was raised. This indicates that N 2 may cause some conversion of Rh6+sites to Rho sites, although we do not understand the mechanism for this process because N2 is not generally considered to be a reducing agent. Although our early work’ seemed to show that the N2 causing the 2250-cm-’ band was present on the same surface sites as those giving rise to the “gem-dicarbonyl” species, which are known* to correspond to Rh+, it is now clear from Figure 3 that the 2250cm-l band must correspond to N2adsorbed on a metallic Rh site. This conclusion was reached earlier by Wang and Yates2 in their elegant low-pressure, low-temperature studies. (8) Rice, c. A.; Worlev, S. D.; Curtis, C. W.; Guin, J. A.; Tarter, A. R. J . Chem. Phys. 1981, 74, 6487 and references cited therein.
Conclusions All IR-detectable traces of CO and C 0 2 can be removed from high-pressure N 2 (99.999%) if it is passed through a catalytic converter containing Rh/A1203 powder maintained at 373 K and if trapping at 158 K is employed. The resulting pure N2provides the same Rh/AI2O3 surface species a t 8000 Torr and 298 K as does pure N2 at low temperatures and pressures. An infrared band at ca. 2250 cm-’ can be assigned to the adsorption of N2 on metallic Rh. A band at 2301 cm-’ observed for preoxidized Rh/A1203 or prereduced Rh/A1203 when substantial impurity CO is present can be assigned to adsorption of N 2 on supported Rh*+, with CO serving as the oxidizing agent when it is present as an impurity on a prereduced surface. Acknowledgment. We thank the Strategic Defense Initiative Organization’s Office of Innovative Science and Technology through Contract N60921-86-C-A226 with the Naval Surface Warfare Center and the U S . Army Advanced Concepts and Technology Committee through Contract DAAAl5-88-K-0001 (J.P.W. and W.C.N.) for support of this work. Registry No. Rh, 7440-16-6; N2. 7727-37-9; CO, 630-08-0; C02, 124-38-9.
Infrared Spectroscopy at High Pressure: Interaction of H, and D, with Rh/Ai,O, J. P. Wey, W. C. Neely, and S. D. Worley* Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: May 6, 1991)
A novel high-pressure-infrared-cell reactor has been employed to study the interaction of ultrapure, high-pressure H2 and D2 with Rh/A1203films. The frequencies for the Rh-H and Rh-D stretching modes were measured to be 2013 and 1441 cm-I, respectively. In addition, a band at 1618 cm-’ for water on the A1203support was observed to develop concomitantly with the 201 3-cm-’ band. It was postulated that H2 dissociates on the Rh sites to produce a weakly bound Rh-H surface species, with the remaining H spilling over to the support where it reacted with surface hydroxyl groups to produce H20. The new Rh-H species was easily removed by reduction of H2 pressure above the catalyst. This weakly bound species is probably relevant to the catalytic chemistry for reactions involving H2 as a reactant over supported Rh. It was further suggested that the heretofore elusive Rh-H stretching mode for the previously postulated Rh carbonyl hydride surface species, observed spectroscopically during catalytic methanation, is present as a weak low-frequency shoulder on the band due to the C-O stretch for that species.
Introduction Infrared spectroscopy has become one of the most, if not the most, important analytical probes for identifying and monitoring surface species on supported transition-metal catalysts. While the vast majority of work reported in this area has concerned surface species produced at low pressures (generally 1200 Torr), recent studies in these laboratories have focused on those species generated at higher pressures (up to 104 Torr) such as commonly exist in industrial reactors. For these investigations a novel high-pressure-infrared-cell reactor was designed and constructed. This reactor, which is capable of operation in the 104-104-Torr pressure regime and a t temperatures ranging from 100 to 600 K, has been described in detail recently; it was employed in a study of the interaction of high-pressure N 2 with Rh/AI20, at ambient temperature.’ Detection of an infrared band corresponding to Rh-H for supported Rh has proved elusive. When CO is hydrogenated over supported Rh catalysts at ca. 100 Torr or less total pressure, infrared bands for the usual CO/Rh species (“gem-dicarbonyl”, ‘linear”, and “bridged”)2 are not detected; rather, only a new band centered between 2020 and 2045 cm-I dependent upon coverage is ob~erved.~The same band is detected upon hydrogenation of Author to whom correspondence should be addressed.
0022-365419112095-8881$02.50/0
C 0 2 over supported Rh, its frequency dependent upon the nature of the support (2020-2025 cm-l for Rh/A1203, 2028-2032 cm-l for Rh/Si02, 2038-2042 cm-’ for Rh/Ti02).4 Solymosi and co-workers have attributed this band to the C-O stretching mode of a ‘rhodium carbonyl hydride” species.s*6 U ‘ I \
-0
/”
Rh
Work in these laboratories has supported this assignment. Deuterium substitution caused a red shift of ca. 5-10 cm-l which is close to the expected shift for D-isotopic substitution two bonds removed from the C O oscillator (high quality a b initio computations for Rh(C0)H vs Rh(C0)D in the gas phase predict a red shift of ca.3 cm-’).’,* However, a puzzling point has always been (1) Wey, J. P.;Burkett, H. D.; Neely, W. C.; Worley, S.D. J . Am. Chem. Soc. 1991, 113, 2919. (2) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957,61, 1504. (3) Worley, S.D.; Mattson, G. A.; Caudill. R. J . Phys. Chem. 1983, 87, 1671. (4) Henderson, M. A.; Worley, S. D. J . Phys. Chem. 1985. 89, 1417. (5) Solymosi, F.; Erdohelyi, A.; Kocsis, M. J . Cural. 1980, 65, 428. (6) Solymosi, F.; Erdohelyi, A. J . Curul. 1981, 70, 451. (7) Henderson. M. A.; Worley, S.D. J . Phys. Chem. 1985.89, 392. (8) McKee, M. L.; Dai, C. H.; Worley, S. D. J . Phys. Chem. 1988, 92, 1056.
0 1991 American Chemical Society
