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Langmuir 1998, 14, 3820-3824
Spectroscopic Observation of a Precursor Complex to Rh-NCO and Al3+-NCO Formation during CO/NH3 Reactions on Rh/Al2O3 Dilip K. Paul* and Chad D. Marten Department of Chemistry, Pittsburg State University, Pittsburg, Kansas 66762-7553 Received January 27, 1998 Low-temperature in situ IR spectroscopy has been used to investigate the reaction of CO and NH3 over Rh/Al2O3. The presence of a strong vibrational feature at 2076 cm-1 due to linear Rh0x -CO is clearly evident at 100 K during coadsorption experiments. However, beginning at 230 K, the formation of isocyanate species was detected on both rhodium and the Al2O3 support by two distinct infrared featrures at 2170 and 2246 cm-1, respectively. The parallel growth of these νa(NCO) modes is consistent with the gradual decomposition of a new band at 1970 cm-1, which is assigned to a perturbed ν(CO) mode bridge-bonded by NH surface species. This bridge-bound complex Rh-(OCrNH) is considered to form by nucleophilic attack of Rh-NH on the carbonyl C of Rh0x (CO) surface species. Upon warming, the decomposition of this complex leads to the formation of rhodium isocyanate (Rh-NCO), and the NCO ultimately migrates to the support.
Introduction 1-3
In his pioneering work, Unland observed the formation of isocyanates during NO/CO reaction1-6 on different oxide-supported transition metal catalysts. Because of the need to explore more efficient synthetic routes to dicyandiamide, a few recent studies focused on catalytic raction of CO and NH3. Dicyandiamide7 is emerging as a very effective nitrification inhibitor that significantly enhances the nitrogen fertilizer efficiency and has potential for reducing ground water pollution by nitrites and nitrates. Cyanamide, an intermediate in the production of dicyandiamide, is a possible product for reactions involving CO or CO2 with NH3.8,9 However, a few recent studies10-14 indicated the formation of isocyanate for the CO/NH3 reaction instead of cyanamide on metal supported catalysts and showed that the presence of an active metal component is absolutely necessary for isocyanate formation. In addition, the nature of the support plays a dominant role for the location and extent of isocyanate formation. Although the mechanism of isocyanate formation is also of particular interest during the CO/NH3 reactions, very little is known other than the two proposals made by Paul and co-workers.11,13 For Ru/Al2O3, one mechanism involves the formation of an amide intermediate (MCONH2) during low-temperature reaction, whereas the (1) Unland, M. L. J. Catal. 1973, 31, 459. (2) Unland, M. L. J. Phys. Chem. 1973, 77, 1952. (3) Unland, M. L. Science 1973, 179, 567. (4) Hecker, W. C.; Bell, A. T. J. Catal. 1984, 85, 389. (5) Solymosi, F.; Sarkany, J.; Schauer, A. J. Catal. 1977, 46, 297. (6) Solymosi, F.; Volgyesi, L.; Rasko, J. Z. Phys. Chem. N.F. 1980, 120, 79, and references within. (7) Gautney, J. TVA Memorandum, May 30, 1985. (8) Boatright, L. G.; Macay, J. S. U.S. Patent 2,721,786, 1955. (9) Rollingson, W. R. U.S. Patent 2,975,032, 1961. (10) Paul, D. K.; McKee, M. L.; Worley, S. D.; Hoffman, N. W.; Ash, D. H.; Gautney, J. J. Phys. Chem. 1989, 93, 4598. (11) Paul, D. K.; Worley, S. D.; Hoffman, N. W.; Ash, D. H.; Gautney, J. Surf. Sci. 1989, 223, 509. (12) Paul, D. K.; Worley, S. D.; Hoffman, N. W.; Ash, D. H.; Gautney, J. Chem. Phys. Lett. 1989, 160, 559. (13) Paul, D. K.; Worley, S. D. J. Phys. Chem. 1990, 94, 8956. (14) Paul, D. K.; Worley, S. D.; Hoffman, N. W.; Ash, D. H.; Gautney, J. J. Catal. 1989, 120, 272.
second mechanism involves the reaction of adsorbed N with CO at higher temperatures. However, elegant studies by Anderson and Rochester15 favored the second mechanism for Rh/Al2O3 surfaces without any spectroscopic evidence. Their conclusion was based on the flash desorption studies of NH3 desorption from a Rh crystal by Vavere and Hansen,16 who reported that NH3 dissociates as low as 250 K, and the surface species is predominantly Rh2NH. Thus, the present work extends the study of the CO/NH3 reaction to a Rh/Al2O3 surface at low temperature using the molecularly sensitive IR spectroscopy as a diagnostic tool to explore the mechanism of the isocyanate formation. Experimental Section Infrared spectra were measured using a purged PerkinElmer model PE 783 infrared spectrometer interfaced with a 3600 data station for data storage and manipulation. The spectra were obtained with a slit program yielding a maximum resolution of 5.4 cm-1 acquired with typical data acquisition times of 0.93-1.9 s cm-1 depending upon the spectral region scanned. Smoothing functions were not used during data handling and manipulation. The stainless steel IR cell used for spectroscopic measurements has been described previously.17-19 Briefly, it consists of a main cell body (double-sided conflat flange) containing a CaF2 disk that serves as a sample holder, positioned in place by a copper support ring which may be temperature controlled with either cooled or heated N2(g). The cell body is contained between two CaF2 windows sealed in stainless steel conflat flanges, permitting IR measurements in the 4000-1000-cm-1 spectral range. The IR cell is attached to the bakeable all-metal gas handling system and is typically maintained at a base pressure of 1 × 10-8 Torr. Pumping is carried out by (15) Anderson, J. A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3809. (16) Vavere, A.; Hansen, R. S. J. Catal. 1981, 69, 158. (17) Beebe, T. P., Jr.; Gelin, P.; Yates, J. T., Jr. Surf. Sci. 1984, 148, 526. (18) Beebe, T. P., Jr.; Yates, J. T., Jr. Surf. Sci. 1985, 159, 369. (19) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J. Chem. Phys. 1979, 71, 3908.
S0743-7463(98)00108-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/19/1998
Al3+-NCO Formation on Supported Rh
Figure 1. Infrared spectra for (a, b) the reaction CO and NH3 on Al2O3 and Rh/Al2O3 and (c) the reaction of CO on Rh/Al2O3.
means of a liquid nitrogen-cooled zeolite sorption pump and a 20 L/s ion pump. Al2O3-supported Rh catalyst samples were prepared in a slurry containing RhCl3‚3H2O (Johnson & Matthey) and Al2O3 (Degussa aluminum oxide C, 100 m2/g) in the appropriate ratio to produce 2.2% Rh/Al2O3. The slurry was suspended in a liquid consisting of 9 parts of spectroscopic grade acetone and 1 part doubly distilled water and was sprayed with an atomizer onto a 2.5-cmdiameter CaF2 disk maintained on a hot plate at ∼360 K. The solvents evaporated rapidly, leaving a thin film of RhCl3‚xH2O/Al2O3 adhering to the CaF2 disk. A total sample weight of 42-45 mg was sprayed uniformly over the disk area of 2.53 cm2, yielding a final deposit surface density of (4.3-4.5) × 10-3 g/cm2 for the various samples. The catalyst sample was then mounted in the stainless steel UHV IR cell, which was subsequently evacuated for ∼12 h, followed by further outgassing at 475 K for ∼72 h. The sample was then subjected to 15-, 30-, 45-, and 60-min cycles of exposure to 400 Torr H2 at 475 K, each cycle being terminated by evacuation to 1 × 10-6 Torr for 30 min. After the reduced sample was allowed to remain under vacuum at 475 K for a period of ∼8 h, the cell was cooled to 300 K before scan of the background spectrum. For low-temperature experiments, background spectra were also taken at low temperatures to minimize the effect of any change in the background. The sample was then exposed to reacting gases, and the IR spectra were monitored as a function of time and temperature. The CO used in this study, (99.99% purity, Scientific Gas Products) was obtained in break-seal glass bulbs and used without further purification. The NH3 gas (99.9% purity) and the H2 gas (99.995% purity) were obtained from Matheson Gas Products in high-pressure cylinders. Ammonia gas was transferred to a glass bulb for convenient use and purified by three freeze-pump-thaw cycles. Results A. Comparative Studies of CO/NH3 Reaction on Al2O3 and Rh/Al2O3. Figure 1 shows the C-O stretching region (2300-1700 cm-1) for adsorption of CO and coadsorption of CO and NH3 on Al2O3 and Rh/Al2O3. The
Langmuir, Vol. 14, No. 14, 1998 3821
1700-1100-cm-1 region illustrates (not shown in Figure 1) 1622- and 1260-cm-1 features due to δasym(NH3) and δsym(NH3), respectively.15,20-21 These bands are characteristic vibrational features of both NH3 complex (Hbonded through isolated OH groups) and NH3 bound to Lewis acid sites (Al3+). In addition, a broad band at ∼1450 cm-1 due to δ(NH2) surface species was observed for both Al2O3 and Rh/Al2O3 films and is consistent with our earlier work on Al2O3-supported Ru and Pd.11,13 Spectrum a shows the support influence indicating no bands in the ν(CO) region except the ν(NH) and δ(HNH) modes in the other regions (not shown), which were not very revealing. The absence of any IR features for the Al2O3 support for the experiments carried out under identical conditions (shown in spectrum a with an enhanced absorbance scale, that is, 10× compared to spectrum c) rules out the possibility of any CO adsorption onto the support. Spectrum c indicates adsorption of CO (1.3 Torr, 298 K, 30 min.) alone on Rh causes two strong predominant infrared features at 2094 and 2023 cm-1, which can be assigned to the symmetric and antisymmetric stretching modes of RhI(CO)2 species, respectively, as shown below. The weak unresolved peak at 2060 cm-1 is assigned to linear CO bound to Rh0x . The broad band 1920-1700 cm-1, centered around 1840 cm-1 is due to bridge-bonded CO.22-26
Gem
Spectrum b shows the reaction of CO and NH3 on Rh/ Al2O3 in which the gem dicarbonyl vibrational features at 2094 and 2025 cm-1 are greatly reduced while development of new features are observed at 2170 and 2246 cm-1. In agreement with the earlier work for the CO/NH3 reaction, the features at 2246 and 2170 cm-1 are assigned to ν(NCO) bound to Al2O3 and Rh, respectively.15,27 The lack of the development of the doublet feature due to the RhI(CO)2 feature is strikingly evident compared to the surface treated with CO only. The reader should note that the absorbance scales differ by a factor of 2 in spectra b and c. For the coadsorption experiment, the development of a band at 1970 cm-1 as a shoulder in the low-frequency side of the antisymmetric Rh-gem dicarbonyl feature is of particular interest (see below). The absence of these features in the spectra for Al2O3 alone indicates that the presence of Rh is necessary for the formation of the isocyanate species on surfaces. It should be noted that a 2246-cm-1 band was not observed for the CO/NH3 reaction over Al2O3 (Figure 1a). Thus, NCO must initially form on Rh or the Rh/Al2O3 interface and then rapidly spill over to the Al2O3 support. It may (20) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley and Sons: New York, 1972; Chapter 8. (21) Little, L. H. Infrared Spectra of Adsorbed Species Academic Press: London, 1966; Chapter 7. (22) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (23) Zaki, M. I.; Kunzmann, G.; Gates, B. C.; Knozinger, H. J. Phys. Chem. 1987, 91, 1486. (24) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789. (25) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1986, 90, 5312. (26) Yates, J. T., Jr.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J. Chem. Phys. 1979, 70(3), 1219. (27) Gutschick, D.; Meissner, H. React. Kinet. Catal. Lett. 1983, 22, 221.
3822 Langmuir, Vol. 14, No. 14, 1998
Figure 2. Infrared spectra for the reaction of CO and NH3 over a 2.2% Rh/Al2O3 film at 298 K and a total pressure of 4.5 Torr. The exposure time indicates the time after the previous scan.
be noted that the coexistence of all three additional IR features (1970, 2170, and 2246 cm-1) for the CO/NH3 reaction is obvious compared to the reaction of CO only on Rh. No observable changes in the ν(NH) and δ(HNH) modes were observed. B. Time and Temperature Dependence of Isocyanate (NCO) Formation. Figure 2 shows the sequence of IR spectra taken at different time intervals during the formation of isocyanate species from the CO/NH3 [P(CO) + P(NH3) ) 4.5 Torr; P(CO)/P(NH3) ) 0.3, 298 K ] reaction on Rh/Al2O3. Upon exposure for the first 17 min, the infrared spectral features due to bridge bonded (1825 cm-1) and terminally bonded (2053 cm-1) CO on Rh0x sites are developed along with the Rh-gem dicarbonyl adspecies (2095 and 2025 cm-1). Upon prolonged CO/NH3 exposure, the intensities of all IR modes (Figure 2a-e) corresponding to ν(CO) and ν(NCO) consistently increased indicating a gradual growth of adsorbed isocyanate and increased CO interaction with Rh. Figure 3 shows the effects of temperature on the formation of isocyanate surface species. It is apparent that observation of the 2076-cm-1 peak at 100 K is attributed to ν(CO) for linear bonded CO on Rh0x , which dominates the entire spectrum with a small shoulder at 2022 cm-1 due to a Rh-carbonyl hydride species.28-31 Upon warming the cell, dramatic spectral changes are noted at 230 K, where two new bands at 2170 (Rh-NCO) and 2246 cm-1(Al3+-NCO) have appeared. In addition, the 2022(28) Dai, C. H.; Worley, S. D. J. Phys. Chem. 1986, 90, 4219. (29) Solymosi, F.; Erdo¨helyi, A.; Kocsis, M. J. Catal. 1980, 65, 428. (30) Solymosi, F.; Erdo¨helyi, A. J. Catal. 1981, 70, 451. (31) McKee, M. L.; Dai, C. H.; Worley, S. D. J. Phys. Chem. 1988, 92, 1056.
Paul and Marten
Figure 3. Infrared spectra for the reaction of CO and NH3 over a 2.2% Rh/Al2O3 film at different temperatures and a total pressure of 3.6 Torr.
cm-1 infrared feature grew in intensity with a new lowfrequency shoulder centered around 1970 cm-1. The symmetric mode of the gem dicarbonyl feature at 2094 cm-1 was not clearly observed until the cell was warmed to 298 K, and the bands at 2246, 2022, and 1970 consistently grew in intensity. Discussion The following surface reactions can be envisaged upon careful investigation of the low-temperature infrared spectral development (Figure 3). (1) The slow activation of degradation of Rh0x clusters begins at 230 K in the presence of surface hydroxyl groups. (2) The formation of a Rh-carbonyl hydride species (band at 2022 cm-1) takes place due to reaction between rhodium carbonyls and hydrogen (here, the source of H being the decomposition of NH3). (3) The gradual development of a bridge-bound precursor complex (band at 1970 cm-1) is clearly observed due to the nucleophilic attack on NHx species on rhoduim carbonyl species. The precursor complex thus decomposes to produce the isocyanate species. The first two surface processes mentioned above have been observed spectroscopically for CO/Rh and during CO and CO2 hydrogenation reactions.28-31 During the degradation of Rh0x to RhI by a complex oxidation process in which isolated surface hydroxyl groups are consumed, and in the presence of CO(g), the spectroscopically identifiable RhI(CO)2 species is produced.32,33 Studies of this oxidation process have been made using a number of physical (32) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988, 110, 2074.
Al3+-NCO Formation on Supported Rh
measurement techniques, such as an extended X-ray absorption fine structure (EXAFS), where van’t Blik et al.34 conclusively demonstrated that Rh0x crystallites were destroyed in the presence of CO(g). The second process involves the formation of a Rhcarbonyl hydride, which was detected spectroscopically in the reactions of H2 with CO or CO2 over supported Rh films with a broad infrared feature at 2020-2045 cm-1 28-31. Here the band at 2022 cm-1 due to Rh-carbonyl hydride is very likely formed by the interaction of Rhcarbonyl and atomic H, which is available from the decomposition of NH3. Earlier flash desorption experiments for ammonia on rhodium crystals indicate that no NH3 desorption occurs following NH3 desorption at 250 K, and the surface species is presumed to be Rh2NH. The presence of the hydride moiety causes a shift of frequency for the CO stretching mode to a lower value than that for the linear Rh0x -CO species (2040-2080 cm-1) due to electron donation into the π* CO orbital. The decrease in intensity ratio I(νs)/I(νa) of gem dicarbonyl modes for CO/ NH3 reactions compared to CO alone is significant (compare spectra b and c in Figure 1). The intensity distribution alteration may be due to the formation of a Rhcarbonyl hydride species (at 2025 cm-1), which overlaps with the νas(CO) mode of the gem dicarbonyl feature. The oxidative degradation process is partially blocked by (a) adsorption of NH3 on Rh, which hinders the formation of mobile linear Rh-carbonyl species, a precursor for oxidation, and by (b) the formation of weakly bound H-bonded ammonia complex with OH groups. This weak H-bonded complex breaks down as the cell is warmed; thus OH groups become free. These hydroxyl groups subsequently interact with metallic rhodium in the presence of CO to cause the formation of gem dicarbonyl species. The third significant surface reaction involves the formation of the isocyanate species on both metal and support. The simultaneous appearance of all three surface species representing 2170-, 2246-, and 1970-cm-1 infrared bands at 230 K indicates that there exists a correlation among these surface species. Careful investigation of time and temperature dependence in the infrared studies (Figures 2 and 3) indicates that the formation of these surface species (Rh-NCO at 2170 cm-1 and Al3+-NCO at 2246 cm-1) takes place during gradual disappearance of the 1970-cm-1 feature. Earlier studies suggest that the support-bound isocyanate (Al3+-NCO) results from the migration of metal-bound isocyanate, which forms either from the CO/NH3 or CO/NO reaction. Thus, the RhNCO formation depends on an elusive surface species characterized by a 1970-cm-1 band. The interesting question to be answered concerns the assignment of the 1970-cm-1 band, which upon slow decomposition leads to the formation of the isocyanate species. The strong vibrational feature at 2076 cm-1 that developed at 100 K is attributed to the formation of linear CO species on Rh0x , and no infrared bands for physisorbed CO were detected. This demonstrates the absence of any oxidized Rh sites. Therefore, the catalyst was originally in a fully reduced state, an observation that had been reported by others.23 There are ample precedents for the formation of NH2 during adsorption of NH3 on Al2O3. In addition, the NH3 decomposition takes place on Rh forming adsorbed NH2 and NH at room temperature. Earlier studies using matrix isolated rotational and infrared (33) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Phys. Chem. 1987, 91, 3133. (34) van’t Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139.
Langmuir, Vol. 14, No. 14, 1998 3823
spectroscopy show no H-bonding between CO and NH3.36 Thus, a reasonable proposal for the precursor complex involving adsorbed CO and NHx may be the formation of a transient bridge-bonded complex via nucleophilic attack of the NH on the carbonyl C species/Rh (shown below) with ν(CO) at 1970 cm-1. The frequency red-shift of 55 cm-1 can be explained by considering the formation of a bridge complex by nucleophilic attack of NH onto the C atom of carbonyl CO, thereby increasing the electron density in the π* orbital of CO.
This bonding configuration leads to the formation of a C-N bond as shown in η1(N)-CONHx surface species, which ultimately decomposes to Rh-NCO. On the other hand, if the nucleophilic attack is not strong enough to form a C-N bond, no isocyanate species would have been formed. This observation is consistent with the elegant study by Gao et al.,37 who noted the formation of isocyanic acid at 285 K upon decomposition of HCONH2 on Ni(111). The proposed precursor for this reaction is η1(N)-HCONH2, whereas the products for the η2(C,O)-HCONH2 reaction are CO and NH3. It is quite interesting to note that Weinberg et al.38,39 were unable to detect any isocyanate formation from formamide decomposition on a Ru substrate. However, high-frequency shifts of ν(CO) in the range of 1600-1700 cm-1 have been reported for Rh-CO complexes in which the oxygen end of the CO molecule is bonded or is strongly interacting with another surface species. Inferences to this type of interaction have been made recently for anomalously low-frequency CO peaks on supported metal catalysts with electrophilic additives.39-42 Thus, the formation of isocyanate species and its migration during the CO/NH3 over Rh/Al2O3 reaction is consistent with the findings of other metal-supported catalysts. These low-temperature studies indicate that formation of a bridge-bonded complex is essential for isocyanate formation. This complex is formed by the nucleophilic attack of the NHx/Rh surface species onto the carbonyl C, which ultimately decomposes to produce the NCO species. This precursor complex has not been observed for other substrates investigated earlier, such as Pd and Ru. Conclusions It has been shown that several surface species result during the low-temperature reaction of CO and NH3 over Rh/Al2O3 which can be detected by infrared spectroscopy. Those causing IR bands at 2172 and 2256 cm-1 may be identified as RhNCO and Al3+-NCO, respectively, which (35) Peri, J. B. J. Phys. Chem. 1965, 69, 231. (36) Hagen, W.; Tielens, G. G. M. Spectrochim. Acta 1982, 38A, 1203. (37) Gao, Q.; Erley, W.; Sander, D.; Ibach, H.; Hemminger, J. C. J. Phys. Chem. 1991, 95, 205. (38) Parmeter, J. E.; Schwalke, U.; Weinberg, W. H. J. Am. Chem. Soc. 1988, 110, 53. (39) Parmeter, J. E.; Schwalke, U.; Weinberg, W. H. J. Am. Chem. Soc. 1987, 109, 5083. (40) Blackmond, D. G.; Kesraoui, S. In Catalysis 1987; Ward, J. W., Ed.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1988. (41) Ichikawa, M.; Fukushima, T. J. Phys. Chem. 1985, 89, 1564. (42) Ichikawa, M.; Lang, A. J.; Shriver, D. F. Sachtler, W. M. H. J. Am. Chem. Soc. 1985, 107, 7216.
3824 Langmuir, Vol. 14, No. 14, 1998
gradually accumulate upon slow decomposition of the bridge-bonded surface species identified by an IR band at 1970 cm-1. The 1970-cm-1 band is attributable to the formation of the precursor complex for isocyanate formation. No surface reaction between CO and NH3 was observed at 100 K. However, the reaction leading to isocyanate was detected at the temperature of 230 K.
Paul and Marten
Acknowledgment. We acknowledge the full support of Professor John T. Yates, Jr., University of Pittsburg, Pittsburg, PA, for providing opportunities for carrying out this research project. We are also grateful to the Research Corp. for the support of this work. LA980108G
