1671
J. Phys. Chem. 1983, 8 7 , 1671-1673
An Infrared Study of the Hydrogenation of CO on Supported Rh Catalysts S. D. Worley;
0. A. Mattson, and R. Caudlll
Department of Chemistry, Auburn University, Auburn University, Alabama 36849 (Received: February 16, 1983)
The hydrogenation of CO over supported rhodium films has been investigated by using infrared spectroscopy as a probe. Rhodium loading and support material were variables in this work. Data for 0.5 wt % Rh/X (X = TiOz,A1203,Si02) films have been interpreted to indicate that a rhodium monocarbonyl hydride species is an important intermediate in catalytic methanation in agreement with an earlier proposal by Solymosi and co-workers. A support effect was evident with Rh/Ti02 films being more active for catalytic hydrogenation of CO than were Rh/A1203or Rh/Si02 films.
species (species I) which exhibits two sharp infrared bands
Introduction
A massive amount of data has been reported recently concerning the reaction of hydrogen or water with carbon monoxide over various supported transition-metal catalysts, including a number of informative studies of the reaction catalyzed by supported rh0di~m.l-I~Recent infrared work in these laboratories has addressed the different types of adsorbed CO species on supported Rh films as a function of Rh loading, reduction conditions, precursor salt, and support material. It has been established that primarily three different types of Rh sites exist on supports depending upon oxidation state and degree of dispersion of the rh~dium.'~-~OThese are a geminal dicarbonyl
(1) M. Niwa, R. Iizuka, and J. H. Lunsford, J. Chem. SOC.,Chem. Commun., 684 (1979). (2)K. Fujimoto, M. Kameyama, and T. Kunugi, J.Catal., 61,7(1980). (3)R. M. Kroeker, W. C. Kaska, and P. K. Hansma, J. Catal., 61,87 (1980). (4)D. G. Castner, R. L. Blackadar, and G. A. Somorjai, J.Catal., 66, 257 (1980). (5)D. C. Grenoble, M. M. Estadt, and D. F. Ollis, J. Catal., 67,90 (1981). (6)P. R. Wataon and G. A. Somorjai, J . Catal., 72,347 (1981). (7)J. T. Yates and R. R. Cavanagh, J. Catal., 74,97 (1982). (8)M. A. Vannice, J. Catal., 74,199;37,449 (1975). (9)P. R. Watson and G. A. Somorjai, J. Catal., 74,282 (1982). (IO) F. Solymosi, I. Tombacz, and M. Kocsis, J. Catal., 75,78 (1982). (11)M. Niwa and J. H. Lunsford, J . Catal., 76,302 (1982). (12)T. Iizuka, Y. Tanaka, and K. Tanabe, J. Catal., 76, 1 (1982). Jpn., 51,2268,2273(1978);M. M. (13)M. Ichikawa, Bull. Chem. SOC. Bhasin, W. J. Bartley, P. C. Ellgen, and T. P. Wilson, J. Catal., 54,120 (1978). (14)C. A. Rice, S. D. Worley, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J. Chem. Phys., 74,6487 (1981). (15)S.D.Worley, C. A. Rice, G. A. Mattaon, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J. Chem. Phys., 76,20 (1982). (16)S.D. Worley, C. A. Rice, G. A. Mattaon, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J.Phys. Chem., 86,2714 (1982). (17)A. C. Yang and C. W. Garland, J.Phys. Chem., 61,1504 (1957). (18)J. T. Yates, T. M. Duncan, S. D. Worley, and R. W. Vaughan, J. Chem. Phys., 70,1219 (1979). (19)R. R.Cavanagh and J. T. Yates, J. Chem. Phys., 74,4150(1981). (20)H. C. Yao and W. G. Rothschild, J.Chem. Phys., 68,4774(1978). (21)H. Arai and H. Tominaga, J . Catal., 43,131 (1976). Faraday Trans. 1, 74,2570 (1978). (22)M. Primet, J. Chem. SOC., (23)M. Primet and E. Garbowski, Chem. Phys. Lett., 72,472(1980). (24)E.W. Thornton, H. Knozinger, B. Tesche, J. J. Rafalko, and B. C. Gates, J. Catal., 62, 117 (1980). (25)S.L. T. Anderson and M. S.Scurrell, J . Catal., 59,340 (1979). (26)A. K.Smith, F. Hagues, A. Theolier, J. M. Basset, R. Ugo, G.M. Zanderighi, J. L. Bilhou, V. Bilhou-Bougnol, and W. F. Graydon, Inorg. Chem., 18,3104 (1979).
0 OC
/ / Rh
I
I/
0 C
co -R
I h-
I1
-Rh-
/c\
R h-
111
near 2030 and 2100 cm-' which do not shift in wavenumber with coverage and remain for preoxidized films, a linear monocarbonyl species (species 11) which exhibits an infrared band between 2040 and 2075 cm-' depending upon coverage and is not present following preoxidation, and a bridged carbonyl species (species 111) which exhibits a broad infrared band between 1840 and 1920 cm-' depending upon coverage and also is not present for preoxidized films. Work in these laboratorie~'"'~ and else~ h e r e ~ has ~ i established ~ ~ p ~ ~ that species I refers to Rh+, most probably as isolated rhodium ions,'*J9 while species I1 and I11 refer to metallic Rh ~ l u s t e r s . ' ~Although J~ there remains some controversy regarding these points,31 it has been demonstrated that supported Rh films can be prepared which exhibit the two extremes-only species I, or primarily species I1 and 111-by use of appropriate loadings, reduction conditions, Rh precursors, and support^.'"'^ The purpose of this paper is to report recent observations concerning the hydrogenation of CO over the various supported Rh films.
Experimental Section The supported catalyst films used in this study were prepared by spraying and reduction techniques discussed earlier.14-16 Following reduction in hydrogen 'and subsequent evacuation at 473 K, the films were allowed to cool to room temperature and were exposed to a mixture of hydrogen and carbon monoxide normally a t a ratio of 3:l and a total pressure of 100 torr. Then the infrared spectra were monitored at room temperature and at several reaction temperatures depending upon the nature of the experiment. The reaction yield of methane was approximated by measuring the intensities of its infrared bands at 1306 and 3018 cm-I and comparing with known con(27)H. Knozinger, E. W. Thornton, and M. Wolf, J . Chem. SOC., Faraday Trans. 1, 75,1888 (1979). (28)J. L.Vidal and W. E. Walker, Inorg. Chem., 19,896 (1980). (29)K. W. Watters, R. F. Howe, T. P. Chojnacki, C. M. Fu, R. L. Schneider, and N. B. Wong, J. Catal., 66,424 (1980). (30)M. Primet, J. C. Vedrine, and C. Naccache, J . Mol. Catal., 4,411 (1978). (31)D.J. C. Yates, L. L. Murrell, and E. B. Prestridge, J. Catal., 57, 41, (1979).
0022-3654/83/2087-1671$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
1672
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Letters
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WAVENUMBER
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Figure 1. Infrared spectra for the interaction of H, and CO over TiO,: (a) 4 1 mg cm-2 of TiO, exposed to 75 torr of H2 and 25 torr of CO for 4 h at 483 K; (b) 3.9 mg cm-' of 10% Rh/TiO, exposed to 75 torr of H, and 25 torr of CO at 298 K; (c) sample in b heated for 1 h at 483 K; (d) sample in b heated for 4 h at 483 K.
centrations of methane in the infrared cell in the presence of the catalyst film following the experiment. For all of the work described here the rhodium precursor salt was RhC13.3H20. The support materials were alumina (Degussa Aluminum Oxide C, 100 m2 g-l), titania (Degussa Titanium Dioxide P25, 50 m2 g-l), and silica (Cabot Cab0-si1 M-5, 200 m2 g-'). The Pyrex infrared cell used has been described previ~usly.'~-'~ All spectra were accumulated by a Perkin-Elmer 580 spectrometer operated at a resolution of 2.8 cm-' in the 2000-cm-' region. Results and Discussion Typical spectra concerning the interaction of H2 and CO over a 10 wt % Rh/TiOz film are shown in Figure 1. Figure l a indicates that a small amount of methane is produced even over a blank TiOz film; approximately twice as much is produced for a blank A1203 film, but no methane could be detected by infrared for a blank SiOz film. In Figure l b the usual bands for adsorbed CO species I, 11, and I11 are observed for the mixture of Hz and CO at 298 K with little or no change in wavenumber or intensity pattern from those observed for pure CO on 10% Rh/Ti02.16 The CO band intensities in Figure 1 are weak relative to those for Rh/Al2O3 and Rh/SiOz. This observation has been noted by others8~32~33 following hightemperature reduction of Rh/Ti02 and attributed to the SMSI (strong metal-support interaction) behavior for Ti02 supports. Upon heating the 10% Rh/Ti02 film, the geminal dicarbonyl species I bands completely disappeared (near 423 K) before any methane was detected. This was observed also for A1203and Si02supports. Spectra ICand Id resulted after heating for 1and 4 h, respectively, at 483 132) S.J . Tauster, S. C. Fung, and R. L. Garten, J. A m . Chem. Soc.,
(1978). (33) S.H. Chien, R. N. Shelimov, D. E. Resasco, E. H. Lee, and G. L. Haller, J . Catai., 77, 301 (1982).
, "W'
Figure 2. Infrared spectra for the interaction of H, and CO over TiO,: (a) 4.1 mg cm-' of TiO, exposed to 75 torr of H, and 25 torr of CO for 4 h at 483 K; (b) 4.0 mg cm-' of 0.5 % Rh/TiO, exposed to 75 torr of H, and 25 torr of CO at 298 K; (c) sample in b heated for 4 h at 483 K.
K. In spectrum ICa new band is evident at 2045 cm-' which declined in intensity and shifted to lower wavenumber upon further heating (Id), while the species I11 bridged carbonyl band remained unchanged in intensity while also shifting to lower wavenumber. After long periods of heating (24 h) the species I11 band decayed also with little increase in methane production. For A120, and Si02the two bands change very little in intensity over a 4-h heating period and ultimately decay in intensity together after the methane production has declined to near zero. Fujimoto and co-workers have reported that the bridged species I11 is hydrogenated at lower temperature on Rh/A1203 than is "linear CO", which is presumably species II.* This would not appear to be true for Rh/Ti02 at least. In any case we do not believe that the band a t 2045 (2030) cm-' refers to species 11, as will be discussed later. Several other workers2J0 have reported that species I disappears without being hydrogenated, and at significantly lower temperture (ca. 423 K)34than in the absence of H2 (ca. 493 K). It should be noted that for longer periods of heating at 483 K for all of the supported 10% Rh films bands develop in the 2850-3000-cm-' region indicating the formation of higher hydrocarbons. Methane is the only detectable product following brief heating. Figure 2 shows the spectra for the interaction of H2 and CO over a 0.5% Rh/Ti02 catalyst film. Figure 2b clearly indicates that only species I exists at 298 K in accord with earlier observations for pure C0.l6 This is also the case for 0.5% Rh/AlZO3and Rh/Si02. Upon heating to 483 K, the species I bands disappeared, and a new band formed at 2043 cm-l; there was no evidence for species I11 on the 0.5% Rh catalysts. Solymosi and co-workers have attributed the new band near 2040 cm-I for supported Rh cat-
100, i70
(34) This work.
The Journal of Physical Chemistry, Vol. 87,No. 70, 7983
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T A B L E I: Infrared Absorption Frequencies a n d Turnover Frequencies for t h e Hydrogenation of C O over S u p p o r t e d R h o d i u m Catalysts freq a t 4 8 3 K*
freq a t 2 9 8 Ka catalyst
species I
10% R h / T i O , 10% R h / A 1 , 0 , 10% Rh/SiO, 0.5% R h / T i O , 0.5% R h / A l , O , 0.5% R h / S i O , a
Values in e m - ' .
2035, 2030, 2040, 2026, 2023, d
2102 2100 2095 2095 2095
-
species 11
species I11
hydrideC
species I11
2063 2060 2065
1905 1886 1900
2045 2045 2045 2043 2045 2040
1875 1858 1875
turnover freq a t 4 8 3 Kb 19.1 11.2 13.8 27.5 7.3 5.3
Values in molecules of CH, ( R h atom).' s-' x l o 5 (averaged over several hours with CH, contribution See t e x t f o r possible s t r u c t u r e . Obscured b y gas-phase CO b a n d ; could n o t be measured.
from blank s u p p o r t e x c l u d e d ) .
alysts which appears during hydrogenation of CO to a monocarbonyl hydride species such aslo H
\ /
Co
Rh
Electron donation from hydride into the antibonding CO orbital would be expected to cause a weakening of the C-0 bond with a concomitant reduction in C-0 stretching frequency (relative to species 11)and increased tendency for CO to dissociate to surface carbides.1° Many workers agree that such surface carbides are the most logical precursors to methane and higher hydrocarbons in a hydrogenation reaction.l~+ll#~~ On the other hand, Iizuka and co-workers have stated that the band near 2040 cm-' should be attributed to species I1 at low coverage.12p36 We believe that the data in Figure 2 support the conclusions of Solymosi and co-workers.1° For CO adsorption on supported Rh, species I1 has always been observed in conjunction with species 111,most probably because both refer to monocarbonyl adsorption on metallic crystallites. The fact that no species 111 is observed for 0.5% Rh on any of the supports during hydrogenation a t 483 K is strong evidence that species I1 is not present either, and that the 2040-cm-' band refers to a new species which is most probably the hydride suggested by Solymosi and co-workers.1° Furthermore, we have observed complete desorption of species I upon evacuation near 493 K in the absence of hydrogen without the formation of any new band in the 2000-cm-' region. It should also be noted that hydrocarbons other than methane were not detected by infrared for hydrogenation of CO over the 0.5% Rh films used in this work; this may indicate that crystallite sites are necessary for the formation of higher hydrocarbons. In general for the hydrogenation of CO over supported transition-metal catalysts the activities of the catalysts are (35) For a few examples see D. L. King, J. Catal., 6 1 , 7 7 (1980);J. G. Ekerdt and A. T. Bell, ibid., 62,19 (1980). (36)See also T. Iizuka and Y. Tanaka. J. Catal., 70,449 (1981);F. Solymosi and A. Erdohelyi, ibid., 70,451 (1981).
quite dependent upon the nature of the support, the order being T i 0 2 > A1203 > Si02.115,8J0-12,37 Table I gives the turnover frequencies observed in this work together with the pertinent infrared data. Since for supported rhodium catalysts it is difficult to estimate the number of catalytic sites because of varying crystallite sizes, the turnover frequencies in the table represent molecules of CHI (Rh atom)-'s-I, and as such are minimum values. The values are also low (ca. because the infrared cell used as a reactor did not contain a circulating pump. Nevertheless, it is apparent that Rh/Ti02 was more active for methanation at 483 K than Rh/A1203or Rh/Si02 in accord with reports of others; the latter two supported Rh films had similar activities.
Conclusions We believe that our data for 0.5% supported Rh films interacting with CO and H2 indicate that the geminal dicarbonyl species I is converted to a rhodium carbonyl hydride at temperatures exceeding 423 K as the first step in the methanation reaction. This new rhodium carbonyl hydride originally proposed by Solymosi and co-workers1° with its weakened C-0 bond then probably decomposes to a carbide or CH species which can be hydrogenated to methane. The fact that only methane was detected over 0.5% supported Rh films lends credence to earlier prop ~ s a l s ~ ~that J * Jsuch ~ catalysts having low Rh loading may contain isolated Rh sites. It would seem that crystallite sites would be necessary for polynuclear carbides which could be hydrogenated to higher hydrocarbons. Acknowledgment. The authors gratefully acknowledge the support of the Research Corporation, the National Science Foundation through Grant No. CHE-7920825, and the Auburn University Energy Grant-In-Aid Program for this work. Registry No. Rh, 7440-16-6; CO, 630-08-0; TiOz, 13463-67-7. (37) S. Y. Wang, S. H. Moon, and M. A. Vannice, J. Catal., 71, 167 (1981).
