ANDREWC. YANGAND CARLW. GARLAND
1504 TABLE V
Vol. 61
that chemical analysis of an acid of approximately
the hemihydrate composition indicated a non-ortho HBAT CAPACITYOF PHOSPHORIC ACID HEMIHYDRATE, P 2 0 6 content of 0.2%. ~H~POI-HIO (LOTB),CAL.D E G . -MOLE-1 ~ T,O K . 10
CR
T,OK.
T,O K .
CR
CP
0.245 115 26.94 220 46.52 15 0.996 120 27.95 47.40 225 2.26 20 125 28.95 230 48.27 3.83 25 130 29.94 49.15 235 5.52 30 135 30.92 240 50.02 7.16 35 140 31.89 245 50.89 8.81 40 145 32.86 250 51.76 10.37 45 150 52.63 33.81 255 11.80 50 34.76 155 53.51 260 13.12 160 55 35.70 265 54.40 14.47 60 165 55.28 36.63 2 70 15.85 170 65 56.17 37.56 275 17.20 175 70 57.05 38.48 280 18.40 39.40 180 75 57. 93a 285 19.57 80 40.31 290 185 58.81 20.72 85 41.20 295 I90 59.68 21.82 195 90 60.56 42.10 300 22.87 200 95 42.97 23.88 100 55.84 205 273.16 43.87 24.90 105 298.16 44.76 60.24 210 110 25.92 45.64 215 Values of Cp a t temperatures from 285°K. upward were obtained by extrapolation from the values for the "low energy" (all solid) form at lower temperatures.
*
An impurity content of 0.066 0.008 mole % was calculated'* from five pairs of points in the premelting range. If the acid was assumed t o be represented by the system 2H3P04.H20-H3P04, the area under the eutectic peak yielded 0.27 mole % HaP04. The impurity content is left somewhat uncertain by these values, together with the fact
Entropy.-Treatment of the data for derivation of the entropy of the hemihydrate was the same as for the anhydrous acid. The calorimeter contained 181.4207 g. (vacuum) or 0.84770 mole of 2HaP04.HzO(c). Tables are availablel6 showing the observed heat capacities and smoothed heat capacities at 1' intervals between 282 and 299'K. The heat capacities at 5' intervals are listed in Table V. The entropy a t 298.16'K. for 2H3P04"2O(c) is 61.73 e. u., of which 0.09 e.u. represents a Debye extrapolation (B = 123) from 0 to 10.26OK., and 61.64 e. u. represents integration from 10.26 to 298.16OK. The entropy increment under the eutectic peak, 0.17 e. u. between 282 and 298.16'K., is not included in the listed entropy. The entropy a t 273.16'K. is 56.65 e. u. Both values for entropy were based on the lower energy curve between 230 and 298.16'K., which is considered t o represent the solid state of the acid. The heat content, Ho- HoO, at 298.16OK is 9569 cal. mole-'. The increment in heat content under the eutectic peak, 50 cal. mole-l, is not included. The heat content a t 273.16'K. is 8118 cal. mole-'. The precision of the measurements between 16 and 280'K. was within 0.04%; below 16'K., the deviations increased progressively to 6% a t 10'N. The error in the region of the eutectic peak was about 0.2%. Acknowledgment.-J. A. Brabson, W. D. Wilhide, R. L. Dunn and Inez J. Murphy made the chemical analyses.
INFRARED STUDIES OF CARBON MONOXIDE CHEMISORBED ON RHODIUM1 BY ANDREWC. YANG~ AND CARLW. GARLAND Department of Chemistru and Spectroscopy Laboratory, Massachusetts Znstitu.teof Technology, Cambridge 39,Massachusetts Received March 81, 1967
The infrared spectrum of CO chemisorbed on rhodium surfaces has been investigated in the region from 1700-4000 cm.-l. The rhodium is supported on a high area alumina and the nature of this deposit can be varied by controIIing the per cent. metal and the heat treatment. On a series of rhodium surfaces, spectra have been studied as a function of coverage for adsorption and desor tion. Reaction of the chemisorbed CO with oxygen, hydrogen and water have been investigated spectroscopically. Tie results are interpreted in terms of several structureR for chemisorbed CO on rhodium.
I. Introduction I n recent years there has been a growing interest in techniques for determining the infrared spectra of molecules adsorbed on the surface of a solid, both for physical adsorption and chemisorption. One of the most significant studies for chemisorbed species has been the work of Eischens, Francis and Pliskin3 on the spectrum of CO adsorbed on (1) This work was supported in part by the Office of Naval Research. (2) Consumer's Union Predoctoral Fellow, 1954-1957.
(3) R. P. Eischens, S. A. Franois and 60, 194 (1958).
W. A. Pliskin, THISJOURNAL,
supported platinum, palladium and nickel surfaces. The work reported below is concerned with the infrared spectrum of CO on rhodium surfaces. There are several reasons for this choice of rhodium. It is a transition metal with an unfilled d band, thus a good chemisorber and catalyst. I n addition, it has a different electronic structure than Pt, Pd or Ni; Rh has nine electrons (4ds5s ground state) outside a closed Kr core whereas Pt, Pd and Ni all have ten outer electrons. Finally, rhodium salts are easily reducible t o the metal, the metal does not dissolve hydrogen4 and adsorbed
c
Nov., 1957
INFRARED STUDIESOF CARBON MONOXIDE CHEMISORBED ON RHODIUM
hydrogen may be desorbed readily.6 A disadvantage in working with R h is that not a great deal is known about the bulk properties of the pure metal. The infrared spectrum has been studied as a function of coverage for CO on surfaces of rhodium supported on a high area, non-porous alumina (Alon CS). This alumina support is free from interfering bands in the region of CO absorption and does not chemisorb CO. Sample composition was varied from 2 to 16% R h by weight. Changes in the spectrum have been followed for the reaction of chemisorbed CO with 02, Ht, and H20. 11. Experimental Equipment Design .-In its general features the equipment used for this work is similar to that described by Eischens, et ~ l . who , ~ have discussed the problems to be considered in the construction of an adsorption cell for infrared work and the necessary changes in a single beam infrared spectrometer. The cell used is shown in Fig. 1. It is constructed from a medium length standard taper 40/35 Pyrex joint and has 1.5 inch diameter CaFz windows (A) which are sealed to the cell with clear glyptal. The sample is supported on a 30 mm. diameter CaFz plate (1.5 to 3 mm. thick) (B) which is positioned a t the center of the cell by a Pyrex ring (C) which rests on the rim of the inner part of the standard taper joint. Heating is provided by a winding of Chrome1 wire (D) around the outside of the cell. The sample temperature can be measured with a chromel-alumel thermocouple which enters the cell at the lower glyptal seal (E) and whose junction is placed in a slot in plate B. The cell has a side arm (F) with a stopcock and ball joint so that it may be disconnected from the vacuum system if desired. The taper joint is sealed with silicone stopcock grease and may be cooled by circulating wa.ter through the coils (G) for high temperature operation. This cell can be used up to 450'. To make use of this cell, which must be mounted vertically, it was necessary to modify a Perkin-Elmer model 12C infrared spectrometer by the addition of a new set of source optics and a Nernst glower mounted above the spectrometer. The source beam was focussed on the sample in the center of the cell and then refocussed on the entrance slit of the monochromator. The entire spectrometer including the new source and the cell could be enclosed and flushed with dry nitrogen to remove atmospheric water vapor and carbon dioxide. Sample Preparation.-The starting material used was hydrated rhodium trichloride, presumably RhClr4Hz0, which is water soluble. To an aqueous solution of the salt, some high area alumina (Alon C) was added, and this mixture was sprayed slowly onto a 30 mm. CaFz plate which was heated on a hot plate. A uniform deposit without cracks can be obtained using a fine commercial perfume sprayer, by adding some acetone to the mixture, and maintaining the temperature of the plate a t approximately 70'. Sample deposits of about 11 mg./cm.* were used which after reduction contained from 2 to 16% by weight of rhodium. The cell was then assembled and the sample dried by heating to 150" while pumping a t mm. for an hour or more. Reduction was carried out by heating in hydrogen gas, then pum ing out the cell and repeating the process with another h i n g of Hz. Following Hz reduction, the sample was degassed a t 10-6 mm. for several hours. Two types of reduced rhodium surfaces were prepared which will he designated as unsintered and sintered. Each type was dried prior to reduction in the same way. The unsintered samples were never heated over 200' during the reduction or degassing of Hz. The reduction was carried out from 125 to 150' for a total time of 30 to 60 minutes; degassing was done from 150 to 200' for periods up to 10 hours. This is sufficient to remove adsorbed hydrogen, but traces of water (4) E. Muller and K. Schwabe, Z . physilc. Chem., 8 1 6 4 , 143 (1931); I. E. Adadurov and N. I. Pevnyi, J . A p p l . Chem. (U.S.S.R.),10, 1216 (1937). (5) P. I. Bel'kevich, J . Gen. Chem. ( U . S . S . R . ) , 9, 944 (1939). (6) Alon C is a product of Godfrey L. Cabot, Ino., Boston 10, Mass. We are indebted to Mr. K . A. Loftman for providing u$ with a gample having 8 B.E.T. area of 95 rnqeg.-q
1505
T 7.3 i n .
C
I
Fig. 1.-Cell
\
for infrared study of chemisorbed CO.
were still seen in background spectra. The sintered samples were heated to approximately 400'. I n some cases reduc; tion took place a t 150' followed by degassing a t 375 to 400 for periods up to 10 hours; in other cases both the reduction and the degassing were carried out at about 400'. No difference was observed for these two kinds of sintered surface and neither showed molecular water in the background spectra although, of course, a sharp band a t 3675 crn.-l due to hydroxyl groups on the alumina is still present. A few samples were reduced either by heating in CO at 150' for hour or heating in vacuo at 400' for 1.5 hours to avoid the use of hydrogen. Comparable results were obtained on surfaces prepared in all these ways aside from a tem erature effect of sintering. N! electron micrographs of the samples have been obtained; however, it is probable that the metal is concentrated in small patches on the alumina as reported by Eischens8 for platinum on silica. X-Ray diffraction shows no lines due to Rh for the 2% samples, but a broad line corresponding to a spacing of 2.19 b. (probably the (111) R h lattice spacing which is 2.1906 A. in the bulk metal) for the 8 and 16% samples. Microscopic examination shows that considerable aggregation of the alumina particles has taken place; these aggregates can be easily broken up by grinding and are clearly very porous in nature. Recording of Spectra.-The spectral regipn from 1700 to 4000 cm.-l was investigated using a CaFz prism. The background, which is quite free from interfering bands, was always measured for the reduced sample prior to adsorption. No trace of C-0 or C-H vibrational bands is seen in the background spectra, indicating that all the acetone used in the spraying preparation has been removed. Background transmission a t 2000 cm.-' varied from 2 to 5% of the incident energy depending on sample thickness, metal content and heat treatment. Accordingly slit widths of 0.2 to 0.3 mm. were used in this work. The background was frequently checked a t the end of a run after CO had been removed by pumping, oxidation or reduction. Reproducibility is indicated by an uncertainty figure assigned to each band.
111. Results Spectra of CO as a Function of Increasing Coverage.-To obtain the spectra of chemisorbed CO at increasing surface coverages, small quantities
ANDREW C. YANGAND CARLW. GARLAND
1506
Vol. 61
which case the 2095 and 2027 bands were of equal intensity even at their weakest). No other bands were observed. The stepwise addition of CO was, therefore, studied for a series of Rh samples : 2% sintered, 8% unsintered and sintered, 16% unsintered and sintered. All of these samples showed a more pronounced low coverage band a t about 2045 em. -1 and a new band at 1905 t o 1925 cm.-l. The resulting spectra for GO on an 8% sintered Rh surface are shown in Fig. 3. In this case, a band at 2045 cm.-l is quite pronounced before either the 2108 & 1 or 2040 f 4 cm.-l bands appear and is still visible as a shoulder at 2062 h 5 cm.-l in the final spectrum. It should be noted that the frequency of this band increases with increasing coverage. The band at 1925 5 cm.-' begins to appear at slightly lower coverage than the bands a t 2108 and 2040; it also shifts with coverage. Another run was carried out on the stepwise addition of CO on an 8% sintered surface where the spectrum is recorded as rapidly as possible after each CO addition without allowing time for equilibrium to be achieved. I n this experiment a 2055 em.-' band becomes quite pronounced and then upon mm. standing with CO gas in the cell at 2 X actually decreases in intensity while the 2108 and 2040 cm.-' bands increase. The bands which occurred a t 2095 and 2027 on unsintered surfaces appear at higher frequency on sintered surfaces by about 13 cm.-l. This is true of all the sintered samples and is due to the effect of adsorbed water. The unsintered samples have not been degassed at as high a temperature and contain some residual water as shown by background spectra. To test this point, samples were prepared by adding a small amount of H 2 0 vapor t o a sintered sample prior t o CO adsorption. With these samples, the bands occur a t 2095 and 2027 cm.-' as they do on unsintered samples. (Preadsorbing hydrogen on sinter samples does not cause a shift in these bands, eliminating adsorbed hydrogen as a possible cause of the shift.) Water also has an effect on the other bands. The band appearing from 2045 i 2 to 2062 f 5 cm. with increasing coverage on sintered samples appears from 2040 f 3 to 2055 f 5 cm.-l with increasing coverage on concentrated unsintered samples; the band appearing at 1925 f 5 ern.-' on sintered samples appears a t 1905 5 cm.-l on concentrated unsintered samples. These shifts were also confirmed by the adding of a small amount of water vapor to a sintered sample prior to CO adsorption. The 1905 or 1925 cm.-' band is the most sensitive to sample treatment and the least reproducible of all the bands; also, correcting for the background transmission in this region is often difficult. Aside from this frequency shift caused by water, the series of surfaces studied form a sequence. The spectra of CO on 2% sintered and 8% unsintered Rh are intermediate between those shown in Figs. 2 and 3. I n the case of 16% sintered Rh, there is a medium band a t 2108 cm.-l and the 2040 cm.-l band appears as a shoulder on a strong band at 2062 cm.-'. I n general sintering is more effec-
*
20'
I 2050 FREP. (ern;').
21AO
I
2000
Fig. 2.-Spectra of CO chemisorbed on a 2% unsintered Rh sample for increasing coverages. The equilibrium CO gas pressures are 1.28, 2.22, 3.60, 4.50, 6.85, 5.10 and 25.6 X 10-3 mm. IO0
i 0 80 Ln Ln
H Ln
z 60
-I
ae 40
2100
2000
1900 F R E P (cm:'),
1800
Fig. 3.-Spectra of CO chemisorbed on an 8% sintered Rh sample for increasing coverages. The equilibrium CO gaB pressures are 1.60, 2.36, 3.43, 5.71, 8.09 and 20 X 10-3 mm.
of the gas were admitted to the cell, time was allowed for equilibrium to be achieved, and then both the gag pressure in the cell and the spectrum were recorded. Direct measurement of coverage, 0, has not been made due to the large dead space volume of the cell; however, the coverage of each chemisorbed species should be closely proportional to the absorbance of the infrared band which arises due to that species. Results for CO adsorption on a 2% unsintered Rh surface are shown in Fig. 2. There is no change in the final spectrum on pumping out the cell to mm. at room temperature following adsorption. The essential feature of these spectra is the presence of two strong bands at 2095 f 1 and 2027 f 2 cm.-l which appear with almost equal intensities and whose positions are not a function of coverage. At the lowest coverages, there is another band which appears a t about 2045 cm.-'. Although the appearance of the 2095 and 2027 em. -' bands was very reproducible, the intensity of the band at 2045 was sensitive to sample preparation and sometimes was hardly observable (in
Nov., 1957
I N F R a R E D STUDIES O F C A B O N
tive in changing the spectrum than is an increase in the rhodium content. In discussing further results, the main emphasis will be placed on runs made with sintered samples t o avoid the complicating influence of adsorbed water. Spectra of CO on Desorption.-To achieve a reasonable rate of desorption, it is necessary to pump on the chemisorbed CO at elevated temperatures. The conditions chosen were pumping at 10-4 mm. with the sample maintained a t a constant temperature of about 150". For desorption of CO from a 2% unsintered surface the 2095 and 2027 cm.-l bands disappear and have equal intensities at all times. I n addition, there is a slight shift to lower frequency (about, 7 cm.-l) during desorption. This desorption is not completely a reversible removal of CO as shown by the fact that the surface after desorption will not readsorb as much CO as it did initially, and the change is greater than that due to loss of area on sintering. The behavior for desorption of CO from an 8% sintered surface is more complex, as shown by Fig. 4. I n this case, the bands do not simply disappear in the reverse order of their appearance on stepwise addition. The bands decrease somewhat in intensity on prolonged pumping a t room temperature. On pumping a t 150°, the band at 2108 cm.-' is removed readily (with a slight shift to lower frequency as in the case of the 2% sample) and the band a t 1925 ern.-' disappears somewhat more slowly with pronounced broadening. There remains a single band which disappears on pumping very slowly with a steady shift to lower frequency. Behavior of the 1925 cm.-' band is somewhat variable in its rate of disappearance but that shown in Fig. 4 is typical of the broadening and shifting toward lower frequency which occurs. Oxidation of Chemisorbed C0.-After the complete chemisorption of CO on a Rh surface, the cell was filled with 0 2 gas at approximately 0.3 atm. The cell was then closed off and heated; spectra were recorded throughout the oxidation process. For CO on 2% unsintered samples, there is no change at room temperature but the 2095 and 2027 cm.-l bands decrease in intensity on heating in oxygen a t 170". At the same time COZ gas is being formed in the cell as indicated by the growth of its 2349 cm.-l doublet band. During the oxidation, there is no shift in frequency of the two surface bands nor any appreciable change in their relative intensities. Also a new band appears at 2127 i 3 cm.-l which is finally the most intense surf ace band. This 2127 band is not a physically adsorbed or weakly held species; when oxidation is stopped before completion and the cell is pumped a t 111111. a t 170", the 2127 band decreases in intensity more slowly than the bands a t 2095 and 2027 cm.-l. It does not seem to be COZ chemisorbed on Rh, since a separate experiment showed no infrared bands for adsorbed species in the region studied when COZwas admitted at room temperature to a cell containing a freshly prepared R h surface. This band is also observed during the initial
MONOXIDE CHEMISORBEn ON RHODIUM
1
I
2100
1
I
I I I900 FREQ. (crn:'~,
2000
I
1507
I
1800
Fig. 4.-Spectra of CO chemisorbed on a 8% sintered Rh sample during desorption at 10-4 mm. Full coverage of CO prior to pumping (curve 1); pumping a t 20' for 16 hr. (curve 2); subsequent pumping a t 150' for 15 min. (curve a), 4 hr. (curve 4), 8 hr. (curve 5 ) , 22 hr. (curve 6).
01
i
I
2100
I
2000
I
I 1900
I
1
1
I800
F R E Q . (ern;').
Fig. 5.-The effect of oxygen on the spectrum of GO chemisorbed on an 8% unsintered Rh sample. Full coverage of CO prior to adding 02 (curve 1); heating in 0.25 atm. O2 at 170' for 10 min. (curve 2), 20 min. (curve 3), 60 min. (curve 4).
stages of the reaction of CO with the supported rhodium chloride salt which has been degassed at room temperature. The results for oxidation of CO on an 8% unsintered R h surface are given in Fig. 5. I n this case there is initially a shoulder at about 2050 cm.-' and a band at about 1900 cm.-l in addition to the 2095 and 2027 cm.-l bands. The band at 1900 disappears at room temperature in 0 2 , while that at 2050 disappears slowly and is shifted to 2077 cm.-'. A s before COz gas is formed in the cell. Additional results were obtained by the oxidation of CO on an 8% sintered surface. The initial spectrum of chemisorbed CO showed a strong 2062 cm.-l band and only medium bands a t 2108 and 2040 cm.-' as well as a band at 1925 cm.-'. At room temperature with oxygen, there was no change in the 2108 and 2040 bands but the 1925 band disappeared completely while the 2062 band decreased in intensity also and shifted to higher frequency. On heating, the 2127 band which appeared was weaker than that shown in Fig. 5.
ANDREW C. YANGAND CARLW.GARLAND
1508
I
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2 100
I
I
2000
I
1900 FREQ I cm-'),
I 1800
1
I700
Fig. 6.-The effect of hydrogen on the spectrum of CO chemisorbed on a 2% unsintered Rh sample. Full coverage of CO prior to adding Hz (curve 1); heating in 0.5 atm. Hz at 170' for 20 min. (curve 2), 60 min. (curve 3).
2oL
I
2100
I
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2000 FREQ.
I
1900
I
I
I800
I
Icrn:').
Fig. 7.-The effect of hydrogen on the spectrum of CO chemisorbed on an 8% sintered Rh sample. Full coverage of CO prior to adding HZ(curve 1); heating in 0.5 atm. HZ at 170' for 6 min. (curve 2, CO gas formed in cell), 48 min. (curve 3, CO gas gone), 57 min. (curve 4).
The shift of the 2062 em.-' band was confirmed by preparing an 8% sintered sample with only a partial coversge of CO. Initially there was only a single infrared band at 2052 (similar t o that shown by Fig. 3 at partial coverage). When O2 was added to the cell at room temperature, this band decreased in intensity rapidly with a frequency shift to 2087 cm.-l and then slowly disappeared. A weak band at 2127 crn.-l was observed. Reaction of Hydrogen with Chemisorbed C0.As in the oxidation runs, CO is first chemisorbed on a R h surface, the spectrum is recorded, then the cell is filled with H f gas a t approximately 0.5 atmosphere and closed off. The sample is allowed to remain in contact with Hz for some time at room temperature, another spectrum is recorded and then the cell is heated to about 170" and spectra are recorded throughout the reaction. Results for the reduction of CO chemisorbed on a 2% unsintered Rh surface are presented in Fig. 6. No appreciable spectral changes were seen for this sample on contact with Hz at room temperature. On heating, the band a t 2095 em.-' disappears very rapidly, the infrared absorption in the region around 2000 changes from the sharp band a t
Vol. 61
2027 to a broad asymmetrical band at about 2045 cm.-' whose shape may be due to the presence of an unresolved band around 1980 em.--'. Most important is the appearance of a new broad band which occurs first at 1855 cm.-l and finally at 1830 ern.-'. With further heating in HP all bands due to surface species disappear slowly (about 7 hours for complete reaction). Since Hz is present in large excess of the CO the final products of the reaction are CH4 and H20. The H20 is of course readily physically adsorbed on the alumina present and gives rise to its typical v3 band at 3490 em.-'. Methane is present in the gas phase as indicated by the observation of t.he PQR branches of the C-H stretching vibration in the region of 3000 em.-'. In some cases, in the initial stages of reaction of GO with HZweak bands appear a t 2960 and 3030 em. -l. It is not possible t o decide whether this initial CH species is in the gas phase or weakly adsorbed on the surface since it is rapidly removed by pumping. This work was repeated using D2gas and the results are identical in the behavior of the spectrum from 1700 t o 2150 cm.-l, showing that none of the new vibrations seen there involve hydrogen motion. The v3 band for D20 on alumina appears around 2500 cm.-l and CD4 gas at 2256 cm.-l (&-branch). Reduction of CO chemisorbed on an 8% sintered R h surface is given in Fig. 7. I n this case, Hz at room temperature has a slow effect on the spectrum, primarily in a shift of the 1925 ern.-' band to 1875 em.-'. On heating in H2, it is possible to observe an increase in the band intensity at 2058 cm.-l as the bands at 2108 and 2040 em.-' disappear rapidly, and a small band appears a t 1860 cm,-l. Some CO gas appears in the cell during this initial heating. While it is present (time 6 t o 40 minutes for Fig. 7), the 2058 band and bands in the 1800 to 1920 em.-' region do not change significantly. On further heating in Hz, the low frequency bands become very broad and flat, and the 2058 band decreases in intensity with a shift to 2025 em.-'. Tn contrast to the results on unsintered samples, no asymmetric broadening of this band occurs for sintered samples and reaction proceeds much more rapidly. Broadening and slower reaction are both effects of residual adsorbed water on unsintered samples. I n all cases, hydrogen reaction does not shift the 2108 or 2040 em.-' bands but does strongly shift the band a t 2062 em.-'. To cotroborate this shift without the interference of 2108 or especially 2040 bands, a l G % sintered surface was studied. For such a surface there are only medium 2108 and 2040 cm.-l bands and a strong 2062 cm.-l band at full coverage. I n the sample used for Fig. 8, addition of CO t o the cell was stopped as soon as the band at 2108 em.-* first appeared so that the effect of Hz on the 2062 cm.-l band could be studied alone. The shifting of the initial 2062 cm.-l band to 2025 em.-' without any broadening is seen clearly, and no appreciable band at 1860 em.-' is seen during the reaction. Runs also were made with 8% sintered surfaces
Nov., 1957
INFRARED STUDIES OF CARBON MONOXIDE CHEMISORBED ON RHODIUM
on which hydrogen had been adsorbed prior to CO adsorption. For these samples, the 2108 and 2040 cm.-l bands were very weak. Also the low frequency band appeared at 1875 cm.-l instead of 1925 cm.-l, and the 2062 em.-' band appeared a t lower frequency in agreement with the effect at room temperature of H2 on CO chemisorbed on a regular sintered sample. Effect of HzO on Chemisorbed C0.-Studies of the reaction of adsorbed CO with Ht gas are complicated by the fact that HzO is formed as a product. This water may also react with CO in the latter stages of reduction with Hz. Discussed below are the effects of large amounts of water. When HzO vapor a t about 20 mm. pressure was introduced into a cell containing CO adsorbed on a 2% unsintered R h surface, the bands at 2095 and 2027 cm.-' decreased somewhat in intensity and were shifted to 2087 i= 1 and 2019 f 1 cm.-l. Also an unresolved broad shoulder appeared on the low frequency side of the 2019 em.-' band. On heating a t 170", the intensity of all bands decreased rapidly and COz gas was observed in the cell by its 2349 cm.-l doublet. Clearly a reaction is taking place between CO and H20 t o form COZ and Hz. Of course the H2 gas formed by this reaction can react with chemisorbed CO and also contribute to the disappearance of surface bands. The effect of H20 vapor (at about 20 mm.) on CO chemisorbed on a 16% unsintered surface is much more complex as shown by Fig. 9. I n general features it appears that H20 is effective in reacting with the surface species having bands at 2095 and 2027 crn.-l, not effective in removing the band around 2055 but does shift it by as much as 50 cm.-l, and has a pronounced effect on the infrared absorption in the region of 1900 cm.-I. There is no further change in the final spectrum shown in Fig. 9 with time. Preadsorbing HzO on an 8% sintered sample prior to CO adsorption, gave the low frequency band a t 1860 em. -l also. Since H20 will be present only in the latter stages of a treatment of adsorbed CO with H2 gas, its effect is most probably a contribution t o the rather variable behavior of the bands around 1900 em.-'. T o obtain a band as low as 1830 ern.-' both water and hydrogen seem to be required. This is shown in Fig. 6 for Hztreatment of CO on an unsintered sample (containing residual water) and confirmed by sintered samples where Hz, CO and H20 (or H20,CO, Hz) were added to the cell in the order given. IV. Discussion At the present time little direct evidence is available on the structure and bonding of chemisorbed gas molecules with a metal surface. There is general agreement that the bonding of a gas like CO with a transition metal surface involves the interaction of the lone pair electrons on the carbon atom with the d electrons of the surface metal atoms, but the details of this interaction are still unclear. Various theoretical models for chemisorption bonding have been discussed by Dowden? (7) D. A. Dowden. J. Chem. Sac., 242 (1950): In& Ene. Chem., 44, 977 (1952); paper a t Chemieorption Conference at Keele, England, July 1956.
I
I
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2100
1
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2000
1509
I
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/
I800
I900
FREQ. (ern.'), Fig. %-The effect of hydrogen on the spectrum of CO chemisorbed at artial coverage on a 16% sintered R h sample. Initial 80 prior to adding Hz (curve 1); heating in 0.5 atm. Hz a t 160" for 10 min. (curve 2), 20 min. (curve 3).
I
2100
I
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1
1900
2000
FREO
I
I I800
(cm:l),
Fig. 9.-The effect of water on the spectrum of CO chemisorbed on a 16% unsintered Rh sample. Full coverage of GO prior to adding HzO (curve 1); HzO vapor at 20 mm. a t 20" for 2 hours (curve 2), then heated a t 180' for 2 hr. (curve 3).
recently. I n the case of CO adsorbed on Ni and Pd, Eischens3interpreted his spectra in terms of two types of structure: a "linear" CO occupying a single surface site (-2050 cm.-l) and a "bridge" CO between two adjacent metal atoms in the surface (-1900 cm.-l). This is by analogy with the known metal carbonyls which have infrared bands in the region 1973-2080 cm.-' due to linear CO groups which are bonded to only one metal atom (as in nickel tetracarbonyl) and in the region 18291860 em.-' due to bridged CO groups which are bonded t o two metal atoms (as in iron nonacarbonyl).8 I n the work presented here, the interpretation has been intentionally separated from the presentation of experimental results since it may be possible to improve the interpretation in the light of future work. Clearly, the character of the R h surface varies with the conditions under which it is prepared. Since the 2% unsintered surface presents the simplest spectra, it will be considered first. The observed C-0 stretching frequencies (2095 and 2027 (8) J. W. Cable and R. H. Sheline, Chem. Ruua., 66, 1 (1956).
ANDREW C. YANGAND CARLW. GARLAND
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Vol. 61
and the other two bands at 2002 and 2104 em.-' would be overtones. If the complex does not have a center of symmetry, as in the structure
c1
\
c1
/
OC-Rh-Rh-CO
06
bo
there would be four infrared active CO fundamentals. Assumption that the higher frequency component always arises from the antisymmetrical coupling of the symmetrical (or antisymmetrical) CO vibrations would account for the observed order of intensities (2033 and 2088 cm.-l strong, 2002 and 2104 em.-' weak). I n any case, the results for this rhodium carbonyl chloride agree well with our assignment of the 2027 and 2095 cm.-l bands on unsintered samples (or 2040 and 2108 cm.-l bands on sintered samples) as due t o two CO's bonded t o a single Rh atom. 0 2100 2050 2000 When the metal concentration is increased to 8 or 16% and especially on sintered surfaces it is FREP. (ctn-.'), reasonable to expect a more regular and compact Fig.. !O.-The spqctrum of Rh2(C0)4C12. The upper metal surface. This is verified by the appearance curve 18 in CCla solution; the lower curve as a powder in a of a broad X-ray line due t o Rh which represents Nujol mull. a spacing of 2.19 A. This distance is very close to em.-') are in the expected region for linear CO vi- the lattice spacing of the (111) planes for bulk facebrations and they are of equal intensity for all cover- centered cubic rhodium. The infrared spectra of ages on both adsorption and desorption. They do CO on these sintered surfaces show two new feanot vary in frequency with increasing coverage and tures: the 2045 em.-' band which appears strong vary only slightly with decreasing coverage. This at low coverages and whose frequency varies with strongly suggests that they represent the symmetri- coverage to 2062 em.-' and the band a t 1925 em.-'. cal and antisymmetrical modes for two CO's bonded The 2045-2062 em. -' band is interpreted as arising to a single metal atom in a rather highly dispersed from a single linear CO bonded to one surface surface. Such a dispersed Rh surface is com- metal atom. This band has a frequency between patible with the low metal concentration and lack the two bands arising from 2CO's per Rh atom, of X-ray evidence for well-developed lattice spac- which is a t it should be. Support for this concluings. It is believed that this 2% unsintered sur- sion is seen in the stepwise addition of CO on an face is highly irregular in structure with small 8% sintered surface where the spectrum is recorded groups of metal atoms essentially isolated from each as rapidly as possible after each CO addition withother. As a test of this interpretation, the spec- out allowing time for equilibrium to be achieved. trum of rhodium carbonyl chloride was taken. I n this experiment a 2055 em.-' band becomes This compound can be prepared easily and has the quite pronounced and then upon standing with CO molecular formula Rhz (CO)&12> gas decreases in intensity while the 2108 and 2040 The spectrum from 1980 t o 2125 cm.-l of this cm.-' bands increase. It would appear that some complex in CC14solution and as a powder in a Nu- sites are adsorbing a second CO molecule thus injol mull is given in Fig. 10. The spectrum in ben- tensifying 2108 and 2040 bands a t the expense of zene solution is identical to that shown for CCL the 2055 band associated with single linear CO. solution. There are no infrared bands in the reAssignment of a surface species which gives rise gion from 1800 to 1980 em.-', thus eliminating t o the band at 1925 cm.-' on concentrated samples any bridge carbonyl groups in the molecule. The is rather difficult. This band is never very strong structure of this complex is not known, but might initially and is the least reproducible feature of the be spectrum. However, it is more intense for GO adsorption on 8 and 16% unsintered samples than oc c1 co on sintered samples. The frequency seems too low \Rd R 'h' for it to be a second kind of single linear GO species. Nor does a simple bridge structure between two Rh \c< 'co surface atoms appear likely since sintering imSince this structure has a center of symmetry, there pairs rather than enhances its intensity and the should be only two infrared active CO funda- band does not appear first on stepwise adsorption mentals. I n this case, the strongest bands at as one might expect for a simple bridge. It is pro2033 and 2088 em.-' would be the symmetrical posed that this 1925 em.-' band may be due t o a and antisymmetrical CO stretching fundamentals bridge structure between two Rh surface atoms each of which have a single linear CO already (9) W. Hieber and H. Lagally, 2. anorg. aEEgem. Chem., 251, 96 bonded t o them (1948).
i
od
.
Nov., 1957
INFRARED STUDIESOF CARBON MONOXIDE CHEMISORBED ON RHODIUM
1511
TABLE I 0 -.hSample prepn.
2% Rh, unsintered 8 or 16% Rh, unsintered 8 or 16% Rh, sintered
c
0
d
All coverages
2027 f 2, 2095 =!= 1
0
c
0
b
\/
Two CO’s per Bite
... ...
0 C
/c
Coverage
Low coverage High coverage Low coverage High coverage 0
0
...
2027 i2, 2095 i 1
...
2040 f 4, 2108 f 1
/
-Rhhll--
... 2040 f3 2055 i5 2045 i2 2062 i5
Bridged CO
...
... 1905 f 5
...
1925 i5
on a dispersed surface which is independent of other sites. However, the linear and bridged CO are held on sites in a more regular part of the rhodium surface and would be expected to shift to higher frequency with increasing coverage as they do (due to interaction of a site with other adjacent sites). I n particular, the adsorption of bridged species might shift the band for linear species. On desorption of CO from a 2% unsintered sample, no band due to single linear CO species was observed. Thus it appears that both CO’s are lost at once from a doubly occupied site. Also, after desorption the sample did not readsorb as much CO as it did initially. These facts suggest a disproportionation mechanism to form COZ gas and a surface carbide as in the case of CO on Fe.1° The results for desorption from an 8% sintered surface (Fig. 4) are difficult to interpret since the 2062 cm.-l band for single linear CO is not well resolved from the 2040 cm.-l component of the two bands assigned to the species having two CO’s bonded to one metal atom. For a 2% Rh sample, which shows only species having two CO’s bonded to one metal atom, the two components (2027 and 2095 em. -l) disappeared at the same rate on desorption. With this as a guide, it is felt that the behavior of the 2040 cm.-I band on the 8% sample can be inferred from the 2108 cm.-I band. The band which desorbs very slowly is thus associated with single linear species which gave the shoulder a t 2062 em.-’. The marked shift toward low frequency of this band on desorption probably is caused by large changes in the surface occurring on desorption. Probably a surface carbide is formed, as discussed above, which shifts this single linear CO band as does adsorbed 0 2 and Hzdiscussed below. On oxidation the new band appearing a t 2127 cm.-l could be due to a CO bonded to a Rh atom which is also bonded to an oxygen atom as shown by
-\/ \ /-Rh-RhThis model is in agreement with the order of appearance on stepwise addition. Although it does not seem possible at this stage to make a firm theoretical statement concerning the bonding between CO and the metal surface, the picture presented above is qualitatively reasonable on the assumption that each surface Rh atom would obey the “rule of 18,” if possible, to achieve a closed shell configuration. Each R h atom has nine outer electrons; if there were, say, five neighboring metal atoms, four of these electrons would go into two non-bonding atomic orbitals and five would be used in metal bonding orbitals leaving two orbitals vacant which could accept the lone pair of electrons from two CO molecules t o give a total of eighteen. If there were seven metal neighbors, two electrons could go into one nonbonding atomic orbital, seven would be used in metal bonding orbitals leaving one vacant orbital which would accept the lone pair from a single CO molecule. Thus an irregular and dispersed Rh deposit would favor the adsorption of two CO’s per metal surface atom, while a more regular and lattice-like deposit would favor single linear and bridged adsorption. Thus it is proposed that there exist three types of CO adsorption: two CO molecules per site, a single linear CO, and a bridged CO between two sites already having a linear CO. The infrared bands for chemisorbed CO which have been assigned to these three surface species are summarized in Table I for both low and high coverages on various rhodium surfaces studied. In the following discussion, the behavior of these species will be examined in terms of the experimental results. For adsorption, it is necessary to make the reasonable assumption that when one CO is chemi0 0 0 sorbed on a site capable of taking two CO’s, a c c c o second CO molecule is very readily adsorbed. \/ \/ + COn(g) -Rh+ Oz(g) -RhThis will account for the equal intensities at all coverages for the 2095 and 2027 cm.-l bands on a This reaction involves doubly occupied sites, which 2% unsintered surface. In interpreting the more agrees with the fact that the intensity of the 2127 complex spectra on concentrated samples, be- crn.-l band is stronger on oxidation when the 2108 havior of the CO adsorbed on doubly occupied sites and 2040 cm.-l bands are stronger initially. This can be inferred from the 2095 cm.-l band (or the assignment of the 2127 cm.-l band is perhaps prefequivalent 2108 cm.-l band for sintered samples) erable to a possible Rh-0-CO species since the since the 2027 cm.-l band is overlapped by the band is also observed during the initial stages of 2055 band. The fact that there is no frequency the reaction of CO with supported RhCL, and the shift with coverage for the bands of two CO’s per (10) R. P. Eischena and A. N. Webb, J . Chem. P h y e . , a0, 1048 site agrees with the picture of a “saturated” site (1952).
ANDREWC. YANGAND CARLW. GARLAND
1512
effect of a chlorine should be much like that of an oxygen atom in shifting the CO vibration to higher frequency. There is no shift in the 2108 and 2040 cm.-' bands during oxidation in agreement with the isolated nature of the sites with 2CO's. The shift of the 2062 cm.-l band to 2087 cm.-' as it disappears on oxidation would be due to the effect on a single linear CO of adjacent sites having adsorbed oxygen (Le., a change in the work function). Sites with 2CO's adsorbed react with 0 2 only when heated, even at partial coverage; the linear CO reacts somewhat for full coverage and completely for partial coverage at room temperature due t o the presence of adjacent sites for adsorption of 0 2 as reactive oxygen atoms. The bridged CO reacts rapidly with oxygen a t room temperature. I n the reaction of chemisorbed CO with hydrogen, those sites with two CO's bonded to a single R h atom react very slightly a t room temperature but quite rapidly at 150'. Some CO is displaced into the gas phase as indicated by the appearance of a weak doublet a t 2143 cm.-l. During the reaction a band appears a t 2048 f 3 cm.-' due to a single unreacted CO remaining on the original site 0
c
0
c
\/
R h + Hz
OC
+
x
\/ Rh
where X might be H, CHO or CH20H. Another broad band appears a t 1855 cm.-l which seems to be an intermediate species in the reduction of CO from doubly occupied sites but not in the reduction of single linear CO (compare Figs. 6 and 8). It is very difficult to assign this band due to the complication of the bridged CO band which is also shifted to lower frequency by hydrogen. Perhaps a type of bridge species is created from doubly occupied sites by hydrogen or perhaps this is an aldehyde-like intermediate. I n the case of the linear CO, hydrogen definitely shifts the 2062 cm.-' band to lower frequency during the reaction as expected for CO adsorbed on a regular surface where every CO is influenced by the nature of the absorbates on adjacent sites. The shift here is in the opposite direction from that caused by oxygen as one would expect since on metals oxygen is an electron-withdrawing adsorbate and hydrogen is an electron-donating absorbate. In all cases of reduction, the final products are clearly CH, gas and H20. The formation of these products makes it very difficult to identify any possible CII- or OHcontaining intermediates during the reaction. The effects of water on chemisorbed CO have not been studied in detail, but it has been shown that small amounts of residual water on unsintered
Vol. 61
samples shift all the bands to somewhat lower frequency. This water also slows down the rate of oxidation and reduction. Larger amounts of water shift the bands more (especially the linear and bridged CO) and will react to form COZ. It should be pointed out that it is very unlikely that any bulk rhodium carbonyls have been formed on adsorption of CO, since they are formed only a t very high pressures (above 100 atm.).g Measurement of the amount of Hz and CO adsorbed on Rh films has led Trapnell'l to propose a simple bridge structure 0
II
/\
Rh
Rh
for chemisorbed CO. This model is based on low mm. for temperature (-183'), low pressure H2 and less than mm. for CO) adsorption with the assumption that one hydrogen atom is chemisorbed per surface metal atom. With the same assumption, his adsorption data a t 0' give 0.8 molecule of CO per Rh atom. Considering the low pressures and the assumption used in Trapnell's work, the probable differences between chemisorption bonding a t high and low temperatures,* and the difference between our supported surfaces and metal films, it is not surprising that a rather different structure for chemisorbed CO has been proposed here. Finally, no infrared bands have been observed for chemisorbed hydrogen or deuterium on Rh. It is possible that a covalent surface hydride is not formed, but rather that hydrogen atoms donate their electrons to the metal and exist non-localized on the surface as protons. Support for this view is found in hydrogenation reactions occurring on rhodium catalysts.12 V. Summary On the basis of extensive infrared studies for CO chemisorbed on rhodium, it is proposed that there exist three types of surface species: two CO molecules bonded to one surface Rh atom, a single linear CO bonded t o one R h atom, and a bridged CO between two adjacent Rh atoms which already have a single linear CO bonded to each. The reaction of chemisorbed CO with 0 2 , HZand H 2 0 has been investigated, and interpreted in terms of these CO surface species. Acknowledgment.-The authors wish to thank Professor R. C. Lord for many helpful discussions on instrumentation and interpretation. (11) hf. A. H. Lanyon and B. M. W. Trapnell. Proc. Row Soc. u,ondon). ~ a a i 387 , ~955). (12) L. Hernandez and F. F. Nord, Ezperientia. 3, 489 (1947).
