898
J . Phys. Chem. 1984, 88, 898-904
Catalytic Oxidation of Carbon Monoxide over RhISiO,. Study
An in Situ Infrared and Kinetic
Janos T. Kisst and Richard D. Gonzalez* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881 (Receiced: April 15, 1983; Zn Final Form: July 29, 1983)
The oxidation of CO on Rh/Si02 has been studied by using an "in situ" infrared cell reactor. The catalytic activity for the rate of CO oxidation was shown to decrease in the sequence Rh(0) > Rh(1) > Rh(1-HI). The results of a temperature-programmed-oxidation study show that both linearly adsorbed (2065 cm-') and bridge-bonded (1920 cm-') CO react with O2 at a faster rate than the CO dicarbonyl species (2092, 2033 cm-I). A higher oxidation state of Rh, presumably Rh(I-III), which gave rise to an adsorbed CO species absorbing at 2103 cm-l, was found to be relatively inactive. The results of a temperature-programmed-reduction study show that the adsorbed CO dicarbonyl species can be converted into the linearly adsorbed species by the addition of H2 at room temperature. Unlike Ru/Si02 and Pt/Si02,reaction rate hysteresis did not occur. However, in an oxygen-rich reactant gas mixture, some modification of the reaction rate did occur as a result of the formation of Rh(1). This led to a modest catalyst deactivation. Both heat-transfer effects and diffusion-controlled processes occur at high CO conversions. Meaningful kinetic parameters can only be obtained by considering steady-state conversions which are less than 10%.
Introduction This represents the second in a continuing series of studies in which "in situ" infrared spectroscopy coupled with kinetic rate measurements is being used to obtain information regarding the catalytic oxidation of C O over supported noble metal catalysts under reaction conditions. In the first paper of this series,' the deactivation of a series of Ru/Si02 catalysts during the CO oxidation was studied. The results of this study strongly suggested that this deactivation was due to the formation of a more tightly bound oxygen species, perhaps a subsurface oxide or lattice oxygen which produced coordinatively unsaturated Ru surface atoms. The catalytic oxidation of C O over supported noble metals other than Pt and Pd has not received the attention that it deserves. Cant el aL2 have performed a comparative study of five noble metals in an attempt to bridge the gap between high-pressure studies on highly dispersed metal powders and low-pressure studies on well-defined, single-crystal surface^.^,^ These studies suggest reasons why results obtained in different laboratories under different experimental conditions in general give results which are not always comparable. In particular, the following variables may have a profound effect on the catalytic activity: (1) initial feed stream composition with which the catalyst is contacted,l,2 (2) heat- and mass-transfer effects which can alter metal particle temperatures,2 (3) dispersion effect^,^ (4) the presence of multiple isothermal steady-state oscillation^,'^^^^ ( 5 ) reaction rate hysteresis caused by intraparticle diffusivities and nonequilibrium adsorption e q ~ i l i b r i a ,(6) ~ , ~the Occurrence of segregated islands of reactivand (7) the role of oxygen as a modifier of the active sites responsible for the CO oxidation reaction.' Supported Rh appears to be the best behaved of the noble metal catalysts for the catalytic oxidation of C 0 . 2 However, the surface chemistry of Rh is complex and strongly dependent on its oxidation state." The use of infrared spectroscopy as a tool to study the adsorption of C O on Rh has been the subject of a large number of studies on both supported catalysts'2-'5 and well-defined crystal surfaces.I6 Because both the structure and the oxidation state of supported Rh can be reasonably well interpreted by its infrared spectra, we felt that an in situ catalytic study of the oxidation of C O on Rh/Si02 could provide useful information regarding the C O species present on the surface during reaction.
Experimental Section The flow system which enables the use of the infrared cell as either a pulse microreactor or a single-pass differential reactor +On leave from the Department of Organic Chemistry, Jozsef Attila University, Szeged, Hungary.
0022-3654/84/2088-0898$01.50/0
has been described in detail in a previous report.I7 The infrared cell reactor was constructed by the Byron-Lambert Co. of Franklin Park, IL, and has the important feature that the reactant gases are forced through the sample disk with little or no leakage occurring around the edges of the sample. For this reason, the infrared cell reactor can effectively be used as a differential single-pass reactor. In several experiments, a conventional Pyrex microreactor constructed from 12-mm Pyrex tubing having a volume of 4.4 mL was used. It has a length of 70 mm and was connected to the flow system by using 3.1-mm tubing and swagelock fittings. Product analysis was performed by using a gas chromatograph (Perkin-Elmer Model Sigma 3-B) located downstream of the reactor. A stainless steel column 1 m in length and having an outside diameter of 3.175 mm packed with Carbosieve S (100-200 mesh) was used to perform the separation. All infrared spectra were recorded on a Perkin-Elmer Model 28 1 infrared spectrophotometer interfaced to a Perkin-Elmer data station to facilitate processing of the data. The absorption bands due to the support and the gas phase were substracted from the resulting transmittance spectra and the result was either replotted as absorbance or reconverted to transmittance. Materials. The silica-supported Rh catalysts were prepared from solutions containing the appropriate weight of RhC13.3H20 (Strem Chemical). The solutions were mixed with Cab-0-Sil, grade M-5 (Cabot Corp., Boston, MA). The subsequent handling of the resultant slurry has been previously described.' The metal loading was 0.3 mmol of Rh/g of catalyst. For use in the spec-
(1) Kiss, J. T.; Gonzalez, R. D., submitted to J . Phys. Chem. (2) Cant, N. W.; Hicks, P. C.; Lennon, B. S. J . Catal. 1978, 54, 372. (3) Engel, T.; Ertl, G. Adu. Catal. 1979, 28, 2. (4) Creighton, J. R.; White, J. M. "Catalysis Under Transient Conditions"; Bell, A. T., Hegedus, L. L., Eds.; American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 178, p 1. (5) Sarkany, J.; Gonzalez, R. D. J . Appl. Caral. 1983, 5, 85. (6) Varghese, P.; Carberry, J. J.; Wolf, E. E. J . Caral. 1978, 55, 76. (7) Sheintuch, M.; Schmitz, R. A. Catal. Reu. 1977, 15, 107. (8) Hegedus, L. L.; Oh, S . H.; Baron, K. K. AIChE J . 1977, 23, 632. (9) Herz, R. K.; Marin, S. P. J . Caral. 1980, 65, 281. (10) Haaland, D. M.; Williams, F. L. J . Catal. 1982, 76, 450. (11) Oh, S. H.; Carpenter, J. E. J . Catal. 1983, 80, 472. (12) Yates, J. T.; Duncan, T. M.; Worley, S. D.; Vaughn, R. W. J. Chem. Phys. 1979, 70, 1219. (13) Arai, H.; Tominaga, H. J. Cars/. 1976, 43, 131. (14) Yates, D. J. C.; Murrell, L. L.; Prestrige, E. B. J . Catal. 1978, 54, 41. (15) Yang, A. C . ; Garland, C. W. J . Phys. Chem. 1957, 61, 1504. (16) Dubois, L. H.; Somorjai, G. A. Surf, Sci. 1980, 91, 514. (17) Miura, H.; Gonzalez, R. D. J . Phys. E 1982, 15, 373.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 899
C O Oxidation over Rh/SiOz
i n
co/o
= 4
x:
I
I
c0/02.2
3 % 2o
v 20.
W
VI W
363K 373K
t
383K
393K ]413K
393K: 383K
373K
363K
2
:
L IO
z
21% !
I
9 8%
9 0%
2%
4 t%
D-s-a
+
-,-
IO0
4 I%
200
9%
I
a"
3 00
T(MIN)
Figure 1. Reaction of a CO/Oz reactant gas mixture having a ratio of 4 on a Rh/SiOz catalyst as a function of temperature. Steady-state
conversions at each temperature are shown in the figure. Metal loading was 0.3 mmol/g of catalyst.
troscopic reactor, the dried catalyst was ground into a powder, less than 45 wm, and pressed into self-supporting disks 25 mm in diameter with an optical density of approximately 25 mg/cm2. The gases used in this study were subjected to purification treatments as previously described.' It was found convenient to use 0, and C O premixed with He to give the following compositions: 5.25% CO in He and 5.20% O2in He. The O2concentration in the carrier gas at the catalyst sample was checked by placing activated MnO in the microreactor. The absence of a color change suggested to us that trace amounts of 0, were in the ppb range. Water levels were much lower. These very low levels of 0,in the He carrier gas stream were attained by using a molecular sieve maintained at 77 K and an oxygen purifier (Supelco Co.) backed by an MnO trap activated in H2 at 673 K. Procedure. Fresh, silica-supported Rh catalysts were treated according to the following pretreatment schedule: the catalyst was heated in flowing H2 (25 mL/min) at 403 K for 0.5 h, the temperature was then increased at a rate of 10 K/min from 403 to 673 K, and the catalyst was reduced at this temperature for 2 h. The catalyst was then outgassed in flowing He at 673 K for 1 h followed by cooling in flowing He to room temperature. Subsequent reductions were for 1 h at 673 K in flowing H2. Treatment of the sample disks used in the infrared cell reactor was identical with that used in the Pyrex microreactor except that the final reduction was performed at 650 K. The reason for the slightly lower temperature used in the in situ infrared studies was to minimize damage to the polycrystalline CaF2 optical lenses. The in situ temperature-programmed studies were performed by flowing the appropriate gas, He, O2or H,, through the infrared cell reactor at a flow rate of 25 mL/min while the temperature of the infrared cell was linearly increased by means of an oven connected to a linear temperature programmer (Valley Forge Model 6000). A temperature program consisting of a 5 K/min rise in temperature was generally used as the infrared spectrum was continuously recorded. Reaction products were monitored by means of the gas chromatograph located downstream of the infrared cell reactor. Chemisorption measurements were performed by using the dynamic pulse method.18 Metal dispersions were calculated on the basis of a CO/Rh adsorption ratio of 1. The CO/Rh ratio of 1 is only valid when the surface concentration of the C O dicarbonyl species is small, as is the case for well-reduced Rh/SiO,. For the case of Rh/A1203or for very highly dispersed Rh catalysts this assumption is not valid.14 (18) Sarkany, J.; Gonzalez, R.D. J . Catal. 1982, 76, 75.
2%
,?x-
'-
8 a./
,
, 100
200 T (MIN)
-.z 2%
300
Figure 2. Reaction of a CO/O2 reactant gas mixture having a ratio of 2 on a Rh/Si02 catalyst as a function of temperature. Steady-state conversions at each temperature are shown in the figure. Metal loading was 0.3 mmol/g of catalyst.
Results and Discussion Turnover frequencies for the rate of CO, formation as a function of both temperature and CO/Oz ratio were measured with the Pyrex microreactor. The results for a C O / 0 2 ratio of 4 are shown in Figure 1. With the exception of an initial deactivation amounting to about 30% of the initial rate, little or no deactivation occurred at each temperature with time during the run. This observation is in marked contrast to similar studies on Pt,S Ru,' and PdI9 in which deactivation occurred for extended periods following each increase in temperature. In addition to this observation, we also observed no reaction rate hysteresis when the reaction temperature was decreased following 100%conversion at 433 K. Turnover frequencies obtained under conditions of increasing temperature were nearly identical with those obtained under conditions of decreasing temperature. At high CO conversions (413 and 433 K) heat-transfer effects became important as evidenced by the slow increase in the C 0 2 turnover frequency with time. Under these conditions, the heat of reaction could not be dissipated, resulting in all likelihood in a substantial increase in the temperature of the metal particles. When the catalyst temperature was increased to 433 K, full ignition (defined as 100% conversion of the limiting reagent) occurred only after the catalyst was contacted with the reaction mixture for 30 min. Turnover frequencies for C 0 2formation as a function of temperature for a reactant feed stream having a CO/Oz ratio equal to the stoichiometric ratio of 2 are shown in Figure 2. Because of the positive order of the reaction in 02,turnover frequencies for C02 formation were considerably larger a t the higher O2 partial pressures used under these reaction conditions. Ignition occurred rapidly at 413 K and the reaction rate could not be decreased when the temperature was decreased to 393 K. When the temperature was decreased to 383 K, the reaction rate decreased rapidly, showing no appreciable hysteresis. The turnover frequency obtained at 383 K under conditions of decreasing temperature was very similar to that obtained at 383 K under conditions of increasing temperature. We, therefore, conclude that the apparent hysteresis observed at 393 K is most likely due to heating effects and not to changes in reaction kinetics. When the CO/O2 ratio in the reactant feed stream was reduced to 0.5, significant changes in the turnover frequencies for C 0 2 formation a s a function of temperature were observed to occur (see Figure 3). Ignition was now observed to occur at 373 K and the reaction rate could only be decreased following a 20-min reaction period at 353 K. In addition, hysteresis was observed following the decrease in the rate of the reaction. When the temperature was increased to 373 (19) Kiss, J. T.; Gonzalez, R. D. J . Phys. Chem., submitted.
900 The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 A L L 02 CONV
co/02= 0 5
Kiss and Gonzalez TABLE I: Summary of Kinetic Data for the Oxidation of CO on Rh/SiO, Catalysts"
--.
8
'I
% 0
: i
2o
~l
> IOOXCONV
z 2
10
363K
1373K
3 6 3 ~ 1353,
343K
353K
363K
1
373K
6-i
1053 1756 3513
Increasing Temperature 0.211 0.504 1.129 0.292 0.674 1.897 1.319 15.3'
99.2 10.5
24.3 28.5
4256 3546 1773
1053 1756 3513
Decreasing Temperature 0.199 0.474 1.129 0.278 0.765 1.933 15.2c 15.3'
02.8 15.3
25.5 29.9
1773
3513
343 K 0.186
96.2
25.3
200 T(MIN)
300
353 K 0.482
363 K 1.335
383 K 14.8'
" Metal loading = 0.3 mniol of Rh/g of catalyst; dispersion =
ccu 6% 100
E,, kJ/ mol 1nA
383 K
4256 3546 1773
383K
31% '
373 K
Pa
u
Y
363 K
Pa
"
.
~ o ~ molecule/(site N , ~ s)
pco, Po,,
30
400
Figure 3. Reaction of a CO/02 reactant gas mixture having a ratio of 0.5 on a Rh/Si02 catalyst as a function of temperature. Steady-state conversions at each temperature are shown in the figure. Metal loading was 0.3 mmol/g of catalyst.
43%; weight of catalyst = 200 mg; flow rate = 25 mL/min. Reactant gases were 5.25% CO in He and 5.2% 0, In He. Steady-state rate. Diffusion-controlled rates due to 100% conversion.
383 K (40MIM 383K (10MIN)
K during a second heating schedule, ignition did not occur. A further increase in the temperature to 383 K resulted in rapid ignition with the additional observation of the formation of a transient pulse of COz. Because this transient pulse of C 0 2was only observed for the case of an oxygen-rich reactant mixture, its origin must be. attributed to the reaction of either surface carbon or strongly adsorbed C O present on the surface prior to the start of the ignition. We are inclined to believe that this C 0 2 pulse is due to the presence of surface carbon for two reasons: (1) As we will show later, infrared experiments show that, under conditions in which the CO/O2 reactant mixture has a stoichiometric excess of 02,only trace amounts of infrared-active C O are observed on the surface of the catalyst during ignition, and (2) the turnover frequency for C 0 2 formation immediately following the onset of the diffusion-controlled reaction is about twice that which would be expected for 100% conversion of the reactant gas mixture. These observations suggest that the Boudouard reaction occurs even under oxidative conditions and that the resultant carbon which is deposited on the surface builds to a coverage which is close to or greater than that corresponding to a monolayer. When the CO/O2 ratio is either net reducing or close to that corresponding to a stoichiometric mixture, there is an insufficient amount of O2 available to react with the excess surface carbon and the transient C 0 2 pulse is not observed. A complete summary of the kinetic data is shown in Table I. Activation energies obtained under conditions of increasing temperature were consistently lower than those obtained under conditions of decreasing temperature. The reason for this is most likely due to a slight inhibition by adsorbed oxygen following ignition. The apparent activation energies obtained in this study are in excellent agreement with the value of 103 f 5 kJ/mol reported by Cant et a1.2 Because of these results, it was hoped that an in situ infrared study would shed some light on the nature of the adsorbed species present on the Rh/Si02 catalyst under reaction conditions. Infrared Studies. The Pyrex microreactor was replaced by the infrared cell reactor and the experiments described above were repeated. The results obtained by using a CO/O2 reactant gas mixture having a ratio of 4 are shown in Figure 4. The main feature in the infrared spectrum of C O obtained during reaction at 383 K is a sharp band centered at 2066 an-'(Figure 4A). This band is assigned to the stretching vibration of CO linearly adsorbed on metallic Rh. Other features in the infrared spectrum include a broad band centered at 1924 cm-' assigned to bridge-bonded C O in addition to two poorly developed shoulders on each side of the sharp CO stretching band centered at 2066 cm-I. These bands have been the subject of some controversy12-15but it is generally believed that they are due to the symmetric and an
2066
!
453 K IGNITION
2063
b
Figure 4. In situ infrared spectra obtained during the reaction of a CO/O, reactant gas mixture having a ratio of 4: (A) infrared spectrum obtained at 383 K, (B) infrared spectrum obtained during ignition at 453 K; ( C ) infrared spectrum obtained at 383 K following ignition.
antisymmetric stretching vibrations of a C O dicarbonyl species adsorbed on Rh(1) sites.20 The fact that these infrared bands are poorly developed suggests that the Rh has been thoroughly reduced. This is in general agreement with the results of Yates et al.,I4 who found that a Rh/Si02 catalyst having a dispersion of about 50% compared to a dispersion of 43% for the catalyst used in this study gave a CO/Rh, chemisorption ratio of unity. Rh/A1203catalysts or very highly dispersed Rh/Si02 catalysts are more difficult to reduce and, therefore, give CO/Rh, chemisorption ratios in excess of unity. During ignition of the reaction at 453 K, the absorbance of the infrared band assigned to linearly adsorbed CO was observed to decrease and shift to 2056 cm-' (Figure 4B). This is in general agreement with the observation that the position of the infrared band assigned to linearly adsorbed C O shifts to higher frequencies with increasing surface c ~ v e r a g e . ~This ~ - ' is~ the ~ ~ result ~ of CO dipole-dipole coupling. C O dicarbonyl species are not involved in dipole4ipole coupling and for this reason the positions of the infrared bands do not change with surface coverage. Following 100%conversion at 453 K, a reduction in the reaction temperature back to 383 K essentially restored the original spectrum (Figure 4C). Turnover frequencies for C 0 2 formation measured by using the infrared cell reactor at 383 K differed by no more than 10% before and after ignition. Infrared spectra corresponding to a stoichiometric CO/O2 reaction mixture are shown in Figure 5. The position and the (20) Worley, S. D.; Rice, C. A.; Mattson, G.A.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1982, 76,20.
C O Oxidation over Rh/Si02
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 901 (A)
CO/O2=05
2063 '06'
373K
1-
1\
w 0 2
U m
383K -
IGNITION
2106
1923
L
cc
0
2065
v)
m
IGNITION
a
373K
2k._/'Mt
Figure 5. In situ infrared spectra obtained during reaction of a CO/O2 reactant gas mixture having a ratio of 2: (A) increasing temperature; (B) ignition at 413 K; (C) decreasing temperature. ~~
absorbance of the infrared band due to linearly adsorbed CO as a function of temperature are shown in Figure 5A. The slight increase in the absorbance observed when the reaction temperature is increased from room temperature to 363 K is due to an increase in the intensity of the symmetric and antisymmetric stretching vibrations of the CO dicarbonyl species giving rise to the features at 2090 and 2036 cm-'. The decrease in frequency of the infrared band assigned to linearly adsorbed CO from 2067 to 2063 cm-' is evidenced that the surface coverage due to this surface species has decreased as the result of an increase in temperature. When the temperature was increased to 403 K, CO diffusion limitations became important and the absorbance of all infrared bands assigned to surface species was observed to decrease. Figure 5B shows the infrared bands present on the surface during ignition at 413 K. It is important to note that, although ignition occurs immediately following the increase in temperature, the complete disappearance of the infrared bands due to the presence of the CO surface species during ignition of the reaction required about 20 min. This observation is explained on the basis of heat effects which cause a slow increase in the temperature of the metal particles above that of the support. Following ignition of the reactant gas mixture for about 20 min, only a trace of the infrared spectrum due to the surface species could be observed; the dominant spectral feature now corresponded to the CO dicarbonyl species. Figure 5C shows the redevelopment of the infrared bands under conditions of decreasing temperature following ignition. There is now a noticeable increase in the absorbance of the infrared bands due to the CO dicarbonyl species in addition to a new feature centered at 2103 cm-I. These results suggest an increase in the number of sites responsible for C O adsorption in the dicarbonyl configuration, possibly Rh(I), in addition to the formation of a higher oxidation state of Rh, possibly Rh(III), which gives rise to the infrared band centered at 2103 cm-l. Turnover frequencies for the formation of C 0 2 at 363 K were 1 X and 0.9 X molecule/(site s) before and after 100% conversion, respectively, indicating very little change in the catalytic activity. The infrared results corresponding to a CO/O2 ratio of 0.5 are shown in Figure 6. Under conditions of increasing temperature (Figure 6A), a sharp drop in the absorbance of all the infrared bands was observed. It should be noted that the infrared spectra of the adsorbed CO species can be used as a useful diagnostic probe to predict the onset of diffusion limitations when the temperature is increased. When the absorbance of the infrared band due to adsorbed CO is large, we can be reasonably sure that CO diffusion limitations are not important. During ignition, only one small infrared band centered at 2106 cm-I was observed in the spectrum. This band is in all likelihood due to CO adsorbed on a higher oxidation state of Rh and appears to be the most unreactive state of Rh involved in catalytic oxidation. It is formed either by the exposure of a Rh/Si02 catalyst to O2 at high temperatures or during the CO oxidation reaction in an oxygen-rich feed stream. Under conditions of decreasing temperature (Figure 6B) all in-
(0 MIN)
2098 2067
~~
Figure 6. In situ infrared spectra obtained during reaction of a CO/Oz reactant gas mixture having a ratio of 0.5: (A) increasing temperature; (B) temperature decreased to 363 K.
I
2102
2102
I1
Ill'
Figure 7. In situ infrared spectra and reaction rates for the formation of CO,on the different oxidation states of Rh: (A) reaction rate measured following exposure to 0,at 573K for l h; (B) ignition at 393 K as a function of time in a C O / 0 2 reactant gas mixture of 0.5; (C) reaction at 363 K following ignition. Turnover numbers are expressed in molecules of CO,/(site s).
frared bands reappear. However, the doublet assigned to the CO dicarbonyl species is now noticeably more intense. In addition to this observation the turnover frequency measured at 363 K decreased from 6 X molecule/(site s) under conditions of increasing temperature to 3.7 x molecule/(site s) following ignition at 383 K. In addition to this observation, the absorbance of the infrared band assigned to the linearly adsorbed CO speices decreased considerably suggesting a substantial decrease in C O surface coverage brought about by the interaction between Rh and O2during full conversion. These results are in good agreement with those of Kim et aL21 These authors studied the oxidation of CO on polycrystalline Rh and found that CO adsorption was inhibited by chemisorbed oxygen at temperatures in excess of 530 K. In addition, they also observed that oxygen adsorbed at high temperatures was catalytically less reactive than oxygen adsorbed at room temperature. This led them to invoke the possibility that this oxygen was trapped below the surface either as lattice oxygen or as a subsurface oxide. (21) Kim,
Y.;Shi, S . K.; White, J. M.J. Catal. 1980, 61, 374.
902
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
Because these results suggest different catalytic CO oxidation rates on the different states of Rh, we performed a series of experiments aimed at measuring these rates using in situ infrared spectroscopy. The results of this experiment are shown in Figure 7. Following pretreatment in O2 at 573 K for 1 h, the catalyst was cooled to room temperature and contacted with a C O / O 2 reaction mixture having a ratio of 0.5. The infrared spectrum of the adsorbed C O species following this pretreatment is shown in Figure 7A. The main feature corresponding to this spectrum is the strong band centered at 2100 cm-I previously assigned to a higher oxidation state of Rh. When the temperature was increased to 363 K, the absorbance of all three infrared bands increased, indicating a partial reduction of the Rh surface. However, the infrared band at 2101 cm-I was still the dominant feature in the spectrum. We suggest that under these conditions the higher oxidation state of Rh predominates. However, some Rh(1) is also present, as evidenced by the infrared bands at 2032 and 2090 cm-I. The turnover frequency for C 0 2 formation was measured under these conditions and found to be 0.68 X molecule/(site s). It did not change significantly during a 30-min reaction period. When the temperature was increased to 393 K, the turnover frequency for C 0 2formation increased slowly over a 60-min period from 4.5 X molecule/(site s) immediately following the increases in temperature to a maximum of 44 X molecule/(site s) during ignition. During this time, the infrared spectrum of adsorbed C O corresponding to an increase in reaction rate underwent significant changes. The most important spectral change was the intensitification of the infrared doublet assigned to the C O dicaibonyl species. The observation that the turnover frequency for C 0 2formation measured by using the infrared cell reactor was greater by a factor of 2 during ignition than that measured by using the Pyrex microreactor is simply due to the diffusion-controlled nature of the reaction during ignition. No infrared bands due to gas-phase C O were observed during ignition under conditions in which excess O2was present. The factor of 2 is, therefore, due to the fact that about 200 mg of catalyst was charged to the Pyrex reactor while an average of 100 mg of catalyst was used in preparation of the catalyst disk charged to the infrared cell reactor. Following ignition, the temperature was reduced to 363 K and the turnover frequency for C 0 2 formation was remeasured following a 60-min reaction period and found to be 1.02 X molecule/(site s). When the infrared spectrum obtained under these conditions (Figure 7C) is compared to that immediately following O2treatment at 573 K (Figure 7A), it is apparent that the most important difference centers on the increase in the intensity of the C O dicarbonyl species following ignition. Under these conditions, the dominant feature in the infrared spectrum of adsorbed C O is the dicarbonyl doublet at 2091 and 2031 cm-l with some Rh in a higher oxidation state still remaining as evidenced by the infrared band centered at 2104 cm-I. When the catalyst was subjected to the standard pretreatment and recontacted with the CO/O2 reaction mixture, only one infrared band centered at 2065 cm-I was observed and the turnover frequency for C 0 2formation was remeasured and found to be 2.7 X molecule/(site s). We conclude, therefore, that the oxidation rate of CO decreases in the sequence Rh(0) > Rh(1) > Rh(1-111). These results are in good agreement with a recent study published by Oh and Carpenter." On the basis of ESCA measurements, these authors determined that the higher oxidation state of Rh responsible for a decline in the C O oxidation rate was Rh(II1). However, we are still open-minded on the assignment of this specific oxidation state of Rh other than suggesting that it is probably greater than Rh(1). In order to obtain a better understanding of the reactivity of the different adsorbed species of CO with 0, a temperatureprogrammed-oxidation study was performed following ignition in a 0.5 CO/O2 reaction mixture at 383 K for 0.5 h. Under these conditions infrared bands at 2103, 2092, 2063, 2034, and 1920 cm-' were observed in flowing He (Figure 8A). When the He was changed to a 5% O,-He gas mixture, the 2063- and 1920-cm-' bands immediately disappeared and C02was detected on the gas chromatograph connected downstream of the infrared cell reactor.
Kiss and Gonzalez I
(A)
t
2063
5092
% -2 298K IN HE
2 9 8 IN 5 X 0 2
403K
493K
(B) 10 MIN IN FLOWING U 2
COzEVOLVED
373 T I 3 7 3
Figure 8. Temperature-programmed-oxidationstudy performed following the ignition of a CO/02 reactant gas mixture having a ration of 0.5: (A) spectra of adsorbed CO as a function of temperature in a 5% 0,-He gas mixture; (B) CO, evolved as a function of temperature. TABLE 11: Titration of the CO Adlayer with 0, and H , Following the Ignition of a C O / O , Reactant Gas Mixturea 298 K
0, consumption CO, evolved H, consumption CO, evolved
uptake, WL 298-383 K 3 8 3 K
199
0
71
161
62
21
360 14
Metal loading = 0.3 mmol/g of catalyst; Rh dispersion = 38%; CO/O, = 0.5; reaction at 383 K for 1 h followed by cooling to 298 K prior to each titration; volume of H, and 0, pulses added = 100 WL.
The corresponding gas-chromatographic response as a function of temperature is shown in Figure 8B. The catalyst temperature was then increased at a rate of 5 K/min in the flowing He-0, gas mixture while the infrared spectrum was simultaneously recorded. The infrared spectra of the adsorbed species clearly show that the CO dicarbonyl species reacts with O2at tempertures which are considerably higher than those which are observed for the reaction of the linearly adsorbed and bridge-bonded CO species with 0,. The results of the temperature-programmed-oxidation study are, therefore, in good agreement with the steady-state CO oxidation results shown in Figure 7. C@02-H2 Titration Experiments. The results shown in Figure 8 suggest that it might be possible to obtain a rough estimate of the relative amounts of linearly adsorbed and bridge-bonded C O as compared to the dicarbonyl species by titrating the CO surface species present with 0, at 298 K following ignition. The results of this experiment for a Rh/Si02 catalyst having a dispersion of 38% Rh are summarized in Table 11. In these studies it is important to stress that the CO surface coverage following 100% conversion cannot be compared to the CO surface coverage following the standard pretreatment in H2 for two reasons: (1) on the reduced Rh/Si02 catalyst only linearly adsorbed and bridge-bonded C O is present, and (2) the CO surface coverage is considerably lower following ignition due to the inhibition of CO adsorption by coadsorbed 0,. When 1 0 0 - ~ LO2 pulses were added to a Rh/SiO, catalyst which has been reacted in a 0.5 CO/O2 reaction mixture at 383 K for 1 h, 199 r L of O2was consumed, resulting in the evolution of 161 WLof C 0 2 . The catalyst was then heated to 383 K in flowing He with the evolution of about 62 additional MLof C 0 2 . Additional pulses of O2at 383 K resulted in the evolution of 21 p L of C 0 2 . When these results are considered together with the infrared observations, we can conclude that the ratio of linearly adsorbed and bridgebonded CO to the dicarbonyl species is about
C O Oxidation over Rh/Si02
I
I G N I T I O N OF A
I O O p I OF H 2
(0)
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 903
C O l O ~ O S M I X T U R E F O R I nR
F L O W Hp A T 2 9 8 K
ADDED AT 298K
F l O W H2AT 3 5 8 ~ 1
W 0
Z
a
m
LL
0
m
m
(C)
a
FLOW
HZ A 1 3 3 5 1
p
4
FLOW+
AT473K
7
l
2025
Figure 9. Interaction between adsorbed CO and H2: (A) infrared spectrum of adsorbed CO following the ignition of a C O / 0 2reactant gas mixture having a ratio of 0.5; (B,C) in flowing H2 at increasing tem-
peratures. 161/83 or roughly 2. On the basis of the O2titration experiments the 244 p L of C 0 2evolved represents a surface coverage of about 10.2 pmol of C O in comparsion to 21.34 pmol of C O adsorbed on the H2-pretreated surface. The decrease in the absorbance of the infrared bands following ignition at 383 K (Figure 6A,B) suggests that this result is quite reasonable. The literature regarding the formation of the C O dicarbonyl species has been somewhat inconsistent. One school of thought suggests that the surface concentration of the C O dicarbonyl species is larger on centers of low surface coordination, Le., edges, steps, kinks, and small parti~1es.l~ However, the predominant point of view, which we also ascribe to, is that the dicarbonyl species is predominantly adsorbed on Rh(1) sites.20 Because of these observations we felt that it would be interesting to treat a Rh/Si02 catalyst with H2 following reaction at 100% conversion at 383 K for 1 h in a CO/O2 reaction mixture having a ratio of 0.5. Under these conditions, all five infrared bands due to adsorbed CO were present. The addition of H2at room temperature resulted in the rapid formation of H 2 0 . However, only a trace amount of C02(g) (14 pL or 0.58 pmol) was evolved. The infrared spectrum of the adsorbed C O species following the addition of 100-pL pulses of H2 at room temperature is quite revealing. The results shown in Figure 9B show that, following the addition of a 100-pL pulse of H 2 at room temperature, all of the C O dicarbonyl species disappeared. In flowing H2, the absorbance of the C O band at 2065 cm-I shifted to 2050 cm-' as the result of the electron-donating properties of the H 2 0 formed during the reaction. It was observed to intensify and continued to increase when the temperature was increased in flowing H, at 5 K/min. Appreciable amounts of CH4 were formed only after the temperature exceeded 373 K reaching a maximum of 473 K (Figure 10). These results strongly suggest that the Rh sites capable of adsorbing C O in the dicarbonyl configuration can readily be converted into Rh sites which adsorb C O in the linear configuration through the addition of H, at room temperature. The structure of the adsorbed C O species present following treatment in H2 has been attributed to a monocarbonyl hydride species having the s t r ~ c t u r e ~ ~ ~ ~ ~ H
' Rh'
co
T M) Figure 10. Temperature-programmed-reactionstudy. Methane evolved as a function of temperature.
The results of the present study are not conclusive with respect to this assignment. However, the intensification of the 2050-cm-l band as the temperature is increased in flowing H2 is in accord with the observation that adsorbed O2 inhibits the adsorption of CO. The removal of this surface oxygen, therefore, promotes the formation of linearly adsorbed CO. The extent to which carbon was formed as the result of the disproprotionation of C O was estimated by using the following procedure: a monolayer of CO was adsorbed at room temperature and then submitted to a temperature-programmed-desorption study which was terminated at 673 K. Under these conditions, only trace amounts of C O could be detected on the surface by using infrared spectroscopy. A pulse of O2 was added at this temperature and the C 0 2 evolved was measured. Because only 1 pmol of C 0 2 was evolved, we conclude that the amount of carbon formed was not very large, but certainly measurable. These results are consistent with our observation that the turnover frequency for C 0 2formation increased to a high value immediately following the ignition of a C 0 / 0 2 reaction mixture having a ratio of 0.5. We direct one final comment to the reactivity of the linearly adsorbed CO on a well-reduced Rh/Si02 catalyst. Following the adsorption of CO to monolayer coverage, a 100-pL pulse of O2 was added at 298 K and the C 0 2 evolved was measured (Figure 11A). The C 0 2 evolved following the addition of the first pulse of O2 was 70 pL and 35 p L of O2 of the original 100-pL pulse did not react to form C 0 2 . With the addition of subsequent pulses of 02,a larger percentage of the incoming pulse was observed to react, reaching about 100% following the addition of the fourth pulse. As the C O adlayer was continuously depleted as a result of the reaction, the volume of the unreacted 0, pulse continued to increase. These results are similar to those obtained on Pt by Sarkany et al.24in this laboratory. We interpret them as follows: the initial pulse of O2reacts slowly due to the requirement that O2must dissociate on vacant Rh dual sites before it can react with CO by a Langmuir-Hinshelwood mechanism. The Eley-Rideal mechanism for 02(g) + CO(ad) has generally been ruled out on ~ the basis of the labeled CO experiments of M a t ~ u s h i m a . ~Be(22) 1671. (23) (24) (25)
Worley, S. D.; Mattson, G. A,; Candill, R. J . Phys. Chem. 1983, 87, Solymosi, F.; Tombacz, I.; Docsis, M. J . Caral. 1982, 75, 78. Sarkany, J.; Bartok, M.; Gonzalez, R. D. J . Cafal. 1983, 81, 347. Matsushima, T. J . Cafal. 1980, 64, 38.
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8
Kiss and Gonzalez
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
reaction between adsorbed C O and adsorbed oxygen by a Langmuir-Hinshelwood mechanism or the formation of a precursor state similar to that proposed by Bennet et a1.26 It is important to note that dual Rh sites are not required for the adsorption of CO. During the addition of subsequent pulses of CO, a slightly larger fraction of the incoming pulse was absorbed than that which reacted. This can be explained by a slow Langmuir-Hinshelwood reaction between the adsorbed surface species which occurred between the addition of pulses of CO. This created additional vacant surface sites capable of adsorbing additional CO. Considerable tailing in the eluted C 0 2 peaks gave further evidence of the Occurrence of this slow surface reaction. The mechanism which we feel to be consistent with these observations can be summarized as follows:
d I
801-
I
CO(g)
PULSE NUMBER
02(g) I 00
-( B)
CO-Rh(s) oa + Cog
+ 0-Rh(s)
UNREACTED CO
+ Rh(s)
- + --*
+ 2Rh(s)
C02(g)
CO-Rh(s)
(1)
20-Rh(s)
(2)
Rh(s)
(slowstep) (3)
Conclusions
w
1 r
,oi.
I
I
ADSORBED CO
zI:
REACTED CO'
0 0
'>
'
\
.-.*
p--0. a-9
2
4
6
8
IO
12
14
16
PULSE NUMBER
Figure 11. (A) Addition of 0, pulses (100 rL) to preadsorbed CO on a Rh/Si02 catalyst at 298 K: (0)adsorbed 0,; (0) reacted 0,; (m) unreacted 0,. (B) Addition of CO pulses (100 ML) to preadsorbed oxygen on a Rh/Si02 catalyst at 298 K: ( 0 )adsorbed CO; (0) reacted CO; (w) unreacted CO.
cause dual Rh sites can be created only by the displacement of weakly adsorbed C O by 02,the initial rate is slow. As the C O surface coverage is depleted by the formation of C 0 2 , additional vacant dual sites capable of dissociating molecular O2 become available, resulting in a larger consumption of the added O2pulse. When the titration procedure is reversed (Figure 1 lB), the results are somewhat different. Following the addition of the first CO pulse to a monolayer of adsorbed oxygen, 55% of the CO pulse reacted, 40% was adsorbed, and 5% did not react. This is consistent with previous results on PtZ4and suggests either the rapid
The following important conclusions emerge from this study: (1) In situ infared studies show that the rate of the catalytic oxidation of C O on Rh/Si02 is faster when Rh(0) predominated on the surface. The catalytic activity sequence was shown to decrease in the order Rh(0) > Rh(1) > Rh(1-111). (2) In situ infrared temperature-programmed-oxidation studies show that both linearly adsorbed (2065 cm-I) and bridge-bonded (1920 cm-') C O react with O2 at a faster rate than the CO dicarbonyl species (2092, 2033 cm-'). (3) The C O dicarbonyl species is readily converted to the linearly adsorbed species through the addition of H2 at room temperature. (4) Both heat-transfer effects and diffusion-controlled processes occur at high C O conversions. Meaningful kinetic parameters can only be obtained by considering steady-state conversions which are less than 10%. (5) Unlike C O oxidation on Ru/Si02 and Pt/Si02, reaction rate hysteresis does not occur. However, in an oxygen-rich reactant gas mixture, some modification of the reaction rate does occur as a result of the formation of Rh(1). This leads to a modest catalytic deactivation. Acknowledgment. Acknowledgment is made to the National Science Foundation, who provided funds under grants DMR 78-18917 and CPE-7920155 for the support of this research. Registry No. Carbon monoxide, 630-08-0; rhodium, 7440-16-6. (26) Bennet, C. 0.;Dwyer, S. M. J . Catal. 1982, 75, 75.
