J . Phys. Chem. 1990, 94, 4617-4622 as positively or negatively charged. The interaction between surfaces bearing adsorbed gelatin is obviously very complex and much remains to be done before a complete understanding of the behavior can be reached. From the results presented here, it is clear that gelatin is able to provide significant colloidal stabilization over a wide pH range, even if its charge has the same sign as that of the surface. Conclusion Direct surface force measurements between layers of gelatin adsorbed on mica surfaces shed some light on the complex nature of adsorption and the configuration of adsorbed chains. The response of these layers to variations in pH and electrolyte concentration can be explained utilizing the ’three-layer model” of Norde and Lyklema,,* which leads to some understanding of the structure of the adsorbed layers. The most important factor appears to be the degree of dissociation of ionic groups in the
4617
gelatin molecule. This determines the affinity between the surface and gelatin segments as well as between segments. Because the type and the magnitude of these electrostatic interactions vary with the pH, the effect of electrolyte concentration is not uniform. On the acidic side of the isoelectric point, where gelatin and mica surfaces are oppositely charged, the steric repulsion increases with electrolyte concentration as the electrostatic attraction between gelatin and the surface is shielded, and the adsorbed layer becomes expanded. On the basic side of the isoelectric point, where gelatin and mica surfaces are similarly charged, the range of the repulsion decreases significantly with electrolyte concentration as the electrostatic repulsion between the gelatin segments are shielded, and the adsorbed layer contracts. At the isoelectric point the adsorbed layer seems to be very compact at low electrolyte concentrations due to intersegmental charge coupling. This electrostatic effect is also shielded as electrolyte concentration is raised, and the adsorbed layer expands.
Rhodium Surface Chemistry on a Chemically Modified A1,03 Support Dilip K. Paul, Todd H. Ballinger, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: September 18, 1989; In Final Form: January 4, 1990)
It has been shown that the OH groups present on a Rh/AI2O3 catalyst may be reacted away using dation by exposure to gas-phase (CH Sic1 at 450 K. The removal of these OH groups strongly suppresses the tendency of CO to destroy Rh: sites forming Rh j)?(CO), species, since this reaction involves surface OH groups as oxidizing agents. It is also demonstrated that CO chemisorption capacity of the Rhx0sites is maintained following the silation reaction, suggesting that the catalytic behavior of nonoxidized Rh may be preserved after chemical modification of the catalyst surface by dation. In addition, studies of the reversible bonding of (CH3)3SiCIto isolated OH groups at 300 K are reported.
I. Introduction Catalysts for controlling automotive exhaust emission are currently comprised of the noble metals Pt, Pd, and Rh.’ Of these metals, Rh is the most effective for reducing the oxides of nitrogen to nitrogen gas.’v2 Typically, Rh is highly dispersed on A1203supports and is active as the metal, designated Rh,: for the desired catalytic chemistry. It is now well-known that active Rh,: may be converted to Rh’ by a complex oxidation process in which isolated surface hydroxyl groups are consumed and, in the presence of CO(g), the spectroscopically identifiable Rh’(CO), species is p r o d ~ c e d . Studies ~ of this oxidation process have been made using a number of physical measurement techniques such as extended X-ray absorption fine structure (EXAFS), where van7 Blik4 conclusively demonstrated that Rh: crystals were destroyed in the presence of CO(g). It is known that both CO(g) and isolated surface O H species must simultaneously participate in the destruction of Rh: sites. This process can be reversed by using H2(g), which reduces Rh’(CO), species back to Rh: while regenerating the isolated hydroxyl group^.^*^ Thus, the interesting reversible redox chemistry involving supported Rh may be summarized as follows: (1 /x)Rh:
+ OH(a) + 2CO(g)
z Rh’(CO),
+ ( 1 /2)H2(g) (1)
The objective of the work reported here is to remove the isolated hydroxyl groups from the Alz03support by chemical reaction with a silation agent so that the conversion of Rho to Rh’ cannot occur by reduction of the active hydroxyl groups. Additional objectives include the determination of the adsorption characteristics of the *Author to whom correspondence should be addressed.
0022-3654/90/2094-46 17$02.50/0
silation agent on Rh: sites and the study of the thermal stability of the alkylsiloxyl species on the AI2O3surface. This work builds upon earlier studies of Knozinger? Zaki,’ and Solym~si,*-~ who have shown in different ways that O H groups are involved in the surface reaction which destroys Rh: sites in the presence of CO(g). The first investigation of CO adsorbed on a Rh/AI2O3catalyst was reported by Yang and Garlandloin which they identified three different types of rhodium sites on the alumina support: 0
oc\Rh/co I
C
I
Rh 11
0
II
C
Rh/ ‘Rh 111
Infrared bands at 2095 and 2027 cm-’ were assigned respectively to symmetric and antisymmetric stretching modes for adsorbed species I, the gem-dicarbonyl, Rh’(CO),. These two bands do not shift with increasing C O coverage and correspond closely in wavenumber and character to those (2095 and 2043 cm-I) observed for the chlorine-bridged Rh2(CO),CI2 molecule which (1) Taylor, K. C. In Studies in Surface Science and Catalysis; Crucq, A., Frenet, A., Eds.; Elsevier Science Publishers: New York, 1987; Vol. 30, pp 97-1 16. (2) Wan, C. Z.; Dettling, J. C. In ref 1, pp 369-395. (3) Basu, P.; Panaytov, D.; Yates, Jr., J. T. J . Am. Chem. SOC.1988, 110, 2074. (4) van? Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Am. Chem. SOC.1985, 107, 3139. ( 5 ) Basu, P.; Panayotov, D.; Yates, Jr., J. T.J . Phys. Chem. 1987, 91, 3133. (6) Knozinger, H.; Ratnasamy, P. Caral. Reu.-Sci. Eng. 1978, 17, 31. (7) Zaki, M. 1.; Kunzmann, G.;Gates, B. C.; Knozinger, H. J . Phys. Chem. 1987, 91, 1486. (8) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789. (9) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1986, 90, 5312. (IO) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957, 61, 1504.
0 1990 American Chemical Society
4618
Paul et al.
The Journal of Physical Chemistry, Vol. 94, No. I I , 1990
contains two Rhl(C0)2 moieties. The other two bands for I1 and 111 species at 2045-2062 and 1905-1925 cm-l are associated with Rh; sites and are due to terminal-CO and bridged-CO species, respectively. 11. Experimental Section
4k Re~elutien 5 4cm-1
T = 300K Evacuated ot I x 1 0 ‘ ~ Torr (Desorption I
, J
Al,O,-supported Rh catalyst samples were prepared”-” in a slurry containing RhC13.3H20 and A1203(Degussa aluminum oxide C) in the appropriate ratio to produce 2.2 wt % Rh/AI2O3. The slurry was suspended in a liquid consisting of nine parts spectroscopic grade acetone and one part distilled water. The slurry was deposited by spraying through an atomizer onto one-half of a 25-mm-diameter CaF2 disk maintained by a hot plate at 340 K . The solvents evaporated rapidly, leaving a thin adherent film of RhCI,.3H20/AI20, on the support disk. The other half of the CaF, support disk was sprayed with metal-free AI2O3 support material in an identical manner. This “half plate” method allows us to observe any chemistry occurring on the support itself which has been treated under identical conditions as the catalyst. A total sample weight of (25-29) X 10” g was deposited in separate sample preparations, yielding a final surface density (Rh/AI2O3) of (8.7-10.7) X IO-, g/cm2 of the geometrical area for the various samples employed here. The surface density of Rh is therefore in the range (1.9-2.4) X 1 0-4 g of Rh/cm2. The catalyst sample was then mounted inside the stainless steel ultrahigh-vacuum infrared celI1IJ2 which was subsequently evacuated by a bakeable all-metal vacuum system employing a liquid nitrogen cooled sorption pump and a 20 L/s ion pump. The sample was outgassed under vacuum at 475 K for about 72 h and subjected to 15, 30, 45, and 60 min cycles of exposure to -400 Torr of H2 at 475 K, each cycle being terminated by evacuation to 1 X 10” Torr. The reduced samples were allowed to remain under vacuum (1 X IO” Torr) at 475 K for a period of -8 h. Following catalyst reduction, silation was performed by admitting IO Torr of (CH,),SiCl(g) to the cell at 450 K for about 8 h. The silated sample was then evacuated for 2 h at 475 K to remove any unreacted (CH3)3SiCI. In controlled studies of the irreversible silation process, such as that shown in Figure 2, the treatment of the AI2O3and Rh/A1,03 surface at 450 K was performed using four cycles of (CH,),SiCI exposure at 0.3, 1.45, 4.05, and 4.15 Torr for an hour, each cycle followed by evacuation at 475 K for 30 min. Then the cell is cooled to 300 K to obtain the IR spectrum. The adsorption isotherm of CO on 2.2% Rh/AI20, and on the silated Rh/AI20, catalyst was measured at 300 K in the IR cell on a substrate prepared under the conditions described above. A total sample (Rh/AI,O,) weight of 5.86 X g was deposited on the full CaF, sample plate area (5.07 cm2). Pressure measurements within the range of 0-10 Torr were made using a Model 22 I A Baratron absolute pressure transducer (MKS Instruments) with an accuracy of *5% of the reading. The transducers were carefully insulated to prevent rapid temperature changes from influencing the measured pressure. Infrared spectra were obtained with a purged Perkin-Elmer Model PE-783 double-beam grating infrared spectrophotometer coupled with a 3600 data acquisition system for data storage and manipulation. The spectra presented here were obtained with a slit program yielding a resolution of 5.4 cm-I acquired at 1 point per cm-I with a typical data acquisition time of 1-10 s/cm-I. The (CH,),SiCI (Petrarch Systems Inc., purity >99.9%) was packaged under a N, atmosphere. I t was transferred to a glass/metal storage bulb inside a drybox purged constantly with N 2 gas. The reagent gas was purified by using several freezepump-thaw cycles in a preparative glass vacuum line operating at a base pressure of 1 X IO-’ Torr. The high purity of this gas was verified by means of mass spectrometry. The H, gas (Matheson) was obtained in a high-pressure cylinder with a purity
-
( I I ) Yates, Jr., J. T.; Duncan, T. M.; Vaughan, R. W . J . Chem. Phys. 1979, 71, 3908. (12) Beebe, Jr., T. P.; Gelin, P.; Yates, Jr., J. T. Surf. Sci. 1984, 148, 526. (13) Beebe. Jr.. T. P.; Yates. Jr., J . T. Surf. Sci. 1985, 159, 369.
3800
I 3600
3400
3200 3000
2800 1300
1200
Wovenumber (cm-I)
Figure 1. Infrared spectra showing characteristic vibrational modes of (CH3)3SiCIon A1203for reversible adsorption and desorption at 300 K.
of 99.995%. The CO (Matheson) was obtained in a break-seal glass bulb at a purity level of 99.9%. 111. Results A . Reversible Interaction of Surface Hydroxyl Groups with
(CH,),SiCI. IR spectra in the v(0H) spectral region before and after 4.0-Torr exposure of (CH,),SiCl(g) to AI2O3at 300 K are shown in Figure 1, In agreement with the model for O H groups on AI20, proposed by Peri,I4-l6the bands at 37 16 and 3670 cm-’ are assigned to isolated hydroxyl groups, whereas the broad u(OH) feature centered at -3580 cm-’ is believed to be caused by vibrations of H-bonded (associated) hydroxyl groups. Upon adsorption-desorption of (CH,),SiCI at 300 K the following reversible spectral changes are observed. ( I ) The intensity of v(0H) features due to isolated hydroxyl groups decreases to near zero. (2) An increase in the absorbance in the associated O H stretching frequency region occurs. (3) Upon evacuation, the spectral features at 3716 and 3670 cm-l (isolated O H groups) are restored and a simultaneous decrease of the intensity due to associated OH group is observed. These observations are consistent with a model that involves the interacton of the CI moiety of (CH,),SiCI with isolated O H groups to produce modified O H species in which the v(0H) is shifted downward. Such shifts have been observed by Kiselev and co-workers!’ for ethers adsorbed on SiO, containing isolated O H groups. In addition, shifts of this type have been observed by Beebe et al.I23l3for the physisorption of CO and N2 near isolated O H groups and for the adsorption of (CH3)20,18(CF2H),0,I9 and CH3C120*21 on isolated O H groups on AI2O3. Figure 1 also shows the characteristic spectral regions for the u(CH3) and 6(CH3) modes of physically adsorbed (CH,),SiCI species, and these spectral frequencies are compared with spectral measurements of these frequencies for pure (CH3),SiC1 gas in Table I. The reversible behavior noted from the observations of the OH modes during evacuation of the surface is confirmed by the observations of the desorption of (CH,),SiCI at 300 K as seen in Figure 1, where the infrared intensity due to both CH3 modes is strongly reduced upon evacuation. B. Irreversible Reaction of Surface Hydroxyl Groups with (CH3),SiCI. Figure 2A shows the effect of heating a hydroxyl-covered A1,03 surface to 450 K under various pressures of (CH3),SiCI. It is observed that the isolated O H modes at 3716 and 3670 cm-’ gradually disappear. In addition, there is an observable decrease in the intensity of the broad associated O H band centered near 3630 cm-I. This behavior of the associated OH band contrasts with the reversible weak adsorption behavior (14) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960, 64, 1526 ( 1 5 ) Peri, J . B. J . Phys. Chem. 1965, 69, 220. (16) Peri. J. 8. J . Phvs. Chem. 1965. 69. 211. ( 1 7 j Kiselev, A. V.;Ligin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (18) Chen, J . G.; Basu, P.; Ballinger, T. H.; Yates, Jr., J. T. Langmuir 1989, 5, 352. (19) Basu, P.; Ballinger, T. H.;Yates, Jr., J. T. Lungmuir 1989, 5, 502. (20) Crowell, J. E.; Beebe, Jr., T. P.; Yates, Jr., J. T. J . Chem. Phys. 1987, 87, 3668 (21) Beebe, Jr., T. P.; Crowell, J. E.; Yates, Jr., J. T. J . Phys. Chem. 1988, 92. 1296
Rhodium Surface Chemistry on an A1203Support
a
4- Resolution
The Journal of Physical Chemistry, Vol. 94, No. I!, 1990 4619 TABLE I: Vibrational Frequencies of Trimetbylcblorosilane and Tbose Observed for (CH3)$iCI/AI,03 following Adsorption at 300 and 450 K frequency, . . cm-' mode
(CH,),SiCl(g) 2912 1454 1415 1260
UCH3)
-a
WH,)
(CH3)3SiC1/A12030(CH3)3SiCI/Al20~ (300 K) (450 K) 2906 1450
2907 1450 1412 1267 -1075
1261
u,,(SI-O-AI)
v
0)
V
a
c
Reversibly adsorbed. "Chemically bound.
0
2n
18,,01
u)
a
-
N
h
a
f
1-1 T.450
K
Resolution T0.01~
To.01 A
5.4"'
v
W V
c 0
, V I
,
,
,
,
,
3800 3500 3200 Wavenumber (cm-' 1 Figure 2. ( A ) Infrared spectra of hydroxyl groups on A1203for various (CH,),SiCI gas exposures at 450 K. (B) Corresponding difference spectra with the AI20, background spectrum subtracted. The spectra were obtained after evacuation at 1 X 10" Torr at 475 K for 30 min and cooling to 300 K . 8
Stretching Modes f o r (CH,), Si-0-AI: CH,
-
W
m
A
a 1.45 (C)
"
0
Symmetric Deformation Mode (CH,), Si-0-AI
AI-OH + CI-Sit >AI-0-Sit + HCI (2)
-
,
This reaction scheme involving surface hydroxyl groups is well-known in the area of organosilicon compounds as reported in many textbooks.32 ~
~~
~
~~
(29) Hertl, W.; Hair, M. L. J . Phys. Chem. 1968, 72, 4676. (30) Goubeau, V. J.; Sommer, H. Z . Anorg. Allg. Chem. 1957, 289, 1. (31) Tsutsumi, K.; Takahashi, H. Colloid Polym. Sci. 1985, 263, 506.
4622
The Journal of Physical Chemistry, Vol. 94, No. I I , 1990
Although reaction 2 seems to occur for both isolated and associated OH groups on the A1203surface, it is clear from the sequence of spectral developments shown in Figure 2 that the isolated OH groups are more reactive since they disappear first. This behavior is observed on pure A1203as well as on Rh/A1203. The fate of the HCI is not known at present; it may be liberated as the gas or it may adsorb on the A1203surface. Somewhat similar observations made by Armistead and Hockey33indicate that trimethyl- and dimethylchlorosilanes react selectively and completely with isolated or single surface hydroxyl groups on Si02(aerosil) whereas the monomethyl and tetrachloro compounds react with some of the interacting hydrogen-bonded surface OH groups in addition to isolated hydroxyl groups. The mechanism of formation of alumasiloxane species during the functionalization of surface OH groups is of interest. Reaction of alkylchlorosilanes with surface hydroxyl groups may be considered to involve a bimolecular transition state in which trimethylchlorosilane molecule forms a five-coordinate bonding structure at the surface.32 Four of these valencies are satisfied by the initial substituents, with the fifth apical position occupied by the oxygen atom of a surface hydroxyl group, utilizing one of its initially nonbonded lone pairs, to form the bond. Elimination of HCI from this complex and reversion of the Si atom of the parent molecule, (CH3)3SiCI, to 3sp3 yields a stable product bonded to the surface by an Si-0-AI bridge. Thus, the single hydroxyl groups, with the oxygen atom pointing away from the plane of the surface, will react in stoichiometric ratio ( I : 1 ) with the silane. C. Influence of Silane Chemical Modification on the COInduced Disruption of Rh: Sites. It has been found that the removal of surface hydroxyl groups from Rh/AI2O3catalysts by reaction with the chlorosilane has a profound effect on the surface reaction with CO which in the absence of chemical modification produces Rh'(CO)*. Thus, in Figure 7 it may be clearly seen that the surface reaction producing strong intensity in the symmetric and asymmetric mode for the Rh'(C0)2 species is almost completely suppressed. A second observation is that both the terminaland bridged-CO species associated with nonoxidized Rh: sites are enhanced in their intensity and surface concentration on the treated catalyst. This observation is very significant, since it may offer a chemical route to the preservation of nonoxidized Rh sites in catalytic applications where both surface O H groups and carbon monoxide are present. The use of compounds like (CH,),SiCI to specifically functionalize OH groups on A1203supports containing clean Rh: sites is even more striking because of the demonstrated tendency for the clean Rh sites to be essentially unreactive toward the chlorosilane. Thus, the development of both terminal- and bridged-CO species on the treated catalyst (Figure 7 ) , and the preservation of CO chemisorption capability as demonstrated by the isotherms shown in Figure 6, illustrate either that the chlorosilane is not reactive with Rh: sites or that it is easily displaced by C O from a weakly bonding mode of interaction with Rh.: It is not possible to make an accurate quantitative analysis of the CO isotherms for the treated and untreated Rh/AI2O3 catalysts. This is because in both cases a mixture of adsorbed CO species is present [Rh1(CO),:2CO/Rh; Rh°CO:I CO/Rh; Rh20(CO):O.SCO/Rh]. The spectroscopic data indicate that the untreated catalyst produces significant Rhl(CO), mixed with the (32) Bazant, V.; Chvalovskg, V.;Rathouskg, J. Organosilicon Compounds; Academic Press: New York, 1965. ( 3 3 ) Armistead, C. G . ;Hockey, J. A. Trans. Faraday Soc. 1967,63,2549.
Paul et al. two other species; the treated catalyst produces much less Rh'(CO),, and the other two surface species are prominent. The CO/Rh ratio for the untreated catalyst is 1.09, and for the treated catalyst it is 0.3 as shown in Figure 6 where the CO adsorption on the untreated AI2O3support has been subtracted from both isotherms. This change of the CO/Rh ratio due to chlorosilane treatment may be attributed to the dominance of the RhO(C0) and Rh2O(C0) species on the treated catalyst as compared to the Rhl(CO), species on the untreated catalyst (as seen in Figure 7 ) . This is caused by the suppression of conversion of Rho to Rh' that normally occurs on the untreated catalyst. The presence of bulk Rh sites which would not adsorb CO will tend to cause a decrease in the measured average saturation values of the CO/Rh ratio below the idealized values associated with the three surface species discussed above. These results suggest that the chlorosilane exhibits little or no reactivity on the Rh; sites and that molecules of this general type may be used for removal of surface hydroxyl groups without influence on the metallic Rh sites of catalytic importance. D. Thermal Stability of (CH,),Si-@AI= Species on R h l A1203Catalysts. Figures 8 and 9 deal with the thermal stability of the chemically modified catalyst surface. During thermal treatment a significant loss of intensity is observed in the v(0H) (not shown in the figures), v(CH3), and 6(CH3) spectral regions. Comparable loss of all characteristic vibrational features [u(CH3) and 6(CH3)] for siloxyl species is also observed for metal-free A1203catalysts under identical thermal treatment, indicating no significant role of metal particles in decomposing siloxane species. The mechanism of decomposition of (CH3)3Si-O-AI= species is also of interest. There may be several possible ways of decomposing the surface species, (CH3)3Si-OAI=, but the limited spectroscopic range being studied here precludes further comment at this time. V. Summary of Results The results of this investigation are summarized below: 1. The dation agent, (CH3),SiCI, interacts reversibly at 300 K with isolated OH groups on Alz03,causing the u(0H) frequency to decrease into the frequency range of associated O H groups. This interaction probably involves weak hydrogen bonding through the CI moiety to the protons in the hydroxyl groups. 2. Irreversible reaction of (CH3),SiCI with surface O H groups (both isolated and associated) occurs at 450 K with the production of strongly bound (CH,),Si-O-AI= surface species. 3. The removal of isolated surface O H groups through chemical modification of the surface leads to a striking suppression of the Rh? destruction by the redox process producing Rh'(CO), in a CO atmosphere. 4. Little or no adsorption of the silating agent occurs on clean Rh? sites during catalyst treatment at 450 K . Thus, the Rh: sites retain their capability to adsorb CO as bridged- and terminal-CO species. 5 . The thermal stability of the (CH3)3SiUAI= surface species under vacuum has been studied. It is found that the siloxyl species gradually disappear above 500 K from both the Rh/AI2O3 and A1203surface, indicating no active role of metal particles. Thermal removal of siloxyl species facilitates Rh: disintegration and conversion to Rh' upon CO adsorption at 300 K.
Acknowledgment. We acknowledge with thanks the full support of this work by the General Motors Corp. We also thank Dr. Galen Fisher of the General Motors Research Laboratory for helpful comments on this work. Registry No. Rh, 7440-16-6; (CH,),SiCI, 75-77-4; CO, 630-08-0.
