Langmuir 1986,2, 677-682
- Gas Mobility *
677
has a striking correlation with the effective gas mobility in the porous medium.
- Dilational Modulus
Conclusions
“
\f
I
I
2
1. Dynamic surface tension, surface viscoelasticity and thin film drainage times and foam stability have been measured for determining the basic foaming Characteristics of a-olefin surfactants. It was found that c16 AOS has the highest surface shear viscosity and dilational modulus followed by C1,and Clz AOS. 2. A good correlation was obtained between surface properties of surfactant solutions and foam stability. The data suggested that the larger the dilational modulus, the more stable the foam. 3. Dynamic gas-liquid saturations in consolidated porous media were measured using the y-ray absorption technique. The saturation distribution revealed that the c16 AOS in situ foam has a pistonlike displacement in the core, while C14and ClzAOS are less effective. 4. The role of dynamic surface properties in foam-enhanced fluid displacement was determined. The results showed that the higher the modulus of elasticity, the slower the movement of the gas, thus increasing the displacement efficiency of the foam in the porous medium.
2ouo 10
5
0
c12 c14 AOS Chain Length
c16
Figure 12. Correlation of gas mobility with dilational modulus with varying AOS chain length. and Q is the cumulative volume of liquid produced at gas breakthrough, L is the length of the core, tb is the breakthrough time, A is the cross-sectional area, P1 and Pz are the upstream and downstream pressures, and P,, is the standard absolute pressure. The lower the effective gas mobility the higher the blocking capability of the foam since the effectiveness of the foam depends on its flow resistance. Figure 12 illustrates the effective gas mobility and dilational modulus with three surfactants. It appears that the chain length of the surfactant influencing the mobility control of foam can be directly explained by the dilational modulus. The results show that the dilational modulus
Acknowledgment. This study was funded by the National Science Foundation and in part by the Department of Energy. Registry No. NaCl, 7647-14-5.
Surface Chemistry of Sodium, Chlorine, and Oxygen on Chromium and Chromium(II1) Oxide J. S. Foord Department of Chemistry, The University, Southampton SO9 5NH, U.K.
R. M. Lambert* Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, U.K. Received October 22, 1985. I n Final Form: June 28, 1986 Chemisorption and coadsorption of sodium, chlorine, and oxygen on Cr(100) and on epitaxially grown Cr203(OOOl)surfaces have been studied by LEED,AES, TDS, and A4 techniques. Na adsorption on Cr(100) results in the formation of positively charged, ordered monolayers, exhibiting strong lateral repuisions and a heat of adsorption of 175 kJ mol-’ at low coverages;subsequent exposure to oxygen brings about oxidation of the Na phase and the underlying Cr(100) lattice. Coadsorption of Na and C1 on Cr(100) causes island formation of (100)-oriented NaCl and implies an appreciable lateral mobility for the surface species at 300 K at high chlorine coverages evidence is presented suggesting formation of CrClZand a mixed CrNaxC12+x corrosion phase. Na adsorption on outgassed Crz03exhibits characteristics similar to the interaction of alkalis with transition metals, although the Na is more strong6 bound than on Cr(100). When heated in the presence of oxygen, reaction of adsorbed Na with the underlying CrzO3 lattice takes place, resulting in formation of a mixed Na-Cr-0 compound. Chlorine adsorption on the outgassed Cr203surface results in the appearance of two TDS states, chlorine being evolved at low temperatures and chromium chloride(s) desorbing at high temperatures. This latter process is suppressed by coadsorbed oxygen and the nature of the reactions occurring is discussed. Electron-stimulated desorption of chlorine is observed in the Cr(100)/Na/C1and Cr20,(0001)/C1chemisorption systems and cross sections for the stimulated desorption processes are presented. 1. Introduction Alkali and halogen species play important roles as promoters in heterogeneous catalysis1 ana as a consequence
* To whom correspondence
should be addressed.
0743-7463/86/2402-0677$01.50/0
their surface chemistry is of particular interest. While the properties of the individual species adsorbed on transition metals has now been studied in some detail (e.g., ref 2-7), (1)Martin, G. A. In Metal-Support and Metal-Additiue E f f e c t s in Catalysis; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 315.
0 1986 American Chemical Society
678 Langmuir, Vol. 2,No. 5, 1986
Lambert and Foord
350 550 7 5 0
350 550 75 Temperature (K)
Figure I. (A) TDS spectra of Na on Cr(100). Traces 1-7 refer respectively to Na coverages of 0.36, 1.1,2.3,4.4, 6.0, 8.9, and 13 X atoms m?. (B) Variation in work function induced hy Na adsorption on Cr(100). much less is known concerning their behavior on oxide surfaces. Other related areas of interest concern the surface interactions that occur between adsorbed alkalis and halogens and between these surface species and the gas phase species present in particular catalytic processes. We address these points in this paper which deals with adsorption and coadsorption studies of Na, CI,, and O2on the Cr(100) and Cr203(0001) surfaces. 2. Experimental Section All experiments were carried out in a stainless steel UHV chamberdescribed in detail elsewhere? equipped with a t h r e g i d RFA, CMA Auger spectrometer, quadrupole masa spectrometer (with direct line-of-sight to the specimensstudied),and inert gas ion etching facilities. Adsorption experiments on Cr employed a Cr(100)crystal which displayed a contaminanefree well-orderd (1x1)surface after extensive Ne+ion bombardment and annealing in vacuo at IO00 K? Experiments on Cr20, were carried out on the (0001)oriented oxide film, formed on Cr(ll0) during in situ oxidation (1050 K, Pa of Os,10 min).l0J1 The structure and compositionof the oxide surface were verified by AES and LEED techniques. Temperature measurement was by Pt, Pt-10% Rh thermocouples spot-welded to the rear faces of the specimens, which were heated resistively. Oxygen dosing was carried out by pressurizingthe chamber with B.O.C. Research Grade gas through a variable leak valve. Exposure to chlorine was by in situ solidstate electrolysis of the appropriate silver chloride'2 while sodium dosing employed a heated aluminosilicate head,13which emitted approximatelyequal fluxes of neutral and ionized sodium species. Control experimentsindicated that the experimentalobservations were essentially independent of the form in which the sodium was dosed and measurementof the ion fluxprovided a convenient monitor of the total sodium exposure. Correlation of observed LEED patterns, Na exposures, and quantitative TDS yields indicated that Na adsorbed with a sticking probability close to unity in all the systems studied and the Na exposures have been converted to surface atom concentrations throughout the work on this basis. Reported A4 measurementsemployed the beam-stop method" and the AES and electron-stimulateddesorption (ESD)data were (2) Grunze, M.; Dowben. P. A. Appl. Surf. Sei. 1982, 10,209. (3) Gerlach, R. L.: Rhodin. T. N. Surf. Sei. 1970, 19, 403. (4) Marbrow, R. A.; Lambert, R. M. Surf. Sei. 1976.61.329. ( 5 ) Lee, S. B.: Weiss, M.; Ertl, G. Surf. Sei. 1981, 108. 357. (6) Pirug, G.; Bonzel, H. P.;Braden, G . Surf. Sei. 1982, 122, 1. (7) Doering, D. L.; Semsrak, S. Surf. Sei. 1983. 129, 177. (8) Foord, J. S.; Goddard, P. J.: Lambert, R. M. Surf, Sei. 1980,94, 110
""l.
(9) Foord. J S.; Lamben. R. M..Sur/. Sei. 1982. 115. 141 (10) Kennett. H. M.; Lee, A. E. S u / . Sn. 1972.33. 377. (11) Foord. J. S.: Lambert. I1 M. Sur/, S~~~, r i . on DIEBI. (12) Goddard, P: J.: Lambert, R. M. Surf. Sei. 1977.67, 180. (13) Weber, R. E.; Cordes, L. F. Reo. Sei. IRstrum. 1966, 37, 617. (14)Knapp. A. G. Surf. Sei. 1973, 34, 289. ~~~
Figure 2. LEED patterns. (A) Cr(100)-c(2x2) Na, 141V. (B) Cr(100)-(4X4)NaC1, 83 V. recorded using a 2.5-keV, 0.5-mm2, 5-MAexciting beam. Auger spectra were recorded in the first derivative mode and signal intensities refer to the measured peak-to-peak heights in such derivatized spectra. All thermal desorption spectra were recorded by using a 25 K s-' heating rate and quoted desorption energies were derived from Arrhenius plots of the leading edge of the desorption traces near takeoff where the surface coverage remains relatively constant. 3. Results 3.1. Na, N a + O2 Adsorption on Cr(100). Na was adsorbed on Cr(100) at 300 K and the resulting adlayers examined by LEED, AES, TDS, and A+ techniques. Na atoms were the only desorption product and a series of TDS spectra are presented in Figure 1A. A single peak is exhibited which broadens to lower temperatures as the Na exposure rises; the desorption activation energy varies from 176 k J mol-' at zero coverage down to 106 k J mol-l at coverages greater than 8.3 x atoms m-2. This latter energy approximates to the sublimation energy of bulk Na (108.2 k J mol-'15). The variation in A@ induced by Na adsorption is illustrated in Figure 1B. The work function falls rapidly when Na is adsorbed up to coverages of 5 X 1Ol8 atoms m-2, after which it rises slightly. The 42x2) LEED pattern shown in Figure 2A was observed after dosing the surface with (5-7) X 10l8atoms m--?Na; at lower Na coverages no change from the clean surface LEED pattern could be detected while at higher coverages, the surface phase became disordered.
~~~~~
(15) CRC Handbook of Chemistry ond Physics: Weast. R. C., Ed.; CRC Boea Raton, FL, 1981.
Langmuir, Vol. 2, No. 5, 1986 619
Surface Chemistry of Na, C12, and O2 on Cr and Cr20,
I
B
B
A
nI (J
480 880 880 1080 Temperature (K)
1,,jb.,
390 630 870 1110 Temperature (K)
Figure 3. (A) Na TDS spectra following a 40-langmuir O2 exposure to Cr(100)subsequent to adsorption of (spectra 1-5) 3.0, 6.0,11,13, and 23 X 10l8atoms m-2 Na. (B) As A, but the order of dosing is reversed. Na coverage (spectra 1-3): 4.8, 8.3, 18 X W8atoms m-2.
Coadscrption experiments were performed by exposing the surface to 40 langmuirs of 02,prior or subsequent to the adsorption of Na. Control experiments indicated that this O2dose was sufficient to bring about saturation of the surface layers. Again Na constituted the only desorbing species in TDS and the recorded spectra are presented in Figure 3. Adsorbed oxygen has the principal effect of raising the Na desorption temperature; three desorption peaks are visible if the oxygen is predosed, while two peaks are visible if the order of dosing is reversed. A$ measurements revealed that oxygen adsorption increased the work function of the Cr surface previously exposed to Na; at oxygen saturation, A# ranged from 2.4 (sodium absent) down to 0.2 eV (sodium coverage = 1.3 X 1019atoms m-2). Heating to 630 K (i.e., below the desorption temperature) caused a rapid decrease in the work function down to values close to those of the oxygen-free surface. However, AES measurements confirmed that there was very little decrease in the 0 (508 eV) Auger signal intensity during this annealing procedure. After further heating to desorb all Na, LEED and AES measurements indicated that a Cr(100)-c(2X4)-0structure developed on the surface; this pattern is also seen during the interaction of O2 with Cr(100) in the absence of Na.'l 3.2. Na, Na + O2 Adsorption on Cr,O,. Similar experiments to those described above were carried out on the outgassed Cr203(0001)surface. TDS spectra monitored after Na deposition on the oxide are shown in Figure 4A. At low coverages, desorption occurs in a single peak with an activation energy of 240 kJ mol-'. As the coverage rises, the desorption temperature drops rapidly until a second peak emerges, characteristic of desorption of bulk Na. If the outgassed oxide is exposed to 40 langmuirs of O2before or after the Na dose, the desorption profile changes significantly (Figure 4B). A most interesting feature is the narrow (fwhm) peak which develops in the desorption spectra (6 peak) independent of whether the oxygen is dosed before or after the Na. In contrast to the case of Cr metal, oxygen chemisorbed on the oxide surface undergoes desorption (as opposed to surface bulk transport) when the surface is heated. O2 desorption occurs from two states, a molecularly bound form being evolved at 400 K and atomically bound form at 1050 K.16 Predosing the surface with Na does not alter the shape of the desorption profile of oxygen subsequently
-
(16) Foord, J. S.; Lambert, R. M., submitted for publication in Surf. Sci.
111
300 620
940
1201
300 600
Temperature (K)
900
1200
Tempe-ature (K)
Figure 4. TDS spectra of Na on Cr20,. Spectra 1-7 refer respectively to Na coverages of 0.83, 1.3, 2.7, 4.2, 8.3, 13, and 16 x l0l8 atoms m-2. (B) TDS spectra of Na from Cr203exposed to 40 langmuirs of 02.In spectra 1-3, Na adsorption was carried out subsequent to O2 exposure, while in spectra 4 and 5 the order of adsorption was reversed. Na coverages for spectra 1-5: 1.4, 7.1, 18, 1.4, and 18 X 1OI8 atoms m-2.
a
6
I
B
t 570
8 7 0 1170 5 7 0 8 7 0 Temperature (K)
1171
Figure 5. TDS spectra recorded after exposure of chlorine to Cr(100) with 13 X 10" atoms m-2Na adsorbed. (A) (Left part of figure) ChIorine exposure = 3 X 10l8molecules m-2. (B) (Right part of figure) Chlorine exposure = lozo molecules m-2.
adsorbed, but the O2yield falls off smoothly, down to zero at Na coverages 18.3 X 10l8 atcms m-2 although AES observations confirmed that oxygen was adsorbed during such exposures. However, if the surface was exposed to Na and 02,heated to 1000 K to desorb all species except Na, then no oxygen desorption following subsequent O2 exposure could be detected for Na coverages as low as 1.2 x IO1* atoms 3.3. Coadsorption of Na and C1 on Cr( 100). Coadsorption experiments were carried out by dosing the surface with Na and then exposing it to chlorine. TDS data are presented in Figure 5 for two differing chlorine exposures on a surface dosed with 1.3 X 1019atoms m-2 Na. The traces were recorded at 23 (Na), 58 (NaCl), 81 (CrCl),and 122 amu (CrC12),which correspond to the parent ions of the desorbing species observed, although the occurrence of ion source fragmentation (NaC1- Na+, CrC1, CrC1')
-
680 Langmuir, Vol. 2, No. 5, 1986
Lambert and Foord
C 2.
- 2.
D c
0
2
4
6
8
Charoe C o l l e c t e d (mC,
Figure 6. Electron-stimulateddesorption decay plots. (1)ESD of C1 from Cr(lW)-NaCl surface (2) ESD of C1 from Cr203-C1 surface. should be borne in mind when examining the spectra. The following points should be noted. First, obvious differences exist between the Na and NaCl desorption profiles at the lower chlorine exposure, while the two profiles are identical at the higher chlorine coverage. This indicates that the thermal loss channel for Na switches over entirely to NaCl desorption, provided excess chlorine is dosed onto the surface. Second, some high-temperature CrCl desorption (#I peak) is observed, even when the Na is in excess. This peak is identical with the one arising from CrCl desorption of C1 overlayers on Na-free Cr.9 Finally, the CrC12evolved (apeak) a t high chlorine exposures exhibits a TDS peak which is split into two components; the lower temperature component aligns with the NaCl desorption temperature whereas the other component is exclusively observed on the Na-free ~urface.~ TDS spectra monitored for lower Na coverages exhibited these same qualitative features. LEED observations showed that chlorine adsorption on surfaces predosed with (3-6) X 10l8 atoms m-2 Na produced the (4x4) LEED pattern in Figure 2B. In the lower Na coverage range, further chlorine exposure resulted in the appearance of the range of compression patterns, observed also in the absence of Na.9 At high Na coverages, no LEED patterns could be observed from the mixed adlayer, although structures characteristic of adsorbed C1 alone appeared if the crystal was heated to desorb the NaC1. Although the Cr/C12adsorption system shows no ESD effects, rapid ESD of chlorine was observed in the presence of Na. A semilogarithmicplot of the decay in the C1 Auger signal intensity (181eV) for a surface exposed to 7 X 1OI8 atoms m-2 Na and lozomolecules m-2 C12is presented in Figure 6. After a nonlinear initial region, a straight line graph is obtained, yielding an ESD cross section of -5 X lo-% m2. It was found that all chlorine could be removed from the surface by ESD,even when the chlorine coverage was greatly in excess of the Na coverage. 3.4. Clz, C12-02 Adsorption on Cr203. Comparative studies of the adsorption kinetics of chlorine on Crz03and Cr(100) were carried out by AES using the C1 (181 eV) Auger signal intensity to determine the relative amounts of chlorine adsorbed. The results indicated that chlorine adsorbs on the oxide surface with a similar high sticking probability up to monolayer to that observed on Cr( Chlorine adlayers were found to be unstable when irradiated with the electron beam ind the recorded ESD data are presented in Figure 6. The semilogarithmicplot shows two linear regions with effective ESD cross sections of 6.4 x loTz3and m2. AES observations of the interaction
300 600 900 1 2 0 0 Temperature (K)
300 600 900 1200 T e m p e r a t u r e (K)
Figure 7. TDS spectra relating to adsorption of chlorine on Cr203. (A) recorded at 35 m u , no exposure to oxygen. Spectra 1-6 refer to chlorine exposures of 0.7,2.3,4.1,8.2, 11,55 X 10l8molecules m-2. (B)recorded at 52 m u , no exposure to oxygen. Spectra 1-4 refer to chlorine exposures of 0,4.1,8.3, 22 X lo1* molecules m-2. (C) recorded at 35 m u , a 40-langmuir O2 preexposure. Spectra 1-3 refer to chlorine exposures of 1.6, 5.5, and 28 X 10l8
molecules m-2.
of chlorine with the Cr203 surface in the presence of preadsorbed oxygen indicated that the apparent C1 uptake was very much reduced. However, this was contrary to results derived from TDS data described below, so it appears that the ESD cross section of chlorine is greatly increased in the presence of coadsorbed oxygen. TDS data recorded following chlorine exposure to the outgassed oxide surface and in the presence of predosed oxygen are illustrated in Figure 7. In the absence of adsorbed oxygen, the 35-amu spectra (C1+fragments) show two peaks (a,#I) and spectra at 52 amu (Cr') only show the corresponding #I peak. Comparison of these spectra with those recorded for the other mass fragments suggests that the a state desorbs as a mixture of C12and C1, whereas the 0 state yields a mixture of CrC12 and CrC1, although it cannot be absolutely ruled out that in each case lower mass species do not result exclusively from ion source fragmentation. If the surface is exposed to 02, prior to chlorine adsorption, then no chromium-containing species are subsequently evolved during TDS; rather two peaks are seen, both of which correspond to desorption of C12and possible C1. 4. Discussion
4.1. Na, Na
+ C1 Adsorption on Cr(100). The rapid
changes in desorption energy and dipole moment of adsorbed Na with coverage, which may be deduced from the TDS and Ad measurements for Na/Cr(100), are common features of alkali surface chemistry on transition metals in the submonolayer regime.3-5.7 The data indicate formation of a partially positively charged overlayer of Na species between which strong lateral repulsive forces operate. The low coverage adsorption energy (175 kJ mol-') is rather lower than typical corresponding values for Na Ni adsorption on Ag (195 kJ mo1-l): Ru (240 kJ (220-250 kJ m ~ l - l )reflecting ,~ perhaps the more electropositive nature of Cr. The work function curve and the appearance of a shallow minimum is qualitatively ac-
Langmuir, Vol. 2, No. 5, 1986 681
Surface Chemistry of Na, C12, and O2 on Cr and Cr203
surprising, it does indicate appreciable mobility of the species concerned at 300 K. The TDS data are in full agreement with the idea that NaCl is formed on the surface, since it constitutes the major Na desorption product in the presence of chlorine. The relative desorption energies of NaCl and Na can be deduced from a closed energy cycle yielding the equation ENa
- ENaCl = DNaCl - E C I
EXrefers to the desorption energy of species X and DNaC1
-
Figure 8. Proposed model for the Cr(100)-(4X4) NaCl overlayer. Open circles, underlying Cr atoms; small shaded circles, Na atoms; large shaded circles, C1 atoms.
counted for by the jellium calculations of Lang.17 However, quantitatively, the derived zero-coverage dipole moment (2.9 D) and work function minimum (2.65 eV on the basis that +cr = 4.35 eV5) both indicate rather less charge transfer than the “substrate independent” values of 4.3 D and 2.1 eV predicted by the theory. The discrepancy must in part arise from the open structure of the transition-metal surface (bcc (100)) which will cause significant deviations from any jellium model and which e~perimentally~?~ reduces the charge transfer, as deduced from A+ measurements. Thermal desorption data for Na on Ni and Ru3p7show two peaks in the spectra associated with monolayer desorption, due to the operation of strong repulsive forces. Only one peak is resolved in the spectra in the present instance. This trend is expected since the adsorption energy of Na on Cr is closer to the sublimation energy of Na and thus multilayer formation takes over before very strong repulsive interactions can develop. This feature is also presumably responsible for the failure to observe any incommensurate hexagonal-close-packed overlayer structures which are common features of alkali surface chemistry.18 It is unclear whether multilayer formation occurs after completion of the well-ordered c(2x2) structure (5.9 X 10l8atoms m-2)or whether further (disordered) monolayer formation occurs beyond this point. Analysis of the TDS data indicates that the heat of adsorption remains constant at the sublimation energy of Na at coverages 18.3 X 10l8 atom m-2 so multilayer formation has certainly occurred by this stage at least. The (4x4) LEED pattern seen in the presence of adsorbed Na and C1 is evidently a true Na-C1 structure since it is only observed when both adsorbates are present. A solution is presented in Figure 8 in terms of a coincidence lattice between Cr(100) and a (100)-orientedphase of NaCl with a lattice parameter 3% below that in bulk NaC1. The nominal surface density of either species in a uniform layer of this structure is 6.7 X 10l8atoms m-2. Since the LEED pattern was observed for Na dosages extending down to 3x atoms m-2 it may be concluded that island formation occurs causing locally high concentrations of NaCl to develop. Additional evidence for NaCl island formation comes from the observations that LEED patterns associated with the Cr(100)/C12system alone are observed at higher chlorine coverages, which presumably arises from C1 adsorption on areas depleted of Na. While this is un(17) Lang, N. D. Phys. Reu. B 1971,4, 4234. (18) Broden, G.; Bonzel, H. P. Surf. Sci. 1979, 84, 106.
is the bond dissociation energy of gaseous NaCl (410 kJ mol-115). On clean Cr, Ea 320 kJ mo1-l: suggesting that NaCl desorption is favored by 90 kJ mol-l over Na desorption. Nevertheless, it may be recalled that not all the chlorine is removed in the form on NaC1, even when Na is in excess. This suggests that the heat of adsorption of chlorine is substantially increased in the presence of Na. While the results show that adsorbed chlorine is very rapidly converted to NaC1, it is apparent that a CrC1, corrosion phaseg forms at high chlorine loadings as witnessed by the development of the cy desorption peak (Figure 5). The peak is, however, modified in the presence of Na, in that a lower temperature component arises at the point at which the onset of NaCl desorption takes place. This is explicable if a mixed phase CrNa,C12+, is formed in addition to a pure CrC1, compound. While it is unsurprising that the presence of Na induces pronounced electron beam effects, in view of the sensitivity of alkali halides to radiation damage,19p20it is interesting that all the C1 may be removed by ESD in situations where TDS and/or LEED data indicate that pure Cr/C12 phases are present. This serves to emphasize the appreciable lateral mobility displayed by mixed Na/C1 adlayers at 300 K. 4.2. Na, Na + O 2Adsorption on Cr(100) and Crz03. The TDS relating to Na adsorption on the outgassed oxide surface (Figure 4) is rather reminiscent of similar data recorded for adsorption on transition metal^.^-^,^ A single desorption peak occurs at low coverages, broadening to lower temperatures as the coverage increases, while a second peak appears at Na coverages in excess of one physical monolayer with desorption characteristics of bulk Na. Thus the model for the adsorbed phase appears to be similar to that proposed earlier for deposition on Cr(loo), except that the Na binding energy at low ONa (240 k J mol-’) is rather greater on the oxide. A significant feature of the Na, O2 coadsorption studies on the oxide surface is the 6 peak which appears in the desorption spectra (Figure 4) independent of whether the oxygen is dosed prior or subsequent to Na exposure. The profile of this peak is quite unlike that associated with Na overlayers, which exhibit broad peaks due to the operation of repulsive forces; instead the narrow fwhm is more characteristic of the decomposition of ionic Na compounds (where the cooperative forces are attractive). Furthermore, the data relating to suppression of oxygen adsorption (section 3.2) indicate that the 6 state does not form in mixed Na/02 adlayers at 300 K; rather it results from heating such adlayers. It is reported elsewhere that heating Na adlayers on Cr203in the presence of oxygen results in the formation of Cr(V1) species in the surface layers.16 We therefore propose that the 6 state represents reaction of the Na, O2 adlayer with the underlying Cr203lattice to form a highly oxidized Na-Cr mixed phase. A t higher Na coverages a number of additional Na desorption peaks develop to lower temperatures dependent upon whether oxygen is dosed (19) Friedenburg, A.; Shapka, Y. Surf. Sci. 1979, 87, 581. (20) Overeijnder, H.; Symonski,M.; Haring, A,; de Vries, A. E. Radiat. Eff. 1978, 36, 63.
682 Langmuir, Vol. 2, No. 5, 1986
prior or subsequent to Na adsorption. It seems likely that these peaks arise from the decomposition of different forms of sodium oxides but no futhter light can be shed on this. The spectra indicate that the entire adsorbed phase of Na in the coverage range studied is oxidized if exposure to O2 is carried out after adsorbing Na. In contrast, rather less than 1.8 X 1019atoms of Na are perturbed by the coadsorbed oxygen if the order of dosing is reversed. With regard to coadsorption studies on Cr(100), it is notable that low-coverage Na desorption is similar, irrespective of the order of dosing of Na and 02,and occurs at approximately the same temperature as on outgassed Cr203. This latter similarity is expected since combined LEED and AES data, recorded after Na desorption, show the presence of a c(2X4)-0 phase, which is believed to consist of a thin pseudomorphic layer of Cr203.11 The highly oxidized Cr(V1) phase does not form, presumably because it is unstable with respect to reduction by the underlying Cr metal. Although exposure of the Na-covered Cr(100) surface to oxygen produces large increases in A@, these are very readily reversed by mild heating of the adlayer. This suggests that oxidation of the Na phase occurs preferentially at 300 K, but subsequently oxygen diffuses into the underlying Cr lattice, eventually releasing Na into the gas phase; Na desorption at higher alkali coverages displays many of the characteristics previously discussed in relation to the adsorption studies on CrzO3. 4.3. C12,C12 + O2Coadsorption on Cr203. The AES data show that chlorine adsorption on outgassed Cr203 occurs with a high sticking probability reminiscent of the interaction of halogens with metallic surface^.^,^ On the outgassed surface, thermal desorption occurs in two temperature regimes (-600 K as C12/C1 and -1200 K as CrCl,/CrCl). The very differing desorption temperatures suggest substantially different heats of adsorption for the two forms and thus it might be expected that the hightemperature state would populate initially. In fact the two states appear to populate simultaneously. We believe that this is because chlorine is initially adsorbed in one chemical state and that it may either desorb or undergo reaction to populate the more strongly bound state during subsequent thermal desorption. It is a particularly surprising result that chlorine adsorption results in the subsequent desorption of chromium chlorides, which presumably affects the chemical composition of the oxide phase. The data indicate that this process is efficiently blocked by coadsorbed oxygen, although again two chemical states (both evolving chlorine) are evident in TDS. The low-temperature chlorine desorption state appears rather similar to that discussed above, irrespective of the presence of adsorbed oxygen. No direct evidence is available concerning
Lambert and Foord the chemical form of this chlorine. Halogen dissociation normally takes place during initial adsorption on metals, but the low desorption temperature ( a peak) observed here is indicative of a much weaker interaction, which could involve molecularly bound chlorine. A very simple explanation of the experimental data could be that chlorine adsorbs molecularly at 300 K, and this species either desorbs or undergoes decomposition to a more strongly bound atomic form upon subsequent heating. 5. Concluding Remarks This work has sought to examine surface interactions between coadsorbed alkali, halogen, and oxygen species and to extend current knowledge on the properties of adsorbed halogen and alkali phases to oxide surfaces. With regard to the interaction of sodium with Cr203,the most striking observations are (i) the increased heat of adsorption of Na on Cr203in comparison with Cr metal and (ii) the tendency for reaction to occur between adsorbed alkali and the underlying oxide lattice resulting in formation of a sodium chromate phase. The W02/Cs system has been found to behave in an analogous fashion,21suggesting that our observations may be of rather general significance. Further studies to establish this point would be desirable. No other directly relevant studies of halogen chemisorption at metal oxide surfaces appear to have been published. The data reported here indicate that one ( a ) state of adsorbed halogen on Cr203 is bound far more weakly than on Cr metal and on transition metals in general.2,9 Also noteworthy is the @-statedesorption as CrCl,, observed from the outgassed oxide surface, which is also seen from Cr metalg but not from the oxide in the presence of adsorbed oxygen; apparently the outgassed oxide possesses character intermediate between that of the metal and the oxygen-saturated oxide. The coadsorption data clearly indicate that the halogen and alkali are strongly associated with each other at the Cr203surface. In this respect the behavior of the doubly-promoted oxide resembles that of transition-metal surfacesZ2J3where well-defined binary surface compounds are formed.
Acknowledgment. We are grateful to Johnson Matthey Ltd. for a loan of precious metals. Registry No. Cr203,1308-38-9; NaCl, 7647-14-5; Cr, 7440-47-3; Na, 7440-23-5; 02,7782-44-7; Clz, 7782-50-5. (21) Desplat, J. L.; Papageorgopoulos, C. A. Surf. Sei. 1980, 92, 97. (22) Goddard, P. J.; Schwaha, K.; Lambert, R. M. Surf. Sci. 1978, 71, 351. (23) Goddard, P. J.; Lambert, R. M. Surf. Sci. 1981, 107,519.
