Langmuir 1991, 7, 678-682
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Adsorption of Water Vapor by Iron Oxides. 2. Water Isotherms and X-ray Photoelectron Spectroscopy N. S. Clarke+ and P. G. Hall'J Department of Chemistry, University of Exeter, Exeter E X 4 4QD, United Kingdom, and Atomic Weapons Research Establishment, Aldermaston, United Kingdom Received April 2, 1990. I n Final Form: September 11, 1990 Comparison between nitrogen and water vapor isotherms on a- and yFe2Os suggests that at the BET point B stage in both cases t h e structure of the water adlayer is close-packed and nonlocalized. Significant hysteresis a n d irreversibility on desorption probably reflect, particularly with y-Fe203, reaction or rehydration effects. Wide energy (survey) X-ray photoelectron spectroscopy (XPS) scans of haematite and mbar after magnetite surfaces show t h e presence of adsorbed water down t o pressures as low as pumping overnight. This conclusion is confirmed by laser induced ion mass analysis (LIMA) experiments. With magnetite the estimated ratio of surface OH to surface 02-from XPS is about 2.
Introduction T h i s p a r t concerns the s t u d y of water vapor adsorption b y iron oxides using the techniques of isotherm measurem e n t and X-ray photoelectron spectroscopy (XPS). McCafferty and Zettlemoyerl have investigated t h e adsorption of water vapor o n dehydroxylated cu-FenOs. T h e y proposed a model i n which chemisorbed hydroxyls s u p p o r t a n immobile first physisorbed layer followed b y a mobile second physisorbed layer. Blyholder and Richardson2studied the infrared spectra of surface hydroxyl groups o n haematite; water adsorption was dissociative and a t e m p e r a t u r e of 475 "C was required t o remove all surface hydroxyls. Takezawa3 using infrared reflectance spectroscopy showed that with h a e m a t i t e the Fe-OH stretching vibration was still observable after heating at 300 "C in a s t r e a m of He. Rochester and T o p h a m 4 assigned 11 infrared m a x i m a i n the range 3000-4000 cm-l t o 11 different t y p e s of surface hydroxyl groups chemisorbed on various crystal planes; molecular water however could b e completely desorbed by evacuation at room temperature. Busca a n d Lorenzelli5 demonstrated t h e existence of infrared maxima d u e to surface hydroxyl groups at 3670, 3640 and 3460 cm-'. S o m e workers1z6 have suggested that t h e mole ratio of physisorbed water to surface hydroxyls in the first monolayer is 1:2 a n d that each physisorbed water molecule is bonded to t w o adjacent surface hydroxyls. W i t h the o t h e r adsorbents referred t o i n the present work, i.e. y-Fe203 and FesOd, t o o u r knowledge t h e r e has been n o previous reported work o n water vapor adsorption.
Experimental Section Water Adsorption Isotherms. The water adsorption was measured gravimetrically by the method of McBain and Bakr,7 with the sample container suspended by a system of rods attached to a helical fused silica spring (Thermal Syndicate Ltd). The sensitivity of the spring was found to be 26.3 punitsg'. +
Atomic Weapons Research Establishment. Exeter.
1 University of
(1) McCafferty, E. A,; Zettlemoyer, A. C. Discuss. Faraday Soc. 1971, 52, 258. (2) Blyholder, G.; Richardson, E. A. J . Phys. Chem. 1962, 66, 2597. (3) Takezawa, N. Chem. Commun. 1971, 1451. (4) Rochester, C. H.; Topham, S. A. J.Chem. SOC.,Faraday Trans. I 1979, 75, 1073. (5) Busca, G.; Lorenzelli, V. React. Kinet. Catal. Lett. 1980,15, 273. ( 6 ) Morimoto, T.; Yokata, Y.; Nagao, M. J . Colloid Interface Sci. 1978, 64, 188.
(7) McBain, J. W.;
Bakr, A. M. J . Am. Chem. Soc.
1926, 48, 690.
0743-7463191 12407-0678$02.50/0
The sample and spring were thermostated at 303 K by water, a t the appropriate temperature from a header tank, circulating through the two thermostating jackets. Heating was achieved by a 350-W carbon filament bulb immersed in the tank, controlled by an ITT F14D thermistor. An additional fine control (or heat sink) was provided by a glass coil through which cold water circulated from a constant head apparatus. The range of thermostating was 268-303 K with a stability of 0.01 K over 4 h (observed). The whole apparatus was contained within a thermostated box which reduced thermal gradients across the apparatus to >2 K. The box was thermostated a t 305 K, heating was by two 200-W carbon filament bulbs controlled by a I T T F14D thermistor. Air circulation was achieved by the use of two fans, one in the top and the other in the bottom of the box. The equilibrium vapor pressure measurement was carried out by using either the silicone oil manometer (SOM) as for the sample preparation technique, or the MKS baratron gauge (145M-100, 0-100 mmHg head type 170M-6A indicator). This device is a capacitance manometer measuring small pressure differences between the pumping system (reference) and the equilibration chamber. The deflection of a metal diaphragm caused by these pressure differences causes a change in capacitance of a dielectric chamber. It was found that the capacitance readings were linear with the saturated vapor pressure (SVP) as measured by the SOM. XPS. The instrument used was a VG ESCALAB Mk I11 using A1 K, radiation. The spectrometer was operated by Dr. J. Watt of the University of Surrey Surface Analysis Laboratory. Samples were pumped to 10-10 T overnight before examination. Samples were mounted as powders on an 8 X 1 cm3 portion of doublesided adhesive tape and placed on a suitable carrier before insertion into the spectrometer. Wide energy (survey) spectra of the samples were recorded to obtain the core level binding energy carbon (a ubiquitous contaminant, the sources of which are atmospheric hydrocarbons and oxides of carbon*). EB(carbon Is) is used as an internal standard, the positions of other peaks in the spectrum are measured relative to this carbon 1s peak. and H20 Contributions to the oxygen Is signal from OH-, 02-, can be obtained by deconvolution using literature values of the chemical shifts8 relative to the oxygen 1s (isolated atom) peak. The background due to inelastically scattered electrons must be subtracted. These calculations were performed by using the University of Surrey program CAMET. The kinetic energy of an electron ejected by such a process is a function of the exciting radiation (hu)and the core level binding energy (EB)according to the relationship KE = hu - EB, where hu is a constant for a given X-ray source. EB is characteristic of a given element and different elements can be identified because (8) Walls, J. M.; Christie, A. B. Surface Analysis and Pre-treatment of Plastic and Metals; Brewis, D. M., Ed.; Applied Science Publishers: Essex, U.K., 1982.
0 1991 American Chemical Society
Adsorption of Water Vapor by Iron Oxides
Langmuir, Vol. 7, No. 4, 1991 679
25
2 0
-
D
nD 0
n
,"
1 5
0
0 YI
c
101
05 01
01
03
O L
05
06
07
08
09
10
PIP0
Figure 2. Water adsorption isotherm y-FezOs: 0,adsorption branch; 0 desorption branch. Solid was outgassed a t 200 "C for 24 h prior to commencement of isotherm. I
1
05
Figure 1. Water adsorption isotherm a-FezOa: 0,adsorption branch; 0 , desorption branch.
of the discrete values of the core level binding energies. Different chemical species can be identified because of the effects of chemical environment on the core level binding energies. For example E g for the 1s level for oxygen as 02-is different to that of oxygen as OH and again different for oxygen as H20. Surface sensitivity is conferred upon the technique by the limiting factor of the electronic mean free path (A) within the solid. The exciting radiation penetrates far into the bulk causing photoionization. However only those electrons within a few X of the surface can escape and he detected. A Beer-Lambert type law is operative and significant sampling is taken from only the first 3X of the surface. The electronic mean free path in the solid is related to the kinetic energy as X
-
with X in 8, and KE in eV
(?)l''
The oxygen 1s binding energy is 536 eV for an isolated atom, and the exciting radiation used in the present work was A1 K a , hi) = 1487 eV, thus X 15 A. Contributions to the detected electronic signal decrease with increasing depth as a result of the Beer-Lambert law. Atoms within the first X of the surface contribute 6570 of the signal, those in the first 2X, 85c,jof the signal, and those in the first 3h, 95r, of the signal. In the present work therefore, the first 45 8, of the solid contribute 957" of the detected signal, and sampling may be regarded as being restricted to this depth. In view of this finite sampling depth, the signal due to atoms in the surface layers must be weighted by a factor, f ( 6 h per point were necessary. The adsorption branch begins a t (0, 01, the curve rises to a plateau a t 0.75 X 10-2 g-g-' and p / p o 0.25. The plateau extends to p / p o 0.43 when the curve rises to another plateau a t 1.55 X geg-' and p / p g = 0.66. This 0.81 when the curve second plateau extends to p / p o turns sharply upward indicating the onset of condensation; a t p / p o = 0.91 the uptake is 3.02 X loT2g-g-l. As desorption is effected, hysteresis occurs; the desorption branch is shown by the closed circles. The intercept of the desorption is a t 0.62 X gsg-l. Since this value is finite, it is considered initially that some surface rehydroxylation had occurred during the isotherm. The "point B" a t the "knee" of the first plateau corresponds to an uptake of 0.775 X loW2gsg-l a t p / p o = 0.25. The nitrogen surface area of this material was 27 m2 g-l, and thus a t the onset of the first plateau, the uptake corresponds to U H ~ O= 10.4 A2 compared with 10.6 A2 from bulk liquid density. Thus, as with cu-FezO3the adsorbate a t this stage would appear to be a close-packed, nonlocalized monolayer. The onset of the second plateau region occurs a t just twice the uptake of the first layer i.e. a t 1.55 X gsg-' and p / p o = 0.66. The situation a t higher coverage than that corresponding to point B and particularly during desorption is no doubt complicated by slow reaction or rehydration. In fact there is separate evidence" to show that water vapor can react with yFezO3 to form goethite. XPS. Wide energy (survey) scans of the surfaces of haematite and magnetite are shown in Figures 3 and 4.
-
SO0
LOO
BINDING ENERGY
-
-
atom
energy/eV
lit. valuelev
core level
Fe C Na 0 Fe Fe Fe 0
60 290" 500 540 720 740 850 980
56 287 497 531 710 723 847 976
3s 1s Auger 1s 2P3jz 2P1p 2s and Auger KNM Auger KLL
approx peak
Actual value 290.3 eV.
Table 11. Haematite Survey Scan atom
Fe
Fe C 0 Fe Fe Fe 0
approx peak energy/eV
lit. valuelev
60 100 290 540 720 740 850 986
56 93 287 531 710 723 847 976
core level 3P
3s
1s
1s 2P3P 2P1/2 2s and Auger KNM
Auger KLL
Table 111. Magnetite Oxygen 1s Signal ~~
oxygen species
position of signal max/eV
02-
533.8 536.1 538.0
OHHz0 02-
~
~~~~~
'(
area of total
78.73 17.88 3.39 weighted area 7.87 -8
Table IV. Haematite Oxygen 1s Signal oxygen species signal max/eV 5 area of total 02OH-
534.5
10.55
536.1
69.13
Hz0
538.1
30.32 weighted area 1.06
02-
=l
Salient features of these spectra are tabulated in Tables I and 11. These spectra identify the elements present in the samples. The samples used were from the same batches as used for the neutron scattering experiments. The deconvoluted 0 1s spectra for magnetite and haematite are shown in Figures 5 and 6 and tabulated in Tables I11 and IV. Certain assumptions are made in the interpretation of the XPS results. The first and most important is that the OH and H2O entities are true surface species; hence no
Langmuir, Vol. 7, No. 4, 1991 681
Adsorption of Water Vapor by Iron Oxides r
++
m
z
3 0 -
