Langmuir 1989,5, 1340-1343
1340
Ie/Im]is ob~erved.~ However, since pyrene groups strongly associate with the hydrophobic surface, dependence of the fluorescence observables on PEG molecular weight disappears.
Summary The fluorescence properties of pyrene-end-labeled poly(ethylene glycol) (Py-PEG-Py) adsorbing on monodisperse polystyrene colloidal particles have been investigated. We conclude that hydrophobic chromophores associate with the PS surface more strongly than PEG segments. This contrasts with adsorption of Py-PEGPy on hydrophilic silica particles previously studied. Emission spectra, excitation spectra in the region of the pyrene monomer emission, fluorescence decay, and photostation-
ary fluorescence depolarization consistently provide evidence of pyrene adsorption on the PS surface. The experimental system studied here is similar to an associative thickener, where the hydrophobic part associates with hydrophobic colloidal particles to improve rheological properties.
Acknowledgment. This work was supported by the NSF-MRL Program through the Center for Materials Research at Stanford University. We thank Yiannis Monovoukas for donating polystyrene particles for this study. A.P.G. gratefully acknowledges the support of Xerox Foundation and a Faculty Development Award from IBM. Registry No. Py-PEGPy, 82870-83-5;PS,9003-53-6;DPH,
1720-32-7.
Retention of Molecular Oxygen in Zeolites at High Temperature S. L. Suib*>tp* and B. E. Morset Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, and Department of Chemical Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268 Received August 18, 1988. In Final Form: June 2, 1989 X-ray photoelectron spectroscopy and residual gas analysis experiments were carried out on NaY zeolite at temperatures between -196 and 400 "C. The objective of this work was to further understand the 0 1s transition for zeolites since contributions can arise from zeolitic oxygen as well as trapped water. The residual gas analysis data show that molecular oxygen is retained by NaY zeolite to temperatures as high as 400 O C . Qualitative trends in the RGA data for 0, match those of a shoulder in the XPS 0 1s transition, which is assigned to molecular oxygen. These data indicate that zeolites can retain small amounts of 0, to very high temperature, and this may be one reason for the typical broadness of the 0 1s data for zeolites found throughout the literature.
Introduction Several surface science studies of zeolites have shown that Si/Al ratios, cation content, and oxidation states can vary considerably in these materials. Since zeolites are used in heterogeneous catalysis and in adsorption devices, the nature of their surfaces is extremely important. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, Auger electron spectroscopy, ionscattering spectroscopy, and Rutherford backscattering are all surface techniques which have been used to study the catalytic surfaces of zeolites.'-'' XPS is the most ~~
~
* Author to whom correspondence should be addressed. Department of Chemistry.
* Department of Chemical Engineering and Institute of Materi-
als Science. (1) Vedrine, J. C. Surface Properties and Catalysis by Non-Metals; Bonnell, J. P. et al., E%.; D. Reidel: New York, 1983;pp 159-187. (2) Suib, S.L.; Winiecki, A. M.; Kostapapas, A. Langmurr 1987,3, 483. (3) Barr, T. L.;Lishka, M. A. J. Am. Chem. SOC.1986,108,3178. (4) Minachev, K. M.; Antoshin, G. V.; Shapiro, E. S. Russ. Chem. Reu. 1978,47,2097.
general of these methods; it allows the determination of oxidation states and may provide information related to the number of different types of species on a surface. XPS has been widely utilized for elemental analyses of near-surface regions (150A). In XPS, X-rays are used to eject photoelectrons. The kinetic energy (KE) of the ejected photoelectron is equal to the incident photon energy (hv)minus the binding energy of the electron (BE) minus the work function of the spectrophotometer (4). The binding energy is dependent on the nature of the bonding of the atomic species and can be calculated as follows: BE = hv - KE - 6 .'' (5) Mataumoto, Y.; Soma, M.; Onishi, T.; Tamaru, M. J. Chem. SOC.,Faraday Trans. 1 1980,76,1122. (6) Corma, A,; Fornes, V.; Pallota,0.; Cruz, J.; Ayerbe, A. J.Chem. SOC.,Chem. Comrnun. 1986,333. (7) Baumann, S.;Strathman, M. D.; Suib, S. L. J. Chem. SOC., Chem. Commun. 1986,308. ( 8 ) Turner, N. H.; Colton, R. J. Anal. Chern. l982,54,393R. (9) Suib, S. L.;Coughlin, D. F.; Otter, F. A.; Conopask, D. F. J. Catal. 1983,84,410. (10) Turner, N.H.; Dunlap, B. I.; Colton, R. J. Anal. Chern. 1984, 56,373R. (11) Ertl, G.;Kuppers, J. Low Energy Electrons and Surface Chemistry; VCH Weinheim, FDR, 1985; Chapters 2 and 3.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 6, 1989 1341
Retention of Molecular Oxygen in Zeolites Since zeolites are insulators, a positive charge may accumulate on the sample from the ejection of photoelectrons. The positive charge can result in a change both in peak position toward lower kinetic energy and in peak shape.l One method of handling this charging effect is through the use of a flood gun. The flood gun bombards the sample with additional electrons to counter the positive charges2 The beam intensity of the flood gun is difficult to control with precision; this lack of control is a disadvantage of this technique.' In addition, since the sample can differentially charge, broad peaks or multiple peaks may not be able to be readily identified even by using standard complexes. Using a reference material, such as carbon (often a contaminant on samples), is a convenient method of mathematically compensating for the charging effect. If it is assumed that the carbon is in electrical equilibrium with the rest of the sample, then a shift in the C 1s peak from its assigned binding energy of 284.6 eV can be applied to the other peaks.12 Recently, we have shown13 that Na may be a better reference for Na-rich zeolites than carbon since different carbon contaminants can be retained on zeolites. Studies by Barr and Lishka3 have also shown that accurate data on zeolite surfaces can be obtained and surface contaminants identified by XPS. These studies indicated the presence of excess alumina or sodium aluminate residues on the surface of the zeolites. For these studies, a flood gun was used to compensate for the charging effect. In our continuing search for better reference materials and studies of contaminants on zeolites, we have focused our attention on oxygen. The object of this work was to study the 0 1s peak as a function of treatment temperature since temperature-dependent contributions to this peak could come from framework 02-ions, H,O molecules known to be present, and molecular 0, in the form of adsorbed contaminant species. This investigation in turn could yield a better understanding of the 0 1s peak in zeolites and other aluminosilicate materials.
Experimental Section Zeolite Y was used as received from Alfa Products, Thiokol, Ventron Division. The surface of zeolite NaY was studied with XPS. A Leybold-Heraeus LHS-10surface analysis system was used. This system was interfaced with a Hewlett-Packard 1000 computer for data acquisition and analysis. The X-ray source was Mg Ka radiation (1253.6 eV). The beam current was 10 mA, and the voltage was 13 kV. The bipolar amplifier was calibrated with the gold 4f7,, peak (binding energy = 83.8 eV) and the copper 2p, peak (binding energy = 932.4 eV). The C 1s peak was set at d84.6 eV for reference purpose^.'^ Data were collected in both a survey mode (constant relative energy) and a narrow mode (constant absolute energy). The reproducibility of peak positions is k0.05 eV.13 This system was equipped with a heatable/coolable sample rod. The samples were frozen to liquid nitrogen temperature (-196 "C) at atmospheric pressure in the preparation chamber. Dry nitrogen gas was passed over the sample as it was cooled to prevent frost from forming. The sample was then transferred into the ultra-high-vacuum chamber of the system. XPS data were successively collected at 50 "C intervals from -200 to 400 "C. The pressure in the analysis chamber was kept below 8 x lo* mbar. At each interval, the sample was moved back into the preparation chamber ( P = lo4 mbar) and heated. The preparation chamber was equipped with an Inficon residual gas analyzer (RGA). The RGA was used to monitor relative amounts of desorbing gases at 25 "C intervals.
,
(12) Turner, N.Anal. Chem. 1986,58, 153R. (13) Winiecki, A. M.; Suib, S. L.; Occelli, M. L. Langmuir
512.
1988, 4,
Table I. Binding Energies of NaY Zeolite 0 Is Peak temp, "C BE: eV -196 532.0" -150 532.1" -100 532.0" -50 531.9" +25 531.9" +25 531.8' +75 531.6' +125 531.9' +175 531.9'1527.3' +225 532.0'1527.4' +275 531.9'1527.6' +325 531.7'1527.7' +400 531.8'1527.6' ~~~
~
" NaY pressed into indium foil. 'Shoulder. Corrected.
'NaY pellet on Cu mount.
Table 11. Binding Energies of NaY temp, "C +25 +225 +225' +400'
" NaY pellet on Cu mount. Corrected.
Zeolite. 0 Is Peak BE: eV 531.8 531.71526.8' 531.61526.6' 531.81526.2'
'Exposed to oxygen.
'Shoulder.
Two sample preparations were used corresponding to lowand high-temperature experiments. For the temperature range -196 to +25 "C, NaY was pressed into indium foil to reduce pump-down times between temperatures. Because indium melts at 156 "C, a thin pellet of NaY on a copper mount was used for the high-temperature studies. The pellet was frozen at -196 "C under dry nitrogen, heated to +25 "C in the preparation chamber, and then analyzed at 50 "C intervals to +400 "C. The pellet was frozen to duplicate the treatment of the zeolite on indium foil. A third type of experiment exposed the sample to oxygen. A NaY pellet was cooled to -196 "C under dry nitrogen and then warmed to +25 "C in the preparation chamber. XPS data were collected at +25 and +225 O C . At T = +225 "C, the sample was removed from the analysis chamber and exposed to a flow of oxygen for 15 min in the preparation chamber. The sample was then returned to the analysis chamber for data collection at +225 and +400 O C .
Results XPS data are presented in Tables I and I1 and Figures 1and 2. Figure 1shows XPS data at different temperatures. The 0 1s peak does not change in shape or position (Table I) until +175 "C (Figure IC). At this temperature, a shoulder begins to appear on the low binding energy side of the main 0 1s peak. This shoulder can be observed at +225 and +400 "C (Figure ld,e). The shoulder remains at the same position (Table I) but is the greatest in size at +225 "C. Figure 2 shows XPS data from the experiment in which the sample was exposed to oxygen at +225 "C. In Tables I and 11, the data were corrected by calculating the amount which the C 1s peak shifted from 284.6 eV and applying this value to the 0 1s peak. These spectra were collected with the following parameters: constant absolute energy (narrow mode), pass energy of 50 eV, step E (energy) of 50 eV, step t (time) of 10 s, and 200 scans. These spectra were also corrected by a satellite subtraction technique. The copper mount was studied separately to establish a blank. No oxygen was detected on this blank sample above +175 "C. The oxygen 1s peak was not significant, and the copper 2p peaks showed no sign of the presence of CuO at +175 "C. RGA data are presented in Table 111. Amounts of desorbed gases are relative, not quantitative. The percent-
1342 Langmuir, Vol. 5, No. 6, 1989
Suib and Morse
'> 0
0
I
546 6
541 8
536 6
531 6
528 8
546 6
521 6
541 6
536 6
528 8
521 6
-l
0 546 8
531 6
Binding Energy (eV)
Binding Energy (eV)
541.6
536.6
531 6
526.8
546 6
521 8
541 6
536 6
531 6
526 6
521 6
Binding Energy (eV)
Binding Energy (eV) 10-
0 6..
-E g
06.-
..=
2,
04..
01s
c m I
-c
0 2..
546 6
541 6
536 6
531 6
528 6
521 6
Binding Energy (eV)
Figure 1. XPS spectra of 0 1s for zeolite Nay: (a) on In foil a t -196 "C; (b) on a Cu mount a t 25 "C; (c) on a Cu mount a t 175 "C; (d) on a Cu mount a t 225 "C; (e) on a Cu mount a t 400 "C.
ages reported are based on a maximum peak height of water at +225 "C. Water, nitrogen, and oxygen were identified from mass-to-charge ( m / e )peaks of 18,28, and 32, respectively. The large amount of desorbing nitrogen at T = -175 "C was expected because the sample was frozen in a nitrogen environment. At temperatures above +125 "C, equal-intensity m / e peaks were observed at 18 and 17, suggesting that both molecular water and OHwere desorbing at these higher temperatures. Discussion The XPS data show a change in the 0 1s peak beginning at +175 "C (Figure IC). Before +175 "C, the 0 1s
peak does not change in shape or position (Figure la,b). At this point, a shoulder appears on the low binding energy side of the main 1s peak. This corresponds to an increase in the oxygen ( m / e 32) peak of the RGA data (Table 111). A t lower temperatures, very little to no oxygen is detected. The shoulder increases in size a t +225 "C (Figure Id) and then begins decreasing as the temperature is increased further. A t +400 "C (Figure le), the shoulder is still evident, although it is smaller than at +175 "C. The XPS spectrum of the copper mount alone does not show this shoulder on the 0 1s peak. In fact, no oxygen is observed above +175 "C on the copper mount, and no RGA spectral features are seen when the copper mount is studied alone.
Langmuir, Vol. 5, No. 6,1989 1343
Retention of Molecular Oxygen in Zeolites
Table 111. Residual Gas Analyses ~
:1
relative amount of gaso temp, "C -175 -150 -125 -100
2.
.-
I
- 2
540 6
0 15
541.6
536.6
531.6
526 6
521.6
Binding Energy (eV)
g4j
-75 -50 0 +25 +75 +125 +175 +225 +275 +325 +375 +400
H2O
NZ
24 28 0 4 4
28 56
8
4 4 4 60 60 100
40 72 76 72
02
4
0
8 0 0 0 0 0 0 0 0
16 100 8 20 20 24
4 24 0 4 4 4
0
4 4 4 0 0 0
Percent of maximum peak height (relative to 100% = water desorbed at +225 "C).
'T
oxygen-preexposed sample at +400 OC is also larger than at +400 "C for the sample not exposed to oxygen. Only a small quantity of oxygen would be expected to adhere to the surface of the sample at such an elevated temperature as 400 "C. This experiment, however, does support the conclusion that the shoulder observed at a binding energy of 527.0 eV between +175 and +400 "C is due to molecular oxygen being released from the bulk of the zeolite and retained at its surface. The ratios of intensities of the 0 1s shoulders qualitatively match trends of intensities of the RGA 0,data.
0
3
546.6
~~
541 6
536.6
531.6
526.6
521.6
Binding Energy (eV)
Figure 2. XPS spectra of 0 1s for zeolite NaY on a Cu mount at 225 "C: (a) prior to oxygen exposure; (b) following oxygen exposure.
The above observations suggest that molecular oxygen is being released by the zeolite beginning at +175 "C. X-ray diffraction patterns before and after the XPS experiments are identical, indicating that the bulk structure of the zeolite is not being affected by the X-rays or the heating. Accordingly, the oxygen being generated from the zeolite must be molecular oxygen originating from deep within the pores of the structure. The excessive energy (heat) required to liberate the oxygen indicates that this species is strongly absorbed to the zeolite. When the NaY sample is exposed to oxygen at +225 "C (Figure 2b), only a slight increase is observed in the shoulder intensity, but the binding energies of the peak and shoulder are not shifted from the previous experiment (Table 11). The intensity of the shoulder of the
Conclusion The changes in the 0 1s peak over the temperature range -196 to +400 "C (Figure 1)indicate that molecular oxygen is being detected leaving the surface starting at +175 "C. Figure 2 supports the conclusion that the shoulder is due to molecular oxygen, because the shoulder intensity increases slightly after exposure to oxygen. Further evidence that the shoulder is molecular oxygen is provided by the RGA data in which the peak a t mle 32 increases at +175 "C and reaches a maximum at +225 "C. These data suggest that the 0 1s transition would not be a good standard for calibration of the binding energy scale for zeolites and that 0, can be retained in the zeolite to a very high temperature. Such retention of O2 could be important in gas separation, catalytic processes, and other applications. Acknowledgment. We gratefully acknowledge support of the Office of Basic Energy Sciences, Division of Chemical Sciences, of the Department of Energy. Registry No.
O,, 7782-44-7.
