Temperature adsorption, spectra

The features due to adsorbed water and carbonates observed on the boehmite and y-alumina deserve further attention as they differ from results published by previous investigators. Figure 4 shows a series of difference spectra for adsorption on y-alumina. Spectra were taken after drying the y-alumina at 350 C, cooling to room temperature and carrying out room temperature adsorption. The spectra are the difference of the sample before and after adsorption. Spectrum 4e is the spectrum for the as received alumina differenced with the dried alumina. The positive band at 3400 cm" is due to adsorbed water, and the small negative feature at 3740 cm" is due to isolated hydroxyls on the dried surface. Besides the three... 

Grassian and Pimentel (210) prepared such surface groups by thermal decomposition of cis- and frans -dichloroethenes at temperatures >200 K or by their photolysis at 110 K on Pt(lll). An ethyne type B spectrum was obtained, as had also been obtained from the direct low-temperature adsorption of ethyne on this surface (Section II.B.l). 

The effects of chemisorption on the two spectral doublets further elucidate the nature of Fe2+ in the zeolite framework. Ammonia, which is small enough to enter both the main channels and the side pockets of the zeolite, is expected to affect both spectral doublets, as experimentally observed. Of the molecules methylamine, dimethylamine, trimethylamine, and piperidine, only the first has any effect on the Mdssbauer spectrum after room temperature adsorption (resulting in decreased spectral area for both doublets). At 340 K, however, dimethylamine adsorption resulted in a spectral area de-... 

In Figure 3.21 the effect of low-temperature adsorption of CO on the spectrum of the surface OH groups of silica, silica-alumina and a zeolites (H-MFl) is shown. The adsorption of CO on the silanol groups present on silica results in the forma-... 

It was confirmed that the presence of the carrier gas (helium) did not affect the diffusion measurements. When helium was replaced by neon, argon, or krypton, no change in the results was observed only xenon caused some deviation of the results (Niessen W, private communication). The main flow could be very rapidly connected with streams of the adsorbates in helium (8 ml min ) in such a way that the total flow and pressure remained constant. The adsorbate partial pressures could be varied, i.e., increased or decreased, almost instantaneously by small jumps. The experiment was started by scanning the spectrum of the pure, activated adsorbent. After a first pressure jump, e.g., from zero to 115 Pa, at a chosen adsorption temperature, the spectrum of the adsorbate/adsorbent was monitored in short intervals. An FTIR spectrometer of Perkin-Elmer type 1800 was employed. An example with sets of spectra of ethylbenzene adsorbed into H-ZSM-5 is shown in Fig. 3. 

All important peculiarity of solid surfaces is the occurrence of adjacent Lewis acid (unsaturated cation) and Lewis base (unsaturated anion) sites in repetitive structures. If they interact in a concerted manner with the adsorbed species, the result is dissociative adsorption. For example, adsorption of hydrogen on ZnO is molecular at -195°C, but at room temperature adsorption is dissociative. The mechanism is most likely heterolytic, as suggested by new Zn —H and O—H vibrations observed in the IR spectrum [reaction (VIII)][37]. 

Wielers and co-workers have investigated the infrared spectra of carbon monoxide adsorbed onto alumina-supported iron catalysts. The infrared adsorption spectrum of carbon monoxide adsorbed on an alumina-supported iron catalyst is represented in Fig. 5.7. In addition to maxima lying between 2010 and 2050 cm" a peak at 2155 cm is also seen. The effect of evacuation on the sample is shown in Fig. 5.8. It can be seen that the carbon monoxide adsorbed on the Fe(II) ions was removed by evacuation for 0.5 h at room temperature. 

During adsorption at low temperature, benzene molecules interact only weakly (van der Waals adsorption) with Ni(lll) and as a result the UPS difference spectrum (c) at 150 K looks very similar to the gas-phase spectrum. The species on the surface is basically unaltered condensed benzene. Comparison of difference spectrum (c) with that in (b), for room-temperature adsorption, reveals both that the benzene n orbitals have shifted with respect to the other orbitals, and that most of the nickel rf-electron density has been lost. Clearly there has been a dissociative and strong interaction of the benzene with the Ni(ll 1) surface since the electronic structure of the benzene molecule has been disrupted and the Ni cf-electrons have been involved in the bonding. This type of change in electronic structure is indicative of chemisorption. 

Figure 5 presents data for the non-lnteractlng Rh/S102 catalyst at similar pressures and at 48, 158, and 333 °C. Even with the scatter In the 333°C data, there Is an obvious transition In the spectra as the temperature Is Increased. The predominant peak around 10 rad/sec diminishes and the one around. 5 rad/sec Increases to dominate the spectrum, a trend similar to that observed In the Rh/T102 spectra. Presumably, these trends are the result of differences In apparent activation energies for H2 adsorption and desorption on the various types of sites. 

The samples were submitted to the sulfidation procedure described above, followed by 2 h of heating at 673 K, under vacuum (about 2x10 3 Pa). After cooling under vacuum, pyridine was adsorbed at room temperature for 30 minutes. The samples were then outgassed in three steps of 1 h the first one at room temperature and the others at 423 K and 523 K. Spectra were taken before pyridine adsorption and after each outgassing step, with a FTIR spectrometer Bruker IFS-88 (spectral resolution set at 1 cm ). Each spectrum represented the average of at least 50 scans. 

Adsorption and evacuation at r.t. gives rise to a spectrum with bands at 1636 cm-i (vc=c), 1494 cm- (vas -CO2 ), 1436 cm-i (scissoring CH2 overlap to Vs -CO2-), 1376 cm-i (8ch), 1277 cm-i (vcc) and 1070 cm- (rocking CH2) indicating the rapid formation of an acrylate species. Weak bands are also observed at 1660 and 1600 cm-i which may be attributed to weakly bonded acid. These bands disappear by evacuation above r.t.. The acrylate species is stable up to an evacuation temperature of about 200°C and then decreases in relative intensity at... 

In situ FTIR spectroscopy was used to study the adsorbed species generated on the catalyst surface in the presence of Hj and Oj. Before the experiment, the catalyst wafer was pretreated by O, (5.3 kPa) at 723 K for 1 h followed by evacuation at the same temperature in vacuum ca. 6x10 Pa) for 2 h. After the pretreatment, the temperature was decreased to a desired one in vacuum and IR spectrum was recorded at that temperature. The spectra of the catalyst wafer recorded at different temperatures were used as the background ones for the adsorption studies described below. 

Figure 11. Infrared spectrum of CO adsorption at 295K for the Pt/SBA-15 catalyst series (a) 2.33% Pt(1.7nm)/SBA-15, (b) 2.69% Pt(2.9 nm)/SBA-15, (c) 2.62% Pt(3.6nm)/SBA-15, and (d) 2.86% Pt(7.1 nm)/SBA-15. Inset is the peak position and FWHM of the atop CO stretching vibration as a function of particle size at room temperature. Peak heights have been modified for clarity [16]. (Reprinted from Ref [16], 2006, with permission from American Chemical Society.)...

The existence of various temperature intervals characterized by predominant manifestation of one of above interactions can be detected from thermal desorption spectra. For instance, the thermal desorption spectrum obtained in [71] for a cleaved ZnO (1010) monocrystal following its interaction with oxygen (Fig. 1.4) indicates the availability of such typical temperature intervals as interval of physical adsorption (a), chemisorption (b), interval of formation of surface defects (c) and, finally, the domain of formation of volume defects (d). 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

Recently the spectrum of O- has been observed on MgO by Lunsford and co-workers (50, 51). The species was formed by adsorption of N20 at low temperatures onto MgO which contained trapped electrons. By using N2170 it was shown that the species was indeed 0 The spectrum shown in Fig. 21 is characterized by gx = 2.042 and g = 2.0013 with = 19.5 and an = 103 G. From the hyperfine coupling it may be shown that the unpaired electron is localized mainly in one 2p orbital. Both the g values and the hyperfine coupling constants are consistent with the energy level diagram of Fig. 20a. The results are also consistent with the spectrum of... 

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