Acidity of SiO₂-Supported Metal Oxides in the Presence of H₂O Using the Adsorption Equilibrium Infrared Spectroscopy Method: 1. Adsorption and Coadsorption of NH₃ and H₂O on SiO₂

Abstract

The present study is dedicated to the characterization (identification, heats of adsorption and coverages) of the adsorbed species formed by the adsorption and coadsorption of NH and H 2 O on two SiO 2 solids. The Adsorption Equilibrium InfraRed spectroscopy (AEIR) which allowed (a) to show that NH 3 and H 2 O are mostly adsorbed on free SiOH groups via H-bonds and (b) to determine their individual heats of adsorption: 53 kJ/mol and 49 kJ/mol whatever their coverages (Langmuir adsorption model) for NH 3ads and H 2 O ads , respectively. These values consistent with microcalorimetry literature data explain that their coverages are decreased upon NH 3-H 2 O co-adsorption considering a competitive Langmuir model. However, the Temperature Programmed Adsorption Equilibrium (TPAE) procedure achieved from MS data indicated that a minor amount of other NH 3 species (not detected using FTIR) are more strongly adsorbed and that hydrolysis of SiOSi siloxane by H 2 O could occur in parallel. NH 3-H 2 O coadsorption leads to formation of NH 4 + species which involves H 2 O adsorbed species. Both NH 3 and H 2 O are not adsorbed above 450 K which means that the SiO 2 contribution to the characterization of the acidity of metal oxides catalysts supported on SiO 2 using NH 3 as probe molecule in the presence of H 2 O is negligible above this temperature.

Acidity of SiO2-Supported Metal Oxides in the Presence of H2O Using the AEIR Method: 2. Adsorption and Coadsorption of NH3 and H2O on TiO2/SiO2 Catalysts

“…At ratio 6:1, a shoulder near 3720 cm –1 and a band at 3660 cm –1 were observed and ascribed to ν­(O–H) bands of TiO 2 . This suggests that the band at 3673 cm –1 in Figure A, which is not detected on SiO 2 supports for similar experimental conditions, is due to TiO 2 domains. An assignment to v­(H 2 O) stretching vibrations of H 2 O adsorbed species like Ti-μ 1 OH 2 can be found between 3646 and 3710 cm –1 .…”

“…The band observed at 3736 cm –1 (spectrum a) at 723 K was ascribed to the ν­(O–H) stretching vibration of free OH groups (see ref and references therein). Note that they could correspond either to SiOH and TiOH species. , In fact, it was shown for bare SiO 2 that this asymmetric band contains a second component located ca 5 cm –1 lower than the maximum attributed to very weakly interacting silanols. , For the sake of clarity, the term “free OH groups” corresponds in the following either to free OH groups and to very weakly interacting silanols. The band raises during the cooling stage associated with a shift to 3745 cm –1 (spectrum b).…”

“…Literature data show that TiO 2 /SiO 2 solids prepared by different methods pretreated at high temperatures provide similar FTIR spectra in the wavenumbers range characteristic of ν­(O–H) stretching vibrations. − For instance, Bonelli et al reported FTIR spectra of different x % TiO 2 /SiO 2 solids ( x in the 3–18 range) pretreated at 723 K. The authors note that the ν­(O–H) band of the SiO 2 support (Grace) (assigned to terminal/geminal free silanols) is slightly shifted from 3742 cm –1 (a position also observed in part 1 on a pure SiO 2 support) to 3745 cm –1 after TiO 2 deposition. A study of the change in the ν­(O–H) bands of TiO 2 /SiO 2 solids similar to ref has been performed on x % TiO 2 /SiO 2 with x in the 8–48 range: whatever the x value, the main band was observed at 3745 cm –1 with a tail on the side of the low wavenumbers.…”

“…Fourier transform infrared (FTIR) spectra of the compressed disk of solid (ϕ = 1.8 cm, m ≈ 40–80 mg) were recorded with a Nicolet 6700 spectrometer (Thermo Scientific) using a small path length (≈ 2.2 mm) homemade in situ IR cell in transmission mode with CaF 2 windows and under gas flow rates containing a partial pressure of NH 3 as described in details in Part 1 of the series . Only bands above 1250 cm –1 were observed in the present study because of the low IR transmission of SiO 2 samples and CaF 2 cell windows.…”

“…The heats of adsorption of NH 3 - and H 2 O-adsorbed species were calculated according to procedures detailed in the part 1 of the series . Briefly, the different θ ex = f ( T a ) curves are compared to theoretical curves obtained by adsorption models (Langmuir, Temkin) considering localized adsorbed species for the adsorption coefficient provided by the activated complex theory and statistic thermodynamics (the different mathematical expressions are recalled in the following when they are used).…”