Quantification of Water and Silanol Species on Various Silicas by Coupling IR Spectroscopy and in-Situ Thermogravimetry

Abstract

Five silica samples (four precipitated silicas provided by commercial suppliers and one with the MCM-41 structure) have been studied by infrared spectroscopy and by a homemade thermogravimetry-infrared spectrum (TG-IR) setup. The silanol amount, accessibility to water, and different alcohols, and the affinity to water of these various silicas were compared and quantified. TG-IR measurements allowed the precise determination of the integrated molar absorption coefficient of the (nu+delta)OH band, epsilon(nu+delta)OH=(0.16+/-0.01) cm micromol(-1). It is independent of the sample origin and the concentration of silanol groups on silicas. For the precipitated dried samples evacuated at room temperature, the silanol concentration COH varies between 3.6 and 7.0 mmol g(-1). It is 5.3 mmol g(-1) in the case of the MCM-41 sample. Exchange experiments with D2O, followed by back-exchanges with different alcohols (methanol, propan-2-ol, 2-methyl-propan-2-ol, and 3-ethyl-pentan-3-ol) have been followed by infrared spectroscopy. All of the silanols of the MCM-41 sample are accessible to water and alcohol molecules. By contrast, about 20% of the silanols in precipitated samples are not exchanged by D2O (internal silanols). Accessibility decreases with alcohol size; the main effect is relative to methanol. Taking into account the sample specific surface areas and the silanol accessibility to D2O, the surface silanol density of precipitated silicas is close to 8 OH per nm2, at maximum coverage. At variance, the silanol surface density of the MCM silica is much lower, 4 OH per nm2. The TG-IR setup has also been used to determine the amount of water adsorbed on silicas through the intensity of the deltaH2O band. It varies linearly with the concentration of adsorbed water, whatever the silica sample. The integrated molar absorption coefficient of two bands, epsilondeltaH2O=(1.53+/-0.03) cm micromol(-1) and epsilon(nu+delta)H2O=(0.22+/-0.01) cm micromol(-1), have been determined. The number of H2O molecules adsorbed per nm2 has been compared on the five samples under an equilibrium pressure of 13 hPa at room temperature. Taking into account the number of silanols accessible to D2O for each sample, the silica-water affinity has been defined by the H2O/(SiOHsurf) ratio. It is close to 0.8-0.9 for the precipitated samples but lower (0.7) in the case of the MCM one. This result is explained by the more important amount of isolated silanol groups presented by this sample.

Speciation of adsorbates on surface of solids by infrared spectroscopy and chemometrics

“…Again, it is only by careful interpretation of spectroscopic pure profile and corresponding adsorption profiles that the single solution proposed by the algorithm, among all the space of mathematically possible solutions, can be judged to be relevant. According to these procedures, the quantitative predictions of water content and kinetic parameters showed a very good agreement with previously published work [14,27], (see Table S2. 4…”

“…In this case, the rank deficiency issue is tacitly accepted by considering that a pure adspecies spectrum exhibits simultaneously negative bands due to consumed adsorption sites and positive bands related to characteristic bands of the adsorbed species including bond formation with the surface site (section 3.2.1). It is illustrated in the case of physisorbed molecular water species over a precipitation silica surface [14,15,27].…”

“…While the procedure should allow one to speciate surface silanol groups, their pure spectra in the (υ+δ)SiOH region -which is of interest for quantification purposes -are completely overlapped by the pure spectra of silane adspecies [27]. cm -1 , attributed to vicinal or geminal free silanol groups [15].…”

“…27 surface concentration of silanol groups can be quantified with the help of the (υ+δ)SiOH band [27]. Hence, this band was isolated from other interferences on 1 SiOHg pure spectrum , allowing the determination of the associated concentration profile (Eq .12) and was calibrated according to Eq.10 using the published integrated extinction coefficient of (υ+δ)SiOH band (ε (υ+δ)SiOH = 0.16 ± 0.01 cm.µmol -1 ) [27].…”

Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

“…Assuming spherical particles, the characteristic length will be r/3, which will somewhat diminish the problem, but still, when working with highly active samples (k′ values for the NH 3 −SCR reaction can sometimes be as high as <2000 mL/gs), 43 this manipulation could certainly become an issue. Although it should be obvious, here, all scientists should be warned to take good care and explain to students the importance of using pressures as low as possible when creating sieved fractions of highly active catalysts such as, for example, V 2 O 5 /TiO 2 , 35 Cu-zeolites, 44 etc. For the operando studies using pellets, it might be impossible in most cases to achieve sufficiently high efficiency numbers to render this effect negligible.…”