Deuteroxyls

Before each run, the hydroxyl spectra were scanned at room temperature and reaction temperature. The spectrometer wavelength scale was locked at the maximum of the optical density of the hydroxyl (deuteroxyl) band, and the decrease (increase) of the OH (OD) band maximum was recorded continuously as a function of time. No shift in the maximum of the optical density occurred during the reaction. For most of the reactions the pressure of D2 was 100 torr. The temperature range from 200° to 400° C was investigated in intervals of 50°C. 

The value of the atom fraction H (of the OH + OD), initially, at equilibrium, and at time t, is denoted by x0, xm, and x, respectively. Allowance was made for the fact that the apparent optical densities of the hydroxyl and corresponding deuteroxyl groups are different by as much as 10%. The excess of D2 was at least 5-fold, so that the variation of the different types of hydroxyls could be evaluated separately (17). From Equation 1 the characteristic slope parameter is obtained (19) ... 

Physisorption may involve the amine-side as well as the silicon-side of the silane molecule. Silanol groups (on the surface as well as on the silane) therefore are very important in the reaction sequence. In this paragraph we aim to clarify the role of both surface and silane silanols. A clear distinction between both types is possible with the complementary use of solid-state NMR and FTIR, and with conversion of the surface hydroxyls to deuteroxyls (K) prior to modification with the silane. 

In order to be able to distinguish non-reacted surface silanols from silanols formed in the reaction, the silica surface was deuterated before modification. After thermal pretreatment, a maximal amount of surface hydroxyls is exchanged to deuteroxyls. If the silane is applied to this deuterated substrate, deuteroxyls found after reaction are non-reacted surface groups. Detected hydroxyls must have been formed in the course of the modification procedure. The deuteration was performed with D20 vapour, as discussed above. 

An impression of the quantity of these surface hydroxyls can be obtained from their FTIR spectra. In figure 9.36 a the FTIR-PAS spectrum for the 973 K deuterated modified silica is displayed. In the deuteroxyl region (2800 - 2500 cm 1) no absorption band is observed. All deuteroxyl groups have reacted. 

OD represents a deuteroxyl group, i.e. one oxygen and one deuterium atom. 

Figure 1. Hydroxyl and Deuteroxyl bands for a "US-Y" zeolite. A. US Sieve steamed 65 hr at 500 C and dried in vacuum at 500 C. B.- A. after exchange with D20 and redrying at 500 C.

Figure 5 Effects of rehydration (with deuterium oxide) on deuteroxyl bands of ZSM-5 [1.35% A120 3). Pretreatment a.

Fs (D) centres are analogues to Fs (H) centres but obtained by irradiation in D2 atmosphere. The hyperfine interaction of the trapped electron with the deuteroxyl D nucleus (1=1) leads to an unresolved triplet due to the small magnetic moment of deuterium. The comparison between the N2" signal obtained by adsorption of N2 on Fs (D) centres reveals that each of the main lines in the N2 spectrum (Fig 3a) are split into doublets in the case of N2 formed over the Fs (H) centres (Fig 3b). This weak superhyperfme splitting of less than 1 Gauss indicates that a single proton (I = 1/2) interacts with the N2 radical, and this proton... 

Figure 2.5 Difference FTIR spectra of silica (A, C) and deuteroxylated silica (B, D) after adsorption of CH4 (a, d), CO (b, e), and CD3CN (c, f). Equilibrium pressures p(CO) = 500 Pa, p(CH4) = 1500 Pa, and pfCDsCNj AOO Pa. Temperature of adsorption is 293 K for CD3CN and 100 K for CH4 and CO. (A) v OH) region, (B) v OD) region (C) 2v(OH) region, and (D) 2v OD) region. Reproduced with permission from Ref. (21). Copyright 2013 American Chemical Society.

FIG. 1 Diffuse reflectance FTIR (DRIFT) spectra of the deuteroxyl stretching vibrations for several oxides after partial surface dehydration at 500 C. 

FIG. 3 The relationship between stretehing frequencies of isolated deuteroxyls on partially dehydrated surfaces and the acidity constants determined by PAD at the oxide/solution interface for several oxides. ( ) Alumina (A) sihca ( ) nutile ( ) anatase (O) ceria (A) zirconia. 

Frost MJ, Sharkey P, Smith IWM (1993) Reaction between hydroxyl (deuteroxyl) radicals and carbon monoxide at temperatures down to 80 K experiment and theory. J Phys Chem 97 12254... 

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