What About Hydride Ions

There have been some speculations about the existence and transport of hydride ions (H ) in oxides under reducing conditions, but according to thermodynamic data of hydrides, the conditions for a hydrogen-separation membrane will be much too oxidizing for hydride ions to be stable [9]. Moreover, the apparent indications of hydride ions in the literature have now been rationalised by other, more credible phenomena, actually arising from transport of neutral hydrogen [2]. Therefore, it seems, at present, that hydride ions play no role in hydrogen permeation in oxides. 

Surface Kinetics of Hydrogen Permeation in Mixed Proton-Electron Conductors 

As the mixed proton and electron conductive oxide membrane becomes sufficiently thin, surface kinetics will become important, and difiusion of protons across the membrane will no longer be rate deterrriining for the overall hydrogen flux. Bouwmeester et al. [10] defined a characteristic thickness, L, for membranes where surface kinetics and bulk kinetics are equally important to the flux. Decreasing a membrane s thickness below gives essentially no increase in the flux. 

To the best of our knowledge, there is yet no literature example where surface kinetics has been proven to limit the hydrogen permeation across a mixed pro-ton-electron conducting oxide, and seemingly membrane thicknesses below Lc have thus never been reached. 

As an alternative to forming electrons, hydrogen may be oxidized by annihilation of electron holes. 

---

On the basis of what we have already learned about the reactions of lithium aluminum hydride with aldehydes and ketones (Chapter 18) and the mechanisms presented so far in this chapter, we can readily predict the product that results when hydride reacts with a carboxylic acid derivative. Consider, for example, the reaction of ethyl benzoate with lithium aluminum hydride. As with all of the reactions in this chapter, this reaction begins with attack of the nucleophile, hydride ion, at the carbon of the carbonyl group, displacing the pi electrons onto the oxygen (see Figure 19.7). Next, these electrons help displace ethoxide from the tetrahedral intermediate. The product of this step is an aldehyde. But recall from Chapter 18 that aldehydes also react with lithium aluminum hydride. Therefore, the product, after workup with acid, is a primary alcohol. 

E5-2 When 13C-labelled formaldehyde, 13CH20, is fed to live cultures of bacteria in an NMR spectrometer, the metabolism of the label can be followed by 13C NMR. Many bacterial species produce roughly equal amounts of formate (HCOO ) and methanol (CH3OH). This is reminiscent of the purely chemical Cannizzaro reaction in which a hydride ion (H ) is transferred directly from one formaldehyde molecule to another. The accompanying 61 MHz deuterium NMR spectra are of methanol that results from the metabolism of deuterium-labelled formaldehyde, CD2O, by Escherichia coli and Pseudomonas putida. What do they tell us about possible Cannizzarase enzymes in those organisms ... 

---