H2-D2 isotopic exchange

Figure 8.9. H2-D2 isotope exchange over Ag/alumina catalyst (H. Backman, J. Jensen, F. Klingstedt, J. WarnJ, T.Salmi, D.Yu. Murzin, Kinetics and modeling of H2/D2 exchange over Ag/Al203, Applied Catalysis A. General, 273 (2004) 303)...

The H2 + D2 isotopic exchange is interesting as one of the simplest chemical reactions of H2 on the catalyst surface. 

H2 is a reactant in several reactions of the catalyst surface. The role of H2 in the reduction of the surface has been treated above. The role of H2 as a reactant in the synthesis of NH3 will be treated below. The present section will treat the adsorption and desorption of H2, the ortho-para conversion, and the H2 4- D2 isotopic exchange. 

Fischer Christian-H, Hart Edwin J, Henglein Amim (1986) H/D isotopic exchange in the D2 - H2+ system under the influence of ultrasound. J Phys Chem 90 222-224... 

In the early nineteen-sixties Halpem, James and co-workers studied the hydrogenation of water-soluble substrates in aqueous solutions catalyzed by rathenium salts [6]. RuCh in 3 M HCl catalyzed the hydrogenation of Fe(III) to Fe(II) at 80 °C and 0.6 bar H2. Similarly, Ru(IV) was autocatalytically reduced to Ru(III) which, however, did not react further. An extensive study of the effect of HCl concentration on the rate of such hydrogenations revealed, that the hydrolysis product, [RuCln(OH)(H20)5. ] " was a catalyst of lower activity. It was also established, that the mechanism involved a heterolytic splitting of H2. In accordance with this suggestion, in the absence of reducible substrates, such as Fe(in) there was an extensive isotope exchange between the solvent H2O and D2 in the gas phase. 

In addition to reactions (a)-(d), ideas on tunneling have been also used (see references cited in ref. 53) to explain isotopic effects in the exchange reactions of Cl and F atoms with H2, D2 and HD molecules, in the reactions... 

The rates of these reactions (R -R2) have been determined by the quantitative analysis of the reduction of H3PM12O40 (M = Mo, W) by a mixture of H2 and D2. With H3PW12O40, the isotopic equilibration of H2 and D2 in the gas phase, as well as the isotopic exchange between the entire solid and the gas phase, is very rapid, so that, to our surprise, the content of H in the gas phase increased rapidly (Fig. 50). The detailed kinetics analysis shows that the reactions of Eqs. (29) and (30) are very rapid and that of Eq. (31) is the slow step, the equilibrium strongly favoring the reactions on the left-hand side of Eq. (29) (Fig. 51, left). 

If the metal had only a beneficial catalytic role necessary (i) to evolve H2 (or HD), and (ii) to regenerate OD groups (C5H10-D2 exchange), the reaction rates should increase with increasing metal contents, possibly up to a limit, and no increase should be expected in the case of oxygen isotope exchange. 

In a recent review work (117) on the chemical and nano-structural characterization of NM/CeO catalysts, a detailed study of the H interaction with a Pt/CeO catalyst reduced at temperatures ranging fh)m 473 K to 773 K is reported. The experimental techniques used in this work were TPD-MS and Isotopic Transient Kinetics (ITK) of the H2/D2 exchange at 298 K. The catalyst sample was carefully selected in order to minimise the Pt and support sintering effects in the investigated range of reduction temperatures. Likewise, a chlorine-free metal precursor, [Pt(NH3)4](OH)2, was used in the preparation of the catalyst. 

This group is currently exploring the use of CO/H chemisorption ratios, H2 D2 exchange rates, and isotopic patterns of ethylene deuterogenation as probes of the nature of supported metals. 

H2/D2 exchange involves type (/) species, but their exact role is still under debate. A kinetic isotope effect operates such that HD is produced — 1.5 times faster from a 2H2 D2 mixture than from a H2 2D2 mixture. Naito et al favour an Eley-Rideal mechanism at 200 K involving a gas-phase molecule and a type (/) chemisorbed atom. Kokes et a/. initially favoured a similar mechanism involving a type Hi) chemisorbed molecule instead of one in the gas phase. However, in a later paper they show that type Hi) species are only important in allotropic ortho-para) conversion and that the exchange reaction involves a Bonhoeffer-Farkas mechanism, using type (/) adatoms, at all temperatures. Richard et al., on the basis of the variation of the first-order rate constant with pressure, deduced that the mechanism must involve atomic H on surface pair sites that most probably were adjacent ion pairs thus, a Rideal type of mechanism was favoured. 

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