Hydrogen spillover, detection

Hydrogen spillover was also often detected with even more complicated catalysts such as Ru-Ni-La203 supported by active carbon, as reported by Inui et al. (36,37). They measured the adsorption capacities with H2 at room temperature to 320°C at a H2 pressure of 0.04 atm. Once more the slow uptake was attributed to the spillover of hydrogen species from (Ni -I- Ru)/La203 to the active carbon. 

When the support contains cations that are not easily reducible (i.e. those of Al, Si, Mg, Zr), hydrogen spillover occurs above 573 K without observable chemical reaction (Class A). However if it contains ferric ions as impurities, as is often the case with alumina, reduction to ferrous ion is detectable by EPR and if it contains sulfate ion, as may be the case with titania, reduction of the precursor with hydrogen automatically generates hydrogen sulfide which poisons the metal (Class B). If deuterium is used in place of hydrogen, support hydroxyls... 

In order to develop more informative and direct method of studying the spillover effect of active particles, the authors of [37] suggested to use the sensor method of detecting migrating particles based on separation of sensor and emitter (donor) of active particles. The latter consists of small metal globules, or clusters (with a diameter of about 20-30 A) of Pt, Pd, Ni, etc. (activator) deposited on quartz or sapphire (AI2O3) plate in the form of a strip less than 1 cm wide. The sensor for detection of hydrogen atoms consisted of a zinc oxide strip (with a width of about 0.1 cm and thickness wlOO nm) deposited on the same plate at a distance of 0.03 or 0.6 cm (two versions) from the inner boundaries of activator strips [38]. 

Hydrogen is suggested to come from the spillover from copper.42 The last step is the rapid hydrolysis of the surface methoxy to form the product methanol and restore the surface OH species required for the adsorption of the next CO2. The water-gas shift reaction occurs predominantly on Cu and it is not affected by the presence of Zr02. The suppressed rate of methanol formation over Cu on Zr02 from CO as compared to C02 is ascribed to the absence of water formation, which prevents the more facile release of methoxy by hydrolysis. Hydrogenation of formates detected on Cu on Ti02 was also observed to occur.43... 

The amount of CO adsorbed on the palladium particles can be deduced from CO-TDS (Fig. 3 Id). The CO-TDS spectrum was identical to that observed after dosing of the same amount of CO on the clean particles, demonstrating that the particles were fully covered with CO and that Ft was replaced from the palladium surface. FFowever, FF had not desorbed because the FF2-TDS experiment (Fig. 3Id) indicated that the overall amount of hydrogen was unchanged. Apparently, CO had displaced surface FF to the subsurface and bulk of the palladium nanoparticles (see schematics in Fig. 31 partial Ft spillover to the support is unlikely because no OFF groups or H2O were detected). 

NMR adsorption isotherms for Ru/SiOi catalysts have been obtained using explicit calibration (89). Although the pressure over the sample could be adjusted in situ, no volumetric data were taken simultaneously, probably because of the important spillover effects in this catalytic system (see Section III.A). The NMR study was performed at pressures between 10 and 760 Torr and at temperatures between 323 and 473 K (only the 323-K results are reviewed here). The dispersion of the catalyst was determined from the irreversible H NMR signal as 0.29. The metal loading was 8 wt% so that a monolayer coverage on 1 g of catalyst corresponds to 2.8 cm of H2 under standard conditions. It is typical for an NMR sample to contain 0.5 g of material in a 1-cm sample volume, and the pores in the powder make up about half the volume. If such a sample of this catalyst is under 760 Torr of hydrogen, the gas phase corresponds to one-third of a mono-layer, and it can make a detectable contribution to the NMR signal. 

Donors and Donees Contacted and Separated.—Teichner was the first to recognize the fundamental importance to the problem of spillover of experiments in which the donee is ultimately separated from the donor and the adsorptives, and examined separately for evidence of the presence of spillage. Hydrogen is, so far, the only adsorptive to have been investigated in this manner and its detection on the donee has been accomplished by way of the chemical reactions it induces. 

Hydrogen atoms returning to the metal by reverse spillover can however initiate isomerisation between the butenes and isotopic exchange between labelled and unlabelled alkenes, after which the reaction is self-propagating. Since the amount of the initiator may be immeasurably smaU, it cannot necessarily be detected by... 

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