Activated Carbon H2O2 Time

Four other Norit SXRO activated carbon catalysts that essentially differ only in their H2O2 activity time have been tested in the glyphosate process. As expected, the reaction rate increased with a decreasing H2O2 time (Figure 17.3a). 

Figure 3.34. Galvanostatic potential-time curves (/ = 5 mA/cm ) of oxygen electrodes with an active material from active carbon P33 + 15% CoTAA. Temperature 25°C, electrolyte 4.5 NH2SO4. (1) Without thermal treatment of the active material, (2) without thermal treatment of the active material adding H2O2 up to 1% in concentration in the electrolyte every 100 h, (3) with thermal treatment of the active material in Ar at 630 C adding H2O2 up to 1% in concentration in the electrolyte every 100 h (according to Figure 6 in ref. [29] reproduced with permission of Elsevier).

The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

As reported so far, one of the best platinum-free ORR catalysts of chalcogenide-type structure is a selenium-modified mthenium catalyst (RuScx/C) [9-20], State-of-the-art catalysts are composed of carbon-supported nano-scaled ruthenium particles whose surface was modified with selenium [9-14], The modification leads to 10 times higher ORR activity, protects the ruthenium particles against electrooxidation, and suppresses the H2O2 formation. As RuSe /C is insensitive to methanol, it might be particularly suitable as an alternative cathode material in direct methanol fuel cells (DMFC) where platinum shows potential losses due to the methanol crossover [15-18]. However, ruthenium is still a costly and rare noble metal and seems not to be a feasible alternative to platinum. Therefore, readers who are interested in this type of catalyst are referred to the cited literature. 

Sodium percarbonate (PCS, NazCOa 1.5 H2O2) is not a true persalt but a simple addition compoimd and, consequently, is less storage stable, as is, than perborate (PBSl or PBS4). It has an active oxygen content of 15% (approximately equal to that of PBSl). Its solubility at 20°C is 120 g/L (i.e., about five times more soluble than PBS4). Its rate of dissolution, in the uncoated form, is about the same as that of PBSl, and its bulk density is somewhat higher (0.9-1.2 kg/L, depending on the method of manufacture). The carbonate liberated contributes to the alkalinity of the wash bath. 

---