Role of chloride or bromide anions and protons for promoting the selective oxidation of H2 by O2 to H2O2 over supported Pd catalysts in an aqueous medium

Microchannel Reactor System Design & Demonstration For On-Site H2O2 Production by Controlled H2/O2 Reaction

“…This is significant because, according to Choudhary et al, 2007b Table 5.4. The first mechanism was proposed by Choudhary and Samanta, 2006, and the second was proposed by our group. The surface reaction steps (step 3 of each mechanism) were assumed to be rate-determining because the reaction rate depended on the concentration of both H 2 and H 2 O 2 .…”

“…The fact that O-O bond does not dissociate during H 2 O 2 formation was experimentally confirmed by Dissanayake and Lunsford, 2003, and the presence of HO 2 -on catalyst surface during the reaction was discovered by Sivadinarayana et al, 2004. In the recent decade, much work has been done to investigate in greater detail the effect of various parameters on the DC reaction. The group of Choudhary demonstrated the effect of the nature and concentration of acid and halide promoters on reactions (1), (2) and (3), as well as the effect of the oxidation state of palladium (Choudhary and Samanta, 2006;Samanta and Choudhary, 2007a;Choudhary et al, 2006aChoudhary et al, , 2007. A mechanism for reactions in Scheme 1 was proposed that accounted for the promoting effect of halides and protons (Choudhary and Samanta, 2006).…”

“…The group of Choudhary demonstrated the effect of the nature and concentration of acid and halide promoters on reactions (1), (2) and (3), as well as the effect of the oxidation state of palladium (Choudhary and Samanta, 2006;Samanta and Choudhary, 2007a;Choudhary et al, 2006aChoudhary et al, , 2007. A mechanism for reactions in Scheme 1 was proposed that accounted for the promoting effect of halides and protons (Choudhary and Samanta, 2006). The mechanism of inhibition of H 2 O 2 consumption reactions by halides and protons was also investigated by the group of Lunsford (Dissanyake and Lunsford, 2003;Chinta and Lunsford, 2004) and Landon (Landon et al, 2003).…”

“…When H + is present in sufficient concentration in the reaction medium, reaction (3) is suppressed and can be neglected, so that H 2 O 2 is consumed only through reaction (4) (Landon et al, 2003, Chinta andLunsford, 2004). Reaction (1) is less affected by the halides than reactions (2)-(4), so that the halides significantly increase the selectivity for hydrogen peroxide (Choudhary and Samanta, 2006). This effect applies to halides dissolved in the reaction medium, as well as to those incorporated into the catalyst support .…”

“…(4)) is inhibited by the presence of acids in the reaction medium, particularly H 2 SO 4 , H 3 PO 4 , and HCl (Choudhary, et al 2006b). The inhibition effect of acids is attributed to the formation of a bond between protons and H 2 O 2 , which would give H 2 O 2 a partial positive charge and prevent it from adsorbing onto the catalyst (Choudhary and Samanta, 2006 ). Acids and halides promote the formation of H 2 O 2 when either is present in the reaction medium.…”