Reductive Dehalogenation of Monobromo‐ and Tribromoacetic Acid by Sodium Borohydride Catalyzed by Gold Nanoparticles Entrapped in Sol–Gel Matrices Follows Different Pathways

Abstract

An efficient, green, and reusable system comprising gold nanoparticles entrapped in an organically modified sol–gel silica matrix was found to catalyze the reduction of monobromo‐ and tribromoacetic acid by NaBH4. Interestingly, the reduction of tribromoacetic acid yielded succinic acid along with acetic acid, whereas monobromoacetic acid was reduced only to acetic acid. The relative yields of succinic acid and acetic acid depended on the rate of addition of BH4–. Slow addition resulted in the formation of succinic acid as the major product. A variable product distribution was achieved as a result of the different pathways for dehalogenation.

The Chemical Properties of Hydrogen Atoms Adsorbed on M0‐Nanoparticles Suspended in Aqueous Solutions: The Case of Ag0‐NPs and Au0‐NPs Reduced by BD4−

“…These results are analogous to those recently reported for the M 0 -NP-catalyzed de-halogenation of Br 3 CCO 2 À . [47,48] These conclusions are also relevant for electrochemical and photochemical processes and catalytic water reduction by radicals.T hus,( M 0 -NPs) nÀ are the starting point for catalytic reductions,b ut for BH 4 À and other hydride-transfer reagents,( M 0 -NPs-H n ) nÀ is the starting point. Figure 1.…”

“…It should be noted that the value of n is unknown. However,i t clearly increases with the increase in [BD 4 À ], [47,48] and its maximal value probably depends on the nature of Mand the radius of the NPs.T he value of m is also unknown and depends on the pH and the nature of M). Thus,i ft he hydrogen evolution proceeds via the Heyrovsky mechanism, then:…”

“…Theresults of the Au 0 -NP-catalyzed H 2 evolution process are analogous to those recently reported for the Au 0 -NPcatalyzed de-bromination of Br 3 CCO 2 À . [47,48] This suggests that acetate,formed via reaction (9), is the major product when high concentrations of BH 4 À are used, whereas succinic acid, formed via reaction (10), is the major product when low concentrations of BH 4 À are used.…”

“…Noticeably,r eaction (9) is analogous to the Heyrovsky mechanism and reaction (10) is analogous to the Tafel mechanism. Hence,t he results indicate that the factors affecting the decomposition of {[(M 0 -NP)-H m ]-(CR 1 R 2 CO 2 À ) f } (nÀm+2f)À and {(M 0 -NP)-H m } (nÀm)À are analogous.A th igh concentrations of BH 4 À /BD 4 À the Heyrovsky mechanism predominates,which indicates that when the M 0 -NPs are highly charged, i.e., when n is large,and the M 0 -NPs are strong reducing agents, [47,48] reaction (4) is considerably faster than reaction (À5). This conclusion suggests that when M 0 -NPs are used as catalysts for reduction processes involving hydride transfer,o ne should use high steady-state concentrations of BH 4 À ,w hereas if the mechanism involves hydrogen atom transfer processes,l ow steady-state concentrations of BH 4 À are preferable.T he second interesting conclusion is that Ag 0 -NPs prefer the Heyrovsky mechanism considerably more than the Au 0 -NPs do.T his observation is attributed to the fact that the over-potential for HER process is considerably higher for Ag than for Au.…”

The Chemical Properties of Hydrogen Atoms Adsorbed on M0‐Nanoparticles Suspended in Aqueous Solutions: The Case of Ag0‐NPs and Au0‐NPs Reduced by BD4−

“…As mentioned previously, the starting point for this process is {(M 0 ‐NP)‐D n } n − , formed via (M 0 ‐NP) + n /4 BD 4 − + 3/4 n H 2 O → {(M 0 ‐NP)‐D n } n − + n /4 H 3 BO 3 + 3/4 n H + (It should be noted that the value of n is unknown. However, it clearly increases with the increase in [BD 4 − ], and its maximal value probably depends on the nature of M and the radius of the NPs. The value of m is also unknown and depends on the pH and the nature of M).…”