Nitrite Reduction to Nitrous Oxide and Ammonia by TiO₂ Electrons in a Colloid Solution via Consecutive One-Electron Transfer Reactions

Abstract

The mechanism of nitrite reduction by excess electrons on TiO2 nanoparticles (eTiO2(-)) was studied under anaerobic conditions. TiO2 was loaded with up to 75 electrons per particle, induced by γ-irradiation of acidic TiO2 colloid solutions containing 2-propanol. Time-resolved kinetics and material analysis were performed, mostly at 1.66 g L(-1) TiO2. At relatively low nitrite concentrations (R = [eTiO2(-)]o/[nitrite]o > 1.5), eTiO2(-) decays via two consecutive processes; at higher concentrations, only one decay step is observed. The stoichiometric ratio Δ[eTiO2(-)]/[nitrite]o of the faster process is about 2. This process involves the one-electron reduction of nitrite, forming the nitrite radical (k1 = (2.0 ± 0.2) × 10(6) M(-1) s(-1)), which further reacts with eTiO2(-) (k2) in competition with its dehydration to nitric oxide (NO) (k3). The ratios k2/k3 = (3.0 ± 0.5) × 10(3) M(-1) and k2 > 1 × 10(6) M(-1) s(-1) were derived from kinetic simulations and product analysis. The major product of this process is NO. The slower stage of the kinetics involves the reduction of NO by eTiO2(-), and the detailed mechanism of this process has been discussed in our earlier publication. The results reported in this study suggest that several intermediates, including NO and NH2OH, are adsorbed on the titanium nanoparticles and give rise to inverse dependency of the respective reaction rates on the TiO2 concentration. It is demonstrated that the reduction of nitrite by eTiO2(-) yields mainly N2O and NH3 via consecutive one-electron transfer reactions.

A Ti(III) reduction method for one‐step conversion of seawater and freshwater nitrate into N2O for stable isotopic analysis of 15N/14N, 18O/16O and 17O/16O

“…The chemistry of the Ti(III) reduction of NO 3 − and NO 2 − to N 2 O has been previously investigated and the major intermediates and the kinetics of each reductive step are reasonably well known . It is also known that the end products include N 2 O, N 2 and NH 4 + , verifying our observations that N 2 O is a major product at low pH.…”

“…NO 3 − reduction generally proceeds as NO 3 − ⇒ NO 2 − ⇒ NO with N 2 O, NH 4 + or N 2 as end products with other possible short‐lived intermediates . Ti(III) chloride was previously used to convert NO 3 − into NO (sparged out of solution before further reduction) or NH 4 + for colorimetric or chemiluminescent analysis, but was never applied to stable isotope N and O assays.…”

Restoring the Nitrogen Cycle by Electrochemical Reduction of Nitrate: Progress and Prospects

“…The direct charge injection to NO 2 − leads to the formation of dianion radical NO 2 2− by Reaction (16). [52,53] Similar to the…”

“…The direct charge injection to NO 2 − leads to the formation of dianion radical NO 2 2− by Reaction (16). [ 52,53 ] Similar to the unstable NO 3 2− , the dianion radical NO 2 2− will quickly hydrolyze and produce NO (ad) (Reaction (17)) [ 54,55 ] …”

Unusual Reduction of Graphene Oxide by Titanium Dioxide Electrons Produced by Ionizing Radiation: Reaction Products and Mechanism

“…Under the conditions of the present work, e TiO 2 – concentration is stable in the time range of hours. Reactions of e TiO 2 – produced by ionizing radiation with a number of oxidants are well known. − In general, free oxidant molecules in the solution react with one electron per TiO 2 nanoparticle, according to a simple second-order rate law. In contrast, in the presence of excess reactants adsorbed onto the TiO 2 , e TiO 2 – has been reported to decay by an apparent multiexponential process, extending over long time ranges …”