Examining selenium reduction mechanisms on Ni-Fe bimetallic nanoparticles using non-stationary kinetic modeling

Abstract Bimetallic nanoparticles (BNPs) are being increasingly used for the remediation of organic and inorganic contaminants. Most heterogeneous reactions on BNPs occur primarily due to: (1) adsorption of contaminant species on the active surface sites; (2) chemical reduction on the catalyst surface; and (3) sorption of the products from the catalyst surface. In this work, we identify the rate determining step for selenium, particularly Se (VI) reduction on Ni-Fe BNPs using kinetic data modeling. A non-stationary kinetic model, without any preliminary assumptions of a rate limiting step was developed. The kinetic data on Se (VI) reduction was fitted to the non-stationary to determine the rate constants. The non-stationary model suggested adsorption limiting step for Se (VI) reduction on Ni-Fe BNPs. With increase in BNP loading (i.e., increase in number of active surface sites), reaction rates also increased linearly. Deviation from linearity was observed due to the deactivation of the BNPs. The loss in catalytic activity can be attributed to the formation of metal oxide on the catalyst particle, resulting in loss of active sites. Even though selenium reduction may no longer occur on the newly formed metal oxides, selenium removal via adsorption can occur. Prediction of the occupied site concentration on the Ni-Fe BNPs suggested possible in-situ regeneration of the active site due to galvanic coupling of the bimetals.