Elucidation of nitrate reduction mechanisms on a Pd/In bimetallic catalyst using isotope labeled nitrogen species

Zhang, Rui

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https://hdl.handle.net/2142/24421 Copy

Description

Title

Elucidation of nitrate reduction mechanisms on a Pd/In bimetallic catalyst using isotope labeled nitrogen species

Author(s)

Zhang, Rui

Issue Date

2011-05-25T14:40:23Z

Director of Research (if dissertation) or Advisor (if thesis)

• Strathmann, Timothy J.
• Werth, Charles J.

Department of Study

Civil & Environmental Eng

Discipline

Environ Engr in Civil Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• catalytic hydrogenation
• nitrate
• reduction
• selectivity
• intermediates

Abstract

Catalytic hydrogenation over Pd-based catalysts has emerged as an effective treatment approach for nitrate (NO3-) removal, but its full-scale application for direct treatment of drinking water or ion exchange regenerant brines requires improved selectivity for the end-product dinitrogen (N2) over toxic ammonia species (NH4+, NH3). A key to improving N2 versus NH4+ production is to elucidate nitrate reduction pathways and identify the key intermediate(s) that determine selectivity. To address this challenge, aqueous reduction experiments with an Al2O3-supported Pd/In bimetallic catalyst were conducted using isotope-labeled nitrite (15NO2-), the first reduction intermediate of NO3-, alone and in combination with unlabeled proposed reduction intermediates (N2O, NO), and using N2O and NO alone, each as a starting reactant. Use of 15N-labeled species eliminated interference from ambient 14N2 when assessing mass balances and product distributions. Simultaneous catalytic reduction of 15NO2- and 14N2O showed no isotope mixing in the final N2 product, demonstrating that N2O does not react with other NO2- reduction intermediates. N2O reduction alone also yielded only N2, verifying that N2O reduction occurs after the reaction step controlling final N2/NH4+ product distribution. In contrast, simultaneous catalytic reduction of 15NO2- and 14NO yielded mixed-labeled N2 (mass 29), and 15NO reduction alone yielded both N2 and NH4+, indicating that NO is a key intermediate involved in determining final product selectivity. N2/NH4+ product selectivity was also evaluated as a function of varying initial 15NO concentration, and results show that selectivity for N2 increases with initial NO concentration to a point, above which product selectivity remains unchanged. This trend is attributed to the increasing importance of N-N pairing reactions leading to N2O formation as the concentration of catalyst-adsorbed NO (NOads) increases to a point of saturating available adsorption sites, above which no further increases in N2 selectivity occur. These results are important because they yield mechanistic insights into the NO3- reduction pathway and information on how catalytic reduction processes can be optimized to maximize N2 production over NH4+.

Graduation Semester

2011-05

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http://hdl.handle.net/2142/24421

Copyright and License Information

Copyright 2011 Rui Zhang

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