An official website of the United States government
TiO 2 supported catalysts have been widely studied for the selective catalytic reduction (SCR) of NO x ; however, comprehensive understanding of synergistic interactions in multi-component SCR catalysts is still lacking. Herein, transition metal elements (V, Cr, Mn, Fe, Co, Ni, Cu, La, and Ce) were loaded onto TiO 2 nanoarrays via ion-exchange using protonated titanate precursors. Amongst these catalysts, Mn-doped catalysts outperform the others with satisfactory NO conversion and N 2 selectivity. Cu co-doping into the Mn-based catalysts promotes their low-temperature activity by improving reducibility, enhancing surface Mn 4+ species and chemisorbed labile oxygen, and elevating the adsorption capacity of NH 3 and NO x species. While Ce co-doping with Mn prohibits the surface adsorption and formation of NH 3 and NO x derived species, it boosts the N 2 selectivity at high temperatures. By combining Cu and Ce as doping elements in the Mn-based catalysts, both the low-temperature activity and the high-temperature N 2 selectivity are enhanced, and the Langmuir–Hinshelwood reaction mechanism was proved to dominate in the trimetallic Cu–Ce–5Mn/TiO 2 catalysts due to the low energy barrier.
more »
« less
Membrane-modified electrocatalysts for nitrate reduction to ammonia with high faradaic efficiency
In light of the enormous energy footprint of the Haber–Bosch process (1–2% of global energy consumption), alternative green routes of generating ammonia (NH 3 ) are needed. The electrochemical reduction of NO 3 − from waste streams is a promising method to produce NH 3 using renewably-sourced electricity. However, catalyst selectivity is a grand challenge that hinders NO 3 − to NH 3 conversion technologies. In this manuscript, we fabricate Nafion-modified metal catalysts for NO 3 − reduction. Although Nafion composites are commonly used to facilitate proton transfer, this work investigates electrodes covered by Nafion overlayers, which possess unique reactivity. We find that Cu versions of these catalysts reduce NO 3 − to NH 3 with a faradaic efficiency of up to (91 ± 2)%, making them among the most selective catalysts reported. Voltammetry studies, surface-enhanced Raman spectroscopy, and density functional theory calculations indicate that the Nafion overlayer activates the N–O bond of a key Cu–NO intermediate, thus facilitating NH 3 production. Lastly, we demonstrate that these catalysts are effective at denitrifying polluted groundwater samples in the field.
more »
« less
- PAR ID:
- 10400604
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 10
- Issue:
- 42
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 22428 to 22436
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Herein, aqueous nitrate (NO3−) reduction is used to explore composition‐selectivity relationships of randomly alloyed ruthenium‐palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO3−reduction proceeds through nitrite (NO2−) and then nitric oxide (NO), before diverging to form either dinitrogen (N2) or ammonium (NH4+) as final products, with N2preferred in potable water treatment but NH4+preferred for nitrogen recovery. It is shown that the NO3−and NO starting feedstocks favor NH4+formation using Ru‐rich catalysts, while Pd‐rich catalysts favor N2formation. Conversely, a NO2−starting feedstock favors NH4+at ≈50 atomic‐% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO3−and NO feedstocks, the thermodynamics of the competing pathways for N–H and N–N formation lead to preferential NH4+ or N2production, respectively, while Ru‐rich surfaces are susceptible to poisoning by NO2−feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N2production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.more » « less
-
Electrochemical reduction of nitric oxide (NOER) is a promising technology for the removal of harmful N-containing species in groundwater under mild conditions. In this work, by means of density functional theory computations, we systematically investigated the potential of utilizing experimentally feasible transition metal–N 4 /graphenes as NOER catalysts. Our results revealed that NO molecules can be moderately activated on a Co–N 4 moiety embedded into graphene, and the subsequent NOER steps can proceed to form either NH 3 at low coverages or N 2 O at higher coverages. Especially, the computed onset potential of NOER on Co–N 4 /graphene ( ca. −0.12 V) is comparable to (or even better than) those on well-established Pt-based catalysts. Thus, Co–N 4 /graphene is a promising single-atom-catalyst with high efficiency for NO electrochemical reduction, which opens a new avenue for NO reduction for environmental remediation.more » « less
-
Abstract Cu‐containing metalloenzymes are known to catalyze oxygen activation through cooperative catalysis. In the current work, we report the design of synthetic polymer Cu catalysts using pyrene‐labelled poly(2‐hydroxy‐3‐dipicolylamino) propyl methacrylate (Py‐PGMADPA) to coordinate multiple Cu sites along polymer chains. The catalysts feature a pyrene end group that can form supramolecular π‐π stacking with conductive carbon to allow efficient immobilization of catalysts to the graphite electrode. Cu‐containing Py‐PGMADPA was examined for electrocatalytic oxygen reduction. The hybrid catalyst showed an onset potential of 0.5 V (vs. RHE) at pH 7 and 0.79 V at pH 13. The kinetic study indicated that the catalyst had a 2e−reduction of oxygen mainly mediated by Cu+centers. We demonstrated the importance of cooperative catalysis among Cu sites which did not exist for other transition metal ions, like Mn2+, Fe2+, Co2+, and Ni2+. The confinement of polymer chains promotes the activity and stabilizes Cu catalysts even at an extremely low Cu loading. The rational design of bioinspired polymer catalysts offers an alternative way to prepare synthetic mimics of metalloenzymes.more » « less
-
Thin films of Ni 3 Al and Ni 3 Ga on carbon solid supports have been shown to generate multi-carbon products in electrochemical CO 2 reduction, an activity profile that, until recently, was ascribed exclusively to Cu-based catalysts. This catalytic behavior has introduced questions regarding the role of each metal, as well as other system components, during CO 2 reduction. Here, the significance of electrode structure and solid support choice in determining higher- versus lower-order reduction products is explored, and the commonly invoked Fischer–Tropsch-type mechanism of CO 2 reduction to multi-carbon products is indirectly probed. Electrochemical studies of both intermetallic and non-mixed Ni–Group 13 catalyst films suggest that intermetallic character is required to achieve C2 and C3 products irrespective of carbon support choice, negating the possibility of separate metal sites performing distinct yet complementary roles in CO 2 reduction. Furthermore, Ni 3 Al and Ni 3 Ga were shown to be incapable of generating higher-order reduction products in D 2 O, suggesting a departure from accepted mechanisms for CO 2 reduction on Cu. Additional routes to multi-carbon products may therefore be accessible when developing intermetallic catalysts for CO 2 electroreduction.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2TA06938E
