Dataset Information

Nanometric Iridium Overlayer Catalysts for High-Turnover NH₃ Oxidation with Suppressed N₂O Formation.

ABSTRACT: In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 ?m thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH₃-O₂ reactions more environmentally benign due to a much lower selectivity for undesirable N₂O (<5%) than that of a Pt overlayer (?70%) at 225 °C. Despite its small surface area, Ir/SUS exhibited promising activity as an ammonia slip catalyst according to a turnover frequency (TOF) >70-fold greater than that observed with conventional Ir nanoparticle catalysts supported on ?-Al₂O₃. We found that the high-TOF NH₃ oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O₂ present in simulated diesel exhaust. Additionally, we found that the Ir overlayer structure was thermally unstable at reaction temperatures ?400 °C and at which point the Ir surface coverage dropped significantly; however, thermal deterioration was substantially mitigated by inserting a 250 nm thick Zr buffer layer between the Ir overlayer and the SUS foil substrate (Ir/Zr/SUS). Although N₂O formation was suppressed by NH₃ oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO₂) were readily converted to N₂ by coupling with a V₂O₅-WO₃/TiO₂ catalyst in a second reactor for selective catalytic reduction by NH₃. These results demonstrated that this tandem reactor configuration converted NH₃ to N₂ with nearly complete selectivity at a range of 200-600 °C in the presence of excess O₂ (8%) and H₂O (10%).

SUBMITTER: Machida M

PROVIDER: S-EPMC7758949 | biostudies-literature | 2020 Dec

REPOSITORIES: biostudies-literature

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Publications

Nanometric Iridium Overlayer Catalysts for High-Turnover NH3 Oxidation with Suppressed N2O Formation.

Machida Masato M Tokudome Yurika Y Maeda Akihide A Koide Tomoyo T Hirakawa Taiki T Sato Tetsuya T Tsushida Masayuki M Yoshida Hiroshi H Ohyama Junya J Fujii Kenji K Ishikawa Naoya N

ACS omega 20201211 50

In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 μm thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH3-O2 reactions more environmentally benign due to a much lower selectivity for undesirable N2O (<5%) than that of a Pt overlayer (∼70%) at 225 °C. Despite its small surface area, Ir/SUS exhibited promising activity as an ammonia slip catalyst according to a turnover frequency (TOF) ...[more]

PMID: 33376920

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Project description:Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO4, turnover number up to ∼10,000). Nanomaterials 1-3 display the highest ever reported turnover frequency values (up to 402 min-1) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 μm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.

Dataset Information

Nanometric Iridium Overlayer Catalysts for High-Turnover NH₃ Oxidation with Suppressed N₂O Formation.

Publications

Nanometric Iridium Overlayer Catalysts for High-Turnover NH<sub>3</sub> Oxidation with Suppressed N<sub>2</sub>O Formation.

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