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 μm thick Fe–Cr–Al metal (SUS) foil. Using this thin-film catalyst made NH 3 –O 2 reactions more environmentally benign due to a much lower selectivity for undesirable N 2 O (<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 γ-Al 2 O 3 . We found that the high-TOF NH 3 oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O 2 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 N 2 O formation was suppressed by NH 3 oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO 2 ) were readily converted to N 2 by coupling with a V 2 O 5 –WO 3 /TiO 2 catalyst in a second reactor for selective catalytic reduction by NH 3 . These results demonstrated that this tandem reactor configuration converted NH 3 to N 2 with nearly complete selectivity at a range of 200–600 °C in the presence of excess O 2 (8%) and H 2 O (10%).

Mixed Catalyst SmMn2O5/Cu-SAPO34 for NH3-Selective Catalytic Oxidation

“…NH 3 oxidation over individual Cu-SAPO34 is rather inactive below 300 °C. The by-product N 2 O is widely known to be composed of ammonium nitrate formed from NH 3 and NO. ,, As the temperature increases, the catalyst gradually shows stronger oxidizing ability and more NO is formed during the reaction. In the case where NH 3 is not completely consumed, the amount of N 2 O also tends to increase with temperature.…”

An Ammonization-Based Transformation of Hexagonal Boron Nitride on Ir(111) from Surface to Near-Surface Regions

“…51,52 The suggested oxidation mechanism is that O atoms actively embedded into the h-BN lattice replace N atoms, generating solid B 2 O 3 structures adsorbed on the surface, and NO gases desorbed due to the difference in their volatility. 53 In the transition from B 2 O 3 to h-BN, H atoms from the NH 3 -dehydrogenated reaction on the Ir(111) surface combine with O atoms to form H 2 O, accompanying incorporation of N atoms into the lattice. Ir(111) surfaces enhance the catalytic activity for the oxygen reduction reaction of h-BN and facilitate NH 3 dehydrogenation, driving the entire structural transition to be dynamically favorable.…”

“…The experimental observations indicate that a structural transformation follows a h-BN–BO x –h-BN path. O 2 molecules are readily dissociated into active atomic oxygen catalyzed by Ir(111) surfaces. , The suggested oxidation mechanism is that O atoms actively embedded into the h-BN lattice replace N atoms, generating solid B 2 O 3 structures adsorbed on the surface, and NO gases desorbed due to the difference in their volatility . In the transition from B 2 O 3 to h-BN, H atoms from the NH 3 -dehydrogenated reaction on the Ir(111) surface combine with O atoms to form H 2 O, accompanying incorporation of N atoms into the lattice.…”

Catalytic Properties of Nanometric Metal Overlayers with Two‐Dimensional Structures

“…A similar structural advantage of the metal overlayer was also observed for Ir catalysts in the same reaction. The Ir overlayer exhibited a more than 70‐fold higher TOF than Ir nanoparticles (Ir/Al 2 O 3 ) [8b] . These results suggest that the advantage of the metal overlayer catalyst exhibiting high TOF depends on the types of metals and catalytic reactions.…”