Nanometric Platinum Overlayer to Catalyze NH₃ Oxidation with High Turnover Frequency

Abstract

A pulsed cathodic arc-plasma deposition of a few-nanometer-thick Pt(111) overlayer on a 50 μm-thick Fe−Cr−Al metal foil produced a thin-film catalyst; this catalyst exhibited high activity for low-temperature NH 3 oxidation and was superior to that of a conventional powder catalyst (Pt/Al 2 O 3 ). A metal honeycomb that was fabricated using a metal foil catalyst successfully demonstrated a light-off performance at a practical gas hourly space velocity of 1.2 × 10 5 h −1 . Despite its nonporosity and small surface area, the Pt overlayer is a promising alternative to Pt/Al 2 O 3 for more efficient ammonia-slip catalysts that use less Pt loading; this is because the turnover frequency for the NH 3 oxidation at 200 °C is more than 180-fold greater than that achieved with Pt/Al 2 O 3 consisting of Pt nanoparticles. Although the nanometric Pt overlayer structure was thermally unstable at reaction temperatures of ≥600 °C, inserting a 250 nm-thick Zr buffer layer between the Pt overlayer and the metal foil substrate significantly mitigated the thermal deterioration. The undesired byproducts of NH 3 oxidation, NO and NO 2 , can be efficiently converted to N 2 by a selective catalytic reduction over a V 2 O 5 −WO 3 /TiO 2 catalyst in a tandem reactor system.

The Effects of Platinum Dispersion and Pt State on Catalytic Properties of Pt/Al2O3 in NH3 Oxidation

“…To achieve maximal activity and reduce contribution from the by‐products (N 2 O, NO x ), it is necessary to establish factors governing the operation of Pt/Al 2 O 3 during ammonia oxidation. At the same time it is important to reach the highest degree of Pt usage thus reducing the cost of aftertreatment systems [9] …”

“…At the same time it is important to reach the highest degree of Pt usage thus reducing the cost of aftertreatment systems. [9] Activity and selectivity of the Pt/Al 2 O 3 catalyst are affected by Pt dispersion because of the difference in the physicochemical properties of small and large Pt particles. For example, Pt metallic particles smaller than 3 nm exhibit semiconductor and dielectric properties due to the modification of electronic structure in comparison with bulk platinum.…”

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

“…By contrast, we recently reported another possible approach to designing an ASC, where this method uses a nonporous metal thin film prepared via pulsed arc plasma (AP) deposition. 30 We found that the as-prepared nanometer-thick Pt overlayer structure formed on the metal foil substrate exhibited high NH 3 oxidation activity comparable or superior to that of Pt/Al 2 O 3 . Furthermore, we demonstrated a light-off performance at ≥200 °C using a monolithic honeycomb comprising the metal foils at a gas hourly space velocity (GHSV) of 1.2 × 10 5 h –1 .…”

“…The NH 3 oxidation activity of the as-prepared Ir/SUS was compared with a reference Pt/SUS, which was studied previously. 30 Figure 2 shows the temperature dependencies of the concentrations of NH 3 , N 2 , N 2 O, NO, and NO 2 in the effluent gas from these catalyst beds. Both catalysts initiated NH 3 conversion at a low temperature of ∼200 °C, which rapidly increased along with an increase in temperature up to 300 °C.…”

“…Because of its excellent mechanical behavior and resistance to oxidation and corrosion at high temperatures, the metal foil has been widely used as a substrate for an automotive catalytic converter. 33 Catalytic NH 3 oxidation was then conducted under a simulated diesel exhaust condition in the presence of 8% O 2 and 10% H 2 O using the as-prepared Ir overlayer (Ir/SUS); the performance of which was compared with that of the Pt overlayer (Pt/SUS) described in our previous study 30 and focusing on differences in product selectivity. We discussed the structure–activity relationship in terms of TOF, oxidation stability, and thermal stability under simulated exhaust gas conditions.…”