ChemInform Abstract: A Steady‐State Isotopic Transient‐Kinetic Analysis of Iron‐Catalyzed Ammonia Synthesis

Abstract

ChemInform Abstract conducted over a commercial Fe catalyst yields the first direct measurements of coverages in reactive intermediates of the working catalyst surface and assessments of the reaction mechanism and surface heterogeneity at steady-state reaction conditions. The most abundant reaction intermediate in the reaction, studied between 623 and 773 K at a total pressure of 204 or 513 kPa, and for a H2/N2 ratio of 3 is suggested to be *N. The line shapes of the transient data of this study lead to the conclusion that a single rate-limiting step exists in the synthesis pathway involving nitrogen and that hydrogen facilitates this step which involves N2 dissociation. The working catalyst surface is found to be kinetically nonuniform at the steady-state reaction conditions employed.

Kinetic and mechanistic analysis of NH3decomposition on Ru(0001), Ru(111) and Ir(111) surfaces

“…In order to develop more efficient catalysts, many experimental studies of ammonia decomposition on Ru and Ir catalysts focused on the relationship between the composition and atomicity of catalysts and product yields. Both temporal analysis of products (TAP) 39,40 and steady-state isotopic transient kinetic analysis (SSITKA) [41][42][43] can be applied to study the characteristics of the active sites and provide information on the adsorptions and reactions. García et al carried out multi-pulse TAP experiments to understand the main mechanistic features involved in the catalytic decomposition of NH 3 over carbonsupported Ru and Ir catalysts.…”

“…John and co-workers found that NH x species are the primary surface intermediates in the temperature range from 623 K to 673 K (204 kPa) and adsorbate N is the most abundant intermediate from 623 K to 773 K using SSITKA. 42 To date, a systematic and detailed comparison of the exact mechanism and microkinetic model for NH 3 decomposition on Ru and Ir supported nanoparticles is scarce in the literature, especially including the model describing the Ru fcc surface, which is observed in the Ru nanoparticle size range of 2.0-5.5 nm. 44 Due to the complexity and difficulty in observing the adsorbed reaction intermediates, many aspects concerning the reaction processes at the atomic level remain unclear.…”

Isotopic transient analysis of ammonia synthesis over Ru/MgO catalysts promoted by cesium, barium, or lanthanum

“…Ammonia synthesis catalyzed by both Fe [46] and Ru [10,28,47] has been studied previously with SSITKA. This technique is very powerful because it allows for the characterization of a catalyst surface under working conditions.…”

Transient Catalytic Studies

“…This has been further developed by Goodwin and coworkers into the state-of-the-art steady-state isotopic transient kinetic analysis (SSITKA) method [168][169][170]. In numerous studies, SSITKA was applied to determine in situ the microkinetics and to detect the reaction intermediates in heterogeneously catalyzed reactions (cf.…”