Role of B5-Type Sites in Ru Catalysts used for the NH3 Decomposition Reaction

Abstract

A series of activated carbon supported Ru catalysts have been reduced at different temperatures under hydrogen flow, and in some cases under ammonia flow, in order to modify the morphology and the particle size of the metallic active sites. CO chemisorption and transmission electron microscopy have been applied to follow the variations of these particles. The samples have been tested in the ammonia decomposition reaction, where systematic differences in catalytic activities as consequence of the support modification as well as due to the changes in the Ru particle sizes have been detected. Furthermore when potassium is added as catalyst promoter the sintering of Ru particles is significantly diminished and thus the changes in catalytic activities are inhibited. The electronic states of the Ru particles have been evaluated by determination of the chemisorption heats of the CO probe molecule. A part of other promoter or support effects it seems to exit a critical mean size for Ru particles on where maximum of catalytic activity is achieved. This behavior can be rationalized by the presence of surface highly active B5 sites, which consist of an arrangement of three Ru atoms in one layer and two further Ru atoms in an internal layer. These especial surface sites are expected to be in a higher proportion over Ru crystallites of those critical sizes, namely for Ru diameter sizes in the rage of 3-5 nm.

H2 Production via Ammonia Decomposition Using Non-Noble Metal Catalysts: A Review

“…Interestingly, this particle size falls into the range where the presence of B5 sites, a particular configuration of 5 atoms, is maximised as speculated by several authors [66,69] although without experimental verification. It is important to mention here that this specific B5 sites are well known to be related to the high activity of ruthenium nanoparticles with sizes between 3-5 nm [71].…”

Catalytic Ammonia Decomposition over High-Performance Ru/Graphene Nanocomposites for Efficient COx-Free Hydrogen Production

“…Compared with the other recently reported metal particle/graphene composites, the Ru/graphene composites prepared in the present work also showed much smaller and more uniform metallic particles [7,8], indicating the formation of high-quality Ru/graphene composites. Due to the structure-sensitive feature of ammonia decomposition on Ru [31,32], the Ru/graphene prepared from the CS system simultaneously allowed good control of the Ru particle size and shape on graphene nanosheets even at extremely high levels of Ru loading (>50 wt %), which was favorable for highly enhanced catalytic activity for ammonia decomposition. Carbon nanotubes (CNTs) were thought to provide very efficient support for ammonia decomposition [35][36][37][38] because of their high surface area and excellent electronic conductive properties.…”

“…On the other hand, CS-40, CS-60, and CS-80 Ru/graphene nanocomposites showed almost the same catalytic performance, although the variation in Ru loading in the nanocomposites occurred over a wide range, the Ru particle size and shape were the same. However, with comparable, or even much lower, levels of Ru loading, the Ru/graphene nanocomposites prepared in a CS system showed much better catalytic performance than those prepared in a SS system, which was attributed mainly to the improved Ru dispersions as well as to the optimal Ru microstructures, since the ammonia decomposition on Ru is structure-sensitive [31,32]. Figure 7 shows the catalytic performance of CS-60 Ru/graphene at 450 °C under different GHSVs.…”

“…Although research effort focused on the catalysis of ammonia decomposition has increased rapidly in recent years [21][22][23][24][25][26][27][28][29][30], and the catalytic activity of current catalysts still requires significant improvement because the current level of ammonia conversion remains much lower than the equilibrium, particularly at reaction temperatures lower than 400 • C. Research progress has identified that Ru is currently the most active component for ammonia decomposition. The control of Ru particles with optimal size and morphology is very important in ammonia decomposition since ammonia decomposition with Ru is structure-sensitive due to the variation of highly active B 5 -type sites [31,32]. These sites arrange three Ru atoms in one layer with two more in the layer directly above, which amounts to a monoatomic step on a Ru (0001) terrace for different structures [33,34].…”

Catalytic decomposition of ammonia over silicon-carbide nanotube: a DFT study

“…Several research groups have studied NH 3 adsorption and decomposition reaction on different metal surfaces [16][17][18][19]. Most of the catalysts used for ammonia decomposition are based on noble metals, especially Ru [20][21][22]. Various supports have been used to increase the dispersion and surface area of active components [16,[23][24][25].…”