Agglomeration Suppression of a Fe-Supported Catalyst and its Utilization for Low-Temperature Ammonia Synthesis in an Electric Field

Abstract

Fe-supported heterogeneous catalysts are used for various reactions, including ammonia synthesis, Fischer–Tropsch synthesis, and exhaust gas cleaning. For the practical use of Fe-supported catalysts, suppression of Fe particle agglomeration is the most important issue to be resolved. As described herein, we found that Al doping in an oxide support suppresses agglomeration of the supported Fe particle. Experimental and computational studies revealed two tradeoff Al doping effects: the Fe particle size decreased and remained without agglomeration by virtue of the anchoring effect of doped Al. Also, some Fe atoms anchored by Al cannot function as an active site because of bonding with oxygen atoms. Using an appropriate amount of Al doping is effective for increasing the number of active Fe sites and catalytic activity. This optimized catalyst showed high practical activity and stability for low-temperature ammonia synthesis in an electric field. The optimized catalyst of 12.5 wt % Fe/Ce0.4Al0.1Zr0.5O2‑δ showed the highest ammonia synthesis rate (2.3 mmol g–1 h–1) achieved to date under mild conditions (464 K, 0.9 MPa) in an electric field among the Fe catalysts reported.

Theoretical prediction by DFT and experimental observation of heterocation-doping effects on hydrogen adsorption and migration over the CeO2(111) surface

“…5 wt% of Fe was loaded over synthesized CeO 2 -based materials using a liquid-phase reduction method as reported elsewhere. 45 First, Fe(NO 3 ) 3 Á9H 2 O (Kanto Chemical Co. Inc.) was poured into distilled water. The support was dispersed in the obtained Fe aqueous solution by stirring for 1 h. Subsequently, the solution was heated at 393 K for 1 h under Ar flow.…”

Enhanced activity of catalysts on substrates with surface protonic current in an electrical field – a review

“…For this purpose, fluorite binary oxides such as CeO 2 34 and perovskites such as SrZrO 3 41, 42 are good candidates. The choice of catalyst support should be considered based on its physical properties 45 : conductors such as Fe 2 O 3 can sustain an electric current but do not promote catalytic activity, whereas insulators such as Al 2 O 3 cannot sustain sufficient surface protonic current, and the electric field results only in discharge. Only semiconducting type supports such as CeO 2 and SrZrO 3 appear to satisfy the required physical properties: Surface protonic conductivity is a key factor, but also the presence of redox-active cations and possibly electronic transport seem important.…”

Recent progress in use and observation of surface hydrogen migration over metal oxides

“…Hydrogen migration plays an important role in various catalyses such as hydrogenation-dehydrogenation of hydrocarbon, 1-5 CO 2 reduction [6][7][8][9] and NH 3 synthesis. [10][11][12][13][14][15][16][17] However, the utilisation of hydrogen migration is not limited to catalysis. It is also fundamentally important for catalyst preparation, 18,19 hydrogen storage, 20,21 fuel cells and sensors.…”

“…[39][40][41] This active migration is called surface protonics. It has been revealed that surface protonics plays a key role in activating the cleavage of strong bonds, as in the NRN bond in N 2 , [13][14][15][16][17]42,43 and the C-H bond in hydrocarbons, as in CH 4 [42][43][44][45][46] and methylcyclohexane (MCH). 4,5 Those activations make it possible to catalyze hydrogen production and formation of the hydrogen carrier, even at low temperatures (o500 K).…”

“…To date, this method has been applied to various catalytic reactions, revealing that surface ion conduction participates in low-temperature catalysis. 39,40,84 Herein, we have described reaction mechanisms that are relevant to surface protonics under a H 2 atmosphere: NH 3 synthesis proceeds via N 2 H [11][12][13][14][15][16][17] and dehydrogenation of methylcyclohexane (MCH) facilitated by H + collision. Both compounds are promising hydrogen carrier candidates.…”