Active Tetrahedral Iron Sites of γ-Fe₂O₃ Catalyzing NO Reduction by NH₃

Abstract

Maghemite (γ-Fe 2 O 3 ) with a spinel structure, consisting of tetrahedral Fe 3+ (Fe 3+ Td ) and octahedral Fe 3+ (Fe 3+ Oh ) sites, has been intensively investigated as an environmentally benign catalyst for selective catalytic reduction (SCR) of NO x with NH 3 . In most cases, Fe 3+ Oh sites were regarded as catalytically active sites (CASs). Here we identify the CASs in SCR by substituting Fe 3+ Oh or Fe 3+ Td sites of γ-Fe 2 O 3 with catalytically inactive Ti 4+ or Zn 2+ , respectively. The SCR activity of γ-Fe 2 O 3 is preserved after Ti 4+ doping but drastically decreases when the catalyst is doped with Zn 2+ , demonstrating that Fe 3+ Td sites serve as CASs in SCR. Synchrotron X-ray absorption spectra coupled with density functional theory calculations reveal that the transfer of an electron from inactive Fe 2+ to active Fe 3+ in the tetrahedral site is easier than that in the octahedral site, making the tetrahedral iron sites active in SCR.

Simultaneously Enhancing Stability and Activity of Maghemite via Site‐Specific Ti(IV) Doping for NO Emission Control

“…Therefore, the combination of the above structure characterizations and the SCR activity and stability tests demonstrate that the site‐specific Ti 4+ doping significantly improves not only the thermal stability of γ‐Fe 2 O 3 , but also the catalytic performance in SCR. As reported in our recent work, Fe 3+ Td sites serve as catalytically active sites in SCR due to much easier electron transfer from inactive Fe 2+ to active Fe 3+ in the tetrahedral site than that in the octahedral site. In fact, lattice oxygen species also play an important role in SCR, especially in activating NH 3 molecules .…”

“…As shown in the SXRD patterns of the samples calcined at 400 °C instatic air (Figure S2), the spinel structure of Ti‐γ‐Fe 2 O 3 remains, whereas γ‐Fe 2 O 3 has been transformed to α‐Fe 2 O 3 with a more stable hexagonal structure, thus leading to a partial loss of catalytic activity at low reaction temperatures (Figure S3). Besides, our recent work showed that Ti‐γ‐Fe 2 O 3 exhibited slightly better SCR performance than γ‐Fe 2 O 3 at a relatively low gaseous hourly space velocity (GHSV) of 300,000 h −1 . In order to clearly distinguish the differences of the stabilities and activities of Ti‐γ‐Fe 2 O 3 from γ‐Fe 2 O 3 , the catalysts were tested under a more demanding condition, i.e., at a high GHSV of 600,000 h −1 in the presence of H 2 O and SO 2 .…”

“…The presence of the vacancies and the random distribution of Fe 3+ Oh often result in a relatively high entropy with respect to a regular spinel structure, thereby making the crystal structure of γ‐Fe 2 O 3 unstable. Our recent work demonstrated that Fe 3+ Td served as catalytically active sites in SCR because the electron transfer from inactive Fe 2+ to active Fe 3+ in the tetrahedral site is easier than that in the octahedral site . Thus, it is feasible to enhance the stability by occuping the vacancies and substituting the Fe 3+ Oh sites by suitable metal cations ,…”

“…Moreover, Ti 4+ cations are environmentally friendly and show strong SO 2 ‐tolerance. Thus, from a practical viewpoint, Ti 4+ may serve as an excellent doping ion for enhancing the stability and activity of γ‐Fe 2 O 3 in SCR …”

Structure–Activity Relationship of Iron Oxides for NO Reduction in the Presence of C3H6, CO, and O2

“…To elucidate the role of the spinel structure in NiFe 2 O 4 , the structure–activity relationship among iron oxides needs to be understood. The structure‐dependent activity of iron oxide has been studied for other reactions . Although it was concluded that acid–base or redox properties were important factors for the catalytic activity, the impact of the difference in circumstances around iron has not been discussed in depth.…”