Laszlo Kotai

Abstract:

Nanosized mixed-metal-oxide catalysts are used in various industrially relevant processes like Fischer-Tropsch synthesis or NOx removal from gases. Transition metal ions, coordinated by reducible ligands (NH₃, pyridine or urea) that make salts with oxometalate anions (MnO₄^-, CrO₄^2-, Cr₂O₇^2-), are used to prepare nano-sized mixed spinel-type metal-oxides with pre-selected composition, properties, and structure. The applicability of a mixed oxide catalyst is determined by the distribution of the two metals among valence states, and crystallographic positions (in spinels T-4 and OC-6). The methods currently used for the synthesis of mixed metal-oxides do not allow the control of these properties. This is because the methods used for synthesis of mixed metal-oxides include processes that take place at high temperatures, where the mobility of atoms leads to the formation of a thermodynamically stable structure. This stable structure is characterized by its unique distribution of metal atoms among positions and valence states. The unique feature of the catalyst synthesis method developed by us [1-4] is based on the thermal decomposition of tetraoxometalates of transition metal ions, coordinated by reducible ligands at relatively low temperatures (100-200 ^oC). The thermal decomposition of tetraoxometalates then releases gas-phase products formed from the ligands. This is a solid-phase reaction which forms mixed oxides with metastable structures because at low temperatures, the metal ions remain in the crystallographic positions of the precursor salt. For example, hexaaquairon(III) permanganate results in (Fe,Mn)O type and (Fe,Mn)₃O₄ type mixed oxides, depending on the atmosphere and temperature of decomposition. Furthermore, the original spinel structure can be oxidized into defect-spinel structures and finally to (Fe,Mn)₂O₃ type oxides.

[Fe(urea)₆(MnO₄)₃] --> (Fe,Mn)₃O₄ --> (Fe,Mn)₃O_4.5 --> (Fe,Mn)₂O₃
--> (Fe,Mn)O

Due to a large number of crystal defects, nanocrystallites are formed which is favorable for catalysis. Our method enables one to set the ratio of the metal ions arbitrarily by starting from an isomorphous solid solution in which we partially replaced the metal ion by another one, and/or the anion by another tetraoxometalate or by an "innocent" anion (which forms gaseous products due to the lack of metal atoms, e.g., MnO₄^- by ClO₄^-).
[Fe(urea)₆](MnO₄)₃ -- (Fe,Mn)-oxides with Fe:Mn=1:3 overall ratio
[Fe(urea)₆(MnO₄)₂(ClO₄)₂ -- (Fe,Mn)-oxides with Fe:Mn=1:2 overall ratio
[Fe(Urea)₆](MnO₄)(ClO₄)₂ -- (Fe,Mn)-oxides with Fe:Mn=1:1 overall ratio
[(Fe_0.5Cr_0.5)(urea)₆](MnO₄)_0.5(ClO₄)_2.5 -- (Fe,Cr,Mn) oxides with Fe:Cr:Mn=1:1:1 overall ratio

[1] Sajó I.E., Bakos P. Szilágyi I.M., Lendvay Gy., Magyari J., Mohai M., Szegedi A., Farkas A., Jánosity A., Klébert Sz., Kótai L., Inorg. Chem., 2018, 57(21), 13679-13692.
[2] Kotai, L., Banerji, K. K., Sajo, I., Kristof, J., Sreedhar, B., Holly, S., Keresztury, G., Rockenbauer, A.
Helv. Chim. Acta, 2002, 85(8) 2316-2327
[3] Kotai L., Fodor J., Jakab E., Sajo I., Szabo P., Lonyi F., Valyon J., Gacs, I., Argay G., Banerji K., Trans. Metal Chem., 2006, 31(1) 30-34
[4] Sajó I. E., Kotai L., Keresztury G., Gacs I., Pokol Gy., Kristóf J., Soptrayanov B., Petrusevski V. M., Timpu D., Sharma P. K., Helv. Chim. Acta 2008, 91, 1646