**3. Materials and Methods**

## *3.1. Catalyst Preparation*

All catalysts were prepared using the citrate method [14]. Nitrate salts were used as starting materials: La(NO3)3·6H2O (Alfa Aesar, 99.9%, Kandel, Germany), Mn(NO3)2·4H2O (Alfa Aesar, 99.98%, Kandel, Germany), Co(NO3)3·6H2O (Acros Organics, 99%, Geel, Belgium) and Ni(NO3)2·6H2O (Acros Organics, 99%, Geel, Belgium). An appropriate amount of nitrate salts of the desired A and B sites were dissolved in deionized water with 10 wt.% excess citric acid (Sigma Aldrich, 99.5%, Munich, Germany). The solution was stirred for 1 h at room temperature and further stirred in an oil bath at 80 ◦C until a viscous gel was obtained. The gel was dried in static air overnight at 90 ◦C and then heated to 150 ◦C for 1 h. The black, spongy material was crushed and calcined at 700 ◦C for 5 h in static air.
