In the last years exsolved nanoparticles from perovskite oxide matrices demonstrated to be high performance catalysts due to their thermal stability, sintering resistance and regenerability. Particular focus has been placed on the development of FeNi bimetallic exsolved systems due to their higher catalytic activity and product selectivity [1,2]. One of the reasons for their higher performance was attributed to the formation a defective crystalline FeO_x shell around a Fe_xNi_y core [1]. Recently, we demonstrated that the exsolved Fe:Ni ratio can be controlled by the extent of A-site deficiency in the parental La_0.4Sr_0.6-xTi_0.6Fe_0.35Ni_0.05O_3±δ (0 ≤ x ≤ 0.2) matrix  thus providing catalytic systems with tunable selectivity with respect to the CO₂-mediated partial oxidation of ethane [2].

In the present report, we show that the exsolved Fe:Ni ratio and so the catalytic selectivity in La_0.4Sr_0.4Ti_0.6Fe_0.35Ni_0.05O_3±δ can also be controlled by the exsolution temperature and how these processes can be fully switched. Low reduction temperature (T = 400 °C) favors the formation of Ni-rich nanoparticles (NiFe), whereas at high temperature (T = 850 °C) the exsolution of Fe is predominant (FeNi). The combination of spectroscopic analyses (Mössbauer, XANES) with XRD and in situ HR-TEM provided full explanation of the temperature-dependent exsolution process. For the CO₂-mediated partial oxidation of ethane, NiFe nanoparticles were selective for the dry reforming pathway by 95%, because of their high metallic character. On the contrary, the FeNi system showed very high selectivity (over 70%) for the oxidative dehydrogenation, i.e. ethylene formation, due to the formation of a defective crystalline wustite (FeO_1+x) shell as demonstrated by TEM and Mössbauer spectroscopy analyses. Long-term catalysis tests revealed the high performance stability of bimetallic exsolved nanoparticle, due to the poor formation of carbon depositions.

Controlling the nanoparticle composition using a process parameter like temperature presented us a unique opportunity: to switch between the NiFe and FeNi nanoparticles in a single matrix through the oxidation step (Figure 1). We demonstrated that this is fully achievable and that both nanoparticle properties and their catalytic selectivity are interchangeable in a single perovskite compound. This opens up to a completely new application landscape for exsolved materials in catalysis and further show the manifold opportunities that the exsolution process offers for designing advanced energy materials.