SFNM04 as oxygen carrier

68

and at the moment the main focus of the research rely precisely in finding new and alternative materials to reach a higher level of efficiency: both the presented technology for chemical looping and the listed material present some limitations that want to be overcome. The optimal material we are searching for will be able to perform properly at lower temperature, as example, leading to benefits regarding the structural stability of the OC, the sintering and agglomeration phenomena and also the component life of the reactor.

Beside the aspect of temperature, also the properties of the oxygen carrier are taken into account: their ability to create O₂ vacancies and the susceptibility to acquire and release oxygen molecule are very important and the possibility to enhance them thanks to doping.

The cost of the material is also important, especially in the future perspective to adopt them for a bigger industrial scale. At the moment, the state of art for chemical looping applications is ceria because of its high kinetics and redox properties; however, ceria requires operating temperature too high and thus moving the interest through more interesting material, such as perovskites precisely. This thesis work, which comes from a collaboration between the Polytechnic of Turin, Massachusetts Institute of Technology (MIT) and University of Udine, focuses on the study of a new perovskite material, namely 𝑆𝑟 𝐹𝑒𝑁𝑖 _. 𝑀𝑜 _. 𝑂 (SFNM04), in order to analyze its performance and chemical behavior by changing operating parameters.

69

Another fundamental phenomenon which characterizes this new material is exsolution: 𝐹𝑒 and 𝑁𝑖 tend to exsolve from the lattice in a reducing ambient, implying in this way a creation of a 𝑁𝑖 − 𝐹𝑒 alloy on the surface; the main advantage brought from exsolution is that this alloy presents a bigger resistance to coke and sulfide poisoning than just metallic nickel and so the electro-catalytic performance of the perovskite are enhanced. The nanoparticles of nickel can be incorporated by using mainly two different procedures. The infiltration method is the most mature but presents some limitation for the high cost and doesn’t ensure an optimal control on the new created morphology. On the other hand, nanoparticles growth in situ are characterized by a uniform distribution, they are faster and less expensive [155].

The synthesis of the sample is realized in this case using Self-Combustion Synthesis (SCS) with citric acid as complexing agent, followed by a calcination at 1110°C in order to obtain a pure cubic structure. X-ray diffraction (XRD) test was performed to characterized different phases of the perovskite, both for the starting material 𝑆𝑟 𝐹𝑒 _. 𝑀𝑜 _. 𝑂 (used as reference material) and for the new synthesized ones. From the results of the composition spectrum, it is possible to notice a limitation in nickel solubility in the double perovskite lattice in SFNM-05 sample: in this case indeed a 𝑁𝑖𝑂 segregation happens on the surface, meaning that not all the nickel is entered in the structure. Since for SFNM-04 this phenomenon doesn’t show, the solubility limit of 𝑁𝑖 in the molybdate structure is found to be between 0.4 and 0.5 molar compared to the other metallic cations.

Figure 2.28: XRD pattern of a) Sr2Fe1.5Mo0.5O6-δ (SF1.5M0.5) and Sr2FeMoO6 (SF1M1) cubic phases at different calcination temperatures. (●) SFM, cubic; (Ⓧ) SrMoO4, tetragonal. b) SF1.5M0.5

(Sr2Fe1.5Mo0.5O6-δ at 1200 °C), SFNM-04 (Sr2FeNi0.4Mo0.6O6-δ) and SFNM-05 (Sr2FeNi0.5Mo0.5O6-δ) [152].

70

An X-ray photoemission spectroscopy (XPS) and an energy dispersive X-ray (EDX) analysis have been performed too on SFNM-04 and SFNM -05 samples, in order to verify the real chemical composition: the results showed a small surface segregation of Sr in the first one, while a significant amount of nickel on the surface of the second one, due to the overcome of the solubility limit.

Once the structural characterization of the sample has been done, a further functional study has been carried out. This was possible thanks to a temperature programmed reduction analysis (TPR), which helped to investigate the evolution and stability of the material during redox cycles. The chosen procedure for the study followed 5 steps: a first pre- treatment until 500°C (10°C/min) in air, an hour dwelling at this temperature, a TPR analysis up to 900°C using 4.5% of hydrogen (in N₂) with a volumetric flow of 35 ml/min and finally a cooling ramp until room temperature with nitrogen, followed by a final re-oxidation ramp as the beginning. The process has been repeated for four cycles, in order to check the repeatability too. It has to be noticed that the initial pre-treatment in air has been done to remove impurities and other species from the surface, as well to perform the oxidation of the sample after the reduction. The same approach in air will be used also for our test.

Figure 2.29: TPR analysis of SFNM-04 in 4.5% H₂/N₂ [152].

71

From the results we can see a difference between the first cycle and the others: after the first cycle the profile starts to show more stability and constancy and the same for the following ones. The explanation of that was found in the creation of a transition phase more stable in reduction during the first cycle, and it was confirmed thanks to XRD analysis: it is noted the present of a Ruddlesden-Popper (RP) phase, more precisely 𝑆𝑟 𝐹𝑒𝑀𝑜𝑂 _. . This phase is not absolute but is present simultaneously in the structure with the double perovskite phase. Generally, RP phases are indicated as 𝐴 𝐵 𝑋 : indeed, more types of this structure exist and some of them are reported in the following figure (Figure 2.30).

Figure 2.30: RP perovskite structures represented by single or multiple perovskite layers between rock salt layers. From the left: n=1,2,3 [156].

It has to be noted that also formation of 𝐹𝑒𝑁𝑖 is detected in the XRD analysis, as consequence of the occurring of ex solution phenomenon. Increasing the number of cycles, the RP phase shows a decrease in the signal entity from XRD, while the peak signal associated to 𝐹𝑒𝑁𝑖 tends to increase. This behavior clearly indicates that the exsolution

72

process doesn’t show immediately, but it proceeds during the cycles, thus justifying the increment in hydrogen consumption with TPR evolution. This phenomenon is clearly shown in the magnification of Figure 2.31.

Figure 2.31: XRD of SFNM-04 after 4 cycles. On the right: magnification in the range 43° and 45°, showing a comparison of the sample after 2 and 4 reductions [152].

Stability of SFNM-04 in CO₂ environment was investigated too. Indeed, the main concern about the sample was that nickel presence in the lattice could accelerate 𝑆𝑟𝐶𝑂 formation because of the reaction of 𝑆𝑟𝑂 with the CO₂ surface absorption; this brings to the creation of a stabile phase of 𝑆𝑟𝑀𝑜𝑂 , which is an insulant and so compromises the performances in presence of a hydrocarbon fuel. The formation of these two phases was both reported in literature [157] and also found by the University of Udine during XRD and TGA analysis of untreated sample and on one treated with 30% 𝐶 𝐻 at 800°C for two hours: the difference between the two case is evident from Figure 2.32.

73

Figure 2.32: TGA for treated SFNM04 (black curve) and untreated (black curve) [80].

In conclusion, the SFNM-04 double perovskite material has been described in this chapter.

Further characterization will be provided in the following chapters, thanks to experimental tests performed in the laboratory.

74

Chapter 3