## 3 Results and Discussions
### LaFe1-xNix Oxygen Carrier
#### 3.2.3 Steam Enhanced Syngas Production  
Though LaFeNi exhibited a good performance on toluene pyrolysis, the yield of H\({}_{2}\) and C\({}_{1}\) is still very low during the toluene step at 900 \({}^{\circ}\)C. The C\({}_{1}\) yield was much lower than the H\({}_{2}\) yield, which can be attributed to carbon deposition. Therefore, steam was added during the toluene step to enhance syngas yield. Figure 10 shows the gas production during 1 typical cycle of chemical looping conversion of toluene using LaFe\({}_{0.6}\)Ni\({}_{0.4}\) at 900 \({}^{\circ}\)C with and without steam. Toluene conversion using LaFe\({}_{0.6}\)Ni\({}_{0.4}\) at 900 \({}^{\circ}\)C without steam could be referred to Section 3.2.1. Since the Fe and Ni exhibited a catalytic effect on promoting toluene conversion, it would inevitably cause serious carbon deposition on the oxygen carrier. The carbon deposition inevitably covered the catalytic center and reduced toluene conversion during longtime operation. In the chemical looping process, the coke was oxidized by air to form CO and CO\({}_{2}\) during the air step (Figure 10a). Due to a large amount of carbon deposition during the toluene step, only CO was formed in the initial 10 min of the oxidation. Afterward, CO\({}_{2}\) and O\({}_{2}\) appeared successively. Steam addition would obviously change the reaction, and the gas evolution is shown in Figure 10b. Two important reactions occurred at the presence of steam, i.e., carbon gasification (C + H\({}_{2}\)O = CO + H\({}_{2}\)) and water gas shift reaction (CO + H\({}_{2}\)O = CO\({}_{2}\) + H\({}_{2}\)). Simultaneously, the presence of steam also caused the reforming of toluene or the intermediate C\({}_{n}\)H\({}_{2m}\) + nH\({}_{2}\)O = nCO + (m+n)H\({}_{2}\). They would efficiently prevent coke formation, promoting CO and H\({}_{2}\) yield. Correspondingly, the concentration of CO and H\({}_{2}\) maintained over 15% and 27%, respectively. Additionally, the reduced oxygen carrier LaFeO\({}_{3\text{-}x}\) would also be oxidized by water splitting (LaFeO\({}_{3\text{-}x}\) + xH\({}_{2}\)O=LaFeO\({}_{3}\) + xH\({}_{2}\)) [38]. The reduction of oxygen carrier and water splitting would set up an inner circulation of LaFeO\({}_{3}\) in the  
Figure 9: Gas evolution during chemical looping toluene conversion using LaFe\({}_{1\text{-}x}\)Ni\({}_{\text{x}}\) at 900 \({}^{\circ}\)C: (a) LaFe\({}_{0.8}\)Ni\({}_{0.2}\); (b) LaFe\({}_{0.6}\)Ni\({}_{0.4}\), (c) LaFe\({}_{0.4}\)Ni\({}_{0.6}\), (d) LaFe\({}_{0.2}\)Ni\({}_{0.8}\). fuel reactor, and it greatly depended on the amount of steam. The inner loop of the oxygen carrier made it as a catalyst enhancing the yield of C\({}_{1}\) and H\({}_{2}\). A sharp CO\({}_{2}\) peak was observed in the initial toluene step, and it was ascribed to the full oxidation effect of oxygen carriers, probably due to the presence of NiO. It is worth noting that considerable CO was observed during the N\({}_{2}\) purging stage after the toluene step, and the duration was about 15 min. A similar phenomenon was not observed in the experiment without steam. Therefore, it should not be attributed to the residual gas product from the toluene step. The presence of steam would cause the recovery of lattice oxygen in the oxygen carrier to form LaFeO\({}_{3}\). The lattice oxygen in the oxygen carrier will migrate to the surface where coke is deposited. Similar oxygen migration characteristics were also reported before [39]. As a result, it caused the solid-solid reaction between coke and oxygen carrier forming CO. Correspondingly, the concentration of CO and CO\({}_{2}\) in the oxidation step was smaller than that in Figure 10a. The effect of steam on toluene conversion was also carried out using other oxygen carriers, and Figure 11 compares the gas concentration using typical LaFe\({}_{1\text{-x}}\)Ni\({}_{\text{x}}\) at 900 \({}^{\circ}\)C with and without steam (S/C = 0.8).