LaNiO perovskite

F. (2014) Nanocast LaNiOs perovskites as precursors for the preparation of coke-resistant dry reforming catalysts. 

The catalytic activity of the perovskite LaNiOs was investigated at 600-800 °C without reduction prior to the reaction by Provendier et al. [9] and Nam et al. [10]. The authors showed that the reduction occurred in situ under the reaction condition (CH4/CO2 = 1). The catalytic activity increases with time and reaches a steady state, which is attributed to the transformation of the LaNiOs perovskite into the active species composed of highly dispersed Ni metal over the lanthanum-based support... 

Crespin, M., Levitz, P., and Gatineau, L. (1983) Reduced forms of LaNiOs perovskite. /. Chem. Soa, Faraday Trans. 

Wang, N., Yu, X., Wang, Y., Chu, W., and Liu, M. (2013) A comparison study on methane dry reforming with carbon dioxide over LaNiOs perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier. Catal Today, 212,98-107. 

Figure 6.1. (d) Perovskite transformations of LaNi03 thermogravimetriac analyses (TGA) data in air (broken curve) and in oxygen (b) ED of anion-deficient LaNiOs in [100], showing superstructure due to anion vacancy ordering arrowed. (After Gai and Rao Z. Naturforsch. a 30). 

If the BVS rule is not satisfied (i.e. when the BVS are not very near to the formal charges for the ions) this may indicate metastability. In LaNiOa.s, for example, BVS calculations give a lanthanum valence of -1-2.63 and valences of -1-2.20 and -1-2.13 for the octahedral and square planar nickel cations, respectively. Although the Ni cation prefers square planar coordination, this oxide readily takes up oxygen upon heating in undergoing a stmctural transition to the perovskite LaNiOs, where the BVS are -1-3.05 and -1-3.01 for lanthanum and nickel, respectively (Alonso et al., 1997). The following worked example illustrates how to use Eqs. 3.20 and 3.21 with Table 3.7. 

Only values, data points not reported. The name "perovskite, which usually denotes calcium titanate, was used in [2598] for LaNiO, and a series of its analogs with Ni replaced by Co, Fe, Mn, Cr, or V, and La partially replaced by Sr, Ca, Gd, or Nd. 

The combustion of other VOCs by perovskites, besides alkanes and alkenes, has also been investigated. Ling et al. [64] have studied LaNiOs catalysts for the combustion of ethanol and acetaldehyde, comparing activity of that for methane combustion. Oxidation of 1 vol.% VOC in air (total flow-100 ml min O.lg catalyst) followed the order for ease of combustion ... 

Initial monitoring of the catalytic activity was performed using as-synthesized perovskites without any reduction step. In the case of conventional LaNiOs, very low values of conversions were observed for both CH4 and CO2 until 800 °C. For nanocast LaNiOs, comparatively higher conversions (approximately 40%) were... 

The catalytic activity was also investigated after reduction of the perovskite LaNiOs under hydrogen. Temperature-programmed reduction (TPR) experiments... 

The different studies performed using the perovskite LaNiOs revealed that the catalytic activity was strongly affected by the size of nickel particles generated during the reduction step performed before reaction or under the flow of products at high temperature. In order to improve the nickel dispersion, partial substitution in the B site of the perovskite was investigated. 

The substitution of nickel by iron was also studied by De Lima et al but the reaction was performed at lower temperature 650 °C [22]. The authors showed that the perovskite LaNio.8Feo 03. among the substituted catalysts, was the most active in the dry reforming of methane. The perovskite phase LaFeOs was observed after 10 h of reaction at 650°C for 0.2 < <0.7 in LaNi Fei c03- Similar results were reported by Kapokova et al. [23] who attributed the high catalytic activity of the substituted materials to the presence of the Ni-Fe alloy released from the perovskite lattice and stabilized on its surface. 

Despite the low activity displayed by LaCoOs, Valderrama et al. [25] showed high catalytic activity for the LaNii Co 03 perovskites whatever the value (but lower than 1). However, the data were collected at thermodynamic equilibrium. Sierra et al [24] obtained the best catalytic activity with the perovskite LaNiOs as catalyst precursor, the presence of cobalt leading to a decrease of the catalytic activity. Nevertheless, the stability toward carbon deposition was enhanced in the presence of cobalt. The formation of a Q>-Ni alloy was suggested by the... 

It is known that noble metal-based catalysts are less sensitive to coke deposit than nickel-based one. Therefore, ruthenium was used as substituent of nickel in the perovskite. De Araujo et al [26] showed that the most interesting catalyst was LaNio.8Ruo.2O3, which, in spite of being less active than LaNiOs, showed the highest resistance against carbon deposition. 

The stability but also the catalytic activity of nickel-based perovskite was improved with a low amount of magnesium or rhodium in the structure. The solubility of Mg in the perovsldte structure is limited phases such as NiO and MgO are evidenced by XRD when >0.1 in LaNii Mg >,03 [24]. High catalytic activity was reached at 700 °C, with LaNio.9Mgo.1O3 under severe reaction conditions 10 mg of catalysts, CH4/CO2 50/50 ml/min without dilution gas. The CH4 and CO2 conversion were respectively equal to 57 and 67% and maintained during 15 h on stream even if carbon deposition was detected by thermogravimetric analysis at the end of the reaction (27 wt%). 

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