In situ spectroscopic characterization of some LaNi1-xCoxO3 perovskite catalysts active for CH4 reforming reactions

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This version of the article has been accepted for publication, after peer review and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: http://dx.doi.org/10.1557/opl.2012.1107 In situ spectroscopic characterization of some LaNi1-xCoxO3 perovskite catalysts active for CH4 reforming reactions Rosa Pereñiguez, Victor M. Gonzalez-Delacruz, Fatima Ternero, Juan P. Holgado, Alfonso Caballero Instituto de Ciencia de Materiales de Sevilla and Dep. Química Inorgánica (CSIC-Universidad de Sevilla). Avenida Américo Vespucio, 49, 41092 Sevilla (España) ABSTRACT Lanthana-supported Ni and Co catalysts were investigated by “operando” techniques (XAS and APPES) for methane reforming reactions. The samples were prepared by the “solid phase crystallization” method (spc), where the precursors La(Ni1-xCox)O3 contains homogeneously distributed metals (Ni, Co) in the crystal structure (perovskite), which, on further reduction, result in the formation of catalytic system Ni1-xCox/La2O3. The monometallic samples (NiLaO3, CoLaO3) have been compared with a bimetallic system of an intermediate composition Ni0.5Co0.5LaO3. This “operando” study has allowed us to obtain important conclusions about the bimetallic particles and the metal-support interactions. The data revealed the formation of bimetallic particles (NiCo); on these ones, the Ni avoids the Co oxidation during the reaction. However, this protection does not induce an improvement in the activity, which presents an intermediate behaviour between Ni/La2O3 and Co/La2O3. These bimetallic particles form a pseudo-alloy with the surface enriched in cobalt (under reduced conditions), resulting nearly in a core-shell structure (Ni@Co). INTRODUCTION The dry reforming of methane has awaked a great interest in the last years versus the traditional steam reforming, especially for the syngas (CO+H2) production. This process offers important advantages compared to steam reforming of methane. In this context, the low H2/CO ratios obtained by dry reforming, makes this process interesting for the liquid hydrocarbons production (Fischer Tropsch reaction) and for the production of others products as formaldehyde, polycarbonates or methanol. In general, this process is not yet industrialized, but it should be remarked that the high concentration of CO2, in the natural gas yield and in the biomass for fermentation (ca. 30-60%), together with the methane, makes this process attractive to the industrialization. However, the main problems are the coke formation and the high temperatures (ca. 800 ºC) required for the reaction. There are several factors under study for understanding the carbon formation as the support used or the temperature of reduction, but the principal investigation is focused on the metallic phase. Although the good results obtained for the noble metals, their high cost makes necessary the searching of cheaper alternative, like nickel or cobalt due to their inherent availability, low cost and high activity. However, these metals present also some disadvantages: the Ni produces an important coke deposit; while the Co is less resistant to oxidize in the reaction conditions [1]. In fact, some studies are directed to prevent the Co oxidation through the addition of noble metals or an alloy formation as NiCo. In this context, we have focused the study to bimetallic Ni-Co systems supported on La2O3 prepared from the reduction of the precursor LaNi1-xCoxO3 (by solid phase crystallization) with a perovskite structure. It is worthy to stand out that, there is still a lot of controversy on aspects such as the nature/state of the active phase under “operando” conditions, in spite of the effort made in the investigation of these systems. EXPERIMENTAL The LaNi1-xCoxO3 perovskites were prepared by the spray pyrolysis method [2], using a solution of La(NO3)3 and Ni(NO3)2/Co(NO3)2, which is passed through two on-line furnaces at 250ºC and 600ºC respectively, producing a powder that was later calcined in air at 600ºC. The physicochemical state of the powders was characterized by means of SEM, XRD, TPR, XPS, etc. The measurements of the catalytic performance in the DRM/SRM reactions were accomplished using an atmospheric flow reactor. The feed gas was a methane/carbon dioxide (or water)/helium mixture with a space velocity of 300000 ml/hg. XAS spectra were collected in transmission mode at the BM25 station of the ESRF (Grenoble, France), while the Ambient Pressure Photoemission Spectroscopy (APPES) experiments were performed at beam line U49/2-PGM1 at BESSY II (Berlin, Germany). RESULTS AND DISCUSSION The catalytic performances of the perovskite samples have been determined for the dry and steam reforming of methane. From these results, it can be concluded that these catalysts are more stable for the dry reforming reaction, as the catalysts deactivate very quickly under steam reforming reaction conditions. So, while under dry reforming conditions the activity remains stable after 10 hours, a decrease of 20% in the methane conversion occurs under steam reforming reaction conditions. On the other hand, the catalytic activity is higher as the proportion of nickel increases: 5% for the cobalt monometallic sample, 40% for the bimetallic one, reaching values of 90% conversion for the monometallic LaNiO3 catalyst at 800ºC. The XAS spectra obtained for the different samples at the Ni and Co K edges are presented in Figure 1. As shown, the nickel phase in the reduced LaNiO3 and LaNi0.5Co0.5O3 samples, analyzed by operando XAS, evolves from a mixture of Ni3+ and Ni2+ to metallic Ni0 under both, hydrogen reduction treatment and DRM reaction. This behavior contrasts with the partial oxidation of nickel observed under SRM reaction [3]. The results obtained by in situ XAS for Co phase for DRM and SRM in the LaCoO3 and LaNi0.5Co0.5O3 reveal that in both cases the cobalt phase is partially oxidized, remaining visibly less oxidized in the LaNi0.5Co0.5O3 that in the monometallic catalysts. These results could be explained considering the formation of a NiCo bimetallic alloy after hydrogen reduction of the original Ni-Co perovskite.  normalized (E) (a.u.) 8320 8350 8380 8410 7700 7730 7760 7790 E (eV) E (eV) Figure 1 - Ni K edge (left) and Co K edge (right XANES spectra obtained in situ for the Ni, Co and NiCo perovskites. The APPES spectra obtained for the LaNi0.5Co0.5O3 sample submitted to a hydrogen reduction treatment agree with the XAS results. As shown in Figure 2, the XPS data indicate that both metallic phases (Ni and Co) remain reduced after hydrogen treatment and dry reforming reaction, while a slight oxidation process occurs under steam reforming reaction conditions, where both metals are partially oxidized to Ni(II) and Co(II) respectively. Even more interestingly, the XPS spectra obtained for this sample with different incident photon energies, corresponding to kinetic energies (KE) of the photoelectron of 200 and 600 eV, shows an important decrease in the intensity of the Ni 3p signal with 200eV photoelectrons. This finding allows us to propose a structure for the bimetallic particles, where the metals are arranged as a “pseudo core-shell” Ni@Co. Ni-LaNi Co O 0.5 0.5 3 LaNiO 3 original-RT H 2 H -Dry RM-800 ºC 2 H -Steam RM-800 ºC 2 Co-LaNi Co O 0.5 0.5 3 LaCoO 3 original-RT H 2 H -Dry RM-800 ºC 2 H -Steam RM-800 ºC 2  normalized (E) (a.u.) 798 793 788 783 778 Binding Energy (eV) 80 78 76 74 72 70 68 66 64 Binding Energy (eV) Figure 2 - APPES spectra (Co 2p and Ni 3p regions) obtained for the LaNi0.5Co0.5O3 sample submitted to the indicated treatments. CONCLUSIONS The catalytic tests revealed that the catalysts based on nickel are more active and selective for the DRM reaction, meanwhile the presence of cobalt results in a lower activity and the enhancement of the RWGS. Besides, the studied systems present better results for the DRM than for the SRM. The nickel phase behaviour in the LaNiO3 and LaNi0.5Co0.5O3 analyzed by operando XAS evolves from the mixture of Ni3+ + Ni2+ in the original samples, to Ni0 under reducing conditions and DRM reaction, in contrast with the partial oxidation registered in SRM reaction. The effects detected in the cobalt phase for DRM and SRM in the LaCoO3 and LaNi0.5Co0.5O3 reveal a bigger oxidability in the cobalt phase than the observed for the nickel species, and this oxidability is lower in the LaNi0.5Co0.5O3 sample. We propose that this higher resistance to oxidation in the mixed sample is promoted by the nickel phase and is transferred to the cobalt phase due to the formation of bimetallic alloy of NiCo, which has been previously observed in similar systems by other authors (4-6). The operando APPES data for LaNi0.5Co0.5O3 agree with the XAS results, detecting this slight oxidation under the SRM reaction. Registering the spectrum of LaNi0.5Co0.5O3 at H2-600 ºC using different photon energies (200 and 600 eV) help us to propose a possible structure for the bimetallic alloy where the metals are arranged as a “pseudo core-shell” Ni@Co.. Co3+ Co0 Co3+ Co0 Ni 2+/3+ Ni0 Co2+ SAT. Steam-650 ºC Dry-650 ºC H -600 ºC O -250 ºC Ni 3p Co 2p Counts/s (a.u.) ACKNOWLEDGMENTS We thank the Ministry of Education and Science of Spain and Junta de Andalucía for financial support (Projects ENE2011-24412 and P07-FQM-02520), the ESRF and BESSY II facilities and the BM25 Spline beamline staff for their experimental support. REFERENCES 1. K. Takanabe, K. Nagaoka, K. Nariai, K. Aika; J. Catal., 232 (2005) 268. 2. E. López-Navarrete, M. Ocaña; J. Europ. Cer. Soc., 22 (2002) 353-359. 3. R. Pereñíguez, V.M. González-DelaCruz, J.P. Holgado, A. Caballero; Appl. Catal. B: Env., 93 (2010) 346-353. 4. V.M. González-DelaCruz, R. Pereñíguez, F. Ternero, J.P. Holgado, A. Caballero; J. Phys. Chem. C, 116 (2012) 2919. 5. J. Zhang, H. Wang, A.K. Dalai, J. Catal. 249 (2007) 300. 6. K. Nagaoka, K. Takanabe, K. Aika, Appl. Catal. A 268 (2004) 151.