Understanding the differences in catalytic performance for hydrogen production of Ni and Co supported on mesoporous SBA-15

Abstract

Three mono and bimetallic Nix Co1−x /SBA-15 catalysts (x = 1, 0.5 and 0) with a total metallic content of 10 wt% have been prepared by a deposition–precipitation (DP) method. The catalytic performances on the dry reforming of methane reaction (DRM) have been determined and correlated with their physical and chemical state before and after the catalytic reaction. So, while the nickel monometallic system presents a high activity and stability in the DRM reaction, the Co/SBA-15 catalytic system turns out completely inactive. For its part, the Ni0.5Co0.5/SBA-15 has initially a catalytic performance similar to the Ni/SBA- 15 monometallic system, but rapidly evolving to an inactive system, therefore resembling the behavior of the cobalt-based catalyst. The characterization by TEM and in situ XPS techniques has allowed us to ascribe these differences to the initial state of metallic particles after reduction and their different evolution under reaction conditions. So, while after reduction both nickel containing Nix Co1−x /SBA-15 catalysts (x = 1 and 0.5) present a well dispersed metallic phase, the cobalt monometallic catalyst yields big metallic particles with a heterogeneous distribution of sizes. Additionally, unlike the Ni/SBA-15, the NiCo/SBA-15 system increases during reaction the metallic particle sizes. Besides indicating that the particle size is a major reason determining the catalytic performances, these results suggest that in the Ni–Co system both metals form after reduction a bimetallic phase mainly located inside the mesoporous channels of SBA-15 support. Under DRM reaction conditions, the cobalt is segregated to the surface of the bimetallic particles, which seems to determine the interaction with the support surface SBA-15. This feature gives rise to a much less stable metallic phase which suffers an important sintering process under DRM catalytic conditions. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Supported nickel catalytic systems are currently one of the most important industrial heterogeneous catalysts because its remarkable performance in a number of economically strategic processes [1–5]. Among them, the steam reforming of methane (SRM, CH4 + H2O ↔ 3H2 + CO) can be outlined as the main industrial process for obtaining hydrogen and synthesis gas, used to syn- thesize various important chemicals and fuels [6–9]. Although it is not yet commercially exploited, the dry reforming of methane (DRM, CH4 + CO2 ↔ 2H2 + 2CO) is an especially interesting reac- tion that transforms two of the most harmful greenhouse gases, methane and carbon dioxide, into a mixture of hydrogen and car- bon monoxide [10–12]. Once again, Ni-based catalysts are the most ∗ Corresponding author. E-mail address: caballero@us.es (A. Caballero). widely tested in the literature for this reforming reaction, even though noble metal based catalysts such as Pt, Ru and Rh are much more performance toward methane conversion. The principal issue comes from the fact that Ni typically undergoes severe deactivation processes, mainly due to coke formation, but also due to sinter- ing of the metallic phase, generating big metallic particles which at the same time, favors the coke formation processes [13–17]. As an alternative to overcome these issues, a number of publications have shown as the use of bimetallic systems, as the combination of nickel and cobalt modified the catalytic performance in steam and dry reforming of methane [18–23]. But, depending on the support or the preparation methods both effects, improvements and wors- ening of the efficiency, have been reported. Main reasons explaining these contradictory findings are probably related with differences in the interaction of metals with support surface, which has been recognized as an essential factor affecting the stability of metal [24–26]. So, a strategy to avoid the growth of metallic particles is the use of special supports, and in particular mesoporous supports.