Steam reforming of simulated biogas over plate NiCr catalysts: Influence of pre-oxidation on catalytic activity
Graphical abstract
Introduction
To address current energy and environmental concerns, the biogas reforming for either hydrogen or syngas production has received considerable attention in recent years [1], [2], [3], [4], [5]. Biogas reforming with added steam provides an opportunity to employ carbon dioxide, one of the main constituents of biogas, as a C1 building block for fuel production [6]. Synthetic gas for high temperature fuel cells (HTFCs) including molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) can be produced directly from biogas using Ni-based catalysts. These catalysts are widely employed for reforming reactions using H2O and/or CO2 as oxidants [7], [8]. However, utilization of high operating currents in HTFCs results in increased cell temperatures and temperature gradients, which can decompose Ni catalysts in a stack [9], [10]. Internal reforming of methane over a Ni-based catalyst is one strategy to recycle waste-heat potentially produced from a stack. For example, a Ni catalyst was efficiently used for the combined indirect and direct internal methane reforming to remove excess heat from an MCFC stack [11]. However, the physical instability of pure Ni powders results in rapid deactivation under HTFC operating conditions. Therefore, enhancing the thermal stability of stack components (e.g., catalysts) is necessary to improve the activity and durability of HTFCs under various operating conditions.
One strategy to increase the thermal and structural stability of bulk Ni catalysts for hydrogen production and HTFCs is the incorporation of Cr into the Ni material with structuralization to improve heat dissipation. For example, a disk-type porous Ni-10% Cr alloy catalyst, for the autothermal reforming of methane, has shown relatively high activity for H2 generation and long-term stability [12]. For practical applications, however, the activity of a Ni alloy catalyst for methane reforming reaction requires improvement. Catalyst activity can be enhanced by either modification with additives or pretreatment. In the first instance, Sabirova et al. [13] reported 25% CH4 conversion over a porous metallic Ni plate during steam reforming at 750 °C (H2O/CH4 = 2). Introduction of MgO into the Ni plate by the impregnation method helped increase the catalyst activity to 50–65%. In the case of pretreatment, a similar type of porous Ni catalyst had significantly increased activity for CO oxidation after an oxidation-pretreatment [14]. Moreover, Yan Ma et al. [15] used a sequential acid and alkali leaching process to enhance the catalytic activity of atomized Ni3Al alloy. King et al. [16] claimed that a Ni–YSZ cermet in a SOFC produced NiO species dissolved in YSZ formed a solid solution during steam reforming and generated small Ni particles following hydrogen pretreatment. However, the active sites produced by pretreatment in this case were unstable under the reaction conditions and subsequently deactivated. In a more recent study, Bonura and coworkers developed a highly active NiCu alloyed catalyst supported on Ce0.9Gd0.1O2−δ (NiCu/CGO) for integrated biogas SOFC process and proved that NiCu/CGO suppressed coke deposition as well as metal sintering upon its utilization as a SOFC anodic material, thereby effectively stabilizing the active metal sites for biogas dry reforming [17].
In this study, we elucidated the influence of the pretreatment process, pre-oxidation at ≥600 °C with sequential reduction at high temperature (700 °C), on the catalytic activity of a NiCr plate catalyst for the steam reforming of biogas. Pretreated catalysts showed increased activity over the as-prepared NiCr catalysts for reforming reactions conducted at 700 °C. In addition, the NiCr plate catalyst activated by pretreatment exhibited superior stability over a time period of 100 h. Controlling factors including temperature, inlet H2O/CH4 ratio, and CH4/CO2 ratios for the steam reforming of biogas were also identified.
Section snippets
Preparation of the porous plate-type NiCr catalyst
Isopropyl alcohol, binder (Methyl Cellulose #1500, Junsei Chemical. Co., Japan), plasticizer (Glycerol, Junsei Chemical Co., Japan), defoamer (SN-154, San Nopco Korea), deflocculant (Cerasperse-5468, San Nopco Korea), dispersant (polyacrylic acid), Ni metal (INCO. #255, INCO), and Cr metal (Alfa > 99.9%) were purchased as received. A mixture containing the Ni and Cr powders (or only Ni powder), plasticizer, defoamer, deflocculant, dispersant, and distilled water was initially ball-milled for 15 h.
Influence of pretreatment on catalytic activity
Steam reforming reactions of simulated biogas were initially conducted over bulk NiCr catalysts at 700 °C with the H2O/CH4/CO2/N2 ratios of 2/1/0.57/1.7. The ratio of H2O/CH4 = 2 was chosen to prevent significant carbon deposition during reforming. As depicted in Fig. 1a, the bulk NiCr plate catalyst showed very low activity with an approximate CH4 conversion of 5%. Next, a pretreatment process was employed to enhance the activity of the catalyst by partly oxidizing the as-prepared NiCr material
Conclusions
The porous NiCr plate manufactured by a conventional procedure exhibited very low catalytic activity. Pretreatment of the NiCr plate catalyst by pre-oxidation at ≥600 °C and successive reduction at 700 °C enhanced the activity and stability of the plate catalyst. The surface of the bulk NiCr material was reconstructed during pretreatment to produce reactive Ni sites identified by SEM, TEM, and XPS-depth profiling studies. In the NiCr catalyst, chromium oxide was proposed to play a crucial role in
Acknowledgements
This research was supported by the Global Research Laboratory Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning of Republic of Korea. Part of this research was also supported by the Fundamental Technology Development Programs for the Future through the Korea Institute of Science and Technology as well as by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (University-Institute cooperation
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