Preparation of ceramic structured catalysts with Ni and Ni-Rh/CeO2 catalytic layers for syngas production by biogas reforming processes

Electricity generated by fuel cells is considered green energy as they can potentially lower greenhouse gas emissions. Fuel cells require hydrogen or H2-rich gas mixtures (syngas) as fuel and, consequently, numerous technologies for hydrogen production are under investigation in the world. Cost of building of hydrogen distribution and trasportation infrastructures represents the major economic barrier to the implementation of hydrogen-based technologies, particularly in the transport sector. Thus, in the near term, the reforming technologies of fossil and renewable fuels for hydrogen/syngas production will play a crucial role toward the transition to the "hydrogen-based economy", especially for low-scale reforming distribution [1]. The conversion of biogas by reforming processes (principally Steam Reforming (SR) and Oxy Steam Reforming (OSR)) into hydrogen/syngas to feed fuel cells represents an alternative route to the traditional utilization of this renewable fuel in less efficient and polluting engines for producing energy and heat. Some of the central issues of reforming processes are the development of efficient, stable and low cost catalysts and the design of more compact and lightweight fuel processors. In this respect, different catalyst configurations (foams, honeycombs, gauze, microchannels) are proposed as alternative to traditional packed-bed reactors [2]. The principal advantages of structured catalysts are related especially to their large surface-to-volume ratio that leads to good heat and mass transfer properties and low pressure drop [3]. In addition, robustness, strength and low cost are other important advantages, especially for the ceramic structured catalysts. In this work, the catalytic performances of structured catalysts (Fig. 1) in SR and OSR of biogas were investigated. Structured systems were prepared by washcoating of 7.5wt%Ni/CeO2 and 7.5wt%Ni-0.5wt%Rh/CeO2 powders (previously prepared by Solution Combustion Synthesis) on cordierite monoliths (500 cpsi, diameter 1 cm, length 1.5 cm) and alumina foams (20-30-40 ppi, diameter 1 cm, length 1.5 cm). Particular efforts were directed to the development and optimization of the washcoating procedure to overcome problems related to the non-chargeable low surface area (? 11 m2/g) of the prepared catalysts, employing acid media to obtain an opportune solid particle dispersion [4]. Then, the use of acid-free stable dispersions was studied to obtain thin catalytic layers onto substrates of complex geometry. Indeed, the evaluation of hydroxyl-based organic compounds for the dispersion via steric-like mechanism was investigated. Sedimentation tests pointed out an increase in dispersion stability with increasing the number of the hydroxyl groups, while rheological behavior was modulated by polyvinyl alcohol (PVA). Results were evaluated in terms of coating load and adhesion performance. On both substrates, coating load in the range 15-20% (w/w) was obtained increasing PVA content and performing multiple depositions. Washcoat layers were found to be quite homogeneous and well adherent to the surface of the support. Adhesion tests in ultrasounds bath showed a good washcoat-support interaction: weight losses (evaluated by measuring the weight of the structured sample before and after the ultrasonic treatment) of about 2wt.% were measured. The catalysts prepared by combustion synthesis were characterized by XRD, TPR and CO chemisorption analyses, while the final structured systems were characterized by SEM-EDX, TEM and XRD techniques. The results evidenced not only a homogeneous distribution of the catalytic layers on the support surface but also a good distribution of the active metallic phase on CeO2. Pressure drop measurements were also carried out on bare and coated systems. Monolith catalyst showed pressure drop lower than foam systems, for which the value increases with the increase of ppi. Biogas OSR and SR of biogas (60% CH4 and 40% CO2) experiments were conducted in a fixed-bed quartz reactor at atmospheric pressure. The influence of temperature (700-900°C) and weight space velocity (WSV=36,000-254,000 Nmlgcat-1h-1), at fixed S/C=3 for the SR reaction and S/C=1 and O/C=0.2 for the OSR reaction, were addressed. High purity CH4, CO2, O2 and N2 gases were employed. Steam was feed by using an isocratic pump and a specially designed evaporator. Between the two catalytic formulations the bimetallic samples (Ni-Rh/CeO2) performed better than monometallic systems (Ni/CeO2), especially at higher space velocity. Instead, considering the influence of the structured support on the catalytic activity, foams showed slightly higher performances toward SR and OSR processes at all the WSV investigated, in terms of CH4 conversion (99.9-98.0% for SR and 99.8-96.0% for OSR), H2 concentration (64-62% for SR and about 58% for OSR, in dry and nitrogen-free basis). In addition, the utilization/production of CO2 and the H2/CO molar ratio in both reforming processes were also studied. OSR process leads a superior CO2 conversion (49-42%) compared to the SR (10-3%), coupled with a lower H2/CO ratio (1.8 for OSR, 2.8 for SR) principally due to the presence of O2 and lower amount of steam that allow to produce more CO from partial oxidation of methane and to convert more CO2 trough the dry reforming reaction that is disadvantaged in SR condition by the large presence of steam.