Catalytic combustion of residual methane on alumina monoliths and open cell foams coated with Pd/Co3O4
Graphical abstract
Introduction
Natural gas (NG) is a promising alternative fuel to gasoline and diesel. The main NG component is CH4. However, NG includes other hydrocarbon compounds, such as propane, butane, and pentanes [1]. The environmental impact of CH4 is related to its large contribution to the overall greenhouse effect, which is 16% [2]. In fact, the global warming potential of CH4 is larger than that of CO2. The main contributions to anthropogenic CH4 emissions come from NG systems, such as gas & oil industry, landfills, rice and other agriculture cultivations, wastewater, stationary combustion systems, compressed NG vehicles, and coal mines [3], [4]. Most of these processes involve combustion of CH4 in lean conditions (CH4 less than 2 vol.%), where the residual methane must be abated before its release into the atmosphere [4], [5], [6], [7], [8]. The catalytic combustion of CH4 is the most feasible way to reduce CH4 emissions [9]. The full oxidation of methane can be performed at a relatively low temperature over noble metals supported on transition metal oxides [10], [11], [12], [13], [14]. For this purpose, Pd-based catalysts supported on simple metal oxides and mixed metal oxides (such as spinels or perovskites) have been extensively studied [15], [16], [17]. In particular, recently there has been increased attention focused on the Co3O4 spinel-based catalysts [18], [19], [20], [21], [22]. The increased interest results from the high catalytic activity of Co3O4 towards CH4 combustion, even without its chemical modification [19], [22], [23], [24], [25], [26], [27]. A high reactivity of this material is attributed to the high lattice oxygen mobility, which influences the reduction–oxidation processes of the catalyst [18], [23], [27]. This property is of fundamental importance since the oxidation-reduction cycles between Co2+ and Co3+ play an important role in the catalytic combustion of CH4 [19], [28].
Structured supports for catalysts are widely used in environmental applications, such as automotive emission control, diesel particulate filters, stationary emission control systems, woodstove combustors, molten metal filters, indoor gas purification systems, ozone abatement systems, catalytic incineration, industrial heat recovery, ultrafiltration, and water filtration [29], [30], [31]. Monolith/foam based structures offer a series of advantages compared to packed beds. In comparison to packed bed reactors, monoliths/foams have a higher geometric surface area (surface/volume ratio, m2 m−3) and can operate at higher space velocity [30], [32]. These characteristics provide better catalytic performance using a lower catalyst loading and lower pressure drop. Compared to monoliths, open cell foams offer an attractive alternative as catalyst supports due to their high porosity, tortuosity, and lower pressure drop compared to monoliths of similar dimensions, with sustained high thermal stability [32], [33], [34], [35]. These characteristics are highly desirable not only for highly exothermic and endothermic reactions (combustion or reforming processes), but also for low contact time reactions (partial oxidation processes) [30], [32], [33], [34]. However, there are some disadvantages related to the materials that can cause a brittle and costly product [32], [36], [37].
One of the critical issues for the structured catalysts is the deposition of a carrier oxide that hosts the active phase on the surface. The wash-coating, from a slurry containing the active elements on ceramic structures, is the most often used method, even at the industrial scale [38], [39], [40], [41]. A proper use of the wash-coating technique implies the optimization of many parameters such as pH, viscosity, the concentration of the slurry, the speed of dipping, etc. In our previous works we showed that the solution combustion synthesis (SCS) followed by wetness impregnation (WI) is a viable way for the preparation of active Co3O4-based catalysts [8], [42], [43]. The SCS method proceeds in a fast, self-sustained, and strongly exothermic reaction. This procedure involves the use of cheap precursors that allow us to obtain catalysts with metal particles well distributed on the carrier [44], [45], [46]. Moreover, SCS technique has already been used successfully for the preparation of catalysts supported on monoliths [47], [48], [49], [50], [51], [52], [53].
In this work, we investigate the catalytic methane oxidation in lean conditions on an alumina monolith, of 100 cpsi, and two alumina open cell foams, of 30 and 45 ppi, coated with Co3O4 spinel via SCS and doped with 3 wt.% of Pd by WI. The 3% Pd/Co3O4 catalyst appears to be a promising material, according to our previous studies [8], [42], [43]. All of the coated structures were physically characterized by measuring the pressure drop and evaluating the volumetric heat transfer coefficients. They were then tested towards the combustion of methane, in lean conditions, to evaluate their influential geometric properties of the structure and catalyst loading for catalytic activity. To evaluate the effectiveness of the Pd/Co3O4 usage, we coated the structures with the same amount of active phase.
Section snippets
Structures and chemicals
Ceramic monoliths made of alumina, 100 cpsi, square channels, were purchased from Chauger Honeycomb Ceramics Co Ltd. (Taiwan). The monoliths (original size: diameter 40 mm, length 30 mm) were cut down to 9 mm in diameter to fit the reactor. Ceramic open cells foams made of alumina (Al2O3 with traces of SiO2), 30 and 45 ppi (original size: diameter 9 mm, length 30 mm), were purchased from Lanik s.r.o. (Czech Republic).
Cobalt(II) nitrate hexahydrate Co(NO3)2·6H2O (≥98% purity), palladium(II) nitrate
SEM and FESEM analysis
Fig. 1 shows the three structures used in this work, with the characteristic geometric dimensions overlayed on SEM pictures, and calculated according to [54], [67] for the monolith, and [68] for the foams. The dimensions measured via SEM were averaged among four different bare monoliths, and six different bare foams per type. The average pore dimension for the foams, dp, was measured considering the average pore diameters of equivalent holes with circular shape [69]. The dp of the foams
Conclusions
Overall, 3 wt.% Pd-doped cobalt spinel catalyst was deposited on an alumina monolith and two alumina foams via solution combustion synthesis and wetness impregnation using glycine and metal nitrates as precursors. The catalytic activity of the coated structures was tested toward methane oxidation in lean conditions, in a gas mixture containing 0.5 and 1 vol.% CH4 at two different WHSV, 30 and 60 NL h−1 gcat−1. The presence of Pd allowed for full CH4 conversion at a temperature lower than 400 °C, for
Acknowledgments
This study was financially supported by the Italian project PRIN IFOAMS (“Intensification of catalytic processes for clean energy, low-emission transport and sustainable chemistry using open-cell FOAMS as novel advanced structured materials”, protocol n. PRIN-2010XFT2BB) funded by the Italian Ministry of Education, University and Research, and the Executive Programme for Scientific and Technological Cooperation CANALETTO (protocol n. M00478) between Italy (Italian Ministry of Foreign Affairs)
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2022, Chemical Engineering JournalCitation Excerpt :Finally, the PdO/Co3O4 OCFs were calcined at 600 °C for 4 h in static air. Considering that the three SiCZir OCF combinations were different in the length of each single foam (SiC1Zir2, SiC1.5Zir1.5, SiC2Zir1), but equal in terms of overall length (3 cm), for sake of comparison we deposited on each entire system a targeted amount of Co3O4 + PdO equal to approx. 250 mg (PdO: 3 wt% of the Co3O4 amount [52–55]), proportionally shared on the two single SiC or Zir OCF depending on their respective lengths in the SiCZir configuration. Fig. 1 shows the test rig we used for testing the CH4 catalytic activity.