Abstract
The catalytic decomposition of methane (CDM) process produces hydrogen in a single stage and avoids CO2 emission thanks to the formation of high added value carbon nanofilaments as a by-product. In this work, Ni monometallic and Ni–Co, Ni–Cu, and Ni–Fe bimetallic catalysts are tested in the CDM reaction for the obtention of fishbone carbon nanofibers (CNF). Catalysts, in which Al2O3 is used as textural promoter in their formulation, are based on Ni as main active phase for the carbon formation and on Co, Cu, or Fe as dopants in order to obtain alloys with improved catalytic behaviour. Characterization of bimetallic catalysts showed the formation of particles of Ni alloys with a bimodal size distribution. For the doping content studied (5 mol. %), only Cu formed an alloy with a lattice constant high enough to be able to favor the carbon diffusion through the catalytic particle against surface diffusion, resulting in higher carbon formations, longer activity times, and activity at 750 °C; whereas Ni, Ni–Co, and Ni–Fe catalysts were inactive. On the other hand, Fe also improved the undoped catalyst performance presenting a higher carbon formation at 700 °C and the obtention of narrow carbon nanofilaments from active Ni3Fe crystallites.
Highlights
Among the different emerging hydrogen production technologies, such as water splitting [1,2] or biomass reforming [3], catalytic decomposition of methane (CDM) represents a realistic alternative to the conventional hydrogen production methods where CO2 sequestration is still the best approach to deal with the large quantities produced of this gas, for example, 0.3–0.4 m3 per m3 of hydrogen is produced in the methane steam reforming process [4]
Reducibility (Figure 1) and crystal structure (Figure 2) of undoped and doped catalysts were measured by temperature-programmed reduction reduction (TPR) and XRD (X-ray diffraction), respectively
The TPR profiles present a small H2 consumption at temperatures above 650 ◦ C, this can be covered by a longer exposure under H2 flow at 650 ◦ C avoiding higher reduction temperatures that may result in Ni particle sintering [39,40]
Summary
Among the different emerging hydrogen production technologies, such as water splitting [1,2] or biomass reforming [3], catalytic decomposition of methane (CDM) represents a realistic alternative to the conventional hydrogen production methods where CO2 sequestration is still the best approach to deal with the large quantities produced of this gas, for example, 0.3–0.4 m3 per m3 of hydrogen is produced in the methane steam reforming process [4]. Due to its reported higher activity in the CDM [7,10], Ni represents one of the best options as active phase in the catalyst, as its activity is limited by the deactivation at high temperature [6], where the methane conversion is favoured thermodynamically and the graphitic order of the carbon nanofilaments obtained is improved [11]. To face this challenge, bimetallic catalysts, with the Catalysts 2018, 8, 300; doi:10.3390/catal8080300 www.mdpi.com/journal/catalysts. For the dopant content studied (5 mol. %), only Cu formed an alloy with a lattice constant high enough to be able to favour the carbon diffusion against surface diffusion, resulting in a higher carbon formation and the obtention of denser carbon nanofibers
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