Abstract
Bimetallic catalysts can provide enhanced performance, and Co-based catalysts in particular have been studied in various respects for their activity in the deposition of carbon nanofibers (CNFs). The majority of studies on CNF catalysis use co-precipitation to create alloys, but recent work has demonstrated the suitability of mechanical alloying (MA) by ball milling to reduce cost and increase catalytic activity. This work establishes the unique ability of MA to control the microstructure to produce bimetallic composites, which retain distinct metallic phases that improve catalytic activity. It is demonstrated that Co-Pd alloys reach a maximum in catalytic activity at an intermediate time of mechanical activation, where 30 min of milling outperformed samples milled for 5, 15, 60, and 240 min at a reaction temperature of 550 °C and a 1:4 C2H4:H2 reactant ratio. This indicates there is benefit to retaining the metals in distinct phases in close proximity. Ball milling provides a relatively simple and scalable method to achieve these unique microstructures, and in the optimal condition tested here, the activity toward carbon deposition is increased fourfold over prior work. Furthermore, the minimum temperature for deposition is also reduced. The characteristics of these materials, the effects of milling and annealing, and the underlying mechanisms of deposition are discussed.
Highlights
IntroductionBimetallic catalysts are known to have distinct performance advantages at lower cost than pure catalysts [1]
Bimetallic catalysts are known to have distinct performance advantages at lower cost than pure catalysts [1]. The performance of these alloys has been attributed to the varied elemental distribution at the surface, which is often controlled through composition [2], though temperature is important [3]
Most pertinent to this work, Co has been alloyed with Ag [8] and Cu [9,10,11] for carbon nanofibers (CNFs) deposition, and when alloyed with Ag, as little as 1% dramatically increases the conversion of ethylene to solid carbon, but larger quantities do not increase this benefit [8]
Summary
Bimetallic catalysts are known to have distinct performance advantages at lower cost than pure catalysts [1]. The performance of these alloys has been attributed to the varied elemental distribution at the surface, which is often controlled through composition [2], though temperature is important [3]. Most pertinent to this work, Co has been alloyed with Ag [8] and Cu [9,10,11] for CNF deposition, and when alloyed with Ag, as little as 1% dramatically increases the conversion of ethylene to solid carbon (as CNFs), but larger quantities do not increase this benefit [8]. The controlled addition of sulfur to Co has been noted to increase CNF deposition rate by surface modification [12], but again, the addition provides diminishing returns
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