Catalisadores Victor Teixeira: desenvolvimento e avaliação de carbetos bimetálicos e trimetálico de níquel, molibdênio e tungstênio na reação de reforma seca de metano

Abstract

Bi and trimetallic carbides (NiMo, NiW and NiMoW) were synthesized, characterized and evaluated as catalysts in dry reforming of methane - DRM (T = 800 °C, P = 1 atm) employing reactional feed with different molar ratios (CH4/CO2 = 1.5; 1.0 and 0.67), simulating the compositions of natural gas reserves contaminated with CO2 or biogas. The NiMoW trimetallic carbide application in DRM is unpublished in the literature and the carbides of NiMo and NiW, scantly investigated, were used in comparison. The catalyst precursors, synthesized by coprecipitation, presented low crystallinities due to their layered turbostratic nickel hydroxides, intercalated by oxometalates of molybdenum and/or tungsten. These precursors were activated by Temperature Programmed Carburization resulting in polycrystalline structures Ni-Mo2C, Ni-WC e Ni-MexC (where Me = Mo+W, x = 1-2). Although bimetallic carbides deactivate in some situations (Ni-Mo2C by oxidation, if CH4/CO2<1; Ni-WC by carbon deposition, if CH4/CO2≥1), the trimetallic carbide NiMoW proved to be active, stable and versatile, even when employed reactional feeds of different molar ratios. Activity and stability of these catalysts in DRM were linked to continuous cycles of oxidation/(re)carburization, where the carbide promotes the activation of carbon dioxide (CO2 → CO +O*) which may lead to their oxidation (Me2C + 5O* →2MeO2 + CO; MeC + 3O* → MeO2 + CO); nickel (Ni0) promotes the methane decomposition (CH4 → C* + 2H2) and the solid carbon formed could lead to oxide (re)carburization (2MeO2 + 5C* → Me2C + 4CO; MeO2 + 3C* → MeC + 2CO). Thus, the pyrolytic carbon deposited over nickel metal sites, the main cause of conventional catalysts deactivation, in these cases, performed a fundamental role in the catalytic activity maintenance, being responsible for the carbothermic (re)carburization of Mo and/or W, partially oxidized by CO2.