Exsoluted Ni from Hexaaluminates for CO_<i>x</i> Free Hydrogen and Carbon Nanotubes via the Catalytic Decomposition of Methane

In this study, a variety of Ni-substituted hexaaluminate catalysts were synthesized and evaluated for methane decomposition (CDM) using concentrated methane (PCH4 = 0.9 at 1 atm). Pre-treatment of the catalyst in a reducing environment (900 °C) led to the exsolution of Ni nanoparticles ranging between 40.79 ± 6.48 and 77.52 ± 9.23 nm and high CMD activity, primarily through the formation of tip-grown carbon nanotubes. Lowering the pre-treatment temperature to 700 °C results in smaller Ni particles exsolved from the parent hexaaluminate structure, which causes changes in the apparent growth mode of carbon nanotubes from a tip to a base-growth mechanism. Despite their low surface area, it is demonstrated that (Ca,Sr,Ba,La)-NixAl12–xO19 hexaaluminates pretreated at the standard temperature are surprisingly active for low-temperature CDM at T = 600 °C and GHSV = 5 L·g–1·h–1 with the initial turnover frequency (TOFinitial) ranging between 2.90 and 4.96 s–1, higher than most reported TOFs for Ni-based catalysts. Carbon yields were on the order of CaNi1Al11O19 (17.36 gC·gNi–1) > BaNi1Al11O19 (15.19 gC·gNi–1) > SrNi1Al11O19 (6.65 gC·gNi–1) > LaNi1Al11O19 (5.44 gC·gNi–1) with the predominant growth mode of carbon being tip growth. Density functional theory calculations reveal that Ni substitution (ENi-sub) and oxygen vacancy formation (EO-vac) are favored in the spinel regions of the hexaaluminate structure as opposed to the mirror plane.