The Kinetic Consequences of Water on Catalytic Methane Pyrolysis

TL;DR

This study explores how water affects methane decomposition and carbon nanotube growth using a Ni–Mo/MgO catalyst, revealing both inhibitory and enhancing effects depending on timing.

Contribution

The novel contribution is the discovery that water's impact on methane pyrolysis depends on its introduction timing, either inhibiting or enhancing the process.

Findings

Water introduced at the start of methane decomposition reduces methane conversion rates.

Adding water after a stabilization period enhances conversion rates and reduces catalyst deactivation.

Water preferentially interacts with carbon deposits, lowering activation energy and improving catalyst longevity.

Abstract

Hydrogen production
from biomass and natural gas has
emerged as
a prominent research area in response to the growing demand for energy
from alternative sources that minimize CO2 emissions. In
this study, we investigate the impact of water, which is present in
and generated from biomass-derived streams, on carbon nanotube (CNT)
growth and hydrogen production during methane decomposition using
Ni–Mo/MgO as a catalyst. We reveal here that the role of water
on CNT growth is highly complex; its effect depends on the stage of
growth at which the water is incorporated. When water is introduced
at the beginning of methane decomposition (t = 0
h), methane conversion rates are negatively impacted. We hypothesize
that water inhibits the significant phase changes the Ni–Mo/MgO
catalyst undergoes during catalyst carburization. In contrast, the
incorporation of a small percentage of water after a stabilization
period (t = 3 h) results in methane conversion rate
enhancements that scale with the introduced water partial pressure
as water selectively reacts with amorphous carbon deposits that lead
to catalyst deactivation, thus prolonging the lifetime of some of
the most active sites. Moreover, water incorporation after stabilization
significantly reduces the apparent activation energy. Density Functional
Theory (DFT) calculations reveal that water preferentially interacts
with carbon fragments on the catalyst surface to remove carbon deposits
with a barrier lower than that required for methane activation, further
supporting its role in cleaning active sites on the catalyst surface.
Characterization of the resulting carbon nanotubes reveals the formation
of more graphitic materials produced in the presence of water, highlighting
the impact of water on nanotube properties. These results provide
clarity toward the many ways in which water, or cofeeding of biomass-derived
materials, may impact catalytic methane pyrolysis rates.

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