(60e) Sustained Oscillations Caused By Feed Modulation during Methane Oxidation on Four-Way Catalysts

Sustained
Oscillations Caused by Feed Modulation During Methane Oxidation on Four-Way
Catalysts

Kyle
Karinshak¹, Steve Golden², Michael P.
Harold^1*

*corresponding
author e-mail: mpharold@central.uh.edu

¹Department of Chemical &
Biomolecular Engineering, University of Houston, Houston, Texas 77204

²CDTi Advanced Materials Inc,
Oxnard, California 93033

Introduction:

Cars and trucks accounted for nearly 25%
of all greenhouse emissions in the US in 2016 [1].
Natural gas vehicles (NGVs) are a promising greener alternative; natural gas has
a more favorable C:H ratio than diesel or gasoline. However, uncombusted
methane – produced in non-negligible amounts by NGVs – presents a serious
problem; methane has a Global Warming Potential twenty-times that of
carbon dioxide [2].
Until recently, methane has been a non-regulated exhaust component and current
emission control catalysts such as the Three-Way Catalyst (TWC), while
effective for CO, NOx, and non-methane hydrocarbons, are ineffective for
methane oxidation.

            To this end,
current work in our group is focused on the development of the Four-Way Catalyst
(FWC) to additionally target methane in addition to CO, NOx and other
hydrocarbons. Current TWCs modified to catalyze methane oxidation require high
loadings of precious group metals (PGM; Pt, Pd, Rh). There is a need to develop
FWCs with lower PGM loading while achieving effective methane oxidation
activity. We are examining a class of FWCs that contain transition metal oxides
with spinel structure, AB₂O₄, in combination with PGM
which have shown some promise in TWC applications. This study examines the
impact of spinel on the methane oxidation activity of a dual component catalyst
containing Mn_0.5Fe_2.5O₄ and Pt/Pd as well as
interesting sustained oscillatory behavior that arises during modulated feed
conditions.

Results
and Discussion:

In earlier work done in our group, we have
shown that the addition of spinel to PGM results in higher catalytic activity with
a near-stoichiometric feed that is modulated at ~1 Hz.  Experiments varying the
amplitude of the modulation around a fixed O₂/methane feed ratio
indicate that the enhancement by the Spinel may be tuned. In order to
understand the impact of the proximity of the Pt+Pd/Al₂O₃
and spinel components, we have systematically separated the spinel and PGM
layers with an inert Al₂O₃ layer (up to 80 g/L). The data
suggests that the addition of the intermediate layer does not impact the
catalytic activity. While the oscillatory feed – avg. λ=0.995, Δλ=±0.014, f=0.7Hz – leads to
higher activity when compared to steady-state feed conditions with the same
avg. λ, it also introduces significant, sustained secondary-oscillations
during methane light-off (Fig. 1). The secondary-oscillations have a period of
7-10 minutes, much longer than the ~1 s period associated with the forced modulation.
Further, the variation of the methane conversion during the oscillation can
vary by as much as 60% (absolute). Additional experiments (example in Fig. 2)
vary the avg. λ while maintaining a constant Δλ of 0.014 and
show that these oscillations appear only at specific avg. λ values.

            Ongoing
experiments will explore how adjusting the feed oscillation frequency and feed
oscillation amplitude affect these secondary oscillations. Additional
experiments utilizing spatially-resolved mass spectroscopy (SpaciMS) will show
how the secondary oscillations develop along the length of the catalyst. We
will discuss the underlying mechanism for the enhanced methane oxidation and
oscillatory behavior.   

Figure 1. CH₄ light-off demonstrating
secondary oscillations.

Figure 2. Methane
conversion at different temperatures as a function of varying avg. λ.

Acknowledgment

            This work was
supported by DOE-EERE (DE-EE0008233).

[1]      US EPA,
Fast Facts on Transportation Greenhouse Gas Emissions, (n.d.).
https://www.epa.gov/greenvehicles/fast-facts-transportation-greenhouse-g...
(accessed April 9, 2019).

[2]      O.
Boucher, P. Friedlingstein, B. Collins, K.P. Shine, The indirect global warming
potential and global temperature change potential due to methane oxidation,
Environ. Res. Lett. 4 (2009) 044007. doi:10.1088/1748-9326/4/4/044007.