Tin dioxide supported catalysts for wet lean methane combustion
Creators: Sajiv Kumar, Roshni
Publisher: name: University of Alberta Library
Year: 2023

<p>Owing to the factors of low cost, abundance and lower emissions from natural gas as compared to diesel and gasoline, there has been a surged interest in utilizing natural gas for transportation<br>
purposes. Though it is known for natural gas to burn in a clean manner with considerably fewer greenhouse gases (GHGs) and near-zero particulate matter, methane itself has a greenhouse gas potential that exceeds CO2 by 23 times. Hence, its release from natural gas vehicles (NGVs) exhaust into the atmosphere needs to be averted. The presence of catalytic converters in the NGV exhaust could result in an abatement of methane emissions, however, it faces several challenges. The exhaust stream from a lean-burn natural gas vehicle (NGV) typically has a maximum<br>
temperature of about 550 ˚C, a low concentration of methane (500 – 1000 ppmv) and a large amount of water vapour (around 15 % by volume) which is a major cause of Pd catalyst<br>
deactivation. Such conditions pose significant challenges in the development of a small-footprint catalytic converter to mitigate methane emissions. Thus, the need to develop a stable combustion catalyst for mobile applications is of paramount importance.</p>

<p>The ability to overcome the negative effects of water is highly dependent on the catalyst formulation including the choice of the support material. In an attempt to develop stable combustion catalysts, water-resistant Pt was first incorporated into the catalyst formulation. To further enhance the resistance to deactivation, SnO2 was employed as a support material due to<br>
its known ability to serve as a sink to the hydroxyls. The kinetics of methane combustion was investigated on the 1 wt.% Pd/SnO2 and 1 wt.% (1:1 Pd:Pt molar ratio) Pd-Pt/SnO2 catalysts for a lean-burn wet feed and compared against similar catalyst formulations deposited on the conventional γ-Al2O3 support. It was demonstrated that Pt addition to Pd provided stable conversions with Pd-Pt/SnO2 catalyst demonstrating the least deactivation during hydrothermal ageing. Both SnO2 and γ-Al2O3-supported Pd-Pt catalysts demonstrated -1 order to water and reduced activation energies of 134 kJ mol-1, proposing the choice of support was inconsequential with Pt addition from the viewpoint of the activation energy. The Pd/SnO2 catalyst on the other hand reported a partial -0.11 order to water. A rate law was suggested with two active sites, one affected by H2O with -1 order and activation energy of 158 kJ mol-1 (similar to that of the Pd/γ-Al2O3 catalyst), and the other unaffected by water with an activation energy of 108 ± 7 kJ mol-1.</p>

<p>To further enhance the stability and activity of the Pd-Pt/SnO2 catalyst, cobalt was chosen as a promoter as CoOx has been demonstrated to participate in wet methane combustion. To the best of our knowledge, kinetic studies nor detailed investigations on such a catalyst combination have not been reported in the literature. Therefore, as a first step, a systematic study to optimize the promoter loading on the 1 wt.% Pd-Pt/SnO2 was conducted. Hydrothermal stability tests revealed that the 10 wt.% Co-promoted Pd-Pt/SnO2 catalyst exhibited enhanced stability and catalytic activity even at 400 ˚C. Kinetic investigations on the Co-promoted catalyst revealed that the<br>
catalyst reported a partial -0.37 order to water, which is the same as for the Pd/SnO2 catalyst. Utilizing an additive rate model (like that of the Pd/SnO2 catalyst), the site unaffected by water reported an apparent activation energy of 89 ± 9 kJ mol-1. While SnO2 dictated the kinetics in terms of activation energy, higher water tolerance on the Co-promoted catalyst could be attributed to synergistic interactions between the various components resulting in the presence of different active sites (or) components on the surface.</p>

<p>In terms of turnover frequency, the Pd-Pt/10Co3O4/SnO2 catalyst surpassed all the catalyst formulations under consideration. Reaction rates normalized per catalyst mass at 500 ˚C and 10 vol% added water were found in the order of Pd/γ-Al2O3(“x”) &lt; Pd-Pt/γ-Al2O3 (1.5x) &lt; Pd- Pt/SnO2 (1.6x) &lt; Pd-Pt/10Co/SnO2 (14x), consequently suggesting that the utilization of the Pd-Pt/10Co/SnO2 catalyst could result in smaller catalytic converter volumes.</p>