(758e) Complete Methane Combustion over Ni/CexZr1-XO2 catalysts

Complete methane combustion over
Ni/Ce_xZr_1-xO₂ catalysts

Junjie Chen¹, Benjamin
D. Carlson¹,
Jae-Soon Choi², Zhenglong Li², Todd J. Toops²,
Eleni A. Kyriakidou^1,*

¹Department
of Chemical and Biological Engineering, University at Buffalo, The State
University of New York, Buffalo, NY 14260, USA

²National
Transportation Research Center, Oak Ridge National Laboratory, Oak Ridge, TN
37831, USA

*elenikyr@buffalo.edu

As the push for alternative energy
sources grows, natural gas has become a promising, significantly cleaner
burning resource compared to traditional fossil fuels.  Methane (CH₄),
as the main component of natural gas, has about 20 times higher global warming
potential compared to CO₂ [1].  Thus, unburnt CH₄
emissions from combustion engines or turbines have attracted significant
attention.  Although Ni/CeO₂-ZrO₂ has been shown to be an
effective catalyst for CH₄ reforming reactions, the development of
Ni/CeO₂-ZrO₂ catalysts that are highly active for the
oxidation of CH₄ has been poorly investigated.  [17]The goal of this work is to develop a fundamental
understanding of the synergistic effect between the active metal (Ni) and a
series of ceria-zirconia redox supports, as well as the impact of the oxygen
storage capacity (OSC) of Ce_xZr_1-xO₂ supports
on the CH₄ combustion performance.

Ce_xZr_1-xO₂
(x=1, 0.83, 0.5, 0.17, 0) supports were synthesized by co-precipitation.  Nickel
was deposited on the as-synthesized supports by dry impregnation.  The textural and redox properties
of the synthesized 2 wt.% Ni/Ce_xZr_1-xO₂catalysts were
characterized by N₂ physisorption, STEM, EDS, XRD, Raman, H₂-TPR
and O₂ pulse chemisorption.  The
CH₄ oxidation performance of both Ce_xZr_1-xO₂
supports and 2 wt. % Ni/Ce_xZr_1-xO₂ catalysts was
tested under a feed gas ratio of CH₄: O₂: Ar = 1: 5: 9, with
30 sccm total flow (90,000 h^-1).  The main low-temperature reduction
peak data of 2 wt.% Ni/CeO₂ is centered at
214^oC (Fig. 1a) and is assigned to Ni_xO_y reduction. 
Zirconia doping led to shifting of the Ni_xO_y reduction
peak higher temperatures. Specifically, the Ni_xO_y
reduction temperature increased with increasing Zr doping as follows: Ni/CeO₂
(214℃) < Ni/Ce_0.83Zr_0.17O₂ (235℃)
< Ni/Ce_0.5Zr_0.5O₂ (264℃) < Ni/Ce0_.17Zr_0.83O₂
(293℃) < ZrO₂ (401℃).  Additionally, the CH₄
oxidation performance of 2
wt.% Ni/Ce_xZr_1-xO₂
catalysts (Fig. 1b) followed the same trend as the Ni_xO_y reduction
temperature, with the 2 wt.% Ni/CeO₂ outperforming (T₅₀ =
54℃, T₉₀ = 72 ^oC) the 2 wt.% Ni/ZrO₂
catalyst.  There results indicate that enhancing the support OSC by Zr doping (ZrO₂
(1.9 μmol/g) < CeO₂ (33.7 μmol/g) < Ce_0.17Zr_0.83O₂
(41.3 μmol/g)  < Ce_0.83Zr_0.17O₂ (75.7
μmol/g) < Ce_0.5Zr_0.5O₂ (119.7
μmol/g) ) does not enhance the CH₄ oxidation performance,
indicating that only the surface or sublayer oxygen species of the catalyst
participate in the lean CH₄ combustion.  Although Zr doping does not
improve the CH₄ oxidation performance of Ni/CeO₂, it
enhances the thermal stability of the catalyst.  Three consecutive CH₄
oxidation cycling experiments over Ni/Ce_xZr_1-xO₂
showed that Zr containing catalysts have better thermal stability than 2 wt.% Ni/CeO₂.
Combining the redox properties and methane combustion performance of Ce_xZr_1-xO₂
and Ni/Ce_xZr_1-xO₂ catalysts, a dual-site
methane combustion mechanism was proposed as well as a promotion effect between
nickel and ceria-zirconia supports.

Figure 1. H₂ –TPR (a) and CH₄
combustion performance as a function of inlet temperature (b) of 2 wt.% Ni/Ce_xZr_1-xO₂
catalysts and 2 wt.% Ni/Quartz sand.

[1]    Peng, H.,
Rao, C., Zhang, N., Wang, X., Liu, W., Mao, W., Han, L., Zhang, P. and Dai,
S., Angew. Chem. lnt. Ed., 2018.