(573b) Novel Fe-Promoted MgAl2O4 Support Material for Control of Carbon Deposition during Reforming Reactions

Introduction

The role of
the support in syngas production from methane has been widely investigated for
various reforming technologies, like steam reforming (MSR), dry reforming
(DRM), partial oxidation of methane (POM) and autothermal reforming (ARM) [1]. Inert supports, e.g. SiO₂,
although ensuring active metal dispersion, will result in quick deactivation
due to carbon deposition, since both CH₄ and CO₂ are
activated on the same active site (mono-functional mechanism) [2-4]. A bi-functional
mechanism exist on acidic/basic supports (Al₂O₃, La₂O₃,
MgO etc.), where CH₄ is activated on active metals and CO₂
on the support, thus enhancing the oxidation of deposited carbon [5-7].

To improve the
latter mechanism, the synthesis of supported reforming catalysts should be adjusted
in such a way that the support inhibits carbon formation by actively
participating in the reforming reaction scheme. This should include (1) high
temperature resistance, (2) basicity which will enhance the adsorption of CO₂
and (3) good redox properties, in particular oxygen storage capacity [8, 9].

MgAl₂O₄
was widely investigated in literature as a suitable support material for
reforming reactions due to its low cost. The addition of MgO to Al₂O₃
prevents interaction between active metals and alumina forming inactive spinel
phases. Guo and co-workers [10] compared Ni/γ-Al₂O₃,
Ni/MgO-γ-Al₂O₃ and Ni/MgAl₂O₄
during methane dry reforming. They found that Ni/MgAl₂O₄
exhibited higher activity and better stability compared to the other samples.
The MgAl₂O₄ spinel can effectively suppress the phase
transformation to form NiAl₂O₄ and stabilizes Ni
crystallites. In order to lower significantly the carbon deposition during DRM
conditions at high temperature (1023 K), the present study proposes a MgFeAlO_x
support, where Fe is incorporated in the magnesium aluminate lattice, as a
cheaper alternative oxygen storage material.

Results

A series of Mg(a%)FeAlO_x
support materials (where α =0, 2.5, 5 ,7.5 ,10 and 20 wt%) were prepared
by one-pot synthesis, using an aqueous solution of the corresponding nitrates
and calcined at  high temperature (1073 K). Supported 8wt.%Ni catalysts were
prepared by incipient wetness impregnation on the Mg(α%Fe)AlO_x materials,
resulting in Ni/Mg(αFe)AlO_x
and were investigated during methane dry reforming at 1023 K and 121.3 kPa.

The evolution of the
support and the catalyst structure during H₂-TPR and CO₂-TPO
was examined using time-resolved in-situ XRD and operando-XAS. The
XANES region of the support MgFeAlO_x showed reduction through a
shift of edge energy position towards lower values. The reduction remained
partial as complete reduction to reference Fe⁺² and Fe⁰ was
not observed. This could indicate spinel reduction from MgFe⁺³AlO_xto
MgFe⁺²AlO_x without complete segregation of iron from the
support [11], while it can be
re-oxidized to MgFe⁺³AlO_x under CO₂ flow. This
redox ability of the support, with iron incorporated in the spinel lattice, is
beneficial for suppressing the carbon deposition during reforming reactions. The
in-situ XRD during H₂-TPR of the catalyst, Ni/Mg(αFe)AlO_x,
showed the formation of Fe-Ni alloy, via Fe delivery from the support material
to the catalyst surface. The presence of Ni enhanced the reducibility of iron
to Fe⁰, indicating that part of Fe could be segregated from the
support.

Figure 1
displays the space time yield, STY, of the products for the best candidate of
Fe-promoted samples along with the non-promoted catalyst during a stability
test (TOS of 12 h). The amount of deposited carbon was determined by O₂-TPO.

Figure 1: Stability tests during DRM after
12h time-on-stream (TOS) at 1023 K (total pressure of 121.3
kPa and CH₄/CO₂=1/1).♦,♦: STY_CO (solid
line); □, □: STY_H2 (dashed line). Blue line: Ni/Mg(0Fe)AlO_xwhere X_CH4 from 72% to
60%, green line: Ni/Mg(2.5Fe)AlO_x where X_CH4 from 75%
to 62%. Error bars were calculated after three independent experiments
representing standard deviation (66% probability confidence interval).

Among
the different supports, Ni/Mg(2.5Fe)AlO_xhas an
optimal composition as it results in more syngas production with a ratio of
CO/H₂ equal to 1.2, due to reverse water gas shift reaction, while
further increase of Fe content leads to catalyst activity decrease. The Fe addition in the support
material increases the oxygen mobility through the support lattice, suppressing
carbon formation during DRM. While 1.12 mol of carbon was deposited on Ni/Mg(0Fe)AlO_xafter 12h, no carbon was detected on samples with Fe incorporated in the
support.

The novel
Fe-promoted MgAl₂O₄ material could be used as a support
for reforming reactions, replacing conventional oxygen storage materials (i.e
CeO₂) due to its low price. The difference in carbon deposition
between Ni/Mg(0Fe)AlO_x and Ni/Mg(2.5Fe)AlO_x indicates the
beneficial effect of Fe incorporation into the magnesium aluminate spinel
lattice.

Acknowledgments

This work was supported by the FAST
industrialization by Catalyst Research and Development (FASTCARD) project,
which is a Large Scale Collaborative Project supported by the European
Commission in the 7^th Framework Programme (GA no 604277), by the
“Long Term Structural Methusalem Funding by the Flemish Government”, the
Interuniversity Attraction Poles Programme, IAP7/5, Belgian State – Belgian Science
Policy, and the Fund for Scientific Research Flanders (FWO; project 004613N). XAS
experiments were performed on the ROCK beam line, at the SOLEIL Synchrotron,
France (proposal number 20150256). The authors acknowledge the SOLEIL staff for
smoothly running the facility, support from Prof. C. Detavernier with the in
situ XRD equipment (Department of Solid State Sciences, Ghent University) and
from Dr. Vitaliy Bliznuk (Department of Materials Science and Engineering,
Ghent University) for the HRTEM measurements.

References

1.            Littlewood, P., X. Xie, M. Bernicke, A. Thomas, and R.
Schomäcker, Ni_0.05Mn_0.95O catalysts for the dry
reforming of methane. Catal. Today, 2015. 242, Part A(0): p.
111-118.

2.            Kroll, V.C.H., H.M. Swaan, S. Lacombe, and C.
Mirodatos, Methane Reforming Reaction with Carbon Dioxide over
Ni/SiO2Catalyst: II. A Mechanistic Study. Journal of Catalysis, 1996. 164(2):
p. 387-398.

3.            Bitter, J.H., K. Seshan, and J.A. Lercher, Mono and
Bifunctional Pathways of CO₂/CH₄ reforming over Pt and Rh
Based Catalysts. J. Catal., 1998. 176(1): p. 93-101.

4.            Ferreira-Aparicio, P., I. Rodrı́guez-Ramos,
J.A. Anderson, and A. Guerrero-Ruiz, Mechanistic aspects of the dry
reforming of methane over ruthenium catalysts. Appl. Catal., A, 2000. 202(2):
p. 183-196.

5.            Tsipouriari, V.A., A.M. Efstathiou, and X.E. Verykios,
Transient Kinetic Study of the Oxidation and Hydrogenation of Carbon Species
Formed during CH₄/He, CO₂/He, and CH₄/CO₂
reactions over Rh/Al₂O₃ Catalyst. J. Catal.,
1996. 161(1): p. 31-42.

6.            Zhang, Z. and X.E. Verykios, Carbon dioxide
reforming of methane to synthesis gas over Ni/La₂O₃ catalysts.
Appl. Catal. A 1996. 138(1): p. 109-133.

7.            Pakhare, D. and J. Spivey, A review of dry (CO₂)
reforming of methane over noble metal catalysts. Chem. Soc. Rev., 2014. 43(22):
p. 7813-7837.

8.            Hecht, E.S., G.K. Gupta, H. Zhu, A.M. Dean, R.J. Kee,
L. Maier, and O. Deutschmann, Methane reforming kinetics within a Ni–YSZ
SOFC anode support. Appl. Catal., A, 2005. 295(1): p. 40-51.

9.            Theofanidis, S.A., V.V. Galvita, H. Poelman, and G.B.
Marin, Enhanced Carbon-Resistant Dry Reforming Fe-Ni Catalyst:
Role of Fe. ACS Catal., 2015: p. 3028-3039.

10.         Guo, J., H. Lou, H. Zhao, D. Chai, and X. Zheng, Dry
reforming of methane over nickel catalysts supported on magnesium aluminate
spinels. Appl. Catal. A 2004. 273(1–2): p. 75-82.

11.         Dharanipragada, N.V.R.A., L.C. Buelens, H. Poelman, E.
De Grave, V.V. Galvita, and G.B. Marin, Mg-Fe-Al-O for advanced CO₂
to CO conversion: carbon monoxide yield vs. oxygen storage capacity. J.
Mater. Chem. A, 2015. 3(31): p. 16251-16262.