(425h) Highly Effective and Economically Viable Catalysts for Gas Phase CO2 Upgrading

16.0pt;line-height:106%">Highly Effective and Economically Viable Catalysts for
Gas Phase CO2 Upgrading

E.
le Saché¹, L. Pastor-Pérez^1,2, T. R. Reina¹

normal">¹ Department of Chemical and Process Engineering,
Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2
7XH, United Kingdom.

normal">² Laboratorio de Materiales Avanzados, Departamento de
Química Inorgánica - Instituto Universitario de Materiales de Alicante,
Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain.

normal">

Introduction

margin-bottom:6.0pt;margin-left:0cm;mso-pagination:none">CO₂ conversion
into valuable products is the most efficient way to reduce the CO₂
fingerprint. Among the viable alternatives, dry reforming of methane, DRM (CH₄+
CO₂ ↔ 2H₂ + 2CO, ΔH°₂₉₈ = 247.3
kJ.mol^-1) is an efficient route to convert CO₂ into
syngas. mso-bidi-font-weight:bold">Thermodynamically DRM requires high reaction
temperatures to achieve maximum CH₄/CO₂ conversions
introducing deactivation due to sintering of the metallic phase. Moreover,
carbon deposition inevitably occurs due to the Boudouard reaction and CH₄
decomposition. Thus, there is a need to develop a thermally stable and catalyst
that will resist deactivation due to carbon deposition and sintering [1].
Pyrochlores are mixed oxides of general formula: A₂B₂O₇.
The A-site typically represents a large rare-earth trivalent metal such as La
and a tetravalent transition metal of smaller diameter such as Zr occupies the
B-site. The substitution of active metals in the lattice can lead to
catalytically active materials with enhanced inherent oxygen mobility thus
leading to sintering resistant and carbon formation robust catalysts for
reforming [1]. The present study focuses on the synthesis, characterisation and
catalytic activity testing of Ni-substituted lanthanum zirconates pyrochlores
for DRM. Specifically, 2, 5 and 10 wt.% of Ni are substituted on the B site of
the La₂Zr₂O₇ pyrochlore leading to a new
generation of highly active catalysts for CO₂ upgrading [2].

Results
and Discussion

12.0pt;font-family:" times new roman>The
catalytic behavior of the catalysts over time at 650 °C are shown in Figure normal"> " times new roman>1. The un-doped pyrochlore LZ and the 2% doped catalyst LNZ2 do not show
any activity in DRM. 10.0pt;font-family:" times new roman>The 5 wt.% font-family:" times new roman>doped catalyst
shows activity but rapidly deactivate mso-bidi-font-size:10.0pt;font-family:" times new roman en-gb>s. On the other hand, the 10% doped catalyst displays
very good performance both in terms of catalytic activity and stability. LNZ10
achieves a conversion of 79% for CO₂ and 65% for CH₄ and
reaches the steady state in less than 1 hour. No deactivation is observed after
30 hours on stream. A stability test was conducted to check the behaviour of
the 10% Ni-doped pyrochlore over a period of two weeks. Despite the reaction
and deactivation mechanism, LNZ10 has demonstrated excellent stability since it
was able to maintain outstanding levels of CO₂/CH₄
conversions in continuous operation during two weeks with less than 10%
conversion loss.

Figure
2 represents the XRD
patterns of the 10% Ni pyrochlore-based catalyst as prepared, after reduction
under hydrogen, after DRM reaction for 30 hours and 360 hours. Although the
pyrochlore phase remained intact confirming the high thermal stability of this
material, small peaks centred at 2θ = 44.4° and 51.7° corresponding to
metallic Ni phase appeared gradually after reaction.  No Ni was detected before reaction nor after
reduction. An elbow appeared after 30h of reaction and a clear peak of Ni was
distinguished after 360 h of reaction, suggesting the exsolution of Ni from the
pyrochlore to the surface of the catalyst during reaction [3]. Carbon
deposition was investigated by TEM. A TEM image of LNZ10 after 360 h of reaction
is shown in Figure 3. After 360 h of
reaction a significant amount of carbon nanotubes is found. Interestingly the
growth of CNTs starts from the interface between the metallic particle and the
catalyst and separates the nickel from the surface as it grows. The CNTs tend
to grow longer, rather than expanding on the surface of the catalyst, allowing
the active sites to remain accessible and functional.

Conclusions

A novel series of Nickel based catalysts was
prepared, characterised and tested for DRM. Nickel was stabilised within a
thermally stable oxide structure, to prevent Ni from sintering and therefore
from forming large clusters that favours carbon formation. XRD of the calcined
samples confirmed the formation of the pyrochlore phase as well as Ni substitution
in the pyrochlore. The 5 and 10% Ni doped pyrochlore were found to be active
for DRM. LNZ5 deactivated quickly whereas LNZ10 displayed outstanding catalytic
activity and stability over a long term stability test of 360 hours. Structural
analysis was conducted on the catalyst after 360 hours of reaction using XRD,
revealing that the pyrochlore structure remained intact during reaction and
that Ni was exsolved to the surface during reaction. TEM revealed the formation
of MWCNTs however they had a limited impact on the catalyst’s activity proving
the robustness of the 10% doped pyrochlore.

References

justify;line-height:normal"> 12.0pt">[1] D. Pakhare, J. Spivey, Chem. Soc. Rev., 43 (2014) 7813-7837.

justify;line-height:normal"> 12.0pt">[2] T.R. Reina, E. le Saché, D. Watson, L. Pastor Perez, A. Sepulveda
Escribano, Catalysts for the reforming of gaseous mixtures, patent application
WO2018167467/ PCT/GB2018/050621

justify;line-height:normal"> 12.0pt">[3] O. Kwon, S. Sengodan, K. Kim, G. Kim, H.Y. Jeong, J. Shin, Y.-W.
Ju, J.W. Han, G. Kim, Nature Communications, 8 (2017) 15967.

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