Communications
Mg1.3AlOx.[38–40] In the first step of the reaction, this structure is
supposed to accelerate the CO2 dissociation at the metal-
support interface as well as the CH4 cracking on Ni atoms,[41–42]
thereby releasing the first CO and H2 molecules.[11] With La2O3/
La2O2CO3 enriched around these metal atoms (Figure 2),
defective sites (e.g. oxygen vacancies) are generated (Fig-
ure 1c), promoting dissociative CO2 adsorption which subse-
quently forms surface oxygen species.[41,43–46] The increased
availability of these oxygen species transforms carbonaceous
intermediates from CH4-rich mixture and releases second CO
and H2 (Table S. 1). This behavior enhances both the reaction
rate and the carbon resistance of the catalyst even at low CO2
partial pressure. Such solid solution was recently also supposed
to be beneficial for CH4-rich DRM by suppressing carbon
formation.[47] However, concerning the loading of Ni and the
catalyst productivity in that investigation (Table S. 3), the
catalyst in the present study is superior. It should be noted that
the mentioned NiOÀ MgO interaction is expected in Ni/MgO as
well. However, due to low Ni loading, this sample showed
almost no activity due to low Ni surface concentration.
terms of active metal price and loading, productivity and
stability against coking, La.Ni(CA)/Mg1.3AlOx.1000 is one of the
most promising candidates for DRM under CH4-rich conditions
(Table S. 3).[47,49–51]
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We conclude that catalysts supported on Mg1.3AlOx.1000
possess improved coking resistance without losing the DRM
activity and are therefore suitable for the reaction with CH4-rich
feed. Citric acid induces a high dispersion already during the
catalyst preparation. The NiOÀ MgO solid solution domains
excellently stabilize small Ni particles throughout all catalyst
pre-treatment steps and DRM. Highly dispersed Ni activates CO2
as an oxidant for carbon gasification, thereby reducing the
coking rate in CH4-rich DRM even at low CO2 partial pressure. La
generates additional oxygen vacancies that help to activate CO2
as well. La.Ni(CA)/Mg1.3AlOx.1000 appears to be the best catalyst
as it has high and stable activity over at least 8 h on stream and
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°
its coking rate is lowest at both 630 and 750 C. Moreover, this
catalyst exposes quite stable activity in CH4-rich DRM over
100 h on stream with little coking. Such exceptional perform-
ance is certainly ruled by high Ni dispersion and enhanced
reducibility.
La.Ni(CA)/Mg1.3AlOx.1000 was employed in a long-term test
over 100 h at typical conditions to evaluate the application
potential (Figure 5). In known literature,[9] a long-term CH4-rich
DRM run with such high feed rate was not reported.
Experimental Section
Complete avoidance of carbon accumulation at high CH4/
CO2 ratio with Ni catalysts was previously considered
infeasible.[48] However, over 100 h of CH4-rich DRM in the
present study, the H2/CO ratio remained constantly near unity
whereas the conversions slightly decreased, but low carbon
amount (~5 wt%) was found on the spent sample. This fraction
was predictably higher than the values in DRM tests over 8 h,
but not proportional to total run time. Interestingly, while
carbon accumulation on spent La.Ni(CA)/Mg1.3AlOx.1000 was
observed in STEM annular bright field (ABF) image after 8 h on
stream, carbon was hardly found on the spent sample after
100 h (Figure S. 13), highlighting the exceptionally stable coking
resistance due to gasification. This behavior is in accordance
with the stable dispersion of small Ni particles (5–10 nm) which
are also partially attached to the support (Figure S. 13). Besides,
STEM-HAADF images prove that the mentioned preferred
localization of Ni in the MgO-enriched structures is preserved
during the reaction (Figures S. 16 and S. 17). These factors are
crucial for both carbon removal by CO2 and stably high DRM
performance with high H2 yield of La.Ni(CA)/Mg1.3AlOx.1000. In
MgÀ Al mixed oxide supports were prepared from MgÀ Al hydro-
talcite (Pural MG50, Sasol). The default precursor Mg1.3AlOx was
°
obtained by calcining the MgÀ Al hydrotalcite at 550 C. This
°
material was thermally pre-treated at 1000 C with a rate of 2 K/min
to prepare Mg1.3AlOx.1000 support.
In order to prepare the final catalysts, both supports were treated
with Ni(NO3)2 ·6H2O (99%, Alfa Aesar) and La(NO3)3 ·6H2O (99%,
ABCR GmbH) by wet impregnation (nominal Ni content 2.5 wt%).
Citric acid (>99%, Alfa Aesar) was added simultaneously in some
cases. The molar ratio of La and Ni was set to 0.8, and the CA/metal
molar ratio was fixed at 1.5. The calculated amounts of Ni, La
precursors and CA were dissolved in deionized water and the
°
solution was stirred for 4 h at 50 C. The MgÀ Al supports were then
°
added and the slurry was stirred at 60 C for 15 h. Water was
gradually removed by a rotary evaporator for 4 h and the samples
°
°
were dried overnight at 120 C and calcined at 400 C for 3 h and
then at 800 C for 6 h both in air with a rate of 2 K/min. MgO
(FLUKA) as well as its corresponding Ni-loaded samples
La.Ni(CA)/MgO and Ni/MgO served as reference materials. Pure NiO
°
°
was prepared by calcining Ni(NO3)2 ·6H2O at 800 C.
XRD powder patterns were recorded on a Panalytical X’Pert
diffractometer equipped with a Xcelerator detector using automatic
divergence slits and Cu Kα1/α2 radiation (40 kV, 40 mA; λ=
0.15406 nm, 0.154443 nm). Cu beta-radiation was excluded using a
nickel filter foil. The samples were mounted on silicon zero
background holders. The obtained intensities were converted from
°
automatic to fixed divergence slits (0.25 ) for further analysis. Peak
positions and profile were fitted with Pseudo-Voigt function using
the HighScore Plus software package (Panalytical). Phase identifica-
tion was done by using the PDF-2 database of the International
Center of Diffraction Data (ICDD).
The low-temperature N2 adsorption was performed on a Micro-
°
meritics ASAP 2010 apparatus at À 196 C. The samples were
°
degassed at 200 C in vacuum for 4 h before the analysis.
Figure 5. CH4, CO2 conversions and H2/CO ratio in long-term CH4-rich DRM
°
with La.Ni(CA)/Mg1.3AlOx.1000 (750 C, 1 bar, CH4/CO2 =2, GHSV=
°
170 L/(gcat ×h)). Catalyst was pre-reduced in situ at 700 C for 1.5 h.
ChemCatChem 2020, 12, 1–8
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© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA