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Abstract:
Methanol steam reforming (MSR) catalysts are derived from
perovskite-type oxides LaCo1-x-yPdxZnyO3 +/-delta by reductive
pretreatment. The unsubstituted LaCoO3 +/-delta (LCO) and
LaCo1-x-yPdxZnyO3 +/-delta (Co substituted with Pd and/or Zn) are
synthesized by a citrate method and characterized by different
techniques. The perovskite-type oxides exhibit a rhombohedral crystal
structure and a comparable surface area (approximate to 8.5 (+/- 2) m(2)
g(-1)). The temperature-programmed reduction (TPR) shows low (100
degrees C < T < 450 degrees C) and high (T > 450 degrees C) temperature
reduction events that correspond to partial and complete reduction of
the non-rareearth metal ions, respectively. At high temperatures, Pd-Zn
alloy nanoparticles are formed exclusively on Pd-and Zn-containing
LaCo1-x-yPdxZnyO3 +/-delta, as evident from high angular annular
dark-field scanning transmission electron microscopy (HAADF-STEM). The
CO2-selective MSR performance of the catalysts strongly depends on the
reductive pretreatment temperature, catalyst composition (i.e., the Pd :
Zn molar ratio and the degree of Co substitution) and reaction
temperature. Only LaCo1-x-yPdxZnyO3 +/-delta catalysts show a
low-temperature CO2 selectivity maximum between 225 and 250 degrees C,
while all catalysts present similar high-temperature selectivity maxima
at T > 400 degrees C. The former is missing on LCO, LaCo1-xPdxO3
+/-delta or LaCo1-yZnyO3 +/-delta. Pd-Zn nanoparticles facilitate
Zn(OH)(2) and Co(OH)(2) formation exclusively on LaCo1-x-yPdxZnyO3
+/-delta, as evident from in situ XRD under steam atmosphere. This
indicates the important role of Pd-Zn nanoparticles in the
low-temperature CO2 selectivity, which is improved from 0 to 76% at 225
degrees C on LCO and LaCo0.75Pd0.125Zn0.125O3 +/-delta, respectively.
The high-temperature CO2 selectivity is governed by the bulk catalyst
composition and the occurrence of reverse water gas shift reaction.
