1 research outputs found
Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO<sub>2</sub> Hydrogenation
Highly
monodispersed CuO nanoparticles (NPs) were synthesized via
continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts
for CO<sub>2</sub> hydrogenation. The catalytic behavior of unsupported
11 nm sized nanoparticles from the same batch was characterized by
diffuse reflectance infrared fourier transform spectroscopy (DRIFTS),
extended X-ray absorption fine structure (EXAFS), X-ray diffraction
(XRD), and catalytic testing, under CO<sub>2</sub>/H<sub>2</sub> in
the temperature range 25–500 °C in consistent experimental
conditions. This was done to highlight the relationship among structural
evolution, surface products, and reaction yields; the experimental
results were compared with modeling predictions based on density functional
theory (DFT) simulations of the CuO system. In situ DRIFTS revealed
the formation of surface formate species at temperatures as low as
70 °C. DFT calculations of CO<sub>2</sub> hydrogenation on the
CuO surface suggested that hydrogenation reduced the CuO surface to
Cu<sub>2</sub>O, which facilitated the formation of formate. In situ
EXAFS supported a strong correlation between the Cu<sub>2</sub>O phase
fraction and the formate peak intensity, with the maxima corresponding
to where Cu<sub>2</sub>O was the only detectable phase at 170 °C,
before the onset of reduction to Cu at 190 °C. The concurrent
phase and crystallite size evolution were monitored by in situ XRD,
which suggested that the CuO NPs were stable in size before the onset
of reduction, with smaller Cu<sub>2</sub>O crystallites being observed
from 130 °C. Further reduction to Cu from 190 °C was followed
by a rapid decrease of surface formate and the detection of adsorbed
CO from 250 °C; these results are in agreement with heterogeneous
catalytic tests where surface CO was observed over the same temperature
range. Furthermore, CH<sub>4</sub> was detected in correspondence
with the decomposition of formate and formation of the Cu phase, with
a maximum conversion rate of 2.8% measured at 470 °C (on completely
reduced copper), supporting the indication of independent reaction
pathways for the conversion of CO<sub>2</sub> to CH<sub>4</sub> and
CO that was suggested by catalytic tests. The resulting Cu NPs had
a final crystallite size of ca. 44 nm at 500 °C and retained
a significantly active surface