The use of methanol as a fuel and chemical feedstock could become very important in the development of a more sustainable society if methanol could be efficiently obtained from the direct reduction of CO2 using solar-generated hydrogen. If hydrogen production is to be decentralized, small-scale CO2 reduction devices are required that operate at low pressures. Here, we report the discovery of a Ni-Ga catalyst that reduces CO2 to methanol at ambient pressure. The catalyst was identified through a descriptor-based analysis of the process and the use of computational methods to identify Ni-Ga intermetallic compounds as stable candidates with good activity. We synthesized and tested a series of catalysts and found that Ni5Ga3 is particularly active and selective. Comparison with conventional Cu/ZnO/Al2O3 catalysts revealed the same or better methanol synthesis activity, as well as considerably lower production of CO. We suggest that this is a first step towards the development of small-scale low-pressure devices for CO2 reduction to methanol.
A nanodispersed
intermetallic GaPd2/SiO2 catalyst
is prepared by simple impregnation of industrially relevant high-surface-area
SiO2 with Pd and Ga nitrates, followed by drying, calcination,
and reduction in hydrogen. The catalyst is tested for CO2 hydrogenation to methanol at ambient pressure, revealing that the
intrinsic activity of the GaPd2/SiO2 is higher
than that of the conventional Cu/ZnO/Al2O3,
while the production of the undesired CO is lower. A combination of
complementary in situ and ex situ techniques are used to investigate
the GaPd2/SiO2 catalyst. In situ X-ray diffraction
and in situ extended X-ray absorption fine structure spectroscopy
show that the GaPd2 intermetallic phase is formed upon
activation of the catalyst via reduction and remains stable during
CO2 hydrogenation. Identical location–transmission
electron microscopy images acquired ex situ (i.e., micrographs of
exactly the same catalyst area recorded at the different steps of
activation and reaction procedure) show that nanoparticle size and
dispersion are defined upon calcination with no significant changes
observed after reduction and methanol synthesis. Similar conclusions
can be drawn from electron diffraction patterns and images acquired
using environmental TEM (ETEM), indicating that ETEM results are representative
for the catalyst treated at ambient pressure. The chemical composition
and the crystalline structure of the nanoparticles are identified
by scanning TEM energy dispersive X-ray spectroscopy, selected area
electron diffraction, and atomically resolved TEM images.
The CO 2 hydrogenation to methanol is efficiently catalyzed at ambient pressure by nanodispersed intermetallic GaPd 2 /SiO 2 catalysts prepared by incipient wetness impregnation. Here we optimize the catalyst in terms of metal content and reduction temperature in relation to its catalytic activity. We find that the intrinsic activity is higher for the GaPd 2 /SiO 2 catalyst with a metal loading of 13 wt.% compared to catalysts with 23 wt.% and 7 wt.%, indicating that there is an optimum particle size for the reaction of around 8 nm. The highest catalytic activity is measured on catalysts reduced at 550°C. To unravel the formation of the active phase, we studied calcined GaPd 2 /SiO 2 catalysts with 23 wt.% and 13 wt.% using a combination of in situ techniques: X-ray diffraction (XRD), X-ray absorption near edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS). We find that the catalyst with higher metal content reduces to metallic Pd in a mixture of H 2 /Ar at room temperature, while the catalyst with lower metal content retains a mixture of PdO and Pd up to 140°C. Both catalysts form the GaPd 2 phase above 300°C, albeit the fraction of crystalline intermediate Pd nanoparticles of the catalyst with higher metal loading reduces at higher temperature. In the final state, the catalyst with higher metal loading contains a fraction of unalloyed metallic Pd, while the catalyst with lower metal loading is phase pure. We discuss the alloying mechanism leading to the catalyst active phase formation selecting three temperatures: 25°C, 320°C and 550°C.
Metallic alloy nanoparticles (NPs) are synthesized in situ in an environmental transmission electron microscope. Atomic level characterization of the formed alloy NPs is carried out at synthesis conditions by use of high-resolution transmission electron microscopy, electron diffraction and electron energy-loss spectroscopy.
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