The Catalyst Review November 2019 - 17

EXPERIMENTAL
desorption of H2O molecular and the modification of
ZnO on the surface of zeolite occurs, further resulting
in the reconstruction of surface Zn species and
accompanied by the H2 dissociation on the ZnO cluster
and the formation of BAS. Therefore, the acid sites are
redistributed with the Zn trans-formation. At a higher
temperature, ZnOZn2+ is partially reduced to Zn and
the isolated Zn2+ ions, which has a strong interaction
with the oxygen present in the framework. Hence,
the transformation of Zn complexes such as ZnO,
ZnOH+, and Zn+, with the aid of H2, can be expressed by
Scheme 1. Source: Gao J, Wei C, Dong M, et al. (2019).
ChemCatChem, 11: 3892-3902.

Figure 1. The selectivity for aromatic
compounds as a function of the amount of
ZnOH+ in Zn-containing HZSM-5 zeolites.

Scheme 1. The evolution of Zn species on
an MFI-type zeolite in an H2 atmosphere.

Fischer−Tropsch Synthesis to Olefins: Catalytic Performance and Structure Evolution of
Co2C-Based Catalysts under a CO2 Environment...
The conversion of syngas has been generally considered to be a promising process for producing clean fuels and value-added
chemicals. However, the syngas feedstock may contain substantial amounts of CO2, which may have a detrimental impact on catalyst
performance, especially for the Co-based Fischer−Tropsch (FT) reactions. Herein, the authors describe experiments undertaken
to elucidate the effects of CO2 on the catalytic performance and structural evolution of CO2 C-based catalysts under FTO reaction
conditions. The catalyst precursor, CoMn composite oxide with a Co/Mn molar ratio of 1/2, was synthesized by coprecipitation.
The structural evolution and the changes in the surrounding micro-environment of the active site were characterized by combining
techniques such as X-ray diffraction (ex-situ XRD and in situ XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and
temperature-programmed desorption of CO/CO2 (CO/CO2-TPD). Table 1 summarizes the catalytic results of CO hydrogenation over
the CoMn catalyst in the presence of CO2 with different contents in the feed. As shown in entry 1, the CoMn catalyst exhibited a
promising catalytic performance with
Table 1. Catalytic results obtained in the presence of CO2 with different contents in the feeda
high olefin selectivity (70.7 C%) and low
CH4 selectivity (3.1 C%). Notably, the total
selectivity to value-added chemicals,
including olefins and oxygenates, reached
88.4 C%. Once syngas with 11.7 vol % of
CO2 was introduced into the system (Table
1, entry 2), the CO conversion slightly
decreased from 30.2 C% to 28.6 C%, and
it seemed that the olefin selectivity was
Figure 1. TEM and HRTEM images of the spent CoMn catalysts under different feed gas atmospheres:
(a1−a3) H2/CO feed; (b1−b3) H2/CO/CO2(24.6) feed; (c1−c3) H2/CO/CO2(39.4) feed; (d1−d3) H2/CO2 feed.
not much affected, though the O/P ratio
dropped significantly. The products for CO2
hydrogenation, however, were dominated
by the methane slate. Increasing the
CO2 content in the syngas feed led to
reductions in the activity, olefin selectivity,
and O/P ratio, while the methane fraction
increased significantly. The Na-promoted
Co2C nanoprisms (Figure 1), serving as
the active phase for the FTO reaction,
remained stable in the presence of syngas.
From DRIFTS spectroscopy, weak linearly and bridge adsorbed CO molecules were observed once the temperature reached 250°C
in a flow of CO-containing gas, and the CO2 environment could inhibit CO adsorption over the Co2C surface. The strong adsorption
strength of CO2 on the catalyst-free site reduced the CO coverage and created an H2/CO surface ratio higher than that without CO2
addition. The changes in surface microenvironment gradually altered the Co2C morphology from nanoprisms with exposed facets of
(101) and (020) to nanospheres with (111) as the dominant facet, and some Co2C even decomposed into metallic Co. The as-obtained
results were not conducive to the adsorption of surface intermediates and could decrease the chain growth probability, resulting in
the product distribution shifting toward a lower carbon number. The competitive adsorption between CO and CO2 on the Co2C-based
catalyst under the CO2 environment was suggested to contribute to the changes in the structure of the active sites and the catalytic
behaviors. Source: Lin T, Gong K, Wang C, et al. (2019). ACS Catal., 9: 9554−9567.
The Catalyst Review

November 2019

The Catalyst Review November 2019

Table of Contents for the Digital Edition of The Catalyst Review November 2019