Reactivity of OH and hydride species in oxide-catalyzed
hydrogenation
reactions has attracted great interest. Herein, we report a combined
in situ spectroscopic characterization and density functional theory
(DFT) calculation study of ceria-catalyzed acetylene semihydrogenation
reaction. The ceria surface is fully hydroxylated during the adopted
reaction condition. C2H2 adsorbs molecularly
on the stoichiometric CeO2 surface and hydrogenates with
OH groups selectively to produce C2H4. Semihydrogenation
of C2H2 to C2H4 with either
OH groups or hydride species on ceria surfaces with surface oxygen
vacancies proceeds more facilely than on a stoichiometric CeO2 surface, but C2H4 adsorbs more strongly
and further hydrogenates to C2H6 more facilely;
moreover, dissociative adsorption of C2H2 to
C2H species occurs, which facilely hydrogenates with the
hydride species eventually to form C2H6 and
react with each other to produce oligomers, decreasing the catalytic
selectivity and stability, respectively. These results demonstrate
that the ceria catalyst with a stoichiometric surface is extremely
selective in catalyzing C2H2 semihydrogenation
reaction to C2H4, whereas surface oxygen vacancies
or hydride species on ceria are harmful to the catalytic performance.
Selective acetylene hydrogenation is a strongly exothermic process, easy to cause coking and metal agglomeration, and thus leads to deactivation. In this work, Pd/TiO 2 with different oxygen vacancies (V o ) were synthesized by controlling reduction temperature in 300-700 C, in which Pd/TiO 2 -HT300 (HT is reduction temperature) possessed the highest V o content. It was found highly dispersed Pd nanoparticles adjacent to more V o exhibited enhanced catalytic behavior (near 100% conversion at 55 C with 80% selectivity and turnover frequency of 0.12 s À1 ) due to hydrogen spillover generation and electron donation originating from V o sites, confirmed by in situ x-ray photoelectron spectroscopy, in situ Raman, and H 2 -temperature programmed desorption. More importantly, the increasing V o sites trap the released heat and devote to a decrease of heat accumulation over a single active Pd site, and consequently inhibit Pd agglomeration and polymerization, affirmed by high-resolution transmission electron microscopy, CO chemisorption, and thermogravimetric analysis.
The
structure sensitivity of CO2 activation in the presence
of H2 has been identified by ambient-pressure X-ray photoelectron
spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical
reaction conditions. Based on the APXPS results and computer simulations,
we propose that, around room temperature, the hydrogen-assisted activation
of CO2 is the major reaction path on Ni(111), while the
redox pathway of CO2 prevails on Ni(110). With increasing
temperature, the two activation pathways are activated in parallel.
While the Ni(111) surface is fully reduced to the metallic state at
elevated temperatures, two stable Ni oxide species can be observed
on Ni(110). Turnover frequency measurements indicate that the low-coordinated
sites on Ni(110) promote the activity and selectivity of CO2 hydrogenation to methane. Our findings provide insights into the
role of low-coordinated Ni sites in nanoparticle catalysts for CO2 methanation.
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