Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2 O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu-ZnO-Al2 O3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In2 O3 -based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.
Methanol synthesis by CO 2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstocka nd mitigating global warming.H owever,t here still is al acko fe fficient catalysts for such alternative processes. Herein, we unveil the high activity,1 00 %s electivity,a nd remarkable stability for 1000 ho ns tream of In 2 O 3 supported on ZrO 2 under industrially relevant conditions.T his strongly contrasts to the benchmark Cu-ZnO-Al 2 O 3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In 2 O 3 -based materials points towards am echanism rooted in the creation and annihilation of oxygen vacancies as active sites,w hose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier.T hese results constitute ap romising basis for the design of ap rospective technology for sustainable methanol production.Methanol is akey building block in the chemical industry, [1] with prospects as asustainable energy carrier if its production is accomplished from CO 2 (captured from large-point emitters) and H 2 (retrieved from renewable sources). [2] This application demands novel catalysts as the ternary Cu-ZnO-Al 2 O 3 system currently employed for methanol synthesis from mixed syngas (CO/CO 2 /H 2 )e xhibits limited activity in CO 2 hydrogenation, because of the inhibiting effect of the water byproduct, [3] low selectivity,owing to its significant activity in the parasitic reverse water-gas shift (RWGS) reaction, [4] and insufficient stability,d ue to water-induced sintering of the active phase. [5] Furthermore,t he intricate network of syner-gistic structural and electronic effects between its components hampers the rational optimization of this material. [4a, 6] Among other catalysts studied, [7] only Cu-ZnO-Ga 2 O 3 /SiO 2 and LaCr 0.5 Cu 0.5 O 3 displayed improved methanol formation rates and high selectivities (up to 99.5 %), but their scalability and long-term stability have not been assessed. Recent experiments on Cu/CeO x /TiO 2 model surfaces [8] also showed promising results,but no attempt has been made to translate this material into ap ractically relevant polycrystalline solid.In our quest for as uitable catalyst, we were intrigued by the much simpler In 2 O 3 system. This reducible oxide is commonly used together with SnO 2 as av ery stable conductive transparent layer in organic light-emitting diodes and thin-film transistors. [9] Moreover,i th as demonstrated high activity and selectivity in multiple catalytic transformations involving CO 2 ,i ncluding electrochemical conversion into formic acid, [10] photocatalytic reduction to CO, [11] and methanol steam reforming. [12] Recently,d ensity functional theory (DFT) studies on CO 2 hydrogenation over non-defective [13] and defective [14] In 2 O 3 (110) surfaces suggested that methanol is the most favorable product and that the reaction follows am echanism comprising the cyclic c...
CO hydrogenation, CO 2 hydrogenation, and water-gas shift (WGS) reactions have been simultaneously investigated over industry-like catalysts based on Cu-ZnO-Al 2 O 3 , under methanol synthesis conditions (513 K, 5.0 MPa). For this, a novel methodology has been applied: the concentration of carbon dioxide in the syngas feed was consecutively increased (R = CO 2 :(CO + CO 2 ) = 0-100) resulting in a volcano-type plot of the rate of methanol formation and forming a hysteresis loop when decreasing the CO 2 concentration again. H 2 O co-feeding experiments revealed that the enhancement of activity can be correlated with the WGS activity linking both hydrogenation paths of CO and CO 2 . On the other hand, excessive amounts of surface hydroxyls seem to inhibit methanol production, explaining the drop in activity at high CO 2 concentrations. An investigation of the catalytic performance was accompanied by an extensive characterisation of the fresh and used catalytic materials by X-ray diffraction, temperature-programmed reduction by H 2 , N 2 O pulse chemisorption, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. It was shown that the copper surface area affects the CO 2 hydrogenation; however, this parameter is unambiguously not the key descriptor for CO 2 -promoted methanol synthesis, which is a consequence of the synergistic interaction of zinc oxide and copper. This structural feature is further promoted by Al 2 O 3 through stabilisation of the surface.The position of the activity maximum is determined by the surface ratio Cu : Zn. The hysteresis behaviour is a result of the continuous decrease of Cu dispersion and the fixation of copper species in its monovalent oxidation state, both detrimental for CO 2 hydrogenation. CO hydrogenation is strongly affected by the Cu : Zn bulk ratio and thus the reducibility of the catalyst. These facts could be substantiated by the use of impregnated model catalysts.
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