a b s t r a c tUtilizing ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the surface segregation and oxidation of Pt 3 Ni(1 1 1) alloys are investigated as a function of temperature and oxygen pressure. The in situ AP-XPS measurements of oxygen oxidation process show that the Pt "skin" surface is not stable under the exposure of oxygen pressure of 100 mTorr at room temperature. As the temperature and pressure are elevated, the formations of Ni 2 O 3 , NiO x , and NiO are observed on surface while Pt atom starts to reduce its adsorbed oxygen, which is a clear sign of surface segregation of Ni to surface. Upon the evacuation of oxygen gas, i.e. ultrahigh vacuum condition, both of NiO x and NiO oxide get reduced and Ni 2 O 3 remains on the surface. The DFT calculation is employed to explain the formation of surface oxides under oxidation condition.
While furfural has great potential as a platform chemical through diverse conversion pathways, 2‐methylfuran is one of the key components as a fuel additive and a precursor of derived biofuels. However, catalyst design to control reactivity of active species to furfural has been challenging. Herein, we demonstrate that a mesoporous Cu−Al2O3 catalyst prepared by solvent‐deficient precipitation shows the enhancement of furfural hydrodeoxygenation performance. Not only is the unique pore structure provided, but also the synergistic effects of improved Cu accessibility, co‐existence of Cu0/Cu+ states, and strong metal‐support interaction contribute to the superior activity and stability. Additionally, essential parameters for the synthesis and the test reaction are studied to improve the selectivity to 2‐methylfuran. The employed catalyst preparation method opens a new channel for tailoring the bifunctionality of supported Cu catalysts.
Heterogeneous catalysts of bifunctionality offer improved process efficiency and enhanced product yield in multistep chemical transformations conducted in a consecutive manner. However, this has seldom been realized in coprecipitated catalysts owing to the complexity of the synthesis. By adjusting the synthesis protocol of an industrial methanol‐synthesis catalyst as a model system, herein we demonstrate the bifunctional effect of Al2O3/Cu/ZnO catalysts to enable the direct production of dimethyl ether from synthesis gas with yields that are 2–3‐fold higher than those obtained with their conventional counterparts, with a greater number of accessible Cu surface atoms and more Al2O3 acid sites existing at the external particle surface. This is achieved by preparing the fully developed Cu,Zn precursor of a specific structure with surface‐decorated Al species. Our approach paves the way towards the rational design of multicomponent precipitated catalysts with tunable bifunctionality for practical cascade reactions.
In recent years, methanol has attracted much attention since it can be cleanly manufactured by the combined use of atmospheric CO2 recycling and water splitting via renewable energy. For the concept of "methanol economy", an active methanol synthesis catalyst should be prepared in a sophisticated manner rather than by empirical optimization approach. Even though Cu/ZnO-based catalysts prepared by coprecipitation are well known and have been extensively investigated even for a century, fundamental understanding on the precipitation chemistry and catalyst nanostructure has recently been achieved due to complexity of the necessary preparation steps such as precipitation, ageing, filtering, washing, drying, calcination and reduction. Herein we review the recent reports regarding the effects of various synthesis variables in each step on the physicochemical properties of materials in precursor, calcined and reduced states. The relationship between these characteristics and the catalytic performance will also be discussed because many variables in each step strongly influence the final catalytic activity, called "chemical memory". All discussion focuses on how to prepare a highly active Cu/ZnO-based catalyst for methanol synthesis. Furthermore, the preparation strategy we deliver here would be utilized for designing other coprecipitation-derived supported metal or metal oxide catalysts.
Bifunctional Al2O3/Cu/ZnO catalysts with Al composition of between 30 mol% and 80 mol% were prepared by sequential precipitation (SP) for the conversion of CO2 into dimethyl ether (DME). In the SP synthesis, the concentration of a precipitation agent managed to be high enough to induce the complete precipitation of Al3+. The prepared precipitates were composed of zincian malachite and amorphous AlO(OH). Furthermore, the calcined mixed metal oxide materials of 60% and 80% Al exhibited a higher acidity than commercial Al2O3 and the H2-reduced catalysts showed the similar Cu dispersion of 6%–7% at all Cu loadings. In the activity test at 573 K and 50 bar, the SP-derived catalyst of 80% Al (SP-80) displayed the best performance corresponding to CO2 conversion of 25% and DME selectivity of 75% that are close to equilibrium values. In order to overcome the thermodynamic limitation, a dual-bed catalyst system was made up of SP-80 in the first layer and zeolite ferrierite in the next. This approach enabled DME selectivity to be enhanced to 90% while CO2 conversion increased a little. Consequently, the studied catalyst system based on the SP-derived catalysts can contribute greatly to selective DME production from CO2.
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