Operating Conditions and Composition Effect on the Hydrogenation of Carbon Dioxide Performed over CuO/ZnO/Al2O3 Catalysts

Abstract

A series of catalysts constituted of mixed copper and zinc oxides supported on alumina were prepared by co-precipitation method. The cooper content was in the 10-90 wt.% range. Their catalytic behavior in the hydrogenation of carbon dioxide to methanol was investigated at high pressure (up to 75 bars). The catalysts were characterized by elemental analysis, N2-adsorption, N2O-chemisorptions, and X-ray diffraction (XRD). The catalysts showed a clear activity in the hydrogenation reaction that could be correlated to the surface area of the metallic copper and to the reaction pressure. The CuO/ZnO/Al2O3 catalyst with a Cu/Zn/Al weight ratio of 60/30/10, exhibits the highest carbon dioxide conversion and methanol selectivity. Finally, a mechanism pathway has been proposed on copper active sites of (Cu 0 /Cu I ) oxidation state.

Differences in Deterioration Behaviors of Cu/ZnO/Al2O3 Catalysts with Different Cu Contents toward Hydrogenation of CO and CO2

“… 35 , 36 Furthermore, the peak appeared at ≈78 eV in Figure 2 e,f for all catalysts, corresponding to the characteristic peak of Al 2p spectra (Al 3+δ species). 17 , 37 , 38 This is shifted from low to high binding energy when compared to the original peak for pure Al 2 O 3 (Al 3+ species, 74.1 eV), according to previous literature studies. 37 , 38 As reported by Li et al 38 and Fu et al., 37 they explained that the shifted higher binding energy occurred from the chemical state of some Al 3+ species that increased to the state of Al 3+δ species to maintain the loss of electron because of Cu interacting with Al 2 O 3 (Al–O–Cu bond).…”

“…Furthermore, these results indicated that the hydrogen consumption of spent CZA catalysts in both hydrogenation reactions decreased with the increase in the number of Zn contents in catalyst composition [sCZA-L(CO) and sCZA-L(CO 2 )], contributing to the dwindling of CO and CO 2 hydrogenation ability. According to the literature studies, 1 , 4 , 17 , 47 , 48 the stability of Cu 0 sites during the CO and CO 2 hydrogenation is the main key factor for H 2 dissociation to adsorb feed molecules in the product formation step, resulting in the increase in the catalytic ability of Cu-based catalysts. In addition, the high Zn contents in the catalyst result in the formation of more surface Zn sites and blocking of the formation of Cu-ZnO interfaces, which acted as a source of the H atom storage for the intermediate molecules in the hydrogenation process.…”

“…It revealed that the CZA-H catalyst exhibited high catalytic activity in both CO and CO 2 hydrogenation reactions due to the high Cu content and high Cu dispersion (as seen in Table 3 ), which retarded the carbon formation (anti-coking ability). 17 , 28 In addition, the deactivation of CZA catalysts under both CO and CO 2 hydrogenation reactions was observed, in which the spent catalysts after CO 2 hydrogenation [sCZA-L(CO 2 ) and sCZA-H(CO 2 )] presented large amounts of carbon deposition when compared to those after CO hydrogenation [sCZA-L(CO) and sCZA-H(CO)]. This is probably due to CO 2 hydrogenation producing more CO and H 2 O [reverse water-gas-shift (RWGS)], which deactivates the catalysts.…”

“…Our previous study determined the catalytic activity of Cu/ZnO/Al 2 O 3 catalysts with different amounts of copper loading, including CZA-L (Cu:Zn = 0.8) and CZA-H (Cu:Zn = 3.0), under atmospheric pressure at 250 °C. These conditions have attracted attention for industrial processes due to their advantage of low energy consumption and low cost, which are facile to control upon CO and CO 2 hydrogenation to methanol. , It is well known that the high reaction pressure for methanol synthesis from CO or CO 2 hydrogenation results in extremely high energy consumption and high cost of a mechanical process; therefore, more instrument systems must be installed for safety control to prevent the hazards in industrial processes such as process damage, environmental damage, loss of human life, loss of cost, and explosion . Noticeably, the methanol synthesis under low pressure is an alternative route to decrease the cost of manufacturing in terms of the cost of production and technical processes. , Both catalysts were chosen for catalytic performance screening with time on stream, and the summary of previous results is listed in Table .…”

“…Basically, the active components are very important for the catalytic performance of catalysts, so the change of active components by loss of active species (such as the blockage active species and catalyst sintering) will directly influence the catalytic activity for catalytic reaction processes. In all these recent reports, there are many literature studies that investigated the activity of Cu/ZnO/Al 2 O 3 via CO and CO 2 hydrogenation to methanol, whereas there are only few reports that discuss in depth the mechanism of deactivation of Cu/ZnO/Al 2 O 3 catalysts. ,, However, active metal (Cu crystallites) sintering, agglomeration of ZnO species, oxidation of surface metallic Cu, byproduct deposition, and carbon deposition over the active component are usually the significant key factors of Cu/ZnO/Al 2 O 3 catalyst deactivation according to the report of Fichtl et al . and Peláez et al that discussed the phenomenon of the mechanism of catalytic deactivation.…”