Tailoring of Hydrotalcite-Derived Cu-Based Catalysts for CO2 Hydrogenation to Methanol

Abstract

Ternary Cu x Zn y Al z catalysts were prepared using the hydrotalcite (HT) method. The influence of the atomic x:y:z ratio on the physico-chemical and catalytic properties under CO 2 hydrogenation conditions was probed. The characterization data of the investigated catalysts were obtained by XRF, XRD, BET, TPR, CO 2 -TPD, N 2 O chemisorption, SEM, and TEM techniques. In the "dried" catalyst, the typical structure of a hydrotalcite phase was observed. Although the calcination and subsequent reduction treatments determined a clear loss of the hydrotalcite structure, the pristine phase addressed the achievement of peculiar physico-chemical properties, also affecting the catalytic activity. Textural and surface effects induced by the zinc concentration conferred a very interesting catalyst performance, with a methanol space time yield (STY) higher than that of commercial systems operated under the same experimental conditions. The peculiar behavior of the hydrotalcite-like samples was related to a high dispersion of the active phase, with metallic copper sites homogeneously distributed among the oxide species, thereby ensuring a suitable activation of H 2 and CO 2 reactants for a superior methanol production.Catalysts 2019, 9, 1058 2 of 15 hydroxycarbonate precursors [8][9][10]. On this account, studies on binary Cu x Zn y preparations enabled a deep understanding of the more complex ternary (Cu/Zn/Al) systems. Therefore, in binary preparations, typical crystalline phases range from Cu-rich to Zn-rich compositions, including malachite Cu 2 (OH) 2 CO 3 , zincian malachite (Cu x Zn y ) 2 (OH) 2 CO 3 , aurichalcite (Cu x Zn y ) 5 -(OH) 6 (CO 3 ) 2 , and hydrozincite Zn 5 (OH) 6 (CO 3 ) 2 . In particular, zincian malachite is currently considered to be the optimum precursor for the methanol synthesis catalyst, which facilitates formation of a highly porous meso-structure [11]. Considering that the maximum amount of Zn which can be substituted into the malachite lattice is 27 at.%, the optimum Cu:Zn molar ratio is close to 2:1 [12]. Higher Zn contents are desirable to facilitate further dilution of the Cu component, thereby enhancing metal dispersion, although this can result in a lower crystallinity of the zincian malachite phase. The final thermal decomposition of the hydroxycarbonates leads the formation of an intimate mixture of oxides, while an in situ reduction step is required to obtain the active catalyst.Recent approaches to synthesize methanol via catalytic hydrogenation of CO 2 , as the key for an anthropogenic chemical carbon cycle along with the need to reduce the economic impact of the conventional high-pressure synthesis of methanol via syngas, now address the research efforts toward novel preparation methods suitable for obtaining catalytic systems characterized by high copper dispersion and surface basicity to enhance CO 2 adsorption [13][14][15], as the competitive reaction pathway of reverse water gas shift (RWGS) leads to a loss of methanol production [16,17]. Therefore, appropriate adsorption amount a...

Enhanced methanol production over non-promoted Cu-MgO-Al2O3 materials with ex-solved 2 nm Cu particles: Insights from an Operando spectroscopic study

“…In this study, the molar ratio Cu 2+ /Mg 2+ /Al 3+ has been fixed to 6/74/20, which, based on our previous work,26 results in materials of large surface area (SBET 200 m 2 /g), exhibiting high metal dispersion and moderate basic sites. These features, that are critical for an enhanced CO2 hydrogenation activity and methanol selectivity, are found in the range of previously reported catalysts24,34 and that of a commercial like Cu/ZnO/Al2O3 sample synthetized as reference catalyst (see Section 4 and 5 of SI). The co-precipitated Cu-Mg-Al-hydrotalcite catalysts were first calcined at 550 °C under an air atmosphere, and then reduced in H2 at two temperatures: 230 and 450 °C.…”

Tuning the intermediate reaction barriers by a CuPd catalyst to improve the selectivity of CO2 electroreduction to C2 products

“…S6). The main CO2 desorption peak of the Cu catalyst was located at 296 °C [60], while that of the Pd catalyst was positioned at 608 °C [61]. In contrast, three main CO2 desorption peaks at 288, 355, and 598 °C were observed for the CuPd(100) interface catalyst.…”

Ex-LDH-Based Catalysts for CO2 Conversion to Methanol and Dimethyl Ether

“…As a result, considerable attention has been recently paid to hydrotalcite-like compounds as catalyst precursors with the general formula of [M 2+ 1-x M 3+ x (OH) 2 ] x+ (A n− ) x/n •mH 2 O. Such layered double hydroxide (LDH) materials are characterized by a homogeneous dispersion of metal cations at an atomic level, high stability against sintering, high specific surface area, and appropriate basic properties [31,32]. However, to the best of the authors' knowledge, very few papers dealing with the use of redox catalysts obtained by calcination of LDH systems (ex-LDH) for the CO 2 hydrogenation to dimethyl ether have been published so far.…”