Design of Interfacial Sites between Cu and Amorphous ZrO₂ Dedicated to CO₂-to-Methanol Hydrogenation

Abstract

We examined the formation mechanism of active sites on Cu/ZrO2 specific toward CO2-to-methanol hydrogenation. The active sites on Cu/a-ZrO2 (a-: amorphous) were more suitable for CO2-to-methanol hydrogenation than those on Cu/t-ZrO2 (t-: tetragonal) and Cu/m-ZrO2 (m-: monoclinic). When a-ZrO2 was impregnated with a Cu­(NO3)2·3H2O solution and then calcined under air, most of the Cu species entered a-ZrO2, leading to the formation of a Cu–Zr mixed oxide (Cu a Zr1‑a O b ). The H2 reduction of the thus-formed Cu a Zr1‑a O b led to the formation of Cu nanoparticles on a-ZrO2, which can be dedicated to CO2-to-methanol hydrogenation. We concluded that the selective synthesis of Cu a Zr1‑a O b , especially amorphous Cu a Zr1‑a O b , is a key feature of the catalyst preparation. The preparation conditions of the amorphous Cu a Zr1‑a O b specific toward CO2-to-methanol hydrogenation is as follows: (i) Cu­(NO3)2·3H2O/a-ZrO2 is calcined at low temperature (350 °C in this study) and (ii) the Cu loading is low (6 and 8 wt % in this study). Via these preparation conditions, the characteristics of a-ZrO2 for the catalysts remained unchanged during the reaction at 230 °C. The latter preparation condition is related to the solubility limit of Cu species in a-ZrO2. Accordingly, we obtained the amorphous Cu a Zr1‑a O b without forming crystalline CuO particles.

Influences of particle size and crystallinity of highly loaded CuO/ZrO2 on CO2 hydrogenation to methanol

“…As shown in Figure , methanol was weakly adsorbed on the surface of Cu/ZrO 2 , prepared at P = 1 mL min −1 , which was not affected by the presence of carbonous residue. The weak adsorption of methanol can be a phenomenon characteristic of amorphous ZrO 2 . The presence of amorphous ZrO 2 in the catalyst was also suggested by the study of the ZrO 2 content (Table ).…”

“…There is a possibility that approximately 15–20 at% of Cu could be incorporated into t‐ZrO 2 lattice . In the case of the FSP‐made catalysts, such incorporation was negligible as discussed below.…”

“…The PXRD peak position of t‐ZrO 2 shifts from 30.2° to 30.5–30.7° by 15–20 at% of the Cu substitution (ca. 0.02° per Cu atomic percent) . The deviation of the t‐ZrO 2 peak position (ca.…”

“…So far, various catalysts for this reaction, such as Cu/ZrO 2 , Cu‐Ag/ZrO 2 , Cu/ZrO 2 /SiO 2 , Cu/ZnO/ZrO 2 , Cu/CeO 2 /TiO 2 , MnO x /m‐CoO x , InO 2 /ZrO 2 , Cu‐In/SiO 2 , Ni‐Ga/SiO 2 , ZnO/ZrO 2 , Pd/ZnO, Pd‐Cu/SiO 2 , and Cu/ZnO/Al 2 O 3 , have been proposed. Among them, the Cu/ZrO 2 catalyst would be a promising candidate because of its activity and selectivity for CO 2 hydrogenation to methanol (S. Tada et al, unpublished data) …”

“…In CO 2 hydrogenation to methanol (CO 2 + 3H 2 → CH 3 OH + H 2 O) by Cu/ZrO 2 catalyst, the interface of Cu‐ZrO 2 is considered the active site . In contrast, the metallic Cu surface is known as an active center for direct reduction of CO 2 to undesirable CO via reverse water gas shift reaction (CO 2 + H 2 → CO + H 2 O) . Thus, smaller Cu particles on ZrO 2 are more favorable for selective methanol generation via CO 2 hydrogenation because of the higher ratio of the interface of Cu‐ZrO 2 sites to the metallic Cu sites .…”