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.

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The mechanistic cause of enhancement of C 3 H 8 -SCR activity by addition of H 2 over Ag/Al 2 O 3 was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C 3 H 8 + O 2 , nitrates were mainly formed on Ag/Al 2 O 3 . An addition of H 2 into a C 3 H 8 -SCR atmosphere increased the concentration of surface acetate significantly, but decreased the concentration of surface nitrates. Formation and consumption rates of acetate and nitrates were estimated with transient in-situ IR measurement. By the addition of H 2 , both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C 3 H 8 + O 2 and acetate in a flow of NO + O 2 were also increased by the addition of H 2 . From a comparison between the evolutions of adsorbed species (nitrate and acetate) and gaseous species (NO and C 3 H 8 ), it was clarified that the NO reduction activity is controlled by partial oxidation of C 3 H 8 to mainly surface acetate. The addition of H 2 results in remarkable promotion of partial oxidation of C 3 H 8 to mainly surface acetate, which is the rate-determining step of C 3 H 8 -SCR in the absence of H 2 .

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We prepared Cu/a-ZrO2 (a-ZrO2: amorphous ZrO2), Cu/m-ZrO2 (m-ZrO2: monoclinic ZrO2), Cu/a-ZrO2/KIT-6, and Cu/t-ZrO2/KIT-6 (t-ZrO2: tetragonal ZrO2) by a simple impregnation method and examined the effect of the ZrO2 phase on CO2-to-methanol hydrogenation. We discovered a-ZrO2-containing catalysts with high activity and selectivity in CO2-to-methanol hydrogenation. Next, we focused on Cu species formation on the above-described catalysts. While pure CuO was observed on Cu/m-ZrO2 and Cu/t-ZrO2/KIT-6, copper-zirconium mixed oxide (Cu x Zr y O z ), not pure CuO, was formed on Cu/a-ZrO2 and Cu/a-ZrO2/KIT-6, as evidenced by X-ray absorption spectroscopy (XAS) and the powder color. After reducing a-ZrO2-containing catalysts with H2 at 300 °C, we observed highly dispersed Cu nanoparticles in close contact with a-ZrO2 (or Cu x Zr y O z ). In addition, methanol vapor sorption revealed that methanol adsorbed more weakly on a-ZrO2 than on m-ZrO2. Therefore, the high dispersion of Cu species and weak adsorption of methanol led to high activity and selectivity in CO2-to-methanol hydrogenation.