Insights into the High Activity and Methanol Selectivity of the Zn/ZrO₂ Solid Solution Catalyst for CO₂ Hydrogenation

Abstract

Selective hydrogenation of CO2 to methanol is vital for mitigating the massive CO2 emission by utilizing the captured CO2 for chemical and fuel productions. Here, the key intermediates and mechanism of CO2 hydrogenation to methanol over the Zn/ZrO2 solid solution catalyst are thoroughly investigated by density functional theory calculations. Our calculations show that CO2 is highly activated when strongly adsorbed on the surface in a carbonate-like configuration, which may be the reason for the high CO2 conversion rate in methanol synthesis. In addition, CO formation from the dissociation of CO2 or COOH* is suppressed because of the stability of carbonate or the high energy barrier of COOH* formation, respectively. When compared with the traditional bi-HCOO route, where breaking the C–O bond is predicted to be the rate-determining step (RDS) with a modest energy barrier of 1.11 eV, a novel route is found to be kinetically much more favorable with a much lower energy barrier of 0.76 eV for the RDS of bi-H2CO* → mono-H2CO*. This alternative route starts from a newly found HCO3 * species in a tetrahedral configuration with the central C atom surrounded by an H atom and three O atoms, denoted as tri-HCOO*. It can be stepwise hydrogenated to methanol through the bi-HCO*, bi-H2CO*, mono-H2CO*, and H3CO* intermediates. These theoretical predictions suggest the high activity of the carbonate species in methanol synthesis over the Zn/ZrO2 solid solution catalyst, different from that on the pure metal oxide surfaces.

Combining X-ray Diffraction and X-ray Absorption Spectroscopy to Unveil Zn Local Environment in Zn-Doped ZrO2 Catalysts

“…Among all the dopants, Zn has been recently investigated for several reactions. ,− Its ionic radius, comparable to the Zr 4+ one (0.60 vs 0.84 Å), permits Zn ions to easily dissolve in the ZrO 2 lattice. The addition of Zn 2+ likely creates oxygen vacancies, modifying oxygen mobility and tuning the surface physicochemical properties.…”

“…The addition of Zn 2+ likely creates oxygen vacancies, modifying oxygen mobility and tuning the surface physicochemical properties. In the past decade, Zn-doped ZrO 2 (ZZR) has been studied for its properties in CO/CO 2 hydrogenation ,,, and ethanol/propene dehydrogenation. ,, As H 2 can be activated through an heterolytic dissociation on ZnO, the presence of Zn–O pairs or ZnO clusters enhances the hydrogenation/dehydrogenation properties of the catalyst. In the last years, ZZR was also investigated for its catalytic activity and selectivity when used as the oxidic component forming bifunctional catalysts for the direct conversion of CO or CO 2 with H 2 to olefins and aromatics. ,,, Particularly, a balanced Zn/Zr ratio allows to strongly activate CO 2 and CO forming carbonates which are selectively hydrogenated to methanol by H 2 activated over Zn–O. ,, …”

“…The addition of Zn 2+ likely creates oxygen vacancies, modifying oxygen mobility and tuning the surface physicochemical properties. In the past decade, Zn-doped ZrO 2 (ZZR) has been studied for its properties in CO/CO 2 hydrogenation 13,18,20,21 and ethanol/propene dehydrogenation. 11,12,17 As H 2 can be activated through an heterolytic dissociation on ZnO, the presence of Zn−O pairs or ZnO clusters enhances the hydrogenation/dehydrogenation properties of the catalyst.…”

Hydrogenation of CO2 to methanol over In-doped m-ZrO2: a DFT investigation into the oxygen vacancy size-dependent reaction mechanism

“…Given that thermal reduction occurs during the experiment, it is, therefore, very difficult to realize oxygen vacancy formation on a perfect ZrO 2 surface, which is consistent with previous theoretical results. 30…”

“…42 This function has been reported to well describe the CO 2 hydrogenation for systems with oxygen vacancies. 30,43 For In and Zr, the core electrons remained frozen within the scalar-relativistic PAW approach, while 5s5p and 4d5s were treated as the valence state. An energy cut-off of 400 eV and a Gaussian smearing width of 0.05 eV were employed.…”