The issue of CO₂, that causing global environmental change has become a worldwide concern. Meanwhile CO₂ is a sustainable carbon resource. It is highly desired to develop technologies to convert CO₂ into valuable chemicals. CO₂ hydrogenation to methanol is one of the promise processes to realize the technologies. Methanol is not only an excellent fuel, but also can be transformed to olefins and other high-value added chemicals. It should be emphasized that the needed hydrogen can be produced from water using any renewable or atomic energy source (Fig. 1)[1].

Methanol synthesis via CO hydrogenation has been industrialized for about 50 years using CuZnOAl₂O₃ catalyst, which also seems like the best catalyst for methanol synthesis for CO₂ hydrogenation. However, methanol selectivity on CuZnOAl₂O₃ from CO hydrogenation is near 99%, while that from CO₂ hydrogenation is less than 60% caused by reverse water gas shift reaction (RWGS). The even more severe problem is the rapid deactivation caused by produced water, which accelerates the sintering of Cu active component during the CO₂ hydrogenation. Therefore, new catalyst is needed to develop to realize the process of CO₂ hydrogenation to methanol with high selectivity of methanol.

Much progress has been made in the development of supported metal catalysts for CO₂ hydrogenation, such as Cu/ZnO/ZrO₂[2], Pd/ZnO [3], "georgeite" Cu [4], Cu(Au)/CeO_x/TiO₂ [5, 6], Ni-Ga [7] and MnO_x/Co₃O₄ [8] catalysts. However, the selectivity towards methanol is lower than 60% under their reported conditions.

Recently, Prof Can Li's Group at Dalian Institute of Chemical Physics presents a binary metal oxide, ZnO-ZrO₂ solid solution catalyst [9], which can achieve methanol selectivity up to 86%-91% with CO₂ single-pass conversion over 10% under the reaction conditions of 5.0 MPa, 24000 mL/(g h), H₂/CO₂ =3/1-4/1, 320-315 ℃ (Fig. 2A).

In their experiments, ZnO phase was not observed in XRD when adding ZnO (5-33%) into ZrO₂, but leads to the phase change of ZrO₂ from monoclinic to tetragonal. The (011) of ZrO₂ shifts to a higher angle when Zn concentration is increased, indicating that ZnO-ZrO₂ is in a solid solution state with Zn incorporated into the ZrO₂ lattice matrix when ZnO contents in the range below 50% (Fig. 2B-D). Their CO₂-TPD experiments show that most of the CO₂ adsorbed by 13%ZnO-ZrO₂ is on Zr sites, however, H₂-D₂ experiments suggest that there is a strong synergetic effect in the H₂ activation between the two sites, Zn and Zr. The solid solution structure affords dual active sites for H₂ activation by Zn sites and CO₂ activation by its neighboring Zr sites respectively, and are responsible for the excellent catalytic performance. The ZnO-ZrO₂ catalyst was also evaluated for CO + H₂. Besides methanol as the major product, some additional products including DME and methane were detected. The space time yield (STY) of methanol on the 13% ZnO-ZrO₂ catalyst from CO₂ hydrogenation is 2.5 times of that from CO hydrogenation at their optimized temperatures for methanol production. These facts indicate that the ZnO-ZrO₂ solid solution catalyst is especially active for CO₂ hydrogenation to methanol.

There is a current debate about the mechanism of CO₂ hydrogenation [10, 11]. Wang et al. [9] propose a mechanism of the direct hydrogenation of CO₂ likely via a formate intermediate, rather than consecutive RWGS and CO hydrogenation on ZnO-ZrO₂ solid solution catalyst. HCOO* and H₃CO* species were observed and identified by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Their further isotopic experiments demonstrate that the surface HCOO* and H₃CO* species can be hydrogenated to methanol (Fig. 2E-F). Furthermore, their DFT calculation results suggest that CO₂ hydrogenation to methanol on the surface of ZnO-ZrO₂ is through the formate pathway, which is in line with the results of experiment.

An important work in heterogeneous catalysis is to bridge the gap between model and applied catalysts especially relative to DFT calculation. In Wang et al.' work [9], the DFT modeling is based on their structure characterization, the intermediates considered in the pathway of DFT is based on the IR spectrum. The work of Wang et al. gives us an encouraging example. However, CO₂ hydrogenation to methanol is carried out under high pressure, while IR test and DFT calculation are preceded under atmosphere. In addition, the concentration varying regular of HCOO* and H₃CO* species is not in line with the methanol varying regular. Further study should be done to confirm whether there are other intermediates for CO₂ hydrogenation to methanol. If we continue to do research in heterogeneous catalysis with a holistic approach involving model materials for structural definition, theoretical modeling for micro kinetic function, and real catalysis data for the integration of high performance data, there is a good chance that this and other problems will be solved in the near future.

The ZnO-ZrO₂ catalyst reported by Wang et al. shows no deactivation in CO₂ hydrogenation and no deterioration in methanol selectivity for over 500 h on stream at least. The further thermal stability test shows that no deactivation is observed after annealing the catalyst at 400 ℃ for 24 h. Furthermore, the catalyst shows the resistance to sulfur-containing molecules. The sulfur-containing molecules are always present in CO₂ sources from flue gas produced from coal or biomass burning. So the high stability of the catalyst toward the sulfur-containing molecules directly decreases the purification cost of the feed gas and results in a superior performance of solid solution catalyst comparing to supported metal catalysts.

The work is of interest to both industry and academia. The high selectivity and stability of the ZnO-ZrO₂ catalyst show potential application in carbon capture and utilization (CCU). The dual active sites nature of the solid solution catalyst offers a new opportunity in fine-tuning catalytic activity and selectivity of a wide range of catalysis.