Insights into the Influence of CeO₂ Crystal Facet on CO₂ Hydrogenation to Methanol over Pd/CeO₂ Catalysts

CeO₂ is an excellent potential material for CO₂ hydrogenation attributed to the highly tunable properties including metal–support interaction and abundant oxygen vacancy. In this work, four CeO₂ supports with structurally well-defined different shapes and crystal facets are hydrothermally prepared, and their effects on the composition of Pd species and oxygen vacancy over Pd/CeO₂ catalysts have been intensively investigated in the reduction of CO₂ to methanol. The 2Pd/CeO₂-R (rods) shows the highest concentration and number of oxygen vacancies, where the (110) facet with high surface oxygen mobility and low oxygen vacancy formation energy is exposed over the CeO₂-R surface. The oxygen mobility at the interface of (111) and (100) facets mainly observed on 2Pd/CeO₂-P (polyhedrons) is higher than the single (111) and (100) facets mainly observed on 2Pd/CeO₂-O (octahedrons) and 2Pd/CeO₂-C (cubs), respectively. The presence of Pd highly promotes the formation of oxygen vacancies by providing dissociated H atoms to facilitate the removal of surface O in ceria support under a H₂ atmosphere. Both the Pd_xCe_1–xO_δ solid solution dominated on CeO₂-R and the PdO species dominated on CeO₂-O are reduced to metallic Pd after reduction with 6–10 nm average particle size. As revealed by density functional theory (DFT) calculations, in contrast to the single Pd⁰ atom on CeO₂ and the thermodynamically most unstable Pd_xCe_1−xO_δ solid solution, the Pd⁰ nanoparticles are the most stable species under the realistic reaction conditions. The 2Pd/CeO₂-R shows the highest catalytic activity as the abundantly available oxygen vacancies function as CO₂ adsorption and activation sites. Moreover, oxygen vacancy reactivity is correlated with its formation energy. The lower formation energy facilitates the formation of oxygen vacancy; however, the reactivity of each oxygen vacancy is lower as the TOF_{oxygen vacancy} of 2Pd/CeO₂-O is 15 times as that of 2Pd/CeO₂-R. Thus, a suitable oxygen vacancy formation energy is likely favorable for enhancing CO₂ reactivity. DFT calculations indicate that the CH₃OH formation is most probably from the formate (HCOO*) pathway via the C–O bond cleavage in H₂COOH*, with the reduction of HCOO* to HCOOH* as the rate-limiting step. These results would provide experimental and theoretical insights into the rational design of an effective catalyst for CO₂ hydrogenation.