Insights into the Influence of CeO₂Crystal Facet on CO₂Hydrogenation to Methanol over Pd/CeO₂Catalysts

Abstract

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 x Ce1–x Oδ 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 x Ce1−x Oδ 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 TOFoxygen 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.

Multi-dimensional designer catalysts for negative emissions science (NES): bridging the gap between synthesis, simulations, and analysis

“…Beyond simply controlling local coordination via compositional modification in flexible 3D chalcogenide frameworks, it is also understood—albeit underexplored—that even morphological changes in 3D systems can introduce profound changes in locally observed electric fields for CO 2 reduction intermediates ( Gao et al., 2012 , 2020 ). This strongly motivates the exploration of additional synthetic techniques that will enable control over particle morphology on well-defined chalcogenide compositions ( Perryman et al., 2020a ), similar to the established hydrothermal and solvothermal routes that have been employed for metal oxide nanoparticle catalysts for thermal CO 2 reduction ( Jiang et al., 2020a ; Liang et al., 2014 ).…”

Facet-Dependent Reactivity of Ceria Nanoparticles Exemplified by CeO2-Based Transition Metal Catalysts: A Critical Review

“…Similarly, Cu/CeO 2 -NR shows the highest rWGS activity at atmospheric pressure as compared to nanospheres, since the formation of active intermediates is facilitated on the {110} facets of nanorods [84]. Moreover, among different morphologies (rods, cubes, octahedrons and polyhedrons), Pd supported on ceria nanorods has shown the highest activity and space-time yield of methanol, due to their abundance in oxygen vacancies [149]. In this point, it should be mentioned that CO 2 hydrogenation catalysts of high activity and selectivity can be also obtained through the development of multicomponent systems [150,151].…”

Probing the morphological effects of ReOx/CeO2 catalysts on the CO2 hydrogenation reaction

“…They also found that the catalytic activity was mainly determined by the CO 2 activation on the oxygen vacancies on the CeO 2 surface, and the lower oxygen vacancy formation energy of Pd/CeO 2 -R could give a higher concentration of oxygen vacancies. 14 Classically, CO 2 hydrogenation over CeO 2 -based catalysts is considered to follow a formate pathway, whereby CO 2 is adsorbed with H 2 to form formate and further hydrogenated to CO or CH 4 . [13][14][15] The catalytic performance can be influenced by adsorbed intermediate species, and their adsorption strength or sites.…”

“…14 Classically, CO 2 hydrogenation over CeO 2 -based catalysts is considered to follow a formate pathway, whereby CO 2 is adsorbed with H 2 to form formate and further hydrogenated to CO or CH 4 . [13][14][15] The catalytic performance can be influenced by adsorbed intermediate species, and their adsorption strength or sites. Density functional theory (DFT) calculations found that oxygen vacancies were crucial for the adsorption and stabilization of reactive species, 16 and CO 2 was readily adsorbed as carbonate (CO 3 2− ) near oxygen vacancy sites on the CeO 2 (111) surface.…”