Effect of Surface Hydroxyls on CO₂Hydrogenation Over Cu/γ-Al₂O₃Catalyst: A Theoretical Study

Abstract

Catalytic hydrogenation of CO2 to methanol is a promising way to recycle and utilize CO2. In this study, the elementary steps leading to HCOO and CO formation have been explored to identify hydroxylation effect of the oxide support on the selectivity in CO2 hydrogenation on Cu/γ-Al2O3 catalyst by the density functional theory (DFT) slab calculations. Two models: Cu4 cluster supported on the dry γ-Al2O3(110) surface, D(Cu4), and on the hydroxylated γ-Al2O3(110) surface, H(Cu4), have been used to model Cu/γ-Al2O3. On D(Cu4), the formation of HCOO is preferred kinetically. On H(Cu4), HCOO formation is still kinetically favorable. These results indicate that the hydroxylation of γ-Al2O3 support cannot alter the pathway of CO2 hydrogenation forming the dominate product HCOO, and ultimately, the selectivity of CO2 hydrogenation for HCOO formation on Cu/γ-Al2O3 is higher, which supports the experimental fact that Al2O3-supported Cu catalyst is widely used to synthesize methanol by CO2 hydrogenation.

CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

“…In fact, introducing surface Zr IV or Ti IV sites on SiO 2 at the interface with Cu nanoparticles via surface organometallic chemistry (SOMC) increases the CH 3 OH activity and selectivity,f urther supporting the importance of Lewis acid sites in this reaction. [13,14] Regarding CO,i th as mainly been proposed to be formed on copper-based catalysts via two different mechanisms:T he so-called redox and formate pathways.T he former consists of the direct conversion of CO 2 to CO (that may involve M À COOH intermediates) and the subsequent reduction of Cu 2 Osurface species to Cu 0 metal forming H 2 O, while the latter involves the decomposition of formate into CO. [9,[15][16][17][18][19][20][21][22][23][24][25][26][27] Additionally,the formation of not only CO but also CH 3 OH is often proposed to be related to the presence of oxygen defect sites originating from the oxide support. [28][29][30][31][32][33][34][35][36] ForC u/ZnO/Al 2 O 3 ,t he origin of the promotional effect towards selective CH 3 OH formation is still amatter of debate.…”

Study on carbon deposition associated with catalytic CH4 reforming by using density functional theory

“…Thus, computational chemistry as a catalyst evaluation tool has become increasingly important. It has been used to study the reaction mechanism and predict the optimal composite catalysts during reactions [23][24][25]. For instance, density functional theory (DFT) has been successfully used to predict the performance of Ni-Pt-Pt (1 1 1) catalyst with one monolayer of Ni atoms residing on a Pt (1 1 1) substrate for ammonia decomposition based on the change of nitrogen binding energies [26].…”

Interface effects for the hydrogenation of CO2 on Pt4/γ-Al2O3

“…Moreover, we have found an artful detail that in the conversion of CO 2 to CO, the H adatom is migrated to the adsorbed CO 2 , similar to that of Ni [8] and Cu−based [12] catalysts. However, the conversion of CO 2 to HCOO is due to the migration of the adsorbed CO 2 molecule from the metal−support interface to the H adatom, which requires much more energy than the former.…”

“…The Ru-(CO) ad species was further hydrogenated by co-adsorbed or gaseous hydrogen to form methane. Pan et al [8] and Zhang et al [12] studied the effect of surface hydroxyls on selective hydrogenation of CO 2 on Ni/␥-Al 2 O 3 and Cu/␥-Al 2 O 3 catalysts, respectively, also indicating that HCOO and CO are crucial intermediates to methanol and methane. All these studies suggest that the products of the initial steps in CO 2 hydrogenation determine the final products.…”