Tuning Adsorption Energies and Reaction Pathways by Alloying: PdZn versus Pd for CO₂ Hydrogenation to Methanol

Abstract

The tunability offered by alloying different elements is useful to design catalysts with greater activity, selectivity, and stability than single metals. By comparing the Pd(111) and PdZn(111) model catalysts for CO 2 hydrogenation to methanol, we show that intermetallic alloying is a possible strategy to control the reaction pathway from the tuning of adsorbate binding energies. In comparison to Pd, the strong electron-donor character of PdZn weakens the adsorption of carbon-bound species and strengthens the binding of oxygen-bound species. As a consequence, the first step of CO 2 hydrogenation more likely leads to the formate intermediate on PdZn, while the carboxyl intermediate is preferentially formed on Pd. This results in the opening of a pathway from carbon dioxide to methanol on PdZn similar to that previously proposed on Cu. These findings rationalize the superiority of PdZn over Pd for CO 2 conversion into methanol and suggest guidance for designing more efficient catalysts by promoting the proper reaction intermediates.

A Computational Study of Direct CO2 Hydrogenation to Methanol on Pd Catalysts

“…δ-) on the Pd (111) surface reported (1.09 eV) matches the work of Zhang et al (0.85 eV), though differs somewhat from the results of Brix et al (2.23 eV); we believe this difference stems from use of a physisorbed CO2 geometry , i.e. an Eley-Rideal mechanism, with a chemisorbed structure considered in our work and the calculations by Zhang et al26,50 Eact(HCOOH) is observed to decrease with increasing Esurf, i.e. follows the trend (111) > (100) > (110).The highest Eact(HCOOH) of 1.35 eV, calculated for the Pd (111) surface, is in agreement with the value of 1.13 eV reported by Brix et al Given that the adsorption energy of HCOOH on the Pd (111) surface is calculated -0.67 eV, and desorption is considered as the reverse process, the high Eact(HCOOH) observed (1.35 eV) for the Pd (111) surface suggests that HCOOH is more likely to desorb than react further.…”

“…25 As part of the reaction pathway via formate, the decomposition of H2COOH* into H2CO* and OH* was included, as previously considered for metal catalysts containing Cu, Pd and Zn. [23][24][25]50 Figure 12. The activation energies (Eact, eV) of reaction steps in the pathway for CO2 hydrogenation to methanol, presented for Pd surfaces in order of increasing Esurf ,i.e.…”

Promoting effect of PdZn alloy for selective hydrogenation of 5‐hydroxylmethylfurfural: An experimental and density functional theory study

“…[62] On the contrary, FCH 2 O displays higher thermodynamic stability on PdZn faces, which is related to the strong electron‐donating and weak electron‐accepting ability of PdZn. the mechanism is the same as that of CO 2 hydrogenation reported by Brix et al The first step of CO 2 hydrogenation more likely leads to the HCOO intermediate on PdZn(111), while the COOH intermediate is preferentially formed on Pd(111) [52]. Compared with the preferential first reaction step (FCHO + H → FCHOH) on Pd(111), the first H atom addition to form FCH 2 O on PdZn(111) and PdZn(200) surfaces shows much lower E a (0.61 and 0.43 eV) and E r (−0.38 and −0.41 eV) values, which indicates that the hydrogenation of HMF prefers to be conducted on the PdZn sites.…”

A Computational Study of Direct CO2 Hydrogenation to Methanol on Pd Catalysts

“…26 As part of the reaction pathway via formate, the decomposition of H2COOH* into H2CO* and OH* was included, as previously considered for metal catalysts containing Cu, Pd and Zn. [24][25][26]55 All TS have also been validated by vibrational analysis, displaying only one imaginary frequency each. The elementary step towards which each energy barrier refers are presented in Table 4.…”