Electronic Effects of Al Doping on the Mechanism of Methanol Formation on an Al Doped Cu/ZnO Interface Model

Computational techniques are used to understand the electronic doping character of Al added to the Cu/ZnO catalyst system during interfacial methanol formation. Notably, some formate species behave as intermediates, others as spectators while the methoxy intermediate is a mechanistic dead end. Water forms at a different active site than that for MeOH formation. Key differences arising from Al-doping and the least energy demanding path for methanol synthesis are discussed.

Abstract

The mechanism of CO₂ hydrogenation to methanol is modelled using plane-wave DFT applied to a representative Cu₈-ZnO(CZ) model, reported previously, with aluminium substituting a bulk Zn (= Cu/ZnO/Al₂O₃(CZA)). On CZA, CO₂ adsorption and activation are enhanced at the active Cu/ZnO interface compared to systems with a Cu-based or CZ-based interface, demonstrating Al's electronic effect. Methanol formation at CZA follows the formate path: CO₂*→ HCOO*→ H₂COO*→ H₂COOH*→ H₂CO*→ H₂COH*→ H₃COH, with small contributions from the RWGS mechanism. Methoxy's binding is enhanced, making it a dead-end and not an intermediate as on CZ. Formate intermediate at the Cu/Zn interface in CZA is electronically destabilized through Al. By contrast, other surface formates are stabilized and act as spectators. The most energy demanding step is the hydrogenation of formate to dioxomethylene (E _a = 1.08 eV) and not methoxy hydrogenation as on CZ. Multiple species are able to scavenge O* regenerating the active interfacial site. OH* was found to poison the active site, although its formation is energy demanding, making the CZA system overall more selective to MeOH than CZ. Water formation occurs on the Cu site as on the CZ system, although Zn sites can stabilize adsorbed water consistent to on experiments at CZA.

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