Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes

Shenzhen Xu; Lesheng Li; Emily Ann Carter

doi:10.1021/jacs.8b09946

Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes

Shenzhen Xu
, Lesheng Li
, Emily Ann Carter

Research output: Contribution to journal › Article › peer-review

21 Scopus citations

Abstract

Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH ^- ) has been postulated to enable selective CO ₂ photoelectroreduction to CH ₃ OH. This hypothesis is supported by recent estimates of sufficient 2-PyH ^- ∗ lifetimes and low barriers for hydride transfer (HT) to CO ₂ . However, the complete mechanism for reducing CO ₂ to CH ₃ OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO ₂ reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH∗ reacting with 2-PyH ^- , a new catalytic role for the surface. HT to HCOOH∗ produces CH ₂ (OH) ₂ , but subsequent HT to CH ₂ (OH) ₂ forming CH ₃ OH is hindered. However, CH ₂ O, dehydrated CH ₂ (OH) ₂ , easily reacts with 2-PyH ^- , producing CH ₃ OH. Further reduction of CH ₃ OH to CH ₄ via HT from 2-PyH ^- ∗ encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH∗ explains why the primary CO ₂ reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH ^- ∗ (vs its protonation product DHP), the relative dominance of CH ₂ (OH) ₂ over CH ₂ O, and the required desorption of CH ₂ (OH) ₂ ∗ are the most likely limiting factors, explaining the low yield of CH ₃ OH observed experimentally.

Original language	English (US)
Pages (from-to)	16749-16757
Number of pages	9
Journal	Journal of the American Chemical Society
Volume	140
Issue number	48
DOIs	https://doi.org/10.1021/jacs.8b09946
State	Published - Dec 5 2018

All Science Journal Classification (ASJC) codes

General Chemistry
Biochemistry
Catalysis
Colloid and Surface Chemistry

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title = "Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes",

abstract = " Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH - ) has been postulated to enable selective CO 2 photoelectroreduction to CH 3 OH. This hypothesis is supported by recent estimates of sufficient 2-PyH - ∗ lifetimes and low barriers for hydride transfer (HT) to CO 2 . However, the complete mechanism for reducing CO 2 to CH 3 OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO 2 reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH∗ reacting with 2-PyH - , a new catalytic role for the surface. HT to HCOOH∗ produces CH 2 (OH) 2 , but subsequent HT to CH 2 (OH) 2 forming CH 3 OH is hindered. However, CH 2 O, dehydrated CH 2 (OH) 2 , easily reacts with 2-PyH - , producing CH 3 OH. Further reduction of CH 3 OH to CH 4 via HT from 2-PyH - ∗ encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH∗ explains why the primary CO 2 reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH - ∗ (vs its protonation product DHP), the relative dominance of CH 2 (OH) 2 over CH 2 O, and the required desorption of CH 2 (OH) 2 ∗ are the most likely limiting factors, explaining the low yield of CH 3 OH observed experimentally.",

author = "Shenzhen Xu and Lesheng Li and Carter, \{Emily Ann\}",

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AU - Li, Lesheng

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Y1 - 2018/12/5

N2 - Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH - ) has been postulated to enable selective CO 2 photoelectroreduction to CH 3 OH. This hypothesis is supported by recent estimates of sufficient 2-PyH - ∗ lifetimes and low barriers for hydride transfer (HT) to CO 2 . However, the complete mechanism for reducing CO 2 to CH 3 OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO 2 reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH∗ reacting with 2-PyH - , a new catalytic role for the surface. HT to HCOOH∗ produces CH 2 (OH) 2 , but subsequent HT to CH 2 (OH) 2 forming CH 3 OH is hindered. However, CH 2 O, dehydrated CH 2 (OH) 2 , easily reacts with 2-PyH - , producing CH 3 OH. Further reduction of CH 3 OH to CH 4 via HT from 2-PyH - ∗ encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH∗ explains why the primary CO 2 reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH - ∗ (vs its protonation product DHP), the relative dominance of CH 2 (OH) 2 over CH 2 O, and the required desorption of CH 2 (OH) 2 ∗ are the most likely limiting factors, explaining the low yield of CH 3 OH observed experimentally.

AB - Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH - ) has been postulated to enable selective CO 2 photoelectroreduction to CH 3 OH. This hypothesis is supported by recent estimates of sufficient 2-PyH - ∗ lifetimes and low barriers for hydride transfer (HT) to CO 2 . However, the complete mechanism for reducing CO 2 to CH 3 OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO 2 reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH∗ reacting with 2-PyH - , a new catalytic role for the surface. HT to HCOOH∗ produces CH 2 (OH) 2 , but subsequent HT to CH 2 (OH) 2 forming CH 3 OH is hindered. However, CH 2 O, dehydrated CH 2 (OH) 2 , easily reacts with 2-PyH - , producing CH 3 OH. Further reduction of CH 3 OH to CH 4 via HT from 2-PyH - ∗ encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH∗ explains why the primary CO 2 reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH - ∗ (vs its protonation product DHP), the relative dominance of CH 2 (OH) 2 over CH 2 O, and the required desorption of CH 2 (OH) 2 ∗ are the most likely limiting factors, explaining the low yield of CH 3 OH observed experimentally.

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Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes

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