Martina Lessio

What Is the Role of Pyridinium in Pyridine-Catalyzed CO2 Reduction on p-GaP Photocathodes?

Experimental evidence suggests that pyridinium plays an important role in photocatalytic CO2 reduction on p-GaP photoelectrodes. Pyridinium reduction to pyridinyl has been previously proposed as an essential mechanistic step for this reaction. However, theoretical calculations suggest that this step is not feasible in solution. Here, cluster models and accurate periodic boundary condition calculations are used to determine whether such a reduction step could occur by transfer of photoexcited electrons from the p-GaP photocathode and whether this transfer could be catalyzed by pyridinium adsorption on the p-GaP surface. It is found that both the transfer of photoexcited electrons to pyridinium and pyridinium adsorption are not energetically favored, thus making very unlikely pyridinium reduction to the pyridinyl radical and the proposed mechanisms requiring this reduction step. Given this conclusion, an alternative and energetically viable pathway for pyridinium reduction on p-GaP photoelectrodes is proposed. This pathway leads to the formation of adsorbed species that could react to form adsorbed dihydropyridine, which was proposed previously to play the role of the active catalyst in this system.

Stability of surface protons in pyridine-catalyzed CO2 reduction at p-GaP photoelectrodes

Adsorbed protons that develop hydride character have been proposed to play a role in the mechanism of CO2 reduction catalyzed by pyridine on GaP photoelectrodes. Investigating their stability represents an important step towards vetting this mechanism. In this contribution, the relative stability of the adsorbed protons is determined using cluster models with dispersion-corrected density functional theory and continuum solvation. Proton acidity constants computed under typical experimental conditions are compared to the acidity constants of other relevant species. The adsorbed protons are predicted to be very stable, suggesting that they will be present on the surface and available to be reduced to surface hydrides that could possibly react with adsorbed pyridine to form adsorbed dihydropyridine, a previously proposed co-catalyst. However, the high stability of such protons also suggests that the surface does not represent a significant proton source; as a consequence, protons required in the proposed mechanism must be provided by a different source such as the acidified aqueous solution in contact with the electrode surface.

Cobalt (II) oxide and nickel (II) oxide alloys as potential intermediate-band semiconductors: A theoretical study

Solar cells based on single pn junctions, employing single-gap semiconductors can ideally achieve efficiencies as high as 34%. Developing solar cells based on intermediate-band semiconductors (IBSCs), which can absorb light across multiple band gaps, is a possible way to defy this theoretical limit and achieve efficiencies as high as 60%. Here, we use first principles quantum mechanics methods and introduce CoO and Co0.25Ni0.75O as possible IBSCs. We show that the conduction band in both of these materials is divided into two distinct bands separated by a band gap. We further show that the lower conduction band (i.e., the intermediate band) is wider in Co0.25Ni0.75O compared with CoO. This should enhance light absorption from the valence band edge to the intermediate band, making Co0.25Ni0.75O more appropriate for use as an IBSC. Our findings provide the basis for future attempts to partially populate the intermediate band and to reduce the lower band gap in Co0.25Ni0.75O in order to enhance the potential of this material for use in IBSC solar cell technologies. Furthermore, with proper identification of heterojunctions and dopants, CoO and Co0.25Ni0.75O could be used in multi-color light emitting diode and laser technologies.

The Role of Surface-Bound Dihydropyridine Analogues in Pyridine-Catalyzed CO2 Reduction over Semiconductor Photoelectrodes

We propose a general reaction mechanism for the pyridine (Py)-catalyzed reduction of CO2 over GaP(111), CdTe(111), and CuInS2(112) photoelectrode surfaces. This mechanism proceeds via formation of a surface-bound dihydropyridine (DHP) analogue, which is a newly postulated intermediate in the Py-catalyzed mechanism. Using density functional theory, we calculate the standard reduction potential related to the formation of the DHP analogue, which demonstrates that it is thermodynamically feasible to form this intermediate on all three investigated electrode surfaces under photoelectrochemical conditions. Hydride transfer barriers from the intermediate to CO2 demonstrate that the surface-bound DHP analogue is as effective at reducing CO2 to HCOO– as the DHP(aq) molecule in solution. This intermediate is predicted to be both stable and active on many varying electrodes, therefore pointing to a mechanism that can be generalized across a variety of semiconductor surfaces, and explains the observed electrode dependence of the photocatalysis. Design principles that emerge are also outlined.

Hydride Shuttle Formation and Reaction with CO2 on GaP(110)

Adsorbed hydrogenated N-heterocycles have been proposed as co-catalysts in the mechanism of pyridine (Py)-catalyzed CO2 reduction over semiconductor photoelectrodes. Initially, adsorbed dihydropyridine (DHP*) was hypothesized to catalyze CO2 reduction through hydride and proton transfer. Formation of DHP* itself, by surface hydride transfer, indeed any hydride transfer away from the surface, was found to be kinetically hindered. Consequently, adsorbed deprotonated dihydropyridine (2-PyH- *) was then proposed as a more likely catalytic intermediate because its formation, by transfer of a solvated proton and two electrons from the surface to adsorbed Py, is predicted to be thermodynamically favored on various semiconductor electrode surfaces active for CO2 reduction, namely GaP(111), CdTe(111), and CuInS2 (112). Furthermore, this species was found to be a better hydride donor for CO2 reduction than is DHP*. Density functional theory was used to investigate various aspects of 2-PyH- * formation and its reaction with CO2 on GaP(110), a surface found experimentally to be more active than GaP(111). 2-PyH- * formation was established to also be thermodynamically viable on this surface under illumination. The full energetics of CO2 reduction through hydride transfer from 2-PyH- * were then investigated and compared to the analogous hydride transfer from DHP*. 2-PyH- * was again found to be a better hydride donor for CO2 reduction. Because of these positive results, full energetics of 2-PyH- * formation were investigated and this process was found to be kinetically feasible on the illuminated GaP(110) surface. Overall, the results presented in this contribution support the hypothesis of 2-PyH- *-catalyzed CO2 reduction on p-GaP electrodes.