Is the Surface Playing a Role during Pyridine-Catalyzed CO₂ Reduction on p-GaP Photoelectrodes?

Abstract

The role of the photoelectrode surface during pyridinecatalyzed CO 2 reduction on p-GaP photocathodes is currently under debate. Understanding the extent of the photoelectrode's direct participation in the catalytic CO 2 reduction mechanism is essential to improving the design of such photoelectrochemical systems. Here, we present new theoretical results demonstrating that the previously proposed pyridinyl radical intermediate is unlikely to form and that reduction of pyridinium to adsorbed pyridine and H species remains the most favorable reduction pathway, even when accounting for the aqueous environment. Furthermore, we conclude, based on recently reported experimental evidence and our new computational results reported herein, that the mechanism of CO 2 reduction operating in this system is likely heterogeneous. We also introduce a new heterogeneous mechanism involving a recently proposed radical species, which we predict will be stable on the electrode surface and that may serve as the active catalytic species in this system.

Hydride Shuttle Formation and Reaction with CO2on GaP(110)

“…[22] They proposed that this radical is generated by ao ne-electronr eduction of PyH + in solution and that photoexcited electrons from the p-GaP photoelectrode have enough energy to promote this process. Finally,t he Musgrave-Hynes mechanismi se ntirely homogeneous, whichisi nconsistentwith our recent findings [16] combined with the experimental observations by Bocarsly andc oworkers [19] showingt hat adsorbed intermediates must be involvedi nt he CO 2 reduction mechanism. However, these two levelsl ie very close and, especially when considering the uncertainty of these computed values, we concluded that the process is likely kinetically hindered.…”

“…[19] Bocarsly and co-workerso bserved that the GaP(110) surface is more active towards CO 2 reduction than the GaP(111)s urface. [16] The hypothesis that the precursors needed for the adsorbed catalystf ormation are generated via PyH + reduction [10] is consistent with the experimental observation that an acidic pH is an essential condition for CO 2 reduction to occur. However, we found that the conduction band minima( CB min )o ft he two surfaces lie very close, with ad ifference of less than 0.1 V. [16] Therefore, we concluded that ah omogeneous mechanism cannote xplain these experimental observations, and that the mechanism must involvea dsorbed intermediates.…”

“…[14,15] Furthermore, in as eparates tudy we demonstrated that adsorbed protons generated by heterolytic water dissociation on the GaP(110) surface are highly stablea nd thusc an be furtherr educed to H À *. Such ah ypothesis is supportedf urther both by recent predictions from our group [16] and measurements by the Bocarsly group. [18] The heterogeneous mechanism for DHP*-catalyzed CO 2 reductiono np -GaP photoelectrodesp roposed by our group is consistentw iths everale xperimental and theoreticalf indings.…”

“…First, it has been observedt hat the selectivity of the CO 2 reductionp rocess, as well as the appliedp otential required for the process to occur,d epends on the electrode material, [1][2][3][4][5][6] thus supporting the hypothesis of ah eterogeneousm echanism. However, we found that the conduction band minima( CB min )o ft he two surfaces lie very close, with ad ifference of less than 0.1 V. [16] Therefore, we concluded that ah omogeneous mechanism cannote xplain these experimental observations, and that the mechanism must involvea dsorbed intermediates. [19] Bocarsly and co-workerso bserved that the GaP(110) surface is more active towards CO 2 reduction than the GaP(111)s urface.…”

“…Musgrave, Hynes, and coworkers proposed ah omogeneous mechanism in which the active catalyst is solvated DHP,f ormed from a1 -pyridinyl radical (1-PyH · )i ntermediate. [16] In addition, the alternative pathway for PyH + reduction introduced above (PyH + + e À !Py* + H*) has am uch lower reduction potential, thus suggesting thatP yH + + e À !1-PyH · is unlikely to compete with it. In ar ecent study, [16] we confirmed that GaP(110) CB min has amore negative reduction potential than PyH + + e À !1-PyH · (À1.44 V, [23] À1.44 V, [24] À1.31 V, [25] and À1.58 V [26] vs. SCE).…”