Synergistic Cyanamide Functionalization and Charge-Induced Activation of Nickel/Carbon Nitride for Enhanced Selective Photoreforming of Ethanol

Abstract

Photoreforming is a promising alternative to water splitting for H 2 generation due to the favorable organic oxidation half-reaction and the potential to simultaneously produce solar fuel and value-added chemicals. Recently, carbon nitride has received significant attention as an inexpensive photocatalyst for the photoreforming process. However, the application of carbon nitride continues to be hampered by its poor photocatalytic performance. Herein, we report for the first time a synergistic modification of an in situ photodeposited Ni cocatalyst on carbon nitride via cyanamide functionalization and solid/liquid interfacial charge-induced activation using excess Ni 2+ ions. Synergism between the cyanamide functionalization and charge-induced activation by the excess Ni 2+ ions invokes enhanced activity, selectivity, and stability during ethanol photoreforming. A H 2 evolution rate of 2.32 mmol h −1 g −1 in conjunction with an acetaldehyde production rate of 2.54 mmol h −1 g −1 was attained for the Ni/NCN-CN. The H 2 evolution rate and elevated acetaldehyde selectivity (above 98%) remained consistent under prolonged light illumination. To understand the origin of the complementary promotional effects, the contributions of cyanamide groups and excess Ni 2+ ions to selective ethanol photoreforming are decoupled and systematically investigated. The cyanamide functionality on carbon nitride was found to promote hole scavenging for the ethanol oxidation reaction, thereby enabling effective electron transfer to the Ni cocatalyst for H 2 evolution. Concomitantly, excess Ni 2+ ions remaining in solution created a positively charged environment on the photocatalyst surface, which improved charge carrier utilization and ethanol adsorption. The work highlights the importance of both carbon nitride functionality and charge on the photocatalyst surface in developing a selective photocatalytic reforming system.

Improved carrier dynamics in nickel/urea-functionalized carbon nitride for ethanol photoreforming

“…The ensuing performance of Ni/ NCN-CN (201.6 µmol H 2 h −1 g −1 and 352.5 µmol acetaldehyde h −1 g −1 ) is higher than for Ni/p-CN. The improved photocatalytic activity of Ni/NCN-CN compared to Ni/p-CN is driven by the role of cyanamide groups in invoking effective photogenerated hole transfer to the ethanol, as discussed in our previous study [21]. This leads to a higher electron uptake by the Ni species for self-reduction into Ni deposits and subsequently, for H 2 generation.…”

“…Moreover, cyanamide groups in NCN-CN were found to be a better ligand for Ni attachment on the surface compared to amino groups in p-CN, providing a stronger metal-support interaction. This is evidenced by the higher Ni loading of Ni/NCN-CN (5.87 wt %) compared to Ni/p-CN (0.97 wt %) [21,25].…”

“…Photoreforming tests were conducted in a spiral photoreactor as illustrated in Fig. S1 [21,26] (illumination area ~ 218.4 cm 2 ). A 100 mg load of photocatalyst was dispersed in 100 mL of 10 vol % ethanol and 1 mM Ni(NO 3 ) 2 .6H 2 O aqueous solution under ultrasonication.…”

“…Among them, surface functionalization has been demonstrated as a promising technique to suppress electron-hole recombination in carbon nitride by inducing an uneven charge distribution. Cyanamide-functionalized carbon nitride, for example, was reported to be more effective in transferring the holes for organic oxidation, thus leading to an enhanced electron uptake for H 2 production [21,22]. More recently, urea-functionalized carbon nitride derived from cyanamide-terminated carbon nitride has been found to further improve photocatalytic H 2 evolution performance although it is less studied compared to other functionalized carbon nitrides [23].…”

“…Alternatively, Ni has been shown as a promising and comparatively inexpensive metal cocatalyst for both H 2 production and selective alcohol oxidation. On a cyanamide-functionalized carbon nitride, in situ photodeposited Ni cocatalyst in the presence of excess Ni 2+ ions was previously reported to give an H 2 production rate of 2.32 mmol h −1 g −1 and an acetaldehyde production rate of 2.54 mmol h −1 g −1 with negligible CO 2 production [21]. However, slow Ni photodeposition was observed as indicated by a 4 h lag time at the start of the reaction due to the strong electron-storing property of cyanamide-functionalized carbon nitride and the energetically challenging Ni 2+ reduction process [21].…”