Controlled Photoplasmonic Enhancement of H₂ Production <i>via</i> Formic Acid Dehydrogenation by a Molecular Fe Catalyst

Abstract

Plasmonic nanoparticles (PNPs) constitute a significant category of photoresponsive materials whose exploitation in photoboosted catalysis is a forward-looking strategy. Here, it is demonstrated that photoexcited core–shell Ag 0 @SiO 2 PNPs can dramatically enhance formic acid dehydrogenation (FADH), catalyzed by the molecular catalyst [Fe(BF 4 ) 2 ·6H 2 O/P(CH 2 CH 2 PPh 2 ) 3, PP 3 ]. In the presence of photoexcited Ag 0 @SiO 2 PNPs, the optimized catalytic system [(Fe/PP 3 )/HCOOH/Ag 0 @SiO 2 / h v] achieves an almost 10-fold increase of the H 2 -gas-production rate vs [(Fe/PP 3 )/HCOOH] (173 vs 17 mL H 2 min –1, using 12.5 μmol of catalyst), while the turnover numbers (TONs) are boosted by ∼400% (35,643 vs 9615) and the turnover frequencies (TOFs) by ∼600% (17,821 h –1 vs 2885 h –1 ). Selective excitation at wavelengths (λ ex ) spanning the photoresponse profile of Ag 0 @SiO 2 NPs demonstrates that the FADH enhancement is maximal at λ ex = 405 nm, which is at the peak of the photoplasmonic response of Ag 0 @SiO 2 NPs. Monitoring of the solution potential ( E h ) under catalytic conditions reveals that the photoexcitation of Ag 0 @SiO 2 PNPs injects hot electrons, as reducing agents, into the reaction solution. Varying the SiO 2 -shell thickness of Ag 0 @SiO 2 PNPs in the range of 3–5 nm allowed control of the hot-electron injection rates and the ensuing FADH rates. The present results are discussed in the context of the catalytic cycle of the [(Fe/PP 3 )/HCOOH] system, where plasmonically generated hot electrons boost H 2 production via FADH by molecular catalysts, in distinction to the thermoplasmonic effects that seem to play a secondary role. The present H 2 -production rate data demonstrate the possibility to approach industrial-scale H 2 -production rates via FADH, using low-cost Fe-based catalysts and no sacrificial cocatalysts. We consider that the phenomenon exemplified herein for a standard molecular-catalysis system, such as [(Fe/PP 3 )/HCOOH], can be valid for many other pertinent molecular FADH catalysts.

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