Size‐ and Shape‐Dependent Activity of Metal Nanoparticles as Hydrogen‐Evolution Catalysts: Mechanistic Insights into Photocatalytic Hydrogen Evolution

Abstract

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen-evolution system composed of the 9-mesityl-10-methylacridinium ion (Acr(+)-Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one-electron-reduced species of Acr(+)-Mes (Acr·-Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen-evolution rate was virtually the same as the rate of electron transfer from Acr·-Mes to PtNPs. The rate constant of electron transfer (k(et)) increased linearly with increasing proton concentration. When H(+) was replaced by D(+), the inverse kinetic isotope effect was observed for the electron-transfer rate constant (k(et)(H)/k(et)(D)=0.47). The linear dependence of k(et) on proton concentration together with the observed inverse kinetic isotope effect suggests that proton-coupled electron transfer from Acr·-Mes to PtNPs to form the Pt-H bond is the rate-determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen-evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr·-Mes to FeNPs.

Proposed Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation at a Noninnocent Mononuclear Metal–Thiolate

“…30 A similar inverse value observed by Fukuzumi was attributed to rate-determining metal-hydride formation via proton coupled electron transfer. 31 The relatively high KIE values for ReL 3 as compared to the inverse KIE observed for metal-hydrides by Gray, indicate a clearly different mechanism for H 2 evolution. Markedly, Fukuzumi recently reported a KIE of 40 for H 2 evolution with [Ir III (Cp * )(H 2 O)(bpm)Ru II (bpy) 2 ](SO 4 ) 2 {Cp * = η 5 pentamethylcyclopentadienyl, bpm = 2,2'-bipyrimidine, bpy = 2,2'-bipyridine} attributing the unusually high KIE to large tunneling effects during catalytic H 2 evolution reactions.…”

Earth-abundant metal complexes as catalysts for water oxidation; is it homogeneous or heterogeneous?

“…[15][16][17][18][19][20][21][22][23][24][25] Earth-abundant metal oxide or hydroxide nanoparticles can act as robust catalysts for water reduction tohydrogen. [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] CO 2 fixation by hydrogen has conjointly been created doable by exploitation of homogeneous catalysts under normal pressure of H 2 and carbon dioxide and at room temperature as well. [42][43][44][45][46] The step of water oxidation is the bottleneck of artificial photosynthesis because it needs the removal of four protons and four electrons from the catalytic site.…”

Shape‐ and Size‐Controlled Nanomaterials for Artificial Photosynthesis

“…[53][54][55] Pt nanoparticles (PtNPs) are most frequently employed as the water reduction catalyst in homogeneous H 2 -evolution systems because Pt is an excellent H 2 -evolution catalyst, as evidenced by the lowest overpotential for proton reduction in electrochemical measurements. [70] Size-controlled PtNPs in the shape of spheres and cubes were prepared by choosing an appropriate precursor, a capping agent, and a reducing agent. The catalytic activity of PtNPs was enhanced by controlling shape and size in photocatalytic H 2 evolution by using Acr + -Mes and dihydronicotinamide adenine dinucleotide (NADH) as a photocatalyst and a sacrificial electron donor, respectively.…”

“…[60][61][62][63][64][65][66][67][68][69] However, it is desirable to minimize the amount of Pt metal used by increasing the catalytic activity because of its high cost and limited supply. [70] Typical TEM images of PtNPs in the shape of a spheres and cubes are shown in Figure 4; the mean diameters of the PtNPs in the shape of spheres and cubes are (3.6 AE 0.6) and (8.6 AE 0.8) nm, respectively [70] Table 1. [70] Size-controlled PtNPs in the shape of spheres and cubes were prepared by choosing an appropriate precursor, a capping agent, and a reducing agent.…”

“…[70] was evaluated by using Equation (1) Chemical structure of Acr + -Mes and overall cycle for photocatalytic H 2 evolution with Acr + -Mes and PtNPs.…”