Plasmon‐Switched Kinetics for Formic Acid Dehydrogenation: Selective Adsorption Driven by Local Field and Hot Carriers

Abstract

Understanding illumination‐mediated kinetics is essential for catalyst design in plasmon catalysis. Here we prepare Pd‐based plasmonic catalysts with tunable electronic structures to reveal the underlying illumination‐enhanced kinetic mechanisms for formic acid (HCOOH) dehydrogenation. We demonstrate a kinetic switch from a competitive Langmuir‐Hinshelwood adsorption mode in dark to a non‐competitive type under irradiation triggered by local field and hot carriers. Specifically, the electromagnetic field induces a spatial‐temporal separation of dehydrogenation‐favorable configurations of reactant molecule HCOOH and HCOO‐ due to their natural different polarities. Meanwhile, the generated energetic carriers can serve as active sites for selective molecular adsorption. The hot electrons act as adsorption sites for HCOOH, while holes prefer to adsorb HCOO‐. Such unique non‐competitive adsorption kinetics induced by plasmon effects serves as another typical characteristic of plasmonic catalysis that remarkably differs from thermocatalysis. This work unravels unique adsorption transformations and a kinetic switching driven by plasmon nonthermal effects.

Plasmon Modifies Formic Acid Dehydrogenation Kinetics

“…9,26,51 However, the different linear slopes (S) manifests a different photochemical conversion efficiency and light−matter interactions which are closely related to the LSPR property of the plasmonic NP. 47 It is clear that the slope variation trend is in good agreement with the corresponding absorption efficiency as well as the local EF enhancement. Furthermore, the Pd mass normalized dehydrogenation rate of PDA@AgPd-400 at high optical power exceeds that of PDA@AgPd-100, which provides another strong evidence for an intensified plasmonic featureinduced superior chemical transformation over PDA@AgPd-400.…”

“…The optical absorption usually can be expressed as P = k ε 0 E 0 2 ∫ V normald V .25em false| E false| 2 I m [ ε ] where P is the optical absorption within the mean free path length, ε 0 is the vacuum permittivity, E 0 is the modulus of the EF of the incident light, V is the volume of NP, |E| is the modulus of the local EF, and Im[ε] is the imaginary part of the dielectric function of the plasmonic material. , The absorption P is proportional to Im[ε] as well as well as the integral of |E| 2 over V. Compared with other samples, PDA@AgPd-400 owns the highest local EF. In addition, the plasmon energy transfer significantly depends on the metallic dielectric properties in hybrid plasmon metals. , A large imaginary part of the permittivity of Pd and a high imaginary part ratio of Pd/Ag in the visible range, implies that Ag is more supportive of direct energy flow to Pd and then boost the d–s interband transition of Pd . Compared with s–s intraband transition, an efficient d–s transition has an inherently intensive excitation, suggesting that it is highly possible to extract energy to interfacial reactants for photochemical conversions before the energy is thermalized with nanostructure phonon modes.…”

“…As shown in Figure a–c, in dark condition, the hydrogenation generation rate first increases and then decreases with the increase of concentration of either each reactant, implying a competitive Langmuir–Hinshelwood (L–H) mechanism, which means that both reactants competitively adsorb onto the same surface site and finally results in a reduced reaction rate at high reactant concentrations. Such competitive L–H mechanism is quite common in conventional thermocatalytic dehydrogenation and follows the equation ,− r = k normalD normala normalr normalk a A [ c normalA ] a B [ c normalB ] ( 1 + a normalA false[ c A false] + a normalB false[ c B false] ) 2 where r denotes the H 2 evolution rate, k dark stands for the reaction rate constant, a is the adsorption equilibrium constant of the reactant species, and [ c ] represents the concentration of the HCOOH or HCOO – (detailed analysis in the Supporting Information). Upon irradiation, however, there is a tendency to evolve into a noncompetitive mode over PDA@AgPd with the intense plasmon characteristics .…”

“…Based on the above discussion, a perfect noncompetitive adsorption mechanism should be obtained if the plasmonic energy could be efficiently utilized. We then use a plasmonic gold core instead of PDA to fabricate Au@AgPd plasmonic NPs (Figures S17–S20), expectedly, a noncompetitive L-H mode takes place over Au@AgPd-100 with a C value of 0.97 at 25 °C (Figure b). , In this case, such an ensemble adsorption behavior can be expressed as (detailed analysis in the Supporting Information) r = k normalL normali normalg normalh normalt a A ′ [ c normalA ] false( 1 + a A ′ [ c normalA ] false) where the a ′ is the adsorption equilibrium constant of the reactant species under illumination, and k Light stands for the reaction rate constant under illumination. For metallic Au@AgPd-100, the plasmonic Au core can be an excellent charge tunneling as well as an origin of plasmon excitation, which enables plasmon energy to be efficiently utilized within the target interfacial reactants.…”

“…In addition, the plasmon energy transfer significantly depends on the metallic dielectric properties in hybrid plasmon metals. 8,46 A large imaginary part of the permittivity of Pd and a high imaginary part ratio of Pd/ Ag in the visible range, 47 implies that Ag is more supportive of direct energy flow to Pd and then boost the d−s interband transition of Pd. 8 Compared with s−s intraband transition, an efficient d−s transition has an inherently intensive excitation, 24 suggesting that it is highly possible to extract energy to interfacial reactants for photochemical conversions before the energy is thermalized with nanostructure phonon modes.…”