Localized surface plasmon resonance (LSPR) allows nanoparticles (NPs) to harvest light and concentrate it near the nanoparticle surface. Light energy is utilized in the generation of excited charge carriers as well as heat. Plasmonic catalysts used these energetic charge carriers (and the heat) to drive chemical reactions on their surface and allowed the discovery of novel and selective reaction pathways that were not possible in thermal catalysis. This review discusses the fundamentals of plasmonic catalysis and its application for CO2 conversion to fuel and chemicals. We first discussed the fundamentals of LSPR and the mechanism of plasmonic photocatalysis, using the concepts of the dielectric function, charge carrier generation, and relaxation pathways. We then reviewed various charge carrier-mediated activation of molecules (their chemical bonds) on the surface of plasmonic nanocatalysts and how the extraction of charge carriers played a critical role in plasmonic catalysis. The concept of multicomponent plasmonic catalysis, a hybrid catalyst by combining plasmonic metals (Cu, Au, Ag, Al, etc.) with nonplasmonic but active catalytic metals (Pt, Pd, Ru, Rh, etc.), in close proximity to each other, was then discussed. Photocatalytic CO2 reduction reactions using the examples of each of three major pathways, (i) direct transfer of hot charge carriers to the reactant molecules, (ii) providing heat to the reactant molecules by photothermal effect, and (iii) enhancing the photon absorption rate of reactant molecules by optical near-field enhancement close to the nanocatalyst surface, were discussed. In the last section, we reviewed plasmonic photocatalysts for dry reforming of methane (DRM) using CO2, which uses two greenhouse gases as feed to produce industrially significant syngas. Overall, the review is broadly divided into four sections: (1) Fundamentals of Plasmonic Nanomaterials, (2) Mechanism of Plasmonic Photocatalysis, (3) Plasmonic Photocatalysts for CO2 Reduction to Fuels and Chemicals, and (4) Plasmonic Photocatalysts for Methane Dry Reforming using CO2; with each section divided into several subsections.

A highly active and stable Cu-based catalyst for CO2 to CO conversion was demonstrated by creating a strong metal–support interaction (SMSI) between Cu active sites and the TiO2-coated dendritic fibrous nano-silica (DFNS/TiO2) support. The DFNS/TiO2–Cu10 catalyst showed excellent catalytic performance with a CO productivity of 5350 mmol g–1 h–1 (i.e., 53,506 mmol gCu –1 h–1), surpassing that of almost all copper-based thermal catalysts, with 99.8% selectivity toward CO. Even after 200 h of reaction, the catalyst remained active. Moderate initial agglomeration and high dispersion of nanoparticles (NPs) due to SMSI made the catalysts stable. Electron energy loss spectroscopy confirmed the strong interactions between copper NPs and the TiO2 surface, supported by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. The H2-temperature programmed reduction (TPR) study showed α, β, and γ H2-TPR signals, further confirming the presence of SMSI between Cu and TiO2. In situ Raman and UV–vis diffuse reflectance spectroscopy studies provided insights into the role of oxygen vacancies and Ti3+ centers, which were produced by hydrogen, then consumed by CO2, and then again regenerated by hydrogen. These continuous defect generation–regeneration processes during the progress of the reaction allowed long-term high catalytic activity and stability. The in situ studies and oxygen storage complete capacity indicated the key role of oxygen vacancies during catalysis. The in situ time-resolved Fourier transform infrared study provided an understanding of the formation of various reaction intermediates and their conversion to products with reaction time. Based on these observations, we have proposed a CO2 reduction mechanism, which follows a redox pathway assisted by hydrogen.

The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni3N) nanosheets as an alternative to these metals. The Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g−1 h−1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO2 hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni3N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.

In this work, we have designed and synthesized nickel-laden dendritic plasmonic colloidosomes of Au (black gold-Ni). The photocatalytic CO 2 hydrogenation activities of black gold-Ni increased dramatically to the extent that measurable photoactivity was only observed with the black gold-Ni catalyst, with a very high photocatalytic CO production rate (2464 ± 40 mmol g Ni −1 h −1 ) and 95% selectivity. Notably, the reaction was carried out in a flow reactor at low temperature and atmospheric pressure without external heating. The catalyst was stable for at least 100 h. Ultrafast transient absorption spectroscopy studies indicated indirect hot-electron transfer from the black gold to Ni in less than 100 fs, corroborated by a reduction in Au−plasmon electron−phonon lifetime and a bleach signal associated with Ni d-band filling. Photocatalytic reaction rates on excited black gold-Ni showed a superlinear power law dependence on the light intensity, with a power law exponent of 5.6, while photocatalytic quantum efficiencies increased with an increase in light intensity and reaction temperature, which indicated the hot-electron-mediated mechanism. The kinetic isotope effect (KIE) in light (1.91) was higher than that in the dark (∼1), which further indicated the electron-driven plasmonic CO 2 hydrogenation. Black gold-Ni catalyzed CO 2 hydrogenation in the presence of an electron-accepting molecule, methyl-p-benzoquinone, reduced the CO production rate, asserting the hot-electron-mediated mechanism. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO 2 hydrogenation took place by a direct dissociation path via linearly bonded Ni−CO intermediates. The outstanding catalytic performance of black gold-Ni may provide a way to develop plasmonic catalysts for CO 2 reduction and other catalytic processes using black gold.

Activation of organic chlorides is a challenging reaction due to their chemical inertness, while hydrogenation of alkene and alkynes faces poor selectivity. In this work, we have demonstrated the use of nickel-loaded black gold (black Au−Ni) that absorbs broadband light from visible to near-infrared of the sunlight due to plasmonic coupling between Au NPs, for photocatalytic hydrodechlorination and propene and acetylene hydrogenation reactions. Hot carriers, the polarizing electric field, and the photothermal effect generated in these catalysts enabled photocatalytic bond activation of these two challenging reactions using visible light at lower temperatures and atmospheric pressure. The black Au−Ni catalyst showed a multifold increase in its activity as compared to black Au. The plasmon-assisted reaction mechanism of hydrodechlorination was supported by intensity-dependent catalysis, kinetic isotope effect (KIE), competitive C−Cl bond activation with one-electron ferricyanide reduction, finite-difference time-domain (FDTD) simulations, and quantum chemical studies. Cluster model-based density functional theory studies show that the reactions have substantial barriers, which were bypassed via an excited state accessed through plasmonic excitations. FDTD studies indicate substantial enhancement of the electric field at the hotspots, iron reduction competed for hot carriers when run in conjunction with C−Cl bond activation, KIE was marginally higher in light as compared to the dark, and the catalyst showed power-dependent activity. These observations indicated the hot carrier involvement in the hydrodechlorination reactions in addition to the photothermal effect.