Sunlight-Fueled, Low-Temperature Ru-Catalyzed Conversion of CO₂ and H₂ to CH₄ with a High Photon-to-Methane Efficiency

Abstract

Methane, which has a high energy storage density and is safely stored and transported in our existing infrastructure, can be produced through conversion of the undesired energy carrier H 2 with CO 2 . Methane production with standard transition-metal catalysts requires high-temperature activation (300–500 °C). Alternatively, semiconductor metal oxide photocatalysts can be used, but they require high-intensity UV light. Here, we report a Ru metal catalyst that facilitates methanation below 250 °C using sunlight as an energy source. Although at low solar intensity (1 sun) the activity of the Ru catalyst is mainly attributed to thermal effects, we identified a large nonthermal contribution at slightly elevated intensities (5.7 and 8.5 sun) resulting in a high photon-to-methane efficiency of up to 55% over the whole solar spectrum. We attribute the excellent sunlight-harvesting ability of the catalyst and the high photon-to-methane efficiency to its UV–vis–NIR plasmonic absorption. Our highly efficient conversion of H 2 to methane is a promising technology to simultaneously accelerate the energy transition and reduce CO 2 emissions.

Collective photothermal effect of Al2O3‐supported spheroidal plasmonic Ru nanoparticle catalysts in the sunlight‐powered Sabatier reaction

“…Also our group demonstrated an efficient sunlight-powered Sabatier reaction, driven by the LSPR of alumina-supported Ru nanorods. [12] We have demonstrated that individual Ru nanorods efficiently harvest sunlight based on their broadband LSPR. For this Ru nanorod catalyst, we identified a large 'nonthermal' contribution at a slightly elevated sunlight intensity of 8.5 kW m À 2 (8.5 sun), resulting in a high photon-to-methane conversion efficiency (PTM) of 55 % [12] over the whole solar spectrum.…”

“…The PTM is the quotient of the increase in reaction rate upon illumination and the rate of incident photons. [12] It carries the same definition as the so-called 'apparent quantum yield' introduced by Liu and coworkers, [7] and quantifies the 'nonthermal' share of the reaction. In contrast to nanorods, the LSPR of spheroidal Ru nanoparticles is positioned in the UV.…”

“…In contrast to nanorods, the LSPR of spheroidal Ru nanoparticles is positioned in the UV. [12,13] Because the catalyst was capable of harvesting UV light only, the PTM achieved with spheroidal particles (5 % w/w Ru at a reactor temperature 150°C and light intensity of 8.5 sun) was much lower than for rods (same loading and conditions). [12] The difference in PTM under these conditions was 41.5 %.…”

“…[12,13] Because the catalyst was capable of harvesting UV light only, the PTM achieved with spheroidal particles (5 % w/w Ru at a reactor temperature 150°C and light intensity of 8.5 sun) was much lower than for rods (same loading and conditions). [12] The difference in PTM under these conditions was 41.5 %. [12] It is an intrinsic limitation of plasmon catalysis that catalytically highly active metal nanospheres for CO 2 methanation display only a weak plasmon resonance, typically positioned in the UV.…”

“…[12] It is an intrinsic limitation of plasmon catalysis that catalytically highly active metal nanospheres for CO 2 methanation display only a weak plasmon resonance, typically positioned in the UV. [12,13] Strong plasmonic metals like Ag, Au and Al, on the other hand do not catalyse methanation of CO 2 . Here, we report plasmonic Al 2 O 3 -supported Ru spheroidal nanoparticles (d~1 nm) as catalyst for the low-temperature conversion of CO 2 and H 2 to CH 4 .…”