This study introduces a plasmonic reduction catalyst, stable only in the presence of air, achieved by integrating Pt-doped Ru nanoparticles on black gold. The catalyst, named DPC/RuPt, demonstrates high efficiency in acetylene semi-hydrogenation, achieving over 90% selectivity with an ethene production rate of 320 mmol g⁻¹ h⁻¹. Its stability, evidenced by 100 hours of continuous operation with air flow, is attributed to the synergy of co-existing metal oxide and metal phases. The catalyst's stability is further enhanced by plasmon-mediated concurrent reduction and oxidation of active sites. Finite-difference time-domain simulations show a five-fold electric field enhancement near RuPt nanoparticles, crucial for activating acetylene and hydrogen. Kinetic isotope effect analysis indicates the contribution of plasmonic non-thermal effects along with photothermal effects. Spectroscopic and in-situ Fourier transform infrared studies, combined with quantum chemical calculations, elucidate the molecular reaction mechanism, emphasizing the cooperative interaction between Ru and Pt in optimizing ethene production and selectivity. Plasmonic nanochemistry provides approaches to develop light-harvesting nanoreactors that exceed the limitations of conventional catalysts. Photoexcitation of plasmonic metal nanoparticles can trigger direct photochemical reactions. However, the chemical reactivity of these metal surfaces limits their application in catalysis for only a few reactions. Functional nanostructures with plasmonic materials that concentrate light energy and efficiently guide it to more active catalytic sites are desired. These 'hybrid plasmonics' can pave new ways for various catalytic transformations with better activities and selectivities by providing alternative reaction pathways and efficient use of solar light. One such challenging reaction is acetylene semi-hydrogenation in excess ethene in terms of high selectivity and conversion. Surprisingly, reports on photocatalytic acetylene semi-hydrogenation are scarce, even though it has been shown as a promising method for enhancing the activity and selectivity of numerous important reactions like CO₂ hydrogenation, reverse water gas shift reaction, and Fischer-Tropsch synthesis. This work reports a hybrid plasmonic reduction catalyst, synthesized by loading Pt-doped Ru nanoparticles (RuPt NPs) over dendritic plasmonic colloidosomes (DPC) of gold. The catalyst design consists of two components: i) DPC, also known as black gold (Au deposited on dendritic fibrous nano-silica (DFNS) using a cycle by cycle approach), which can harvest a broad region of visible light and generate hot-spots due to the plasmonic coupling between Au NPs, and ii) Pt-doped Ru NPs (Ru:Pt = 90:10) as the catalytic sites, rationally designed to control the extent of semi-hydrogenation. RuPt NPs loaded on DPC (DPC/RuThis study introduces a plasmonic reduction catalyst, stable only in the presence of air, achieved by integrating Pt-doped Ru nanoparticles on black gold. The catalyst, named DPC/RuPt, demonstrates high efficiency in acetylene semi-hydrogenation, achieving over 90% selectivity with an ethene production rate of 320 mmol g⁻¹ h⁻¹. Its stability, evidenced by 100 hours of continuous operation with air flow, is attributed to the synergy of co-existing metal oxide and metal phases. The catalyst's stability is further enhanced by plasmon-mediated concurrent reduction and oxidation of active sites. Finite-difference time-domain simulations show a five-fold electric field enhancement near RuPt nanoparticles, crucial for activating acetylene and hydrogen. Kinetic isotope effect analysis indicates the contribution of plasmonic non-thermal effects along with photothermal effects. Spectroscopic and in-situ Fourier transform infrared studies, combined with quantum chemical calculations, elucidate the molecular reaction mechanism, emphasizing the cooperative interaction between Ru and Pt in optimizing ethene production and selectivity. Plasmonic nanochemistry provides approaches to develop light-harvesting nanoreactors that exceed the limitations of conventional catalysts. Photoexcitation of plasmonic metal nanoparticles can trigger direct photochemical reactions. However, the chemical reactivity of these metal surfaces limits their application in catalysis for only a few reactions. Functional nanostructures with plasmonic materials that concentrate light energy and efficiently guide it to more active catalytic sites are desired. These 'hybrid plasmonics' can pave new ways for various catalytic transformations with better activities and selectivities by providing alternative reaction pathways and efficient use of solar light. One such challenging reaction is acetylene semi-hydrogenation in excess ethene in terms of high selectivity and conversion. Surprisingly, reports on photocatalytic acetylene semi-hydrogenation are scarce, even though it has been shown as a promising method for enhancing the activity and selectivity of numerous important reactions like CO₂ hydrogenation, reverse water gas shift reaction, and Fischer-Tropsch synthesis. This work reports a hybrid plasmonic reduction catalyst, synthesized by loading Pt-doped Ru nanoparticles (RuPt NPs) over dendritic plasmonic colloidosomes (DPC) of gold. The catalyst design consists of two components: i) DPC, also known as black gold (Au deposited on dendritic fibrous nano-silica (DFNS) using a cycle by cycle approach), which can harvest a broad region of visible light and generate hot-spots due to the plasmonic coupling between Au NPs, and ii) Pt-doped Ru NPs (Ru:Pt = 90:10) as the catalytic sites, rationally designed to control the extent of semi-hydrogenation. RuPt NPs loaded on DPC (DPC/Ru