Linking Atomic and Reactor Scale Plasmon Photocatalysis in Acetylene Hydrogenation with Optically Coupled ETEM

Plasmonic photocatalysis, chemistry driven by the resonant, collective oscillation of electrons in response to light, has shown unique chemical selectivity in certain reactions as compared to thermally driven chemistry under analogous conditions [1]. In particular, it has been shown that Pd catalysts coated on plasmonic Al nanoparticles demonstrate increased selectivity for ethylene (C2H4) production over ethane (C2H6) during the partial hydrogenation of acetylene (C2H2) [2]. A hypothesis for the preference for partial hydrogenation is the ability of energetic carriers generated from the decay of plasmon excitation to desorb molecular H2 from the Pd surface. Similarly, the field of thermal catalysis has observed that hydride formation, the uptake of atomic hydrogen into the Pd crystal lattice, can significantly impact chemical selectivity [3]. The observed increased chemical selectivity from plasmon catalysis may not only be a result of molecular desorption, but also atomic reconfiguration from an induced phase transformation caused by the fleeting H2 molecules. In this work, we observe phase transformations governed by plasmon resonances in Pd nanoprisms and correlate this behavior to bench-scale reactor performance. First, we utilize an image corrected environmental transmission electron microscope (ETEM) equipped with an optical holder to observe hydrogen- and light-driven transformations. In parallel, we employ ensemble characterization techniques to probe the chemical activity of monodisperse AgPd alloyed catalysts under laser illumination, utilizing a home-built automated system of mass flow control, heating, illumination, and gas chromatography. Our work lays a foundation to linking atomic-scale structure and atomic-scale catalyst transformations with ensemble-scale performance. Previous works using optically coupled ETEM have shown that Pd hydride undergoes phase transitions when exposed to laser illumination in plasmonic systems, and that these transitions can proceed in ways not seen in the absence of light. However, these studies were performed on antenna-reactor systems, in which light is harnessed by one material and reactions performed on the other [4]. In this work, we design Pd nanotriangles with 50 nm edge lengths and 10 nm thicknesses that support visible plasmon resonances owing to their thin geometry. The particles are synthesized colloidally in an aqueous solution of PVP and Na2PdCl4 [5]. The PVP serves as both reductant and surfactant for the particles. Using electron energy loss spectroscopy (EELS), we measure a 450 nm plasmon resonance on the “tip mode” of the nanoprisms. Therefore, in contrast to previous works, we are able to optically excite these Pd nanoparticles without the need for a second light absorbing structure. Using dark-field TEM and defect contrast imaging, we selectively image one phase of the PdH system to track hydrogen flow in and out of the particle. On these single particle systems, we observe plasmonic effects on Pd hydride control, namely a spatial selectivity of the nucleation of the low-hydrogen phase. We see a strong preference for the phase transformation to occur in the tips of the triangular prisms with the sharpest radius of curvature. Based on electromagnetic simulations, we show that this corresponds to the areas with highest electric field enhancements. We observe 10x enhancement of phase transformation rates with linear kinetics, signaling that we are modulating the surface-limited desorption of H2 molecules from the particle surface. To demonstrate the applicability of these findings to bench-scale reactors, we synthesize Pd containing plasmonic catalysts and test their ensemble scale chemical performance for the partial hydrogenation of acetylene. To boost the plasmon response and in order to make well-controlled monodisperse samples, we use spherical AgPd alloy catalysts in the sub 5 nm range for our ensemble testing [6]. Whereas the Pd nanoprisms cannot be synthesized in a highly uniform fashion, we are able to control the growth of small spherical particles within a 1.5 nm size distribution for a more well controlled study. Ag is doped in to achieve a visible optical response in these smaller particles, and the Ag doping is varied from 0 to 100% to study its impact on the reaction. We show that alloys possessing a plasmon resonance at 450 nm produce higher selectivity under laser illumination than heated, maintaining >90% selectivity with >80% conversion, compared to the thermal results of <70% selectivity. Furthermore, by varying the thickness of the photoactive layer in high Ag and high Pd cases, we demonstrate fundamental differences in the photochemical response. Namely, the high Pd particles demonstrate photothermal heating characteristics, while the high Ag particles show direct photochemical behavior. Our results demonstrate that atomic reconstruction in plasmonic photochemical reactions cannot be ignored. Plasmons can induce phase changes in Pd in H2 environments, and this may have a direct impact on chemical selectivity. We show with our ETEM results that hydrides are most efficiently destroyed when the highest electric fields are achieved. This trend continues in bulk measurements where increasingly high selectivity is obtained with increased Ag content, hence increased electric field enhancement. Ongoing research is focused on matching identical conditions within the ETEM and ensemble reactor for direct comparisons to control for alloy effects. (i) EELS spectra and (ii/iii) maps of Pd nanoprisms in both H-rich β phase and H-poor α phase under 400 Pa and 0 Pa H2 pressure, respectively. The red-shifted tip-mode of the hydride phase Pd prism overlaps well with our 450 nm excitation laser. (iv) Defect contrast imaging of the β to α phase transition “in the dark” in which the β phase is false-colored purple [7]. (v) Nucleation statistics for phase transformations with and without laser illumination showing a strong site preference for sharper tips (higher EM fields) when the system is illuminated. (i) UV/Vis spectra of <5 nm AgPd nanoparticles showing a strong plasmon resonance near 450 nm. The insets show the bright yellow color of NPs dissolved in hexanes and monodisperse particles imaged in TEM. (ii) Ensemble photocatalysis results of acetylene hydrogenation over AgPd nanoparticles. The thermal reaction (left) shows decreasing selectivity for ethylene with increasing conversion. For the light driven reaction (right), under 450 nm illumination, high selectivity is retained with increasing conversion.