Light absorption in metal catalyst nanoparticles can excite charge carriers to generate hot electron (and complimentary hot holes) with energy higher than the Fermi level. When hot electrons possess energy high enough, they exhibit a high tendency to inject into antibonding orbitals of adsorbates on the photoexcited metal nanoparticles, weakening the corresponding chemical bonds to promote chemical reactions with accelerated reaction kinetics and improved selectivity. Such hot-carrier chemistry has been reported on plasmonic metal nanoparticles, such as silver and gold, which exhibit strong surface plasmon resonances (SPRs) and strong light absorption. However, these metal nanoparticles are not suitable catalysts because their affinity toward interesting molecules is limited. In contrast, most transition metals, such as platinum-group metals and early transition metals, are industrially essential catalysts, but light absorption power in metal nanoparticles is low due to the absence of SPRs in the visible spectral range. Therefore, it is intriguing to explore the potential of hot-carrier catalytic chemistry on photoexcited non-plasmonic metal nanoparticles. Upon the absorption of the same optical power, metal nanoparticles with a small size usually exhibit a high probability of hot electron production and high efficiency of injecting hot electrons into adsorbates. It is challenging to have strong light absorption power and operation stability of the catalyst metal nanoparticles with small sizes. In this thesis, dielectric light antenna, i.e., spherical silica nanoparticles with strong surface scattering resonances near their surfaces, is introduced to support the metal catalyst nanoparticles, enabling improved light absorption power in the metal nanoparticles and operation stability. This thesis focuses on ultrafine rhodium (Rh) nanoparticles (with sizes ranging from 1.7 nm to 4.2 nm) that are widely used as thermal catalysts in many important industry reactions, especially for oxygen-containing species conversion, an oxyphilic feature of Rh nanoparticles.
Firstly, this dissertation conducted a comparative study to investigate the influence of silica geometry, nanospheres, and rodlike nanoparticles on the light absorption of Rh nanoparticles. Both silica substrates enhanced the light absorption of loaded Rh nanoparticles due to elongated light scattering paths (random scattering) and enhanced electromagnetic field intensity (resonant scattering). However, silica nanospheres support both resonant scattering and random light scattering modes, exhibiting a higher Rh absorption than the usage of rodlike silica nanoparticles. The light resonant scattering modes on highly symmetrical silica nanospheres enable producing "hot spots" with a much higher electromagnetic field intensity than incident light intensity. This study then investigated the effect of silica geometries on photocatalytic performance. The CO2 hydrogenation was studied as a model reaction. The Rh/silica nanosphere system exhibited a faster photocatalytic kinetic than the case of rodlike silica nanoparticles. It is possibly due to the enhanced light power density around the silica nanospheres. The results give a promise of expanding Rh nanoparticles from thermo-catalysis to photocatalysis.
Secondly, this dissertation moves onto accelerating aerobic oxidation of primary alcohols to aldehydes, which was benefited from activated oxygen molecules by hot electron injection. This study found that photoexcited Rh nanoparticles enabled accelerating the alcohol oxidation kinetics by four times at a light power intensity of 0.4 W cm-2, accompanied by a reduced activation energy of 21 kJ mol-1. The derived Langmuir-Hinshelwood rate equation was used to fit the oxygen partial pressure results. Photo-illumination promotes the cleavage of associatively adsorbed oxygen molecules into adsorbed oxygen atoms, reducing the energy barrier. Besides, the silica-supported Rh nanoparticles exhibited a higher photocatalytic performance because of the good colloidal stability and enhanced light absorption of small-sized Rh particles. This part of the dissertation shows the possibility of hot-electron mediated reaction pathways towards a desirable kinetic of alcohol oxidation.
Thirdly, it will be meaningful to use the abstracted protons from cheap alcohol sources to reduce other organic molecules rather than dangerous hydrogen gas. This dissertation then investigated the possibility of using an isopropanol solvent as a hydrogen source to reduce nitrobenzene and the feasibility of enhancing the selectivity of the reaction with the light illumination. The results showed that the isopropanol was spontaneously oxidized, producing acetone. Light illumination onto Rh particles selectively enhanced the coupling of reduced nitrobenzene intermediates to produce azoxybenzene. The selectivity of nitrobenzene and production rates gradually increased with a higher number of light photons. Photo-illumination promotes both aniline and azoxybenzene production rates. Hot electrons on Rh particles possibly enabled activating nitrobenzene molecules and increasing concentrations of reduced nitrobenzene intermediates. It resulted in a higher possibility of condensation product and azoxybenzene selectivity, which could not be obtained by elevating temperature without light illumination. This part of the work demonstrated the feasibility of hot electrons from Rh nanoparticles to tune the reaction selectivity in a liquid phase.
Lastly, it is challenging to modulate the selectivity of CH4 from CO2 hydrogenation because of the competitive CO production. This dissertation moves towards enhancing both kinetic rates and selectivity of CH4 for gaseous CO2 hydrogenation by photoexcited Rh nanoparticles. Light illumination onto Rh/silica nanosphere particles resulted in the selectivity of CH4 over 99% in contrast to ~70% under dark conditions at 330 oC and with an absorbed light power intensity of 1.5 W cm-2. The activation energy of CH4 production and CO2 consumption gradually decreased with higher light power intensity because of the transient injection of hot electrons into adsorbates to activate intermediates. Increasing operating temperature and light power intensity synergistically enhanced the reaction kinetics.
Besides, a middle-sized Rh nanoparticle showed a better photocatalytic performance than that of the largest-sized Rh nanoparticles because of the balance in hot-electron production efficiency and intrinsic catalytic performance. Partial pressure dependence and in situ infrared characterizations showed that the critical stable intermediates for CH4 production should be hydrogenated CO2 species (HCOO* COOH*) and hydrogenated CO* species (carbonyl hydride or HxCO*). The light illumination exclusively enhanced the dissociation of CO2 and CO* without apparent influence on CO* desorption. Under high reaction temperature, light illumination preferred a faster CO* conversion than CO2 dissociation, leading to high CH4 selectivity. This result was also supported by higher methanation rates of CO gas under light illumination. The infrared result showed a reduced stretching frequency of CO*, which supported the possibility of the electron from Rh back-donating into antibonding orbitals of strongly adsorbed CO* species. However, hot electrons from silver nanoparticles with a weak COOH* or CO* adsorption could not efficiently activate carbon-species and could not promote CO2 hydrogenation kinetics.
This dissertation offers an avenue of enhancing light absorption of small-sized Rh nanoparticles and expanding its usage from thermal catalysis to photocatalysis for driving oxidation and reduction reactions. The reactants share a common feature containing oxygen elements, a strong affinity with rhodium metal for efficient hot electron injection. We studied the light power intensity and temperature-dependence, showing the accelerated reaction kinetics by hot electron-driven pathways. Photo-excited rhodium nanoparticles were believed to promote the cleavage of chemical bonds O-O, N-O, and C-O to drive chemical transformations. The findings offer insights into developing the scope of non-plasmonic metal nanoparticles in photocatalytic reactions for industrial applications. / Chemistry
| Identifer | oai:union.ndltd.org:TEMPLE/oai:scholarshare.temple.edu:20.500.12613/4701 |
| Date | January 2020 |
| Creators | Dai, Xinyan, 0000-0001-7491-871X |
| Contributors | Sun, Yugang, Dai, Hai-Lung, Strongin, Daniel R., Zhang, Xiaoyi |
| Publisher | Temple University. Libraries |
| Source Sets | Temple University |
| Language | English |
| Detected Language | English |
| Type | Thesis/Dissertation, Text |
| Format | 262 pages |
| Rights | IN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available., http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | http://dx.doi.org/10.34944/dspace/4683, Theses and Dissertations |
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