Abstract
This thesis aims at synthesizing, characterizing, and evaluating the catalytic activity of noble metals (Au, Pd, and Pt) in the form of heterogeneous catalysts supported on mesoporous supports such as titanium dioxide (TiO2). Silicon dioxide (SiO2) and aluminium dioxide (Al2O3) were used as reference supports to compare to TiO2. Mesoporous TiO2 was achieved through surfactant-based methods such as pyrolysis and sol-gel method (soft-template inverse micelle approach), with P-123 and F-127 serving as structure controlling agents. The mesoporous TiO2-supported metal nanoparticle (M-NPs) catalysts were prepared using the sol-gel method for the synthesis of the support, followed by the deposition of M-NPs onto the surface of the support through the deposition-precipitation method. On the other hand, mesoporous TiO2-supported single-atom catalysts were prepared using the pyrolysis method for the synthesis of the support and the deposition of metal single-atoms in a single-step synthesis. The deposition of metal nanoparticles was performed using sodium borohydride (NaBH4) to reduce noble metals from Mn+ to M0. The mesoporous TiO2-supported metal nanoparticles are denoted as PdNPS/TiO2, PtNPS/TiO2, and AuNPS/TiO2 catalyst, while the mesoporous TiO2-supported single-atom catalysts are denoted as Pd1/TiO2, Pt1/TiO2, and AuNPS/TiO2.
Prior to catalytic evaluation, the structural and chemical properties of the catalytic materials were analyzed using various characterization techniques such as powder X-ray diffraction (p-XRD) for determining the crystalline phases. Fourier transform infrared spectroscopy (FTIR) was used for elucidating the functional groups of the catalyst, while thermogravimetric analysis (TGA) was used for thermal stability. The inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used for measuring the metal (Pd, Pt, and Au) loading. Furthermore, scanning electron microscope was used for determining the surface morphology, while the transmission electron microscope (TEM) was used for the distribution and measuring the sizes of the nanoparticles and further defining the surface morphology. In addition, the energy dispersive X-ray spectroscopy (EDX) was used for determining the metal components. Lastly, nitrogen physisorption was used to measure physicochemical properties such as the porosity, surface areas, pore sizes, and pore volumes.
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The techniques mentioned above revealed the catalytic materials to be of mesoporous nature, exhibiting adequate structural and textural properties of the mesoporous structure prior and post the deposition of metal nanostructures onto the surface of the support, with high surface areas and pore volumes greater than 3 nm, desired for mesoporous catalysts. In addition, upon the deposition of metal nanostructures on mesoporous support, physicochemical, thermal and crystalline properties alter depending on the nature of metal nanostructure. The high resolution annular dark-field scanning transmission electron microscope (HADF-TEM) was proposed for determining the distribution, average particle sizes, and the morphology of metal single-atoms on an atomic scale. However, the technique requires costly software and a highly trained technician, hence we were unable to obtain the images. We exploited HRTEM instead, even so, we could not spot the single atoms, hence we have not yet confirmed the distribution and average atom sizes.
Titanium dioxide was used in the esterification conversion of levulinic acid (LA) as metal single-atom and nanoparticle catalyst support. This study was carried out to evaluate the efficiency of metal single-atoms (Pt1, Au1, and Pd1) and their corresponding M-NPs counterparts (PtNPS, AuNPS, and PdNPS) catalysts supported on mesoporous TiO2 (mixture of rutile and anatase). This was achieved by modulating reaction parameters such as the catalyst loading, temperature, stability, metal-support interactions, and the reaction system's acidity to study the catalysts' behaviour. Formic acid and water were introduced and designated as acid feeds to enhance the acidity of the reaction system. Some of the synthesized catalysts exhibited excellent activities (95% average conversion, and 90% average selectivity) in the presence of the acid feeds, and selectivity towards isopropyl levulinate (biofuel additive) as the product of interest. Based on the findings, there is no significant difference in conversions when comparing metal nanoparticles to metal single-atoms of (1-5% loading) Au-, Pd-, and Pt-catalysts, hence single-atom catalysis appears to have much potential for lowering the amount of noble metals used in heterogeneous catalysis.
The esterification of LA was further used to study the rearrangement of M-NPs atoms at the surface of the mesoporous support. In this study, we made use of multiple loadings (1-5%) of Pd-NPs on mesoporous TiO2, SiO2, and Al2O3 supports synthesized by the sol-gel method to study the induction period phenomena. The mesoporous SiO2 and Al2O3 were used as reference supports. The combination of the sol-gel method and deposition-precipitation method was
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based on the prospect of achieving high surface areas and better control on the nanoparticle sizes. The findings postulated that the induction period observed can be attributed to the TiO2 support-induced surface atom rearrangement of PdNPS prior to the catalytic reaction.
Another reaction of interest was the conversions of biomass-derived compounds (biomass-derived alcohols) in oxidation reactions. In this study, the pyrolysis method was used to synthesize mesoporous TiO2-supported Au, Pt, and Pd single-atom catalysts and evaluate their excellent catalytic performance over biomass-derived alcohol oxidation in the presence of tertbutyl hydrogen peroxide (TBHP) as an oxidant. The single-atom catalysts exhibited excellent conversion activity (≥ 90% conversion, ≥ 99 selectivity) for benzyl alcohol (BA) to benzaldehyde, furfuryl alcohol (FA) to furfural (FF), and 1-phenylethanol to acetophenone. We postulated that, even though single-atom catalysts exhibit high catalytic activity and selectivity, different metal atoms behave differently in the oxidation of alcohols and give variety of product selectivities.
In conclusion, this thesis provides in-depth findings as to why single-atom catalysis is regarded to as an alternative field for the long-term goal of reducing the use of noble metals in catalysis. The study also reveals the catalytic property behind the induction period phenomenon. Lastly, the study provides in-depth findings on the metal-support interactions between metal single-atom (Pd, Pt, and Au) catalysts and the support material (TiO2).