Carbon dioxide (CO2) utilization is indispensable to reduce high atmospheric CO2 concentration, attributing to global warming and ocean acidification. Reduction of CO2 into high value-added chemicals and fuels is a promising process to mitigate CO2 emissions and opens possibilities to have carbon-based energy production with net-zero carbon emissions. CO2 is the most oxidized form of carbon and thermodynamically stable. To effectively reduce CO2, nanotubular yolk–shell catalysts for methane reforming, and tandem catalysts for direct CO2 hydrogenation to light olefins were developed. In the former process, Ni yolks encapsulated with SiO2 shell demonstrated excellent stability with a high resistance to carbon deposition in the confined morphology due to the efficient CO desorption. Forming Pt–Ni single-atom alloys on the yolks pushed the catalyst operating temperature down to 500 °C and further improved the catalyst stability due to the enhanced Ni reducibility. In the latter process, indium oxide supported on zirconia and SAPO-34 zeolite were operated as a tandem catalyst to produce a high light olefins selectivity by shifting the reaction equilibrium to the right for the CO2 to methanol conversion. Zirconia promoted with yttria (YSZ) inhibited the reduction and hydroxylation of active indium sites. The improved oxygen vacancy formation in YSZ and strong metal–support interaction between indium oxide and YSZ resulted in stable light olefins production. These discoveries can be adopted to the current power generation and manufacturing processes to utilize CO2 emissions to produce high value-added chemicals with net-zero carbon emissions.

CO2 emissions have drastically increased since the first industrial revolution in the 1750’s. Additionally, methane emissions, the second most greenhouse gas, have kept rising since the first measurement in 1983 and have been growing rapidly in the past two years, possibly due to microbial emissions from wetlands or agriculture. These long-lived greenhouse gases significantly impact global climate change. To slow down global warming and achieve the goal of a zero-carbon footprint, many efforts have been made, such as converting greenhouse gases to syngas, using electricity for reactions, and seeking new heating methods for the reactions instead of using fossil fuels to fire the systems to minimize CO2 emissions.Dry reforming of methane (DRM) can be utilized as a sustainable reaction to capture and utilize greenhouse gases. Ni-based supported catalysts have been widely used due to Ni's low cost and high activity. Despite their excellent performance, side reactions such as methane decomposition and Boudouard reaction can cause severe coke formation, leading to catalyst deactivation. Our work reported that both yolk-shell morphology and Pt-Ni interaction in single-atom-alloy (SAA) structures could maintain the DRM activity. However, the effect of morphology and Pt-Ni interaction could not be elucidated. We studied the reaction kinetics of DRM by proposing a detailed elementary reaction mechanism with 12 surface species and 17 elementary steps over the Ptx-NiCe@SiO2 and Pt0.25-NiCe/SiO2WI catalysts. Due to the different DRM activity over the wet-impregnated and yolk-shell structures, we examined multiple reaction models to explain the effect of morphology and Pt-Ni interaction on the DRM activity. Our results show that the reverse Boudouard reaction and dissociative adsorption of CO2 and CH4 are the rate-limiting steps. The desorption of CO* and H* is also critical to products’ yield and selectivity. Compared with the Pt0.25-NiCe/SiO2WI catalyst, the C* removal followed by the fast CO* desorption is more favored on Pt0.25-NiCe@SiO2 SAA, suggesting the reverse Boudouard reaction prevents the catalyst deactivation by coking. The high O* and low H* coverages are observed on Pt0.25-NiCe@SiO2 SAA due to the confined yolk-shell morphology and enhanced Pt-Ni interaction in SAA structures, respectively. Both these effects can lead to facile C* removal in the Pt0.25-NiCe@SiO2 SAA catalyst, leading to a stable DRM activity. To improve the energy consumption of highly endothermic reactions, we investigated a novel heating method, induction heating (IH), often referred to as radio frequency (RF) heating, for ethanol-to-1,3-butadiene (ETB) reaction as an effective and alternate solution for conventional Joule heating. Instead of being obtained from fossil feedstocks, 1,3-butadiene can be produced from the dehydrogenation of ethanol, a sustainable resource from biomass fermentation. However, the commonly used Ostromislensky process for ETB reaction has two steps: 1) ethanol dehydrogenation to acetaldehyde and 2) 1,3-butadiene production from ethanol and acetaldehyde. The two-step reaction requires different temperatures and catalysts, which makes the 1,3-BD production through the Ostromislensky process inefficient when loaded in a single reactor with conventional furnace heating. However, separate reaction temperature zones can be obtained in a single reactor by adjusting the amount of the susceptors with induction heating (IH) so that two-step reactions can be performed in a tandem system to reduce energy consumption and increase selectivity to the desired products. To verify the IH system and its behavior, the first-step reaction, ethanol dehydrogenation to acetaldehyde, was studied prior to the tandem reaction. The dehydrogenation of ethanol can be catalyzed by supported copper catalysts. The reaction is typically carried out at high temperatures, around 250-300 °C, without oxygen. We studied the catalyst activity with IH for the first time and achieved high ethanol conversion and acetaldehyde selectivity at a temperature of 30 ˚C lower than that with conventional furnace heating (CFH). A transport model was applied to design the catalyst bed configuration and improve the catalyst activity, stability, and energy efficiency by minimizing the temperature gradient. The work was then extended to the Ostromislensky process of the ETB with IH in a tandem system, and the C4 selectivity was tuned by individually controlling the reaction temperature and WHSV of each catalyst bed. A high ethanol conversion of 62.9%, acetaldehyde selectivity of 37.5%, and C4 product selectivity of 55.3% were obtained at relatively low reaction temperatures of T1 = 171 ˚C and T2 = 320 ˚C with the tandem system, nine times lower power consumption than the higher reaction temperatures when T1 is 245 ˚C. Our work suggests that applying IH to a tandem system can successfully achieve the two temperature zones inside a single reactor, such that both production and energy efficiency can be improved for a multi-step reaction.

Photocatalytic water splitting is seen as a viable alternative to steam reforming for hydrogen fuel generation. One of the major keys to exploiting photocatalytic technologies as a commercial energy source lies in the development of catalytic materials that can drive photocatalytic reactions using the largest portion of the solar energy spectrum – the visible light region. Gallium zinc oxynitride solid solution has been recommended as a promising candidate for photocatalytic water splitting, however, the current synthesis methods are largely multi-steps and batch wet chemistry techniques that are not easily amenable to scale up. Flame spray pyrolysis (FSP) is a continuous, single step, fast, relatively cheaper and easily scalable nanomaterial synthesis method. This work reports the deployment of flame spray pyrolysis to the synthesis of gallium zinc oxynitride solid solutions. The solid solutions were prepared using two different precursor compositions expressed as urea to total metal ratio, solvent mixtures and synthesis conditions, and X-ray diffraction and UV-Vis spectra were employed for preliminary characterizations. Based on solvent screening results, no single solvent satisfied the solubility, boiling point and combustion enthalpy requirements. The XRD data shows that solvents of weak combustion enthalpy resulted in either amorphous or very low crystalline, bimodal and inhomogeneous phase solid solution. However, solvent mixtures of adequate combustion enthalpy yielded single phase and highly crystalline solid solutions with all peaks indexed to the hexagonal wurtzite ZnO structure irrespective of synthesis conditions. The absence of extra peaks coupled with the slight lattice contraction relative to pristine ZnO satisfied the necessary conditions to qualify the as-prepared samples as solid solutions of GaN and ZnO. Lattice contraction was favored as urea to total metal ratio decreased while peak broadening occurred as the urea to total metal ratio in the precursor solution increased while the crystallinity lowered. This could be possibly due to increased dopant concentration in the ZnO lattice. Samples made with DI H2O + butanol solvent mixture were more crystalline and of larger crystallite size than those made from methanol + butanol mixture which is attributed to the fact that the solvent mixture supplied higher rate of energy (1.17 kJ/s) than the methanol-butanol (1.12 kJ/s) to the flame.

Tauc’s plots were used to estimate the direct bandgaps of the samples. With DI H2O + butanol mixture, the bandgaps are 3.08 and 3.12 eV with smaller bandgap belonging to the sample with higher urea to total metal ratio in the precursor solution. Comparing with the XRD data, the smaller bandgap corresponds with lattice contraction. But for the methanol-butanol mixture, the bandgaps are 3.07 eV and 3.16 eV with the smaller bandgap belonging to the sample with lower urea to total metal ratio. This perhaps, support the claims as reported in Literatures that bandgap is not solely defined by dopant content. Overall, based on bandgaps, the synthesized samples can drive photocatalytic activity in the UV region. Hence, optimization of the synthesis technique is necessary to further narrow the bandgap to the visible light range.

Catalytic oxidation of methane to less potent gases has been the current technology for controlling methane emissions from natural gas-powered vehicles. Pd is commonly used as an active metal for methane oxidation, but its amount is desired to be minimized due to its high cost. The stability and activity of Pd species at wide operating temperatures depend on the Pd dispersion and local structure on the CeO2 surface. Here, we adopt flame spray pyrolysis (FSP), a high-temperature synthesis, to stabilize Pd on CeO2. The Pd/CeO2 catalv4yst synthesized in the oxidizing environment generates highly active Pd species, while the reducing environment has less active Pd species for methane oxidation. Decreasing Pd loading on CeO2 can enhance the reaction rate due to better dispersion. Detailed structural characterizations of catalysts synthesized at different loading and synthesis conditions using CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments identify the highly active sites as atomically dispersed Pd and the less active sites as the combination of highly dispersed Pdn+, Pd0, and Pd0 cluster. These results indicate that the control of Pd structures is a function of its loading and the FSP synthesis condition. Evaluation of the methane oxidation reaction over these structures reveals that the enhanced methane oxidation activity by the atomically dispersed Pd is due to the facilitation of the conversion from CH2 to carbonates during the methane oxidation reaction. This study demonstrates that FSP can be used to control Pd structures for synthesizing atomically dispersed Pd for enhanced methane oxidation activity.

In addition to controlling the chemical property of catalysts by FSP, the uniformity of catalyst particles can be governed by the release rate of precursors in FSP. However, there is a lack of experimental techniques for the direct measurement of the precursor release, and its quantification through the single droplet combustion (SDC) modeling has been based on the immediate release of the precursor from the droplet. Here, a single droplet combustion model that includes film theory has been developed, and the thickness of the mass boundary layer that limits the release of precursor to the combustion zone is coupled with the droplet temperature for a more accurate prediction of the temporal precursor release during synthesis. The model reveals that the mass boundary layer can influence nanomaterial formation and its coupling with droplet temperature allows a more accurate comparison of precursor release rate. We also developed design rules that can guide experiments on the choice of solvent composition and nozzle operating parameters to improve homogeneity. Quantifying the precursor release through thermodynamic phase relation and diffusion model with two moving boundary conditions is further developed to investigate precursor concentration on catalysts homogeneity and more precise prediction. This new methodology for accurate prediction of precursor release is the first application of film theory for the understanding and design of flame-synthesized homogeneous catalysts.