|dc.description.abstract||Reverse water gas shift (RWGS) is an emerging technology for CO2 utilization. The RWGS process catalytically converts CO2 to carbon monoxide (CO), producing syngas (a mixture of hydrogen (H2) and CO) which can be further used to produce higher hydrocarbons. Economically this route is more promising than the carbon capture technology because RWGS converts CO2 to valuable syngas that can offset the cost of CO2 capturing. The main challenge is selecting a suitable catalyst that must be highly active, selective, stable, and durable in converting CO2 to syngas.
In this study, cerium oxide (ceria) prepared through the reverse microemulsion (RME) process is used as a base catalytic material. An extensive investigation has been conducted to assess the potential of the RME-based bulk ceria and supported ceria on γ-alumina towards RWGS reaction, including reaction tests and several characterization techniques including X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR) to determine outlet gas composition, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) to determine the surface elemental composition, gas adsorption to determine the specific surface area (BET-SSA) and inductively coupled plasma optical emission spectroscopy (ICP-OES) to determine the bulk composition, TEM (transmission electron microscope) to look for the particle shape and find the crystalline planes, TPR (temperature-programmed reduction) to check the reducing abilities of the catalyst, CO2-TPD to find the relevant active site and in-situ FTIR studies to find the RWGS reaction mechanism.
First, RWGS reaction was studied over unsupported bulk ceria (CeO2) prepared by reverse microemulsion (RME) method and direct precipitated method. Using a unique microemulsion ratio, highly porous ceria nanoparticles (RME-ceria) with targeted exposed (111) facets and high specific surface area of 142 m2 gcat-1 were successfully synthesized compared to ceria nanoparticles prepare via direct precipitation method (DP-ceria) with specific surface area of 101 m2 gcat-1. Long term stability tests (almost 100 h on stream) showed well stable activity of RME-ceria towards the RWGS. At lower GHSV of 10,000 ml gcat-1 h-1, nearly equilibrium conversion (~ 62%) was observed which stabilizes after 70 h on stream to around 52%. However, DP-ceria showed significant decline in conversion from 53% to 24% in similar time span of 70 h. Compared to DP-ceria, RME-ceria showed excellent activity and stability at all conditions towards the RWGS reaction.
Second, the RWGS reaction was studied for the first time in the field on catalysis over ceria-supported γ-alumina prepared via reverse microemulsion method. Three catalysts were prepared at three different loadings of ceria (20 wt%, 30 wt% and 40 wt%). All the catalyst were test for the application of RWGS reaction. Results confirm that 40% wt ceria-supported γ-alumina (40%Ce/Al) showed similar activity and stability as of unsupported RME-ceria. 40% Ce/Al showed very high specific surface area of 292.06 m2 gcat-1, which is almost doubled compared to what we observe for the bulk RME-ceria. SEM results confirm the cluster like structure of the catalyst that leads to high porosity and high exposed surface area. Long term stability test at GHSV of 10,000 ml gcat-1 h-1 showed stable 55% CO2 conversion to CO with 100% selectivity. Finally, among all the Ce/Al catalyst formulations 40%Ce/Al catalyst appears as the optimum formulation for RWGS applications.
In the third part of this Ph.D. thesis, a thorough investigation was performed for the scope of stainless-steel reactors in the RWGS application. It was observed that at an operating temperature of above 550°C in a highly active carbonaceous environment of CO-H2-H2O stainless steel undergoes severe corrosion known as metal dusting. This disintegration leads to form nanometal particles that facilitate filamentous coke formation on the steel wall. Empty reactor test (without catalyst) confirms the fact that in the absence of CO, stainless steel showed stable behavior (no reactivity for the incoming gas mixture of H2-CO2 below 550°C and only 4% CO2 conversion to CO even after 90 h on stream).
Finally, suggestions for future work include the study of 40%Ce/Al using more advanced techniques like XPS, TME, and Raman spectroscopy for the in-depth surface analysis that would help to significantly enhance the activity at higher space velocities. It was also proposed that the effect of promoters like Cu, Co, and Fe should be studied. Literature showed that these promoters significantly enhance catalyst activity at a lower temperature. Conclusively, All the catalysts (unsupported and supported) showed 100%CO selectivity and stable conversion with excellent coking resistance.||en