Embargoed until 2024-11-22
Author
Date
2023Type
- Doctoral Thesis
ETH Bibliography
yes
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Abstract
Methanol is a key platform compound widely applied at industrial scale to produce a plethora of valuable chemical derivatives, and its great potential as a transportation fuel is envisaged to revolutionize the energy sector landscape in the coming years. Currently, the growing demand for methanol coupled with increasing global efforts to control anthropogenic carbon dioxide (CO2) emissions and lessen our reliance on fossil resources has propelled research into manufacturing processes with potential to effectively close the carbon cycle via CO2 capture and utilization. Methanol synthesis via thermocatalytic hydrogenation of CO2 using renewable resources has emerged as a strategic route to enable its sustainable production. However, the industrial implementation of this approach is largely contingent on identifying efficient heterogeneous catalysts for this transformation. The recent discoveries of indium oxide (In2O3) and zinc zirconium oxides (ZnZrOx) as highly selective catalysts represent a key milestone towards this goal. Still, several practical challenges and fundamental questions need to be addressed to enable the application of such oxide catalysts. Firstly, the impact of carbon monoxide (CO) on these systems has been scarcely investigated, although CO may comprise up to half of the carbon feedstock, depending on the origin of CO2 and process configuration. Secondly, while metal promoters are considered crucial to improve the sluggish hydrogen splitting ability of these reducible oxide catalysts, the lack of fundamental understanding of their speciation and promotional effects hampers the rational design of optimal catalytic architectures. Thirdly, despite metal promotion and deposition on a ZrO2 carrier being effective strategies to boost the performance of pure In2O3, no study has thus far tackled integrating these approaches to generate an optimal catalyst formulation that meets the high activity and stability standards of the industry. Finally, the role of oxygen vacancy formation and dynamics in generating active ensembles in promoted reducible oxide catalysts remains poorly understood. With these premises, this thesis aims at advancing the design of promoted reducible oxide catalysts based on In2O3 and ZnZrOx for the sustainable methanol production via CO2 hydrogenation. Towards this goal, a holistic approach consisting of controlled and precise synthesis, quantitative performance evaluation, ex situ, in situ, and operando characterization, and kinetic analyses coupled with theoretical modelling studies was employed.
In a first step, the impact of CO on methanol synthesis over In2O3 based catalysts in bulk, supported, or palladium or nickel promoted forms is evaluated using cycle experiments with variable CO2 and CO contents at H2/(CO+CO2) = 4. CO addition decreased methanol productivity on all catalysts except In2O3/monoclinic(m)-ZrO2, the activity of which was increased by 10%. Controlled formation of oxygen vacancies and improved resistance to sintering are revealed as the main reasons for the activation of the latter, and an interplay of CO/H2O induced sintering and CO inhibition as the origin of performance loss, which is partially recovered for Pd-In2O3 upon re exposure to CO2 rich streams. Focusing on the most promising systems (In2O3/m-ZrO2 and Pd-In2O3), operation protocols in hybrid CO2-CO feeds are explored to maximize their methanol yield.
Although a plethora of metal promoters have been reported to enhance the performance of pure In2O3 in CO2 hydrogenation to methanol, the lack of systematic catalyst preparation and evaluation hinder direct comparison of their speciation and promotional effects, and consequently, the design of an optimal system. To address this aspect, flame spray pyrolysis (FSP) is employed as a standardized synthesis method to incorporate nine metal promoters (M = 0.5 wt.%) in In2O3. Methanol productivity generally increased on M In2O3 with selectivity following Pd ≈ Pt > Rh ≈ Ru ≈ Ir > Ni ≈ Co > Ag ≈ In2O3 > Au. The promoters display different speciation. Atomically dispersed promoters (Pd, Pt, Rh, Ru, and Ir) grant the highest improvement in performance, particularly Pd and Pt, which markedly promote hydrogen activation while hindering undesired CO formation. In contrast, metals in clustered (Ni and Co) or nanoparticle (Ag and Au) forms display moderate and no promotion, respectively.
To attain a single catalytic system with superior reactivity on the basis of the formerly introduced In2O3/ZrO2 and Pd-In2O3, a suitable synthetic approach capable of harnessing synergic effects of the individual components must be identified. Ternary Pd-In2O3/ZrO2 catalysts prepared by FSP display outstanding methanol productivity, surpassing their binary counterparts (Pd-In2O3 and In2O3/ZrO2). Unlike established impregnation and co precipitation methods, FSP promotes the formation of materials combining low nuclearity palladium species associated with In2O3 monolayers dispersed on the ZrO2 carrier. A pioneering protocol developed to quantify oxygen vacancies using in situ electron paramagnetic resonance (EPR) spectroscopy reveals their enhanced generation because of this unique catalyst architecture, thereby rationalizing its high and sustained methanol productivity.
With the aim to develop Pd-In2O3/ZrO2 catalysts via a more scalable method, ternary systems prepared by impregnation are investigated. The structure of Pd-In2O3/ZrO2 systems is found to evolve under CO2 hydrogenation conditions into a selective and stable architecture. Detailed operando characterization and simulations reveal a rapid restructuring driven by the metal metal oxide interaction energetics. The findings highlight the crucial role of reaction induced restructuring in complex CO2 hydrogenation catalysts and offer insights into the optimal integration of acid base and redox functions for practical implementation.
Shifting the focus to ZnZrOx systems, while highly selective and stable their activity remains moderate, and descriptors to improve their design are lacking. Here, FSP is also applied to synthesize a series of ZnZrOx materials with different zinc contents, which are systematically compared to coprecipitated (CP) analogs to establish synthesis structure performance relationships. FSP systems generally display a threefold higher methanol productivity compared to their CP counterparts. Unlike CP, FSP maximizes the surface area and formation of atomically dispersed Zn2+ sites incorporated in lattice positions within the ZrO2 surface, which is key to improving performance.
The last part of the thesis builds on this knowledge and investigates the promotion of ZnZrOx catalysts with small amounts (0.5 mol%) of diverse hydrogenation metals (Re, Co, Au, Ni, Rh, Ag, Ir, Ru, Pt, Pd, and Cu) prepared via a standardized FSP approach. Cu, an earth abundant and less costly metal, emerges as the most effective promoter, doubling methanol productivity. Operando X-ray absorption, infrared, and electron paramagnetic resonance spectroscopic analysis and simulations reveal that Cu0 species form Zn rich low nuclearity CuZnx clusters on the ZrO2 surface during reaction, which correlates with the generation of oxygen vacancies in their vicinity. This catalytic ensemble promotes the rapid hydrogenation of intermediate formate into methanol while effectively suppressing CO production.
Overall, the concepts and findings presented in this thesis offer design criteria for the development of innovative and efficient catalytic technologies that can propel green methanol production via CO2 hydrogenation at industrial scale. Show more
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https://doi.org/10.3929/ethz-b-000643251Publication status
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Publisher
ETH ZurichSubject
Heterogeneous catalysis; Methanol synthesis; Catalyst design; CO2 hydrogenationOrganisational unit
03871 - Pérez-Ramírez, Javier / Pérez-Ramírez, Javier
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