Periodic Reporting for period 1 - PraMixCat (Operando studies of praseodymium based mixed oxide supported metal catalysts for the direct conversion of methane to methanol: bridge the gap of model catalysis and ambient applications)
Periodo di rendicontazione: 2023-08-01 al 2025-10-31
The sustainable utilization of methane is a major challenge for both science and industry. Methane is abundant in natural gas and biogas, yet its strong C–H bond and the high reactivity of methanol make selective conversion extremely difficult. Current industrial routes rely on multi-step processes, involving high-temperature methane reforming followed by high-pressure synthesis of methanol from syngas, resulting in a significant carbon footprint. Developing a direct and mild reaction pathway therefore strongly aligns with EU priorities such as the European Green Deal, REPowerEU, and the transition towards sustainable chemical production.
Originally, the project planned to explore praseodymium oxide and its mixed oxides for the direct conversion of methane to methanol (DCMM). Preliminary experiments, however, showed that praseodymium-based materials predominantly yield CO2 as the main product. This behaviour can be attributed to the intrinsically high oxygen reactivity of praseodymium oxides, which is difficult to modulate. Even the introduction of water is insufficient to suppress overly reactive surface oxygen species. These findings motivated a strategic shift towards ceria-based systems.
The oxygen species on ceria-based materials exhibit moderate activity, being neither overly reactive nor inert, which makes them well suited for selective oxidation reactions. Beyond pure ceria, mixed oxides such as Ce–ZrOₓ and Ce–LaOₓ offer opportunities to tune oxygen vacancy structures and surface oxygen species, enabling systematic exploration of structure–performance relationships for the direct conversion of methane to methanol. Previous studies have largely focused on ultra-high-vacuum model systems, leaving significant “pressure” and “materials” gaps relative to realistic catalytic environments.
This project aimed to bridge these gaps by integrating continuous-flow catalysis, advanced in situ and operando spectroscopy, and density functional theory (DFT). In addition to establishing ceria as a model oxide for partial methane oxidation, the project investigated Ce–ZrOₓ and Ce–LaOₓ mixed oxides to understand how compositional modification influences methane activation and methanol selectivity. The overall objectives were:
1.To determine the mechanisms of methane activation on ceria and ceria-based mixed oxide surfaces, and to clarify the roles of water and oxygen species under realistic reaction conditions.
2.To derive mechanistic principles linking surface composition, the nature and abundance of oxygen species and oxygen vacancies, thereby guiding the design of more active and selective oxide catalysts for the direct conversion of methane to methanol.
3.To develop transferable scientific insights that underpin the rational design and optimization of oxide catalysts for methane conversion under realistic operating conditions.
Originally, the project planned to explore praseodymium oxide and its mixed oxides for the direct conversion of methane to methanol (DCMM). Preliminary experiments, however, showed that praseodymium-based materials predominantly yield CO2 as the main product. This behaviour can be attributed to the intrinsically high oxygen reactivity of praseodymium oxides, which is difficult to modulate. Even the introduction of water is insufficient to suppress overly reactive surface oxygen species. These findings motivated a strategic shift towards ceria-based systems.
The oxygen species on ceria-based materials exhibit moderate activity, being neither overly reactive nor inert, which makes them well suited for selective oxidation reactions. Beyond pure ceria, mixed oxides such as Ce–ZrOₓ and Ce–LaOₓ offer opportunities to tune oxygen vacancy structures and surface oxygen species, enabling systematic exploration of structure–performance relationships for the direct conversion of methane to methanol. Previous studies have largely focused on ultra-high-vacuum model systems, leaving significant “pressure” and “materials” gaps relative to realistic catalytic environments.
This project aimed to bridge these gaps by integrating continuous-flow catalysis, advanced in situ and operando spectroscopy, and density functional theory (DFT). In addition to establishing ceria as a model oxide for partial methane oxidation, the project investigated Ce–ZrOₓ and Ce–LaOₓ mixed oxides to understand how compositional modification influences methane activation and methanol selectivity. The overall objectives were:
1.To determine the mechanisms of methane activation on ceria and ceria-based mixed oxide surfaces, and to clarify the roles of water and oxygen species under realistic reaction conditions.
2.To derive mechanistic principles linking surface composition, the nature and abundance of oxygen species and oxygen vacancies, thereby guiding the design of more active and selective oxide catalysts for the direct conversion of methane to methanol.
3.To develop transferable scientific insights that underpin the rational design and optimization of oxide catalysts for methane conversion under realistic operating conditions.
1.Continuous-flow methane-to-methanol performance
The catalytic performance of CeO2 and ceria-based mixed oxides (Ce–ZrOₓ and Ce–LaOₓ) was systematically evaluated for the DCMM under continuous-flow conditions. Reaction parameters, including gas flow rate, temperature, and feed composition, were optimized to identify conditions favoring selective methane oxidation.
The influence of key gas components was examined in detail. The roles of oxygen and water were investigated to clarify their impact on methane activation, methanol formation, and catalyst stability, while the effect of hydrogen was assessed to evaluate its influence on surface oxygen species and reaction activity. These studies enabled a consistent comparison of pure ceria and mixed oxide catalysts and provided a robust basis for correlating catalyst composition with performance under realistic operating conditions.
Main achievement:
The project showed that rational tuning of ceria composition enables metal-free catalysts to achieve high activity, high selectivity, and long-term stability for methane-to-methanol conversion under realistic continuous-flow conditions.
2.In situ and operando insights into the reaction mechanism
To elucidate the reaction mechanism, in situ and operando spectroscopic techniques were employed to probe surface chemistry on ceria-based catalysts under realistic reaction conditions. Steady-state in situ infrared spectroscopy revealed the evolution of surface hydroxyl groups and key carbon-containing intermediates, including formate and methoxy species.
Operando ambient-pressure X-ray photoelectron spectroscopy performed at the SPECIES beamline (MAX-IV) enabled direct tracking of surface oxygen and carbon species, as well as changes in the chemical states of cerium, under different gas compositions and temperatures.
Main achievement:
These complementary spectroscopic studies demonstrated that the synergy between oxygen and water dynamically regulates surface oxygen species, promotes rapid methoxy turnover, and suppresses deep oxidation pathways, thereby underpinning the high selectivity observed in continuous-flow methane-to-methanol conversion.
3.Theoretical calculations
Density functional theory (DFT) calculations were performed to investigate methane activation on CeO2 (111) and CeO2 (110) surfaces. The C–H bond activation barriers were evaluated under dry conditions and in the presence of O2 and H2O to quantify how these components modify the reaction energetics. Different reaction pathways were systematically compared to identify the most energetically favourable routes.
By correlating theoretical results with experimental observations, a complete reaction energy profile was established, describing the transformation of methane from initial C–H bond activation to the formation of methanol.
Main achievement:
The theoretical analysis delivered a coherent, energetically consistent reaction pathway for methane-to-methanol conversion on ceria, explaining the experimentally observed activity and selectivity and providing a mechanistic framework to guide the rational design of improved ceria-based oxide catalysts.
The catalytic performance of CeO2 and ceria-based mixed oxides (Ce–ZrOₓ and Ce–LaOₓ) was systematically evaluated for the DCMM under continuous-flow conditions. Reaction parameters, including gas flow rate, temperature, and feed composition, were optimized to identify conditions favoring selective methane oxidation.
The influence of key gas components was examined in detail. The roles of oxygen and water were investigated to clarify their impact on methane activation, methanol formation, and catalyst stability, while the effect of hydrogen was assessed to evaluate its influence on surface oxygen species and reaction activity. These studies enabled a consistent comparison of pure ceria and mixed oxide catalysts and provided a robust basis for correlating catalyst composition with performance under realistic operating conditions.
Main achievement:
The project showed that rational tuning of ceria composition enables metal-free catalysts to achieve high activity, high selectivity, and long-term stability for methane-to-methanol conversion under realistic continuous-flow conditions.
2.In situ and operando insights into the reaction mechanism
To elucidate the reaction mechanism, in situ and operando spectroscopic techniques were employed to probe surface chemistry on ceria-based catalysts under realistic reaction conditions. Steady-state in situ infrared spectroscopy revealed the evolution of surface hydroxyl groups and key carbon-containing intermediates, including formate and methoxy species.
Operando ambient-pressure X-ray photoelectron spectroscopy performed at the SPECIES beamline (MAX-IV) enabled direct tracking of surface oxygen and carbon species, as well as changes in the chemical states of cerium, under different gas compositions and temperatures.
Main achievement:
These complementary spectroscopic studies demonstrated that the synergy between oxygen and water dynamically regulates surface oxygen species, promotes rapid methoxy turnover, and suppresses deep oxidation pathways, thereby underpinning the high selectivity observed in continuous-flow methane-to-methanol conversion.
3.Theoretical calculations
Density functional theory (DFT) calculations were performed to investigate methane activation on CeO2 (111) and CeO2 (110) surfaces. The C–H bond activation barriers were evaluated under dry conditions and in the presence of O2 and H2O to quantify how these components modify the reaction energetics. Different reaction pathways were systematically compared to identify the most energetically favourable routes.
By correlating theoretical results with experimental observations, a complete reaction energy profile was established, describing the transformation of methane from initial C–H bond activation to the formation of methanol.
Main achievement:
The theoretical analysis delivered a coherent, energetically consistent reaction pathway for methane-to-methanol conversion on ceria, explaining the experimentally observed activity and selectivity and providing a mechanistic framework to guide the rational design of improved ceria-based oxide catalysts.
The project delivered several advances beyond the current state of the art:
Metal-free ceria for DCMM under realistic conditions.
The project provided the first mechanistic evidence that pure ceria can catalyse the direct conversion of methane to methanol under continuous-flow and atmospheric-pressure conditions. In contrast, tests with noble-metal-modified ceria showed a strong tendency towards deep oxidation to CO2 rather than selective methanol formation.
Identification of an O2–H2O synergistic mechanism.
Oxygen replenishes reactive lattice oxygen and promotes water dissociation, while water generates surface hydroxyl species that facilitate methane activation, methoxy formation, and methanol release, collectively suppressing deep oxidation.
Design principles for ceria-based mixed oxides.
The results establish that controlling surface oxygen species, hydroxyl mobility, and lattice oxygen regeneration is critical for achieving selective methane conversion.
Together, these advances provide a clear mechanistic basis for the rational design of improved oxide catalysts for methane valorisation.
Metal-free ceria for DCMM under realistic conditions.
The project provided the first mechanistic evidence that pure ceria can catalyse the direct conversion of methane to methanol under continuous-flow and atmospheric-pressure conditions. In contrast, tests with noble-metal-modified ceria showed a strong tendency towards deep oxidation to CO2 rather than selective methanol formation.
Identification of an O2–H2O synergistic mechanism.
Oxygen replenishes reactive lattice oxygen and promotes water dissociation, while water generates surface hydroxyl species that facilitate methane activation, methoxy formation, and methanol release, collectively suppressing deep oxidation.
Design principles for ceria-based mixed oxides.
The results establish that controlling surface oxygen species, hydroxyl mobility, and lattice oxygen regeneration is critical for achieving selective methane conversion.
Together, these advances provide a clear mechanistic basis for the rational design of improved oxide catalysts for methane valorisation.