This Science, re-examine the catalyst reconstruction!

Special Notes:In this paper, by the study of foreign exchange technology The original writing of the center aims to share relevant scientific research knowledge. Due to limited knowledge, there are inevitably omissions and mistakes. Readers are requested to read critically and to criticize and correct them.OriginalTong heart is not lost(Xue Yan Hui Technical Center) EditorFengyun

research background

Supported metal catalysts respond to pretreatment and reaction conditions by recombination, phase changes, and chemical and structural oscillations, which can lead to catalyst activation, deactivation, or changes in selectivity. A good example of catalyst restructuring is the so-calledstrong metal-Carrier Interaction (SMSI), I .e. metal nanoparticles(NPs) are reduced or oxidized at high temperature by reducible carriers (such as TiO₂) of the covering wrap. SMSI can lead to a large change in product selectivity, which is in line with the high activity of mixed metal oxides in the oxygen evolution reaction and industrial Cu/ZnO_x/Al₂O₃The activation of the catalyst.

Key issues

However, the research on the reconstitution process of the catalyst in the reaction process still has the following problems:

1. Little is known about the structure of the cover layer under reaction conditions.Although there is substantial evidence of overburden formation during catalyst pretreatment, including fromH₂,O₂andCO₂Atomic resolution in situ electron microscopy studies of the formation of the capping layer under ambient conditions, but little is known about the structure of the capping layer under reaction conditions.

2. The effect of the covering layer on the catalytic performance under the reaction conditions is still unknown.Recently, in situTEM shows Pt/TiO₂Completely removedTiO_xoverburden, but whether the overburden persists under the reaction conditions, or is used to induceIt remains unclear whether pretreatment of SMSI only indirectly affects catalyst performance.

3, the development of metal NP requires a multi-scale operation method.Multi-scale operation methods (from individual nanoparticles to the overall level) are useful for supporting metals. It is essential for NPs to develop meaningful structure-performance relationships.

new ideas

in view of this,Utrecht University, The NetherlandsBert M. Weckhuysenby a combination of in situ electron microscopy and vibrational spectroscopy, et al.Thin TiO formed on a nickel/titania catalyst during reduction at 400°C_xThe capping layer was completely removed under carbon dioxide hydrogenation conditions. On the contrary, inAfter reduction at 600°C, exposure to carbon dioxide hydrogenation reaction conditions resulted in only partial re-exposure of nickel to form the same TiO_xinterface sites of contact, and by providing a pool of carbon species in favor of carbon-Carbon coupling. The findings of this workChallenged yes.traditional understanding of SMSI and calls for more detailed operational studies of nanocatalysts at the single-particle levelto re-examine the structure-Static model of liveness relations.

Technical scheme:

1. The partial coverage of Ni in the reduction process at 400°C was analyzed.By means of in situ electron microscopy experiments, the authors investigated the loading at the single-particle level The formation of TiOx coating on Ni NPs, the surface selective coverage of Ni NPs is explained by theoretical calculation.

2. Ni/TiO is shown₂InCO₂in the hydrogenation processNi re-exposureThe author followed.CO₂hydrogenation reaction conditionsTiO_xThe change of the overlayer confirms the dynamic evolution of the catalyst during the electron microscope observation to express its more active surface.

3. The reduction of Ni/TiO at 600°C was explored₂ofTiO_xOverburden formation and reorganizationThe author prepared600-Ni/TiO₂, showing a fully encapsulated and relatively thickTiO_xoverlay, inCO₂under the hydrogenation reaction conditions,600-Ni/TiO₂are still mainlyTiO_xThe package.

4. The catalytic performance was compared and the reaction mechanism was analyzed.Catalytic tests showed that both catalysts wereCO at 200°C to 400°C and 5 bar₂Hydrogenation is active,600-Ni/TiO₂C of the catalyst₂ Selective enhancement becomes more pronounced. proposed a recombination under reaction conditionsNi/TiO₂model of the catalyst to explain the variation in catalyst performance.

Technical advantages:

1. For the first time, the recombination of metal oxide coatings under operating conditions has been directly observed.For the first time, the reorganization of metal oxide coatings under operating conditions has been directly observed, and their effects on the reduction temperature have been rationalized.CO/CO₂in hydrogenation reactionEffect of Ni activity and selectivity.

2, reveals the reaction process of Ni/TiO₂On catalystTiO_xEvolution of overburdenThe authors reveal that at low temperatures(400°C) and high temperature (600°C) after reduction, used for CO and CO₂Hydrogenation reactions of industrial relevance in catalytic processesNi/TiO₂On catalystTiO_xEvolution of the overlay.

Uncover information at the single-atom level with unprecedented resolutionCombining multiple characterizations and calculations to observe and understand superlayer reorganization and its impact on catalysis, the combination of nano-and bulk-operation analysis techniques reveals information at the single-atom level with unprecedented resolution.

4. Provides new insights into the properties of the loaded metal nanoparticle systemThe author customized the low-oxide coating to keep it stable under the reaction conditions of different reduction pretreatment temperatures. The results can help understand the performance of many supporting metals in sustainable technical reactions, such as biomass andCO₂High-value utilization, etc.

Technical Details

Partial coverage of Ni during reduction at 400°CIn order to passSMSI-induced oxide coating and its application in CO₂role in hydrogenation, the authors in 400°C (400-Ni/TiO₂) reduced 6 wt% Ni/TiO₂Catalyst. By in situ electron microscopy experiments in a window gas chamber,The loaded are studied at the single-particle level Ni NPs上TiO_xFormation of cover layer. The author analyzedThe particle size of Ni NPs is also confirmed by the particle size of Ni NPs. by 400-Ni/TiO₂in situHAADF-STEM imaging, TiO was detected on Ni NPs_xThe cover layer corresponds to the partially encapsulated double layer. The author calculatedThe strain changes and Ti-Ti and Ni-Ni of the Ni surface atomic column further confirm the continuous TiO_xOverlay. Through theoretical calculations, it is found that inFormation of TiO on Ni(111)₂cover with a lower energy loss, explaining the experimentally observed The surface selective coverage of Ni NPs, and shows that the cover layer is likely to be a low oxide phase.

Figure  在Ni/TiO₂Hydrogenation catalyst through low temperatureH₂reduction formationTiO_xOverburden and its presence inCO₂RECONSTRUCTION IN THE PROCESS OF METHANATION

Ni/TiO after 400°C reduction₂InCO₂in the hydrogenation processNi re-exposureAuthors by usingHAADF-STEM track a single Ni NP while at 400°C and atmospheric pressure from H₂SwitchCO₂:H₂, trackingCO₂hydrogenation reaction conditionsTiO_xOverlay changes. Mass spectrometry was used to track the reactant and product gases from the windowed gas chamber in real time during the reaction. IntroductionCO₂:H₂After the mixture can be detectedCH₄The formation of the catalyst was confirmed during the electron microscope observation.CO₂Hydrogenation.Ni NP formed a TiO_xcapping layer, which, prior to hydrogenation, selectively occupies(111) surface of Ni. Exposure to CO₂After the sameNi NPs reorganize and the NPs take on an overall rounded morphology and retain their Ni metallic phase. Supported Ni catalyst in CO/CO₂The hydrogenation process evolves dynamically to express its more active surface. The author measuredDecay of the Ni 2p signal relative to the Ti 2p signal in NAP-XPS, indicating that exposure to CO₂under hydrogenation conditionsTiO_xThe removal of the cover represents the majority of the sample.

FigureReduction of Ni/TiO at high temperature₂On hydrogenation catalystCO₂stable during methanationTiO_x/Formation of Ni coating

Reduction of Ni/TiO at 600°C₂ofTiO_xOverburden formation and reorganizationIn order to form a more stableTiO_xoverlay, capableCO₂survived under hydrogenation conditions, in 600°C(600-Ni/TiO₂) in-situ reduction of 6 wt% Ni/TiO₂Catalyst. itsThe NP diameter approximately doubled to 14.0 nm. 600-Ni/TiO₂showed a fully encapsulated and relatively thickTiO_xOverlay. Through in-situEELS analysis confirms that TiO_xThe formation of the cover layer, which shows the entireTi L in Ni NP_2,3Ionization edge signal. Compared to the original catalyst,TiO_x-Low oxide overburden is still visible, but in CO₂It became more inhomogeneous after exposure, indicating its partial degradation. In-situSTEM results show that in CO₂under the hydrogenation reaction conditions,600-Ni/TiO₂are still mainlyTiO_xThe package. Therefore, it is expected that inAlmost complete loss of activity of such catalysts was observed in Ni-promoted reactions.

FigureIn situ vibrational spectroscopy reveals CO and₂under hydrogenation conditionsNi/TiO₂re-exposure

comparison of catalytic performanceThe catalyst was tested by in situ Fourier transform infrared spectroscopy under hydrogenation conditions, and the residualTiO_xOverburden pair Ni/TiO₂catalyst catalysisCO/CO₂Effect of hydrogenation performance. The results show that the two catalysts inCO at 200°C to 400°C and 5 bar₂Hydrogenation is active. In CO/CO₂In the co-hydrogenation experiment,C of 600-Ni/TiO2 catalyst₂ Selective enhancement becomes more pronounced. under the reaction conditions,600-Ni/TiO₂The re-exposure of the Ni surface in the reaction affected the product selectivity.
mechanism analysis and structure sensitivityproposed a recombination under reaction conditionsNi/TiO₂The model of the catalyst is used to explain the changes in surface chemistry and ultimately the changes in catalyst performance. In-situ The combination of FTIR spectra and HAADF-STEM results shows the formation of model structures. The authors propose that the increased Ni-TiO_xinterface by providing an interface withThe adsorbate on Ni closely contacts the C species reservoir and assists in C- C coupling by stabilizing intermediate and transition states.
 

Figureon reduction and reaction process of Ni/TiO₂New understanding of catalyst performance and evolution
 

OutlookIn short, the author directly observed the reorganization of metal oxide coatings under operating conditions for the first time, and rationalized their impact on the reduction temperatureCO/CO₂in hydrogenation reactionEffect of Ni activity and selectivity. AuthorprovidedNi-TiO_xOperational evidence for the formation of interfacial sites and demonstrate their effect on catalytic performance. Similar operating methods can be applied to understand many other chemical reactions. The key to this effort is to simultaneously measure catalytic performance and monitor and control the support, such as crystal phase, porosity, oxidation state and exposed surface, and supported active phase nanostructures, suchNP size, shape and composition.

References:MATTEO MONAI, et al. Restructuring of titanium oxide overlayers over nickel nanoparticles during catalysis. Science, 2023, 380(6645): 644-651DOI: 10.1126/science.adf6984https://www.science.org/doi/10.1126/science.adf6984