On the Cobalt Carbide Formation in a Co/TiO2 Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction

Chemical Imaging of Carbide Formation and Its Effect on Alcohol Selectivity in Fischer Tropsch Synthesis on Mn-Doped Co/TiO2 Pellets

X-ray diffraction/scattering computed tomography (XRS-CT) was used to create two-dimensional images, with 20 μm resolution, of passivated Co/TiO2/Mn Fischer-Tropsch catalyst extrudates postreaction after 300 h on stream under industrially relevant conditions. This combination of scattering techniques provided insights into both the spatial variation of the different cobalt phases and the influence that increasing Mn loading has on this. It also demonstrated the presence of a wax coating throughout the extrudate and its capacity to preserve the Co/Mn species in their state in the reactor. Correlating these findings with catalytic performance highlights the crucial phases and active sites within Fischer-Tropsch catalysts required for understanding the tunability of the product distribution between saturated hydrocarbons or oxygenate and olefin products. In particular, a Mn loading of 3 wt % led to an optimum equilibrium between the amount of hexagonal close-packed Co and Co2C phases resulting in maximum oxygenate selectivity. XRS-CT revealed Co2C to be located on the extrudates' periphery, while metallic Co phases were more prevalent toward the center, possibly due to a lower [CO] ratio there. Reduction at 450 °C of a 10 wt % Mn sample resulted in MnTiO3 formation, which inhibited carbide formation and alcohol selectivity. It is suggested that small MnO particles promote Co carburization by decreasing the CO dissociation barrier, and the Co2C phase promotes CO nondissociative adsorption leading to increased oxygenate selectivity. This study highlights the influence of Mn on the catalyst structure and function and the importance of studying catalysts under industrially relevant reaction times.

Direct Evidence of Dynamic Metal Support Interactions in Co/TiO2 Catalysts by Near-Ambient Pressure X-ray Photoelectron Spectroscopy

The interaction between metal particles and the oxide support, the so-called metal-support interaction, plays a critical role in the performance of heterogenous catalysts. Probing the dynamic evolution of these interactions under reactive gas atmospheres is crucial to comprehending the structure-performance relationship and eventually designing new catalysts with enhanced properties. Cobalt supported on TiO2 (Co/TiO2) is an industrially relevant catalyst applied in Fischer-Tropsch synthesis. Although it is widely acknowledged that Co/TiO2 is restructured during the reaction process, little is known about the impact of the specific gas phase environment at the material's surface. The combination of soft and hard X-ray photoemission spectroscopies are used to investigate in situ Co particles supported on pure and NaBH4-modified TiO2 under H2, O2, and CO2:H2 gas atmospheres. The combination of soft and hard X-ray photoemission methods, which allows for simultaneous probing of the chemical composition of surface and subsurface layers, is one of the study's unique features. It is shown that under H2, cobalt particles are encapsulated below a stoichiometric TiO2 layer. This arrangement is preserved under CO2 hydrogenation conditions (i.e., CO2:H2), but changes rapidly upon exposure to O2. The pretreatment of the TiO2 support with NaBH4 affects the surface mobility and prevents TiO2 spillover onto Co particles.

Cobalt Stabilization through Mesopore Confinement on TiO2 Support for Fischer-Tropsch Reaction

Cobalt supported on mesostructured TiO2 catalysts has been prepared by a wet-impregnation method. The Co/TiO2 catalytic system showed better catalytic performance after support calcination at 380 °C. Co nanoparticles appeared well distributed along the mesopore channels of TiO2. After reduction pretreatment and reaction, a drastic structural change leads to mesopore structure collapse and the dispersion of the Co nanoparticles on the external surface. Along this complex process, Co species first form discrete nanoparticles inside the pore and then diffuse out as the pore collapses. Through this confinement, a strong metal-support interaction effect is hindered, and highly stable metal active sites lead to better performance for Fischer-Tropsch synthesis reaction toward C5+ products.

CatMass: software for calculating optimal sample masses for X-ray absorption spectroscopy experiments involving complex sample compositions

This paper presents software for calculating the optimal mass of samples with complex compositions (e.g. supported metal catalysts) for X-ray absorption spectroscopy (XAS) and scattering measurements. The ability to calculate the sample mass and other relevant parameters needed for an XAS measurement allows experimentalists to be better prepared in terms of detector selection, energy range of scan and overall time needed to complete the measurement, thus increasing efficiency. CatMass builds on existing sample mass calculators allowing users to determine the optimum sample preparation, collection geometry, usable energy range for a scan and approximate edge step of the absorption event. Visualization tools present the absorption calculation results in a format familiar to XAS experimentalists, with the added ability to save calculations and plots for future reference or recalculation. CatMass is a program broadly applicable in catalysis and is helpful for users with complex samples due to composition/stoichiometry or multiple competing elements.

Uncovering the reaction mechanism behind CoO as active phase for CO2 hydrogenation

Transforming carbon dioxide into valuable chemicals and fuels, is a promising tool for environmental and industrial purposes. Here, we present catalysts comprising of cobalt (oxide) nanoparticles stabilized on various support oxides for hydrocarbon production from carbon dioxide. We demonstrate that the activity and selectivity can be tuned by selection of the support oxide and cobalt oxidation state. Modulated excitation (ME) diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that cobalt oxide catalysts follows the hydrogen-assisted pathway, whereas metallic cobalt catalysts mainly follows the direct dissociation pathway. Contrary to the commonly considered metallic active phase of cobalt-based catalysts, cobalt oxide on titania support is the most active catalyst in this study and produces 11% C2+ hydrocarbons. The C2+ selectivity increases to 39% (yielding 104 mmol h-1 gcat-1 C2+ hydrocarbons) upon co-feeding CO and CO2 at a ratio of 1:2 at 250 °C and 20 bar, thus outperforming the majority of typical cobalt-based catalysts.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

Operando structural science of functional materials

This editorial gives a brief overview of operando structural science of functional materials with emphasis on catalytic science.