ABSTRACT
The structures of solids can locally differ from the macroscopic picture obtained by structural averaging techniques. This difference significantly influences the performance of any functional material. Measurements of these local structures are challenging. Thus, the description of defects is often disregarded. However, in order to understand the functionality, such irregularities have to be investigated. Here, we present a high resolution scanning transmission electron microscopic (STEM) study revealing local structural irregularities in open structured oxides using catalytically active orthorhombic (Mo,V,Te,Nb)Ox as a complex example. Detailed analysis of annular dark field- and annular bright field-STEM images reveal site specific local structural displacements of individual framework and channel sites in the picometer range. These experimental observables can be considered as an important structural addendum for theoretical modelling and should be implemented into the existing data in order to quantify site specific potential energies and stresses. This information can further be used to describe the impact of the structure on the catalytic performance in greater detail.
ABSTRACT
We present an effective procedure to differentiate instrumental artefacts, such as parasitic ions, memory effects, and real trace impurities contained in inert gases. Three different proton transfer reaction mass spectrometers were used in order to identify instrument-specific parasitic ions. The methodology reveals new nitrogen- and metal-containing ions that up to date have not been reported. The parasitic ion signal was dominated by [N2 ]H+ and [NH3 ]H+ rather than by the common ions NO+ and O2 + . Under dry conditions in a proton transfer reaction quadrupole interface time-of-flight mass spectrometer (PTR-QiTOF), the ion abundances of [N2 ]H+ were elevated compared with the signals in the presence of humidity. In contrast, the [NH3 ]H+ ion did not show a clear humidity dependency. On the other hand, two PTR-TOF1000 instruments showed no significant contribution of the [N2 ]H+ ion, which supports the idea of [N2 ]H+ formation in the quadrupole interface of the PTR-QiTOF. Many new nitrogen-containing ions were identified, and three different reaction sequences showing a similar reaction mechanism were established. Additionally, several metal-containing ions, their oxides, and hydroxides were formed in the three PTR instruments. However, their relative ion abundancies were below 0.03% in all cases. Within the series of metal-containing ions, the highest contribution under dry conditions was assigned to the [Fe(OH)2 ]H+ ion. Only in one PTR-TOF1000 the Fe+ ion appeared as dominant species compared with the [Fe(OH)2 ]H+ ion. The present analysis and the resulting database can be used as a tool for the elucidation of artefacts in mass spectra and, especially in cases, where dilution with inert gases play a significant role, preventing misinterpretations.
ABSTRACT
The well-defined particle morphology of crystalline MnWO4 catalysts investigated in the present study facilitates obtaining insight into the origin of selectivity limitations in alkane oxidation. Hydrothermal synthesis at variable pH values granted access to a series of phase-pure MnWO4 catalysts with particles ranging from cube-like (aspect ratio 1.5) to rod- or needle-like (aspect ratio 6.8) shapes. Kinetic studies reveal a strong dependence of the propane consumption rate on the particle shape. The true origin of the structure sensitivity was unraveled by comprehensive bulk and surface analysis using nitrogen adsorption, XRD, SEM, ADF-STEM, STEM-EELS, XPS, multi-laser excitation Raman and DRIFT/operando FTIR spectroscopies, temperature-programmed oxidation (TPO), in situ NEXAFS, and DFT calculations. The active phase is composed of a thin manganese oxy-hydroxide layer formed on the surface of crystalline MnWO4. The differences in catalytic performance within the series clearly illustrate that the structural motif as the most popular descriptor in oxidation catalysis is not essential, since all MnWO4 catalysts in the series under study exhibit the same bulk crystal structure and bulk chemical composition and are phase pure and homogenous. The variable particle shape serves as a proxy that reflects the formation of varying abundance of redox active Mn2+/Mn3+ surface sites, which correlates with catalytic activity. Operando FTIR spectroscopy directly confirms the formation of Mn-OH surface species by abstraction of hydrogen atoms from the propane molecule on nucleophilic oxygen atoms and suggests that active site regeneration occurs via oxidative dehydrogenation of Mn-OH species indicating a single-site nature of the active sites that does not allow four-electron reduction of molecular oxygen. Instead, intermediates are created that cause side reactions and lower the selectivity. The findings highlight fundamental design criteria that may be applied to advance the development of new alkane oxidation catalysts with improved selectivity.
ABSTRACT
We present a versatile grid reactor setup for transmission electron microscopy (TEM), which is able to track catalytic conversion on TEM amounts of sample. It is based on the concept of decoupling catalytic gas-phase reactions from the structural analysis of identical particles before and after reaction. The system has superior properties in terms of image resolution and long-term measurements compared to conventional in situ TEM analysis. Monitoring catalytic conversions on a TEM grid is enabled by proton-transfer reaction mass spectrometry. In addition, identical location imaging benefits from a secure transfer of the sample between TEM and the reactor system by vacuum transfer holders. Using Pt and Cu/ZnO/Al2O3 as an example we show that structural changes of identical particles or areas of a Pt foil before and after reactive experiments can be tracked. During catalytic testing the samples are exposed to homogeneous reaction conditions. The concept minimizes electron-sample and electron-atmosphere interactions and can prospectively be considered as complementary tool to in situ TEM analysis.
