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Ga-containing oxides have been widely investigated as catalysts for acceptorless propane dehydrogenation. During catalyst activation by H2 at elevated temperatures, Ga can migrate. Nevertheless, the PDH active sites have not typically been considered mobile under reaction conditions. Evidence for mobility during activation is indirect and consists largely of changes in the hydroxyl population, observed using IR spectroscopy. In this study, Magic-Angle Spinning NMR and IR spectroscopies, STEM-EDS, and conventional kinetic measurements are enhanced by analysis of the in situ XANES. They establish that Ga(I) is the highly mobile species, and that it can interconvert with active sites. Since the XANES signatures differ significantly for Ga(I)/SiO2 vs Ga(I)/ZSM-5, it was possible using linear combination fitting to monitor and quantify the dynamic redistribution of Ga species in physical mixtures of SiO2 and HZSM-5. Under H2, Ga(III) is reduced to Ga(I), which then diffuses rapidly between SiO2 and ZSM-5 particles whose diameters are ca. 200 μm. This migration can occur in both directions, and is significantly faster than Ga(III) reduction. As a result, a mixture of weakly active Ga/SiO2 and HZSM-5 shows a PDH activity approaching that of highly active Ga/ZSM-5. Thermodynamic analysis of the equilibrium populations on both supports suggests an enthalpic preference for Ga(III) and an entropic preference for Ga(I). Ga ions in both oxidation states are enthalpically more stable on HZSM-5, while Ga association with SiO2 is likely entropically favored. These thermodynamic preferences, in combination with long-range Ga(I) mobility, predict and explain the temperature-dependent redistribution of Ga species between oxidation states, across supports, and even onto catalyst bed diluents under reducing conditions. The high mobility of Ga(I) during catalyst activation and under reaction conditions enables Ga to sample a large range of environments, challenging long-standing assumptions about the ability to precisely synthesize single-site Ga catalysts for alkane dehydrogenation. 

Ga-containing oxides are selective catalysts for high-temperature hydrocarbon oxidations such as alkane dehydrogenation. The active sites are proposed to have an oxidation state of either +1 or +3 and bear a combination of hydride and alkyl ligands, in addition to oxygen-donor ligands associated with the support. Activating Ga/-Al2O3 or Ga/ZSM5 in H2 at or above 473 K causes the Ga K-edge to (1) shift decisively to lower energy, and (2) increase significantly in its white line intensity. Taken together, these behaviors are shown to be signatures for Ga(III) reduction. The extent of Ga(I) formation, assessed by quantitative XANES analysis, varies with temperature, H2 pressure, and the nature of the support. In addition, H2 causes strong suppression of the EXAFS amplitude. The origin of the latter phenomenon was investigated by analyzing Ga(I) EXAFS signatures in two well-defined model compounds: mixed-valent Ga2Cl4 and Ga(I)-β´´-Al2O3. Theoretical simulations show the EXAFS “invisibility” of Ga(I) at non-cryogenic temperatures is a direct result of anomalously large vibrational amplitudes. Due to its valence electron configuration and largely unhybridized 4p orbitals, Ga(I) forms very long, labile bonds whose mean-squared relative displacements (σ2) are an order of magnitude higher compared to the shorter, stronger bonds of Ga(III). Consequently, EXAFS signals for materials containing a mixture of Ga oxidation states are virtually devoid of structural information about the Ga(I) sites, being dominated instead by scattering paths involving residual Ga(III) sites. At reaction temperatures in the H2-activated catalysts, hydrides are at best minority species, accompanying the majority Ga(I) sites. These insights set the stage for quantitative analysis of Ga speciation during alkane dehydrogenation, and elucidation of the structure(s) of the active sites. Furthermore, they have broad implications for the use of X-ray spectroscopy to extract structure-property relations for catalysts and other materials containing Ga(I) or other weakly-bonded and highly dynamic metal ions. 

A sustainable and robust supply chain of rare earth elements (REEs) is necessary to meet our consumer, national security and clean energy goals. However, current intra-REE separation technologies (e.g.solvent extraction) are costly and carry a heavy environmental burden. Therefore, the development of new aqueous based ligands that are selective for individual REEs will be integral in future REE production systems. To develop these ligands, an understanding of how ligand coordination structure relates to selectivity is imperative. We used X-ray absorption spectroscopy (XAS) to observe the local structure around four lanthanide (Ln) ions (La, Ce, Pr and Nd) complexed by water and several relevant chelating ligands [lanmodulin EF-hand 1 peptides (LanM1), ethylenediaminetetraacetic acid (EDTA), aminotris(methylenephosphonic acid) (ATMP) and citric acid]. To collect these liquid-phase XAS spectra, we developed a new flow cell that prevents bubble interference and beam damage to the samples. In the X-ray absorption near-edge structure (XANES), we observed energy shifts in the white line, white line broadening and differences in the white line intensity of different Ln–ligand complexes between ligands. In the extended X-ray absorption fine structure (EXAFS), we distinguished differences in peak intensity and distance between coordinating ligands. Differences in the local coordination structure between Ln–LanM1 peptide complexes were more subtle compared with the other ligands (La–water, La–EDTA, La–ATMP and La–citric acid complexes). Further XANES and EXAFS studies, in combination with modelling and other techniques, could greatly improve our structural knowledge of how these aqueous ligands bind Ln ions and how they can be used to design more selective ligands for more efficient and sustainable REE separations. 

The unique properties of the lanthanide (Ln) elements make them critical components of modern technologies, such as lasers, anti-corrosive films and catalysts. Thus, there is significant interest in establishing structure–property relationships for Ln-containing materials to advance these technologies. Extended X-ray absorption fine structure (EXAFS) is an excellent technique for this task considering its ability to determine the average local structure around the Ln atoms for both crystalline and amorphous materials. However, the limited availability of EXAFS reference spectra of the Ln oxides and challenges in the EXAFS analysis have hindered the application of this technique to these elements. The challenges include the limitedk-range available for the analysis due to the superposition ofL-edges on the EXAFS, multielectron excitations (MEEs) creating erroneous peaks in the EXAFS and the presence of inequivalent absorption sites. Herein, we removed MEEs to model the local atomic environment more accurately for light Ln oxides. Further, we investigated the use of cubic and non-cubic lattice expansion to minimize the fitting parameters needed and connect the fitting parameters to physically meaningful crystal parameters. The cubic expansion reduced the number of fitting parameters but resulted in a statistically worse fit. The non-cubic expansion resulted in a similar quality fit and showed non-isotropic expansion in the crystal lattice of Nd₂O₃. In total, the EXAFS spectra and the fits for the entire set of Ln oxides (excluding promethium) are included. The knowledge developed here can assist in the structural determination of a wide variety of Ln compounds and can further studies on their structure–property relationships. 

CO oxidation in the presence of NO over PdCu alloys takes place with high activity and low inhibition over a 3 : 1 PdCu alloy, which has the optimal Pd : Cu ratio on the surface. 

The requirement for C₂H₂concentrations below 2 parts per million (ppm) in gas streams for C₂H₄polymerization necessitates its semihydrogenation to C₂H₄. We demonstrate selective chemical looping combustion of C₂H₂in C₂H₄-rich streams by Bi₂O₃as an alternative catalytic pathway to reduce C₂H₂concentration below 2 ppm. Bi₂O₃combusts C₂H₂with a first-order rate constant that is 3000 times greater than the rate constant for C₂H₄combustion. In successive redox cycles, the lattice O of Bi₂O₃can be fully replenished without discernible changes in local Bi coordination or C₂H₂combustion selectivity. Heterolytic activation of C–H bonds across Bi–O sites and the higher acidity of C₂H₂results in lower barriers for C₂H₂activation than C₂H₄, enabling selective catalytic hydrocarbon combustion leveraging differences in molecular deprotonation energies.