X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

Abstract

Dispersed Mo oxides associated with an oxide support catalyze a wide variety of hydrocarbon transformations. The supported catalysts typically contain several types of molybdate species. Because a small fraction is catalytically active, characterization of the precise nature of these active sites and their activation mechanisms is challenging. In this work, monomolybdate sites with uniform first coordination spheres were synthesized by a direct solid−solid reaction between MoO 2 Cl 2 and silica. The Mo(O) 2 (OSi) 2 sites were characterized by X-ray absorption spectroscopy, enabled by comparison to several well-defined model compounds. In molecular MoO 2 (OSiPh 3 ) 2 , a prominent feature at ca. 3 Å in the Fourier-transform extended X-ray absorption fine structure arises due to constructive interference between single-, double-, and triple-scattering paths involving the near-linear silanolate ligands. Coordination of additional ligands causes the Mo−O−Si angle to decrease, weakening this feature. Even stronger attenuation is characteristic of Mo(O) 2 (OSi) 2 sites, which are inferred to have smaller Mo−O−Si angles and a range of Mo−Si distances imposed by the nature of their attachment to the silica surface as molybdasiloxane rings; smaller rings with lower Mo−O−Si angles contribute little intensity. While Mo(O) 2 (OSi) 2 sites may be useful in mechanistic studies involving supported molybdate catalysts, they are not single-site materials. The impact of the intrinsic heterogeneity of their attachment to the support on their reactivity will be important to consider in structure-property studies.

Limits of Detection for EXAFS Characterization of Heterogeneous Single-Atom Catalysts

“…While EXAFS data are often modeled using a select few scattering paths that dominate the spectrum, the measured EXAFS is the sum of all scattering paths that give rise to wave patterns with constructive and/or deconstructive interference. Therefore, especially at longer distances (higher R ), where multiple scattering processes are increasingly common, typically one scattering path is not independently responsible for the presence/absence of features in the EXAFS . One method for summarizing the full information content available in an EXAFS spectrum  which is becoming increasingly popular in the analysis of SACs  is a wavelet transform, which separates signal contributions into their k and R -space representations.…”

“…Therefore, especially at longer distances (higher R), where multiple scattering processes are increasingly common, typically one scattering path is not independently responsible for the presence/absence of features in the EXAFS. 59 One method for summarizing the full information content available in an EXAFS spectrum � which is becoming increasingly popular in the analysis of SACs � is a wavelet transform, which separates signal contributions into their k and R-space representations. In particular, the continuous Cauchy wavelet transform (CCWT) has become popular within the EXAFS community.…”

Circumventing CO2 Reduction Scaling Relations Over the Heteronuclear Diatomic Catalytic Pair

“…As shown in Figure 1d, the dominant FT EXAFS peak for Mo 1 -NC and DMCP Fe1-Mo1 -NC appear both at around 1.26 Å, which can be ascribed to the Mo-N/O scattering path. 19,20 The Fourier transformation of the EXAFS spectrum for DMCP Fe1-Mo1 -NC can be well fitted by the structure, as shown in the inset of Figure S14 (optimized by DFT). Tables S5 and S6 summarize the fitting results of the EXAFS spectra for Mo 1 -NC, Fe 1 -NC, and DMCP Fe1-Mo1 -NC.…”

Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WOx/SiO2 Catalyst: A Computational Perspective

“…The geometric factors can also determine the initiation pathway. Therefore, various reduction and activation mechanisms may be simultaneously at work due to the heterogeneity of the surface metal species. ,,,,− …”