We study the behavior of Ge–Ge, Ge–Sn, and Sn–Sn vibrational modes in GeSn semiconductors with Raman Spectroscopy. Raman spectroscopy is a rapid, nanoscale spatial resolution and non-destructive ...approach to accurately determine the composition and strain information of GeSn, and it is thus crucial for the material investigation and device application of GeSn alloys. By using several excitation wavelengths at 532, 633 and 785 nm on a set of fully strained and fully relaxed Ge1-xSnx layers with the Sn composition in the range 2.3 % < xSn < 31 %, all modes are identified and their evolution as a function of strain and Sn content is determined. The Raman shifts of all vibrational modes are found to exhibit the same function versus the composition xSn and in-plane strain ε//, Δω = ωGeSn - ωGe = axSn + bε//, where a is the Sn composition factor and b is the strain shift factor. In addition, the Ge–Sn mode intensity increases with Sn content. It is discovered for the first time that the Sn composition determined from the plot of the intensity ratio of the Ge–Sn mode over the Ge–Ge mode as a function of Sn composition at 785 nm excitation agrees well with that the X-ray Diffraction (XRD) Reciprocal Space Mapping (RSM), offering a novel approach for determining Sn content by Raman spectroscopy.
•A set of fully strained and fully relaxed GeSn layers with 2.3 %<xSn<31 % grown by MBE.•Ge–Ge LO, Ge–Sn and Sn–Sn vibrational modes behavior under multi-wavelength Raman.•Analytic fitting of Raman peak position is obtained to estimate xSn and strain.•First time to find the intensity ratios of Ge–Sn and Ge–Ge modes are related to xSn.
A nonthermal, atmospheric He/O
plasma (NTAP) successfully removed polyvinylpyrrolidone (PVP) from Pd cubic nanoparticles supported on SiO
quickly and controllably. Transmission electron microscopy ...(TEM) revealed that the shape and size of Pd nanoparticles remain intact during plasma treatment, unlike mild calcination, which causes sintering and polycrystallinity. Using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we demonstrate the quantitative estimation of the PVP plasma removal rate and control of the nanoparticle synthesis. First-principles calculations of the XPS and CO FTIR spectra elucidate electron transfer from the ligand to the metal and allow for estimates of ligand coverages. Reactivity testing indicated that PVP surface crowding inhibits furfural conversion but does not alter furfural selectivity. Overall, the data demonstrate NTAP as a more efficient method than traditional calcination for organic ligand removal in nanoparticle synthesis.
Oxidative coupling of methane (OCM) is a promising industrial process to upgrade natural gas to high value chemicals. In this study, Temporal Analysis of Products (TAP) and steady-state experiments ...were conducted to distinguish how the composition of surface and gas phase oxygen influence mechanistic details of the selective conversion of CH4 to C2H4 over the Mn-Na2WO4/SiO2 catalyst. The results from TAP studies indicate that methane activation on this catalyst proceeds predominantly via a short-lived, transient surface oxygen species and there is a competition for this species to form either CO or methyl radicals on the surface. This active species has a total lifetime of 3 s and is identified to have a dioxygen (e.g. O22- or O2-) form. We show that the concentration of the transient surface oxygen species significantly impacts the OCM performance. Oxygen attributed to the catalyst lattice (in a singular form e.g., O-), is found to activate methane to a lesser degree, but exclusively forms CO2. Evidence for these surface pathways for methyl radical, CO and CO2 formation identified by TAP are also validated through steady-state experiments. Finally, by distinguishing different catalyst oxygen species and their role in selective/nonselective pathways, important screening criteria have been identified for the advancement of superior catalyst formulations.
Platinum-tungsten oxides are among the most studied metal–metal oxide pair catalysts for C–O hydrogenolysis reactions. The Brønsted acid density and synergy between Pt and WO x , especially in the ...inverse structure, are critical to reactivity and selectivity. However, a clear molecular-level understanding of the formation and dynamics of Brønsted acid sites (BAS) is lacking. Here, using in situ spectroscopic characterizations (Raman and FTIR), chemical probing (CO chemisorption and pyridine titration), density functional theory (DFT) calculations, and a model reaction (tert-butanol dehydration), we demonstrate the structural evolution of WO x species and associated BAS dynamics in various environments. In situ Raman and DFT calculations show that below monolayer coverage, the WO x species stay as isolated monomers on the SiO2 support and W3O x trimers on Pt. The W3O x trimers on Pt are dynamic and 10× more active toward dehydration than the WO x species on the SiO2 support. H2 plays a complex role: at low temperatures (<473 K), it creates more BAS in the form of W3O7H on Pt by reversible hydrogen spillover, and at higher temperatures (>573 K), it partially reduces the W3O x . We further show that the inverse configuration allows changes in the BAS density via catalyst pretreatments. This study provides a strategy for tuning Brønsted acid density and regenerating sites by pretreatment and catalyst composition.
Oxidative coupling of methane (OCM) is an attractive direct route for upgrading methane to valuable chemicals. In this study, temporal analysis of products (TAP) and steady-state experiments are ...conducted to understand the role of individual oxide phases and their combinations in supported Mn–Na2WO4/SiO2 catalysts for OCM. The results from TAP transient kinetic studies indicate that Mn plays an important role in promoting gas-phase oxygen activation, while NaO x /SiO2 and WO x /SiO2 are relatively inert toward gas-phase oxygen and methane activation. However, the supported catalyst combining Na and W in the form of Na2WO4 shows enhanced gas-phase oxygen activation, exhibiting a much lower oxygen activation energy (148 kJ/mol) and enhanced activity toward methane activation as compared to the individual supported oxide catalysts. The addition of Mn to Na2WO4/SiO2 further decreases the oxygen activation energy by 40 kJ/mol. Moreover, methane activation is also enhanced with CH3 as the main intermediate, but with increasing Mn content, more CH2 intermediates are observed. Different forms of oxygen (both dioxygen and atomic) are detected on the catalyst surface using isotopic pump/probe pulsing and their distribution is found to depend on the catalyst composition. An optimal Mn content in the Na2WO4/SiO2 catalyst system is needed to enhance the amount of dioxide surface species (e.g., superoxide 16O2 – or peroxide 16O2 2–) associated with Na2WO4, leading to high C2 selectivity for OCM. When the Mn content is too high, the larger MnO x domains are shown to contribute to the formation of higher concentrations of monoxide surface species that lead to nonselective OCM pathways. This insight from transient kinetic characterization using TAP combined with conventional steady-state studies provides a deeper understanding of the role of individual oxide phases and their combination on supported catalysts toward the formation of intermediate surface species and their impact on the OCM reaction mechanism. This knowledge is critical for designing superior catalyst formulations for OCM.
Syngas (H2 + CO) is the key to a future circular economy, but decarbonizing its production is challenging. Here, we electrify the dry reforming of methane (DRM) via selective coupling of microwave ...(MW) radiation to metal-free doped ceria catalysts. Reactivity studies on cerium-zirconium mixed oxides and comprehensive characterization reveal that the reduced Ce3+ centers with adjacent oxygen vacancies (Ce3+–VO) are catalytically active and responsible for absorbing the MW energy. We introduce bulk bed spatial temperature measurements to compare MW to furnace heating. The selective deposition at MW energy to the reactive sites leads to excellent performance, unattainable by conventional electrical furnace heating. Step-change experiments, isotopic labeling, and spectroscopic insights elucidate a Mars-van-Krevelen (MvK)-type DRM turnover in concert with a MW-absorbing cycle that relies on the oxidation state of the catalyst. The reaction is periodically excited and quenched at the reactive centers without elevating the bulk measured temperature. These new insights may guide the design of catalysts and process intensification for carbon-free syngas production.
Oxidative coupling of methane (OCM) is a promising process for the single-step conversion of CH4, the major component of natural gas (NG), to value-added C2 products. Among hundreds of alkali, ...alkaline earth and transition metal and metal oxide-based bulk and supported catalysts tested for this reaction, the supported Mn-Na2WO4/SiO2 catalysts have been found very promising due to their excellent thermal and chemical stability (up to 1000 hours on stream) and high C2 product yield (~25%). Despite large numbers of structural and kinetic investigations on this catalyst system, the stable structure, nature of the active site, and reaction mechanism are still under debate due to lack of in-situ/operando characterization and time-resolved chemical probing. These discrepancies have hindered the improvement of this catalyst system for practical implementation in NG utilization.The objective of the current study is two-fold: (i) to establish molecular level structural insights of supported Mn-Na2WO4/SiO2 catalyst structure during the OCM reaction, and (ii) to decipher the complex OCM reaction mechanism over this catalyst with the aid of transient, highly time-resolved chemical characterization and steady-state kinetic studies. The state-of-the-art in-situ Raman, UV-Vis, XRD, and NAP-XPS studies reveal, for the first time, that the freshly calcined supported Mn-Na2WO4/SiO2 catalysts possess surface Na-WOx and MnOx species along with crystalline phases of Na2WO4, Mn2O3 and cristobalite SiO2 support. During the OCM reaction, the crystalline Na2WO4, Mn2O3 phases were unstable due to melting and reduction, respectively, whereas the surface Na-WOx and MnOx species are both thermally and chemically stable. Further investigations via transient analysis of products (TAP) technique reveal that the supported Na2WO4/SiO2 catalysts possess two different types of lattice oxygen species at OCM relevant temperature: (i) molecular O2* type species originate from the lattice of molten Na2WO4 phase, and (ii) atomic O* type is associated with surface Na-WOx sites. Advanced kinetic investigations demonstrate that (i) surface Na-WOx sites are responsible for selectively activating CH4 to C2Hx and over-oxidizing CHy to CO, and (ii) molten Na2WO4 phase is mainly responsible for over-oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. Finally, the promotion mechanism of Mn was deciphered to show that the Mn-oxide phase, in itself (Mn/SiO2 catalyst), is less active and highly unselective towards CH4 oxidation. However, Mn addition (i) improves the reducibility and release rate of lattice oxygen species, (ii) helps in low-temperature activation of lattice oxygen species, and (iii) improves the exchange of gas-phase oxygen and lattice oxygen species present in supported Na2WO4/SiO2 catalysts. These promotional effects of Mn are reflected in higher CH4 activity and C2H4 to C2H6 ratio during steady-state OCM reaction. These new findings will guide the rational design of catalysts with higher activity and product selectivity.
