The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (−273 to >1000 °C) and pressure (UHV to ≫100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman–IR, Raman–UV-vis, Raman–EPR) and the operando Raman spectroscopy methodology (e.g., Raman–MS and Raman–GC) is allowing for the establishment of direct structure–activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

The present work elucidated the morphology and crystal-plane effects of nanoscale ceria on the activity of CuO/CeO2 catalysts toward NO reduction CeO2 Nanopolyhedra were enclosed by (111) and (100) planes; the nanorods predominantly exposed (110) and (100) surfaces, and the nanocubes only showed the polar (100) planes Moreover, the strongest interaction was between CuO and CeO2 rods, followed by CuO/CeO2 polyhedra, and the CuO/CeO2 cubes showed the least interaction Importantly, Cu2+ ions could be incorporated into the pore and surface lattices by occupying the vacant sites in the nanostructure CeO2 rods Partial copper oxide species were segregated on the surface of CeO2 cubes with larger particle sizes As a result, the site geometry and coordination environment of Cu2+ ions were different on the (111), (110), and (100) surfaces of CeO2 This surface structure effect in turn led to a higher surface reducibility, activity and N2 selectivity of CuO/CeO2 nanorods for NO reduction at low temperatures (below 250 °C); the polyhedra and cubes were less active

Morphology and Crystal‐Plane Effects of Nanoscale Ceria on the Activity of CuO/CeO2 for NO Reduction by CO

Copper-containing cerium oxide materials have received considerable attention both in catalysis and electro-catalysis fields due to their unique physicochemical characteristics in conjunction to their lower cost compared to noble metals (NMs)-based catalysts. Nowadays, it is well documented that the complex Copper–Ceria interactions (either geometric or electronic) have a key role on the catalytic performance. Hence, considerable efforts have been devoted on the understanding and the fine-tuning of metal–oxide interactions. Despite the growing progress in the field, several crucial issues related to the influence of: i) particle’s shape and size, ii) active site’s chemical state, iii) charge transfer between interfacial sites, and iv) intrinsic defects (e.g., surface oxygen vacancies) on the interfacial activity are still under investigation. This survey summarizes the recent advances in the last 10 years on the fundamental origin of Copper–Ceria interactions and their implications on the catalytic activity. The insights lately obtained by means of: i) ex situ advanced characterization techniques, ii) in situ sophisticated studies (e.g., operando techniques), iii) theoretical analysis (e.g., DFT calculations), and iv) innovative probing approaches (such as the inverse CeO2/CuO model system) are provided. The state-of-the-art catalytic applications of CuO/CeO2 binary oxides (water gas shift (WGS) reaction, preferential oxidation (PROX) of CO, CO2 hydrogenation, selective catalytic reduction (SCR), N2O decomposition, etc.) in relation to the aforementioned aspects are discussed. Some guidelines towards the fine-tuning of the surface chemistry of CuO/CeO2 catalysts for real life energy and environmental application are provided.

The role of Copper–Ceria interactions in catalysis science: Recent theoretical and experimental advances

Copper-ceria is one of the very active catalysts for the preferential oxidation of carbon monoxide (CO-PROX) reaction, which is also a typical system in which the complexity of copper chemistry is clearly exhibited. In the present manuscript, copper–ceria catalysts with different Cu contents up to 20 wt % supported on CeO2 nanorods were synthesized by a deposition–precipitation (DP) method. The as-prepared samples were characterized by various structural and textural detections including X-ray diffraction (XRD), Vis-Raman, transmission electron microscopy (TEM), ex situ/in situ X-ray absorption fine structure (XAFS), and temperature-programmed reduction by hydrogen (H2-TPR). It has been confirmed that the highly dispersed copper oxide (CuOx) clusters, as well as the strong interaction of Cu-[Ox]-Ce structure, were the main copper species deposited onto the ceria surface. No separated copper phase was detected for both preoxidized and prereduced samples with the Cu contents up to 10 wt %. The fresh copper–...

Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble average...

Design strategies for the molecular level synthesis of supported catalysts.

Study of active sites and mechanism responsible for highly selective CO oxidation in H2 rich atmospheres on a mixed Cu and Ce oxide catalyst

Mechanistic insights into the formation of acetaldehyde and diethyl ether from ethanol over supported VOx, MoOx, and WOx catalysts

UV−visible diffuse reflectance spectroscopy (UV−vis DRS) and the ethanol oxidation probe reaction were used to investigate the structure and function of binary transition metal oxide catalysts containing dispersed WOx and MoOx domains supported together on alumina. The efficacy of UV−vis DRS as a tool to identify segregated MoOx and WOx surface domains along with their growth into larger and interacting binary oxides is demonstrated for the first time. UV−vis absorption edge analysis of physical mixtures of single oxide catalysts indicates that spatially segregated domains of different composition result in multiple edges in the UV−vis absorption spectra. Binary oxide catalysts containing 0.5 Mo atoms/nm2 and 0.5 W atoms/nm2 show two distinct absorption edges at 3.60 and 4.13 eV, corresponding to spatially and compositionally segregated MoOx and WOx domains. At higher surface densities (2−8 total metal atoms/nm2) only one edge is observed, suggesting that MoOx and WOx are molecularly mixing and forming a ...

Effects of Metal Oxide Domain Size, Dispersion, and Interaction in Mixed WOx/MoOx Catalysts Supported on Al2O3 for the Partial Oxidation of Ethanol to Acetaldehyde

Method for quantifying redox site densities in metal oxide catalysts: Application to the comparison of turnover frequencies for ethanol oxidative dehydrogenation over alumina-supported VOx, MoOx, and WOx catalysts

Hari Nair

Papers

Study of active sites and mechanism responsible for highly selective CO oxidation in H2 rich atmospheres on a mixed Cu and Ce oxide catalyst

Mechanistic insights into the formation of acetaldehyde and diethyl ether from ethanol over supported VOx, MoOx, and WOx catalysts

Effects of Metal Oxide Domain Size, Dispersion, and Interaction in Mixed WOx/MoOx Catalysts Supported on Al2O3 for the Partial Oxidation of Ethanol to Acetaldehyde

Method for quantifying redox site densities in metal oxide catalysts: Application to the comparison of turnover frequencies for ethanol oxidative dehydrogenation over alumina-supported VOx, MoOx, and WOx catalysts

Tailored α-methylene-γ-butyrolactones and their effects on growth suppression in pancreatic carcinoma cells.