Cu-exchanged small-pore zeolites (CHA and AEI) form methanol from methane (>95% selectivity) using a 3-step cyclic procedure (Chem. Commun.20155144474450) with methanol amounts higher than Cu-ZSM-5 and Cu-mordenite on a per gram and per Cu basis. Here, the Cu x O y species formed on Cu-SSZ-13 and Cu-SSZ-39 following O2 or He activation at 450 °C are identified as trans-μ-1,2-peroxo dicopper­(II) ([Cu2O2]2+) and mono-(μ-oxo) dicopper­(II) ([Cu2O]2+) using synchrotron X-ray diffraction, in situ UV–vis, and Raman spectroscopy and theory. [Cu2O2]2+ and [Cu2O]2+ formed on Cu-SSZ-13 showed ligand-to-metal charge transfer (LMCT) energies between 22,200 and 35,000 cm–1, Cu–O vibrations at 360, 510, 580, and 617 cm–1 and an O–O vibration at 837 cm–1. The vibrations at 360, 510, 580, and 837 cm–1 are assigned to the trans-μ-1,2-peroxo dicopper­(II) species, whereas the Cu–O vibration at 617 cm–1 (Δ18O = 24 cm–1) is assigned to a stretching vibration of a thermodynamically favored mono-(μ-oxo) dicopper­(II) with a Cu–O–Cu angle of 95°. On the basis of the intensity loss of the broad LMCT band between 22,200 and 35,000 cm–1 and Raman intensity loss at 571 cm–1 upon reaction, both the trans-μ-1,2-peroxo dicopper­(II) and mono-(μ-oxo) dicopper­(II) species are suggested to take part in methane activation at 200 °C with the trans-μ-1,2-peroxo dicopper­(II) core playing a dominant role. A relationship between the [Cu2O y ]2+ concentration and Cu­(II) at the eight-membered ring is observed and related to the concentration of [CuOH]+ suggested as an intermediate in [Cu2O y ]2+ formation.

This study reports the highly selective (more than 95%) dehydrogenation of propane to propylene as well as the reverse hydrogenation reaction by silica-supported single-site Zn(II) catalyst. The catalyst is thermally stable at dehydrogenation temperature (550 °C and above), and catalytic byproducts are small. In situ UV-resonance Raman, XANES, and EXAFS spectra reveal that tetrahedrally coordinated Zn(II) ions are chemisorbed into the strained three-membered siloxane rings on the amorphous silica surface. Under reaction conditions, the Zn(II) ion loses one Zn–O bond, resulting in a coordinatively unsaturated, 3-coordinate active center. The infrared spectrum of adsorbed pyridine indicates that these are Lewis acid sites. Theoretical calculations based on hybrid density functional theory suggest that the catalyst activates H–H and C–H bonds by a nonredox (metal) mechanism consisting of heterolytic cleavage of C–H bonds, in contrast with the homolytic mechanisms such as oxidative addition/reductive elimination pathways. The computed minority catalytic pathway consists of undesired C–C bond cleavage at Zn(II) site, follows a slightly different mechanism, and has a significantly higher activation energy barrier. These mechanisms are consistent with the high olefin selectivity observed for single-site Zn(II) on SiO2.

Resonance Raman (RR) spectroscopy has several advantages over the normal Raman spectroscopy (RS) widely used for in situ characterization of solid catalysts and catalytic reactions. Compared with RS, RR can provide much higher sensitivity and selectivity in detecting catalytically-significant surface metal oxides. RR can potentially give useful information on the nature of excited states relevant to photocatalysis and on the anharmonic potential of the ground state. In this critical review a detailed discussion is presented on several types of RR experimental systems, three distinct sources of so-called Raman (fluorescence) background, detection limits for RR compared to other techniques (EXAFS, PM-IRAS, SFG), and three well-known methods to assign UV-vis absorption bands and a band-specific unified method that is derived mainly from RR results. In addition, the virtues and challenges of surface-enhanced Raman spectroscopy (SERS) are discussed for detecting molecular adsorbates at catalytically relevant interfaces. Tip-enhanced Raman spectroscopy (TERS), which is a combination of SERS and near-field scanning probe microscopy and has the capability of probing molecular adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometre spatial resolution, is also reviewed (300 references).