No abstract

The activity of plasmonic Au−TiO 2 catalysts for solar hydrogen production from H 2 O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition−precipitation). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV−vis spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the chargecarrier separation and stabilization under photocatalytic conditions was explored in relation to H 2 evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of separated electrons and holes, whereas the enrichment of Ti 3+ in the support lattice hampers an efficient electron transport. Under the given experimental conditions, these properties were most efficiently generated by depositing gold particles on anatase/rutile composites using the deposition−precipitation technique.

Supported V2O5/Ce1–x Ti x O2 (3, 5, and 7 wt % V; x = 0, 0.1, 0.3, 0.5, 1) and bare supports have been tested in the selective catalytic reduction (SCR) of NO by NH3 at different gas hourly space velocities (GHSVs) and were comprehensively characterized using XRD, pseudo in situ XPS, and UV–vis DRS as well as EPR and DRIFTS in in situ and operando mode. The best V/Ce1–x Ti x O2 (x = 0.3, 0.5) catalysts showed almost 100% NO conversion and N2 selectivity already at 190 °C with a GHSV value of 70000 h–1, which belongs to the best performances observed so far in low-temperature NH3-SCR of NO. The corresponding bare supports still converted around 80% to N2 under the same conditions. On bare supports, SCR proceeds via a Langmuir–Hinshelwood mechanism comprising the reaction of adsorbed surface nitrates with adsorbed NH3. On V/Ce1–x Ti x O2, nitrate formation is not possible, and an Eley–Rideal mechanism is working in which gaseous NO reacts with adsorbed NH3 and NH4 +. Lewis and Brønsted acid sites, though adsorption of NH3, do not scale with the catalytic activity, which is governed rather by the redox ability of the materials. This is boosted in the supports by replacing Ce with the more redox active Ti and in catalysts by tight connection of vanadyl species via O bridges to the support surface forming −Ce–O–V­(O)–O–Ti– units in which the equilibrium valence state of V under reaction conditions is close to +5.

Photocatalytic hydrogen evolution rates and structural properties as well as charge separation, electron transfer, and stabilization have been analyzed in advanced sol–gel-derived carbon nitrides (SG-CN) pyrolyzed at different temperatures (350–600 °C) and in bulk polymeric carbon nitride reference samples (CN) by XRD, XPS, FTIR, UV–vis, Raman, and photoluminescence as well as by in situ EPR spectroscopy. SG-CN samples show about 20 times higher H2 production rates than bulk CN. This is due to their porous structure, partial disorder, and high surface area which favor short travel distances and fast trapping of separated electrons on the surface where they are available for reaction with protons. In contrast, most of the excited electrons in bulk polymeric CN return quickly to the valence band upon undesired emission of light, which is responsible for their low catalytic activity.