Quantum yield and quantum efficiency (QY) as used in heterogeneous photocatalysis (solid/liquid or solid/gas systems) have too often been used incorrectly to mean the ratio of the rate of a given event to the rate of incident photons impinging on the (external) rector walls, typically for broadband radiation. There is little accord on how to express process efficiency. At times QY is defined, often ill-defined, and more frequently workers fail to describe how it was assessed. This has led to much confusion in the literature, not only because of the different meaning of QY from that in homogeneous photochemistry, but also because this method of describing photon efficiency precludes a comparison of results from different laboratories because of the variations in light sources, reactor geometries, and overall experimental conditions. It cannot be overemphasized that the reported QY is an apparent quantum yield, indeed a lower limit of the true quantum yield. This position paper addresses this issue, and argues that any reference to quantum yields or quantum efficiencies in a heterogeneous medium is ill-advised unless the actual number of photons absorbed by the light harvester (the photocatalyst) has been determined. The extent of light scattering in a solid/liquid heterogeneous medium is significant. A practical and simple alternative for comparing process efficiencies was recently suggested by defining a relative photonic efficiency ζr. A quantum yield can subsequently be determined from ζr, as φ = ζφphenol, where φphenol is the quantum yield for the photocatalyzed oxidative disappearance of phenol (a standard secondary actinometer) using Degussa P-25 TiO2 as the standard catalyst material.

Relative photonic efficiencies and quantum yields in heterogeneous photocatalysis

Highly dispersed Ru nanoparticles loaded on a TiO2 support (Ru/TiO2(B)), which affects the hydrogenation of CO2 to CH4 (methanation), were prepared by employing a “dry” modification method using a barrel-sputtering instrument. The loaded Ru nanoparticles exhibited a narrow particle-size distribution with a mean diameter of ca. 2.5 nm. Methanation of CO2 on the Ru/TiO2(B) catalyst produced a 100% yield at ca. 160 °C, which is more than 200 °C below that required for Ru/TiO2 prepared by a conventional “wet” impregnation method. In addition, the methanation reaction over Ru/TiO2(B) proceeded at temperatures as low as room temperature with a reaction rate of 0.04 µmol min−1 g−1.

CO2 methanation property of Ru nanoparticle-loaded TiO2 prepared by a polygonal barrel-sputtering method

Transient measurement techniques are applied for the kinetic investigation of the reaction mechanism of the carbon dioxide methanation, using diffuse reflectance infrared spectroscopy and mass spectrometry. The coupled information of the surface intermediates and the gas phase components time evolution leads to accurate identification of spectator species on the surface. Reaction intermediates, carbon monoxide and formates have been identified. The former is a key intermediate, and its hydrogenation leads to methane formation. The latter is fixed on the support, in equilibrium with a active formate species on the interface metal-support. A reaction mechanism is proposed including the formation step of the formates through a carbonate species.

In-situ surface and gas phase analysis for kinetic studies under transient conditions The catalytic hydrogenation of CO2

The present invention provides a process for producing chlorine by oxidation of hydrogen chloride with oxygen which process comprises using a ruthenium catalyst.

Process for producing chlorine

In this paper we show that a 0.5 wt.% Ru/γ-Al2O3 catalyst is appropriate to carry out the Sabatier reaction (CO2 methanation) under process conditions relevant for the Power-to-Gas application and we provide a kinetic model able to describe the CO2 conversion over a wide range of process conditions, previously unexplored. To achieve these goals, the effects of feed gas composition (H2/CO2 ratio and presence of diluents), space velocity, temperature and pressure on catalyst activity and selectivity are investigated. The catalyst is found stable when operating over a wide range of CO2 conversion values, with CH4 selectivity always over 99% and no deactivation, even when working with carbon-rich gas streams. The effect of water on the catalyst performance is also investigated and an inhibiting kinetic effect is pointed out. Eventually, the capacity of kinetic models taken from the literature to account for CO2 conversion under the explored experimental conditions is assessed. It is found that the kinetic model proposed by Lunde and Kester in 1973 (J. Catal. 30 (1973) 423) is able to describe satisfactorily the catalyst behavior in a wide range of CO2 conversion spanning from differential conditions to thermodynamic equilibrium, provided that a new set of kinetic parameters is used. It is shown however that a better fitting can be achieved by using a modified kinetic model, accounting for the inhibiting effect of H2O on CO2 conversion rate.

Kinetics of CO2 methanation on a Ru-based catalyst at process conditions relevant for Power-to-Gas applications

A structural investigation of the Ru-RuOx/TiO2 catalyst was carried out in order to determine the role played by the support, the size of the particles, their dispersion, structural relationship with the support and the preparation conditions on the activity. In the ideal catalyst the metallic particles are 10–15A in size. Moreover they are selectively dispersed on the TiO2 crystallites with the rutile structure. This uneven distribution is not found for other metals ( Rh, Pt, Co, Ni, etc...) on P25 though the preparation procedure was similar.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400423432

Selective dispersion of the Ru-RuOx/TiO2 catalyst for methanation of CO2 at room temperature and atmospheric pressure

The structure of ruthenium supported on titania: a catalyst for low-temperature methanation of carbon dioxide

Partially-reduced ruthenium dioxide on titania is an active and selective catalyst for methane formation from carbon dioxide, already at temperatures below 100 C. Results from catalytic screening together with characterization by electron microscopy indicate that the TiO 2 support (Degussa P25 in particular) plays a major role in stabilizing metallic ruthenium in highly-dispersed form (d 10-30 A), provided care is taken in the RuO 2 deposition step. Chemisorption of H 2 and CO 2 has revealed a strong g coadsorption synergy together with extensive hydrogen spillover. Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy in situ has provided confirmatory evidence of the latter, and shown that CO, believed to arise from the reverse water-gas shift reaction accumulates on the metal causing catalyst deactivation. Heating above 100 facilitates the Boudouard disproportionation reaction and the activity is regenerated. Implications for the mechanism of CO 2 hydrogenation are briefly discussed.

Catalyst Characterization and in situ FTIR Studies of Carbon Dioxide Methanation Over Ruthenium Supported on Titania

Oxidative dimerization of CH 4 has been studied using Co 3 O 4 catalysts promoted with chlorides of Li, Na, K and Cs. Chemical modification of Co 3 O 4 - LiCl catalyst at the reaction conditions was evidenced by ESR spectroscopy. A decrease in the optimum temperature, at which C 2 - product yields were maximum, was noticed when the promoter was changed from Li to Cs through group IA alkali metal chlorides.

K. R. Thampi

Papers

Selective dispersion of the Ru-RuOx/TiO2 catalyst for methanation of CO2 at room temperature and atmospheric pressure

The structure of ruthenium supported on titania: a catalyst for low-temperature methanation of carbon dioxide

Catalyst Characterization and in situ FTIR Studies of Carbon Dioxide Methanation Over Ruthenium Supported on Titania

Oxidative Dimerization of Methane on Alkali Chloride Promoted Co3O4

A TEM study of the Ru-RuOx/TiO2 catalyst for methanation of CO2 at room temperature and atmospheric pressure