Title: turnover number
Long Title: IUPAC Gold Book - turnover number
DOI: 10.1351/goldbook.14101
Status: current

Definition
Number of times, \(n\), that the overall reaction (the photochemical transformation) goes through a photocatalytic cycle.

Notes
1) It can be equally applied to homogeneous and heterogeneous systems.
2) TON is a number, whereas in traditional catalysis, e.g., enzyme kinetics, it is defined per certain time period.
3) In heterogeneous photocatalysis it is the ratio of the number of photoinduced transformations for a given period of time to the number of active sites or photocatalytic centres in their ground state. Should the number of active sites be known, TON could be expressed per number of sites.
4) TON is a quantity of dimension one attributed in photocatalysis to the overall system that contains the photocatalyst, even if, in heterogeneous photocatalysis, part of the photocatalyst active centres appears to be not irradiated.
5) In homogeneous photocatalysis it is the ratio of the number of photoinduced transformations for a given period to the number of ground state photocatalyst entities.

Related Terms
- heterogeneous: https://goldbook.iupac.org//terms/view/14018
- homogeneous: https://goldbook.iupac.org//terms/view/14020

Source
- PAC, 2011, 83, 931. 'Glossary of terms used in photocatalysis and radiation catalysis (IUPAC Recommendations 2011)' on page 995 (https://doi.org/10.1351/PAC-REC-09-09-36)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/14101/html
- json: https://goldbook.iupac.org/terms/view/14101/json
- xml: https://goldbook.iupac.org/terms/view/14101/xml

Citation: Citation: 'turnover number' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.14101
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Accessed: 2026-05-07T00:06:01+00:00