## 4 Nanodiamonds as Catalysts  
NDs recently attracted attention in the catalysis field where they have been investigated in two main roles: as a support material to be functionalized with a catalyst or as a proper catalyst material. In the former case, NDs are attractive due to their chemical and mechanical stability, large surface area, and relatively easy surface functionalization, allowing them to load a variety of catalysts. The catalysis function is then only loosely influenced by the NDs' properties and this body of literature will not be considered here. For a recent review we point the reader to [PERSON] et al. [114]. The following section will instead be focused only on those investigations where NDs are reported as catalyst materials themselves. Catalysis by NDs has been reported for a few reactions of industrial interest, including oxidative dehydrogenations and dechlorination reactions. Recently, conductive NDs were also considered as electrocatalysts for CO\({}_{2}\)-reduction schemes with promising results. Dehydrogenation of ethylbenzene is the main route for the industrial synthesis of styrene, employed for more than 80% of its production worldwide [115]. Styrene is then used to obtain a variety of specialty polymers applied as resins, synthetic rubbers, and plastics. The process generally involves a Fe\({}_{2}\)O\({}_{3}\) catalyst and requires superheated steam to limit an otherwise rapid catalyst poisoning by carbon-based species. [116]. When investigated as an alternative catalyst, NDs compared favorably with respect to the benchmark, providing longer duration without requiring superheated steam [117]. The activity was attributed to high surface area and to a partially-ordered geometry promoting reagent adsorption. However, the NDs employed were powders from detonation synthesis, which are of limited industrial interest due to incompatibility with fixed-bed configurations. Steps towards solving this issue by immobilizing NDs were taken by using a SiC foam [118] or few-layered graphene [119] as the supporting material. Dechlorination is a class of reactions where chlorine in an organochlorine compound is removed. Three main routes are commonly employed: the high temperature removal of HCI (dehydrochlorination), reductive replacement of chlorine with hydrogen from catalyst-activated hydrogen gas (hydro-dechlorination), and chlorine gas extraction by a metal-based catalyst (simple dechlorination). These reactions are used in industrial synthesis and are especially important for the disposal of chlorinated chemicals, such as chlorinated solvents. The high temperature and/or metal-based catalysis requirement generated interest towards investigating carbon-based catalysts as replacements. NDs in particular proved to be active for the dechlorination of 1,2-dichloroethane in relatively mild conditions (300 \({}^{\circ}\)C), promoting a combination of hydro-dechlorination and simple dechlorination. Interestingly, hydro-dechlorination took place from hydrogen-terminated surface sites on NDs [120]. Electrocatalytic reduction to fuels (CH\({}_{4}\), CO, ethanol) or value-added chemicals (acetic acid, formic acid, methanol, ethylene, propanols) is currently one of the most promising routes for CO\({}_{2}\) conversion, providing at the same time a storage method for intermittent renewable energy sources and CO\({}_{2}\) extraction from the atmosphere [121; 122]. Materials requirements for a CO\({}_{2}\)-reduction electrocatalyst are quite strict: In addition to long-term stability, a high activity is needed to overcome the difficult kinetics dictated by the multi-electron redox nature of the reaction. Another important point is selectivity, as the hydrogen evolution reaction often competes successfully with the CO\({}_{2}\) reduction pathway [123; 124]. As a result, the field is very active and rapidly evolving with novel materials being investigated at a quick pace.