## 1 Introduction
Great interest has been directed in recent years towards the study and use of mini-invasive, efficient and environmentally friendly methods for cleaning surfaces [41; 42; 43; 44; 45; 46]. Among the innovative cleaning methods of architectural elements, the use of photocatalysis has been largely investigated [47; 48; 49; 50; 51; 52; 53; 54], with the most used photocatalyst being based on titanium oxide (TiO\({}_{2}\)). In a heterogeneous photocatalysis system, the photoinduced transformations occur on the surface of the photocatalyst when light of appropriate wavelengths is absorbed (for example, TiO\({}_{2}\) in the anatase form has a band gap of about 3.2 eV, corresponding to \(\lambda_{\text{max}}\approx 388\) nm) with the production of electron-hole pairs. The generated electrons and holes migrate on the surface of the photocatalyst, where, together with reactive oxygen species produced via redox reactions, they can decompose several organic compounds [55; 56]. Titanium dioxide, present in nature in three polymorphs, rutile, anatase and brookite, is one of the most studied photocatalysts because of its easy availability, high reactivity and chemical stability: it has been used in several applications, such as for the purification of environmental pollutants, for the decontamination of water and for its self-cleaning properties in the conservation of cultural heritage [57; 58; 59; 60; 61; 62; 63]. In recent years, numerous photocatalytic materials associated with superhydrophobic surface coatings have been developed for protecting exposed buildings and monuments. Superhydrophobicmaterials can significantly limit damage due to water adsorption and contribute to the cleaning of the stone surfaces, restoring the aesthetic qualities of the artwork [30; 31; 32; 64; 65; 66]. In this work, we present a preliminary investigation on the possibility of using photocatalysis on painted stone for cleaning purposes. Firstly, the harmlessness and effectiveness of a self-cleaning and protective treatment based on TiO\({}_{2}\) and OrMoSil on a carbonate stone (_Lecce stone_) were evaluated. TiO\({}_{2}\), used as a photocatalytic agent, was synthesized via sol-gel, as reported in [63], and its photocatalytic efficiency was compared with the well-known titanium dioxide P25 (Evonik, Essen, Germany), a mixture of nanocrystalline anatase and rutile. The OrMoSil hydrophobic coating was prepared by modifying the procedure described in [32]. The _Lecce stone_ is typical of the Salento area, in the southeastern end of Italy, and was widely used during the Baroque period for the construction of civil and religious monuments and for the creation of sculptural works of art. It is characterized by a white-cream color, high porosity and excellent workability. _Lecce stone_ consists predominantly of calcium carbonate (93-97%). From a petrographic point of view, it is a homogeneous planktonic foraminiferal biomimetic [67]. In the Salento area, there are some examples of painted _Lecce stone_. Of particular importance are the votive aedic dedicated to [PERSON], entirely stuccoed and decorated, located in Nardo, which has recently been restored [68]; the Abbey of Santa Maria di Cerrate (Squinzano, Lecce, Italy), located in the monastic complex founded by the monks of the \"Rule of Saint Basil\" in the 12 th century [69]; and the Basilica of Santa Caterina d'Alessandria in Galatina (Lecce, Italy), built between 1369 and 1391 [70]. The first part of the experiments was performed on raw _Lecce stone_, on which we evaluated the harmlessness of the TiO\({}_{2}\)/OrMoSil treatment through colorimetric tests, measurements of water absorption through capillarity and of permeability to water vapor and its durability by means of artificial ageing and salt crystallization. Then we tested, with colorimetric methods, the self-cleaning efficacy of the coating in the photodegradation of methyl orange (MeO) and methylene blue (MB) dyes, commonly used as model pollutants, under artificial solar irradiation [71; 72; 73; 74].