**1. Introduction**

A photocatalyst is a material able to absorb light efficiently and thereby induce a chemical reaction [1]. In the 1970s, the photo-response of solid materials became a very important research topic due to the potential applications of this phenomenon in new technologies. This led to heterogeneous photocatalysis encouraging unique developments with applications in alternative energy, organic synthesis and environmental treatment [2], with semiconductor compounds being most widely used for this purpose. Of these, TiO2 and TiO2-based materials have by far been the most attractive economically: they are authoritative photocatalysts also because they are nontoxic and offer among other advantages ready availability, chemostability, and reusability. Other inorganic semiconductors have been successfully used as photocatalysts, principally in environmental heterogeneous photocatalysis. These include CdS, WO3, SnO2, Į-Fe2O3, AgNbO3 and SrTiO3 [3]. Basically, when such semiconductor photocatalysts absorb light, electronic excitation occurs that can be used in chemical or electrical processes.

During all this time, knowledge in disciplines such as photochemistry, catalysis and semiconductor physics has grown and allowed a deeper understanding of the phenomenon of photocatalysis, which depends mainly on the interaction of radiation with the solid and its surface reactivity [4]. The basic concepts underlying the phenomenon of photocatalysis have been obtained from studies done on photocatalytic systems employing microcrystalline (bulk) semiconductor photocatalysts. Recently, nanotechnology has opened up exciting new possibilities to develop more efficient photocatalysts [5]. This modernization of photocatalysis and nanophotocatalysis has been encouraged by progress that has been made in the physics and chemistry of nanoscale semiconductor particles exhibiting quantum size effects, *i.e*., a dependence on the fundamental properties of these nanosized particles. The phenomenon of quantum confinement, which these nanoparticles exhibit, favors changes in the electronic, optical, photochemical and photocatalytic properties of nanocrystalline semiconductors [6,7].

Much work has been published in the last two decades related not only to colloidal semiconductors but to other types of nanostructure materials, including: nanocrystalline powders, coupled nanocomposites, nanocrystalline films, mesoporous semiconductors, *etc*. Clearly, although nanoparticle semiconductors have high surface energy compared to bulk semiconductors, they tend to reduce surface tension by aggregation. In order to prevent or delay aggregation and counter the coarsening of the nanocrystals, a stabilizer or surfactant is added during synthesis that is adsorbed on them, thereby reducing surface tension and creating a steric or electrostatic barrier. In addition, this characteristic shown by the nanoparticles can be used to create a new class of photocatalysts with a porous structure [8]. For this, surfactant type compounds are introduced during synthesis in suitable concentrations, not so much to prevent aggregates from forming but to form a nanoporous structure in the aggregates. As photocatalysts, mesoporous materials have a number of advantages, including: (1) a high value of specific surface area and nanometer pore size; (2) prolonged substrate retention; (3) migration of photogenerated charge carriers through the adjacent semiconductor nanoparticles, and an accumulation at their points of contact, and (4) high possibility of repeat light reflectance encouraging a greater absorption of light [9]. The benefits of these mesoporous structures are illustrated by the titania nanophotocatalysts in the photocatalytic oxidation of dyes [10], oxidative disruption of the cell membranes of *E. coli* bacteria [11] and carbon dioxide reduction to methanol [12], among others. Specifically, mesoporous ZnO [13], CeO2 [14] and Co3O4 [15] have all exhibited high photocatalytic activity in the destruction of dyes when compared with non-porous nanopowders.

Specifically, considering mesoporous niobium compounds, in one of the first works where the mesoporous niobium-containing silicate of MCM-41 was synthesized, the results of X-ray diffraction and transmission electron microscopy confirmed that this material exhibited an ordered mesoporous structure that could be altered by applying external pressure (∼50 MPa) [16]. According to these researchers, the niobium-containing MCM-41 structure is mechanically less stable than pure silica MCM-41. The mesoporous molecular sieves were then modified with ammonium and copper cations and tested in several catalytic reactions [17]. An important result of this study showed that mesoporous NbMCM-41 was an attractive catalyst for the selective oxidation of thioethers to sulphoxides with H2O2 owing to its high activity and to the presence of Nb-O<sup>í</sup> species in the framework [18]. Recently, mesoporous Nb2O5 was used as a photocatalyst instead of the nanocrystalline niobium oxide compound, and its quantum yield of hydrogen evolution from aqueous methanol solutions increased by a factor of 20 [19]. When the mesoporous Nb2O5 samples were loaded with nanoparticles of Pt, Au, Cu and NiO, their photocatalytic activity increased such that the rate of H2 evolution from an aqueous methanol solution under UV irradiation changed, being higher for the Pt-loaded niobium oxide catalysts [20]. Another important application of Nb2O5 photocatalysts has been the oxidation of organic compounds in aqueous media [21]. This involves natural and synthetic Nb2O5 being placed initially in the presence of hydrogen peroxide and methylene blue [22] and in another study niobium pentoxide was modified by doping it with molybdenum or tungsten and then treated with H2O2, significantly increasing the photocatalytic activity of this compound in the oxidation of methylene blue dye [23].

This article reports obtaining Nb/MCM-41 molecular sieves by using two synthesis methods: the traditional incipient impregnation method and the sol-gel process. After characterizing the synthesized powders, their photodegradation capacity was evaluated using methylene blue as a pattern compound. Finally, also with regard to their photodegradation capacity, the effect of the synthesis method for the Nb/MCM-41 photocatalysts was determined, together with that of the niobium content in the different samples.
