## 1 Introduction  
The emission of nitrogen oxides (NO\({}_{\mathrm{x}}\), x = 1, 2) stemming from the combustion of fossil fuels in power plants or from the exhaust gases of mobile sources (i.e., road transport) is an ever-present environmental concern. These gases contribute to air pollution and lead to multiple environmental problems such as acid rain, photochemical smog, the greenhouse effect, among others [1; 2]. Several alternatives for NO\({}_{\mathrm{x}}\) mitigation have been proposed. Since 1973, the selective catalytic reduction (SCR) of NO\({}_{\mathrm{x}}\) with ammonia (NH\({}_{3}\)-SCR) has been widely utilized [3]. However, because of the risks associated with handling and storage of NH\({}_{3}\), NO\({}_{\mathrm{x}}\) reduction with hydrocarbon (HC) technology (HC-SCR) has been extensively studied [4]. There exist numerous works in the literature reporting the use of different hydrocarbons as reducing agents and a wide variety of catalysts under these reaction conditions [4; 5]. While this approach looks very promising, it has not been extensively applied yet due to a series of drawbacks such as: (i) catalyst deactivation; (ii) limited selectivity of NO\({}_{\mathrm{x}}\) to N\({}_{2}\); (iii) high reaction temperatures, (iv) the need of excess HC that must be burnt, and finally (v) incremental air pollution (CO\({}_{2}\)) and energy consumption. Consequently, it is important to find methods for NO\({}_{\mathrm{x}}\) reduction that (i) reduce the CO\({}_{2}\) emissions, (ii) use green reducing agents, (iii) require lower energy costs, and (iv) involve low-cost, greener, active, selective and stable catalysts. In this regard, hydrogen has been considered to be a powerful alternative due to the fact that it is environmentally friendly and it forms H\({}_{2}\)O and N\({}_{2}\) as the major end-products [6; 7; 8; 9]. Equation (1) represents the overall reaction for the selective formation of N\({}_{2}\) using H\({}_{2}\) as reductant. The studied catalytic systems for H\({}_{2}\)-SCR are diverse and mostly based on the study of noble-metal catalysts [10]. For this reason, nowadays there is an increasing interest to develop alternative catalytic systems involving the use of less expensive and more abundant transition metals, with the respective economic benefits [11; 12; 13; 14; 15]. For instance, Pt supported on Al\({}_{2}\)O\({}_{3}\) exhibits high activity for NO reduction. Nevertheless, the selectivity to N\({}_{2}\) decreases with increasing reaction temperatures [16; 17; 18; 19; 20]. In order to improve its catalytic performance, alternative supports have been systematically tested: SiO\({}_{2}\), TiO\({}_{2}\), MnO\({}_{\mathrm{x}}\), and zeolites, together with perovskite type solids, etc. [21]. Pd and Ir deposited on different substrates have also been proposed as active materials towards NO\({}_{\mathrm{x}}\) H\({}_{2}\)-SCR with a strong dependence of the final outcome on the metallic active site-support interactions and the specific reaction conditions [12]. Other catalysts based on noble metals such as Pd and Pt have shown better results even at low operating temperatures (below 200 \({}^{\circ}\)C) [6]. Nevertheless, the search for affordable and inexpensive catalysts based on non-noble transition metals increased the number of studies exploring the use of copper-based alternatives for the selective reduction of NO [22]. Bifunctional Cu-Al-MCM-41 catalysts were studied for both NO reduction and hydrocarbon oxidation [11]. The results showed that well dispersed and isolated Cu ions present in the mesoporous MCM-41 catalyst could act as an active redox center. Hence, hydrogen behaves as a better reducing agent than hydrocarbons, leading to an increased NO conversion at lower temperatures. The use of the Cu/ZSM-5 catalyst rendered a simple and straightforward NO reduction in the presence of hydrogen. However, the activity was almost negligible at reaction temperatures below 400 \({}^{\circ}\)C [23].