Alternative reducers should be decomposed into ammonia without producing harmful products and be easy to store and transport at an affordable cost and a wide availability from a technical and commercial standpoint. The ammonia precursors studied can be classified into active carbon (AC), Yzeolite, ammonium carbamate, ammonium formate, methanamide and guanidinium formate, ammonium salt, and ammoniate. The ammonia capture capacities of AC and Y-zeolite with on-site ammonia synthesis are increased compared with non-treated AC and Y-zeolite due to ammonium ion formation and ammine complex formation, respectively [
19]. However, their ammonia capture capacities are still too low compared with those of ammoniates. The guanidinium salts with higher decomposition temperatures present larger ammonia densities than ammonium carbamate and urea [
20]. The formamide-based mixtures are less efficient compared with urea-based agents but their NH
3 slip processes are better prevented [
21], while ammonium salts and ammoniates offer better ammonia storage performance than solid urea [
22]. The on-board ammonia storage and delivery system (ASDS) with the ammoniate-based SCR technology has the advantages of high density and direct ejecting of ammonia for de-NO
x [
23]. For instance, after being compressed to 1219 kg/m
3, [Mg(NH
3)
6]Cl
2 occupies a volume and weight 3.1 and 2.8 times less than those of AdBlue, respectively [
24]. The thermodynamic stability of the monoamine phase ([Sr(NH
3)]Cl
2), the diamine phase ([Sr(NH
3)
2]Cl
2), and the octopamine phase ([Sr(NH
3)
8]Cl
2) has also been verified to explain the working principle of strontium chloride as the ammonia sorbent for sorption selective catalytic reduction of NO
x (sorption-SCR) [
25]. To improve the heat and mass transfer performance and avoid agglomeration of ammoniates, expanded natural graphite treated with sulfuric acid (ENG-TSA) is added as the matrix for ammoniates (CaCl
2, SrCl
2, BaCl
2, NH
4Cl, and NaBr) to complete sorption-SCR [
26,
27], with an annual required mass/volume and cost generally lower than those of urea-SCR. Furthermore, due to the advantage of the relatively lower starting temperature for desorbing ammonia compared with AdBlue (over 160°C), the NO
x conversion of sorption-SCR with CaCl
2/ENG-TSA is around 45% higher than that of the urea-SCR at 50°C [
28]. Therefore, it can be concluded that ammoniates with the matrix of ENG-TSA bring a bright future for safe and efficient NH
3 selective catalytic reduction of NO
x (NH
3-SCR).