Acidic-Basic Modified Catalysts for Improving NO Oxidation and NH3 Decomposition

Abstract

The study of NO oxidation for air pollution abatement technologies is considered as an essential process to improve the efficiency of NOx removal. In the NO oxidation reaction, metallic Pt formed on the catalyst surface plays an important role for enhanced reactivity. Typically, the formation of metallic Pt through Pt-Ti interactions and the prevention of Pt oxidation are promoted by the surface acidity of TiO2. However, the limited availability of acidic sites (-OH) can lead to the agglomeration of Pt particles. This promotes the formation of oxidized Pt species (Pt²⁺ or Pt⁴⁺), reducing the overall catalytic activity. In this current work, we improved NO oxidation by adding acidic sites on TiO₂ through sulfuric acid treatment of bare TiO₂. The low pH of SA-TiO2 allows for better distribution of sulfate to the positively charged surface. Subsequently, the Pt introduced by the increase of acidic sites significantly improved the dispersion and bound to [P(NH₃)₄]-SO₄. The compounds on the surface bound to SO4 and NH3 were decomposed during calcination, favoring the formation of metallic Pt over oxidation of Pt. As a result, the enhanced acidity of SA-TiO₂ facilitated the formation of highly reactive Pt⁰ and Pt²⁺ species, which influeneced improved NO oxidation performance at lower temperatures. Ammonia is considered a promising liquid hydrogen carrier. It is easy to store and transport and does not emit CO in the process of formation H2. In the NH3 decomposition reaction, the active metal of the catalyst is important for the cleavage of the N-H bond. Generally, the N-H cleavage step and N2 desorption step act as RDS, which limits the performance of the catalyst. In this study, various metals (Ru, Ni, Fe, Co, W) were used as active metals to synthesize the catalysts. Ru loaded on SiO2 exhibited higher catalytic activity than other active metals with an energy advantage in both steps. Furthermore, the performance of Ru/SiO2 catalyst was improved over conventional catalysts by controlling the pH after the active metal screening of these catalysts. At the optimal point of pH, Ru/SiO2 catalyst showed excellent results in ammonia conversion and hydrogen production rate. NH4OH treatment confirmed that Ru particles were highly dispersed in a specific pH range. Subsequently, pH control affected not only the dispersion and crystal size of Ru particles, but also the Ru states. In conclusion, compared with other catalysts, Ru/SiO2 (pH 5) catalyst showed the advantages of low crystal size and metallic Ru formation behavior, and the NH3 decomposition performance was improved at lower temperatures. In NO oxidation, sulfuric acid treatment enhanced the activity of Pt/TiO2 catalysts, and in NH3 decomposition, basic treatment enhanced the activity of Ru/SiO2 catalysts. These studies are expected to provide important implications for subsequent studies involving acid-base treatments.