Pt loaded vanadium based catalyst for NH3 and CO emission control

Abstract

SCR (selective catalytic reduction) is the most efficient and dependable method of reducing nitrogen oxides in industrial gaseous emissions. Excessive NH3 injection to improve NOx conversion efficiency leads NH3-slip, which is a challenge with the NH3-SCR process. Since NH3 is a known hazardous gas, controlling its emissions is a major topic. The solution for these concern is the after-treatment catalyst system, which could selectively convert NH3 to N2 (known as NH3-SCO). The NH3-SCO catalyst acts as a "guard catalyst," allowing excess NH3 dosing to limit both NOx conversion and NH3 emission from the NH3-SCR process. In the terms of catalyst candidates, Pt-based catalysts have previously been reported to be highly active in SCO reactions, according to prior investigations. Although Pt has low N2 selectivity when compared to other non-PGM-based catalysts, it has shown outstanding SCO performance when some promoters are added. Despite these attempts, several feed components such as CO in addition to NH3 have been shown to significantly reduce N2 selectivity. Since CO is constantly present in stationary emission sources that use NH3-SCR, it is essential to understand how the NH3-SCO process interacts with CO and how it affects the environment. This study reports the direct usage approach for the simultaneous oxidation of NH3 and CO of a vanadium-based SCR catalyst with a low Pt content. Over the desired temperature range, a simple and efficient strategy for adding a low Pt content to a vanadium-based NH3-SCR catalyst resulted in enhancement of N2 selectivity with significant NH3 and CO co-oxidation performance. The increased selectivity for N2 was demonstrated as the following surface reactions: (1) selective catalytic reduction of NH3-NO by internally produced NOx species (i-SCR) and (2) CO-SCR in the presence of carbon monoxide in the feed. The close proximity of each metal in a catalytic system provides insight into the formation and its role of distinct active sites over PtVW/TiO2 catalytic system. Tungsten had a minor influence on the rate and selectivity of the ammonia and CO co-oxidation process, whereas the effects of Pt and V loading were clear. Particularly, decline in oxidative activity as V loading increased along with the dramatic increase in N2 selectivity was observed, suggesting the formation of a strong interaction between Pt and V. Catalyst characterizations were carried out to better understand and prove the existence of Pt-V interaction. According to the composition of the catalyst system, XRD pattern analysis reveals that the production of Pt-V interactions enhances the dispersion of Pt. The absence of aggregation of Pt in catalysts containing both Pt and V despite same calcination conditions implies two metals forms a strong interaction. The TEM-EDS study and CO-chemisorption technique both validated this increase in dispersion. According to XPS studies, these interactions are mediated by electrophilic V5+ oxide state that electron donated from Pt. Pt shifted to oxide state when the Pt and V was co-impregnated over TiO2, and V preferred the oxidation state of V4+. Additionally, the absence of a change in the chemical states of Ti and W further indicates the formation of bimetallic Pt-V oxide. The reaction data also numerically provides the formation of these bimetal Pt-V. The TOF and activation energy values obtained from data for the N2 production reaction with less than 20% NH3 conversion indicate that theses bimetal Pt-V act as a distinct active site for the NH3 oxidation and N2 formation reactions. The PtVW/TiO2 catalyst shows much lower activation energy for the N2 formation process than the Pt-only catalyst, indicating that the bimtal Pt-V active sites are more suitable for the N2 formation reaction. The temperature-programmed approach was used to conduct a comparative analysis of metal composition. The analysis of the catalyst properties, such as surface acidity and reducibility demonstrates the benefits of the development of these different active sites. Due to the enhanced surface acidity and reducibility of the catalyst as a result of the Pt-V interaction, selective oxidation of NH3 could be induced. Increased reducibility efficiently dehydrogenates adsorbed NH3, further forms surface NOx sorbent species. Increased catalyst surface acidity maintains NH3 in the form of NHx at elevated temperatures. According to this properties, it has been hypothesized that the i-SCR mechanism could be induced for the high N2 selectivity. CO also has an influence on the improvement of N2 selectivity in this investigation. When CO was not present in the feed, decline in N2 selectivity was observed. To this end, DRIFTs analysis was used to examine the impact of CO in conjunction with the i-SCR mechanism. IR study comparing each catalyst demonstrates that the i-SCR and CO-SCR mechanisms were the major point for the enhanced N2 selectivity. Based on the observations made in this study, the reaction pathway for the N2 selectivity enhancement of the PtVW/TiO2 catalyst is proposed to better understand the role of the Pt-V interaction.