Effect of Formaldehyde in Selective Catalytic Reduction of NO_x by Ammonia (NH₃-SCR) on a Commercial V₂O₅-WO₃/TiO₂ Catalyst under Model Conditions

Abstract

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial V 2 O 5 -WO 3 /TiO 2 catalyst in NH 3 -SCR of NO x under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH 3 to a formamide (HCONH 2 ) surface intermediate. This deprives NH 3 partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N 2 selectivity. Between 250 and 400 °C, HCONH 2 decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO 2 and NO, which enters the NH 3 -SCR reaction.

Impact of gas phase reactions and catalyst poisons on the NH3-SCR activity of a V2O5-WO3/TiO2 catalyst at pre-turbine position

“…[36,37] Regarding the mechanism of byproducts formation, recent studies showed that HCN can be formed over SCR catalysts via the reaction between NH3 and HCHO. [38][39][40] Therefore, also in the present case the observed HCN could originate from the same process. The conversion of 200 ppm propylene over the vanadium catalyst resulted in a maximum emission of 18 ppm HCN and 9 ppm HCHO at ambient pressure and 550 °C (Fig.…”

Inhibition Effect of Phosphorus Poisoning on the Dynamics and Redox of Cu Active Sites in a Cu-SSZ-13 NH3-SCR Catalyst for NOx Reduction

“…Nitrogen oxides (NO x ) emitted from both stationary sources (e.g., coal-fired power plants and industrial boilers) and mobile sources (e.g., gasoline- and diesel-powered vehicles) are harmful to human health as a class of respiratory irritants, , and they participate in the formation of secondary air pollutants such as fine particulate matter (PM 2.5 ), ground-level ozone (O 3 ), photochemical smog, etc . , For NO x in diesel exhausts, selective catalytic reduction with ammonia (NH 3 -SCR), using small-pore Cu-SSZ-13 zeolite with a chabazite (CHA) framework topology as the catalyst, has been widely utilized and proven to be a reliable technology to meet the stringent emission regulations. − Cu-SSZ-13 demonstrates excellent activity over a wide temperature range from 200 to 500 °C, good N 2 selectivity, and superior hydrothermal stability as compared to other Cu-zeolite catalysts (e.g., Cu-beta and Cu-ZSM-5). − In spite of the advantages, the practical use of Cu-SSZ-13 is still restricted by issues like chemical poisoning by sulfur (S), phosphorus (P), alkali and alkaline earth metals (e.g., Na, K, Ca, and Mg), zinc (Zn), etc . − …”

High-Throughput NOx Removal by Two-Stage Plasma Honeycomb Monolith Catalyst

“…Hydrocarbon selective catalytic reduction (HC-SCR) has been successfully used for NO x removal within a narrow temperature window (250–350 °C). − The use of non-thermal plasma in conjunction with a catalyst has been proposed to expand the temperature window with the view of lowering the temperature. − This comes from a plasma state consisting of energetic electrons, excited species, radicals, and ions. Indeed, energetic electrons produced by dielectric barrier discharge (DBD) plasma have energies in the range of 1–10 eV .…”