On the enhancing effect of Ce in Pd-MOR catalysts for NOx CH4-SCR: A structure-reactivity study

Abstract

2 Graphical abstract NO oxida)onin ion-exchange position, probably in two different sites within the MOR framework.The addition of cerium to Pd-HMOR enhances its catalytic performance for NO x CH 4 -SCR.With 1 wt.% Ce, both NO x conversion into N 2 and CH 4 selectivity towards SCR have increased. Small CeO 2 clusters interacting with palladium are likely to play a major role in this catalytic reaction. The number of such species increases up to Ce loading of ca. 2 wt.%.However, above 3 wt%, NO x conversion values decrease with Ce loading, which is attributed to the formation of bulk CeO 2 species not interacting with palladium.

On the Effect of Preparation Methods of PdCe-MOR Catalysts as NOx CH4-SCR System for Natural Gas Vehicles Application

“…However, the reduction of PdO particles in zeolites is commonly described to occur at ambient or even lower temperatures [12,13,16]. The integration of the peaks (Table 1) shows an H2/Pd ratio similar to the unity, which is consistent with approximately all Pd in the samples being presented as Pd In a previous study [11], it was concluded that the reduction peak identified for Pd(x)-HMOR (x = 0.15, 0.3, 0.5, and 0.7 wt. %), which are similar to the ones observed for the catalysts presented herein, was actually composed by two different reduction processes (resolved as Gaussian peaks), likely to correspond to Pd 2+ ions in different exchange positions.…”

“…In a previous work [11], a second reduction peak in H2-TPR profile of Pd(0.3)Ce(x)-HMOR catalysts, between 240 and 560 °C, has been ascribed to the reduction of CeO2 species in interaction with palladium species. Bulk CeO2 species are usually reduced at higher temperatures.…”

“…As previously identified for the monometallic Pd catalysts, the bands near 380 nm are characteristic of Pd 2+ species. The fact that it appears slightly blue-shifted, may be due to the presence of Ce [11]. The bands at 260 and 290 are ascribed to transition 4f-5d of Ce in CeO2 clusters, respectively [23,24].…”

“…As it was described in a previous study considering Pd(0.3)Ce(x)-HMOR catalysts (x = 1-10 wt. % Ce) [11], this is related to the presence of surface Ce 4+ (CeO2) species, whose reduction is known to occur in the same range of temperatures [22]. The highest for Pd(0.15)Ce(2)-HMOR, which means that a considerable amount of H2 consumption is due to the presence of surface CeO2 species.…”

“…In a previous work [11], the optimal loadings of Pd (monometallic formulation) and Ce (bimetallic formulation) were optimized. The purpose of the this study is to complement the previously-mentioned work, namely by studying the effect of the preparation method on palladium stabilization in the MOR zeolite, the effect of Pd presence on the stabilization of Ce species, and the effect of metal introduction order.…”