Mechanistic investigation of the reduction of NOx over Pt-and Rh-based LNT catalysts

The influence of the noble metals (Pt vs. Rh) on the NOx storage reduction performances of lean NOx trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al2O3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NOx, by decreasing the threshold temperature for the reduction of the stored NOx. The Pt-based catalyst promotes the reduction of the adsorbed NOx at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH3 decomposition significantly enhancing N2 selectivity. Moreover, formation of small amounts of N2O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NOx stored at 150 °C, where NOx is likely in the form of nitrites

The Effect of Hydrogen on the Storage of NOx Over Silver, Platinum and Barium Containing NSR Catalysts

Addition of hydrogen significantly increases the amount of stored nitrogen oxides (NOx) over Pt/Ba/Al2O3 at 100 A degrees C and Ag/Al2O3 between 100 and 200 A degrees C. This increase is unique for hydrogen as reductant. A higher apparent NOx storage capacity is also observed by simultaneous reduction of NO in a selective catalytic reduction (SCR) reaction, where, however, the conversion in H-2-SCR i

Silver as Storage Compound for NOx at Low Temperatures

High loaded silver/alumina (16 wt%) was found to be suitable as a temporary NOx trap under cold start, so called passive NOx storage. High amounts of NOx can be stored in the presence of H-2 on this material below 200 degrees C and released below 400 degrees C under lean conditions or under rich conditions at adsorption temperature

Enhanced Low Temperature NOx Reduction Performance Over Bimetallic Pt/Rh-BaO Lean NOx Trap Catalysts

The overall NSR operation was tested over a bimetallic Pt/Rh-BaO lean NO (x) trap (LNT) catalyst in the range of 473-673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/gamma-Al2O3 and 0.5 wt% Rh/BaO/gamma-Al2O3 samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/gamma-Al2O3 and Rh/BaO/gamma-Al2O3, which both were mildly aged, have limited NO (x) reduction capacity at 473 K. However, combining Pt and Rh in the NO (x) storage catalyst assisted the NO (x) reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO2-TPD data showed that the presence of Rh into the Pt/BaO/gamma-Al2O3 system has a considerable effect on the spill-over process of NO (x) , accelerating the release of NO (x) at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/gamma-Al2O3 catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NO (x) reduction efficiency in comparison with that of the monometallic Pt and Rh-BaO LNT catalysts

Micro-calorimetric studies of NO2 adsorption on Pt/BaO-supported on gamma-Al2O3 NOx storage and reduction (NSR) catalysts-Impact of CO2

The adsorption of NO2 on Pt/BaO/gamma Al2O3 catalyst has been investigated by micro calorimetry at atmospheric pressure, NOx storage tests and temperature-programmed desorption (TPD). The heat of adsorption of NO2 (Delta H-ads(NO2)) was determined over a wide range of NOx coverages, as the catalyst was exposed to 500/900 ppm NO2 in the absence/presence of 5% CO2 in the range of 423-773 K. The temperature dependent changes of Delta H-ads(NO2) verified the presence of energetically different NOx storage sites with different binding strength. The Delta H-ads(NO2) was found to follow a linear correlation versus temperature, ranging for example from -134.5 to -178.8 kJ/mol for NOx storage over Pt/BaO/gamma Al2O3 at 423-673 K. Thus, at high temperature mostly strongly bound nitrates were formed, while at lower temperature more loosely bound species were also present. Interestingly, the heat of adsorption was higher when using higher NO2 concentration, indicating more bulk barium nitrate formation. This is consistent with the TPD data where a clear high temperature peak was visible after adsorption using 900 ppm NO2 at 423 and 473 K, which was not the case for 500 ppm NO2. Moreover, the micro-calorimetric data also provided evidence in support of the detrimental effect of CO2 on the NOx uptake process. The heat released during the NOx storage in 500 ppm NO2 + 5% CO2 was determined to be significantly reduced ca. -97.8 kJ mol(-1) at 423 K, but ca. -134.5 kJ mol(-1) without CO2. Furthermore, our results show that it is critical to measure heat of adsorption for surface compounds since they are significantly different compared to thermodynamic data for bulk materials

Enhanced Low Temperature NOx Reduction Performance Over Bimetallic Pt/Rh-BaO Lean NOx Trap Catalysts

The overall NSR operation was tested over a bimetallic Pt/Rh-BaO lean NO (x) trap (LNT) catalyst in the range of 473-673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/gamma-Al2O3 and 0.5 wt% Rh/BaO/gamma-Al2O3 samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/gamma-Al2O3 and Rh/BaO/gamma-Al2O3, which both were mildly aged, have limited NO (x) reduction capacity at 473 K. However, combining Pt and Rh in the NO (x) storage catalyst assisted the NO (x) reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO2-TPD data showed that the presence of Rh into the Pt/BaO/gamma-Al2O3 system has a considerable effect on the spill-over process of NO (x) , accelerating the release of NO (x) at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/gamma-Al2O3 catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NO (x) reduction efficiency in comparison with that of the monometallic Pt and Rh-BaO LNT catalysts

Mechanistic investigation of the reduction of NOx over Pt-and Rh-based LNT catalysts

The influence of the noble metals (Pt vs. Rh) on the NOx storage reduction performances of lean NOx trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al2O3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NOx, by decreasing the threshold temperature for the reduction of the stored NOx. The Pt-based catalyst promotes the reduction of the adsorbed NOx at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH3 decomposition significantly enhancing N2 selectivity. Moreover, formation of small amounts of N2O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NOx stored at 150 \ub0C, where NOx is likely in the form of nitrites

The effect of iron loading and hydrothermal aging on one-pot synthesized Fe/SAPO-34 for ammonia SCR

The current commercially-available technique for NOx reduction for diesel engines is the selective catalytic reduction (SCR) of NOx with NH3 over Cu zeolites. One of the problems of this technique is their limited ability to convert NOx at diesel particulate filter (DPF) regeneration temperatures. In addition, during regeneration of the DPF there is a risk of thermally deactivating the SCR catalyst. Thus, the aim of the current work was the development of a catalytic system that can reduce NOx both at low as well as high temperature and in addition is stable at high temperature. In order to reach this goal, a Fe/SAPO-34 with chabazite (CHA) structure was combined in a system with a commercial Cu/CHA catalyst. Earlier studies have shown that it is difficult to ion-exchange Fe into CHA structures due to steric hindrance, and we have therefore used a novel synthesis procedure which incorporated iron directly into the zeolite structure. Fe/SAPO-34 with three different Fe-loadings (0.27; 0.47 and 1.03 wt.% Fe) were synthesized and the catalysts were characterized using inductively coupled plasma atomic spectroscopy (ICP-AES), N-2 adsorption-desorption isotherms, BET area measurements and X-ray diffraction (XRD). The chemical composition, porous and crystalline structure of the parent SAPO-34 sample were found to be only slightly affected by addition of small amounts of Fe in the framework zeolite structure. However, more visible changes in the crystallinity were observed in the Fe/SAPO-34 catalysts with higher Fe content, which were attributed to the unit cell size expansion provoked by integration of higher amounts of Fe into the zeolite SAPO-34 framework. The Fe/SAPO-34 with the lowest Fe-loading (0.27 wt.%) was found to be the best catalyst when considering activity as well as high temperature stability. The synthesized Fe/SAPO-34 catalyst demonstrated a significantly improved NOx reduction performance at high temperatures (600-750 degrees C) when compared to a commercial Cu/CHA SCR system, and the combined system (Fe/SAPO-34+ Cu/CHA) exhibited a very good performance in a large temperature interval (200-800 degrees C) that encompasses most diesel exhaust gas conditions

Mechanistic Investigation of the Reduction of NOx over Pt- and Rh-Based LNT Catalysts

The influence of the noble metals (Pt vs. Rh) on the NOx storage reduction performances of lean NOx trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al2O3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NOx, by decreasing the threshold temperature for the reduction of the stored NOx. The Pt-based catalyst promotes the reduction of the adsorbed NOx at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH3 decomposition significantly enhancing N2 selectivity. Moreover, formation of small amounts of N2O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NOx stored at 150 °C, where NOx is likely in the form of nitrites

Mechanistic investigations of the promoting role of Rh on the NSR performance of NOx storage BaO-based catalysts

To determine the promoting effect of Rh on the overall NOx storage and reduction (NSR) performance, the studies in the current work were directed toward investigating the storage and release ability over Rh NOx storage BaO-based catalysts compared to Pt. In terms of the metal surface dispersion and the ability of the noble metals to release oxygen at lower temperatures, the synthesized catalysts were characterized by means of dynamic CO chemisorption (RT) and N2O dissociation (RI - 773 K). The NOx storage capacity and the thermal stability of the NOx adsorbed species formed on the surface were analyzed via NOx storage tests and temperature programmed desorption (TPD) without and in the presence of CO2 and H2O. In addition, experiments with lean and rich cycling were conducted at 473,573 and 673 K. The results from the N2O dissociation experiments showed the superior ability of Rh/Al and Rh/Ba/Al catalysts compared to Pt toward O-2 release from the catalytic surface at lower temperatures. In this work, we show that the presence of Rh into the BaO/gamma-Al2O3 system has a considerable effect on the spill-over process of NOx to the precious metal, controlling the subsequent desorption of NOx to occur at lower temperatures in comparison with that of the Pt catalysts. It is suggested a mechanism of NOx desorption where the lower temperature of O-2 release from the surface of Rh catalysts could leave a significant number of noble metal sites accessible for adsorption. Thus this could facilitate the rate of spill-over of NOx from the storage site (the surface sites on gamma-Al2O3 and those on BaO) to the noble metal and their desorption at lower temperatures. The limited NOx storage ability of the Rh-based BaO/gamma-Al2O3 catalysts under lean-burn conditions was found to originate from both low NO oxidation activity and NOx reduction activity, while the main limiting factor for the low NSR performance of the Pt-based catalysts was the limited regeneration ability during rich period