CeO2 WO3 Research Articles

Comprehensive investigation of the mutual effect of Ce and W species in the NH3-SCR catalyst

CeO2–WO3 catalysts were synthesized using the coprecipitation method (CW-C), mixing method (CW-M) and deposition–precipitation method (CW-DP).

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

High calcium resistance of CeO2–WO3 SCR catalysts: Structure investigation and deactivation analysis

The SCR performances of CaO poisoning CeO2–WO3 and V2O5–WO3/TiO2 catalysts were prepared and compared for selective catalytic reduction of NO with NH3 (NH3–SCR). It was found that the CeO2–WO3 catalyst showed better calcium resistance than V2O5–WO3/TiO2 catalyst even when the CaO mass percentage reached up to 5wt%. In order to further understand the resistance mechanism, XRD, Raman, XPS, H2-TPR, NH3-TPD and in situ DRIFTS were used to analyze and explore the changes of CeO2–WO3 catalysts before and after the CaO deactivation. The results manifested that the introduction of Ca species not only affected CeO2 lattice but also led to CaWO4 formation on the surface. Additionally, although CaO increased the surface chemisorbed oxygen and Ce4+ species, the reducibility and Lewis acid sites, NH3 chemisorbed amount and Brønsted acid sites were greatly inhibited after deactivation. Furthermore, the enhanced NO2 adsorption capacity after the alkaline Ca species introduction could partially offset the negative effect in the loss of acid sites over the catalyst surface, which might be the primary reason for stronger CaO resistance of CW catalyst than traditional V2O5–WO3/TiO2 catalyst.

Investigation of the Poisoning Mechanism of Lead on the CeO2-WO3 Catalyst for the NH3-SCR Reaction via in Situ IR and Raman Spectroscopy Measurement.

The in situ IR and Raman spectroscopy measurements were conducted to investigate lead poisoning on the CeO2-WO3 catalysts. The deactivation mechanisms were studied with respect to the changes of surface acidity, redox property, nitrate/nitrite adsorption behaviors, and key active sites (note that the results of structure-activity relationship of CeO2-WO3 were based on our previous research). (1) Lewis acid sites originated from CeO2 and crystalline WO3, whereas Brønsted acid sites originated from Ce2(WO4)3. The poisoned catalysts exhibited a lower surface acidity than the fresh catalysts: the number of acid sites decreased, and their thermal stability weakened. (2) The reducibility of catalysts and the amount of active oxygen exhibited a smaller influence after poisoning because lead preferred to bond with surface WOx species rather than CeO2. (3) The quantity of active nitrate species decreased due to the lead coverage on the catalyst and the partial bridged-nitrate species induced by lead exhibited a low degree of activity at 200 °C. (4) Crystalline WO3 and Ce2(WO4)3 originated from the transformation of polytungstate sites. These sites were the key active sites during the SCR process. The formation temperatures of polytungstate on the poisoned catalysts were higher than those on the fresh catalysts.

Comparison of the Structures and Mechanism of Arsenic Deactivation of CeO2–MoO3 and CeO2–WO3 SCR Catalysts

The mechanism of arsenic poisoning of CeO2–WO3 (CW) and CeO2–MoO3 (CM) catalysts during the selective catalytic reduction (SCR) of NOx with NH3 was investigated. It was found that the ratio of activity loss of the CW catalyst decreases as the temperature increases, while the opposite tendency was observed for the CM catalyst. The fresh and poisoned catalysts were characterized using X-ray diffraction (XRD) temperature-programmed reduction with H2 (H2-TPR), X-ray photoelectron spectra (XPS), NH3-temperature-programmed desorption (NH3-TPD), in situ DRIFTS, and in situ Raman spectroscopy. The results indicate that arsenic oxide primarily destroys the structure of the surface CeOx species in the CM catalyst but prefers to interact with WO3 in the CW catalyst. Additionally, the BET surface area, the number and stability of Lewis acid sites, and the NOx adsorption for these two types of catalysts clearly decrease after deactivation. According to the DRIFTS and Raman investigations, at low temperatures, the greater number of sites with adsorbed NH3 in the poisoned CM catalyst leads to less loss of activity than the poisoned CW catalyst. However, at high temperatures, the greater number of Lewis acid sites remaining in the poisoned CW catalyst may play an important role in maintaining the activity of this catalyst.

Characterization of CeO2–WO3 catalysts prepared by different methods for selective catalytic reduction of NO with NH3

In this work, cerium–tungsten oxide catalysts were prepared by three methods: single step sol–gel (SG), impregnation (IM), and solid processing (SP). The catalysts were used for selective catalytic reduction (SCR) of NOx with ammonia over a wide temperature range. The results indicated that the catalysts prepared by the SP and IM methods exhibited better SCR activity than that prepared via the SG method in 175–500 °C. The excellent activity can be attributed to larger surface area, higher surface concentrations of Ce and Ce3 +, enhanced NO oxidization ability, and greater number of surface acid sites.

Identification of the active sites on CeO2–WO3 catalysts for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study

A series of CeO2–WO3 (WxCe) catalysts were prepared for the SCR of NOx with NH3. Among the three representative samples, the W1Ce catalyst exhibited great activity during SCR reaction and a large amount of N2 from NH3 oxidation, whereas the W0.05Ce catalyst yielded more NO from NH3 oxidation than the W3Ce at high temperature. Three types of metal oxide species (isopolytungsten species, Ce2(WO4)3 compounds and crystalline WO3) were observed on the catalyst surfaces with increased WO3 loading. CeO2, rather than WO3 or Ce2(WO4)3, presented favorable reducibility behavior. Furthermore, CeO2 with intrinsic oxygen vacancies and unsaturated Wn+ cations of crystalline WO3 provided Lewis acid sites; meanwhile, the WOW or WO modes of Ce2(WO4)3 provided Brønsted acid sites. Based on the Raman spectra, the WO species of the [WO4] or [WO6] units could be the active sites. The results suggest a reaction mechanism consisting of two independent cycles, denoted as a redox cycle due to the excellent oxygen storage capability and reducibility of cubic fluorite CeO2 (for NH3 activation) and an acid site cycle, resulting from Brønsted acid sites formed on the WOW species of Ce2(WO4)3 (for NH3 adsorption). Surface nitrate species cannot be involved directly in SCR reaction with gaseous NH3; additionally, the adsorbed NO2 demonstrated activity over the W1Ce catalyst at 200°C.

Ge, Mn-doped CeO2–WO3 catalysts for NH3–SCR of NOx: Effects of SO2 and H2 regeneration

The addition of Ge and Mn to CeO2–WO3 catalysts for NH3–SCR of NOx was investigated. The effect of calcination temperature and the influence of SO2 on SCR activity were studied, and heat treatment with H2 was used to regenerate sulfated catalysts. The results showed that doping with Ge and Mn enhanced the SCR activity of Ce3W2Ox (molar ratio of Ce:W=3:2) at 100–250°C. More than 95% NOx conversions were obtained between 200 and 470°C over (Ce3W2)0.9Ge0.1Ox-500 (molar ratio of (Ce+W):Ge=9:1, calcined at 500°C) catalyst. The characterization results indicated that the formation of amorphous phase and high atomic ratio of W/Ce on the surface might contribute to the high activity over (Ce3W2)0.9Ge0.1Ox-500. The NOx conversions decreased over Ce3W2Ox series catalysts calcined at 700°C, however, the activities of (Ce3W2)0.9Ge0.1Ox calcined at 700°C were higher than those of the others, suggesting the addition of Ge weakened the effect of high calcination temperature. The deactivation of (Ce3W2)0.9Ge0.1Ox-500 by SO2 was much slower than that of Ce3W2Ox-500. The sulfated (Ce3W2)0.9Ge0.1Ox-500 catalysts could be easily regenerated by heat treating with H2 at 300°C since the sulfates species were unstable over these catalysts. H2 regeneration is an effective way to recycle GeO2–CeO2–WO3 catalyst.

Manganese doped CeO2–WO3 catalysts for the selective catalytic reduction of NO with NH3: An experimental and theoretical study

Abstract The CeO 2 –WO 3 (CeW) and Mn-doped CeO 2 –WO 3 (MnCeW) catalysts were prepared by co-precipitation method. The MnCeW catalyst exhibited more activity for NO x conversion than the CeW catalyst did below 200 °C. The experiment results showed that the MnCeW catalyst provided more Bronsted acid sites and higher reducibility than the CeW catalyst. Theoretical studies showed that oxygen vacancies can easily form on the MnCeW (110) surface, resulting in more facile NH 3 adsorption and higher activity. The reaction mechanism mainly followed the L–H mechanism at 150 °C over the MnCeW catalyst: both adsorbed NH 3 and NH 4 + react with nitrite species.

CeO2–WO3 Mixed Oxides for the Selective Catalytic Reduction of NO x by NH3 Over a Wide Temperature Range

A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst. A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO x by ammonia (NH3-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H2-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO2–WO3 with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH3-SCR reactions, N2 selectivity and SO2 durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h−1. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO2–WO3 mixed oxide catalyst.