Investigations on the Antimony Promotional Effect on CeO2-WO3/TiO2for Selective Catalytic Reduction of NOxwith NH3

Understanding the high performance of an iron-antimony binary metal oxide catalyst in selective catalytic reduction of nitric oxide with ammonia and its tolerance of water/sulfur dioxide

In recent years, Fe-based catalysts for the selective catalytic reduction of NO with NH₃ (NH₃-SCR) have been attracting more attention. In this work, a novel Fe-Sb binary metal oxide catalyst was synthesized using the ethylene glycol assisted co-precipitation method and was characterized using a series of techniques. It was found that the catalyst with a molar ratio of 7:3 (Fe:Sb) displayed the best NH₃-SCR activity with 100% conversion of NO_x (nitrogen oxides) over a wide temperature window and with good resistance to H₂O + SO₂ at 250 °C. The X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NO_x adsorption results suggested that strong electron interactions between Fe and Sb in Fe-O-Sb species existed and electrons of Sb could be transferred to Fe through the 2Fe³⁺ + Sb³⁺ ↔ 2Fe²⁺ + Sb⁵⁺ redox cycle. The introduction of Sb significantly improved the adsorption behaviour of NO_x species on the Fe_0.7Sb_0.3O_x surface, which benefitted the adsorption/transformation of NO_x, thereby facilitating the NH₃-SCR reaction. In addition, the Fe_0.7Sb_0.3O_x catalyst demonstrated a good tolerance of H₂O and SO₂, since the decomposition of NH₄HSO₄ on the catalyst surface was promoted by the introduction of Sb.

Getting insight into the effect of CuO on red mud for the selective catalytic reduction of NO by NH3

A series of copper-modified red mud catalysts (CuO/PRM) with different copper oxide contents were synthesized by wet impregnation method and investigated for selective catalytic reduction of NO by NH₃ (NH₃-SCR). The catalytic results demonstrated that the red mud catalyst with 7 wt% CuO content exhibited the excellent catalytic performance as well as resistance to water and sulfur poisoning. The red mud support and copper-containing catalysts were characterized by XRF, XRD, N₂ adsorption-desorption, HRTEM, EDS mapping, XPS, H₂-TPR, NH₃-TPD and in situ DRIFT. The obtained results revealed that well dispersed copper oxide originating from 1 to 7 wt% CuO contents was more facile for the redox equilibrium of Cu²⁺ + Fe²⁺ ↔ Cu⁺ + Fe³⁺ shifting to right to form Cu⁺ and surface oxygen species than crystalline CuO generating from high CuO loading (9 wt% CuO), which was beneficial to the enhancement of reducibility and the formation of Lewis acid sites on the catalyst surface. All these factors made significant contributions to the improvement of NH₃-SCR activities for CuO/PRM catalysts. Moreover, in situ DRIFT analysis combined with DFT calculated results confirmed that the finely dispersed copper species not only enhanced the NH₃ activation but also promoted the NO_x desorption, which synergistically facilitated the NH₃-SCR process via the Eley-Rideal mechanism.

Enhancement of NH3-SCR performance of LDH-based MMnAl (M = Cu, Ni, Co) oxide catalyst: influence of dopant M

Transition metal (Cu, Ni, Co) doped MnAl mixed oxide catalysts were prepared through a novel method involving the calcination of hydrotalcite precursors for the selective catalytic reduction of NO x with NH3 (NH3-SCR). The effects of transition metal modification were confirmed by means of XRD, BET, TEM, XPS, NH3-TPD, and H2-TPR measurements. Experimental results evidenced that CoMnAl-LDO presented the highest NO x removal efficiency of over 80% and a relatively high N2 selectivity of over 88% in a broad working temperature range (150-300 °C) among all the samples studied. Moreover, the CoMnAl-LDO sample possessed better stability and excellent resistance to H2O and SO2. The reasons for such results could be associated with the good dispersion of Co3O4 and MnO x , which could consequently provide optimum redox behavior, plentiful acid sites, and strong NO x adsorption ability. Furthermore, dynamics calculations verified the meaningful reduction in apparent activation energy (E a) for the CoMnAl-LDO sample, which is in agreement with the DeNO x activity.

Boosting the low-temperature activity and sulfur tolerance of CeZr2Ox catalysts by antimony addition for the selective catalytic reduction of NO with ammonia

In this paper, a series of Sb modified CeZr₂O_x mixed oxides (Sb_yCZ) were synthesized by citrate method for the selective catalytic reduction of NO with ammonia (NH₃-SCR). Experimental results exhibited that the Sb addition could bring a great improvement of SCR activity at 200-360 °C owing to the enhancement in surface area, redox ability and surface acidity. More importantly, the sulfur tolerance of the catalyst with proper Sb loading contents was dramatically improved. For instance, above 85% deNO_x efficiency was retained over Sb_0.5CZ catalyst after 24 h reaction in the presence of 100 ppm SO₂ and 5 vol.% H₂O. As for pure CeZr₂O_x and the catalysts with low Sb loading contents, the serious accumulation of ammonium sulfates resulted in the deactivation after SO₂ exposure. However, with excessive Sb addition, more labile oxygen readily reacted with SO₂ and the redox cycle was then disrupted, leading to the decrease of SCR activity. With an appropriate Sb loading contents, the sulfate species preferentially formed around Sb cations could restrain the further consumption of oxygen species in Ce-O-Ce or Ce-O-Zr mode by SO₂ via a space confinement effect. Thus, a certain amount of labile oxygen was preserved to drive the SCR reaction, thereby enhancing the sulfur tolerance of the catalyst.

Fe2O3-CeO2@Al2O3 Nanoarrays on Al-Mesh as SO2-Tolerant Monolith Catalysts for NO x Reduction by NH3

Currently, selective catalytic reduction of NO _x with NH₃ in the presence of SO₂ is still challenging at low temperatures (<300 °C). In this study, enhanced NO _x reduction was achieved over a SO₂-tolerant Fe-based monolith catalyst, which was originally developed through in situ construction of Al₂O₃ nanoarrays (na-Al₂O₃) on the monolithic Al-mesh by a steam oxidation method followed by anchoring Fe₂O₃ and CeO₂ onto the na-Al₂O₃@Al-mesh composite by an impregnation method. The optimum catalyst delivered more than 90% NO conversion and N₂ selectivity above 98% within 250-430 °C as well as excellent SO₂ tolerance at 270 °C. The strong interaction between Fe₂O₃ and CeO₂ enabled favorable electron transfers from Fe₂O₃ to CeO₂ while generating more oxygen vacancies and active oxygen species, consequently accelerating the redox cycle. The improved reactivity of NH₄⁺ with nitrates following the Langmuir-Hinshelwood mechanism and active NH₂ species that directly reacted with gaseous NO following the Eley-Rideal mechanism enhanced the NO _x reduction efficiency at low temperatures. The preferential sulfation of CeO₂ alleviated the sulfation of Fe₂O₃ while maintaining the high reactivities of NH₄⁺ and NH₂ species. Especially, the SCR reaction following the Eley-Rideal mechanism largely improved the SO₂ tolerance because NO does not need to compete with sulfates to adsorb on the catalyst surface as nitrates or nitrites. This work paves a way for the development of high-performance SO₂-tolerant SCR monolith catalysts.

Novel TiO2 catalyst carriers with high thermostability for selective catalytic reduction of NO by NH3

A series of TiO2 catalyst carriers with ceria additives were prepared by a precipitation method and tested for selective catalytic reduction (SCR) of NO by NH3. These samples were characterized by XRD, N2-BET, NH3-TPD, H2-TPR, TEM, XPS and in situ DRIFTS, respectively. Results showed that the appropriate addition of ceria can enhance the catalytic activity and thermostability of TiO2 catalyst carriers significantly. The maximum catalytic activity of Ti-Ce-Ox-500 is 98.5% at 400 °C with a GHSV of 100 000 h-1 and the high catalytic activity still remains even after the treatment at high temperature for 24 h. The high catalytic performance of Ti-Ce-Ox-500 can be attributed to a series of superior properties, such as larger specific surface area, more Brønsted acid sites, more hydrogen consumption, and the higher proportion of chemisorbed oxygen. Ceria atoms can inhibit the crystalline grain growth and the collapse of small channels caused by high temperatures. Furthermore, in situ DRIFTS in different feed gases show that the SCR reaction over Ti-Ce-Ox-500 follows both E-R and L-H mechanisms.

Insights into the CO2 Stability-Performance Trade-Off of Antimony-Doped SrFeO3-δ Perovskite Cathode for Solid Oxide Fuel Cells

One major challenge for the further development of solid oxide fuel cells is obtaining high-performance cathode materials with sufficient stability against reactions with CO₂ present in the ambient atmosphere. However, the enhanced stability is often achieved by using material systems exhibiting decreased performance metrics. The phenomena underlying the performance and stability trade-off has not been well understood. This paper uses antimony-doped SrFeO_3-δ as a model material to shed light on the relationship between the structure, stability, and performance of perovskite-structured oxides which are commonly used as cathode materials. X-ray absorption revealed that partial substitution of Fe by Sb leads to a series of changes in the local environment of the iron atom, such as a decrease in the iron oxidation state and increase in the oxygen coordination number. Theoretical calculations show that the structural changes are associated with an increase in both the oxygen vacancy formation energy and metal-oxygen bond energy. The area-specific resistance (ASR) of the perovskite oxide increases with Sb doping, indicating a deterioration of the oxygen reduction activity. Exposure of the materials to CO₂ leads to depressed oxygen desorption and an increased ASR, which becomes less pronounced at higher Sb doping levels. Origin of the stability-performance trade-off is discussed based on the structural parameters.

The promoting effect of noble metal (Rh, Ru, Pt, Pd) doping on the performances of MnOx–CeO2/graphene catalysts for the selective catalytic reduction of NO with NH3at low temperatures

Rh, Ru, Pt or Pd doping on MnOx–CeO₂/graphene catalysts increased the amount of chemisorbed oxygen and surface acid sites.

Manganese-cerium oxide (MnOx-CeO2) catalysts supported by titanium-bearing blast furnace slag for selective catalytic reduction of nitric oxide with ammonia at low temperature

A series of manganese-cerium oxide (MnO_x-CeO₂) catalysts supported by Ti-bearing blast furnace slag were prepared by wet impregnation and used for low-temperature selective catalytic reduction (SCR) of NO with NH₃. The slag-based catalyst exhibited high nitrogen oxide removal (deNO_x) activity and wide effective temperature range. Under the condition of NO = 500 ppm, NH₃ = 500 ppm, O₂ = 7-8 vol%, and total flow rate = 1600 mL/min, the Mn-Ce/Slag catalyst exhibited a NO conversion higher than 95% in the range of 180-260 °C. The activity of Mn/Slag catalysts was greatly enhanced with the addition of CeO₂. The results indicated that Ti-bearing blast furnace slag had suitable phase composition as good support of SCR catalyst.

IMPLICATIONS

Ti-bearing blast furnace slag is a kind of industrial waste in China. Much slag was underused and piling up, which could cause many environmental issues, such as enormous waste of titanium and groundwater and soil contamination by heavy metals in leachates. The utilization of slag as the support of SCR catalyst will not only make use of solid waste but also cut down the NO_x emitted from power plant.

Mechanistic Investigation into the Effect of Sulfuration on the FeW Catalysts for the Selective Catalytic Reduction of NOx with NH3

Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NO_x with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH₃-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO₂ was hindered, leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir-Hinshelwood reaction pathway between adsorbed NH₃/NH₄⁺ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO₂ poisoning.

Structure and performance of a V2O5–WO3/TiO2–SiO2 catalyst derived from blast furnace slag (BFS) for DeNOx

The excellent DeNOx performances for a slag-based catalyst derived from its presence of proper amounts of Al₂O₃/Fe₂O₃/SO₄²⁻ as the catalyst dopants or impurities from processing Ti-bearing BFS in making the catalyst.