Niobium sulfide as a dopant for Mo/TiO2 catalysts

Perspectives on strategies for improving ultra-deep desulfurization of liquid fuels through hydrotreatment: Catalyst improvement and feedstock pre-treatment

Reliance on crude oil remains high while the transition to green and renewable sources of fuel is still slow. Developing and strengthening strategies for reducing sulfur emissions from crude oil is therefore imperative and makes it possible to sustainably meet stringent regulatory sulfur level legislations in end-user liquid fuels (mostly less than 10 ppm). The burden of achieving these ultra-low sulfur levels has been passed to fuel refiners who are battling to achieve ultra-deep desulfurization through conventional hydroprocessing technologies. Removal of refractory sulfur-containing compounds has been cited as the main challenge due to several limitations with the current hydroprocessing catalysts. The inhibitory effects of nitrogen-containing compounds (especially the basic ones) is one of the major concerns. Several advances have been made to develop better strategies for achieving ultra-deep desulfurization and these include: improving hydroprocessing infrastructure, improving hydroprocessing catalysts, having additional steps for removing refractory sulfur-containing compounds and improving the quality of feedstocks. Herein, we provide perspectives that emphasize the importance of further developing hydroprocessing catalysts and pre-treating feedstocks to remove nitrogen-containing compounds prior to hydroprocessing as promising strategies for sustainably achieving ultra-deep hydroprocessing.

Molybdenum-decorated V2O5–WO3/TiO2: surface engineering toward boosting the acid cycle and redox cycle of NH3-SCR

Submonolayer Mo-decorated V₂O₅–WO₃/TiO₂ provides abundant vanadia species and unsaturated V⁴⁺ species, accelerating the acid and redox cycling of low-temperature NH₃-SCR.

CeO2 grafted with different heteropoly acids for selective catalytic reduction of NOx with NH3

The CeO₂ catalysts grafted with heteropoly acid (i.e., HPA) could enhance their catalytic performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). In comparison to HSiW/CeO₂, HPMo/CeO₂, and commercial V₂O₅-WO₃/TiO_x catalysts, HPW/CeO₂ catalysts showed the best SCR performance. XPS and DRIFTS demonstrated that the amount of HPA on HPW/CeO₂ was more than those on HSiW/CeO₂ and HPMo/CeO₂. H₂-TPR results indicated that reducibility of HPMo/CeO₂ was stronger than those of HSiW/CeO₂ and HPW/CeO₂, resulting in the high-temperature performance loss. According to kinetic results, below 250 °C, k_SCR-ER and k_SCR-LH of HPW/CeO₂ were higher than those of HSiW/CeO₂, meanwhile k_side of both HSiW/CeO₂ and HPW/CeO₂ were low. Therefore, HPW/CeO₂ had the better SCR performance than HSiW/CeO₂. As NH₃ was completely consumed, SCR activity depended on the ratio of SCR reaction in the consumption of NH₃. The selectivity of SCR reaction, NSCR reaction, and C-O reaction of HSiW/CeO₂ were almost the same as those of HPW/CeO₂ above 250 °C, resulting in the NO_x conversion of HPW/CeO₂ was basically the same as that of HSiW/CeO₂ above 250 °C. Due to the lowest k_SCR-ER and k_SCR-LH, and highest k_side, NO_x conversion of HPMo/CeO₂ was the worst compared to HSiW/CeO₂ and HPW/CeO₂ catalysts.

Deactivation by HCl of CeO2-MoO3/TiO2 catalyst for selective catalytic reduction of NO with NH3

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃ was investigated with BET, XRD, NH₃-TPD, H₂-TPR, XPS and catalytic activity measurements. The results showed that HCl had an inhibiting effect on the activity of the CeO₂–MoO₃/TiO₂ catalyst. The deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst could be attributed to pore blockage, weakened interaction among ceria, molybdenum and titania, reduction in surface acidity and degradation of redox ability. The Ce³⁺/Ce⁴⁺ redox cycle was damaged because unreactive Ce³⁺ in the form of CeCl₃ lost the ability to be converted to active Ce⁴⁺ in the SCR reaction. In addition, a decrease in the amount of chemisorbed oxygen and the concentrations of surface Ce and Mo was also responsible for the deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst.

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃.

Effect of MoO3 on vanadium based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

The selective catalytic reduction (SCR) activities of the MoO₃ doped V/WTi catalysts prepared by the incipient wetness impregnation method at low temperature were investigated. The results showed that the addition of MoO₃ could enhance the NO_x conversion at low temperature and the best SCR activity was obtained when the dosage of MoO₃ reached 5wt.%. The NH₃-TPD and DRIFTS experiments indicated that the addition of MoO₃ changed the type and number of acid sites on the surface of catalysts and reaction activities of acid sites were altered at the same time. The redox capacity and amount of active oxygen species got improved for V3Mo5/WTi catalyst, which could be confirmed by the H₂-TPR and transient response experiments. Water vapor inhibited the NO_x conversion at low temperature. Deposition of ammonium sulfate or bisulfate might be main reason for the loss of catalytic activity in the presence of SO₂ at low temperature. Choosing the suitable NH₃/NO ratio and elevation of reaction temperature both could weaken the influence of SO₂ on the SCR activity of the V3Mo5/WTi catalyst. Thermal treatment of the deactivated catalyst at 350°C could get the low temperature activity recovered. The decrease of GHSV improved the deNO_x efficiency at low temperature and we speculated that the rational technological process and operation parameters could contribute to the application of this kind of catalysts in real industrial environment.

Promotional Effects of Ti on a CeO2-MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3

In this study, Ti was doped to CeO₂-MoO₃ to promote the catalytic performance for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The preparation method for CeMo_0.5Ti_aO_x (a = 0, 1, 2, 5, 10) catalysts was a stepwise precipitation method. When Ti was doped, all of the Ce-Mo-Ti catalysts exhibited remarkably improved NO_x conversion and N₂ selectivity than the CeMo_0.5O_x without Ti. The CeMo_0.5Ti₅O_x with excellent activity in a broad temperature range was selected as an optimal catalyst to investigate the effects of Ti addition. The formation process analysis of the CeMo_0.5Ti₅O_x showed that, the Mo and Ti species first precipitated together from the mixed solution with the increase of pH, and then Ce species precipitated onto the Mo-Ti precipitates. The obtained catalyst exhibited remarkably facilitated NO_x and NH₃ adsorption, enhanced charge imbalance, promoted redox property, and improved surface acidity, which are all important reasons for the excellent catalytic performance of an NH₃-SCR catalyst.

Enhanced performance in NOx reduction by NH3 over a mesoporous Ce–Ti–MoOx catalyst stabilized by a carbon template

The mesopore Ce–Ti–MoO_x catalyst stabilized by an in situ formed carbon template showed more than 90% NO_x conversion at 175–425 °C for SCR of NO_x with NH₃. Increasing temperature in the presence of carbon template contributed to the narrow mesoporous distribution and excellent low-temperature SCR activity.