ABSTRACT
NOX rarely binds with labile oxygens of catalytic solids, whose Lewis acidic (LA) species possess higher binding strengths with NH3 (ENH3) and H2O than Brönsted acidic counterparts (BA--H+; -OH), oftentimes leading to elevate energy barrier (EBARRIER) and weaken H2O tolerance, respectively. These limit NH3-assisted wet NOX reduction via Langmuir-Hinshelwood-type or Eley-Rideal (ER)-type model on LA species, while leaving ER-type analogue on BA--H+ species proper to reduce wet NOX. Given hard-to-regulate strength/amount of -OH species and occasional association between ENH3 and EBARRIER, Ni1V2O6 (Ni1) was rationally chosen as a platform to isolate mono-dentate SO32-/SO42- species for use as BA--H+ bonds via protonation to increase collision frequency (k'APP,0) alongside with disclosure of advantages of SO32-/SO42--functionalized Ni1V2O6 (Ni1-S) over Ni1 in reducing wet NOX. Ni1-S outperformed Ni1 in achieving a larger BA--H+ quantity (k'APP,0↑), increasing H2O tolerance, and elevating oxygen mobility, thus promoting NOX reduction activity/consequences under SO2-excluding gases. V2O5-WO3 composite simulating a commercial catalyst could isolate mono-dentate SO32-/SO42- species and served as a control (V2O5-WO3-S) for comparison. Ni1-S was superior to V2O5-WO3-S in evading ammonium (bi-)sulfate (AS/ABS) poison accumulation and expediting AS/ABS pyrolysis efficiency, thereby improving AS/ABS resistance under SO2-including gases, while enhancing resistance against hydro-thermal aging.
ABSTRACT
TiO2-supported antimony oxide-vanadium oxide-cerium oxide (SVC) imparts Lewis acidic (L)/Brönsted acidic (B) sites, labile (Oα)/mobile oxygens (OM), and oxygen vacancies (OV) for selective catalytic NOX reduction (SCR). However, these species are harmonious occasionally, readily poisoned by H2O/sulfur/phosphorus/carbon, thus limiting SCR performance of SVC. Herein, a synthetic means is reported for immobilizing HSOA-/SOA2- (A= 3-4) or H3-BPO4B- (B= 1-3) on the L sites of SVC to form SVC-S and SVC-P. HSOA-/SOA2-/H3-BPO4B- acted as additional B sites with distinct characteristics, altered the properties of Oα/OM/OV species, thereby affecting the SCR activities and performance of SVC-S and SVC-P. SVC-P activated Langmuir-Hinshelwood-typed SCR better than SVC-S, as demonstrated by a greater Oα-directed pre-factor and smaller binding energy between Oα and NO. Meanwhile, SVC-S provided a larger B-directed pre-factor, thereby outperforming SVC-P in activating Eley-Rideal-typed SCR that dictated the overall SCR activities. Compared with SVC-S, SVC-P contained fewer OV species, yet, had higher OM mobility, thus enhancing the overall redox cycling feature, while providing greater Brönsted acidity. Consequently, the resistance of SVC-P to H2O or soot were greater than or similar to that of SVC-S. Conversely, SVC-S revealed greater tolerance to hydro-thermal aging and SO2 than SVC-P. This study highlights the pros and cons of HSOA-/SOA2-/H3-BPO4B- functionalities in tailoring the properties of metal oxides in use as SCR catalysts.
ABSTRACT
Catalysts are severely poisoned by ammonium sulfate (AS) and ammonium bisulfate (ABS) during selective catalytic NOX reduction (SCR) at low temperatures. To circumvent this issue, metal-substituted vanadates (MV2O6, M = Mn, Co, Ni, or Cu) supported on TiO2 were synthesized and functionalized with SOY2- to form M1 (S) catalysts (Y = 3 or 4). The Mn1 (S) could balance pre-factor and energy barrier required for the SCR, thereby exhibiting the highest NOX consumption rate (activity) among the M1 (S) catalysts. The Mn1 (S) also had desirable redox property, leading to the best SCR performance maximum-obtainable at low temperatures. Notably, the Mn1 (S) substantially reduced the thermal energy needed to decompose AS/ABS poisons. Such unique feature of the Mn1 (S) was pronounced when the Mn1 (S) was promoted by Sb (Mn1-Sb (S)). The resulting Mn1-Sb (S) showed the best SCR performance among all catalysts tested. The Mn1-Sb (S) could minimize the deposition of AS/ABS on the surface and unprecedentedly recovered its performance after regeneration even in the presence of NOX, NH3, SO2, and H2O at 260-280 °C. The temperatures required for the regeneration of the Mn1-Sb (S) were reduced by 100 °C or more in comparison with those of SCR catalysts reported previously.
ABSTRACT
In this study, catalytic activity and thermal stability of the arc plasma deposited (APD) Pt nano-particles on A12O3 and CeO2-Al2O3 were compared with that of the conventionally prepared Pt/Al2O3. All the catalysts were characterized by BET-surface area, transmission electron microscopy, X-ray photoelectron spectroscopy, CO-pulse chemisorption, H2-temperarture programmed reduction and X-ray absorption near edge spectroscopy. Through the quantum chemical calculations of different metal oxide support, CeO2 was identified as a suitable anchoring material with high energy level between the Pt species (Pt(0) and PtO(x)) on ceria. Subsequently, the results of XPS and XANES revealed the presence of abundant Pt(0) metal species in APD catalysts. The addition of ceria to Al2O3 support enhanced the dispersion of Pt nano-particles. The H2-TPR of Pt/CeO2-Al2O3 (APD) catalyst showed high-temperature reduction peaks corresponding to the interaction of Pt with ceria on alumina by Pt-O-Ce. Consequently, the Pt nano-particles deposited on CeO2-Al2O3 by APD attained strong thermal resistance at high temperatures. In addition, superior catalytic activities for CO and C3H6 oxidation and NO(x) reduction were obtained for the Pt/CeO2- Al2O3 (APD) catalyst.
