Catalytic Oxidation of Chlorobenzene over Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM‑5 Catalysts: A Study with Practical Implications

Industrial-use catalysts usually encounter severe deactivation after long-term operation for catalytic oxidation of chlorinate volatile organic compounds (CVOCs), which becomes a “bottleneck” for large-scale application of catalytic combustion technology. In this work, typical acidic solid-supported catalysts of Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 were investigated for the catalytic oxidation of chlorobenzene (CB). The activation energy (<i>E</i>_a), Brønsted and Lewis acidities, CB adsorption and activation behaviors, long-term stabilities, and surficial accumulation compounds (after aging) were studied using a range of analytical techniques, including XPS, H₂-TPR, pyridine-IR, DRIFT, and O₂-TP-Ms. Experimental results revealed that the Brønsted/Lewis (B/L) ratio of Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 catalysts could be adjusted by ion exchange of H• (in HZSM-5) with Mnⁿ⁺ (where the exchange with Ce⁴⁺ did not distinctly affect the acidity); the long-term aged catalysts could accumulate ca. 14 organic compounds at surface, including highly toxic tetrachloromethane, trichloroethylene, tetrachloroethylene, <i>o</i>-dichlorobenzene, etc.; high humid operational environment could ensure a stable performance for Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 catalysts; this was due to the effective removal of Cl• and coke accumulations by H₂O washing, and the distinct increase of Lewis acidity by the interaction of H₂O with HZSM-5. This work gives an in-depth view into the CB oxidation over acidic solid-supported catalysts and could provide practical guidelines for the rational design of reliable catalysts for industrial applications

Alkali Potassium Induced HCl/CO₂ Selectivity Enhancement and Chlorination Reaction Inhibition for Catalytic Oxidation of Chloroaromatics

Industrial combustion of chloroaromatics is likely to generate unintentional biphenyls (PCBs), polychlorinated dibenzo-<i>p</i>-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). This process involves a surface-mediated reaction and can be accelerated in the presence of a catalyst. In the past decade, the effect of surface nature of applied catalysts on the conversion of chloroaromatics to PCBs/PCDD/PCDF has been well explored. However, studies on how the flue gas interferent components affect such a conversion process remain insufficient. In this article, a critical flue gas interferent component, alkali potassium, was investigated to reveal its effect on the chloroaromatics oxidation at a typical solid acid–base catalyst, Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5. The loading of alkali potassium was found to improve the Lewis acidity of the catalyst (by increasing the amounts of surface Mn⁴⁺ after calcination), which thus promoted the CO₂ selectivity for catalytic chlorobenzene (CB) oxidation. The KOH with a high hydrophilicity has favored the adsorption/activation of H₂O molecules that provided sufficient hydroxyl groups and possibly induced a hydrolysis process to promote the formation of HCl. The K ion also served as a potential sink for chorine ions immobilization (via forming KCl). Both of these inhibited the formation of phenyl polychloride byproducts, thereby blocking the conversion of CB to chlorophenol and then PCDDs/PCDFs, and potentially ensuring a durable operation and less secondary pollution for the catalytic chloroaromatics combustion in industry

Mercury Re-emission Behaviors in Magnesium-Based Wet Flue Gas Desulfurization Process: The Effects of Oxidation Inhibitors

The effects of four oxidation inhibitors (including ascorbic acid, formaldehyde, hydroquinone, and sodium thiosulfate) on mercury re-emission in simulated magnesium-based wet flue gas desulfurization solution were evaluated. Experimental results demonstrated that the addition of ascorbic acid significantly increased the mercury re-emission, whereas the formaldehyde and hydroquinone could slightly enhance this process at pH 6. Sodium thiosulfate was found to somewhat inhibit the bivalent mercury reduction owing to the strong binding between mercury and thiosulfate ions. pH variation showed dramatic effects on elemental mercury re-emission in the solution containing ascorbic acid or hydroquinone, and the mercury emission amount in 30 min increased about seven- and two-fold, respectively, when pH was adjusted from 6 to 8. These findings indicated that the ascorbic acid was an important reducer of mercury re-emission under acidic condition besides the sulfite ions, whereas it was the main reducer of mercury reduction under alkaline condition. In addition, the investigation on the effects of temperature and the presence of Cl^– ions suggested that the mercury re-emission would be accelerated at elevated temperatures and could be suppressed in the presence of Cl^– ions

Facile Approach for the Syntheses of Ultrafine TiO₂ Nanocrystallites with Defects and C Heterojunction for Photocatalytic Water Splitting

In this paper, a supercritical water (sc-H₂O) reaction medium was employed for the syntheses of ultrafine TiO₂ nanocrystallites (at ca. 5 nm) that were linked with lactate species at surface. The resulting hybrid material was then subjected to an aging at ca. 300 °C for 2 h under N₂ atmosphere. After subjected to spherical aberration corrected STEM and EPR analyses, it was noted that the aged sample was shown with highly distorted crystal lattice with oxygen vacancies at surface and Ti³⁺ in the bulk. The anoxic aging also caused incomplete combustion for lactate species, leading to the formation of C heterojunction with TiO₂. UV–vis, PL and transient photocurrent (TP) measurements revealed that the resulting surface oxygen vacancies and C heterojunction had conferred a combination of advantages in enhancing visible light absorption and promoting electron–hole pair separation for aged sample, which led to significantly promoted hydrogen production efficiency in photocatalytic water splitting under a full-spectrum irradiation (where the aged TiO₂ had yielded ca. 4-fold higher hydrogen production rate than the nonaged one and ca. 40–50-fold higher than commercial Degussa P25). We expected that the work conducted herein could provide a facile and controllable approach to produce simultaneously defects and C heterojunction for ultrafine TiO₂ nanocrystallites, which might lead to scale-up production of them for industry

SO₂ Poisoning Structures and the Effects on Pure and Mn Doped CeO₂: A First Principles Investigation

SO_<i>x</i> poisoning effects in environmental catalysis have long been recognized as a challenge in development of efficient catalysts for industrial applications. In this paper, a theoretic method combining density functional theory and standard thermodynamic data (enthalpy and entropy) was applied to investigate the SO₂ poisoning to pure and Mn doped CeO₂ as model catalysts in realistic temperature and pressure. Surface Ce­(SO₄)₂ rather than Ce₂(SO₄)₃ was identified to be the most stable poisoning structure on pure CeO₂. The SO_<i>x</i> poisoning to the catalysts could not be surmounted simply by heteroatom doping, since the introduction of Mn will enhance the thermal stability of the surface sulfate. The results also indicated that the Lewis acidity of the catalysts could be enhanced by slightly sulfating, which might make some positive effect on catalytic performances for the abatement of environmentally sensitive species including NH₃, NO, CO, and hydrocarbons

The Superior Performance of Sol–Gel Made Ce–O–P Catalyst for Selective Catalytic Reduction of NO with NH₃

In this paper, a sol–gel made Ce–O–P catalyst (referred to as Ce–O–P-SG) was employed for selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃, which was directly compared with two other Ce–O–P samples as synthesized via hydrothermal and coprecipitation routes (referred to as Ce–O–P-HT and Ce–O–P-CP, respectively). Experimental results revealed that the Ce–O–P-SG catalyst yielded a more than 90% NO conversion at 200 °C in the presence of 10 vol % H₂O, whereas Ce–O–P-HT and Ce–O–P-CP catalysts only showed 50% and 20% NO conversions under the same conditions, respectively. After subjected to a series of characterization technologies (e.g., XRD, BET-BJH, XPS, NH₃-TPD, py-IR, and H₂-TPR), it was found that more enriched surface Ce­(4+) species were formed except for the two main CePO₄ phases (monazite and rhabdophane phases) of the Ce–O–P-SG catalyst compared to the other two samples, resulting in the increase of surficial active oxygen ions content. This could lead to an enhancement in surface acidity and redox capacity of the Ce–O–P-SG catalyst, effectively promoting the NH₃–SCR activity of the catalyst. Further analyses on SO₂ and H₂O tolerance revealed that the Ce–O–P-SG possessed a higher sulfur resistance than the other two samples, which could be attributed to the SO₂ trapping effect by the abundant active oxygen species over Ce–O–P-SG catalyst

CePO₄ Catalyst for Elemental Mercury Removal in Simulated Coal-Fired Flue Gas

CePO₄ catalyst (termed Ce–P–O) was for the first time employed to capture elemental mercury (Hg⁰) under simulated coal-fired flue gas conditions. As compared with commercial SCR catalyst (i.e., V–W–Ti), the Ce–P–O catalyst showed a much better performance in Hg⁰ removal. The high Hg⁰ adsorption capacity, abundant active oxygen species, and excellent SO₂ poisoning resistance account for the performance of the Ce–P–O catalyst. When the catalyst was subjected to individual flue gas component conditions, it was found that the presence of NO can significantly improve the Hg⁰ removal efficiency over the Ce–P–O catalyst; however, HCl did not show promotion effect. It is proposed that the former occurs because the generated NO₂ (originated from NO oxidation) could react with Hg⁰ ad-species (e.g., Hg₂O), regenerating the HgO and hence enhancing the Hg⁰ chemisorption. The latter was found to be due to the absence of the Deacon reaction over the catalyst

Cl Species Transformation on CeO₂(111) Surface and Its Effects on CVOCs Catalytic Abatement: A First-Principles Investigation

Cl species transformation and deactivation effects on ceria (111) model catalysts were investigated in the first-principles framework. Conventionally, the strong adsorption of Cl atom in the oxygen vacancy of ceria was believed to be the dominant deactivation factor. However, under the typical conditions of chlorinated volatile organic compounds (CVOCs) catalytic combustion, the deactivation was found to be hindered because of the high O₂/Cl ratio in the reactants’ feed. Then, the possible formation pathways of Cl₂ and HCl during CVOCs catalytic abatement reaction were proposed. It was identified that the H-bond interaction between surface hydroxyls and Cl species was the key factor to control the selectivity in the final product of Cl species (HCl or Cl₂). By introduction of H₂O or other H resources, the coverage of surface OH radicals could be increased, which in turn benefits the conversion to HCl over Cl₂. However, the competitive adsorption between H₂O and oxygen on vacancy would lead to somewhat of a loss of low-temperature catalytic activity

Effective Way to Control the Performance of a Ceria-Based DeNO_<i>x</i> Catalyst with Improved Alkali Resistance: Acid–Base Adjusting

Compared with V₂O₅–WO₃/TiO₂, the ceria catalyst supported on sulfated zirconia (referred to as CeSZ) shows a superior alkali resistance for selective catalytic reduction of NO in flue gases. It reveals an unexpected result that a moderate amount of potassium (normally considered as SCR poisons) could even enhance the activity of CeSZ catalyst. To investigate this exceptional phenomenon, we studied the surface acid–base properties of CeSZ catalysts with different amounts of K and their influences on SCR performances. Although K resulted in a sharp decrease in Brønsted acid sites, the total acidity, especially strong acidity, barely changed when K/Ce was less than 0.4. It was proposed that a small amount of potassium could initially alter some Brønsted acid sites to Lewis ones, therefore retaining the majority of total acidity. Moreover, increased surface basicity due to K depositing led to an enhancement in NO chemisorption and oxidation, which is beneficial to the SCR process via the reaction of NO₂ and NO_<i>x</i> ad-species with adsorbed NH₃ species. This explains why the SCR catalytic activity was improved at lower temperature for CeSZ catalysts after K depositing. Therefore, the catalytic activity and reaction temperature window of CeSZ catalyst could be controlled by simply tuning the surface acid/base sites, which may give some inspiration to improve the catalytic activity and poisoning tolerance

Mercury Re-Emission in Flue Gas Multipollutants Simultaneous Absorption System

Recently, simultaneous removal of SO₂, NO_<i>x</i> and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NO_<i>x</i> absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg⁰-emission through the decomposition of HgSO₃NO₂^–, but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg­(NO₂)_<i>x</i>^2‑<i>x</i>]. In addition, the subsequent addition of Cl^– could further suppress the Hg⁰ emission, where the formation of a stable Hg-SO₃–NO₂–Cl complex was assumed to be the main reason for such strong inhibition effect