Xiaole Weng

Deactivation mechanism of Ce/TiO2 selective catalytic reduction catalysts by the loading of sodium and calcium salts

In this paper, the poisoning effect of alkali and alkaline earth metal on Ce/TiO2 catalysts was investigated for the first time and a deactivation mechanism was proposed. The Ce/TiO2 catalyst was observed to be deactivated seriously by the loading of Na+, K+ or Ca2+ ions. When the Na/Ce, K/Ce or Ca/Ce molar ratio exceeded 0.25, NO conversion of the Ce/TiO2 catalyst at 380 °C decreased from 78% to negligibly low. After subjecting it to a range of analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectra etc., it was found that amorphous ceria was dominant in the Ce/TiO2 sample, and would change to ceria crystals with the loading of Na+ or Ca2+ ions, leading to a poor dispersion of ceria on the surface of TiO2 as well as the transformation of Ce3+ to Ce4+. Both of these led directly to the disappearance of oxygen vacancies in the ceria particles, slowing down the reduction rate of ceria and hence decreasing its rate of oxidation/reduction recycle. We proposed that the enlargement of ceria nanoparticles, the reduced Ce4+/Ce3+ redox cycle rate and the change in the surface acidity could be the three major factors contributing to the decline of selective catalytic reduction (SCR) activity of the Ce/TiO2 catalyst after loading with Na+ or Ca2+ ions.

High-throughput continuous hydrothermal synthesis of an entire nanoceramic phase diagram.

A novel High-Throughput Continuous Hydrothermal (HiTCH) flow synthesis reactor was used to make directly and rapidly a 66-sample nanoparticle library (entire phase diagram) of nanocrystalline Ce(x)Zr(y)Y(z)O(2-delta) in less than 12 h. High resolution PXRD data were obtained for the entire heat-treated library (at 1000 degrees C/1 h) in less than a day using the new robotic beamline I11, located at Diamond Light Source (DLS). This allowed Rietveld-quality powder X-ray diffraction (PXRD) data collection of the entire 66-sample library in <1 day. Consequently, the authors rapidly mapped out phase behavior and sintering behaviors for the entire library. Out of the entire 66-sample heat-treated library, the PXRD data suggests that 43 possess the fluorite structure, of which 30 (out of 36) are ternary compositions. The speed, quantity and quality of data obtained by our new approach, offers an exciting new development which will allow structure-property relationships to be accessed for nanoceramics in much shorter time periods.

Polyethyleneimine functionalized protonated titanate nanotubes as superior carbon dioxide adsorbents

In this paper, protonated titanate nanotubes (PTNTs) were modified with polyethyleneimine (PEI) by wet impregnation method for CO(2) adsorption. Their micro-morphology and structural properties were characterized by a range of analytical techniques, including XRD, TEM, SEM, N(2) adsorption etc. Experimental results revealed that the functionalized PTNTs with 50 wt.% PEI loaded exhibited a high CO(2) adsorption capacity of 130.8 mg/g-sorbent at 100°C. Only a minor loss of its capacity was observed after five consecutive adsorption-desorption runs. The PEI was existed both in the internal and external mesoporous pores of PTNTs via chemical combination between amino group and enriched protons, which accounted for their good thermal stability at elevated temperatures. The results present herein imply that the PEI modified PTNTs could be appealing materials for capturing CO(2) from power plant flue gas.

Ceria supported on sulfated zirconia as a superacid catalyst for selective catalytic reduction of NO with NH3.

In this paper, ceria supported on sulfated zirconia (CeSZ) as a superacid catalyst was synthesized and the resulted performances for selective catalytic reduction (SCR) of NO with NH(3) were investigated. Experimental results revealed that the sulfation of zirconia supports could greatly improve the SCR activity of the catalysts. Among the tested samples, the CeSZ catalyst with Ce/Zr mole ratio at 0.095 possessed the highest NO conversion (i.e., 98.6% at ca. 420 °C and 180,000 h(-1)). The sulfation had led to a formation of pure tetragonal phase of ZrO(2), a well dispersion of CeO(2), abundant stable superacid sites, increasing surface area and enrichment of Ce(3+) on the surface, all of which were responsible for its excellent performance in SCR of NO with NH(3).

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

Mechanisms and reaction pathways for simultaneous oxidation of NOx and SO2 by ozone determined by in situ IR measurements

Ozone (O3) oxidation combined with wet scrubbing is a promising method for the simultaneous removal of SO2 and NOx in flue gas. In this study, the O3 oxidation processes of NO and SO2, as well as their coexistence, were investigated using an in situ IR spectrometer. Experimental results showed that the O3 concentration and the reaction temperature played critical roles in the O3 oxidation process of NO. Around 80°C, when inlet molar ratio of O3/NO was less than 1, NO was mainly oxidized to NO2, while when the ratio was greater than 1, NO would be further oxidized to NO3, N2O5, and HNO3. NO3 was the key intermediate product for the formation of N2O5 and HNO3. However, the subsequent reactions of NO3 were temperature dependence. With the increase of reaction temperature above 100°C, the concentration of NO2 increased whereas the concentrations of N2O5 and HNO3 decreased. The oxidation of SO2 by O3 was negligible and SO2 had little influence on the oxidation of NO in the simultaneous oxidation of NO and SO2. Finally, based on the in situ IR results, the oxidation mechanism is discussed and the reaction pathways are proposed.

Catalyst performance and mechanism of catalytic combustion of dichloromethane (CH2Cl2) over Ce doped TiO2.

TiO2 and Ce/TiO2 were synthesized and subsequently used for the catalytic combustion of DCM. TiO2 had abundant Lewis acid sites and was responsible for the adsorption and the rupture of C-Cl bonds. However, TiO2 tended to be inactivated because of chloride poisoning due to the adsorption and accumulation of Cl species over the surface. While, Ce/TiO2 obtained total oxidation of CH2Cl2 at 335°C and exhibited stable DCM removal activity on 100h long-time stability tests at 330°C without any catalyst deactivation. The doped cerium generated Ce(3+) chemical states and surface active oxygen, and therefore played important roles from two aspects as follows. First of all, the poisoning of Cl for Ce/TiO2 was inhibited to some extent by CeO2 due to the rapid removal of Cl on the surface of CeO2, which has been verified by NH3-IR characterization. In the other hand, CeO2 enhanced the further deep oxidation of C-H from byproducts and retained the certain oxidation of CO to CO2. Based on the DRIFT characterization and the catalysts activity tests, a two-step reaction pathway for the catalytic combustion of DCM on Ce/TiO2 catalyst was proposed.

The effects of surface acidity on CO2 adsorption over amine functionalized protonated titanate nanotubes

In this paper, an infrared study has been performed on CO2 adsorption over polyethylenimine (PEI) functionalized protonated titanate nanotubes (PTNTs) prepared by a simple wet impregnation method. It was found that PTNTs had a large amount of acidity sites, which had strong interaction with the amino groups of the loaded PEI. This interaction could lead to good dispersion of PEI molecules, resulting in good performance in adsorption capacity, amine efficiency and adsorption kinetics. Both weakly and strongly adsorbed CO2 species formed under CO2 exposure, which were attributed to hydrogen-bonded species and carbamate groups, respectively. The existence of strong surface acidity increased the proportion of weakly adsorbed CO2 on the protonated amine species, which was beneficial to the adsorption–desorption cyclic performance. Furthermore, it was confirmed that the interaction between the support surface and amine also resulted in the superior thermal stability of amine-modified sorbents. Overall, our results suggested that suitably tuning surface acidity of the support could effectively facilitate CO2 adsorption.

Novel SCR catalyst with superior alkaline resistance performance: enhanced self-protection originated from modifying protonated titanate nanotubes

In this paper, ion-exchangeable titanate nanotubes were synthesized with a treatment of ethanol washing (referred to as TNTs-eth), which were utilized as supports for ceria (CeO2) catalysts. The resulting catalysts showed excellent resistance to alkali metal and alkaline earth metal poisoning in NH3-SCR applications, where the NO conversions could be maintained at 97%, 88% and 95% (at 350 °C) when Na+, K+ and Ca2+ were added, respectively. After the catalysts were subjected to a range of analyses, it was found that the resistance was mainly attributed to the significant increase in structural ion-exchangeable OH groups in the TNTs-eth catalyst, which led to a remarkable surface acid strength and abundant acid content, effectively neutralizing the basicity induced by the alkali metals and alkaline earth metals and retaining the SCR performance. Moreover, the repeatable Ce4+/Ce3+ redox cycles and the well-maintained nano-tubular structure also accounted for the excellent resistance performance of the Ce/TNTs-eth catalyst. We expect that the self-protection effect of the Ce/TNTs-eth catalyst (that was induced by its ion exchange ability) might provide a novel approach for the design and synthesis of SCR catalysts to relieve their alkali metal and alkaline earth metal poisoning.

Mercury Re-Emission in Flue Gas Multipollutants Simultaneous Absorption System

Recently, simultaneous removal of SO2, NOx 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 NOx 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 Hg(0)-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)x(2-x)]. In addition, the subsequent addition of Cl(-) could further suppress the Hg(0) emission, where the formation of a stable Hg-SO3-NO2-Cl complex was assumed to be the main reason for such strong inhibition effect.