CeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZSM-5 sorbents for high-temperature H<Subscript>2</Subscript>S removal

Performance of CeO₂-La₂O₃/ZSM-5 sorbents for sulfur removal was examined at temperature ranging from 500 oC to 700 oC. The sulfur capacity of 5Ce5La/ZSM-5 was much bigger than that of CeO₂/ZSM-5. H₂ had a negative impact on the sulfidation; however, CO had little influence on sulfur removal. The characterization results showed that CeO₂ and La₂O₃ were well dispersed on ZSM-5 because of the intimate admixing of La₂O₃ and CeO₂, the major sulfidation products were Ce₂O₂S and La₂O₂S, the XRD and SEM results revealed that ZSM-5 structure could remain intact during preparation and sulfidation process, the H₂-TPR showed that the reducibility of CeO₂ can be remarkably enhanced by addition of La.     

Effects of preparation methods for V<Subscript>2</Subscript>O<Subscript>5</Subscript>-TiO<Subscript>2</Subscript> aerogel catalysts on the selective catalytic reduction of NO with NH<Subscript>3</Subscript>

A series of V₂O₅-TiO₂ aerogel catalysts were prepared by the sol-gel method with subsequent supercritical drying with CO₂. The main variables in the sol-gel method were the amounts of V₂O₅ and when the vanadium precursor was introduced. V₂O₅-TiO₂ xerogel and V₂O₅/TiO₂ (P-25) were also prepared for comparison. The V₂O₅-TiO₂ aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅-TiO₂ xerogel and impregnated V₂O₅/TiO₂ (P-25) catalysts. The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), FT-Raman spectroscopy, temperature-programmed reduction with H₂ (H₂-TPR), and temperature-programmed desorption of ammonia (NH₃-TPD). The selective catalytic reduction of NOx with ammonia in the presence of excess O₂ was studied over these catalysts. Among various V₂O₅-TiO₂ catalysts, V₂O₅ supported on aerogel TiO₂ showed a wide temperature window exhibiting high NOx conversions. This superior catalytic activity is closely related to the large amounts of strong acidic sites as well as the surface vanadium species with characteristics such as easy reducibility and monomeric and polymeric vanadia surface species. This work was presented at the 7 ^th Korea-China Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Effect of CeO<Subscript>2</Subscript>-addition sequence on the performance of CeO<Subscript>2</Subscript>-modified Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in autothermal reforming of iso-octane

Two types of CeO₂-modified Ni/Al₂O₃ catalysts were prepared by a consecutive impregnation method with different sequences in the impregnation of Ni and CeO₂, and their performance in autothermal reforming (ATR) of isooctane was investigated. Catalysts prepared by adding CeO₂ prior to the addition of Ni, Ni/CeO₂-Al₂O₃, produced larger amounts of hydrogen than those obtained using catalysts prepared by adding the two components in an opposite sequence, Ni-CeO₂/Al₂O₃. The results of H₂ chemisorption and temperature-programmed reduction revealed that added CeO₂ increased the dispersion of the Ni species on Al₂O₃ and suppressed the formation of NiAl₂O₄ in the catalyst such that large amounts of Ni species were present as NiO, the active species for the ATR. The elemental and thermogravimetric analyses of deactivated catalysts indicated that Ni/CeO₂-Al₂O₃, which showed a longer lifetime than Ni-CeO₂/Al₂O₃, contained lesser amounts and different types of coke on the surface.     

Reduction of CO<Subscript>2</Subscript> to CO via reverse water-gas shift reaction over CeO<Subscript>2</Subscript> catalyst

CeO₂ catalysts with different structure were prepared by hard-template (Ce-HT), complex (Ce-CA), and precipitation methods (Ce-PC), and their performance in CO₂ reverse water gas shift (RWGS) reaction was investigated. The catalysts were characterized using XRD, TEM, BET, H₂-TPR, and in-situ XPS. The results indicated that the structure of CeO₂ catalysts was significantly affected by the preparation method. The porous structure and large specific surface area enhanced the catalytic activity of the studied CeO₂ catalysts. Oxygen vacancies as active sites were formed in the CeO₂ catalysts by H₂ reduction at 400 °C. The Ce-HT, Ce-CA, and Ce-PC catalysts have a 100% CO selectivity and CO₂ conversion at 580 °C was 15.9%, 9.3%, and 12.7%, respectively. The highest CO₂ RWGS reaction catalytic activity for the Ce-HT catalyst was related to the porous structure, large specific surface area (144.9 m²?g^?1) and formed abundant oxygen vacancies.     

Selective catalytic oxidation of H<Subscript>2</Subscript>S using nonhydrolytic vanadia-titania xerogels

A series of vanadia-titania (V-Ti) xerogel catalysts were prepared by nonhydrolytic sol-gel method. These catalysts showed much higher surface area and total pore volumes than the conventional V₂O₅-TiO₂ xerogel. Two species of surface vanadium in the xerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Xerogel catalysts from the nonhydrolytic method showed very high conversion of H₂S without harmful emission of SO₂. The conversion of H₂S increased with increasing vanadia loading up to 10V-Ti; however, it decreased at higher vanadia loading (12V-Ti and 18V-Ti) probably due to the formation of crystalline V₂O₅.     

Reduced graphene oxide supported V<Subscript>2</Subscript>O<Subscript>5</Subscript>-WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> catalysts for selective catalytic reduction of NOx

We present a reduced-graphene-oxide (rGO)-supported V2O₅-WO₃-TiO₂ (VWTi) catalysts for the efficient selective catalytic reduction of NOx. The rGO support provides well-dispersed functional sites for the nucleation of nanoparticles, allowing the formation of VWTi catalysts with high specific surface areas. The dispersion of the nanoparticles, as observed by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), confirmed the uniform dispersion of the particles on the rGO surface. Detailed Fourier-transform infrared (FT-IR) and NH₃ temperature-programmed desorption (NH₃-TPD) analyses indicated that the high density of acidic sites provided by the rGO is key to the observed enhancement of NOx removal efficiency, and the rGO-supported catalysts exhibit improved NOx removal efficiencies with smaller amounts of V₂O₅ and WO₃ compared with the commercially available V₂O₅-WO₃-TiO₂ catalysts.     

The Effect of Sulfur on ZrO<Subscript>2</Subscript>-Based Biomass Gasification Gas Clean-Up Catalysts

The effect of sulfur on biomass gasification gas clean-up over ZrO₂, Y₂O₃–ZrO₂ and SiO₂–ZrO₂ catalysts was examined. Experiments were carried out at the temperature range of 600–900 °C with sulfur free and 100 ppm H₂S containing simulated gasification gas feeds. A mixture of toluene and naphthalene was used as a tar model compound. Results revealed that the sulfur addition affected positively on the catalyst properties mainly at 600 and 700 °C: over Y₂O₃–ZrO₂ and ZrO₂ sulfur addition improved naphthalene and ammonia conversion. However, over SiO₂–ZrO₂ no clear effect with H₂S addition was observed. The effect of sulfur addition on the catalyst properties was connected to the formation of SO₂ from H₂S when oxygen was available. The intensity of the sulfur effect increased with the Lewis basicity strength of the catalysts. This indicates that the sulfur adsorption has a role in generating new type of active sites and/or plays role in changing the redox properties of the zirconia. Since the biomass gasification gas contains usually significant amounts of H₂S the sulfur tolerance of the zirconia based catalysts is a remarkable benefit.     

CO and CO<Subscript>2</Subscript> Methanation Over Ni/SiC and Ni/SiO<Subscript>2</Subscript> Catalysts

Ni/SiC and Ni/SiO₂ catalysts prepared by both wet impregnation (WI) and deposition–precipitation (DP) methods were compared for CO and CO₂ methanation. The prepared catalysts were characterized using N₂ physisorption, temperature-programmed reduction with H₂ (H₂-TPR), H₂ chemisorption, pulsed CO₂ chemisorption, temperature-programmed desorption of CO₂ (CO₂-TPD), transmission electron microscopy, and X-ray diffraction. H₂-TPR analysis revealed that the catalysts prepared by DP exhibit stronger interaction between the nickel oxides and support than those prepared by WI. The former catalysts exhibit higher Ni dispersions than the latter. The catalytic activities for both reactions over Ni/SiC and Ni/SiO₂ catalysts prepared by WI increase on increasing the Ni content from 10 to 20 wt%. The Ni/SiC catalyst prepared by DP shows higher catalytic activity for CO and CO₂ methanation than that of the Ni/SiC catalyst prepared by WI. Furthermore, it exhibits the highest catalytic activity for CO methanation among the tested catalysts. The high Ni dispersion achieved by the DP method and the high thermal conductivity enabled by SiC are beneficial for both CO and CO₂ methanation.     

Catalytic combustion of volatile aromatic compounds over CuO-CeO<Subscript>2</Subscript> catalyst

Ce_1?x Cu _x O₂ oxide solid solution catalysts with different Ce/Cu mole ratios were synthesized by the one-pot complex method. The prepared Ce_1?x Cu _x O₂ catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H₂ temperature-programmed reduction (H₂-TPR). Their catalytic properties were also investigated by catalytic combustion of phenyl volatile organic compounds (PVOCs: benzene, toluene, xylene, and ethylbenzene) in air. XRD analysis confirmed that the CuO species can fully dissolve into the CeO₂ lattice to form CeCu oxide solid solutions. XPS and H²-TPR results indicated that the prepared Ce_1?x Cu _x O₂ catalysts contain abundant reactive oxygen species and superior reducibility. Furthermore, the physicochemical properties of the prepared Ce_1?x Cu _x O₂ catalysts are affected by the Ce/Cu mole ratio. The CeCu₃ catalyst with Ce/Cu mole ratio of 3.0 contains abundant reactive oxygen species and exhibits superior catalytic combustion activity of PVOCs. Moreover, the ignitability of PVOCs is also affected by the respective physicochemical properties. The catalytic combustion conversions of ethylbenzene, xylene, toluene, and benzene are 99%, 98.9%, 94.3%, and 62.8% at 205, 220, 225, and 225 °C, respectively.     

Low-Temperature Complete Oxidation of Ethyl Acetate Over CeO<Subscript>2</Subscript>-Supported Precious Metal Catalysts

Catalytic combustion of ethyl acetate was investigated over various CeO₂-supported precious metal catalysts prepared by impregnation method, and the effect of reduction treatment on the activity was examined. Among the catalysts tested, Ru/CeO₂ achieved the highest activity for ethyl acetate combustion, and the activity was almost unchanged by the heat treatment in a hydrogen atmosphere. In the cases of Pt/CeO₂, Pd/CeO₂, and Rh/CeO₂, the catalytic activity was enhanced by the reduction treatment at 400 °C, though the activity of the reduced catalysts was still inferior to that of Ru/CeO₂. It was confirmed by temperature-programmed reduction that the reduction of the ruthenium species was initiated at the lowest temperature among the CeO₂-supported precious metals. The precious metal species reducible at lower temperatures should be responsible for the high activity in the complete oxidation of ethyl acetate.     

Combined Steam and Carbon Dioxide Reforming of Methane on Ni/MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: Effect of CeO<Subscript>2</Subscript> Promoter to Catalytic Performance

Abstract  

The catalytic performance during combined steam and carbon dioxide reforming of methane (SCR) was investigated on Ni/MgAl₂O₄ catalyst promoted with CeO₂. The SCR catalyst was prepared by co-impregnation method using nickel and cerium metal precursors on hydrotalcite-like MgAl₂O₄ support. In terms of catalytic activity and stability, CeO₂-promoted Ni/MgAl₂O₄ catalyst is superior to Ni–CeO₂/Al₂O₃ or Ni/MgAl₂O₄ catalysts because of high resistance to coke formation and suppressed aggregation of nickel particles. The role of CeO₂ on Ni/MgAl₂O₄ catalyst was elucidated by carrying out the various characterization methods in the viewpoint of the aggregation of nickel particles and metal-support interactions. The observed superior catalytic performance on CeO₂-promoted Ni/MgAl₂O₄ catalyst at the weight ratio of MgO/Al₂O₃ of 3/7 seems to be closely related to high dispersion and low aggregation of active metals due to their strong interaction with the MgAl₂O₄ support and the adjacent contact of Ni and CeO₂ species. The CeO₂ promoter also plays an important role to suppress particle aggregation by forming an appropriate interaction of NiO–CeO₂ as well as Ni–MgAl₂O₄ with the concomitant enhancement of mobile oxygen content.     

Improving the SO<Subscript>2</Subscript> absorption rate of CeFeMg-based sorbent promoted with titanium promoter

To improve the poor SO₂ absorption rate of CeFeMgTi sorbent with high sulfur removal capacity and fast regeneration, a new sorbent, CeFeMgTi-sol was prepared by the modified co-precipitation method and tested in a packed bed reactor at RFCC conditions (sulfation of MgO to MgSO₄ in the presence of low concentration of SO₂ at 973 K, regeneration of MgSO₄ to MgO and H₂S in the presence of H₂ at 803 K). The CeFeMgTi-sol sorbent showed excellent SO₂ absorption and sulfur removal capacity (46.2 sulfur g/g absorbent×100). It was found that the SO₂ absorption rates were related to the structure of the Mg and Ti and the textural properties such as surface area and pore volume. In the case of the fresh state of CeFeMgTi sorbent, CeO₂, MgO and MgTiO₃ structures were observed. But the new CeFeMgTisol sorbent before SO₂ absorption, showed a separated MgO and TiO₂ peak only. These differences in the sorption rates were discussed by the difference in the XRD pattern, surface area and pore volume.     

Catalytic Steam Reforming of Propane over Ni/LaAlO<Subscript>3</Subscript> Catalysts: Influence of Preparation Methods and OSC on Activity and Stability

Abstract  

To develop an efficient catalyst for steam reforming of propane, Ni/LaAlO₃ catalysts were prepared by deposition precipitation, impregnation, and solvo-thermal methods, and characterized by XRD, BET, H₂-TPR, elemental analyses, and TEM. Ni/Al₂O₃ and Ni/CeO₂ catalysts were also synthesized by the solvo-thermal method for comparison. The Ni/LaAlO₃ catalysts exhibited better catalytic performance than both Ni/Al₂O₃ and Ni/CeO₂ catalysts, and activities with Ni/LaAlO₃ were found to be dependent upon the preparation methods. In particular, the Ni/LaAlO₃ catalyst synthesized by the solvo-thermal method exhibited the highest activity presumably because tetrahydrofuran helps distribute generated Ni nanoparticles onto the catalyst surface in a uniform fashion. In addition, the solvo-thermally prepared Ni/LaAlO₃ catalyst was found to be highly stable, with its activity being maintained at least during 100 h. The observed high stability is attributed to the excellent oxygen storage capacity of LaAlO₃, which was first determined by thermogravimetric methods as well as by soot oxidations in the presence of Al₂O₃, CeO₂, and LaAlO₃. Compared to the Ni/Al₂O₃ and Ni/CeO₂ catalysts, Ni/LaAlO₃ exhibited suppressed carbon formation even at lower S/C ratios due to the superior oxygen transport ability of the LaAlO₃ support.     

Experimental study on ZnO-TiO<Subscript>2</Subscript> sorbents for the removal of elemental mercury

ZnO-TiO₂ sorbents synthesized by an impregnation method were characterized through XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and EDS (Energy dispersive spectrometer) analyses. An experiment concerning the adsorption of Hg⁰ by ZnO-TiO₂ under a simulated fuel gas atmosphere was then conducted in a bench-scale fixed-bed reactor. The effects of ZnO loading amounts and reaction temperatures on Hg⁰ removal performance were analyzed. The results showed that ZnO-TiO₂ sorbents exhibited excellent Hg⁰ removal capacity in the presence of H₂S at 150 °C and 200 °C; 95.2% and 91.2% of Hg⁰ was removed, respectively, under the experimental conditions. There are two possible causes for the H₂S reacting on the surface of ZnO-TiO₂: (1) H₂S directly reacted with ZnO to form ZnS, (2) H₂S was oxidized to elemental sulfur (S _ad ) by means of active oxygen on the sorbent surface, and then S _ad provided active absorption sites for Hg⁰ to form HgS. This study identifies three reasons why higher temperatures limit mercury removal. First, the reaction between Hg⁰ and H₂S is inhibited at high temperatures. Second, HgS, as the resulting product in the reaction of mercury removal, becomes unstable at high temperatures. Third, the desulfurization reaction strengthens at higher temperatures, and it is likely that H₂S directly reacts with ZnO, thus decreasing the S _ad on the sorbent surfaces.     

Renewable hydrogen production by steam reforming of glycerol over Ni/CeO<Subscript>2</Subscript> catalyst prepared by precipitation deposition method

Catalytic steam reforming of glycerol for renewable hydrogen generation has been investigated over Ni/CeO₂ catalyst prepared by precipitation-deposition method. The fresh and used catalysts were characterized by surface area and pore size analysis, X-ray diffraction patterns and scanning electron micrographs. Reforming experiments were carried out in a fixed bed tubular reactor at different temperatures (400–700 °C), glycerol concentrations (5–15 wt%) and contact times. (W/F _Ao =2−80 g-cat·h/mol of glycerol). The investigation revealed that the Ni/CeO₂ catalyst prepared by the above method is effective to produce high yield of hydrogen up to 5.6 (moles of H₂/moles of glycerol fed). The formation of methane and carbon monoxide was greatly reduced over this catalyst. Significantly low amount of coke deposition was observed on the CeO₂ supported catalyst. From the kinetic analysis, the activation energy for the steam reforming of glycerol was found to be 36.5 kJ/mol.     

Effect of calcination atmospheres on the catalytic performance of nano-CeO<Subscript>2</Subscript> in direct synthesis of DMC from methanol and CO<Subscript>2</Subscript>

Nano-CeO₂ was prepared through the calcination of Ce(OH)₃ precursor in different atmospheres (H₂, Ar, air, O₂), which was prepared by a hydrothermal method, and then used as catalysts in the direct synthesis of dimethyl carbonate (DMC) from methanol and CO₂. The results indicated that the catalyst calcined in O₂ (CeO₂-O₂) showed an optimum catalytic performance, and the yield of DMC reached to 1.304 mmol/mmol_cat. In addition, reaction temperature and weight of catalyst were optimized. Based on characterizations of the catalysts, the ratio of Ce(IV)/Ce(III) and Lewis acid-base property of nano-CeO₂ catalyst could be adjusted through different calcination atmosphere treatment. It was determined that the higher activity of CeO₂-O₂ catalyst is mainly attributed to its higher ratio of Ce(IV)/Ce(III) as well as abundant and moderate intensity Lewis acid base sites.     

Phase cooperation of V<Subscript>2</Subscript>O<Subscript>5</Subscript> and Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> in the selective oxidation of H<Subscript>2</Subscript>S containing ammonia and water

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium oxide-based catalysts. The investigation was focused on the role of V₂O₅, and phase cooperation between V₂O₅ and Bi₂O₃ in this reaction. The conversion of H₂S continued to decrease since V₂O₅ was gradually reduced by treatment with H₂S. The activity of V₂O₅ was recovered by contact with oxygen. A strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V₂O₅ and Bi₂O₃. Temperature-programmed reduction (TPR) and oxidation (TPO) and two bed reaction tests were performed to explain this synergistic effect by the reoxidation ability of Bi₂O₃. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>-promoted MgO sorbents for CO<Subscript>2</Subscript> capture at moderate temperatures

A series of Al₂O₃ and CeO₂ modified MgO sorbents was prepared and studied for CO₂ sorption at moderate temperatures. The CO₂ sorption capacity of MgO was enhanced with the addition of either Al₂O₃ or CeO₂. Over Al₂O₃-MgO sorbents, the best capacity of 24.6 mg- CO₂/g-sorbent was attained at 100 °C, which was 61% higher than that of MgO (15.3 mg-CO₂/g-sorbent). The highest capacity of 35.3 mg-CO₂/g-sorbent was obtained over the CeO₂-MgO sorbents at the optimal temperature of 200 °C. Combining with the characterization results, we conclude that the promotion effect on CO₂ sorption with the addition of Al₂O₃ and CeO₂ can be attributed to the increased surface area with reduced MgO crystallite size. Moreover, the addition of CeO₂ increased the basicity of MgO phase, resulting in more increase in the CO₂ capacity than Al₂O₃ promoter. Both the Al₂O₃-MgO and CeO₂-MgO sorbents exhibited better cyclic stability than MgO over the course of fifteen CO₂ sorption-desorption cycles. Compared to Al₂O₃, CeO₂ is more effective for promoting the CO₂ capacity of MgO. To enhance the CO₂ capacity of MgO sorbent, increasing the basicity is more effective than the increase in the surface area.

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Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on ZrO<Subscript>2</Subscript> supported Ni catalysts for autothermal reforming of CH<Subscript>4</Subscript>

The effect of La₂O₃ content in Ni-La-Zr catalyst was investigated for the autothermal reforming (ATR) of CH₄. The catalysts were prepared by the coprecipitation method and had a mesoporous structure. Temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) indicated that a strong interaction developed between Ni species and the support with the addition of La₂O₃. Thermogravimetric analysis (TGA) and H₂-pulse chemisorption showed that the addition of La₂O₃ led to well dispersed NiO molecules on the support. Ni-La-Zr catalysts gave much higher CH₄ conversion than Ni-Zr catalyst. The Ni-La-Zr containing 3.2 wt% La₂O₃ showed the highest activity. The optimum conditions for maximal CH₄ conversion and H₂ yield were H₂O/CH₄=1.00, O₂/CH₄=0.75. Under these conditions, CH₄ conversion of 83% was achieved at 700 °C. In excess O₂ (O₂/CH₄>0.88), the catalytic activity was decreased due to sintering of the catalyst.     

Nanoclays as nano adsorbent for oxidation of H<Subscript>2</Subscript>S into elemental sulfur

Modified bentonites were used for the oxidation of H₂S into elemental sulfur. Active phases such as iron and cobalt sulfide were added to supports Cloisite 30B and 15A. The produced nano adsorbents were characterized by X-Ray diffraction, ICP, BET surface area and SEM. Selective oxidation of H₂S was carried out over the nano adsorbent in the experimental setup. The tests were performed at 70 and 180 °C, under atmospheric pressure and in the presence of 5,000 ppm of H₂S in the inlet gas stream. The results confirmed the increase in the distribution of active metals and activity of Cloisite 30B, in comparison with Cloisite 15A. Cobalt-containing support showed significant improvement in the capacity of H₂S removal, and in the outlet stream less than 50 ppm of H₂S was detected.