Decrease In Surface Acidity Research Articles

Microscopic impact mechanism of alkali earth metal poisoning and Ce modification on the deNOx over the γ-Fe2O3 (0 0 1) surface

Ce-doped γ-Fe2O3 catalyst has excellent deNOx and poisoning resistance. However, the NH3-SCR mechanism of improving of Ca poisoning by Ce at atomic level is still unclear. Therefore, the effects of alkali earth metal Ca poisoning on the γ-Fe2O3 (001) surface and Ce modified Ca-γ-Fe2O3 (001) surface in the selective catalytic reduction (SCR) of NOx by NH3 were studied. γ-Fe2O3 catalyst poisoned by Ca results from the decrease of surface acidity and the improvement of stable nitrite/nitrate species. Ce brings extra Lewis acid sites for the NH3 adsorption when it is used to modify the poisoned catalyst, which improves the NH3 adsorption and activation. The adsorption of NO and O2 as well as the decomposition of O2 may be inhibited by the presence of Ce. Most of the stable, inactive nitrite/nitrate species are active and decomposed. The production of NH2 is aided by the enhancement of the dehydrogenation process of NH3. This study provides a direction for prolonging the service life of low-temperature Fe-based deNOx catalysts in industrial cement furnaces.

Synthesis and Acidity Study of Na-ZSM-5 Zeolite

Zeolites are a special class of structured aluminosilicates with a developed inner surface and a large number of micropores, which allows them to be used as sorbents, membrane materials, and catalysts. Zeolite ZSM-5 is an aluminosilicate formed by parallel channels with a diameter of 5.3–5.6Å interconnected by sinusoidal channels with a diameter of 5.1–5.5Å. At the same time, the presence of aluminum atoms in the zeolite crystal lattice makes it possible to carry out dehydrogenation, esterification, isomerization, and aromatization reactions, and the presence of microchannels significantly increases the number of collisions of reacting compounds, which leads to a significant increase in the rate of chemical transformations in such systems. The synthesis of ZSM-5 zeolite using structure-directing agents has been known since the late 70s of the last centuries, while the high cost of structure-directing agents used and the long duration of hydrothermal synthesis and the need for subsequent post-synthetic procedures lead to a high cost of the resulting zeolite. One of the possible ways to reduce the cost of the synthesis of ZSM-5 zeolite is the direct use of ZSM-5 zeolite as a structure-forming agent, while the synthesis conditions play a decisive role in the structure of the resulting zeolite. The article presents the results of a study of the synthesis of ZSM-5 zeolite and a study of its acidic properties using locally produced reagents. Zeolite was prepared using pre-milled silica gel, sodium hydroxide, and sodium aluminate. The pre-weighed silica gel sample was filled with sodium hydroxide solution and stirred at 70 ℃ to form a colloidal solution, after which the pre-prepared sodium aluminate solution was added, and hydrothermal synthesis was carried out at 180 ℃ for 24 hours. During the synthesis, the Si/Al ratio was varied in the range from 7.5 to 2000, while the resulting acidity ranged from 0.002 to 0.265 mmol(NH3)/g(ZSM-5). It was found that the synthesis of ZSM-5 using pre-prepared reagent solutions is superior to the method using direct dissolution of solid reagents in an autoclave. An increase in the Si/Al ratio from 7.5 to 100 leads to a corresponding increase in surface acidity from 0.066 mmol(NH3)/g(ZSM-5) to 0.265 mmol(NH3)/g(ZSM-5). A further increase in the Si/Al ratio to 2000 leads to a decrease in surface acidity to 0.002–0.005 mmol(NH3)/g(ZSM-5). A reliable and simple method for the synthesis of ZSM-5 zeolite has been developed, providing a zeolite yield of up to 97%.

Lead poisoning and regeneration of Mn-Ce/TiO2 catalysts for NH3-SCR of NOx at low temperature

The effect of lead on the catalytic performance of Mn-Ce/TiO2 catalysts in the selective catalytic reduction (SCR) of NOx with ammonia at low temperature was investigated; with the help of nitrogen sorption, XRD, FT-IR spectroscopy, H2-TPR and NH3-TPD characterization, the causes of lead poisoning and acid regeneration were clarified. The results indicate that the doping of Pb in Mn-Ce/TiO2 leads to a significant decrease of the low-temperature SCR activity; with a Pb loading of 11%, the conversion of NO over Mn-Ce/TiO2 at 180°C decreases from original 100% on the fresh catalyst to 44% on the Pd-poisoned catalyst. The presence of Pb may reduce the content of active Mn4+ and Ce3+ species on the Mn-Ce/TiO2 catalyst, which suppresses the redox cycle of Mn4+ + Ce3+ ↔ Mn3+ + Ce4+; moreover, the decrease of surface acidity on the Mn-Ce/TiO2 catalyst by the doping of Pb is also disadvantageous to the adsorption and activation of NH3. The Pd-poisoned Mn-Ce/TiO2 can be regenerated by nitric acid treatment; after the regeneration, the catalytic activity of Mn-Ce/TiO2 in NH3-SCR of NO is almost completely recovered and even exceeds that of the fresh catalyst at 80–150°C. The nitric acid treatment can restore the redox capacity of Mn-Ce/TiO2, increase the surface area, and create new acid sites, which contribute to recovery of the activity of Pb-poisoned Mn-Ce/TiO2 catalyst in NH3-SCR.

The poisoning effect of KCl and K2O on CeO2-TiO2 catalyst for selective catalytic reduction of NO with NH3

Based on experiments and DFT studies, CeO2-TiO2 catalyst was found to be deactivated by KCl or K2O, and the poisoning effect of K2O was more serious than that of KCl. The characterization results showed that compared with KCl, K2O could lead to a greater decrease in surface acidity, reducibility, the ratio of Ce3+/Ce4+ and the concentration of the chemisorbed oxygen on the surface of CeO2-TiO2 catalyst. The results of in situ DRIFT showed that K2O had a stronger inhibition effect on NH3 adsorption on the catalyst surface than KCl. The introduction of K2O or KCl promoted the adsorption of NO on the catalyst surface, but not all NOx adspecies were reactive in the NH3-SCR reaction. These results were further demonstrated by DFT calculations. The PDOS of different K species doped CeO2-TiO2 catalysts and the adsorption energies of oxygen vacancy, chemisorption oxygen, NH3 and NOx were calculated by MS DMol3. It was found that the introduction of K species weakened the reactivity of the catalyst surface, inhibited the formation of oxygen vacancies and chemisorption oxygen, and reduced the adsorption of NH3 on the catalyst surface, all of which led to the decrease in the catalytic activity.

Yttria-Stabilized Zirconia as a High-Performance Catalyst for Ethanol to n-Butanol Guerbet Coupling.

It has been shown that yttria-stabilized zirconia is an effective catalyst for ethanol to n-butanol Guerbet coupling. The variation of the calcination temperature allows an improvement in the catalytic characteristics of this material via stabilization of the tetragonal phase of zirconia, having higher basicity than the monoclinic one. The treatment of yttria-stabilized zirconia at an optimal calcination temperature of 500 °C induces the increase in surface basicity required for the aldol condensation step, along with a decrease in surface acidity, which is responsible for the side reaction such as ethylene formation. The catalyst obtained significantly exceeds in selectivity and n-butanol yield than individual zirconia and other oxide systems which have been studied in this reaction.

Comparison of MoO3 and WO3 on arsenic poisoning V2O5/TiO2 catalyst: DRIFTS and DFT study

The mechanisms of arsenic poisoning on MoO3- and WO3-doped V2O5/TiO2 catalysts are studied and the poisoning resistance effect of MoO3 are revealed. The arsenic at certain amount (above 10% of arsenic from XPS) decreases the surface area and the number of active sites where reduction takes place. From the results of DRIFTS and DFT, arsenic lowers the surface acidity by decreasing the quantity of Lewis acid sites and the stability of Brønsted acid sites. We also found newly formed As–OH groups of poisoned catalysts (at 1440cm−1) have quite low activity at high temperature.MoO3 provides better activity on poisoned catalysts than WO3 does on V2O5/TiO2 catalysts. This may be due to the good dispersion of MoO3 on the support. The reactivity of surface W–O–W and W=O groups is considerably lower than the corresponding Mo–O–Mo and Mo=O groups from the DFT calculations models. This could be one of the explanations on the decrease of surface acidity of poisoned catalysts.

Ceria promotion on the potassium resistance of MnOx/TiO2 SCR catalysts: An experimental and DFT study

The deactivation of catalysts by potassium and the promotion of cerium on MnOx/TiO2 for selective catalytic reduction (SCR) of NO with NH3 are investigated by both experiments and DFT calculations. Catalyst poisoning by K is due to the decrease of surface acidity, the loss of reducibility and the improvement of stable nitrite/nitrate species. When the catalyst is modified by CeO2, SCR activities of both fresh and poisoned catalysts are improved. Cerium provides new Lewis acid sites for NH3 adsorption, which does not seem to be influenced by K. The Ce 4f orbitals of poisoned surfaces are nearly unaffected, which preserve reducibility of catalyst for NH3 activation. Stable, inactive nitrite species do not build up and the reactive nitrate species are still present on the surface. These advantages can be attributed to the unique active sites, the [–O–Ce–O–Mn–O–]units. These active sites induce the movement of alkali metals to assemble together, leaving more surface active sites for SCR reaction.

Deactivation Mechanism of Potassium on the V2O5/CeO2 Catalysts for SCR Reaction: Acidity, Reducibility and Adsorbed-NOx

A series of V2O5/CeO2 catalysts with different potassium loadings were prepared to investigate alkali deactivations for selective catalytic reduction of NOx with NH3. An alkali poisoning mechanism could be attributed to surface acidity, reducibility, and NOx adsorption/desorption behaviors. The detailed factors are as follows: (1) decrease of surface acidity suppresses NH3 adsorption by strong bonding of alkali to vanadia (major factor); (2) low reducibility prohibits NH3 activation and NO oxidation by formation bonding of alkali to vanadia and ceria (important factor); (3) active NOx(-) species at low temperature diminish because of coverage of alkali on the surfaces (minor factor); and (4) stable, inactive nitrate species at high temperature increase by generating new basic sites (important factor).

The effect of phosphate anions on surface and acidic properties of TiO 2 hydrolyzed from titanium ethoxide

TiO 2 obtained from the hydrolysis of titanium ethoxide was impregnated with different amounts of ammonium hydrogen phosphate, and subjected to heat treatment at different temperatures. Textural and acidic properties were determined. It was found that the presence of increasing amounts of phosphate groups within the TiO 2 reduces surface area and total pore volume. The development of the bidentate state of ligation of the phosphate, starting at 400 °C is associated with enhanced surface acidity. Although, the bidendate phosphate groups persist till 800 °C, a significant decrease in surface acidity is observed at this temperature, believed to result from the elimination of water from the structure. The presence of the phosphate groups delays the anatase to rutile phase transformation. Moreover, bidentate phosphate groups appear to play a role in stabilizing the pore structure of TiO 2.

Measurement of brönsted acidity of silica-alumina solid catalyst by base exchange method

Silica-Alumina catalysts having three different Si/Al ratios were prepared. Surface acidity of these catalyst samples was measured by the base exchange method using aqueous ammonium acetate, CH 3 COONH 4 solution and the results were compared with those measured by the base exchange method using KOH and NH 4 OH as adsorbates from aqueous solution. The amount of K 4 and NH 4 + adsorbed was determined titrimetrically and spectrophotometrically, respectively for measuring the concentration of K + and NH 4 + in the solution before and after adsorption. It was found that the surface acidity was more pronounced when ammonium acetate solution was used as an adsorbate. With the use of NH 4 OH and KOH as bases considerable dealumination takes place with a consequent decrease of surface acidity of the catalyst samples. In all cases, the surface acidity increased with decrease of sample size. It is seen that surface acidity measured with NH 4 OH becomes closer to that measured with KOH at the same(?) Si/Al ratio, but contrary to this, surface acidity measured with NH 4 OH becomes closer to that measured with CH 3 COONH 4 at high Si/Al ratio. This phenomenon is attributed to dealumination of silica-alumina catalyst samples by the strong base.

Effect of Clay Binder on Sorption and Catalytic Properties of Zeolite Pellets

Sorption of N2 ,O 2, Ar, and CH4 in zeolites X, Y, and mordenite in powder as well as pellets was studied using gas chromatography. Heats of sorption in pellets formed using clay as a binder increased by 34-78% for N2 and 7-18% for CH4 relative to the respective zeolite powder. Surface acidity of zeolites upon pelletization as measured by ammonia temperature-programmed desorption showed that high-strength acid sites completely disappear for both zeolite Y and mordenite when bound with bentonite and attapulgite. Decreases in weaker acid sites upon pelletization for mordenite and HY are 47-52% and 34%, respectively. Fourier transform infrared spectroscopy also showed a decrease in acidity upon pelletization of zeolite powders as observed from the reduction in the intensity of the peaks corresponding to acidic hydroxyl groups. Influence of the decrease in surface acidity upon pelletization was also observed from conversion values for alkylation of toluene with methanol in zeolite pellets. For example, reduction in total xylene formed was by 50%, 87%, and 100% for HY520, HY510, and HM510, respectively. These observations have been interpreted in terms of solid-state migration of cations present in clay interlayer space to zeolite extraframework sites during pelletization. Reexchange of cations in zeolites HY and HM with an aqueous solution of Na + /Ca 2+ salts shows sorption isotherms closer to the respective pelletized samples. X-ray diffraction studies of zeolites in powders as well as pellets confirm migration of clay cations to zeolite extraframework sites.

Influence of coke formation on the aromatization of isophorone

The effect of coke formation on the aromatization of isophorone to 3, 5-xylenol is studied on alumina and two carbon-covered aluminas of 12 wt.-% and 18 wt.-% carbon, acting as catalysts in the reaction temperature range 673–808 K. Coke formation, which is amorphous in nature, is found to have a beneficial role in improving the selectivity for aromatization and also in decreasing the decomposition of isophorone. The decomposition decreased with the decrease in surface acidity. Selectivity for 3, 5-xylenol improved with the initial carbon content and is observed to be higher on the 18 wt.-% carbon-covered alumina catalysts.

Alkylations of benzene, alkylbenzenes, and halobenzenes catalyzed by protonated mordenite pretreated with chlorofluorocarbons

The chlorofluorocarbon (CFC) treatment of protonated mordenite (HM) was carried out in a flow reactor under ordinary pressure, usually at 600°C for 10 min using CClF 3 as a treatment agent. The mordenite structure was almost completely retained during the CFC treatment. Evidence for dealumination and surface fluorination was observed by XPS examination. Decrease of surface acidity, as seen by NH 3-TPD, accompanied the treatment. The function of HM as a catalyst for alkylations of benzene and alkylated or halobenzenes with methanol was greatly enhanced by the CFC treatment. In particular, the activity maintenance was remarkably improved. Thus, in the alkylations at 300°C under the molar ratios of CH 3OH/Aromatic compound = 1 and W/ F = 81.2 (for toluene) or 97.5 (for 1,2,4-trimethylbenzene (TMB)) g h mol −1, the conversions at the initial stage of the run were increased from 27 to 65% (for toluene) and from 12 to 47% (for TMB) by CFC treatment of the HM catalyst. In addition, the conversions of benzene and TMB were well maintained at about 60 and 40%, respectively, throughout the running time of 3 h in the reactions catalyzed by the treated HM, while the conversions of benzene and TMB rapidly decreased and became almost zero within 90 min in the reaction catalyzed by untreated HM.

Dehydration of ethanol on filtrol and modified filtrols

The commercially available clay cracking catalyst filtrol (SR grade 63) and its heat treated forms have been characterized by chemical, spectral, X-ray and surface measurement methods. The decrease in pore volume of filtrol during heat treatment is accounted for by the phase transition in the system. The catalytic activity of filtrol and its heat modified forms towards dehydration of ethanol have been studied and the catalytic activity has been correlated to the H + ion concentration. The decrease in the percentage conversion of ethanol for the calcined sample coincides with the decrease in surface acidity and loss of pores during calcination. The activation energy for ethanol dehydration reaction on filtrol has been determined.

The effect of base exchange on the oxidizing properties of silica-alumina catalysts

A quantitative study has been made of the effect of base exchange on the oxidizing (radical-forming) power of a silica-alumina catalyst. A single base-exchange-dehydration cycle causes a drop in radical-forming activity to about one-third and subsequent base-exchange-dehydration cycles result in a further stepwise reduction in oxidizing power. The sodium uptake and the decrease in surface acidity follow a similar stepwise pattern. These results are interpreted in terms of the Danforth formulation of the acid sites on a silica-alumina surface. They provide support for the view that oxidizing activity is associated with Lewis acid sites produced by elimination of the elements of water from hydroxyl groups surrounding aluminum ions in the surface. The role of silica is to facilitate the formation of these sites and to contribute to their stability. A study of the effect of activation temperature on the oxidizing properties of the unexchanged and exchanged catalysts supports this interpretation. An explanation of the results based on the blocking of active oxidizing centers by the hydration sheath surrounding metal cations on adjacent sites is also discussed.