Adsorption and oxidation of NH3 over V2O5/AC surface

V₂O₅/AC has been reported to be active for selective catalytic reduction (SCR) of NO with NH₃ at around 200 °C and resistant to SO₂ deactivation. To elucidate its SCR mechanism, adsorption and oxidation of NH₃ over V₂O₅/AC are studied in this paper using TG, MS and DRIFTS techniques. It is found that the adsorption and oxidation of NH₃ take place mainly at VO bond of V₂O₅. A higher V₂O₅ loading results in more NH₃ adsorption on the catalyst. V₂O₅ contains both Brnsted and Lewis acid sites; NH₄⁺ on Brnsted acid sites is less stable and easier to be oxidized than NH₃ on Lewis acid sites. Gaseous O₂ promotes interaction of NH₃ with AC and oxidation of NH₃ over V₂O₅/AC. NH₃ is oxidized into NH₂ and acylamide structures and then to isocyanate species, which is an intermediate for N₂ formation.     

Effect of calcination temperature on the characteristics of SO<Emphasis Type="Italic">sk</Emphasis>4/2-/TiO<Subscript>2</Subscript> catalysts for the reduction of NO by NH<Subscript>3</Subscript>

The catalytic activity of sulfated titania (ST) calcined at a variety of temperatures has been investigated for selective catalytic reduction (SCR) of NO by NH₃. The NO removal activity of ST catalyst mainly depends on its sulfur content, indicating critical role of sulfur species on the surface of TiO₂. The role of sulfur is mainly the formation of acid sites on the catalyst surface. The presence of both BrØnsted and Lewis acid sites on the surface of sulfated titania has been identified by IR study with the adsorption of NH₃ and pyridine on ST. The reduction of the intensity of IR bands representing BrØsted acid sites is more pronounced than that revealing Lewis acid sites as the calcination temperature increases. It has been further clarified by IR study of ST500 catalyst evacuated at a variety of temperatures. The NO removal activity also decreases with the increase of the catalyst calcination temperature. It simply reveals that BrØnsted acid sites induced by sulfate on the catalyst surface are primarily responsible for the enhancement of catalytic activity of ST catalyst containing sulfur for NO reduction by NH₃.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2

A mechanistic scheme of N₂O and N₂ formation in the selective catalytic reduction of NO with NH₃ over a Ag/Al₂O₃ catalyst in the presence and absence of H₂ and O₂ was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N₂ and N₂O are formed via direct or oxygen-induced decomposition of surface NH₂NO species. These intermediates originate from NO and surface NH₂ fragments. The latter NH₂ species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N₂O and O₂. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgO_x particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N₂O production in the presence of H₂ is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N₂O decomposition.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Iron oxide supported sulfated TiO2 nanotube catalysts for NO reduction with propane

Sulfated TiO₂ nanotubes and a series of iron oxide loaded sulfated TiO₂ nanotubes catalysts with different iron oxide loadings (1 wt%, 3 wt%, 5 wt% and 7 wt%) were prepared and calcined at 400 °C. The physico-chemical properties of the catalysts were studied by using XRD, N₂-physisorption, Raman spectroscopy, SEM-EDX, TEM, XPS, and pyridine adsorption using FTIR and H₂-TPR techniques. It was observed that iron oxide was highly dispersed on the sulfated TiO₂ nanotube support due to its strong interaction. The activity of these catalysts in the catalytic removal of NO with propane was also studied in the temperature range of 300–500 °C. Highest activity (90% NO conversion) was observed with 5 wt% iron oxide supported on sulfated TiO₂ catalyst at 450 °C. Selective catalytic reduction of NO activity of the catalysts was correlated with iron oxide loading, reducibility, and the Brönsted and Lewis acid sites of the catalysts. The catalyst also showed good stability under studied reaction conditions that no deactivation was observed during the 50 h of reaction.     

Novel promoting effect of acid modification on selective catalytic reduction of NO with ammonia over CeO2 catalyst

A series of acid modified CeO₂ catalysts were prepared and used for selective catalytic reduction (SCR) of NO with NH₃. The results showed that the SCR activity of pure CeO₂ was greatly enhanced by the modification of acid. The CeO₂ modified by 20% phosphotungstic acid exhibited the best NO conversion in a wide temperature range of 150–550 °C. The SCR activity was slightly influenced by SO₂ and H₂O, while such effect was reversible. The improvement of SCR activity and N₂ selectivity over CeO₂ catalyst modified by acid was attributed to the enhanced amount and intensity of Brønsted or Lewis acid sites.     

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO₂, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm^?1 suggests that besides structural Lewis sites on the surface of ZrO₂, strong Lewis sites are made from chemisorbed SO₃. Upon adsorption of water, SO₃ is converted, partially, into a surface sulfated species which may act as strong Brønsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO₄. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Brønsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.     

Evidence of Correlation between Electronic Density and Surface Acidity of Sulfated TiO2

Correlation between the electronic structure and surface acidity of TiO₂–SO₄ ^2– with different SO₄ ^2– amounts has been investigated by means of NH₃-TPD, NH₃-FT-IR and XPS. With the increase of sulfate loadings, the shift of binding energies of O 1s in hydroxyl and Ti 2p_2/3 increases and is proportional to total acidity. A linear relation is obtained between Ti 2p binding energy shift and Lewis acid sites, while the shift in O 1s binding energy is attributed both to the generation of NH₃ hydrogen bond and of Brønsted acid sites. Accordingly, the results obtained from XPS measurements provide evidence that the ammonia adsorption sites are attributed to the decrease of electron density of O 1s in hydroxyl (Brønsted type and H bonded) and Ti 2p_2/3 (Lewis type) by inductive effect of the neighboring sulfate ion.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

Influence of H4SiW12O40 loading on hydrocracking activity of non-sulfide Ni-H4SiW12O40/SiO2 catalysts

The effect of H₄SiW₁₂O₄₀ loading on the catalytic performance of the reduced Ni-H₄SiW₁₂O₄₀/SiO₂ catalysts for hydrocracking of n-decane with or without the presence of thiophene and pyridine is studied. The catalysts were characterized by BET, XRD, Raman, XPS, H₂-TPR, H₂-TPD, NH₃-TPD and FT-IR of pyridine adsorption. It was found that addition of H₄SiW₁₂O₄₀ to the system increases the catalytic activity and the promoting effect is a function of the H₄SiW₁₂O₄₀ loading. The best result was obtained on 5%Ni-50%H₄SiW₁₂O₄₀/SiO₂ catalyst which shows the highest activity for hydrocracking of n-decane and excellent tolerance to the sulfur and nitrogen compounds in the feedstock. The results showed that a suitable amount of H₄SiW₁₂O₄₀ loading on the 5%Ni/SiO₂ catalyst increases the amount of both hydrogen adsorbed and Brønsted acid and Lewis acid sites on the catalyst. The high catalytic performance of the catalyst can be related to the nature of H₄SiW₁₂O₄₀ and the proper balance between metal and acid functions.     

Study on the NO reduction by NH3 on a SO42 −/AC catalyst at low temperature

The selective catalytic reduction (SCR) of NO with NH₃ at low temperature over a novel SO₄^2 −/AC catalyst (activated coke supported sulfate acid) was investigated especially in the presence of SO₂ and H₂O. A higher SCR activity and stability were attributed to the cooperation of the oxygen-containing function group and carbon. SO₂ promoted SCR activity, which was due to the formation of H₂SO₄ on the catalyst surface and adsorbed more NH₃. The capillary condensation of H₂O in the micropores of the catalyst inhibited SCR activity and the reaction of adsorbed NH₃ and NO.     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

NO x reduction by ethanol on Pd/zeolites-effect of oxygen

The reduction of NO by ethanol on palladium catalysts supported on NaZSM-5 and HZSM-5 zeolites was studied. Temperature programmed techniques, such as desorption of ethanol, NO and surface reaction (TPSR) analyses, besides NH₃ (NH₃–TPD), as well as, FTIR after pyridine adsorption were used to accomplish reaction products and surface properties. Results show that NaZSM5 and HZSM5 present different amounts of Lewis and Brönsted acid sites, which affect strongly the carbon product distribution, but not the nitrogen selectivity. NO reaction with ethanol in the presence of oxygen occurs only over the metal and is independent of the acid sites.     

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO₂ by O₂/H₂O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO₂ adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO₂ was first adsorbed on the bare zigzag sites, followed by reaction with H₂O to form H₂SO₃, which was further oxidized by O₂ to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO₃ or H₂SO₃ as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO₂ adsorption (and oxidation) on the amount of surface oxygen.     

HC-SCR reaction pathways on ion exchanged ZSM-5 catalysts

Metal cation (metal = Cu, In and La) ion exchanged ZSM-5 zeolites as catalysts for the NO selective reduction by propane and propene in excess oxygen. The surface reactions of HC-SCR over catalysts were investigated through in situ DRIFTS method. For C₃H₈-SCR, adsorbed nitrate species (–NO₃) were observed as main reaction intermediates and they could react with gaseous propane to produce N₂, H₂O and CO₂. While for C₃H₆-SCR, adsorbed amine species (–NH₂) were observed as main reaction intermediates and they could react with NO or NO₂ to produce the final products. The different reaction pathways for C₃H₈-SCR and C₃H₆-SCR over catalysts were proposed based on the DRIFTS results and the main factors controlling the activities of catalysts were discussed in details. The competing adsorption between NO–O₂ and HC–O₂ on the Brønsted acid sites of catalysts was responsible for the different reaction pathways in HC-SCR.     

Effect of SO2 on the catalytic activity of Ga2O3–Al2O3 for the selective reduction of NO with propene in the presence of oxygen

The effect of coexisting SO₂ on the catalytic activity of Ga₂O₃–Al₂O₃ prepared by impregnation, coprecipitation and sol–gel method for NO reduction by propene in the presence of oxygen was studied. Although the activity of Al₂O₃ and Ga₂O₃–Al₂O₃ prepared by impregnation (Ga₂O₃/Al₂O₃(I)) and coprecipitation (Ga₂O₃–Al₂O₃(CP)) was depressed considerably by the presence of SO₂, NO conversion on Ga₂O₃–Al₂O₃ prepared by sol–gel method (Ga₂O₃–Al₂O₃(S)) was not decreased but increased slightly by SO₂ at temperatures below 723 K. From catalyst characterization, SO₂ treatment was found to cause two important effects on the surface properties: one is the creation of Brønsted acid sites on which propene activation is promoted (positive effect), and the other is the poisoning of NO_x adsorption sites on which NO reduction proceeds (negative effect). It was presumed that the influence of SO₂ treatment on the catalytic activity is strongly related to the balance between the negative and positive. The activity enhancement of Ga₂O₃–Al₂O₃(S) by SO₂ was accounted for by the following consideration: (1) increase of the propene activation ability by SO₂, (2) incomplete inhibition of NO_x adsorption sites by SO₂.     

Novel promoting effect of SO2 on the selective catalytic reduction of NOx by ammonia over Co3O4 catalyst

Spinel nano-Co₃O₄ was prepared by solid-state reaction at room temperature and investigated for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Although suffering from pore filling and plugging, treatment of this catalyst by SO₂ showed novel promoting effect on NH₃-SCR above 250 °C. Bulk cobalt sulfate was observed over the sulfated Co₃O₄ with XRD, which would be an active component for NH₃-SCR. The sulphated Co₃O₄ catalyst exhibited good resistance to SO₂ (500 ppm, 100 ppm) and 10% H₂O at a space velocity of about 25 000 h⁻¹ at 300 °C, as tested for 12 h.     

Active sites on Pt containing sulfated zirconia

The influence of the Pt and sulfate concentration on the activity of Pt containing sulfated zirconia for n-heptane conversion was investigated. Pt was deposited on the support by impregnation and by photocatalytic deposition. The amount deposited was 2.5 and 0.4 wt% respectively. For comparison a hybrid catalyst consisting of sulfated zirconia and Pt on SiO₂ was prepared. As supports a commercial sulfated zirconia with a fixed sulfate concentration, a commercial and self synthesized Zr(OH)₄ were used. The sulfate content varied between 20 and 60% of a monolayer. The shifts to higher frequency in the IR spectra of CO adsorbed on Pt correlate with the increasing amounts of sulfates on zirconia and are attributable to the changes in the electron density of the supported metal, i.e. the electron deficiency of Pt increases with increasing concentration of acid sites. After activation in air and reduction in hydrogen two SO₂ peaks were detected by a temperature programmed heating procedure (TPE—temperature programmed evolution). The lower the desorption temperature of the first SO₂ peak, the higher the activity. The shift to lower temperature is connected with a higher Pt and sulfate concentration, furthermore with the proximity of the metal to acid sites. The catalysts with a low sulfate concentration possess only Lewis acid sites and are inactive for n-heptane conversion. At higher sulfate concentration, Br?nsted acid sites are present and the catalysts are active. The concentration of these acid sites is related to the concentration of sulfates, which desorb at lower temperature. Dedicated to Professor Konrad Hayek.