Transient TPSR, DRIFTS-MS and TGA studies of a Pd/ceria-zirconia catalyst in CH4 and NO2 atmospheres

In this study, the parameters governing the activity of Pd/ceria-zirconia catalysts in the selective catalytic reduction (SCR) of NO_x assisted by methane are investigated using a combination of temperature-programmed spectroscopic and thermogravimetric techniques and transient SCR conditions. By DRIFTS of adsorbed CO, it is established that Pd species on Ce_0.2Zr_0.8O₂ are mainly present in cationic form but exhibit high reducibility. As found by temperature-programmed surface reaction (TPSR) in CH₄ + NO₂ atmosphere, the CH₄-SCR reaction is initiated at 280 °C on Pd/Ce_0.2Zr_0.8O₂ and yields almost 100% N₂ above 500 °C. DRIFTS-MS and TGA experiments performed under transient SCR conditions show that DeNO_x activity is due to a surface reaction between some methane oxidation products on reduced Pd sites with ad-N_xO_y species presumably located on the support. The detrimental effect of O₂ on DeNO_x is explained by the promotion of the total combustion of methane assisted by the ceria-zirconia component at the expense of the SCR reaction above 320 °C.     

NO/CH4 Reaction over Nanodispersed Pt Particles

The catalytic behavior of the cubic (70%) Pt nanoparticles supported on alumina, with an average diameter of 132nm, was investigated for NO/CH₄ reaction. It was observed that the formation of reaction products (N₂O, CO and NH₃) is related to the size as well to the shape (facet) of the Pt nanoparticles.     

The Remarkable Effect of In2O3 on the Catalytic Activity of In/HZSM-5 for the Reduction of NO with CH4

In/HZSM-5/In₂O₃ catalyst that contained two different kinds of In induced by the impregnating and the physical mixing method respectively has shown remarkable activity for the CH₄-SCR of NO _x comparing with In/HZSM-5. The addition of In₂O₃ into In/HZSM-5 improved the NO conversion through enhancing the adsorption of NO _x over In/HZSM-5.     

Adsorption of NO and NO2 on ceria–zirconia of composition Ce0.69Zr0.31O2: A DRIFTS study

In this study, the nature of surface intermediates generated by adsorption of NO and NO₂ on a commercial ceria–zirconia powder of composition Ce_0.69Zr_0.31O₂ was investigated using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The conditions of occurrence of the main adsorbed species, i.e. nitrites and nitrates, are studied semi-quantitatively as a function of catalyst pre-treatment and/or type of adsorbed NO_x molecule. On the partially reduced ceria–zirconia, the primary role of NO_x is to re-oxidize the surface via adsorption/decomposition on reduced sites. By contrast, the formation of nitrites and nitrates readily occurs on oxidized surfaces, the latter kind of species being strongly promoted in the case of NO₂ adsorption only.     

Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

EPR studies on NO interaction with MoOx/t-ZrO2 catalysts obtained by slurry deposition

Variable temperature adsorption of nitric oxide on MoO₃ supported on tetragonal zirconia (MoO_x/t-ZrO₂), obtained by slurry deposition, was investigated by EPR spectroscopy. The influence of molybdenum loading and co-adsorbed oxygen on the adsorption process of NO was elucidated. Particular attention was devoted to redox character of NO activation. Another important aspect concerned is the nature of surface nitrosyl complexes of molybdenum and their thermal stability. The role of oxygen in NO transformation over catalyst surface was also discussed.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

Methane activation by NO2 on Co loaded SBA-15 catalysts: The effect of mesopores (length, diameter) on the catalytic activity

In a general model of “three-function deNO_x” catalyst, the partial oxidation of methane by NO₂ is an important step (CH₄ + NO₂ → C_xH_yO_z + NO). To study the effect of the length and diameter, in the mesopores of SBA-15, we have synthesized catalysts with 3 wt.% cobalt supported on SBA-15, with differences in length and diameter of channels. Three different cobalt species were detected on all catalysts. We demonstrated by TPSR experiments that the activity of cobalt/SBA-15 catalysts is affected by the length, the diameter and connections between mesopores of the SBA-15 supports. We show that by changing textural properties of silica support the temperature of 100% conversion of NO₂ into NO can decrease by more than 100 °C.     

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Oxidation of carbon by NOx, with particular reference to NO2 and N2O

The reactions reviewed here concern those between elemental carbon and NO₂, N₂O and NO, sometimes in the presence of oxygen. The section on NO includes only updates to recent reviews. Soots, activated carbons and carbon blacks are more reactive than graphite. The magnitudes of the reaction rates are found to be: NO₂ > N₂O ≈ NO ≈ O₂. The presence of a soluble organic fraction (SOF) in soot is found to influence some reactions, and all three reactions suffer from inhibition by surface products. The mechanisms proposed for the surface adsorbates are summarised. All authors found that two types of active site were present; one forming weak bonds (physisorption), and the other undergoing chemisorption to form groupings such as -C-ONO, -C-ONO₂ or -C-NO₂. The latter decompose to give oxides of carbon, and are sometimes called redox reactions. The adsorbates appear to be the same for all NO_x species. Some elemental nitrogen adsorption takes place, and can involve incorporation into the C skeleton. The attack of NO on carbon proceeds via NO₂, so that catalysts that facilitate this oxidation are effective. Gaseous SO₂ and H₂O assist in the process by forming acids which are good oxidants. The change in activation energy with temperature found experimentally for NO and N₂O may be due to the form of nitrogen on the edge carbon atoms.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

Fluidization characteristics of aerogel Co/Al2O3 catalyst in a magnetic fluidized bed and its application to CH4-CO2 reforming

The fluidization characteristics of a nanoparticle catalyst were investigated in a fluidized bed assisted with an axial magnetic field. It showed that slugging and channeling, commonly observed when processing nanoparticles via conventional fluidized bed reactors, could be effectively eliminated, and the size of the agglomerates and bubble diameter could also be reduced with the aid of the magnetic field, leading to much improved gas-solid contact efficiency. Due to the improved gas-solid contact efficiency, the performance of the CH₄-CO₂ catalytic reforming has been significantly enhanced, where the initial conversion of CH₄ was 7.6% and 24.3% higher than those obtained in a conventional fluidized bed reactor and a fixed bed reactor. The catalytic deactivation, caused by carbon deposition on catalyst surfaces, is also slower in the magnetic fluidized bed operation, where the CH₄ conversion is 11.7% and 42.6% greater as compared with those in the conventional fluidized bed operation and the fixed bed operation. The present investigations demonstrated that carbon deposition can be much suppressed through improving the gas-solid contact efficiency with the assistance of the magnetic field.     

CO2 reforming of CH4 over stabilized mesoporous Ni-CaO-ZrO2 composites

Mesoporous Ni-CaO-ZrO₂ nanocomposites with high thermal stability were designed and employed in the CO₂/CH₄ reforming. The nanocomposites with appropriate Ni/Ca/Zr molar ratios exhibited excellent activity and prominent coking resistivity. The Ni crystallites were effectively controlled under the critical size for coke formation in such nanocomposites. It was found that low Ni content resulted in high metal dispersion and good catalytic performance. Moreover, the basicity of the matrices improved the chemisorption of CO₂ and promoted the gasification of deposited coke on the catalyst.     

DRIFT study of the interaction of NO and O2 with the surface of Ce0.62Zr0.38O2 as deNOx catalyst

Gas–solid interactions and surface intermediates evolution after NO adsorption onto calcined Ce_0.62Zr_0.38O₂ were investigated. The results of adsorption and temperature-programmed desorption of NO were explained using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) coupled with temperature-programmed experiments in environmental cell. Surface NO-containing species such as nitrites and nitrates were identified during evolution of NO on the surface of Ce_0.62Zr_0.38O₂ solid solution at low and high temperature. The ceria–zirconia solid solution was found to be active in deNO_x reaction in the presence of a “toluene, propene and propane” mixture.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Correlations for dependence of NOx emissions on heat loss in premixed CH4/air combustion

The present study represents an effort to correlate the dependence of NO_x emissions on heat losses to the atmospheric environment in a CH₄/air fueled combustor. To this end, the numerical analysis was performed over a wide range of residence times, equivalence ratios and heat losses using a perfectly stirred reactor (PSR) code. The numerical results showed that the calculated NO_x concentration initially increased, reached a maximum value and then decreased with increasing residence time when the heat loss was present. The similar variation was observed in changes in the thermal NO concentration that was evaluated by only considering the reactions associated with the thermal (Zeldovich) NO mechanism. With the heat loss increased, the calculated NO_x concentration was substantially reduced for all equivalence ratios investigated. In addition, the reductions in the NO_x concentration with respect to residence time became faster with increasing the equivalence ratio particularly for fuel rich conditions. The observed variations in the calculated NO_x concentration over the residence time (NO_x/τ) were found to fit well to the following correlation:ln(NO_x/τ)=a(HLI)+b. In the correlation, HLI is the dimensionless heat loss parameter and coefficients a and b are constants expressed as a function of adiabatic flame temperature (for a given equivalence ratio) and equivalence ratio, respectively.     

Steam resistant CoLa-mordenite catalysts for the SCR of NOx with CH4

The stabilization of Co-mordenite catalysts through lanthanum exchange is reported here. The effect of exchange order and calcination conditions upon the reduction of NO_x to N₂ at 500 °C was tracked during 400 h on a stream containing NO_x, CH₄, O₂ and 10% H₂O. Both the fresh and used catalysts were characterized through TPR, Raman spectroscopy, FTIR spectroscopy using CO as probe molecule, and XPS. These techniques revealed that the CoLa-mordenite catalysts which were not affected by the severe hydrothermal treatment showed no sign of Co or La migration out of the exchange positions. Instead, those that rapidly deactivated showed the formation of cobalt oxides and, in some cases, the migration of the cations to other exchange positions. The presence of exchanged lanthanum seems to preserve the integrity of the zeolite structure preventing the migration of cobalt ions with the subsequent formation of cobalt oxides which favors the reaction of methane with O₂, thus decreasing N₂ production.