The mechanism of selective NOx reduction by hydrocarbons in excess oxygen on oxide catalysts: V. Adsorption properties of a commercial Ni-Cr oxide catalyst

According to X-ray diffraction analysis data, the test catalyst was a Ni-Cr spinel with an impurity of NiO. With the use of in situ IR spectroscopy, it was found that nitrite, nitrate, and acetate surface complexes occurred under the reaction conditions of the selective catalytic reduction of nitrogen oxides by propane in the presence of oxygen on the nickel-chromium catalyst. As the temperature was increased, the nitrite complexes were converted into nitrate species. The molar absorption coefficient of surface nitrate complexes was determined. According to IR-spectroscopic and TPD data, the nitrate complexes were bound relatively weakly to the surface. The temperature region of their existence was 50–200°C. The temperature region of existence of the surface acetate complexes was 200–400°C. The individual adsorption of oxygen was not observed; however, oxygen-containing surface sites (Cr⁵⁺=O) participated in the formation of the surface complexes of reactants.     

The mechanism of selective NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> reduction by hydrocarbons in excess oxygen on oxide catalysts: VII. The nature of synergism on a mechanical mixture of catalysts

Based on a mechanistic study of the selective reduction of NO _x by propane on NTK-10-1 and Ni-Cr oxide (NCO) catalysts, the reason for synergism in this process on a mechanical mixture of the catalysts was determined. On the NCO catalyst at temperatures higher than 250°C without NO _x activation, C₃H₈ was oxidized with the formation of a considerable amount of hydrogen. This hydrogen migrated to the surface of NTK-10-1 through a gas phase and reduced this surface. On the reduced surface, H₂ reacted with NO _x by a mechanism characteristic of supported platinum group metals. In accordance with this mechanism, nitrogen atoms, which were formed by the dissociation of NO on metal atoms reduced by hydrogen, recombined to form nitrogen molecules in a gas phase, whereas oxygen atoms reacted with the hydrocarbon to form CO₂ and H₂O molecules in a gas phase. The positive effect of H₂, which was formed on the NCO surface, on the reduction of NO _x on NTK-10-1 is the main reason for synergism. An analysis of the experimental data demonstrated that an effectively working mechanical mixture of catalysts can be obtained if one of the mixture components is responsible for the effective activation of nitrogen oxides and the other is responsible for the activation of hydrocarbons.     

Formation of sulfur surface species on a commercial NO<Subscript>x</Subscript>-storage reduction catalyst

The influence of SO₂ exposure under lean (oxidizing) and rich (reducing) reaction conditions on the storage and oxidation/reduction function of a commercial NO_x storage-reduction catalyst was investigated by temperature-programmed uptake experiments and high temperature XRD. Both the storage capacity and the oxidation/reduction function of the catalyst were deactivated by SO₂ exposure under lean and rich reaction conditions. The deactivation of the storage component, i.e. the loss of the NO_x storage capacity, resulted mainly from the formation of Ba-sulfates accumulating in the bulk phase, which have a high thermal stability (>800°C) and, therefore, cannot be removed under the typical operation conditions of a NSR catalyst. For the oxidation function only a temporarily deactivation during lean reaction conditions was observed. Besides the formation of SO^2- ₄ species on the storage component at the beginning of the SO₂ exposure under rich conditions, an adsorption of SO₂ on the noble metal component was observed resulting in the formation of sulfur deposits. The oxidation of these sulfur species with a subsequent spillover of SO^2- ₄ species to the storage component during lean conditions could accelerate the deactivation of the storage capacity.     

Spectrokinetic study of the mechanism of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> reduction with propylene over ZrO<Subscript>2</Subscript> in excess oxygen

It has been demonstrated by quantitative spectrokinetic measurements that, on the surface of zirconia stabilized as a tetragonal phase, the rate-limiting step of the selective catalytic reduction of nitrogen oxides (SCR of NO _x ) with propylene is the interaction of surface nitrates with C₃H₆ yielding organic nitro compounds. It is hypothesized that propylene reacts not with the nitrates themselves but with the activated complex NO₂ ^ads whose structure is intermediate between the structures of the monodentate NO₃ ^? and NO₂ species. Deep C₃H₆ oxidation exerts an adverse effect on the rate of the SCR of NO _x with propylene, and the interaction between O₂ and NO, which yields NO₂ and NO₃ ^? stimulates further nitrogen reduction to N₂. The effect of the reaction between oxygen and O₂N?C _n H _m on the NO _x reduction rate is variable and is determined by the C₃H₆/NO _x ratio. A generalized scheme of the SCR of NO _x with propylene on the surface of ZrO₂ partially stabilized as a tetragonal phase has been developed by comparing experimental data of this study and data available from the literature.     

X-ray photoelectron spectroscopic study of the interaction of supported metal catalysts with NO<Subscript>x</Subscript>

The reactions of the platinum and rhodium model catalysts applied to aluminum oxide with NO_x (10 Torr NO + 10 Torr O₂) were studied by X-ray photoelectron spectroscopy. The reaction conducted at room temperature formed on the surface of the oxide support the NO _3,s ^? nitrate ions characterized by the N1s line at 407.4 eV and O1s line at 533.1 eV and the NO _2,s ^? nitrite ions characterized by the N1s line with a binding energy of 404.7 eV. At the same time, the Pt4f and Rh3d lines of the supported platinum particles are shifted toward higher binding energies by 0.5–1.0 eV and 0.7–1.2 eV, respectively. It is assumed that the binding energies increase due to changes in the chemical state of the platinum metal in which oxygen is dissolved. The reaction of NO_x with Pt/Al₂O₃ at 200°C forms platinum oxide defined by the Pt4f _7/2 line with a binding energy of 72.3 eV.     

Simultaneous removal of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and SO<Subscript>2</Subscript> by low-temperature selective catalytic reduction over modified activated carbon catalysts

A series of modified porous activated carbon (AC) catalysts prepared by impregnation were investigated for the low-temperature (≤250°C) selective catalytic reduction (SCR) of NO _x with NH₃ with simultaneous removal of SO₂. The effects of various preparation conditions and reaction conditions on NO and SO₂ conversions were observed, such as support type, active components, copper loading, calcination temperature and presence of H₂O and O₂. The modified AC catalysts were characterized by BET, XRD, TG and TPX methods. The activity test results showed that the optimal catalyst is 15% Cu/WCSAC which can provide 52% NO conversion and 68% SO₂ conversion simultaneously at 175°C with a space velocity of 30000 h^?1, and the optimal calcination temperature was 500°C. The presence of H₂O could inhibit NO conversion and promote the SO₂ conversion. The effect of O₂ (0–5%) was evaluated, and the NO and SO₂ conversions were best when the concentration of O₂ was 3%. Research demonstrated that Cu/WCSAC catalyst was a kind of potential catalysts due to the amorphous phase, high specific areas and high active ability.     

Generation of Antimicrobial NO<Subscript>x</Subscript> by Atmospheric Air Transient Spark Discharge

Atmospheric pressure air plasma discharges generate potential antimicrobial agents, such as nitrogen oxides and ozone. Generation of nitrogen oxides was studied in a DC-driven self-pulsing (1–10 kHz) transient spark (TS) discharge. The precursors of NO_x production and the TS characteristics were studied by nanosecond time-resolved optical diagnostics: a photomultiplier module and a spectrometer coupled with fast intensified camera. Thanks to the short (~10–100 ns) high current (>1 A) spark current pulses, highly reactive non-equilibrium plasma is generated. Ozone was not detectable in the TS, probably due to higher gas temperature after the short spark current pulses, but the NO_x production rate of ~7 × 10¹⁶ molecules/J was achieved. The NO₂/NO ratio decreased with increasing TS repetition frequency, which is related to the complex frequency-dependent discharge properties and thus changing NO₂/NO generating mechanisms. Further optimization of NO₂ and NO production to improve the biomedical and antimicrobial effects is possible by modifying the electric circuit generating the TS discharge.     

Molecular mechanism of propane oxidative dehydrogenation on surface oxygen radical sites of VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/TiO<Subscript>2</Subscript> catalysts

Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VO _x /TiO₂ were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes C_β–H bond activation of propane followed by the formation of a surface hydroxyl group V–O _t H and a propyl radical n-C₃H₇. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging O _b oxygen atom with the methylene C–H bond of propyl radical n-C₃H₇ followed by the formation of a hydroxylated surface site HO _t –V⁴⁺–O _b H and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V⁵⁺–(O _t O _b )^? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity.     

Role of surface structures on the selective reduction of nitrogen oxides with hydrocarbons on oxide catalysts

The infrared spectrum in situ was used to study the mechanism of selective reduction of nitrogen oxides on a series of oxide catalysts. It is shown that under the reaction conditions, nitrites, nitrates, and nitroorganic and acetate complexes are formed. Qualitative comparison of the rate of decomposition of the complexes and the rate of formation of the process products was performed using the spectral kinetic technique. Based on this comparison, the role of surface complexes in the process was determined. A mechanism showing the main stages of the reduction of nitrogen oxides is proposed.     

Activation of methanol-tolerant carbon-supported RuSe<Subscript>x</Subscript> electrocatalytic nanoparticles towards more efficient oxygen reduction

An electrocatalytic system that utilizes tungsten oxide modified carbon-supported RuSe_x nanoparticles is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic voltammetry and rotating ring-disk voltammetry, as well as upon its introduction (as cathode) to the low-temperature hydrogen–oxygen fuel cell. After the modification of RuSe_x catalytic centers with ultra-thin films of WO₃, the potential of oxygen reduction in 0.5 mol dm⁻³ H₂SO₄ (in the absence and presence of methanol) is shifted ca. 70 mV (under rotating disk voltammetric conditions) towards more positive values, and the percent formation (at ring) of the undesirable hydrogen peroxide has decreased approximately twice when compared to the WO₃-free system. Relative to bare electrocatalyst, an increase of power density from 75 to 100 mW cm⁻² (at 300 mA cm⁻²) has been observed upon utilization of WO₃-modified RuSe_x in polymer electrolyte membrane fuel cell at 80 °C. In comparison to Vulcan-supported Pt nanoparticles, the overall electrocatalytic performance of tungsten oxide modified carbon-supported RuSe_x nanoparticles is lower, but the latter system is practically insensitive to the presence of methanol even at 0.5 mol dm⁻³ level. Dedicated to Professor Dr. Algirdas Vaskelis on the occassion of his 70th birthday.     

The reactions and composition of the surface intermediate species in the selective catalytic reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> with ethylene over Co-ZSM-5

A pretreatment-transient reaction product analysis method was applied to study the reactions and average composition of the possible surface intermediate species in selective catalytic reduction with ethylene of NO _x over Co-ZSM-5. The reactions of the surface species, formed by the pretreatment of Co-ZSM-5 in a NO/C₂H₄/O₂ mixture at 275°C, with the NO/O₂ flow produced much more N₂ than that with the individual NO or O₂ flow. The similarity of N₂/CO _x /H₂O product distribution generated from the above surface species-NO/O₂ reactions and that from the normal NO/C₂H₄/O₂ flow reactions implies that the surface species NC _a O _b H _c formed in the three-component pretreatment process is very likely the primary intermediate surface species generated during the real flow reactions. The in situ FT-IR (DRIFT) spectroscopy measurements of the surface species support the above conclusion.     

On the properties of surface complexes formed upon the adsorption of NO<Subscript>x</Subscript>, C<Subscript>3</Subscript>H<Subscript>6</Subscript>, and their mixtures with oxygen on ZrO<Subscript>2</Subscript> according to EPR,TPD, and fourier transform IR spectroscopy data

The adsorption of reactant mixtures is quantitatively and qualitatively different from the adsorption of the individual reactants. Thus, O₂ is almost not adsorbed on ZrO₂; however, a considerable concentration of molecular oxygen was detected among the products of desorption after the adsorption of a mixture of NO + O₂ and the total amount of desorbed molecules was greater by a factor of 10 than their total amount after the individual adsorption of NO and O₂. Among the qualitative differences is the formation of the O₂^- radical anion on the surface only upon the adsorption of the mixture of NO + O₂. Similarly, the number of desorbed molecules upon the simultaneous adsorption of C₃H₆, NO, and O₂ was much greater than that upon their individual adsorption; this is related to the formation of paramagnetic and nonparamagnetic NO₂–hydrocarbon complexes on the surface, which contained the NO₂ group and a hydrocarbon fragment.     

Oxygen reduction in acid media by a Ru<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se<Subscript>z</Subscript>(CO)<Subscript>n</Subscript> cluster catalyst dispersed on a glassy carbon-supported Nafion film

Electrocatalytic oxygen reduction was studied on a Ru_xFe_ySe_z(CO)_n cluster catalyst with Vulcan carbon powder dispersed into a Nafion film coated on a glassy carbon electrode. The synthesis of the electrocatalyst as a mixture of crystallites and amorphous nanoparticles was carried out by refluxing the transition metal carbonyl compounds in an organic solvent. Electrocatalysis by the cluster compound is discussed, based on the results of rotating disc electrode measurements in a 0.5 M H₂SO₄. A Tafel slope of −80.00±4.72 mV dec⁻¹ and an exchange current density of 1.1±0.17×10⁻⁶ mA cm⁻² was calculated from the mass transfer-corrected curve. It was found that the electrochemical reduction reaction follows the kinetics of a multielectronic (n=4e⁻) charge transfer process producing water, i.e. O₂+4H⁺+4e⁻→2H₂O. Electronic Publication     

The effect of the carrier nature and the method of preparation of oxide catalysts containing indium on their activity in the selective catalytic reduction of nitrogen monoxide with C<Subscript>1</Subscript>-C<Subscript>4</Subscript> hydrocarbons

The activity of samples containing indium in the selective catalytic reduction (SCR) of NO with C₁-C₄ hydrocarbons depends on nature of the carrier, Al₂O₃, ZrO₂, the quantity of indium oxide, and the method of its introduction. The most active catalysts (2.5–5.0% In₂O₃/Al₂O₃) are stable to water and are characterized by a large overall concentration of oxide centers. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 107–111, March–April, 2007.     

Thermo-programmed reduction study of Pt/WO<Subscript>x</Subscript>–ZrO<Subscript>2</Subscript> materials by thermogravimetry

The thermo-programmed reduction study of Pt/WO_x–ZrO₂ materials prepared with different tungsten loading were performed by thermogravimetry. The samples were synthesized by impregnation method and calcined at 600, 700 and 800°C. The characterizations of both un-calcined and calcined materials were carried out using different techniques: thermal analysis (TG and DTA), X-ray diffraction (XRD) and thermo-programmed reduction (TPR). TG and DTA analysis of un-calcined were used to determination of calcination temperatures of the samples. XRD diffractograms were useful to help us in the determination of phase presents. TPR profiles showed between three and four events at different temperatures attributed to platinum reduction and the different stages of tungsten specie reduction.     

An XPS study of the oxidation of noble metal particles evaporated onto the surface of an oxide support in their reaction with NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The interaction of the model catalysts Rh/Al₂O₃, Pd/Al₂O₃, Pt/Al₂O₃, and Pt/SiO₂ with NO _x (mixture of 10 Torr of NO and 10 Torr of O₂) was studied by X-ray photoelectron spectroscopy (XPS). Samples of the model catalysts were prepared under vacuum conditions as oxide films ≥100 Å in thickness on tantalum foil with evaporated platinum-group metal particles. According to transmission electron microscopic data, the platinum-group metal particle size was several nanometers. It was found by XPS that the oxidation of Rh and Pd nanoparticles in their interaction with NO _x occurs already at room temperature. The particles of platinum were more stable: their oxidation under the action of NO _x was observed at elevated temperatures of ～300°C. At room temperature, the interaction of platinum nanoparticles with NO _x hypothetically leads to the dissolution (insertion) of oxygen atoms in the bulk of the particles with the retention of their metallic nature. It was found that dissolved oxygen is much more readily reducible by hydrogen than the lattice oxygen of the platinum oxide particles.     

The Mechanism of Selective NO<Subscript>x</Subscript> Reduction by Hydrocarbons in Excess Oxygen on Oxide Catalysts: III. Adsorption Properties of the Commercial STK Catalyst

According to X-ray diffraction data, the STK catalyst is a mixture of Fe₂O₃ and Cr₂O₃. The temperature-programmed reduction spectrum exhibited two reduction peaks: one, with T _max = 250°C, corresponds to the reduction process Cr₂O₃ → CrO and the other, with T _max = 360°C, corresponds to the reduction Fe₂O₃ → Fe₃O₄. The results of thermal desorption measurements suggest that the individual adsorption of oxygen on the surface of the STK catalyst is low; in this case (according to IR-spectroscopic data), an atomic form is the main species. Surface nitrite-nitrate complexes are formed upon the adsorption of NO. Nitrite and nitrate complexes desorbed at maximum rates at 105 and 160°C, respectively. Unlike the NTK-10-1 catalyst, the NO species, which desorbed at high temperatures (250–400°C), was absent from the surface of STK. Propane adsorbed at room temperature to form surface compounds containing an acetate group. The interaction of propane with the surface of the STK catalyst at reaction temperatures resulted in strong surface reduction.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 550–558.Original Russian Text Copyright © 2005 by Tret’yakov, Burdeinaya, Zakorchevnaya, Matyshak, Korchak.