Formation Of Surface Nitrates Research Articles

Dynamics of low temperature N2O formation under SCR reaction conditions over a Cu-SSZ-13 catalyst

Small pore Cu-SSZ-13 zeolite can effectively catalyze selective catalytic reduction (SCR) for NOx (NO and NO2) using NH3 as a reductant. However, side reactions leading to the formation of N2O, a greenhouse gas, occur in parallel under SCR reaction conditions. This work investigates the fundamental characteristics of N2O formation during SCR reactions over a Cu-SSZ-13 catalyst under both steady-state and transient conditions during reactant transitions at 200 °C. The absence of nitrate bands from in situ DRIFTS experiments and the inconsistencies between high stability of NH4NO3 and N2O formation at low temperature exclude the formation of surface nitrate or NH4NO3 as a precursor of N2O formation as the major path under standard SCR reaction conditions at 200 °C. The analysis from the transient N2O formation behavior during reactant transitions and NO + NH3 titration experiments indicates that more oxidized Cu-SSZ-13 catalyst, containing higher amount of Cu-oxy species, produces higher amount of N2O.

Potassium Titanate Nanobelts: A Unique Support for Au and AuRh Nanoparticles in the Catalytic Reduction of NO with CO

AbstractThe catalytic reduction of NO with CO was examined over monometallic Au and bimetallic AuRh nanoparticles supported on potassium titanate (K2Ti8O17) nanobelts (KTN) and TiO2. The highly crystalline KTN, prepared from TiO2 by a hydrothermal process using KOH, were several μm long and ca. 8 nm wide. Both Au and bimetallic AuRh catalysts showed a striking enhancement of the catalytic performance with KTN as support, compared to corresponding TiO2‐supported catalysts. The reasons for this behavior could be traced back using in situ diffuse reflectance infrared Fourier transform spectroscopy in tandem with modulation excitation spectroscopy, which proved that the KTN support promotes the formation of surface nitrate (−NO3−), carbonate (−CO32−), and isocyanide (−NCO) species. In particular, the combination of KTN with bimetallic AuRh nanoparticles results in the facile formation of isocyanide, which is a pivotal intermediate for producing dinitrogen molecules via the reaction with NO; NCO(a)+NO(a)→N2+CO2.

Absorption of Near UV Light by HNO3/NO3(-) on Sapphire Surfaces.

We have determined absorption of the near UV light (290-345 nm) by nitric acid (HNO3) deposition on sapphire window surfaces as a function of the HNO3 pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO3 surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10(-19) and (0.29 ± 0.03) × 10(-19) cm(2)/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO3 adsorption was reported, a new absorption band appeared in the 320-345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO3(-)) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO3 formed by HNO3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5-7 min. Background transmission changes in the 320-350 nm region after exposing clean sapphire surfaces with many repeated HNO3 deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320-350 nm range. We also modeled photolysis rates of HNO3/NO3(-) on urban grimes. Atmospheric implications of the results are discussed.

NO oxidation on zeolite supported Cu catalysts: Formation and reactivity of surface nitrates

The comparative activities of a small-pore Cu-CHA and a large-pore Cu-BEA catalyst for the selective catalytic reduction (SCR) of NOx with NH3, and for the oxidation of NO to NO2 and the subsequent formation of surface nitrates were investigated. Although both catalysts are highly active in SCR reactions, they exhibit very low NO oxidation activity. Furthermore, Cu-CHA is even less active than Cu-BEA in catalyzing NO oxidation but is clearly more active for SCR reactions. Temperature-programed desorption (TPD) experiments following the adsorption of (NO2+NO+O2) with different NO2:NO ratios reveal that the poor NO oxidation activity of the two catalysts is not due to the formation of stable surface nitrates. On the contrary, NO is found to reduce and decompose the surface nitrates on both catalysts. To monitor the reaction pathways, isotope exchange experiments were conducted by using 15NO to react with 14N-nitrate covered catalyst surfaces. The evolution of FTIR spectra during the isotope exchange process demonstrates that 14N-nitrates are simply displaced with no formation of 15N-nitrates on the Cu-CHA sample, which is clearly different from that observed on the Cu-BEA sample where formation of 15N-nitrates is apparent. The results suggest that the formal oxidation state of N during the NO oxidation on Cu-CHA mainly proceeds from its original +2 to a +3 oxidation state, whereas reaching a higher oxidation state for N, such as +4 or +5, is possible on Cu-BEA.

A comparative study of N2O formation during the selective catalytic reduction of NOx with NH3 on zeolite supported Cu catalysts

A comparative study was carried out on a small-pore Cu-CHA and a large-pore Cu-BEA zeolite catalyst to understand the lower N2O formation on small-pore zeolite supported Cu catalysts in the selective catalytic reduction (SCR) of NOx with NH3. On both catalysts, the N2O yield increases with an increase in the NO2/NOx ratios of the feed gas, suggesting N2O formation via the decomposition of NH4NO3. Temperature-programmed desorption experiments reveal that NH4NO3 is more stable on Cu-CHA than on Cu-BEA. In situ FTIR spectra following stepwise (NO2+O2) and (15NO+NH3+O2) adsorption and reaction, and product distribution analysis using isotope-labeled reactants, unambiguously prove that surface nitrate groups are essential for the formation of NH4NO3. Furthermore, Cu-CHA is shown to be considerably less active than Cu-BEA in catalyzing NO oxidation and the subsequent formation of surface nitrate groups. Both factors, i.e., (1) the higher thermal stability of NH4NO3 on Cu-CHA, and (2) the lower activity for this catalyst to catalyze NO oxidation and the subsequent formation of surface nitrates, likely contribute to the higher SCR selectivity with less N2O formation on this catalyst as compared to Cu-BEA. The latter is determined as the primary reason since surface nitrates are the source that leads to the formation of NH4NO3 on the catalysts.

Photocatalytic carbon oxidation with nitric oxide

The photocatalytic oxidation of carbon black on TiO2 using nitric oxide as an oxidizing agent was investigated. Layer-wise deposited carbon and TiO2 powder was illuminated with UVA light in the presence of NO at parts per million concentrations in dry and hydrated carrier gas at a temperature of 150°C. Carbon was photocatalytically converted mainly into CO2, and NO mainly into N2. Carbon oxidation rates of 7.2μg/h/mgTiO2 were achieved in the presence of 3000ppm NO. Under these experimental conditions in the absence of molecular oxygen, formation of surface nitrates causing TiO2 photocatalyst deactivation is suppressed. Addition of water enhances surface nitrate formation and catalyst deactivation.NO and carbon particulate matter are air pollutants emitted by diesel engines. Elimination of soot collected on a diesel particulate filter through oxidation is a demanding reaction requiring temperatures in excess of 250°C. The present study opens perspectives for a low-temperature regeneration strategy for the diesel particulate filter that simultaneously performs DeNOx reactions.

NO2 adsorption on Fe- and Cu-zeolite catalysts: The effect of the catalyst red–ox state

In the present work we investigate the adsorption of NO2 onto a Fe- and a Cu-promoted zeolite SCR catalyst in the absence of gaseous water, with a particular focus on the effect of the catalyst red–ox state. For both catalytic systems, results from the analysis of the gas phase during step changes of the NO2 concentration, combined with literature FTIR information, emphasize an important role of the catalyst red–ox state in the dynamics of NO2 adsorption: the molar ratio of released NO to adsorbed NO2 is close to 1/3 in the case of oxidized catalysts, in line with a two-steps mechanism leading to the formation of surface nitrates, but is greater when the catalyst is partially reduced. Furthermore the initial catalyst red–ox state influences the amount of nitrates that can be adsorbed on the catalyst surface, the pre-reduced sample exhibiting a greater storage capacity. The Cu-zeolite catalysts are able to store a greater amount of nitrates and are more easily reduced in comparison with the Fe-zeolite.

Formation and removal of Ba-carbonates or carboxylates on Pt/BaO/γ-Al2O3 lean NOx traps

In addition to feed composition, the NOx storage capacity (NSC) of a Pt/BaO/γ-Al2O3 lean NOx trap (LNT) also depends on the chemical state of Ba prior to the start of the lean NOx exposure phase during which the NSC is measured. This state of Ba is a result of the history of the trap's cyclic exposures to lean and rich conditions and is a function of the feed composition and the time intervals of the exposures. We have systematically investigated the role of adsorbed CO2 on the NOx sorption process on traps with 20, 8 and 4wt.% Ba. This was accomplished through a sequence of NOx storage and regeneration cycles with CO2 and H2O in the feed followed by purge in Ar and then by NOx storage and regeneration without CO2 and H2O at 300°C. The amount of pre-adsorbed CO2 released during NOx storage upon the introduction of NO2+O2 correlated well with the amount of NOx stored on the trap with 20wt.% Ba but not on the traps with 8 and 4wt.% Ba loadings. These results were further investigated in DRIFTS sequential adsorption experiments for CO2, NO2+O2, H2 or NO2+O2, CO2, H2 at 300°C. The DRIFTS experiments showed that, similar to bulk vs. surface nitrate formation on samples with high vs. low Ba loading, CO2 adsorption forms either carbonates or carboxylates on high vs. low Ba loadings, respectively. Regeneration by H2 removed all NOx adsorbates and any carboxylates, but not the carbonates. Thus, the formation of carbonates slows the NOx storage process as NOx must compete with pre-adsorbed carbonates for adsorption sites after regeneration. In addition, comparison between the NOx breakthrough curves with 7% CO2 in the lean feed and with pre-adsorbed CO2 on the catalyst showed that the NOx storage before the slip occurs was intimately related to the oxygen spillover from Pt to the surrounding Ba and competition between CO2 and NOx for adsorption sites. These findings provide insights to further improve the LNT formulations by using optimum Ba loadings and dispersions targeted at increasing the zero NOx slip time after each fuel rich pulse.

Structure and Surface Properties of Zirconia-Supported Molybdena Obtained by Slurry Deposition

A series of MoO3/t-ZrO2 catalysts was prepared by slurry deposition. Structural studies revealed that the presence of MoO3 had no influence on the phase composition of zirconia support. IR/RS and EPR results confirmed that both isolated and polymeric (2D) oxomolybdenum species occurred on catalysts surface. Adsorption of NO resulted in the reduction of molybdenum centers with simultaneous formation of surface nitrates.

Heterogeneous Photochemistry Relevant to the Troposphere: H2O2 Production during the Photochemical Reduction of NO2 to HONO on UV‐Illuminated TiO2 Surfaces

Bleached out: The detection of an oxidized co-product, hydrogen peroxide, during an important tropospheric reaction, combined with the detection of photoaccelerated surface nitrate formation (a competing pathway), allows a likely mechanism to be proposed (see figure) which explains previously observed experimental stoichiometries.

XPS study of nitrogen dioxide adsorption on metal oxide particle surfaces under different environmental conditions

The adsorption of nitrogen dioxide on gamma aluminium oxide (gamma-Al(2)O(3)) and alpha iron oxide (alpha-Fe(2)O(3)) particle surfaces under various conditions of relative humidity, presence of molecular oxygen and UV light has been investigated. X-Ray photoelectron spectroscopy (XPS) is used to monitor the different surface species that form under these environmental conditions. Adsorption of NO(2) on aluminum oxide particle surfaces results primarily in the formation of surface nitrate, NO(3)(-) with an oxidation state of +5, as indicated by a peak with binding energy of 407.3 eV in the N1s region. An additional minority species, sensitive to the presence of relative humidity and molecular oxygen, is also observed in the N1s region with lower binding energy of 405.9 eV. This peak is assigned to a surface species in the +4 oxidation state. When irradiated with UV light, other species form on the surface. These surface-bound photochemical products all have lower binding energy, between 400 and 402 eV, indicating reduced nitrogen species in the range of N oxidations states spanning +1 to -1. Co-adsorbed water decreases the amount of these reduced surface-bound products while the presence of molecular oxygen completely suppresses the formation of all reduced nitrogen species on aluminum oxide particle surfaces. For NO(2) on iron oxide particle surfaces, photoreduction is enhanced relative to gamma-Al(2)O(3) and surface bound photoreduced species are observed under all environmental conditions. Complementing the experimental data, N1s core electron binding energies (CEBEs) were calculated using DFT for a number of nitrogen-containing species in the gas phase and adsorbed on an Al(8)O(12) cluster. A range of CEBEs is calculated for various nitrogen species in different adsorption modes and oxidation states. These calculated values are discussed in light of the peaks observed in the XPS N1s region and the possible species that form following NO(2) adsorption and photoreaction on metal oxide particle surfaces under different conditions of relative humidity, presence of molecular oxygen and UV light.

Modeling NOx Storage Materials: On the Formation of Surface Nitrites and Nitrates and Their Identification by Vibrational Spectroscopy

We have studied the reaction of NO2 with BaO nanoparticles supported on an ordered Al2O3 thin film on NiAl(110). Combining chemical analysis by X-ray photoelectron spectroscopy (XPS) and vibrational spectroscopy by infrared reflection absorption spectroscopy (IRAS), performed in combination with molecular beam (MB) techniques, the sequence of appearance of various nitrogen−oxo surface intermediates and their spectral properties are identified. The initial intermediates at 300 K are surface nitrites (NO2-), which are preferentially oriented parallel to the surface. Whereas formation of nitrites is rapid even at 300 K, conversion of nitrites into surface nitrates (NO3-) occurs at a very low rate. After surface nitrate formation, no further reaction is observed. At higher temperature (500 K), conversion into surface nitrates is more facile and is followed by formation of ionic nitrates. All three nitrogen−oxo species can be clearly identified via their characteristic vibrational spectra. No spectroscopic evi...

Identifying surface species by vibrational spectroscopy: Bridging vs monodentate nitrates

Identification of surface species by IR spectroscopy is often based on common knowledge, with a clear risk of misassignment. Combining density functional theory (DFT) and IR reflection absorption spectroscopy (IRAS), we investigated surface nitrate formation on BaO/Al 2O 3 thin films. We found that it is not possible to identify coordination geometries of nitrates based solely on their vibrational frequencies. For metal-supported thin films, however, differences in polarization of the NO stretching modes facilitate unambiguous identification. In contrast to traditional assignments, we show that both bridging and monodentate nitrates are formed on BaO, whereas on Al 2O 3 bridging nitrates dominate.

NO x reduction on a transition metal-free γ-Al 2O 3 catalyst using dimethylether (DME)

NO 2 and dimethylether (DME) adsorption as well as DME and NO 2 co-adsorption on a transition metal-free γ-alumina catalyst were investigated via in-situ transmission Fourier transform infrared spectroscopy (in-situ FTIR), residual gas analysis (RGA) and temperature programmed desorption (TPD) techniques. NO 2 adsorption at room temperature leads to the formation of surface nitrates and nitrites. DME adsorption on the alumina surface at 300 K leads to molecularly adsorbed DME, molecularly adsorbed methanol and surface methoxides. Upon heating the DME-exposed alumina to 500–600 K the surface is dominated by methoxide groups. At higher temperatures methoxide groups are converted into formates. At T > 510 K, formate decomposition takes place to form H 2O(g) and CO(g). DME and NO 2 co-adsorption at 423 K does not indicate a significant reaction between DME and NO 2. However, in similar experiments at 573 K, fast reaction occurs and the methoxides present at 573 K before the NO 2 adsorption are converted into formates, simultaneously with the formation of isocyanates. Under these conditions, NCO can further be hydrolyzed into isocyanic acid or ammonia with the help of water which is generated during the formate formation, decomposition and/or NCO formation steps.

The adsorption of and reaction of NO 2 on Ag(1 1 1)-p(4 × 4)-O and formation of surface nitrate

The adsorption and reaction of nitrogen dioxide on the Ag(1 1 1)-p(4 × 4)-O surface has been investigated with RAIRS, TPRS and STM. At 300 K NO 2 initially reacts with the oxygen overlayer to form nitrate in p(3 × 3) and p(4 × 4) structures, which convert to a new p(3 × 3) at saturation coverage. Surface pitting during nitrate adsorption is suggestive of the incorporation of silver atoms into the NO 3 structure. With heating NO 3 decomposes into NO 2 and O at 396 K and 497 K, and oxygen desorbs at 578 K.

NO x storage on BaO: theory and experiment

We review our understanding of the NO 2 interaction with BaO. The presented picture has evolved from calculations using the density functional theory, reactor measurements and vibrational spectroscopy of surface species during NO 2 storage on a BaO/Al 2O 3 catalyst and BaO powder. The DFT calculations predict nitrite formation on the BaO(1 0 0) surface upon NO 2 adsorption. A particular stable adsorption configuration is a nitrate–nitrite pair geometry, with the nitrate bonded to a barium cation and the nitrite involving a surface oxygen anion. This configuration is supported by vibrational spectroscopy. Only nitrite formation is observed for NO 2 exposure to BaO powder at low temperatures, whereas signatures at ∼250 °C reveal the formation of surface nitrates. Continuous NO 2 adsorption at this temperature results in surface Ba(NO 3) 2, as well as bulk Ba(NO 3) 2, formation.

Direct decomposition of nitric oxide over perovskite-type catalysts: Part II. Effect of oxygen in the feed on the activity of three selected compositions

Direct decomposition of nitric oxide over three selected perovskites, both in the absence and the presence of oxygen added to the reaction gas mixture, was investigated at a steady state under a wide range of experimental conditions in a plug-flow reactor with 1 g catalyst. Temperatures between 723 and 923 K were covered. The catalysts (La 0.87Sr 0.13Mn 0.2Ni 0.8O 3− δ , La 0.66Sr 0.34Ni 0.3Co 0.7O 3− δ , and La 0.8Sr 0.2Cu 0.15Fe 0.85O 3− δ ) were prepared with specific surface area of 12.7, 9.0 and 10.1 m 2/g, respectively. When no oxygen was added to the feed, these perovskites showed an improved activity in comparison with other literature known materials, except Cu-ZSM-5 zeolites. Oxygen has an inhibiting effect, which depends strongly on temperature and varies with flow-rate (contact time) and composition. It is relatively weak at lower temperatures and higher contact times and increases between about 773 and 873 K. Between 873 and 923 K, where the oxygen inhibition decreases with temperature, the rate of decomposition can be best expressed by a simple model: r= k NO P NO/(1+ KP O 2 ). For the low temperature range, the NO decomposition is best described by a model, in which oxygen is an inhibitor but also reactant. Overall data analysis permits to assume that oxygen takes part in the formation of an intermediate giving dinitrogen and that inhibition by oxygen is caused by its parallel participation in the formation of surface nitrates.

Microscopic studies of NO 2 on Ag( [formula omitted])–p(2×1)-O and reactivity of surface nitrate

It has been shown previously with temperature programmed reaction spectroscopy (TPRS) and high resolution electron energy loss spectroscopy that NO 2 adsorption on Ag(1 1 0)–p(2×1)-O at room temperature leads to the formation of surface nitrate, NO 3(a), which, when heated, decomposes into NO(g) and N(a) at 475 K; the latter recombines with O(a) to form NO(g) at 530 K. We have reexamined this reaction using scanning tunneling microscopy in combination with TPRS and low energy electron diffraction (LEED). The present study shows that NO 3(a) formed at room temperature yields straight rows at low coverage and jagged rows at high coverage along the [0 0 1] direction, which exhibit p(4×1) and c(6×2) LEED patterns, respectively. Nitrate formation results in the formation of pits one layer deep on about 18% of the terrace area and severely roughens step edges, indicative of the incorporation of silver atoms into the nitrate structure. This surface nitrate species is found to be reactive toward CO oxidation. Annealing the nitrate adlayer to 485 K results in the formation of zigzag rows comprised of N(a) and O(a) atoms. Oxygen in this structure is not reactive with CO. Surface structure models are suggested for adsorbed nitrate and coadsorbed N and O.

FTIR study of CO and NO adsorption and coadsorption on a Cu/SiO2 catalyst: Probing the oxidation state of copper

At room temperature CO is adsorbed strongly on Cu+ sites (band at 2131 cm−1) whereas NO is preferably adsorbed on Cu2+ sites (band at 1882 cm−1). Coadsorption of CO and NO allows simultaneous and selective detection of both kinds of cations. This observation is used to follow the changes on the sample surface occurring in the presence of oxygen. Addition of small amounts of O2 to the CO–NO–Cu/SiO2 system first leads to the oxidation of the Cu+ sites to Cu2+. This process is followed by formation of surface nitrates which block the Cu2+ sites for NO adsorption. Adsorption of CO at 85 K allows detection of Cu2+ cations (ca. 2200 cm−1) in addition to the Cu+, CO is replaced by NO from these sites. Cu0 sites form carbonyls which, when copper is highly dispersed, can absorb at the same frequency at which Cu+–CO carbonyls are detected. In this case both kinds of species could be distinguished by their stability: the Cu0–CO species are easily destroyed during evacuation.