NO Temperature-programmed Desorption Research Articles

Pd–Ce interactions and adsorption properties of palladium: CO and NO TPD studies over Pd–Ce/Al2O3 catalysts

We have examined the adsorption of CO and NO on powder Pd/Al2O3, Pd–Ce/Al2O3 and CeO2/Al2O3 catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al2O3 TPD profiles are identical to those observed for Pd/Al2O3, suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al2O3 and Pd–Ce/Al2O3. On oxidized catalysts, Pd/Al2O3 is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al2O3 and react to N2O and N2. In comparison with Pd/Al2O3, reduced Pd–Ce/Al2O3catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N2 over N2O.

Reaction of Nitric Oxide Adsorbed on Platinum Clusters in X Zeolites. Effect of Cluster Size and Nature of Alkali Cations

Adsorption and temperature-programmed desorption of NO was studied over Pt/alkali zeolites. Platinum clusters of three different sizes were prepared by different decomposition routes of [Pt(NH3)4]2+ partially ion-exchanged in NaX zeolite. The effect of various alkali ions (Li, Na and K) was also examined. Similarly to platinum supported by alumina, the conversion of adsorbed NO to nitrogen increases with increasing Pt particle size. Increasing electropositivity of the alkali cations shifts the desorption to lower temperatures.

Temperature-programmed desorption of NO adsorbed on Mn 2O 3 and Mn 3O 4

When heated to 873 K, large amounts of H 2O along with lesser amounts of CO 2 and O 2 desorbed from single-phase samples of Mn 2O 3 and Mn 3O 4 to form O vacancies and create coordinatively unsaturated (CUS) cations. Exposure of these surfaces to NO at 300 K produced a certain amount of N 2O, but no other detectable gas-phase products, with more formation over Mn 2O 3 (0.72 × 10 18 molecule N 2O/m 2) than that over Mn 3O 4 (0.10 × 10 18 molecule N 2O/m 2). Temperature-programmed desorption (TPD) spectra with Mn 2O 3 after this exposure gave three principal NO peaks at 390 (α), 470 (β) and 650 K (γ), while only one O 2 peak occurred at 650 K concomitant with the γ NO peak. A weak N 2 peak at 650 K was also observed. This O 2 peak was not obtained with Mn 3O 4. The total amount of NO desorbed from either surface was typically 1.6 × 10 18 − 2.0 × 10 18 molecule/m 2 and the NO/O 2 ratio of the concomitant γ NO and O 2 peaks was routinely near unity. Based on these results and earlier studies, the 650 K peak appears to be the decomposition of surface nitrate groups whereas the α and β NO peaks are assigned to dinitrosyl and mononitrosyl species, respectively, on CUS sites. The nitrate groups do not seem to be involved in the catalytic cycle for NO decomposition above 773 K, whereas the latter two species may well be. A reaction intermediate containing two NO molecules is postulated, and three proposed models provide rate expressions which fit the data well, with the model invoking the decomposition of an (NO) 2 species on a CUS single site giving the best fit.

Interaction between Nitric Oxide and Copper on Copper Zirconia Sulfated Catalysts

The room temperature adsorption and temperature-programmed desorption of NO have been performed on unsulfated and sulfated copper zirconia solids (Cu/SZ). Only CuO clusters are present on Cu/ZrO 2 and Cu/SZ prepared by the ligand exchange technique with Cu acetylacetonate in organic medium. The sulfation of zirconia induces a stabilization of the mononitrosyl species, adsorbed on Cu, up to 333 K, and a shift of v No from 1872 to 1910 cm -l , which shows a decrease of the electron density at the copper sites. Moreover, it inhibits the formation of nitrito or nitrato complexes on copper. This behavior shows the strong interaction between copper and sulfate species and can be viewed either by a dilution phenomenon of the CuO surface by migrating sulfate species or by migration of sulfate anions on the CuO surface.

Combined infrared and mass spectrometric study of reactions of adsorbed NO and CO on 0.5 wt% Rh/SiO 2 catalyst

NO adsorption, CO adsorption, NO temperature-programmed desorption and decomposition (TPD), and temperature-programmed reaction (TPR) of NOCO have been studied over 0.5 wt% Rh/SiO 2 catalysts by a combined infrared and mass spectrometric technique. Infrared study reveals that the high wavenumber RhNO δ− at 1723–1740 cm −1 is the dominant adsorbate during TPD and TPR with NO:CO = 1:1. During TPR, CO reduces part of Rh surface resulting in the formation of the low wavenumber Rh-NO δ−1 at 1634–1680 cm −. Increasing CO partial pressure (i) promotes the formation of Rh 0 sites, producing linear CO, (ii) increases the selectivity to N 2O below the light-off temperature, (iii) raises the light-off temperature, and (iv) promotes the formation of SiNCO and RhNCO. Comparison of results of the present study with those of previous studies on 4 wt% Rh/SiO 2 shows that different dispersion of Rh crystallites on SiO 2 support results in significant variation in chemisorptive and reactive properties of Rh metal for the NOCO reaction.

Catalytic behaviour of Cu/ZrO2 and Cu/ZrO2(SO42−) in the reduction of nitric oxide by decane in oxygen-rich atmosphere

The selective catalytic reduction (SCR) of NO by decane under an oxidising atmosphere has been carried out on Cu/ZrO2 and Cu/ZrO2(SO42−). Zirconia-supported Cu catalysts were prepared by ligand exchange with Cu acetylacetonate followed by calcination at 773 K. The solids obtained were characterised by temperature programmed reduction (TPR) by hydrogen and temperature programmed desorption (TPD) of NO. Cu/ZrO2 is active and selective in the reduction of NO by decane at low temperature (< 600 K) but oxidises NO to NO2 between 640 and 770 K. By contrast, whatever the temperature, a total selectivity to nitrogen is obtained with Cu/ZrO2(SO42−). About 40% NO conversion to N2 is observed with GHSV of 70 000 h−1. The promoting effect of sulfate is attributed to the large increase of acidity and the strong interaction between copper and sulfur species which is evidenced by TPD of NO and TPR by H2.

Infrared studies of NO adsorption and co-adsorption of NO and O 2 onto cerium-exchanged mordenite (CeNaMOR)

Infrared (IR) studies on NO adsorption and co-adsorption of NO and O 2 onto cerium- and lanthanum-exchanged mordenite (CeNaMOR and LaNaMOR, respectively) were performed in order to elucidate the role of redox properties of cerium ( Ce III Ce IV ) in the oxidation of NO to NO 2, an important preliminary step in the NO reduction catalysis. NO adsorption onto both CeNaMOR and LaNaMOR leads to the formation of N 2O (2247 cm −1), NO + (nitrosonium ion; 2162 cm −1) and NO x − species ( nitrito or/and nitrato, 1300–1500 cm −1). These are thought to arise from disproportionation of NO towards N 2O and N 2O 3 and the subsequent ionization of N 2O 3 towards NO + and NO 2 −. This scheme is supported by the transient observation of molecular N 2O 3. The co-adsorption of NO and O 2 onto CeNaMOR and LaNaMOR resulted in the enhanced formation of NO + and NO 3 − (1515, 1488–1497 and 1333 cm −1), which is accounted for by the formation of NO 2 and its subsequent ionization via N 2O 4 towards NO + and NO 3 −. Combining IR and NO temperature-programmed desorption (TPD) data, it is proposed that the formation of NO + is associated with zeolite acid sites, and that NO 3 − ( nitrato) species are coordinated to lanthanide cations. Furthermore, the NO + and NO x − species were found to desorb more easily from CeNaMOR than from LaNaMOR. The redox properties of cerium ( Ce III Ce IV ) may contribute to the easier desorption of these oxidized NO species.

Temperature-programmed desorption of nitric oxide over Cu-ZSM-5

Room temperature adsorption and temperature-programmed desorption (TPD) of NO over a Cu-ZSM-5 catalyst active for NO decomposition were investigated. Even at 25° CN 2O and N 2 were observed as reaction products during NO adsorption, but they ceased when NO adsorption was completed. During NO TPD, N 2O, NO and O 2 desorption peaks were observed. A relatively low temperature oxygen desorption peak was detected at ca. 360° C. No decomposition was detected without copper present. NO TPD was also collected over reduced and oxidized catalysts. In addition nitrous oxide and NO 2 adsorptions and their TPDs were obtained over the catalyst in order to understand the interaction of NO with the catalyst. TPD studies with NO 2 implicate the presence of NO 2 on the catalyst.

The adsorption of nitric oxide and nitrogen dioxide on polycrystalline platinum

NO and NO 2 adsorption on a polycrystalline platinum surface was studied using temperature programmed desorption (TPD). NO adsorption is irreversible for NO exposures of less than 0.12 L at 120 K yielding only N 2 and O 2 upon heating. However, when the NO exposure is increased to 0.12 L and above, reversible NO adsorption is observed. Two NO desorption states occur with peaks at 470 and 300 K. No desorption of NO 2 resulting from oxidation of NO was observed at any coverage. NO 2 adsorption at 120 K is largely irreversible below monolayer saturation coverages. However, a small NO 2 desorption peak is observed at 300 K at a monolayer coverage of NO 2. NO and O 2 TPD spectra following NO 2 exposures are quite different from those spectra obtained following NO or O 2 exposures. The differences in the NO TPD spectra are due to the influence of coadsorbed O atoms on the adsorption and decomposition of both NO and NO 2. Changes in the O 2 TPD spectra are due to the very high coverages of adsorbed O atoms that can be obtained from NO 2 exposures.

NO/NH 3 coadsorption on Pt(111): Kinetic and dynamical effects in rotational accommodation

The strongly interacting coadsorbate system of ammonia/nitric oxide on Pt(111) has been examined using both quadrupole mass spectrometer (QMS) and laser-induced fluorescence (LIF) detected temperature programmed desorption (TPD) methods. The QMS/TPD experiments show simultaneous desorption of NO and NH 3 as reaction-limited products from the dissociation of an adsorbed NONH 3 complex. Although NO TPD kinetics are altered by the formation of this complex, LIF/TPD results show that the rotational accommodation, spin-orbit temperature, lambda doublet populations, and molecular alignment of desorbed NO are insensitive to this attractive interaction. This suggests that, following decomposition of the NONH 3 species, NO resides on the surface sufficiently long that it has no memory of being complexed.

The NO + CO reaction on Pt(100)

The interaction of NO with CO and with H2 on Pt(100) was studied by temperature programmed desorption (TPD), isothermal desorption mass spectrometry, and low energy electron diffraction (LEED), TPD of NO and CO coadsorbed at 120 K yields almost complete reaction with both N2 and CO2 products desorbing as sharp, simultaneous peaks at ≈ 410 K. with full widths at half maximum as narrow as 3 K. Isothermal desorption mass spectrometry yields N2 and CO2 rates that exhibit a maximum with time. Both experiments indicate that the reaction mechanism is autocatalytic. Annealing NO-CO adlayers formed at 120 K to temperatures above 300 K causes the subsequent N2 and CO2 TPD peaks to broaden.'TPD of NO coadsorbed with H2 yields sharp N2 and H2O product peaks that closely resemble the N2 and CO2 peaks observed in the NO + CO reaction. LEED experiments during TPD and isothermal desorption showed that the (1 × 1) → hex substrate phase transformation sometimes accompanies desorption of N2 and CO2. The TPD and isothermal desorption results can be fit by two simple models: chemical autocatalysis, in which an intermediate chemical species participates in a “chain propagation” reaction, and structural autocatalysis, which involves the formation of a reactive intermediate structure involving Pt atom displacements.

Decomposition of NO on clean Pt near atmospheric pressures: II. Adsorbate coverages

Temperature programmed desorption (TPD) and temperature programmed adsorption (TPA) have been used to characterize adsorbate coverages during and after NO decomposition on polycrystalline Pt foils at pressures between 10 −4 and 30 Torr. The densities and stoichiometries of tightly bound species were determined after reaction by TPD of NO, N 2, and O 2 following cooling and pumpdown to <10 −8 Torr. For characterization during reaction at pressures up to 10 −3 Torr the ribbon was flashed inside a 35 cm 3 reaction cell, and desorption and adsorption spectra of all species were recorded. Using digital acquisition of pressures versus time, peaks as small as 10 −3 of the background pressure could be analyzed. By flashing to different fixed temperatures, adsorption isobars during reaction were determined. These measurements show that there is a tightly bound stoichiometric layer of N and O (perhaps undissociated) and that the reactive state is weakly bound and appears to be strongly inhibited by molecular oxygen. This model also agrees with reaction rate measurements at these pressures.