NO Temperature-programmed Desorption Research Articles

The Remarkable Enhancement of CO‐Pretreated CuOMn2O3/γ‐Al2O3 Supported Catalyst for the Reduction of NO with CO: The Formation of Surface Synergetic Oxygen Vacancy

NO reduction by CO was investigated over CuO/γ-Al2O3, Mn2O3/γ-Al2O3, and CuOMn2O3/γ-Al2O3 model catalysts before and after CO pretreatment at 300 °C. The CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst exhibited higher catalytic activity than did the other catalysts. Based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse reflectance spectroscopy (DRS), Raman, and H2-temperature-programmed reduction (TPR) results, as well as our previous studies, the possible interaction model between dispersed copper and manganese oxide species as well as γ-Al2O3 surface has been proposed. In this model, Cu and Mn ions occupied the octahedral vacant sites of γ-Al2O3, with the capping oxygen on top of the metal ions to keep the charge conservation. For the fresh CuO/γ-Al2O3 and Mn2O3/γ-Al2O3 catalysts, the -Cu-O-Cu- and -Mn-O-Mn- species were formed on the surface of γ-Al2O3, respectively; but for the fresh CuO-Mn2O3/γ-Al2O3 catalyst, -Cu-O-Mn- species existed on the surface of -Al2O3. After CO pretreatment, -Cu-□-Cu- and -Mn-□-Mn- (□ represents surface oxygen vacancy (SOV)) species would be formed in CO-pretreated CuO/γ-Al2O3 and CO-pretreated Mn2O3/γ-Al2O3 catalysts, respectively; whereas -Cu-□-Mn- species existed in CO-pretreated CuO-Mn2O3/γ-Al2O3. Herein, a new concept, surface synergetic oxygen vacancy (SSOV), which describes the oxygen vacancy formed between the individual Mn and Cu ions, is proposed for CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst. In addition, the role of SSOV has also been approached by NO temperature-programmed desorption (TPD) and in situ FTIR experiments. The FTIR results of competitive adsorption between NO and CO on all the CO-pretreated CuO/γ-Al2O3, Mn2O3/γ-Al2O3, and CuO-Mn2O3/γ-Al2O3 samples demonstrated that NO molecules mainly were adsorbed on Mn2+ and CO mainly on Cu+ sites. The current study suggests that the properties of the SSOVs in CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst were significantly different to SOVs formed in CO-pretreated CuO/γ-Al2O3 and Mn2O3/γ-Al2O3 catalysts, and the SSOVs played an important role in NO reduction by CO.

Novel binderless zeolite-coated monolith reactor for environmental applications

A novel structured catalyst based on a binderless zeolite film was developed onto a cordierite monolith. The zeolitic film was obtained performing a binderless washcoating which was followed by a hydrothermal treatment, which resulted in a coating with hybrid physicochemical properties, having a porous washcoat-like microstructure with an intergrowth between the aggregates. This combination confers to the zeolite coating an open structure which is an advantage of washcoats and a high mechanical stability characteristic of the zeolite growths. It should be emphasized that these desired properties are obtained without the use of binders. The physico-chemical properties were investigated through MIP (mercury intrusion porosimetry), SEM (scanning electron microscopy), ultrasonic adherence tests, NO-TPD (temperature-programmed desorption of NO) and EPMA (electron probe microanalysis). The catalytic properties of the monolith were evaluated in the selective catalytic reduction of NO x at high spatial velocities, showing promising results.

NO evolution reaction with NO 2 adsorption over Fe/ZSM-5: In situ FT-IR observation and relationships with Fe sites

The evolution of NO following NO 2 introduction onto various Fe/ZSM-5 catalysts was investigated. The activity depended on the method of catalyst preparation used. The most efficient method was CVD with high-Fe loading. Results from in situ FT-IR revealed that NO was initially generated on ion-exchanged Fe sites, temporarily adsorbed, and then desorbed. For the most active samples, the result of temperature-programmed desorption of NO 2, from which ion-exchanged sites may be estimated, indicated that the amount of NO evolution corresponded to nearly half the amount of exchanged Fe. Assuming that most of the exchange sites in these samples are binuclear structures, it may be postulated that one NO molecule evolves from each binuclear site. From this, we propose that NO 2 is decomposed into NO and atomic oxygen at the binuclear Fe sites. The oxygen occupies a bridging vacancy site between Fe atoms. NO evolves and is replaced by follow-on NO 2.

Investigation of oxide catalysts activity in the NO x neutralisation with organic reductants

Temperature-programmed desorption of NO (TPD) and temperature-programmed reduction of NO by propene and ethanol (TPSR) over the catalyst with redox properties and the structure of spinels – Co 3O 4 and CoFe 2O 4 – were investigated. The TPD experiments can determine the temperature range in which the deNO x proceeds and provide information about the heat of adsorption of NO on the surface of catalysts. The values of the adsorption heat are much higher in the case of the catalyst with weaker redox properties (CoFe 2O 4), because of stronger bonds between the adsorbate and the surface of catalyst. The TPD and TPSR experiments show that ethyl alcohol is a more active reductant in the deNO x process than propene. The maximum of NO x conversion is higher with ethyl alcohol used as a reductant in both cases of investigated catalysts. Moreover, the temperature of the maximum degree of NO x reduction is lower in the case of alcohol used as an reductant. Co 3O 4 is a more active and suitable catalyst for the deNO x process than CoFe 2O 4.

A comparative investigation of NdSrCu 1− xCo xO 4−δ and Sm 1.8Ce 0.2Cu 1− xCo xO 4−δ ( x: 0–0.4) for NO decomposition

A series of single-phase T-structured NdSrCu 1− x Co x O 4−δ with oxygen vacancies and T′-structured Sm 1.8Ce 0.2Cu 1− x Co x O 4−δ ( x: 0–0.4) with oxygen excess were prepared using ultrasound-assisted citric acid complexing method, and characterized by means of techniques such as thermogravimetric analysis and NO temperature-programmed desorption (NO-TPD). The catalytic activities of these materials were evaluated for the decomposition of NO. It was found that the NdSrCu 1− x Co x O 4−δ catalysts were of oxygen vacancies whereas the Sm 1.8Ce 0.2Cu 1− x Co x O 4−δ ones possessed excessive oxygen (i.e., over-stoichiometric oxygen); with a rise in Co doping level, the oxygen vacancy density of NdSrCu 1− x Co x O 4−δ decreased while the over-stoichiometric oxygen amount of Sm 1.8Ce 0.2Cu 1− x Co x O 4−δ increased. The NO-TPD results revealed that NO could be activated much easier over the oxygen-deficient perovskite-like oxides than over the oxygen-excessive perovskite-like oxides, with the NdSrCuO 3.702 catalyst showing the best efficiency in activating NO molecules. Under the conditions of 1.0% NO/helium, 2800 hr −1, and 600–900°C, the catalytic activity of NO decomposition followed the order of NdSrCuO 3.702 > NdSrCu 0.8Co 0.2O 3.736 > NdSrCu 0.6Co 0.4O 3.789 > Sm 1.8Ce 0.2Cu 0.6Co 0.4O 4.187 > Sm 1.8Ce 0.2Cu 0.8Co 0.2O 4.104 > Sm 1.8Ce 0.2CuO 4.045, in concord with the sequence of decreasing oxygen vacancy or oxygen excess density. Based on the results, we concluded that the higher oxygen vacancy density and the stronger Cu 3+/Cu 2+ redox ability of NdSrCu 1− x Co x O 4−δ account for the easier activation of NO and consequently improve the catalytic activity of NO decomposition over the catalysts.

Selective Catalytic Reduction of NO in Excess Oxygen by Methane over Mn/ZSM-5 Catalysts

The selective catalytic reduction of NO in excess O 2 by CH 4 was investigated over a series of ZSM-5-supported manganese catalysts (Mn/ZSM-5). The catalytic activity depended on preparation method and Mn loading. An ion exchange method resulted in a higher activity than an impregnation method. Catalytic activity increased with increasing Mn loading until a Mn loading of 2.06% that gave a maximum NO conversion of 57.3%. H 2 temperature-programmed reduction results showed that the ion exchange method and low Mn loadings (≤ 2.06%) restricted the formation of non-stoichiometric MnO x (1.5 < x < 2) species that have higher oxidative activity, and thus suppressed the combustion of CH 4 by O 2 , which increased the selectivity for NO reduction. In a SO 2 containing stream, a substantial decrease in NO conversion was seen at ≤ 550 °C but not at ≥ 600 °C. SO 2 temperature-programmed surface reaction and NO temperature-programmed desorption demonstrated that sulfur species were formed at temperature ≤ 550 °C, and these covered active sites and decreased the catalytic activity. The sulfur species desorbed at ≥ 600 °C, thus the addition of SO 2 did not have a significant impact on the catalytic activity. ：考察了富氧条件下 Mn/ZSM-5 催化剂上 CH 4 选择催化还原 NO 反应, 并采用 H 2 程序升温还原、SO 2 程序升温表面反应和 NO程序升温脱附等手段对催化剂进行了表征. 结果表明, 催化剂活性与制备方法和 Mn 负载量密切相关. 离子交换法制备的 Mn/ZSM-5 催化剂活性明显优于浸渍法制备的催化剂; NO 转化率随着 Mn 负载量的增加而增加, 至 2.06% 时达到最大值 (57.3%), 然后随着 Mn 负载量的增加而降低. 采用离子交换法或较低 Mn 负载量 (≤ 2.06%) 抑制了催化剂中非化学计量的 MnO x (1.5 < x < 2) 物种的形成, 减缓了 CH 4 的氧化燃烧反应, 因而 CH 4 还原 NO 的选择性提高. 在含 SO 2 体系中, Mn/ZSM-5 活性在 550 °C 以下时明显下降, 但在 600 °C 以上基本不受影响. 这是由于在 550 °C 以下时 SO 2 在 Mn/ZSM-5 表面形成了稳定的吸附硫物种, 覆盖了部分活性位, 导致催化剂活性降低; 而在 600 °C 以上时含硫物种基本脱附完全, 因而对催化剂活性影响不大. Mn/ZSM-5 prepared by the ion-exchange method with a Mn loading of 2.06% exhibited the maximum NO conversion and was more tolerant to SO 2 poisoning at higher temperature.

Influence of CeO 2 and La 2O 3 on properties of palladium catalysts used for emission control of natural gas vehicles

Pd/YZ-Al 2O 3 (Y and Zr modified Al 2O 3, and hereafter, labelled as Al) catalysts with 4 wt% additive CeO 2 and/or La 2O 3 were prepared and characterized by X-ray photoelectron spectroscopy (XPS), NO-temperature programmed desorption (NO-TPD), N 2-adsorption/desorption (Brunauer-Emmet-Teller BET method), X-ray diffraction (XRD) and CO-chemisorption. Catalytic activities for CH 4, CO and NO conversion were tested in a gas mixture simulated the emissions from natural gas vehicles (NGVs) operated under stoichiometric conditions. The results indicated that all catalysts exhibited excellent catalytic performances for CH 4 and CO oxidation and the promoting effect of CeO 2 or La 2O 3 was significant for NO conversion. XPS results showed that the electron density around Pd was increased by CeO 2 and/or La 2O 3, the binding energy of Pd 3d decreased as the order: Pd/Al > Pd/Ce/Al > Pd/La/Al > Pd/CeLa/Al. The electron-rich Pd showed Rh-like catalytic properties which exhibited good activity for the reduction of NO. NO-TPD results showed that the addition of CeO 2 and/or La 2O 3 increased NO adsorption on surface, and promoted the conversion of NO.

Detailed kinetic modeling of NOx adsorption and NO oxidation over Cu-ZSM-5

Detailed kinetic modeling was used in combination with flow reactor experiments to investigate the NOx adsorption/desorption and NO oxidation over Cu-ZSM-5. NO oxidation is likely an important step for selective catalytic reduction (SCR) using urea and hydrocarbons, and thus was investigated separately. First the NO2 adsorption on Brönstedt acid sites in H-ZSM-5 was modeled using an NO2 temperature programmed desorption (TPD) experiment. These results, together with the results of the NO2 TPD and NO oxidation experiments, were used in developing the model for Cu-ZSM-5. A substantial amount of NO2 was adsorbed on the catalyst. However, the results from a corresponding NO TPD experiment showed that only very small amounts of NO were adsorbed on the catalyst and therefore this step was not included in the model. The model consists of reversible steps for NO2 and O2 adsorption, O2 dissociation, NO oxidation and two steps for nitrate formation. The first nitrate formation step was disproportionation of NO2 to form NO and nitrates. This step enabled us to describe the NO production during NO2 adsorption. Further, in the reverse step the NO reacts with the nitrates and decreased their stability. Without this step the nitrates blocked the surface resulting in to low NO oxidation activity. However, we observe that nitrates can be decomposed also without the presence of NO and in the second reversible step were the nitrates decomposed to form NO2 and oxygen on the copper. These steps enabled us to describe both the TPD and activity measurement results. NO oxidation was observed even at room temperature. Interestingly, the NO2 decreased when increasing the temperature up to 100°C and then increased as the temperature increased further. We suggest that this low-temperature NO oxidation occurs with species loosely bound on the surface and that is included in the detailed mechanism. An additional NO2 TPD at 30°C was also modeled to describe the loosely bound NO2 on the surface. The detailed model correctly describes NO2 storage, NO oxidation and low-temperature NO oxidation.

DRIFT study of the interaction of NO and O 2 with the surface of Ce 0.62Zr 0.38O 2 as deNO x catalyst

Gas–solid interactions and surface intermediates evolution after NO adsorption onto calcined Ce 0.62Zr 0.38O 2 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 0.38O 2 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.

Formation of the Surface NO during N2O Interaction at Low Temperature with Iron-Containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.

The role of lanthana loading on the catalytic properties of Pd/Al2O3-La2O3 in the NO reduction with H2

The role of La2O3 loading in Pd/Al2O3-La2O3 prepared by sol–gel on the catalytic properties in the NO reduction with H2 was studied. The catalysts were characterized by N2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO.The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV=72,000h−1) and the extent of interaction between Pd and La2O3. At NO/H2=0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250°C. For NO/H2=1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350°C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (∼1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250°C) favoring the formation of N2.

The influence of steps on the dissociation of NO on Pt surfaces: Temperature-programmed desorption studies of NO adsorption on Pt{211}

Temperature-programmed desorption (TPD) has been used to investigate the adsorption of NO on Pt{211} at 300 K and 120 K. Results show that NO dissociation occurs readily on Pt{211}, as evidenced by the observation of N2 and N2O in the TPD spectrum. Following adsorption at 120 K three NO TPD peaks at 338, 416, and 503 K are observed, in agreement with previous observations. In combination with data acquired in a recent reflection absorption infrared spectroscopy and density functional theory investigation of NO/Pt{211}, these peaks are assigned to the desorption of NO from an O–NO complex, the recombinative desorption of N and O atoms, and to desorption of a step-bridged NO species, respectively. These assignments are in disagreement with previous work, where the high-temperature NO peak was assigned to the desorption of step bound NO and the two low-temperature peaks were assigned to the desorption of NO from terrace sites. TPD spectra recorded following adsorption at 300 K, with a heating rate of 1 K s−1, show similar features to those recorded following 120 K adsorption. This is also in disagreement with previous observations, where only two NO TPD peaks were observed following adsorption at room temperature. This disagreement can be accounted for by the different heating rates used in the two experiments.

TEMPERATURE PROGRAMMED DESORPTION-MASS SPECTROMETRY STUDY OF NO DESOrPTION AND DECOMPOSITION BY TITANIA SUPPORTED 12-TUNGSTOPHOSPHORIC ACID

Titania supported 12-tungstophosphoric acid was examined for NO temperature-programmed desorption and decomposition. More than 90 % of absorbed NO decomposed into N2 without evident desorption of O2.

Copper supported on mixed alumina/gallium oxide pillared α-tin phosphate for De-NOx applications

Alumina- and mixed alumina/gallium oxide-pillared α-tin phosphate have been impregnated with different amounts of copper via the incipient wetness method. Their characterization by X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NO temperature-programmed desorption has allowed to gain insight into the nature of the copper species. These are present as small CuO clusters (not detectable by XRD) as well as forming part of spinel-like structures. NO-TPD studies show that only NO is desorbed from all the catalysts. Ga3Al11-SnP with 4.9 wt% Cu interacts most strongly with NO and this catalyst exhibits the highest degree of reduction of Cu2+ to Cu+ after the catalytic reaction. The catalysts are active for the selective catalytic reduction (SCR) of nitric oxide with propane in the presence of excess oxygen.

Adsorption and thermal behavior of CO and NO on Pd{110} and Pd{320}

Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)×(100)]], have been investigated using molecular beam surface scattering (MBSS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temperature range of 250–500 K. No evidence for long-range ordering of CO adlayers on Pd{320} was observed. Saturation coverage for CO/Pd{320} at 300 K is 0.61 ML. The initial sticking probability of CO is 0.91 and is shown to depend only weakly on the surface temperature. It is also found that the initial sticking probability of CO on Pd{110} exhibits a strong dependence on the kinetic energy of CO molecules and the CO adsorption on Pd{320} may also exhibit this same behavior. CO desorbs molecularly from Pd{320} with desorption features near 390 and 500 K. NO is shown to adsorb molecularly below 420 K with a high initial sticking probability close to 0.9. The initial sticking probability is largely temperature independent up to 450 K. No long-range order was observed from NO/Pd{320} in the temperature range of 300–600 K. The saturation coverage of NO is 0.64 ML at 300 K. It is demonstrated that NO partially dissociates upon heating, yielding N2 and N2O. Desorption of molecular NO occurs on both surfaces between 300–600 K, with N2 and N2O desorbing above 450 K. In contrast to Pd{110}, a high temperature N2 peak near 600 K is seen during NO TPD from Pd{320}, which is attributed to tightly bound, adsorbed nitrogen atoms. The relative TPD yields of N2 and N2O from Pd{110} and Pd{320} are found to be strongly dependent on the initial NO coverage with enhanced dissociation at lower coverages. Pd{320} is found relatively unreactive towards the thermal dissociation of NO. The activity of Pd{110} and Pd{320} towards NO dissociation upon adsorption increases with surface temperature. The relative yields of N2 and N2O reach a maximum near 550 K of ∼40% and ∼20%, respectively. By direct comparison to NO/Pd{110}, the present study shows that the presence of (100) steps on the Pd{320} surface does not significantly enhance its reactive properties with respect to adsorption and thermal behavior of CO and NO.

The effect of NO adsorption on the chemical state of promoted hydrotreating catalysts

The chemical state of the active phase (Ni–Mo–S and Ni–W–S) of sulfided NiMo and NiW catalysts during the interaction with NO is investigated by extended X-ray absorption fine structure (EXAFS). The results show that approximately 30 % of the Ni atoms in the active phase are oxidized already at room temperature (RT) upon NO exposure. On the average, the chemical state of the W atoms in sulfided NiW/Al 2O 3 is hardly changed. Essentially all the Ni atoms in the active Ni–Mo–S phase are oxidized above 150°C. Although the present results indicate that significant structural and chemical changes occur when NO is used as a probe molecule in activated hydrotreating catalysts, NO temperature-programmed desorption (TPD) and NO chemisorption can, nevertheless, provide important insight into the nature of the active phase.

Growth and Chemical Properties of Layered, Binary Mixed Oxides: NO on MgO−NiO and CaO−NiO

The growth mode of layered, binary mixed oxides consisting of NiO, MgO, and CaO has been studied using low-energy electron diffraction (LEED), ion scattering spectroscopy (ISS), and NO temperature-programmed desorption (NO-TPD). The NiO/MgO layered system shows greater long-range order relative to NiO/CaO. The superior wetting found for NiO/MgO compared to NiO/CaO is likely due to the close lattice match of NiO/MgO, in contrast to NiO/CaO. The influence of MgO and CaO on NO adsorption on NiO has been studied using TPD and infrared reflection absorption spectroscopy (IRAS) and correlated with the growth mode of the corresponding layered oxide system. A long-range, substrate-mediated interaction has been found between NO and coadsorbed MgO (or CaO) on the NiO(100)/Mo(100) surface.

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH 4+O 2 (lean-NO x) reactions

Mixed oxides of the general formula La 0.5Sr x Ce y FeO z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3− x and oxides α-Fe 2O 3 and CeO 2 depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4+O 2 (“lean-NO x ”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5Sr x Ce y FeO z materials to reduce NO by CO or by CH 4 under “lean-NO x ” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s −1) at 300°C (based on NO chemisorption) revealed values comparable to Rh/α-Al 2O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5Sr x Ce y FeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5Sr 0.2Ce 0.3FeO z and La 0.5Sr 0.5FeO z solids.

Adsorption and Decomposition of NO on Lanthanum Oxide

The adsorption behavior of NO on La2O3, an effective catalyst for selective NO reduction with CH4 at temperatures above 800 K, depends upon the pretreatment as indicated by temperature-programmed desorption (TPD) and diffuse reflectance FTIR spectroscopy (DRIFTS). The use of isotopic 18O2 exchange and adsorption showed that oxygen adsorbed dissociatively by filling oxygen vacancies and that both oxygen vacancies and lattice oxygen were mobile at high temperature. Oxygen pair vacancies were assumed to be created by desorption of molecular oxygen and, upon cooling, a certain distribution of pair and single vacancies exists at the surface as the pair vacancies can rearrange due to oxygen ion migration. After La2O3 was pretreated at 973 K in He, exposure to NO at 300 K caused a brief reaction forming N2O, then gave three NO TPD peaks at 400, 700, and 800 K. The only O2 desorption occurred during the 800 K NO peak and gave an NO/O2 ratio near unity. Oxygen chemisorption prior to NO admission eliminated the formation of N2O during NO adsorption at 300 K, blocked the sites giving NO desorption at 700 K, but enhanced the NO and O2 peaks at 800 K. TPD after 15N16O adsorption on an La2O3 surface containing exchanged 18O lattice anions, but no chemisorbed O atoms, showed that both 15N16O and 15N18O desorbed at 400 K, but only 15N16O was present in the 700 and 800 K desorption peaks, and 16O2 again desorbed at 800 K. When both lattice exchange and chemisorption of 18O2 on the La2O3 surface were allowed before 15N16O adsorption, 15N18O was desorbed at 400 and 800 K while 16O2, 16O18O, and 18O2 were also desorbed at 800 K; thus the NO peak at 400 K involves exchange with surface lattice oxygen atoms, while the 800 K peak involves exchange with chemisorbed oxygen atoms. DRIFTS indicated the presence of anionic nitrosyl (NO−), hyponitrite (N2O2)2−, chelated nitrite (NO2−), nitrito (ONO−), and bridging and monodenate nitrate (NO3−) species. Consequently, the three NO TPD peaks were assigned as follows: 400 K, decomposition of nitrito, nitro, and bidentate nitrate species; 700 K, desorption from NO− and (N2O2)2− species; and 800 K, decomposition of monodenate nitrate species into NO and O2. A model of the La2O3 surface based on the (001) and (011) crystal planes is proposed to account for these different sites. Two types of oxygen pair vacancy sites with a different O–O separation appear to exist, with one forming (N2O2)2− species, and four additional sites—(1) an oxygen single vacancy, (2) a single vacancy and a lattice oxygen atom, (3) a coordinative unsaturated lattice oxygen atom, and (4) adjacent lattice oxygen atoms—are proposed to explain the formation of NO−, nitrito (M–ONO−), chelated nitrites, and bridging nitrate species, respectively. Among these species, (N2O2)2− was detected by DRIFTS under reaction conditions at 800 K and is most likely to be an active intermediate during NO decomposition. Monodentate nitrate species are also observed at 800 K, but are very stable and still present after purging at 800 K.

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.