Temperature-programmed Reaction And Desorption Research Articles

Isotopically labeled oxygen studies of the NO x exchange behavior of La 2CuO 4 to determine potentiometric sensor response mechanism

The harmful nature of nitrogen oxides (NO x ) to environmental systems necessitates sensors to detect their presence. One effective means is the use of solid state sensors. These sensors are compact, rugged, and can be inexpensively fashioned, making them a viable option for measuring gas concentration. A non-Nernstian potentiometric sensor can be used to detect low concentrations of NO x gas in multiple regions of oxygen concentration. Understanding the mechanism for this behavior can assist in optimizing the sensor couple for a given application. Previous studies using Temperature Programmed Reaction and Desorption (TPR/TPD), along with IR and XPS analysis, have identified the formation of charge-building compounds that establish voltage on the sensing electrode when exposed to NO x gas. To further elucidate the mechanism, TPR and TPD were performed using isotopically labeled oxygen with simultaneous exposure to NO x . The material was examined under multiple gas conditions of NO x and 16O 2/ 18O 2 atmospheres, as well as varied levels of 18O enrichment in the lattice itself. Through these studies, it was determined that the formation of charged surface complexes occurs solely through the use of lattice oxygen (vs. gas phase O 2) with adsorbed NO x .

Isotopically Labeled Oxygen Studies of the Exchange Behavior of Nox Over La2cuo4

The harmful nature of nitrogen oxides (NOx) to environmental systems necessitates sensors to detect their presence. One effective means is the use of solid state sensors. These sensors are compact, rugged, and can be inexpensively fashioned, making them a viable option for measuring gas concentration. A non-Nernstian potentiometric sensor can be used to detect low concentrations of NOx gas in multiple regions of oxygen concentration. Understanding the mechanism for this behavior can assist in optimizing the sensor couple for a given application. Previous studies using Temperature Programmed Reaction and Desorption (TPR/TPD), along with IR and XPS analysis, have identified the formation of charge-building compounds that establish voltage on the sensing electrode when exposed to NOx gas. To further evaluate the mechanism, TPR and TPD were performed using isotopically labeled oxygen with simultaneous exposure to NOx.

Temperature-Programmed Reaction and Desorption of the Sensor Elements of a WO3 ∕ YSZ ∕ Pt Potentiometric Sensor

Temperature-programmed reaction (TPR) and desorption (TPD) were performed to determine the reaction and adsorption characteristics of , CO, , , and their mixtures on the sensor elements of a /yttria-stabilized-zirconia (YSZ)/Pt potentiometric sensor. The results are discussed, focusing on how sensor performance is influenced by catalytic and adsorptive behavior of the electrode materials. The sensor response is most likely given by a dissymmetry of the decomposition activity between the and Pt electrode. No significant adsorption of and its related molecules is observed during the TPD of and -containing gas mixtures. Enhanced NO response by coexisting CO agrees with the increased NO adsorption by the addition of CO, indicating that the NO response is related to gas adsorption behavior.

Chloromethane surface chemistry on Fe 3O 4(1 1 1)–(2 × 2): A thermal desorption spectrometry comparison of CCl 4, CBr 2Cl 2, and CH 2Cl 2

The surface chemistry of CBr 2Cl 2 on the Fe 3O 4(1 1 1)–(2 × 2) selvedge of single-crystal α-Fe 2O 3(0 0 0 1) has been investigated using temperature programmed reaction and desorption (TPR/D) measurements. The spectra obtained in this case show that strong chemisorption occurs and that a series of adsorbed halogenated reaction products are present. By comparison, studies of the adsorbed phase of CH 2Cl 2 show that only physisorption occurs. The TPR/D spectra of CBr 2Cl 2 show that dissociative formation of CCl 2 followed by its reaction with lattice oxygen is central to the monolayer reaction chemistry in this chloromethane. The branching ratios of the various desorbed products are compared with those obtained from CCl 4 adsorbed on the same (2 × 2) surface.

The thermal chemistry of model organosulfur compounds on gallium arsenide (110)

Organothiols and related organosulfur compounds hold promise for use as self-assembled ultra-thin electron beam resists, II–VI material growth precursors, and monolayer passivation layers. We report the results of temperature-programmed reaction and desorption (TPR/D) studies of the surface chemistry of three model organosulfur compounds, CH 3SH, (CH 3S) 2 and (CH 3) 2S, on the (110) face of gallium arsenide. Our measurements indicate that each of these species interacts strongly with the GaAs(110) surface. The first monolayer of CH 3SH desorbs at ∼300 K, with a desorption wave peak temperature (extrapolated to the zero coverage limit) corresponding to a desorption activation energy of 0.81 eV. This value is high compared to ∼110 K for the second layer, suggesting a chemical interaction between the thiol and the surface, despite the fact that the monolayer feature in the TPR/D spectra exhibits first-order kinetics, and the molecule is observed to desorb intact. The behavior of (CH 3S) 2 is markedly different; the disulfide decomposes upon adsorption (or during the TPR/D temperature ramp) and desorbs predominantly as (CH 3) 2S at ∼500 K, perhaps as the result of a concerted associative/dissociative desorption process that leaves sulfur at the surface. This result is also evidence that adsorbed CH 3SH and (CH 3) 2S do not assume equivalent surface-bound forms and supports the idea that the hydrogen atom of the thiol remains in close association with the adsorbed parent molecule, while the disulfide undergoes dissociative adsorption. Finally, our results indicate that (CH 3) 2S, like CH 3SH, molecularly adsorbs/desorbs. The desorption activation energy, extrapolated to zero coverage, is 0.79 eV, consistent with a strong adsorbate–substrate interaction. The similarity of this value to that for CH 3SH suggests that a specific sulfur–surface interaction dominates the desorption kinetics at low coverage. In the monolayer regime, with increasing exposure, the monolayer desorption feature in the TPR/D spectra broadens and exhibits strong asymmetry, and ultimately develops a lower temperature feature, indicating complex kinetics consistent with coverage-dependent phase behavior. Finally, by comparison of the measured desorption energies of the organosulfur compounds with those of the alkyl halides, for which the adsorption on GaAs(110) has been extensively investigated, an estimation of the chemical and physical contributions to the molecule–surface interaction is obtained. Our analysis suggests that both types of interactions play important roles in the adsorption energetics.

Numerical simulation of temperature-programmed reaction data: an application in surface chemical kinetics

The application of numerical simulations of temperature-programmed reaction and desorption (TPRD) spectra as a means of supporting mechanistic studies of surface reactions is demonstrated through two simple examples: the chemistry of ethene hydrogenation and the chemistry of methyl moieties co-adsorbed with hydrogen atoms. Where possible, literature kinetic parameters have been employed in the simulations. Where no parameters exist, preliminary estimates of the kinetic parameters were systematically modified until a visual agreement between the empirical TPRD spectra and the numerical simulations was obtained. In this way, these simulations have permitted a preliminary determination of the activation energies and pre-exponential factors for a number of the reaction steps in the chemistry of co-adsorbed methyl moieties and hydrogen atoms.

Titania–Silica Mixed Oxides: V. Effect of Sol-Gel and Drying Conditions on Surface Properties

Mesoporous titania–silica aerogels have been prepared by an alkoxide–sol-gel process with ensuing supercritical drying which involved either semicontinuous extraction using supercritical CO2or transferring the solution–sol-gel liquid directly into the supercritical state. Surface properties of these materials were compared to those of conventionally dried titania–silica xerogel. The influence of several important preparation parameters (hydrolysis route, Ti-content, and drying method) on surface properties of the gels were investigated by temperature-programmed reaction and desorption (TPRD) of isopropanol and X-ray photoelectron spectroscopy (XPS). The information gained by these surface-sensitive methods was interpreted in the light of previous bulk structural investigations, i.e., X-ray diffraction, FTIR, FTRaman, and UV–vis spectroscopy. Up to 723 K, three TPRD peaks were identified and assigned to the different surface species in titania–silica mixed oxides found in previous structural investigations: inactive silica domains, titania domains, and highly dispersed Ti in silica (Si–O–Ti linkages). The Ti-containing surface species catalyzed dehydration of isopropanol to propene at different temperatures. Depending on the dispersion and nature of titanium oxo species on the surface of the titania–silica sol-gel mixed oxides (Si–O–Ti heteroconnectivity), significant differences in the reaction-desorption profiles were observed. Prehydrolysis of the silicon alkoxide had no influence on the relative abundance of Si–O–Ti connectivity inferred from FTIR. However, XPS and TPRD revealed that the Ti-concentration on the surface increased when prehydrolysis had been applied. An increase in the bulk titania content of the mixed oxides resulted in a concomitant rise in both the surface Ti/Si ratio and the relative contribution of Si–O–Ti linkages. All titania–silica gels showed enrichment of the surface with silica. This behavior strongly depended on the drying procedure applied. Low-temperature supercritical drying and evaporative drying yielded higher titania surface concentration (lower Si enrichment) compared to high-temperature supercritical drying.

Alumina-Supported Manganese Oxide Catalysts: II. Surface Characterization and Adsorption of Ammonia and Nitric Oxide

Alumina-supported manganese oxide catalysts (2-8.4 wt% Mn), prepared from manganese acetate, have been characterized by in situ infrared (IR) spectroscopy and temperature-programmed reaction and desorption (TPRD), in relation to the selective catalytic reduction (SCR) of NO with NH3. Two Lewis acid-type coordinatively unsaturated Mn ions are present on the catalyst surface, most likely in the 3+ oxidation state. The Mn catalyst does not show Brønsted acidity other than that of the support. The Mn dispersion amounts to at least 20-30% The molecular interaction with ammonia is relatively strong. No ammonia oxidation is observed if oxygen is absent. The interaction with NO is very weak, although strongly bonded oxidized species can also be formed in the presence of oxygen, resulting in NO2, nitrito, and nitrate groups. These species decompose giving back NO gas. IR spectra of NH3-NO coadsorption suggest that Mn3+ species can bind both one NO and one NH3 molecule. In the absence of oxygen reaction between NO and NH3 is observed in the IR cell in the temperature range 300-423 K. In the presence of oxygen the reaction occurs to completion already at 325 K, provided ammonia is preadsorbed. Oxygen has several roles: it oxidizes the catalyst, favoring NO adsorption; it permits hydrogen abstraction from adsorbed ammonia, thereby activating it for reaction with NO; and it can oxidize gas-phase NO to NO2. Hydrogen abstraction that has proceeded too far results in the formation of N2O, which occurs at higher temperatures and lowers the reaction selectivity for SCR.

Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide IV. Diffuse reflectance FTIR study of NO adsorption and reaction

Adsorption of NO onto the oxidized surfaces of both crystalline and amorphous chromia results in the formation of nitrate complexes, by the reaction of NO with surface oxygen. Up to six differently bound species exhibiting different thermal stabilities have been identified. As the catalyst surfaces are saturated with adsorbed oxygen, the bound NO molecules are not decomposed: Upon heating, NO is molecularly desorbed, and neither formation of N 2O nor of N 2 is observed. Reduced chromia surfaces, which are partially depleted in surface oxygen, are more reactive towards NO. Decomposition of NO, to form N 2O and N 2, is taking place already at room temperature and is accompanied by a partial reoxidation of the catalyst surface. Nitrato complexes are the most abundant surface species also on the reduced chromia; signals of adsorbed mono- and dinitrosyl complexes are comparatively weak. This observation shows that NO preferentially binds to the Lewis basic surface oxygen ions. As a consequence of the decreased surface oxygen concentration, the coverage of bridging surface nitrate species (requiring two adjacent surface oxygens) is smaller than on the oxidized catalysts. The reaction of NO with surface hydroxyl groups, producing N2 and NO 2 −, results in a minor concentration of Cr-NO 2 surface nitro complexes. On the reduced amorphous catalyst, extensive reoxidation of the surface accompanied by N 2O formation is observed between 311 and 411 K in temperature-programmed reaction and desorption (TPRD) experiments, while this process is observed to a much smaller extent on reduced crystalline chromia. Above 473 K, bridging and bidentate surface nitrates are the most abundant species on both morphologies. Further heating results in the decomposition of these nitrates yielding N 2, N 2O (on crystalline chromia only), and surface oxygen.

Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide III. Diffuse reflectance ftir study of ammonia desorption from Brønsted and Lewis acid sites

Ammonia has been adsorbed onto oxidized and reduced surfaces of crystalline α-Cr 2O 3 and of amorphous chromia, respectively. Temperature-dependent changes in the diffuse reflectance FTIR spectrum are monitored and are correlated with temperature-programmed reaction and desorption (TPRD) spectra reported in part I of this study (1). Two types of NH3 molecules bound to Brønsted acidic sites on the surface of the chromia catalysts, as well as two species bound to Lewis sites, are identified from the IR spectra. The oxidized surface of α-Cr 2O 3 is characterized by a high surface coverage of strongly Lewis-bound ammonia molecules, which desorb at temperatures above 400 K. In contrast, weakly bound NH 3 prevails on the oxidized surface of amorphous chromia. The desorption of the latter species starts at 380 K and is completed around 480 K. A reductive pretreatment of the chromia surfaces by hydrogen (10% in Ar) decreases the number of surface hydroxyl groups and devoids the surface of adsorbed oxygen. As a consequence, the number of Brønsted acidic sites is reduced both on the crystalline and the amorphous catalyst. On the reduced surface of α-Cr 2O 3, strongly bound ammonia coordinated to Lewis acidic sites is observed to persist to temperatures beyond 560 K. In contrast, weakly Lewis-bound NH 3 prevails on reduced amorphous chromia, which is quantitatively desorbed at temperatures below 410 K. The formation of NH 3 oxidation products on α-Cr 2O 3 is manifested by the observation of two characteristic surface species, an N 2O 2 dimer and an NO 2 chelate surface complex. Corresponding signals are not observed on the surface of amorphous chromia. Apparently the removal of labile oxygen from the surface of α-Cr 2O 3 at moderate temperatures generates strongly Lewis acidic chromium sites, and the firm bonding of NH 3 to these sites is related to the undesired direct oxidation of ammonia.