TPD Analysis Research Articles

NOx-sorbing metal oxides, MnOx-CeO2. Oxidative NO adsorption and NOx-H2 reaction

(n)MnOx–(1−n)CeO2 binary oxides have been studied for the sorptive NO removal and subsequent reduction of NOx sorbed to N2 at low temperatures (≤150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO−3) and/or nitrite (NO−2) species on the surface depending on temperature, O2 concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NOx toward H2 was examined for MnOx–CeO2 impregnated with Pd, which is known as a nonselective catalyst toward NO–H2 reaction in the presence of excess oxygen. The Pd/MnOx–CeO2 catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H2 at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnOx–CeO2 surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H2–O2 combustion, H2–NO reaction was suppressed to a certain extent in the presence of O2. Nevertheless, Pd/MnOx–CeO2 attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H2, and 6% O2 in He at as low as 150 °C, compared to ca. 30% conversion for Pd/γ–Al2O3 at the same temperature. The combination of NOx-sorbing materials and H2-activation catalysts is expected to pave the way to development of novel NOx-sorbing catalysts for selective deNOx at very low temperatures.

The nature of metal oxide on adsorptive and catalytic properties of Pd/MeO x/Al 2O 3 catalysts

The role of ceria, niobium and molybdenum oxides on the promotion of the NO reduction by CO was studied. A bifunctional mechanism was discussed as a function of both the nature of interaction between metal oxide and palladium and the redox properties of each metal oxide. The NO dissociation was better on the Pd/MoO 3/Al 2O 3 catalyst than on the Pd/CeO 2/Al 2O 3 and Pd/Nb 2O 5/Al 2O 3 catalysts. The explanation for the very high N 2 production on Pd–Mo catalyst during the TPD analysis may be attributed to the NO+Me δ+ stoichiometric reaction. The promoting effect of a reducible oxide for the NO+CO reaction at low temperature can be ascribed mainly to its easiness for a redox interchange and its interaction with the noble metal particles. This would increase the surface redox ability and favor the dynamic equilibrium needed for high N 2 selectivity.

Advantages of base exchanged natural clinoptilolite as a catalyst for the Knoevenagel reaction

The potential use of natural zeolites as base catalysts was ascertained by studying the catalytic behavior of a purified natural clinoptilolite (NZ) and its sodium carbonate modified form (NZC) in the aldol condensation reaction of benzaldehyde with methyl cyanoacetate (Knoevenagel reaction). The natural zeolites activities were compared to that of cesium modified X and Y faujasite type zeolites (CsFAUX and CsFAUY). It was observed that the activity of the NZC zeolite was five times higher than that of NZ. The normalized activity per surface area of the modified clinoptilolites was higher than that shown by synthetic FAUs zeolites. The higher activity of NZC is attributed to the presence of single and double carbonate species, such as Na 2CO 3 and Na 2Ca(CO 3) 2, occluded in the internal cavities of the clinoptilolite zeolite, as confirmed by XRD and IR diffuse reflectance. The CO 2 and NH 3 TPD analysis corroborated the presence of stronger basic sites in the natural zeolites in good agreement with the catalytic behavior shown by these solids in the Knoevenagel condensation reaction.

Comparison between the surface acidity of solid catalysts determined by TPD and FTIR analysis of pre-adsorbed pyridine

A series of both crystalline (Y zeolites, beta zeolites and mordenites) and amorphous alumino-silicates were investigated by means of temperature-programmed desorption with mass spectrometry detection (TPD-MS) and of Fourier transform infra-red (FTIR) analysis of adsorbed pyridine, in order to compare the surface concentration of acid sites potentially involved in a specific catalytic process, as determined through the two techniques. Up to three distributions of acid sites were evidenced in the investigated catalysts by deconvolution of the TPD-MS spectra recorded after pyridine adsorption at different temperatures (150–300°C), depending on the type of catalyst and on its dealumination degree. FTIR analysis showed that pyridine adsorption on acid solids occurs under kinetic control and that equilibrium attainment implies a redistribution of pyridine molecules between Brønsted and Lewis surface sites. The reliability of the two titration techniques was verified by the good match between the values of overall surface acidity determined through them under equilibrium conditions at 150°C.

Revisiting the Effect of Surface Chemistry on Adsorption of Water on Activated Carbons

A sample of Westvaco carbon was oxidized with ammonium persulfate and both initial and oxidized samples were washed with methanol. The samples were characterized using sorption of nitrogen, Boehm titration, potentiometric titration, IGC, FTIR, TPD, and thermal analysis. Then water adsorption isotherms were measured at various temperatures close to ambient (relative pressure from 0.001 to 0.3). The isotherms were analyzed in terms of the thermodynamic consistency, presence of equilibrium, and water-activated carbon surface interactions. The results showed that washing with methanol significantly modifies the surface chemistry of carbons, creating phenols and new, very easily hydrolyzed esters. The latter, when in contact with water, is hydrolyzed, creating carboxylic acids and methanol. This may result in unjustified application of the Clausius−Clapeyron equation to calculate the isosteric heat of adsorption.

Catalytic Features of Cesium Hydrogen Salts and Triammonium Salt of Heteropoly Acid H3PW12O40 in the Alkylation of Ammonia with Methanol to Methylamines.

Catalytic features of CsxH3-xPW12O40 (CsxPW; x=0-3) and (NH4)3PW12O40(NPW) in the methylamine synthesis by sequential alkylations of NH3 with CH3OH were investigated by conducting the reaction (NH3/CH3OH/He=2/1/7) at 0.1MPa by flow method. All the H+- or NH4+-containing heteropoly compounds were effective for the methylamine synthesis at ca. 573-723K. In contrast with SiO2•Al2O3, all the heteropoly catalysts exhibited a long induction period in the initial stage of the reaction, during which significant sorption of CH3OH by the catalysts was observed. For the H+-containing heteropoly catalysts, the uptake of NH3 was also observed in its early stage. The uptakes of CH3OH by Cs2.5PW and Cs2.95PW were in the range of 2-3 moles per mol-polyanion, while those of NH3 as NH4+, estimated by TPD analysis, were 1.0 per H+ in each fresh sample. These results suggest that both CH3OH and NH3 can diffuse into their rigid crystal bulk. After such an induction period, the catalysts showed steady activity. The activity of CsxPW and the distribution of the amines during the steady state greatly varied with Cs-content x. Among the high surface area salts, Cs2.5PW, Cs2.95PW, NPW, etc, that showed high catalytic activity, Cs2.95PW gave almost 100% selectivity to the smaller amines, MMA (monomethylamine: 0.4nm) and DMA (dimethylamine: 0.49nm), with a negligible yield of TMA (trimethylamine: 0.61nm) even at high CH3OH conversions. On the other hand, NPW and CsxPW with x 2nm) in a significant proportion. This fact implies that the high shape-selective salt, Cs2.95PW, has the active sites (H+) mostly in its small macropores (probably, <0.6nm) where no TMA can be formed or diffuse, while non-shape-selective one, Cs2.5PW, has H+ throughout its micro- to macropores. In fact, TPD studies suggested that the both salts easily chemisorb MMA and DMA as well as NH3 (as ammonium ions) even into their bulk or very small pores, but Cs2.95PW can hardly chemisorb TMA, while Cs2.5PW chemisorbs TMA only onto the surface of its relatively large pores in reasonable amounts.

Acidity modification during the agglomeration of ZSM-5 with montmorillonite

Ammonia TPD analyses of ZSM-5 zeolites of three Si/Al ratios (15, 30, 43) agglomerated with sodium montmorillonite revealed a noticeable modification on the acid properties of the bound zeolites as compared with the raw materials. An initial increase of the proportion of the weak acidity was found on the agglomerated zeolites, though the total and strong acidity decreases. The higher the Si/Al ratio of the zeolite, the higher is this increase. After several steps of ion exchange of the bound zeolites, the acidity reaches the expected values again, and the total and strong acidities are enhanced at the expense of the weak acidity. These results were related to a solid-state ion exchange between protons of the zeolite and cations of the clay during the calcination subsequent to the agglomeration.

Computer simulation of derivative TPD

In this paper, the advantages of employing a Derivative Temperature-Programmed Desorption (DTPD) curve in TPD analysis are demonstrated. Based on a series of theoretical DTPD curves obtained by computer simulation with double assumption of zero signal noise and no temperature gradients across the sample, a comparison is made between the TPD and DTPD curves, and it is found that the approach can (a) estimate desorption order, (b) raise resolving power, and (c) eliminate baseline drift. The equations for calculating kinetic parameters from DTPD curves are also presented. The results show that these equations are valid.

Modified CuO Surface Appropriate for Selective CO Gas Sensing at CuO/ZnO Heterocontact

The surface properties of copper oxide (CuO) were discussed for considering the working mechanism of a CuO/ZnO heterocontact gas sensor having carbon monoxide (CO) gas selectivity. We focused on p-type semiconducting CuO, using sodium as a dopant, which is a key material for selective CO gas sensing and its surface properties were characterized by XPS, TPD and FT-IR analyses. It was found that the Cu/O ratio of the 1mol% Na2CO3-added CuO surface was higher than that of pure CuO surface by XPS analysis. By TPD analysis, it was found that CO was adsorbed more strongly on 1mol% Na2CO3-added CuO than on pure CuO. By the in situ IR measurement of CO adsorbed on the surface of 1mol% Na2CO3-added CuO specimen under 300°C, we found two asymmetric bands of CO, whereas an asymmetric streching CO2 band was found on IR measurement of pure CuO specimen. It was confirmed that Cu atoms on pure CuO surface exposed to CO at 300°C changed from Cu2+ to Cu1+ or Cu0, in contrast to Cu atoms on 1mol% Na2CO3-added CuO surface, which changed to Cu2+. The electrical conductivity was measured as a function of temperature for pure CuO, CuO with 1mol% Na2CO3 and ZnO specimens. The resistivity of ZnO was larger than that of pure CuO and CuO with 1mol% Na2CO3 by three and six orders of magnitude at 250°C, respectively. A working mechanism of CuO(Na)/ZnO heterocontact gas sensor was elucidated in order to explain its high CO selectivity.

Electrochemical studies on ionic conduction in Ca-doped BaCeO 3

BaCeO 3-based ceramics exhibit both protonic and oxide ionic conduction at high temperatures. In order to investigate the relation between ionic conduction in these oxides and the valence of the dopant cation, divalent Ca-doped BaCeO 3 oxides were synthesized and their conduction properties were studied comparing them with those of trivalent Y-doped ones. When the dopant cation was changed from Y to Ca, both protonic and oxide ionic conductivities decreased. TPD analysis and electrochemical tracer experiment showed that the decrease in protonic conductivity could be ascribed to the decrease both in proton concentration and its mobility in the oxides.

Kinetics of CO Oxidation on Cu/Rh(100) Model Bimetallic Catalysts

The oxidation of carbon monoxide has been studied on a series of Cu/Rh(100) model bimetallic catalysts using a combined elevated pressure reactor-ultrahigh vacuum surface analysis system. The effects of temperature and oxygen/carbon monoxide partial pressures on the rates of CO 2 formation and surface composition/morphology were determined. Although the addition of copper to Rh(100) altered the rate of CO oxidation, the kinetic behavior of the Cu/Rh(100) catalysts was essentially the same as that for Rh(100) with modifications within the low Cu coverage region, i.e., at θ Cu < 0.5 ML. The altered kinetics are attributed to two types of reaction sites at these low Cu coverages. The primary role of copper in the Cu/Rh(100) bimetallic catalyst is to increase the surface oxygen coverage. The presence of Cu and Rh carbonate-like surface species was evident in postreaction TPD analysis.

The determination of active filter aid adsorption sites by temperature‐programmed desorption

AbstractTemperature programmed desorption (TPD) was used to evaluate the active sites of several filter aids that have been promoted for use by the deep‐frying industry: activated carbon, aluminas, bleaching earths, diatomaceous earth, silica and synthetic magnesium silicate. TPD analysis determines both the total surface concentration of the active sites and the details of their intensity distribution. A correlation analysis was made between the adsorbents' oil treatment performance and the observed acidic/basic sites. For some of the adsorbents, the adsorption of free fatty acids and the percent change in the color of the oil was shown to relate to the total basic sites and acid sites, respectively. The study also showed that other factors might affect these adsorptivities.

Oxidation of Cr(III) to Cr(VI) Species During the Thermal Decomposition Process of Zn/Cr-layered Double Hydroxide Carbonate

Abstract The intermediates obtained from thermal decomposition of Zn/Cr-layered double hydroxide carbonates (Zr/Cr-LDH; Zn0.66Cr0.34(OH)2(CO3)0.170.57H2O) were studied by XRD, TPD, FT-IR, ESCA and chemical analyses. It was found that Cr(III) ions embedded originally in the hydroxide layers were oxidized to Cr(VI) species in the temperature range between 473 and 773 K where the layer structure was lost, and reduced again to Cr(III) in the spinel phase over 773 K.

Calcium catalytic active sites in carbon-gas reactions. Determination of the specific activity

A new procedure to quantify the catalytic active sites (CAS) in calcium-catalyzed carbon gasification is presented. The method is based on TPD analysis after CO 2 chemisorption. The second CO 2 composite peak is related to the calcium-carbon contact and the quantification of this peak provides a measure of the CAS. Turnover frequencies based on this CAS for CO 2 and steam gasification are found constant for different calcium loading and for samples with different sintering degrees

Effect of water vapor on the conductivity of porous Na β/β″-Al 2O 3 ceramics

Porous Na β/β″-Al 2O 3 ceramics was prepared by sol-gel process in combination with the freeze-drying method. The obtained samples were examined by X-ray, SEM, TPD and FTIR analyses. The ionic conductivity was determined in air and vacuum using the complex impedance method. The effect of ambient water vapor on the conductivity was interpreted on the basis of the characterization results.

Oxidation of carbon monoxide catalyzed by manganese-silver composite oxides

The oxidation of carbon monoxide on Mn Ag composite oxides was carried out and their catalytic action was investigated. The composite oxides had much higher activities than either the Mn or the Ag single oxide catalysts. The Ag in the composite catalyst remained in the oxidized state in air even at calcination temperatures as high as 400 °C, as long as its content was less than 50 mol%, whereas it lost lattice oxygen in the absence of Mn at the same temperature. The TPD and TG analyses showed that the composite oxides had active lattice oxygen belonging to Ag which was easily liberated on heating in the absence of oxygen. It was found, however, that Ag still retained the oxidized states even after the desorption of its active lattice oxygen, but reduction of Mn occurred instead. These facts suggested that the active oxygen on Ag was mainly consumed in the oxidation of CO, and Mn served as an oxygen carrier; i.e., vapor phase oxygen was first incorporated into the composite catalyst through Mn and, then, was transferred to the reduced Ag. This concerted action of Mn and Ag in the composite catalyst provided high activity in the oxidation of CO.