Reflectance spectroscopy: critical remarks on the evaluation of diffuse reflectance spectra

Die Reflexionsspektroskopie wird normalerweise zur Bestimmung der Eigenschaften von Oberflächen wie Farbe oder Anteile von diffuser und direkter Reflexion verwen-det. Darüber hinaus können pulverförmige Materialien, die häufig in der Katalyse verwendet werden, in vielen Fällen nur in diffuser Reflexion vermessen werden. In der Literatur geben verschiedene Autoren die unterschiedlichsten Verfahren an, mit denen ein Quasi – Absorptionsspektrum aus der Reflexionsmessung erzeugt werden kann, z.B. durch die Berechnung von optischer Dichte, 1 – R oder F(R). Diese Arbeit befaßt sich mit einigen grundsätzlichen Fragestellungen, die bei solchen Reflexionsmessungen auftreten können: • Welche Informationen sind im Reflexionsspektrum enthalten? • Welche Darstellung der y-Achse kommt einer Absorptionsmessung am nächsten? • Sind Verdünnungsreihen, wie sie in der Transmissionsspektroskopie oft ver-wendet werden, auch in der Reflexionsspektroskopie sinnvoll? Um den Zusammenhang zwischen Absorptions- und Reflexionsspektren zu klären, sind Holmiumoxidfiltergläser zunächt in Transmission bzw. Absorption ver-messen worden; danach wurden die Filtergläser gemörsert, verschiedene Korngrös-sen ausgesiebt und anschließend sowohl unverdünnt wie auch verdünnt mit SiO2-Pulver Reflexionsspektren aufgenommen. Ergebnisse und Schlußfolgerungen: • Die Reflexionsspektren ändern sich in Abhängigkeit von der Korngröße in ähnlicher Weise wie sich Transmissionsspektren in Abhängikeit von der Schichtdicke ändern. • Die Darstellung als F(R)–Spektrum ohne irgendwelche Einschränkungen kommt in ihren Charakteristika (Maxima etc.) dem Absorptionsspektrum am nächsten. • Verdünnungen bringen keine Verbesserung der spektroskopischen Charakte-ristika, sondern nur eine Schwächung des Signalanteils des Probenmaterials

Immiscible fluid: Heat of fusion heat storage system

Both heat and mass transfer in direct contact aqueous crystallizing systems were studied as part of a program desig- ned to evaluate the feasibility of direct contact heat transfer in phase change storage using aqueous salt system. Major research areas, discussed include (1) crystal growth velocity study on selected salts; (2) selection of salt solutions; (3) selection of immiscible fluids; (4) studies of heat transfer and system geometry; and (5) system demonstration

A newly developed attachment to the Lambda 9 spectrometer for in situ reflectance measurements of catalysts at temperatures up to 623 K

A newly developed attachment to the Lambda 9 spectrometer for in situ reflectance measurements of catalysts at temperatures up to 623 K M.Thiede and J.Melsheimer Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin, Germany Abstract The spectroscopic investigation of catalyst powders under reaction conditions is generally possible only in reflectance. Two problems occur: 1. Low signal level Normally the sample is positioned directly at the integrating sphere and all diffusely scattered light from the sample is reflected back into the integrating sphere. A hot reactor cell cannot be attached directly, it must be positioned as far as possible from the integrating sphere to avoid its heating. If the reactor cell is attached at some distance from the measurement window of the integrating sphere only a portion of the diffusely scattered light will be reflected back into the sphere. At a distance of 12 mm, for example, this will only be 20%. In a first setup we bridged the distance with highly reflecting ceramics, to increase the part of light reflected into the sphere (Fig.1). 2. Thermal radiation The thermal radiation of the hot reactor cell also enters the integrating sphere.At higher temperatures it is significantly stronger than the measurement light from the spectrometer itself. This leads to increased noise and saturation of the detector. In the newly developed setup we tried a. to make the distance between integrating sphere and reactor cell with oven as large as possible; b. to increase the signal level c. to decrease the surface which causes the thermal radiation, and d. to bring reactor cell and oven into a vertical position which facilitates the work with powder samples. These requirements could be fulfilled by the application of a specially formed light conductor made of quartz, in which the light is conducted to the sample and back into the integrating sphere by total reflectance. At the same time, the surface which causes the thermal radiation was reduced (Fig.2). In this way, we succeeded in improving the signal-to-noise ratio by a factor of 4-5 compared with the first setup

In Situ Spectroscopic Study of Isomerisation of Light Alkanes over Sulfated Zirconia Catalysts

In situ DRIFT and DR-UV/Vis spectroscopies were performed during n-butane (1 or 5 kPa partial pressure, 358 – 453 K) and n-pentane (1 kPa, 298 – 323 K) isomerization in the presence of two different sulfated zirconia catalysts: a sulfate containing ordered mesoporous zirconia of the MCM-41 structure and a conventional sulfated zirconia catalyst with a tetragonal zirconia bulk structure. After a 100 min induction period, the mesoporous zirconia deactivates slowly during n-butane isomerization at 453 K while an absorption band at 285 nm grows, indicating the formation of unsaturated surface species. The conventional catalyst which is more active and produces the same maximum rate already at 378 K deactivates rapidly and a band grows at 310 nm, indicating the formation of allylic carbocations on the surface. During n-pentane isomerization, both catalysts deactivate rapidly while bands at 285 and 310-320 nm (mesoporous) and 335 nm (conventional) are formed. The spectra clearly show that the surface species on the two catalysts differ, although in principle the same gas phase products are observed, indicating an influence of the underlying wall or tetragonal bulk structure, respectively

Subdynamics as a mechanism for objective description

The relationship between microsystems and macrosystems is considered in the context of quantum field formulation of statistical mechanics: it is argued that problems on foundations of quantum mechanics can be solved relying on this relationship. This discussion requires some improvement of non-equilibrium statistical mechanics that is briefly presented.Comment: latex, 15 pages. Paper submitted to Proc. Conference "Mysteries, Puzzles And Paradoxes In Quantum Mechanics, Workshop on Entanglement And Decoherence, Palazzo Feltrinelli, Gargnano, Garda Lake, Italy, 20-25 September, 199

Evolution of the electronic structure of Cs₂H₂PVMo₁₁O₄₀ under the influence of propene and propene/O₂

Evolution of the Electronic Structure of Cs2H2PVMo11O40 under the Influence of Propene and Propene/O2 J. Kröhnert, F.C. Jentoft, J. Melsheimer, R. Ahmad, M. Thiede, G. Mestl, and R. Schlögl Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Faradayweg 4-6, Germany Changes in the electronic and vibrational spectra of Cs2H2PVMo11O40 in the presence of propene (1) or propene/O2 (2) were followed by in situ UV/Vis/NIR diffuse reflectance spec-troscopy. (1) At 298 K propene leads to reduction as indicated by a broad absorption band extending from the Vis to the NIR range. Iso-propanol was detected at 323 K and the maxi-mum of the broad band shifted from 740 to 700 nm. At higher temperatures the visible ab-sorption band shifted back about 25 nm. (2) Under conditions of catalytic oxidation a propene conversion of ca. 4% was detected with acrolein and CO as major products (670 K). Although the absorption band in the Vis range is less pronounced than in the presence of propene only at the same temperature, the catalyst is not restored to its fully oxidized state. The evolution of a band at 680-700 nm at 620-670 K indicates the formation of a structure with reduced and oxidized metal sites next to each other. This maybe related to the observation of molydenyl and vanadyl species in post mortem Raman spectra. 1. Introduction Cs salts of the vanadomolybdophosphoric acid are, for example, applied as catalysts for oxidative dehydrogenation of isobutyric acid to methacrylic acid [1-3]. The sensitivity of the catalyst under industrial operation suggests that the nature of the active phase may not be identical to the structurally well-defined salts which are molecular solids composed of Keggin ions, Cs cations, and water. Interestingly, the light-off temperature for oxidation reactions coincides with the temperature for the loss of constitutional water [4]. It is thus hypothesized that the water loss is connected to the formation of the active phase, whereby the electronic state of the active phase evolves in an atmosphere that contains both oxidative (O2) and re-ductive (hydrocarbon) components at the same time. In situ UV/Vis/NIR diffuse reflectance spectroscopy offers the unique possibility to si-multaneously investigate electronic features such as d-d transitions, intervalence charge trans-fers (IVCT), and ligand-to-metal charge transfers (LMCT) as well as the vibrational overtones and combination modes of water. From preliminary UV/Vis/NIR experiments, as from other methods (e.g., TG-DTA experiments), it has become clear that catalysts of the type CsxH4-xPVMo11O40 with x = 0-2 are already thermally unstable in the presence of an inert gas. This instability is expressed by the appearance and disappearance of absorption bands. The goal of this work was to investigate the loss of crystal and subsequently constitutio-nal water, and possible concomitant electronic changes of Cs2H2PVMo11O40 under inert, oxi-dative, and reductive conditions over a wide temperature range, as well as under the conditi-ons of oxidation catalysis. Propene was selected as a reactant and the gas phase was monito-red in order to correlate catalytic performance with spectroscopic data. 2. Experimental A Perkin-Elmer Lambda 9 spectrometer with an enlarged integrating sphere was used for in situ UV/Vis/NIR diffuse reflectance spectroscopy on different dilute catalyst samples. So-lutions of Cs2CO3 and heteropoly acid were used for the preparation of the Cs2H2PVMo11O40 samples. Approximately 110 mg of the catalyst (7-10 wt%) were mixed with SiO2 (Heraeus, 0.1-0.4 mm) and placed in a microreactor of in-house design operating under continuous gas flow. Sequential spectroscopic measurements were carried out with a scan speed of 240 nm/min, a slit width of 5.0 nm, and a response time of 0.5 s. Spectralon® was used as a refe-rence. The apparent absorption was evaluated from the diffuse reflectance data using the for-mula 1-Rmixture/RSiO2. The feed mixture was 10 vol-% propene in helium or 10 vol-% propene plus 10 vol-% oxygen in helium with a total gas flow of 71 or 74 ml/min, respectively. The gases were analyzed with two gas chromatographs (Perkin Elmer), equipped with heated au-tomatic gas sampling valves, an FFAP column (Macherey-Nagel) and a packed Carboxen-1000 column using FID and TCD in both GCs. Series A experiments (10% propene): The temperature was held constant for 2 h at room temperature (RT), and then the temperature was increased at a rate of 1 K/min to 323 K, and spectra were recorded over a period of ca. 5 hours. Series B experiments (10% propene): The temperature was increased from RT to 323 K and then to 670 K in steps of ~ 50 K (5 K/min heating rate), with a 2 h isothermal period after each step. Series C experiments (10% propene, 10% O2): The temperature was increased as in Series B with extended isothermal periods of 9 h at 413 K and 19 hours at 670 K. 3. Results The Series A spectra show a strong increase in apparent absorption already at RT. After 40 min on stream (RT3 in Fig. 1) a visible absorption band formed at ~ 740 nm and this band underwent a blue shift to 700 nm when the temperature was increased to 319 K. In contrast to similar experiments using He, the crystal water bands at 1430 and 1925 nm already disappear after 70 min on stream (Figure 1). Formation of iso-propanol was detected at 319 K. Series B spectra showed similarly strong changes in apparent absorption with a red shift of ca. 25 nm for the visible absorption band and the appearance of an additional band in the NIR (at ~ 1050 nm). The NIR band (appearing above 560K) is broad and overlaps with the visible band (Fi-gure 2). The visible band increases with increasing temperature until a single broad visi-ble/NIR band forms. For Series C, increasing temperature leads to a decrease in the intensity of the absorption bands, particularly the NIR band (Figure 3). However, the visible band be-comes clearly recognizable again at 563 K; it is possible that a catalytic reaction begins to occur at this temperature. The products acrolein, propionic acid, acrylic acid and water were first detected at 603 K. At 670 K in addition to these products we also detected propionalde-hyde, acetone, CO and acetic acid, with the conversion of propene being ca. 4% and that of O2 ca. 12 %, and the highest selectivities being for acrolein and CO. In the Series C spectra the defined feature in the UV region does not disappear as it did in the Series B spectra at higher temperatures. Under catalytic reaction conditions above 563 K one observes an increase in the intensity of the shifted visible absorption band at 680-700 nm with increasing temperature (=620 K) and time on stream (Figure 4). 4. Discussion The water bands disappear much more readily in the presence of propene than in inert gas, and at the same time, isopropanol is formed. These observations can be explained by an addition of water from the catalyst to propene, a typical acid-catalyzed reaction. Propene thus appears to draw the crystal water from the catalyst, and when the crystal water is gone the constitutional water is removed as well. The sample apparently underwent considerable re-duction even at the relatively low temperature of propene hydration, which corresponds to the observations in inert gas at higher temperature, and reduction generally seems to accompany the water loss. Hence, water, which is added in the industrial oxidation process, may play an essential role in maintaining a certain, i.e. active, state of the catalyst which is different from a van-der-Waals solid built of isolated Keggin units. The electronic structure change in the pre-sence of propene is dramatic; the defined LMCT band is obscured by an intense, almost con-tinuous absorption which is even more pronounced at higher temperatures (up to 670 K). The catalyst sample was black after treatment with the propene atmosphere, in contrast to He-treated catalyst samples that were blue [5]. In the presence of propene and oxygen, the initial reduction at 555 K is partly reversed at 620-670K; however, although excess oxygen is available the catalyst remains in a reduced state. The decrease in the intensity of the visible absorption band below the catalytic reaction temperature (603K) may be attributed to an oxidation of some Mo5+ and V4+ centers by the gas phase oxygen. Above this temperature the absorption band increases with rising tempera-ture through the stronger reduction of the catalyst and at the same time the conversion also increases. The blue shifted absorption band at ca. 680 nm that was observed at 670K could indicate oxygen vacancies that are important for the oxidation reactions. These species may be the same as a species observed in post mortem Raman analysis of these samples that was charac-terized by a shoulder at about 1002 cm-1 and was interpreted as molybdenyl species [6]. Un-der the same conditions, the free acid H4PVMo11O40 showed a blue shift up to 660 nm [5], which might indicate the presence of molybdenyl and vanadyl species in the catalyst sample, since Raman bands were in turn detected at 1008 and 1030 cm-1 [6]. In summary, the changes in electronic structure appear too dramatic to be just a conse-quence of a partial reduction of the Keggin ion; rather it seems that the geometric structure is partially dissolved leading to a transformation from a molecular solid to more condensed oxi-dic species with semiconducting character. The availability of relatively free electrons that is suggested by the continuous character of the UV/Vis spectra at high temperatures is a prere-quisite for the activation of molecular oxygen and thus for the redox catalytic activity. The structural changes are too severe to allow the restoration of the heteropolyacid through the water that is formed in the propene oxidation; and acidic properties also no longer play a role for the product distribution under these conditions. References 1. M. Misono, N. Nojiri, Appl. Catal., 64 (1990) 1. 2. Th. Ilkenhans, B. Herzog, Th. Braun and R. Schlögl, J. Catal., 153 (1995) 275. 3. L. Weismantel, J. Stöckel and G. Emig, Appl. Catal., 137 (1996) 129. 4. S. Berndt, Dissertation, TU Berlin, 1999. 5. J. Kröhnert, O. Timpe, J. Melsheimer, F.C. Jentoft, G. Mestl and R. Schlögl, to be pub-lished. 6. G. Mestl, T. Ilkenhans, D. Spielbauer, M. Dieterle, O. Timpe, J. Kröhnert, F.C. Jentoft, H. Knözinger and R. Schlögl, Appl. Catal. A, submitted