Spectroscopy under reaction conditions

In Situ Mossbauer Spectroscopy under Reaction Conditions... 

A method for observing intermediates directly in the reaction cycle is in situ IR spectroscopy under reaction conditions. As early as 1975, Penninger published a contribution concerning in situ IR spectroscopic studies of cobalt carbonyl modified by tri-u-butylphosphine as a hydroformylation catalyst [58] at relatively low catalyst concentrations of 2 mmoll-1. The observed carbonyl... 

In-Situ Mossbauer Spectroscopy Under Reaction Conditions 141... 

Grunwaldt J-D, Caravati M, Hannemann S, Baiker A. X-ray absorption spectroscopy under reaction conditions - opportunities and limitations of in situ monitoring and time-resolved studies of heterogeneous catalysts. Phys Chem Chem Phys. 2004 6 3037. 

Although methanol synthesis from COj hydrogenation over supported copper catalysts has been widely investigated, there are still controversies concerning the methanol synthesis mechanism and the effect of copper on the catalytic activity[l-6]. In this work, the influence of the copper dispersion in Cu/ZrO catalyst on the catalytic activity in COj hydrogenation was investigated. In order to understand the reaction mechanism, FT-IR spectroscopy under reaction conditions and TPD of adsorbed methanol were performed. 

Grunwaldt, J.-D., Caravati, M., Hannemann, S. and Baiker, A. (2004) X-ray absorption spectroscopy under reaction conditions suitability of different reaction cells for combined catalyst characterization and time-resolved studies. Phys. Chem. Chem. Phys., 6, 3037-3047. 

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

The second approach is to study real catalysts with in situ techniques such as infrared and Mossbauer spectroscopy, EXAFS and XRD, under reaction conditions, or, as is more often done, under a controlled environment after quenching of the reaction. The in situ techniques, however, are not sufficiently surface specific to yield the desired atom-by-atom characterization of the surface. At best they determine the composition of the particles. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Mflssbauer Spectroscopy. In Figure 1 are shown four MCssbauer spectra recorded under various conditions and after a progression of treatments of sample 1 (see Table I). The first spectrum (spectrum A) is a room temperature spectrum (in flowing helium) taken after the sample had been under water-gas shift conditions for three hours. The second spectrum (spectrum B) was recorded for 22 hours while the sample was under water-gas shift reaction conditions. Immediately following the collection of spectrum B the sample was cooled rapidly (5 minutes) to room temperature in flowing helium. At this point the sample had been under reaction conditions for a total of twenty-five hours. The third spectrum (spectrum C) was then recorded. Next, the sample was treated for an additional 40 hours under water-gas shift reaction conditions, and then cooled to room temperature in flowing helium. The fourth spectrum (spectrum D) was then recorded. 

Figure 10.15. X-ray photoelectron spectroscopy measurements on prereduced Pd/LaCo03 into Pd0/CoO c/La2O3 after successive exposures under reaction conditions 0.15vol.% NO, 0.5vol.% H2 and 3 vol.% 02. Photopeaks Pd 3d and N Is recorded on the calcined sample in air at 400°C (a) after reduction in H2 at 500°C (b) after exposure under reaction conditions at 25°C (c) at 200°C for 30 min (d) at 300°C for 30 min (e) at 300°C for 2h (f) at 500°C for 30min (g) and at 500°C for 2h (h) (reproduced with permission from Ref. [117]).

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

The matrix photochemistry of 2v proved to be fairly complicated.108 The primary product of the photolysis of 2v is carbene lv, which was identified by ESR spectroscopy. Under the conditions of matrix isolation the carbene showed the expected reactivity towards molecular oxygen (formation of carbonyl oxide 7v) and carbon monoxide (formation of ketene lOv) (Scheme 22). In contrast to the oxocyclohexadienylidenes (la and derivatives) carbene lv slowly reacted with CO2 to give an a-lactone with the characteristic C=0 stretching vibration at 1896 cm-1. The latter reaction indicates that lv is — as expected — more nucleophilic than la. 

A strong point of Raman spectroscopy for research in catalysis is that the technique is highly suitable for in situ studies. The spectra of adsorbed species interfere weakly with signals from the gas phase, enabling studies under reaction conditions to be performed. A second advantage is that typical supports such as silica and alumina are weak Raman scatterers, with the consequence that adsorbed species can be measured at frequencies as low as 50 cm-1. This makes Raman... 

Carbonylation of IBPE and other 2-arylethanols with various organosoluble Pd-catalysts was studied in detail with special emphasis on the role of the promoters p-toluenesulfonic acid and LiCl [55], Some of the catalytic species, such as [PdCl(PPh3)2] formed from [Pd(PPh3)4] or from Pd(II) precursors in aqueous methylethylketone (MEK) under reaction conditions (54 bar CO, 105 °C) were identified by P NMR spectroscopy. Ibuprofen was obtained in a fast reaction (TOP = 850 h" ) with 96% yield (3-IPPA 3.9 %), while the carbonylation of l-(6-methoxynaphtyl)ethanol gave 2-(6-methoxynaphtyl)propionic acid (Naproxen) with high selectivity (97.2 %) but with moderate reaction rates (TOP = 215 h" ). 

F.C. Jentoft, Ultraviolet-Visible-Near Infrared Spectroscopy in Catalysis Theory, Experiment, Analysis, and Application Under Reaction Conditions, Adv. CataL, 52, 129-211 (2009). 

Returning to the general liquid phase catalytic system, assume that you have chosen an appropriate spectroscopy to investigate the system under reaction conditions. The spectroscopy provides spectra, i. e. absorbance A(t), at specific intervals in time. If S denotes the complete set of all species that exist at any time in the physical system, then Sjo s is the subset of all observable species obtained using the in situ spectroscopy. This requires that the pure component spectra aj..as obs are obtainable from the multi-component solution spectra A t) without separation of constituents, and without recourse to spectral libraries or any other type of a priori information. Once reliable spectroscopic information concerning the species present under reaction are available, down to very low concentrations, further issues such as the concentrations of species present, the reactions present, and reaction kinetics can be addressed. In other words, more detailed aspects of mechanistic enquiry can be posed. 

High-pressure infrared studies of ruthenium carbonyl solutions under H2/CO at temperatures employed for CO reduction have also been reported. In //-teiradecane solution at 180 C under 1 1 H2/CO, mainly Ru(CO)5 is detected (60). In acetic acid solvent at 200 C, only Ru(CO)5 is detected under 400 atm of 1 1 H2/CO at H2/CO pressures of 200 atm, Ru3(CO)l2 is also observed (166). Reaction solutions have also been studied by sampling under reaction conditions, rapidly cooling the samples to low temperatures, and analyzing them by infrared spectroscopy after reaction at 265 atm of 1 1 H2/CO at 180 C, only Ru(CO)s could be detected (164). At higher temperatures and lower pressures (100 atm of 1 1 H2/CO and 250"C), evidence was seen for the clusters Ru3(CO)l2 and H4Ru4(CO)l2 as well as Ru(CO)s (163). 

N. and P. In all the above-mentioned cases, however, NMR spectroscopy is a powerful tool for studies of working catalysts, their surface sites, and the reactants and adsorbates occurring on their surface under reaction conditions. 

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

In the preceding decade, solid-state NMR spectroscopy has provided important and novel information about the nature and properties of surface sites on working solid catalysts and the mechanisms of these surface reactions. This spectroscopic method offers the advantages of operation close to the conditions of industrial catalysis. A number of new techniques have been introduced and applied that allow investigations of surface reactions by solid-state NMR spectroscopy under both batch and flow conditions. Depending on the problems to be solved, both of these experimental approaches are useful for the investigation of calcined solid catalysts and surface compounds formed on these materials under reaction conditions. Problems with the time scale of NMR spectroscopy in comparison with the time scale of the catalytic reactions can be overcome by sophisticated experimental... 

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due to Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. 

Potassium tetrachloropalladate(II) reacts with 1 to give CH3C1 and metallic Pd as products (48). This reaction apparently follows the same mechanism as the Hg(II) system, with reported rate constants K (= kj A 4) = 150 and k2 = 7.7 x 10 3 M sec-1. Additional support for the proposed methylpalladium(II) intermediate comes from the report of such a species, detected by proton NMR spectroscopy, in the reaction of PdCir with (CH3)3Sn+ (44). There has been a separate study on the base-off complex formed between PdCl4 and 1 (49). Attempts to react K2PdCl6 with 1 proved futile, due to the extremely rapid decomposition of the Pd salt under reaction conditions (46). 

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