Infrared spectroscopy, catalyst

Au-DENs were prepared via literature procedures (2) and deposited onto Degussa P25 Titania by stirring at pH 6 overnight. In situ infrared spectroscopy, catalyst activation, and CO oxidation experiments were performed using previously described procedures.(3) Catalyst activation under CO oxidation conditions were used 23 mg catalyst samples diluted 10 1 with a-Al203. In CO oxidation activity measurements, the feed composition was 1.1% CO, 27% O2 balance He, and the flow rate was kept constant at 20 mL/min. Conversion was measured as a function of temperature and rate data was determined only for conversions between 1 and 12%. 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Polyester composition can be determined by hydrolytic depolymerization followed by gas chromatography (28) to analyze for monomers, comonomers, oligomers, and other components including side-reaction products (ie, DEG, vinyl groups, aldehydes), plasticizers, and finishes. Mass spectroscopy and infrared spectroscopy can provide valuable composition information, including end group analysis (47,101,102). X-ray fluorescence is commonly used to determine metals content of polymers, from sources including catalysts, delusterants, or tracer materials added for fiber identification purposes (28,102,103). 

An unusual method for the preparation of syndiotactic polybutadiene was reported by The Goodyear Tire Rubber Co. (43) a preformed cobalt-type catalyst prepared under anhydrous conditions was found to polymerize 1,3-butadiene in an emulsion-type recipe to give syndiotactic polybutadienes of various melting points (120—190°C). These polymers were characterized by infrared spectroscopy and nuclear magnetic resonance (44—46). Both the Ube Industries catalyst mentioned previously and the Goodyear catalyst were further modified to control the molecular weight and melting point of syndio-polybutadiene by the addition of various modifiers such as alcohols, nitriles, aldehydes, ketones, ethers, and cyano compounds. 

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 most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. Sometimes infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on adsorption sites on a catalyst. We will first summarize the theory behind infrared absorption. 

Transmission infrared spectroscopy is very popular for studying the adsorption of gases on supported catalysts and for studying the decomposition of infrared active catalyst precursors during catalyst preparation. Infrared spectroscopy is an in situ technique that is applicable in transmission or diffuse reflection mode on real catalysts. 

Elementary steps in which a bond is broken form a particularly important class of reactions in catalysis. The essence of catalytic action is often that the catalyst activates a strong bond that cannot be broken in a direct reaction, but which is effectively weakened in the interaction with the surface, as we explained in Chapter 6. To monitor a dissociation reaction we need special techniques. Temperature-programmed desorption is an excellent tool for monitoring reactions in which products desorb. However, when the reaction products remain on the surface, one needs to employ different methods such as infrared spectroscopy or secondary-ion mass spectrometry (SIMS). 

Owing largely to research over the last twenty years, the sulfided C0-M0/AI2O3 system is one of the best-characterized industrial catalysts [H. Topsoe, B.S. Clausen and F.E. Massoth, Hydrotreating Catalysis (1996), Springer-Verlag, Berlin]. A combination of methods, such as Mbssbauer spectroscopy, EXAFS, XPS, and infrared spectroscopy, has led to a picture in which the active site of such a catalyst is known in almost atomic detail. 

Much of the pioneering work which led to the discovery of efficient catalysts for modern Industrial catalytic processes was performed at a time when advanced analytical Instrumentation was not available. Insights Into catalytic phenomena were achieved through gas adsorption, molecular reaction probes, and macroscopic kinetic measurements. Although Sabatier postulated the existence of unstable reaction Intermediates at the turn of this century. It was not until the 1950 s that such species were actually observed on solid surfaces by Elschens and co-workers (2.) using Infrared spectroscopy. Today, scientists have the luxury of using a multitude of sophisticated surface analytical techniques to study catalytic phenomena on a molecular level. Nevertheless, kinetic measurements using chemically specific probe molecules are still the... 

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. 

In order to investigate whether COj reacts in a concerted way with surface carbon or whether it dissociates first to CO and adsorbed oxygen and the adsorbed oxygen reacts, infrared spectroscopy, pulse reactor studies and XANES measuremerrts were used. The i.r. spectrum of a prereduced (Ihour at 675K in S /oHj/Nj) Pt/ZrOj catalyst in contact with CO2 at 775K is shown in Fig 6 The spectrum shows the presence of linearly bound CO on Pt at 2053 cm [15]. Additionally, bands of carbonate type species appeared in the region between 1375 and 1540 cm . Over pure supports (in the absence of Pt) the CO band was not seen, but peaks in the carbonate region were observed... 

At present, most workers hold a more realistic view of the promises and difficulties of work in electrocatalysis. Starting in the 1980s, new lines of research into the state of catalyst surfaces and into the adsorption of reactants and foreign species on these surfaces have been developed. Techniques have been developed that can be used for studies at the atomic and molecular level. These techniques include the tunneling microscope, versions of Fourier transform infrared spectroscopy and of photoelectron spectroscopy, differential electrochemical mass spectroscopy, and others. The broad application of these techniques has considerably improved our understanding of the mechanism of catalytic effects in electrochemical reactions. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

Although acetone was a major product, it was not observed by infrared spectroscopy. Flowing helium/acetone over the catalyst at room temperature gave a prominent carbonyl band at 1723 cm 1 (not show here). In this study, a DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) cell was placed in front of a fixed reactor DRIFTS only monitored the adsorbed and gaseous species in the front end of the catalyst bed. The absence of acetone s carbonyl IR band in Figure 3 and its presence in the reactor effluent suggest the following possibilities (i) acetone formation from partial oxidation is slower than epoxidation to form PO and/or (ii) acetone is produced from a secondary reaction of PO. 

The strength of the Bronsted (BAS) and Lewis (LAS) acid sites of the pure and synthesized materials was measured by Fourier transformed infrared spectroscopy (ATI Mattson FTIR) by using pyridine as a probe molecule. Spectral bands at 1545 cm 1 and 1450 cm 1 were used to indentify BAS and LAS, respectively. Quantitative determination of BAS and LAS was calculated with the coefficients reported by Emeis [5], The measurements were performed by pressing the catalyst into self supported wafers. Thereafter, the cell with the catalyst wafer was outgassed and heated to 450°C for lh. Background spectra were recorded at 100°C. Pyridine was then adsorbed onto the catalyst for 30 min followed by desorption at 250, 350 and 450°C. Spectra were recorded at 100°C in between every temperature ramp. 

All the catalysts contained 1 wt.% platinum, introduced into the zeolite through ion exchange by [Pt(NH3)4)]2+ in competition with NH4+ (NH4+/Pt=100), followed by calcination under dry air flow at 450°C for 4h. The dispersion of platinum, measured using CO adsorption followed by infrared spectroscopy, was about 60%. Whatever the crystallite size, the catalysts were first pelletized, then crushed and sieved to obtain 0.2 - 0.4 mm particles. 

Agnelli, M., Swaan, H. M., Marquez-Alvarez, C., Martin, G. A., and Mirodatos, C. 1998. CO hydrogenation on a nickel catalyst. II. A mechanistic study by transient kinetics and infrared spectroscopy. J. Catal. 175 117-28. 

Perl, J. B. Characterization of catalyst surfaces by computerized infrared spectroscopy. Prepr., Div. Pet. Chem.,... 

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