Adsorbed probe molecules infrared spectroscopy

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. 

Among the spectroscopic techniques, one of the most widely used to characterize the basic properties of alkaline earth metal oxides is infrared (IR) spectroscopy of adsorbed probe molecules (41,47-49) this is described below. 

Vibrational spectroscopy of adsorbed probe molecules is one of the most powerful tools to assess the acidic properties of catalysts. Acidity studies of dealumi-nated Y zeolites (main active component of FCC catalysts) or other zeolitic catalysts are reported using mostly Fourier Transform Infrared Spectroscopy (FTIR) with CO adsorption at 77 K or FTIR-pyridine/substituted pyridines adsorption at 425 K [22-26]. FTIR acidity studies of commercial FCC catalysts are even more scarce... 

The combination of infrared spectroscopy and XAS has been extremely useful in the understanding of site structure. Infrared spectra [13, 50, 52] of adsorbed probe molecules can help to differentiate between different types of site. They are discriminative in the sense that the probe molecules will adsorb with different thermodynamic parameters on the different sites. XAS on the other hand will average over all the different sites present in the zeolite. This can of course be an advantage, but is also a disadvantage in the sense that the active site can be lost in the signal of the other species. Some combined X-ray absorption infrared instrumentation is currently being developed and tested for metal catalysts [53,... 

Nitric oxide was chosen as the adsorption probe for infrared spectroscopy because studies by others indicate that this molecule is a selective probe for iron cations (22,26-31). Volumetric adsorption of NO has been used to determine the dispersion of supported iron samples (27,32). In the present work. Infrared bands for adsorbed NO are shown to distinguish between different types of iron surface sites. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

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. 

The use of infrared spectroscopy of adsorbed molecules to probe oxide surfaces has been reviewed by Davydov and Rochester [23], This approach works on sulfide catalysts as well. The infrared signal of NO has been successfully used to identify sites on the surface of a hydrodesulfurization catalyst, as the following example shows [24]. 

I nfrared (I R) Spectroscopy Infrared spectroscopy is the most widely used technique for studying the surface chemistry of heterogeneous catalysts [103], It can give information about the catalyst structure, as well as about the species adsorbed on the catalyst surface. By using probe molecules like CO, NO and NH3, information is obtained about the nature and environment of atoms and ions exposed on the surface. The method is based on the absorption, transmission, or reflection by a... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

In order to characterize the MgO sites where the Pd atoms are stabilized after deposition by soft-landing techniques, we used CO as a probe molecule [61]. The adsorption energy, Eb, of CO has been computed and compared with results form thermal desorption spectroscopy (TDS). The vibrational modes, (o, of the adsorbed CO molecules have been determined and compared with Fourier transform infrared (FTTR) spectra. From this comparison one can propose a more realistic hypothesis on the MgO defect sites where the Pd atoms are adsorbed. 

Adsorbed carbon monoxide is a matter of special interest in ultrahigh vacuum as well as in electrochemical systems. CO has been used as probe molecule in surface vibrational spectroscopy. For important reviews of CO adsorbed from the gas phase, see [21, 42, 45, 46]. The rather large dynamic dipole moment (9///0Q) of adsorbed CO is particulary suited for infrared spectroscopy at electrochemical interfaces, where submonolayer amounts of species must be observed in the presence of IR-active solution compontents. 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

Nitrogen and carbon monoxide are both candidates as small probe molecules which may interact only with strong acid sites in zeolites and which can be observed by infrared spectroscopy. As an illustration of this method, consider the recent work of Wakabayashi et al. on N2 adsorbed in H-mordenite [29], HY and HZSM-5 [30], References to infrared spectra of adsorbed CO include [31-33]. 

In-situ FTIR of CO (and other molecules as site specific probes) was studied on the H-form and hydrothermally treated samples of H-ZSM-5. These results have been more thoroughly reported elsewhere [10] but are sununarized here for comparison with experimental data from other techniques. Zeolites have been traditionally examined using infrared spectroscopy of N-containing adsorbates such as ammonia and pyridine to assess Bronsted acidity. The use of weaker Lewis bases allows a more discriminating approach to assessing the strength and quantity of Bronsted sites arising from the partial and sequential dealumination of framework A1 in H-ZSM-5. 

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