Chapter 1 Thermal analysis: a guide through catalyst's synthesis and reaction process

Abstract

Various thermal analysis methods trace all stages of catalyst formation. This chapter deals with a sequential study of the main stages of catalyst formation from widely used thermogravimetry to specially developed in situ calorimetry through precursor synthesis to the catalytic reaction. Particular focus is given to Cu, Zn-based catalytic systems for methanol synthesis and Ni, Mg-based catalysts for dry methane reforming. The calcination of hydroxocarbonates and their decomposition kinetics were investigated by the simultaneous thermal analysis-mass spectrometry (STA-MS) method. Using temperature-programmed reduction technique, the composition of oxide systems, their reduction, and activation of the metal catalyst was analyzed. Additional diffraction, spectroscopic, and microscopic methods characterized the change in metal-support interaction during successive oxidative and reducing temperature treatments. Structural-functional relationships can be identified based on thermochemical, structural, and catalytic data. High-pressure thermogravimetry was used to probe the adsorption layer on the catalyst surface under methanol synthesis and coking under dry methane reforming conditions. Finally, the application of in situ calorimetry for studying the catalyst restructuring in oxidative reactions is shown.

1.1 Prologue

Recent discoveries in heterogeneous catalysis in the academic and industrial fields have primarily been accelerated by the advances in analytical tools and the broad application of complementary in situ methods. The instrumental progress has been achieved by developing several areas: hyphenated techniques, multiple coupling options, high-throughput experimentation, and operando methods. These techniques serve to examine the surface and bulk of the catalysts and directly measure the parameters of reaction systems under conditions of relevant catalytic performance. All of the trends above had affected the thermal methods of analysis (TA). On the other hand, when the methods experience technical limitations, a wide range of experimental approaches are still available, determined by the experimentalist's creativity, patience, diligence, and obstinacy. Although the growing presence of conventional machines has outnumbered the custom-designed setups, the research-specific equipment provides more profound insight than analytics with standard configuration.

Calorimetry together with thermogravimetry are techniques with a very long history. In the era of progressively developing synchrotron beam, spectroscopy, and electron microscopy in the alliance of catalysis and surface science, thermal methods have erroneously gained a reputation as supportive methods. A legitimate question arises among the catalytic community: is this still good enough? A look at the past of chemical findings reminded us that Marie Curie was literary "weighting radioactivity" in her experiments with Pierre Curie's piezoelectric balance for discovering new elements Ra and Po. Research of that level is nowadays performed on particle accelerators.

Another critical point is the definition of kilojoule. Is it a big or small value? From the experiments with energy transformation of James Joule, we know that 1 kJ will give the tennis ball a speed of 360 km/h or heat 1 L (55.5 mol) of water to 0.25 K. Given that the energy of common chemical bonds ranges from 150 to 1,000 kJ/mol, it is a very high heat evolution or consumption that is detected upon formation and decomposition of chemical substances. Even though the chemical reaction occurs on a minimal amount of active centers, the heat effects of the strong interaction, such as the formation of covalent bonds, are significant. However, weaker interactions such as vibrations of the participating molecules and long-range interactions are less available.

Even though the spectroscopy methods dominate, they experience significant limitations when quantifying specific surface sites under reaction conditions, which is imperative in catalysis research. Conventional catalysts do not consist of chemically and crystallographically homogeneous surfaces. Adsorption phenomena and thermochemical properties of the topmost layer provide necessary information for the characterization of the surface region. However, it is insufficient to build a clear picture of the chemistry of all involved processes. Even though in situ thermal analysis methods open up the possibility of studying structure-function relationships, one must keep in mind that correlation does not necessarily mean causation. Decisive information on the geometry and electronic structure of the surface layer under in situ conditions is not trivial to extract. In addition, the interplay between the flexibility of the surface and the stability of the bulk structure shapes the dynamic behavior of the catalytically active system. To bridge the gap between these elements, a high-level theory has to accompany the experimental findings. Proceeding from these basic facts, progress, and limits of instrumentation, the following chapters aim to demonstrate and remind the reader of the actual application of thermal analysis as a primary self-consistent or a secondary, complementary method for the characterization of solid catalysts.

1.2 Concerning thermal analysis in catalysis

Thermochemical information in analyzing catalysts and catalytic processes is a significant building block for understanding the active state of the catalyst. Thermogravimetry (TG), temperature-programmed reduction/oxidation (TPR/O), differential scanning calorimetry (DSC), and adsorption microcalorimetry (MC) are the main pillars of thermoanalytical methods in catalysis, and this will continue to be the case. These methods concerning the chemistry of heterogeneous reactions in the solid-gas system are applied for (i) the characterization of solid samples and surfaces and (ii) the study of heterogeneous processes. The first group includes the analysis of sample composition, determination of redox properties (TPR-TPO cycles) [1, 2], determination of temperature limits for catalyst stability (TG) [3], thermokinetics of calcination and reduction (TG, TPR/O), titration of metallic surface area, quantification of basic and acidic centers (TPD and pulse TA) [4, 5, 6], and characterization of strong and weak adsorption sites of the catalyst (MC). The second subdivision implies the application of in situ techniques to study surface adsorbates and deposits (in situ TG) [2, 7, 8], characterization of equilibrium and nonequilibrium states of solid, measurement of the heat effects of reversible phase transitions during the reaction as the catalyst structure is formed, construction of "thermochemical properties-reactivity" diagrams (in situ DSC) [9]. Instrumental basics, theoretical foundation, and numerous examples are summarized and reviewed in the literature [10].

Figure 1.1 provides a standard overview of thermochemical and adsorption processes within the catalytic cycle that an alternate application of these methods could determine. The formation of a catalyst as represented by the phase a is typically a multistage synthetic route involving additional calcination and reduction steps before it is exposed to reaction feed. Ex situ temperature-programmed techniques are employed on almost every stage to characterize the intermediate products in all detail and thus have a clear picture of the catalyst formation from a precursor through a precatalyst and active catalyst to a waste catalyst as represented by deactivated phase ß. Once the surface is exposed to the chemical potential of the gas phase, the formation of vacancies and surface reconstructions occurs. In the current example, the active state in the reaction feed could be considered to be an endothermic formation of the defective state isostructural to the phase a. The E* value does not include the energy of lattice reconstruction; that is, exothermic adsorption of A (EadsA) and desorption of product B (EadsB) do not modify the phase composition of the bulk and involve the formation and refill of vacancies and surface reorganization solely. The adsorbed species A overcomes the activation barrier (E#) and progresses through the intermediate state leading to final product formation and desorption. Meanwhile, the surface endures a row of adaptive states during the catalytic cycle. The formation of the inactive phase ß is related to exothermic Erelx relaxation of the lattice to the more stable structure; thus, the reaction enthalpy of phase transformation could be expressed as Er?=?Erelx - E?.

Fig. 1.1:...