The interest of the scientific community on methylammonium lead halide perovskites (MAPbX₃, X = Cl, Br, I) for hybrid organic-inorganic solar cells has grown exponentially since the first report in 2009. This fact is clearly justified by the very high efficiencies attainable (reaching 20% in lab scale devices) at a fraction of the cost of conventional photovoltaics. However, many problems must be solved before a market introduction of these devices can be envisaged. Perhaps the most important to be addressed is the lack of information regarding the thermal and thermodynamic stability of the materials towards decomposition, which are intrinsic properties of them and which can seriously limit or even exclude their use in real devices. In this work we present and discuss the results we obtained using non-ambient X-ray diffraction, Knudsen effusion-mass spectrometry (KEMS) and Knudsen effusion mass loss (KEML) techniques on MAPbCl₃, MAPbBr₃ and MAPbI₃. The measurements demonstrate that all the materials decompose to the corresponding solid lead (II) halide and gaseous methylamine and hydrogen halide and the decomposition is well detectable even at moderate temperatures (~60 °C). Our results suggest that these materials may be problematic for long term operation of solar devices.

The role of thermodynamics in assessing the intrinsic instability of the CH3NH3PbX3 perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH3NH3X(s) and PbX2(s) is first analysed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH3NH3PbI3 the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH3(g)+CH3X(g), CH3NH2(g)+HX(g) or CH3NH3X(g) is analysed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized.

The nature of the gas phase product released during the thermal decomposition of CH₃NH₃PbI₃ (methylammonium lead iodide) to PbI₂ (lead diiodide) under vacuum is discussed on the basis of thermodynamic predictions, recently published experimental results, and new experiments presented here. From the limited data currently available, the nature of the main decomposition path is not clear because, both, the process releasing HI(g) + CH₃NH₂(g) (1) and that leading to NH₃(g) + CH₃I(g) (2) were observed under different conditions. Our thermodynamic analysis showed that process (2) is largely favoured for all the CH₃NH₃PbX₃ (X = Cl, Br, I) compounds. However, Knudsen effusion mass spectrometry experiments (temperature range 140–240 °C) showed that HI(g) and CH₃NH₂(g) were the predominant species in the vapor, with process (2) occurring to a much smaller extent than suggested by the thermodynamic driving force, thus being of minor importance under effusion conditions. We also found that this process was comparatively enhanced by high temperatures and low effusion rates (high impedance orifice). Our experimental evidence suggested that the thermodynamically favoured process (2) was affected by a significant kinetic hindrance. Overall, the prevailing decomposition path is likely to markedly depend on the actual operative conditions.

The present review addresses the latest findings and limitations in catalytic pyrolysis for the processing of plastic waste into valuable fuels. Compared to thermal degradation of plastics, catalytic pyrolysis provides better results in regards to the quality of the obtained liquid hydrocarbon fuel. Different types of catalysts can be used in order to improve the thermal degradation of plastics. Some of the most used catalysts are different types of zeolites (HUSY, HZSM-5, H$\beta$), Fluid Catalytic Cracking (FCC), silica-alumina catalysts, or natural clays. There is a need to find affordable and effective catalysts in the aim of achieving commercialization of catalytic pyrolysis of plastic waste. Therefore, this study summarizes and presents the most significant results found in the literature in regards to catalytic pyrolysis. This paper also investigates the symmetry effects of molecules on the pyrolysis process.

The vaporization behaviour and thermodynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf₂) were studied by combining the Knudsen Effusion Mass Loss (KEML) and Knudsen Effusion Mass Spectrometry (KEMS) techniques. KEML studies were carried out in a large temperature range (398–567) K by using effusion orifices with 0.3, 1, and 3 mm diameters. The vapor pressures so measured revealed no kinetically hindered vaporization effects and provided second-law vaporization enthalpies at the mean experimental temperatures in close agreement with literature. By exploiting the large temperature range covered, the heat capacity change associated with vaporization was estimated, resulting in a value of −66.8 J K⁻¹ mol⁻¹, much lower than that predicted from calorimetric measurements on the liquid phase and theoretical calculations on the gas phase. The conversion of the high temperature vaporization enthalpy to 298 K was discussed and the value Δ^g _lH_m(298 K) = (128.6 ± 1.3) kJ mol⁻¹ assessed on the basis of data from literature and present work. Vapor pressure data were also processed by the third-law procedure using different estimations for the auxiliary thermal functions, and a Δ^g _lH_m(298 K) consistent with the assessed value was obtained, although the overall agreement is sensitive to the accuracy of heat capacity data. KEMS measurements were carried out in the lower temperature range (393–467) K and showed that the largely prevailing ion species is BMIm⁺, supporting the common view of BMImNTf₂ vaporizing as individual, neutral ion pairs also under equilibrium conditions. By monitoring the mass spectrometric signal of this ion as a function of temperature, a second-law Δ^g _lH_m(298 K) of 129.4 ± 7.3 kJ mol⁻¹ was obtained, well consistent with KEML and literature results. Finally, by combining KEML and KEMS measurements, the electron impact ionization cross section of BMIm⁺ was estimated.