A transient in situ infrared spectroscopy study on methane oxidation over supported Pt catalysts

Catalysts with platinum dispersed on alumina, ceria and mixed alumina-ceria have been prepared by incipient wetness impreg- nation, characterized with transmission electron microscopy and X-ray diffraction, and evaluated for total oxidation of methane under both stationary and transient gas compositions (oxygen pulsing). Further, in situ diffuse reflectance Fourier transformed infrared spectroscopy has been used to monitor the evolution of surface species during precise transient gas composition changes using high-speed gas switching valves. The results show that platinum interacts sufficiently strong with all the supports so as to form small platinum particles. The smallest sizes are observed for the Pt/Ce sample. The alumina con- taining samples show, in contrast to the Pt/Ce sample, a decreased methane conversion with the increasing oxygen concentration and a clear kinetic bistability between increasing and decreasing oxygen concentrations. The bistable kinetics is likely connected to oxidation and reduction of platinum occurring at different stoichiometric gas mixtures depending on the history of the system, for which an oxidation of the platinum particles effectively inhibits the dissociative adsorption of methane leading to low reaction rate. It is shown for the alumina containing samples that the adverse effects of oxygen excess can be circumvented by the use of periodic operation so that the the average methane conversion is improved. Further, Pt/Ce seems to exhibit additional active sites at the platinum-ceria interface explaining the generally higher conversion of methane for this sample

Comparative Study of SO2 and SO2/SO3 Poisoning and Regeneration of Cu/BEA and Cu/SSZ-13 for NH3 SCR

Two copper-exchanged zeolites, Cu/SSZ-13 and Cu/BEA, were studied as catalysts for the selective reduction of NOx by NH3 (NH3-SCR). Their activities for standard SCR (NOx = NO) and fast SCR (NOx = 50% NO + 50% NO2) were measured before and after sulfur poisoning at 250\ua0\ub0C. The effect of 30\ua0ppm SO2 and a mixture of 24\ua0ppm SO3 + 6\ua0ppm SO2 was evaluated. The repetition of subsequent activity measurements served as regeneration method in SCR conditions. SO2 deactivated Cu/SSZ-13 whereas Cu/BEA was only moderately affected. SO3 led to stronger deactivation of both catalysts than SO2. However, also for this case, the Cu/BEA was significantly less affected than Cu/SSZ-13, even though Cu/BEA contained larger amount of stored sulfur. One possible reason for this could be the large pores of Cu/BEA, where the sulfur species possibly resulted in less sterical hindrance than in the small pore SSZ-13 structure. NH3 temperature-programmed desorption (NH3-TPD) showed no loss of storage sites upon sulfur treatment and subsequent regeneration. Partial activity recovery was observed after a period in SCR conditions at 400\ua0\ub0C and 500\ua0\ub0C. Temperature at 300\ua0\ub0C was insufficient to regenerate the catalysts. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO adsorption suggested that SO2 interacts with the ZCuOH sites on Cu/SSZ-13, causing the strong poisoning

Methane oxidation over Pd supported on ceria–alumina under rich/lean cycling conditions

Catalysts with highly dispersed palladium on alumina, alumina doped with 20 wt% ceria and ceria have been prepared, characterized and examined for net-lean methane oxidation. In particular, the activity and selectivity were investigated during rich/lean cycling of the feed. The ceria content is found to influence both the general and the instantaneous activity responses. The results indicate that the active phase of palladium changes between reduced and oxidised Pd during the rich/lean cycling, and that the process is influenced by the presence of ceria

Chemistry of supported palladium nanoparticles during methane oxidation

Time-resolved in situ energy-dispersive X-ray absorption spectroscopy and mass spectrometry are used to correlate changes in chemical state of alumina and ceria supported palladium nanoparticles with changes in activity and selectivity for methane oxidation. Specifically, modulation excitation spectroscopy experiments are carried out by periodically cycling between net-reducing and net-oxidizing reaction conditions. The XANES and EXAFS data show that the palladium nanoparticles are readily bulk oxidized when exposed to oxygen, forming a PdO-like phase, and reduced back to a reduced (metal) phase when oxygen is removed from the feed. The difference between the two support materials is most noticeable at the switches between net-oxidizing and net-reducing reaction conditions. Here, a brief reduction in conversion is observed for the alumina supported catalyst, but for the ceria this reduction in conversion is minor or not observed at all. This difference is attributed to differences in the oxidation kinetics and the oxygen storage capability of ceria

Fault Tree Analysis, Strengths and Weaknesses

Fault Tree Analysis (FTA) is a tool of hazard identification techniques. As a useful method it is applied in various industries, social and environmental problems for reconstruction, failure analysis, and failure frequency estimation. ETA is a graphical and logic combination of causes of a defined undesired event where Boolean algebra is used. It is a backward method which is used to think about the consequences which may occur. This analysis method is mainly used in the field of safety engineering to quantify the probability of an undesired event and is used to reconstruct it. It can also be used to reconstruct an accident. Besides its advantages, there are a number of shortcomings which imply that the method still has rooms for improvement. Among the disadvantages are the uncertainties in covering all failure modes, inaccuracy in human error in investigation of complex man-made systems and inefficiency of the tool in case of scarce or insufficient data. These problems demand some revision study to find the research questions in detail. The purpose of this paper is to conduct a literature review in order to find weaknesses and strengths of FTA to indicate a platform for future research works

Unsteady-state operation of supported platinum catalysts for high conversion of methane

Total oxidation of methane over model monolith catalysts with platinum sup- ported on alumina, alumina-ceria and ceria has been studied under unsteady- state operation of the feed gas stoichiometry. The general activity for methane oxidation follows the order Pt/alumina < Pt/alumina-ceria < Pt/ceria. Thanks to high catalytic activity at the gas composition switches, increased cycling fre- quency between oxygen excess and oxygen free conditions increases the average methane conversion significantly from 11 to 58% for Pt/alumina and from 25 to 87% for Pt/alumina-ceria. The corresponding stationary methane conversion is 10 and 19%, respectively. The underlying reason for the enhanced catalytic activity is likely twofold namely that periods with detrimentally high coverage of either oxygen or carbon are shortened and that the transients induce a highly active (chemical) state of the catalyst, thus, facilitating high average conversion of methane

A simple methodology for environmental risk evaluation: a case study related to water resources management

ABSTRACT: One of the most important strategies to achieve a sustainable development is to control the risk of the environmental threats. Conceptual frameworks for risk assessment are needed in water resource management and this study is aimed to introduce a simple methodology by which the risk of wastes as contaminants could be identified and minimised. Using a qualitative risk analysis, since few quantitative data were known, this methodology is applied to assess the hazard identification and risk assessment at St Miguel, Azores (Portugal). The main conclusions were the need for proper management and control of waste discharges and the need of a surveillance programme to keep track on what is released and thereby be able to define and implement minimisation programmes