Platinum and palladium are frequently used as catalytic materials, for example for the oxidation of CO. This is one of the most widely studied reactions in the field of surface science. Although ...seemingly uncomplicated, it remains an active and interesting topic, which is partially explained by the push to conduct experiments on model systems under relevant reaction conditions. Recent developments in the surface-science methodology have allowed obtaining chemical and structural information on the active phase of model catalysts. Tools of the trade include near-ambient-pressure X-ray photoelectron spectroscopy, high-pressure scanning tunneling microscopy, high-pressure surface X-ray diffraction, and high-pressure vibrational spectroscopy. Interpretation is often aided by density functional theory in combination with thermodynamic and kinetic modeling. In this review, results for the catalytic oxidation of CO obtained by these techniques are compared. On several of the Pt and Pd surfaces, new structures develop in excess O
2
. For Pt, this requires a much larger excess of O
2
than for Pd. Most of these structures also develop in pure O
2
and are identified as (surface) oxides. A large body of evidence supports the conjecture that these oxides are more reactive than the corresponding O-covered metallic surfaces under similar conditions, although still debated in the literature. An outlook on this developing field, including directions that move away from CO oxidation towards more complex chemistry, concludes this review.
Application of surface-science techniques, such as XPS, SXRD, STM, and IR spectroscopy under catalytic reactions conditions yield new structural and chemical information. Recent experiments focusing on CO oxidation over Pt and Pd model catalysts were reviewed.
Moderate intensity Pulsed Electric Fields (PEF) was studied for microbial inactivation as an alternative to high intensity PEF or to classical thermal pasteurization. The process is characterized by ...the application of electric pulses, allowing an increase of the product temperature by the ohmic heat generated by the pulses. A systematic evaluation of the effect of parameters electric field strength (E) and pulse width (τ) on the inactivation of Escherichia coli, Listeria monocytogenes, Lactobacillus plantarum, Salmonella Senftenberg and Saccharomyces cerevisiae in orange juice was carried out in a continuous flow system. A wide range of conditions was evaluated, and both E and τ were shown to be important in the efficacy to inactivate micro-organisms. Remarkably, PEF conditions at E = 2.7 kV/cm and τ = 15–1000 μs showed to be more effective in microbial inactivation than at E = 10 kV/cm and τ = 2 μs.
Inactivation kinetics of the tested PEF conditions were compared to an equivalent thermal process to disentangle non-thermal effects (electroporation) from thermal effects responsible for the microbial inactivation. At standard high intensity PEF treatment a non-thermal inactivation at E = 20 kV/cm and τ = 2 μs pulses was observed and attributed to electroporation. Non-thermal effects could also be resolved with moderate intensity PEF at E = 2.7 kV/cm and pulse width between τ = 15–1000 μs. Microbial inactivation at these moderate intensity PEF conditions was studied in more detail at different pH and medium conductivity for E. coli and L. monocytogenes in watermelon juice and coconut water. Under moderate intensity PEF conditions the effectiveness of treatment was independent of pH for all evaluated matrices in the pH range of 3.8–6.0, whereas under high intensity PEF conditions the pH of the product is a critical factor for microbial inactivation. This suggests that the inactivation proceeds through a different mechanism at moderate intensity PEF, and speculations for this mechanism are presented. In conclusion, moderate intensity PEF conditions at E = 2.7 kV/cm and pulse width of 15–1000 μs has potential for industrial processing for the preservation of fruit juices and pH neutral liquid food products.
•Moderate intensity PEF as alternative to high intensity PEF was studied.•Electric field strength and pulse duration are critical factors for inactivation.•PEF conditions at E = 2.7 kV/cm are more effective than at E = 10 kV/cm.•Non-thermal and thermal effects for inactivation were discriminated•Moderate intensity PEF has the same effectiveness in high- and low-acid fruit juices.
Abstract
The hydrodesulfurization process is one of the cornerstones of the chemical industry, removing harmful sulfur from oil to produce clean hydrocarbons. The reaction is catalyzed by the edges ...of MoS
2
nanoislands and is operated in hydrogen-oil mixtures at 5–160 bar and 260–380 °C. Until now, it has remained unclear how these harsh conditions affect the structure of the catalyst. Using a special-purpose high-pressure scanning tunneling microscope, we provide direct observations of an active MoS
2
model catalyst under reaction conditions. We show that the active edge sites adapt their sulfur, hydrogen, and hydrocarbon coverages depending on the gas environment. By comparing these observations to density functional theory calculations, we propose that the dominant edge structure during the desulfurization of CH
3
SH contains a mixture of adsorbed sulfur and CH
3
SH.
Observing the oxidation of platinum van Spronsen, Matthijs A; Frenken, Joost W M; Groot, Irene M N
Nature communications,
09/2017, Letnik:
8, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Despite its importance in oxidation catalysis, the active phase of Pt remains uncertain, even for the Pt(111) single-crystal surface. Here, using a ReactorSTM, the catalytically relevant structures ...are identified as two surface oxides, different from bulk α-PtO
, previously observed. They are constructed from expanded oxide rows with a lattice constant close to that of α-PtO
, either assembling into spoked wheels, 1-5 bar O
, or closely packed in parallel lines, above 2.2 bar. Both are only ordered at elevated temperatures (400-500 K). The triangular oxide can also form on the square lattice of Pt(100). Under NO and CO oxidation conditions, similar features are observed. Furthermore, both oxides are unstable outside the O
atmosphere, indicating the presence of active O atoms, crucial for oxidation catalysts.Improving platinum as an oxidation catalyst requires understanding its structure under catalytic conditions. Here, the authors discover that catalytically important surface oxides form only when Pt is exposed to high pressure and temperature, highlighting the need to study catalysts in realistic environments.
Gene products of the major histocompatibility complex (MHC) of human and non-human primates play a crucial role in adaptive immunity, and most of the relevant genes not only show a high degree of ...variability (polymorphism) but also copy number variation (CNV) is observed. Due to this diversity, MHC proteins influence the capability of individuals to cope with various pathogens. MHC and/or MHC-linked gene products such as odorant receptor genes are thought to influence mate choice and reproductive success. Therefore, MHC typing of wild and captive primate populations is considered to be useful in conservation biology, which is, however, often hampered by the need of invasive and time-consuming methods. All intact
Mhc-DRB
genes in primates appear to possess a complex and highly divergent microsatellite, DRB-STR. A panel of 154 pedigreed olive baboons (
Papio anubis
) was examined for their
DRB
content by DRB-STR analysis of genomic DNA. Using the same methodology on DNA of feces samples,
DRB
variability of a silvery gibbon population (
Hylobates moloch
) (
N
= 24), an endangered species, could successfully be studied. In both species, length determination of the DRB-STR resulted in the definition of unique genotyping patterns that appeared to be specific for a certain chromosome. Moreover, the different STR lengths were shown to segregate with the allelic variation of the respective gene. The results obtained expand data gained previously on DRB-STR typing in macaques, great apes, and humans and strengthen the conclusion that this protocol is applicable in molecular ecology, conservation biology, and colony management, especially of endangered primate species.
The olive baboon represents an important model system to study various aspects of human biology and health, including the origin and diversity of the major histocompatibility complex. After screening ...of a group of related animals for polymorphisms associated with a well-defined microsatellite marker, subsequent MHC class I typing of a selected population of 24 animals was performed on two distinct next-generation sequencing (NGS) platforms. A substantial number of 21
A
and 80
B
transcripts were discovered, about half of which had not been previously reported. Per animal, from one to four highly transcribed
A
alleles (majors) were observed, in addition to ones characterised by low transcripion levels (minors), such as members of the
A*14
lineage. Furthermore, in one animal, up to 13
B
alleles with differential transcription level profiles may be present. Based on segregation profiles, 16
Paan-AB
haplotypes were defined. A haplotype encodes in general one or two major
A
and three to seven
B
transcripts, respectively. A further peculiarity is the presence of at least one copy of a
B*02
lineage on nearly every haplotype, which indicates that
B*02
represents a separate locus with probably a specialistic function. Haplotypes appear to be generated by recombination-like events, and the breakpoints map not only between the
A
and
B
regions but also within the
B
region itself. Therefore, the genetic makeup of the olive baboon MHC class I region appears to have been subject to a similar or even more complex expansion process than the one documented for macaque species.
Pulsed electrical field (PEF) technology can be used for the inactivation of micro-organisms and therefore for preservation of food products. It is a mild technology compared to thermal ...pasteurization because a lower temperature is used during processing, leading to a better retention of the quality. In this study, pathogenic and spoilage micro-organisms relevant in refrigerated fruit juices were studied to determine the impact of process parameters and juice composition on the effectiveness of the PEF process to inactivate the micro-organisms. Experiments were performed using a continuous-flow PEF system at an electrical field strength of 20kV/cm with variable frequencies to evaluate the inactivation of Salmonella Panama, Escherichia coli, Listeria monocytogenes and Saccharomyces cerevisiae in apple, orange and watermelon juices.
Kinetic data showed that under the same conditions, S. cerevisiae was the most sensitive micro-organism, followed by S. Panama and E. coli, which displayed comparable inactivation kinetics. L. monocytogenes was the most resistant micro-organism towards the treatment conditions tested. A synergistic effect between temperature and electric pulses was observed at inlet temperatures above 35°C, hence less energy for inactivation was required at higher temperatures. Different juice matrices resulted in a different degree of inactivation, predominantly determined by pH. The survival curves were nonlinear and could satisfactorily be modeled with the Weibull model.
•We investigated the inactivation of micro-organisms using PEF in fruit juices.•Spoilage micro-organism was easier to be inactivated than pathogenic ones.•Synergistic effect between temperature and pulses was demonstrated.•The pH of fruit juices has an influence on the inactivation using PEF.•Survival curves could be modeled using the Weibull equation.
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