2,580 research outputs found
Unveiling the Structure Sensitivity for Direct Conversion of Syngas to C2-Oxygenates with a Multicomponent-Promoted Rh Catalyst
No full textAbstract: Mn and Li promoted Rh catalysts supported on SiO2 with a thin TiO2 layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO2 layer on the surface of SiO2 was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh0 and Rhδ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H2/CO ratio in the feed is recommended. Graphic Abstract: [Figure not available: see fulltext.].ChemE/Catalysis EngineeringChemE/O&O groe
Computational Exploration of Rh-III/Rh-V and Rh-III/Rh-I Catalysis in Rhodium(III)-Catalyzed C-H Activation Reactions of N-Phenoxyacetamides with Alkynes
No full textThe selective rhodium-catalyzed functionalization of arenes is greatly facilitated by oxidizing directing groups that, act both as directing groups and internal oxidants. We report density functional theory (B3LYP and M06) investigations on the mechanism of rhodium(III)-catalyzed redox coupling reaction of N-phenoxyacetamides with alkynes. The results elucidated the role of the internal oxidizing directing group, and the role of Rh-III/Rh-I and Rh-III/Rh-V catalysis of C-H functionalizations. A novel Rh-III/Rh-V-Rh-III cycle successfully rationalizes recent experimental observations by Liu and Lu et al. (Liu, G. Angew. Chem. Int. Ed. 2013, 52, 6033) on the reactions of N-phenoxyacetamides with alkynes in different solvents. Natural Bond Orbital (NBO) analysis confirms the identity of Rhy intermediate in the catalytic cycle.National Natural Science Foundation of China [21133002, 21203004]; Shenzhen Peacock Program [KQTD201103]; National Science Foundation of the USA [CHE-1361104]; National Science Foundation under the CCI Center for Selective C-H Functionalization [CHE-1205646]; National Science Foundation [OCI-1053575]SCI(E)[email protected]; [email protected]
Analysis of persistence in daily monsoon rainfall over India.
No full textIn earlier studies the authors have established that monsoon rainfall up to the time scale of at least 5 days is dependent on its past value. In this study we examine the dependence in daily rainfall for 12 stations spread all over India, and in daily rainfall of 10 meteorological regions during the summer monsoon; 1) by analysing the rainfall as a stochastic point process by fitting Markov chains of orders 1 to 4 to the station data and 2) by fitting autoregressive and autoregressive moving average models to the spatially averaged data. The significant conclusion is that the Markov chain of order 3 fits the runs of wet and dry spells better than chains of lower order, and that the autoregressive model of order 1 fits the spatially averaged rainfall amounts satisfactorily as compared to the higher order autoregressive of ARMA models. Examination of rainfall intensities during spells of various lengths has revealed that the rain intensity is greater during spells of 2 or 3 days than on isolated days. The rain intensity is greater on the days lying in the middle of a wet spell than on isolated days or on the first and the last days of a longer wet spell
Monsoon rainfall variations and teleconnections over South and East Asia
No full textSeasonal summer monsoon (June-September) data for 120 stations over East Asia (China, Japan, Mongolia, Korea) varying from 1881 to 1998 are utilized to understand their interannual and climate characteristics and to investigate their teleconnections with South Asian (in particular India's) monsoon rainfall. Contemporaneous relations on an interannual time-scale reveal that the rainfall variations over north China (southern Japan) are in-phase (out-of-phase) with South Asian rainfall. Based on the instrumental data available, regional rainfall anomaly time series for the 118-year period for the two coherent regions, over north China and southern Japan are prepared. All the three series (India, China, Japan) have been subjected to statistical tests. Results reveal that while there are year-to-year fluctuations, the Mann-Kendall rank statistic suggests no significant long-term trends. However, the application of Cramer's statistic to study the short-term climate variability depicts decadal variability with certain epochs of above and below normal rainfall over each region. The epochs tend to last for about three decades over India and China, and about five decades over Japan. The turning points for China follow those of India about a decade later. The relationships of South and East Asian monsoon rainfall exhibit secular variations. The inter-connections between the monsoon-related events (rainfall over South Asia, rainfall over East Asia, Northern Hemisphere circulation, tropical Pacific circulation) appear to strengthen (or weaken) around the same time, implying that the monsoon related events over geographically separated regions seem to get linked (or delinked) around the same time
Rainfall variability over South-East Asia - Connections with Indian monsoon and Enso extremes: New perspectives
No full textSeasonal and annual rainfall data for 135 stations for periods varying from 25 to 125 years are utilized to investigate and understand the interannual and short-term (decadal) climate variability over the South-east Asian domain Contemporaneous relations during the summer monsoon period (June to September) reveal that the rainfall variations over central India, north China, northern puts of Thailand, central parts of Brunet and Borneo and the Indonesian region east of 120 E vary in phase However, the rainfall variations over the regions surrounding the South China Sea, in particular the north-west Philippines, vary in the opposite phase. Possible dynamic causes for the spatial correlation structure obtained are discussed Based on the instrumental data available and on an objective criteria, regional rainfall anomaly time series for contiguous regions over Thailand, Malaysia, Singapore, Brunet, Indonesia and Philippines are prepared. Results reveal that although there are year-to-year random fluctuations, there are certain epochs of the above- and below-normal rainfall over each region These epochs are not forced by the El Niño La Nina frequencies Near the equatorial regions the epochs tend to last for about a decade, whereas over the tropical regions, away from the Equator, epochs last for about three decades There is no systematic climate change or trend in any of the series. Further, the impact of El Niño (La Nina) on the rainfall regimes is more severe during the below (above) normal epochs than during the above (below) normal epochs. Extreme drought flood situations tend to occur when the epochal behaviour and the El Nino La Nina events are phase-locke
The relationship between some large-scale atmospheric parameters and rainfall over southeast Asia: A comparison with features over India
No full textMonthly rainfall data for 135 stations for periods varying from 25 to 125 years are utilised to investigate the rainfall climatology over the southeast Asian monsoon regime. Monthly rainfall patterns for the regions north of equator show that maximum rainfall along the west coasts occurs during the summer monsoon period, while the maximum along the east coasts is observed during the northeast monsoon period. Over the Indonesian region (south of the equator) maximum rainfall is observed west of 125°E during northern winter and east of 125°E during northern summer. The spatial relationships of the seasonal rainfall (June to September) with the large scale parameters - the Subtropical Ridge (STR) position over the Indian and the west Pacific regions, the Darwin Pressure Tendency (DPT) and the Northern Hemisphere Surface Temperature (NHST) - reveal that within the Asian monsoon regime, not only are there any regions which are in-phase with Indian monsoon rainfall, but there are also regions which are out-of-phase. The spatial patterns of correlation coefficients with all the parameters are similar, with in-phase relationships occurring over the Indian region, some inland regions of Thailand, central parts of Brunei and the Indonesian region lying between 120° to 140°E. However, northwest Philippines and some southern pans of Kampuchea and Vietnam show an out-of-phase relationship. Even the first Empirical Orthogonal Function of seasonal rainfall shows similar spatial configuration, suggesting that the spatial correlation patterns depict the most dominant mode of interannual rainfall variability. The influence of STR and DPT (NHST) penetrates (does not penetrate) upto the equatorial regions. Possible dynamic causes leading to the observed correlation structure are also discussed
Rainfall patterns over India: Classification with Fuzzy c-means method
No full textSeasonal (June through September) percentage departure from normal rainfall patterns over India for the period 1871-1994 have been classified using Fuzzy c-means method (FCM) to identify the dominant modes of spatio-temporal variability in the Indian monsoon rainfall. Unlike the hard clustering methods, for example the Map-to-Map (MM) correlation method and the k-means (KM) clustering method, this method does not force a pattern to get classified into only one cluster but assigns varying membership to every cluster. Thus marginal patterns get classified into all clusters with different memberships. Patterns for the 124-year period are represented by the four dominant clusters. The spatial patterns associated with the extreme (deficient/excess) Indian monsoon rainfall (IMR) get high membership in one of the clusters only, while the patterns associated with the normal IMR get almost equal membership to all clusters. Even the spatial patterns during the El Niño/La Nina episodes show high preference to a particular cluster. Time variation of each cluster shows that there are epochs where a particular cluster dominates. Possible dynamic causes leading to the clusters are examined. Merits and demerits of the FCM method are also discussed
The role of natural factors (Part 1): Addressing on Mechanism of different types of ENSO, related teleconnections and solar influence
No full textThis is the final version of the article. Available from Springer Verlag via the DOI in this record.The role of natural factors, mainly the sun, is explored on major tropospheric modes of variability in a holistic way. It formulates a flow chart, depicting coupling in the ocean-atmosphere system, initiated by solar decadal variability that involves El Niño Southern Oscillation (ENSO). Possible mechanisms for Canonic ENSO, Modoki ENSO and Canonic-Modoki ENSO are proposed considering their relevance to the decadal variation of Hadley, Walker circulation and mid-latitude jets. The upper stratospheric feature of the polar vortex is included too. Teleconnections by the ENSO on Indian Summer Monsoon (ISM) with a special emphasis on the later two decades of the last century is discussed. The disruption of usual ENSO-ISM teleconnection during that period is also attended. Subsequent analyses presented some results of solar signature which could possibly trigger different types of ENSO, agreeing with proposed mechanisms of the flow chart. It addressed the changing pattern of ENSO behaviour since the 1970s. The overall study can benefit the modelling community by an improved representation of ENSO in models and a better representation of ISM teleconnection via regional Hadley cell
The role of natural factors (part 2): Indian summer monsoon in climate change period—observation and CMIP5 models
No full textThis is the final version. Available on open access from Springer via the DOI in this recordThis study discusses the role of natural factors and related teleconnections for Indian summer monsoon (ISM) with a special emphasis on later two decades of the last century. The combined influence of the sun and volcanos on ISM is examined using observational data as well as CMIP5 model outputs. Possible mechanisms relating to a disruption of the usual ENSO-ISM teleconnection for those decades are explored. Observation suggested that the regional Hadley circulation, via the NAO in the northern hemisphere and Indian Ocean Dipole in the southern hemisphere, may have a role in the change in ISM behaviour. Such features though captured well in the observation are shown missing in models. Additionally, it indicates that differences among models mainly originate in a regional level, which could be due to inconsistency in representing regional teleconnection features. Interestingly, all models perform reasonably well in terms of global thermodynamic scaling arguments. The overall study underpins important areas, where natural factors influence regional climate, but models miss out and suggest discrepancies among each other. Such knowledge has major implications in regional as well as global scale. The modelling community will also greatly benefit by an improved representation of ENSO and ISM in models
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