Heavy quark fragmentation functions at next-to-leading perturbative QCD

It is well-known that the dominant mechanism to produce hadronic bound states with large transverse momentum is fragmentation. This mechanism is described by the fragmentation functions (FFs) which are the universal and process-independent functions. Here, we review the perturbative FFs formalism as an appropriate tool for studying these hadronization processes and detail the extension of this formalism at next-to-leading order (NLO). Using the Suzuki's model, we calculate the perturbative QCD FF for a heavy quark to fragment into a S-wave heavy meson at NLO. As an example, we study the LO and NLO FFs for a charm quark to split into the S-wave $D$-meson and compare our analytic results both with experimental data and well-known phenomenological models

Bifunctional catalysts for the direct production of liquid fuels from syngas

Design and development of catalyst formulations that maximize the direct production of liquid fuels by combining Fischer-Tropsch synthesis (FTS), hydrocarbon cracking, and isomerization into one single catalyst particle (bifunctional FTS catalyst) have been investigated in this thesis. To achieve this aim, a second functionality (other than FTS) has to be added to the catalyst formulation to break the limitation of a classical Anderson-Schulz-Flory (ASF) distribution of FTS products. Since upgrading the FTS hydrocarbons is mostly based on acid-catalyzed reactions, zeolites are potential candidates for this approach. In this relation, recent literature highlights the use of H-ZSM-5 for the following reasons: (1) it is one of the few zeolites industrially produced and applied for acid-catalyzed hydrocarbon conversion reactions, (2) due to its narrow channel type structure and well distributed acid sites, it represents a (relatively) stable catalytic performance, especially at low-temperature Fischer-Tropsch process conditions, and (3) besides acid-catalyzed cracking, it has a fair isomerization and oligomerization activity at low temperatures which is essential to increase the octane number in case of gasoline cut and improve the cold flow properties of diesel (Chapter 1). All the FTS experiments in this thesis were performed on a homemade lab-scale unit described in Chapter 2. The experimental setup is based on ‘six-flow fixed-bed microreactor’ concept which offers an increased experimental throughput as well as accuracy. The latter is due to equal conditions (in terms of process temperature, feed composition, equipment conditions, etc.) under which the six parallel experiments are performed. The condition is that all the reactors (flows) should behave identical, i.e., provide similar results employing the same catalyst. Design and operation of such piece of equipment confirm that indeed it is possible to obtain reproducible activity and selectivity data within an acceptable experimental error (Chapter 2). Incorporation of separate mass flow and pressure controllers as well as product separation units in each flow allows running reactions with high production of liquid fractions (as in conventional single-flow operations). This is crucial for a complete quantification of FTS product compositions and will represent an advantage over high-throughput setups with more than ten flows where such instrumental considerations lead to elevated equipment volume, cost, and operation complexity. Therefore, a six-flow fixed-bed microreactor unit combines the advantages of high-throughput and conventional FTS setups at the lab-scale (Chapter 2). In Chapter 3, combination of cobalt FTS active phase and acid functionality of H-ZSM-5 zeolite is explored in two different catalyst configurations: (i) H-ZSM-5 as catalytic coating on Co and (ii) H-ZSM-5 as catalytic support for Co. Spherical shaped Co/SiO2 is chosen as a conventional FTS catalyst for comparison and used as precursor to synthesize the H-ZSM-5-coated Co-catalyst. In the first case, various silicalite-1 and H-ZSM-5-coated reference samples were prepared by subjecting Co/SiO2 to a direct hydrothermal procedure (state of the art method to prepare zeolite coatings). Silica in the Co/SiO2 catalyst transforms into the zeolite when subjected to the hydrothermal synthesis while the original shape of the support is preserved after the transformation. By this synthesis approach, Co3O4 agglomerates are enwrapped in an H-ZSM-5 coating on a nanometer scale. The resulting bifunctional catalyst considerably lowers the production of FTS wax (C21+), as compared with Co/SiO2. The membrane effect of this coating, however, results in mass transport limitations that lower the productivity. In the absence of acid functionality, accumulation of carbonaceous species deactivates the silicalite-1-coated reference catalyst. The H-ZSM-5-coated Co-catalyst shows lower CO conversion levels than the conventional Co/SiO2 due to the membrane coating. This lower activity and modification of Co crystallites because of the hydrothermal treatment should be considered as the major drawbacks of this approach. On the other hand, systematic comparison of catalytic performances between physically mixed, coated catalyst, and non-acidic coated catalysts shows that the close proximity between the FTS and acid components is essential for improving the bifunctionality of the catalyst to increase the selectivity towards liquid products and eliminate the FTS heavy hydrocarbons (Chapter 3). Such contact can be maximized when Co is directly dispersed over the zeolite (configuration (ii)). Since the Co accessibility is better in this configuration, limitations associated with the membrane effect of a zeolite coating can be overcome while preserving the important close proximity of the two functionalities. To compensate for the relatively low intrinsic activity of FTS catalysts and to increase their productivity, high metal loadings are typically required in FTS catalyst formulations. In general, microporous zeolites are devoid of mesopore surface area, essential for an optimal dispersion of Co particles at high metal loadings. On the other hand, formation of metal clusters in the micropores is undesired, as Co particles smaller than 6 nm are not optimal for FTS in terms of activity and selectivity. Therefore, mesoporous H-ZSM-5 (‘mesoH-ZSM-5’) is studied as carrier for Co-based FTS catalysts in Chapters 4 to 7. Synthesis optimization of mesoH-ZSM-5 involved demetalation via consecutive base and acid treatments. NaOH (alkaline) and tetrapropylammonium hydroxide (TPAOH, organic) bases were employed as desilicating agents. Consecutive basic-acid treatments provides H-ZSM-5 with high mesopore surface areas and volumes. Under similar treatment conditions, NaOH results in a more severe desilication than TPAOH, creating mesostructures with pore sizes and volumes very similar to the amorphous SiO2 reference support. A more controlled desilication with TPAOH gives rise to more mesoporosity suggesting a higher degree of hierarchy with large cavities communicated with smaller mesopores. Further, TPAOH is preferred over NaOH, since Na+ is a well-known poison for Co-based FTS catalysts and trace amounts results in a lower FTS activity as compared with the organic base treated samples (Chapter 4). The consecutive acid treatment (with HNO3) removes the produced extraframework aluminum, caused by zeolite desilication, and boosts the FTS activity. Moreover, the acid treatment restores the Brønsted acidity of mesoH-ZSM-5 (Chapter 5). The large mesopore surface area of mesoH-ZSM-5 improves the metal dispersion at elevated Co loadings. The Co/mesoH-ZSM-5 catalyst is a much more active catalyst than Co/H-ZSM-5 and the conventional Co/SiO2. Moreover, time-on-stream stability of Co/mesoH-ZSM-5 and Co/SiO2 is comparable in terms of CO conversion, during 140 h of FTS reaction. As compared with Co/H-ZSM-5, the improved transport properties of mesoH-ZSM-5 increase the selectivity of the supported Co-catalyst towards liquid hydrocarbons and lowers that to methane. The high selectivity to liquid hydrocarbons over H-ZSM-5-supported catalysts is visible as a cutoff in the molar distribution above C11 in terms of the ASF distribution of conventional catalysts (e.g., Co/SiO2). Measurements after 140 h on-stream show that Co/mesoH-ZSM-5 is ca. three times more selective than Co/SiO2 towards the C5–C11 cut, producing a large fraction of unsaturated hydrocarbons, other than ?-olefins. Moreover, wax production is considerably suppressed over the zeolite-containing catalyst (513 K, 15 bar total pressure, feed composition H2/CO = 1, and GHSV = 12 m3STP kg-1cat h-1) (Chapters 5 and 6). Origins of methane selectivity over zeolite-supported Co-catalysts are also investigated. mesoH-ZSM-5 was used as carrier for a series of Co-based FTS catalysts of different loadings with ZrO2 and/or Ru added as promoters. By means of advanced catalyst characterization techniques (including quasi in situ dark field transmission electron microscopy, CO adsorption-diffuse reflectance infrared fourier transform spectroscopy, synchrotron-based X-ray absorption spectroscopy (EXAFS and XANES), etc.) in addition to a detailed catalyst performance assessment, a relationship is drawn between structural characteristics of Co (when supported on the zeolite) and its FTS activity and selectivity. Addition of either ZrO2 or Ru considerably increases the Co reducibility upon activation at 773 K and improves the FTS activity during the first 80 h of reaction after which the activity is returned to that of the unpromoted catalyst. This catalyst promotion does not significantly affect the product selectivity (Chapter 6). Methane selectivity over the zeolite-supported Co-catalysts originates from direct CO hydrogenation and hydrocarbon hydrogenolysis as the most important side reactions on coordinatively unsaturated Co sites, which are stabilized as consequence of a strong metal-zeolite interaction (Chapters 5 and 6). In addition to mesoH-ZSM-5, other zeolite topologies were investigated as FTS catalyst carriers: delaminated MWW (H-ITQ-2) and mesoporous FAU (Chapter 7). All the zeolite supports were carefully characterized for their number and strength of acid sites by temperature-programmed NH3 desorption and pyridine adsorption. To explore the role of acid-catalyzed reactions, including hydrocracking and isomerization, in the altered product distribution of zeolite-containing catalysts (with respect to conventional ones), acid-catalyzed model reactions of C6 (n-hexane or 1-hexene) were performed. Zeolite acid density and strength are essential parameters to tune the FTS product selectivity towards liquid hydrocarbons. Only strong acid sites, active for hydrocracking at the operating temperature window of Co-based FTS catalysts, give rise to deviations from a conventional ASF product distribution (Chapter 7). On purpose (partial) deactivation of Brønsted acidity in mesoH-ZSM-5 by carbonaceous species (during catalyst synthesis) decreases the iso- to n-paraffin ratio and selectivity to gasoline fraction which further confirms the above-mentioned role of acid-catalyzed reactions in tuning the product selectivity (Chapter 5). When acid site domains are in a close vicinity of FTS sites at a nanometer scale, ?-olefins, which are primary FTS products, may crack or isomerize before they are hydrogenated. Indeed 1-hexene conversion is considerably higher than that of n-hexane over mesoH-ZSM-5 (Chapter 6). The classical mechanism of such acid-catalyzed reactions, through rearrangement of a secondary carbocation into a protonated dialkylcyclopropane or through a bimolecular mechanism, increases the hydrocarbons’ degree of branching. Since FTS may mainly produce linear ?-olefins, considerable amounts of other unsaturated hydrocarbons in the liquid products are formed over the acid sites. Altogether, our results demonstrate that the use of mesoporous zeolites as FTS supports holds many promises for the direct synthesis of liquid fuels from syngas. The challenges that still need to be addressed include a better control over the product selectivity of bifunctional catalysts. In this respect, it is essential to tackle the aforementioned origin(s) of methane production on the zeolite-supported Co-catalysts. In addition, more insight is required to further separate and define the contributions of ‘the metal’ and ‘the zeolite/acid’ functions in the overall product spectrum of these catalysts. While neglected or poorly described in the open literature, such insight is necessary for further catalyst optimization in relation to the product spectrum and practical applications. Detailed acid-catalyzed hydrocarbon conversion studies, under conditions relevant to that of FTS, together with reference experiments and detailed kinetic investigations are considered essential for a better understanding of bifunctional FTS systems. Finally, the long term stability of these catalysts is largely unexplored. As an ongoing research, a new PhD project has recently started on this topic at the Catalysis Engineering section of Delft University of technology.Chemical EngineeringApplied Science

Probing heavy charged Higgs bosons through bottom flavored hadrons in the

Observing light or heavy charged Higgs bosons $$H^\pm$$, lighter or heavier than the top quark, would be instant evidence of physics beyond the Standard Model. For this reason, in recent years searches for charged Higgs bosons have been in the center of attention of current colliders such as the CERN Large Hadron Collider (LHC). In spite of all efforts, no signal has been yet observed. Especially, the results of CMS and ATLAS experiments have excluded a large region in the MSSM $$m_{H^+}-\tan \beta$$ parameter space for $$m_{H^+}=80{-}160$$ GeV corresponding to the entire range of $$\tan \beta$$ up to 60. Therefore, it seems that one should concentrate on probing heavy charged Higgs bosons ($$m_{H^\pm }>m_t$$) so in this context each new probing channel is welcomed. In this work, we intend to present our proposed channel to search for heavy charged Higgses through the study of scaled-energy distribution of bottom-flavored mesons (B) inclusively produced in charged Higgs decay, i.e., $$H^+\rightarrow t\bar{b}\rightarrow B+X$$. Our study is carried out within the framework of the generic two Higgs doublet model (2HDM) using the massless scheme where the zero mass parton approximation is adopted for bottom quark

Six-flow operations for catalyst development in Fischer-Tropsch synthesis: Bridging the gap between high-throughput experimentation and extensive product evaluation

Design and operation of a “six-flow fixed-bed microreactor” setup for Fischer-Tropsch synthesis (FTS) is described. The unit consists of feed and mixing, flow division, reaction, separation, and analysis sections. The reactor system is made of five heating blocks with individual temperature controllers, assuring an identical isothermal zone of at least 10 cm along six fixed-bed microreactor inserts (4?mm inner diameter). Such a lab-scale setup allows running six experiments in parallel, under equal feed composition, reaction temperature, and conditions of separation and analysis equipment. It permits separate collection of wax and liquid samples (from each flow line), allowing operation with high productivities of C5+ hydrocarbons. The latter is crucial for a complete understanding of FTS product compositions and will represent an advantage over high-throughput setups with more than ten flows where such instrumental considerations lead to elevated equipment volume, cost, and operation complexity. The identical performance (of the six flows) under similar reaction conditions was assured by testing a same catalyst batch, loaded in all microreactors.ChemE/Chemical EngineeringApplied Science

Mechanic, Half-Metallic and Thermoelectric Properties of the PdZrTiAl under pressure: A DFT study

The half-metallic, mechanical, and transport properties of the quaternary Heusler compound of PdZrTiAl is discussed under hydrostatic pressures in the range of –11.4 GPa to 18.4 GPa in the framework of the density functional theory (DFT) and Boltzmann quasi-classical theory using the generalization gradient approximation (GGA). By applying the stress, the band gap in the minor spin increases so that the lowest band is obtained 0.25 eV at the pressure of –11.4 GPa while the maximum gap is calculated 0.9 eV at the pressure of 18.4 GPa. In all positive and negative pressures, the PdZrTiAl composition exhibits a half-metallic behavior 100% spin polarization at the Fermi level. It is also found that applying stress increases the Seebeck coefficient in both spin directions. In the minority spin, the n-type PdZrTiAl, the power factor (PF) for all the cases is greater in the equilibrium state than the strain and stress conditions whereas in the majority spin, the PF value of the stress state is greater than the other two. The non-dimensional figure of merit (ZT) is significant and is about one in spin down in the room temperature for the all pressure states that it remains on this value by applying pressure. The obtained elastic constants indicate that the PdZrTiAl crystalline structure has a mechanical stability. Based on the Yong (E), Bulk (B) and shear (G) modulus and Poisson (n) ratio, the brittle-ductile behavior of this compound has been investigated under pressure. The results indicate that PdZrTiAl has a ductile nature and it is a stiffness compound in which elastic and mechanical instability increases by applying strain

Catalysis engineering of bifunctional solids for the one-step synthesis of liquid fuels from syngas: A review

The combination of acidic zeolites and Fischer–Tropsch synthesis (FTS) catalysts for one-step production of liquid fuels from syngas is critically reviewed. Bifunctional systems are classified by the proximity between FTS and acid functionalities on three levels: reactor, catalyst particle, and active phase. A thorough analysis of the published literature on this topic reveals that efficiency in the production of liquid fuels correlates well with the proximity of FTS and acid sites. Moreover, possible side reactions over the FTS metal, including direct CO hydrogenation and hydrocarbon hydrogenolysis, are addressed. The contribution of these side reactions should carefully be considered and separated from that of the zeolite function when evaluating the performance and product spectrum of zeolite-containing catalysts.Chemical EngineeringApplied Science