C2 Production Rates Research Articles

Unraveling and tuning the linear correlation between CH4 and C2 production rates in CO2 electroreduction

Although many catalysts have been reported for the CO2 electroreduction to C1 or C2 chemicals, the insufficient understanding of fundamental correlations among different products still hinders the development of universal catalyst design strategies. Herein, we first discover that the surface *CO coverage is stable over a wide potential range and reveal a linear correlation between the partial current densities of CH4 and C2 products in this potential range, also supported by the theoretical kinetic analysis. Based on the mechanism that *CHO is the common intermediate in the formation of both CH4 (*CHO → CH4) and C2 (*CHO + *CO → C2), we then unravel that this linear correlation is universal and the slope can be varied by tuning the surface *H or *CO coverage to promote the selectivity of CH4 or C2 products, respectively. As proofs-of-concept, using carbon-coated Cu particles, the surface *H coverage can be increased to enhance CH4 production, presenting a high CO2-to-CH4 Faradaic efficiency (FECH4 ∼52%) and an outstanding CH4 partial current density of –337 mA cm−2. On the other hand, using an Ag-doped Cu catalyst, the CO2RR selectivity is switched to the C2 pathway, with a substantially promoted FEC2 of 79% and a high partial current density of –421 mA cm−2. Our discovery of tuning intermediate coverages suggests a powerful catalyst design strategy for different CO2 electroreduction pathways.

TRAPPIST photometry and imaging monitoring of comet C/2013 R1 (Lovejoy): Implications for the origin of daughter species

We report the results of the narrow band photometry and imaging monitoring of comet C/2013 R1 (Lovejoy) with the robotic telescope TRAPPIST (La Silla observatory). We gathered around 400 images over 8 months pre- and post-perihelion between September 12, 2013 and July 6, 2014. We followed the evolution of the OH, NH, CN, C3 , and C2 production rates computed with the Haser model as well as the evolution of the dust production. All five gas species display an asymmetry about perihelion, the rate of brightening being steeper than the rate of fading. The study of the coma morphology reveals gas and dust jets which indicate one or several active zone(s) on the nucleus. The dust, C2 , and C3 morphologies present some similarities while the CN morphology is different. OH and NH are enhanced in the tail direction. The study of the evolution of the comet activity shows that the OH, NH, and C2 production rates evolution with the heliocentric distance is correlated to the dust evolution. The CN and, to a lesser extent, the C3 do not display such a correlation with the dust. These evidences and the comparison with parent species production rates indicate that C2 and C3 on one side and OH and NH on the other side could be -at least partially- released from organic-rich grains and icy grains. On the contrary, all evidences point to HCN being the main parent of CN in this comet.

TRAPPIST monitoring of comet C/2012 F6 (Lemmon)

We report the results of the long-term narrowband photometry and imaging monitoring of comet C/2012 F6 (Lemmon) with the robotic TRAPPIST telescope (La Silla Observatory). Observations covered 52 nights pre- and post-perihelion between December 11, 2012, and June 11, 2013 (perihelion: 24 March, 2013). We followed the evolution of the OH, NH, CN, C3 , and C2 production rates computed with the Haser model as well as the evolution of the A (θ )fρ parameter as a proxy for the dust production. All five gas species display similar slopes for the heliocentric dependence. An asymmetry about perihelion is observed, the rate of brightening being steeper than the rate of fading. The chemical composition of the comet’s coma changes slightly along the orbit: the relative abundance of C2 to CN increases with the heliocentric distance (r ) below −1.4 au and decreases with r beyond 1.4 au while the C3 -to-CN ratio is constant during our observations. The behavior of the dust is different from that of the gas, the slope of the heliocentric dependence becoming steeper in early February, correlated to a change in the visual lightcurve slope. However, the dust color does not vary during the observations. The application of several enhancement techniques on the images revealed structures in the CN, C3 , and C2 images. These features imply the existence of one or several active zone(s) on the comet nucleus. The shape of the structures is similar in these three filters and changes from a roughly hourglass shape in December and January to a corkscrew shape in February and March. The structures in the continuum filters (sampling the dust) are not correlated to those observed for the gas. During several full nights in February, we observed changes in the CN and C2 structures that repeated periodically because of the nucleus rotation, our derived rotational period being of 9.52 ± 0.05 h.

Integrated autothermal oxidative coupling and steam reforming of methane. Part 2: Development of a packed bed membrane reactor with a dual function catalyst

A numerical feasibility study was performed on the integral performance of dual function catalyst particles in a packed bed reactor equipped with a porous membrane for distributive feeding of oxygen. The exothermic oxidative coupling and the endothermic steam reforming of methane (for simultaneous production of ethylene and synthesis gas) are integrated at the level of a porous catalyst particle with distributed activity, where the presence of the intraparticle heat-sink strongly reduces the total reaction heat and the temperature gradients in the reactor, eliminating the need for expensive conventional cooling of the reactor.Numerical simulations, with reaction kinetics taken from the literature, revealed that with distributive oxygen feeding via membranes indeed the local oxygen concentration in the packed bed membrane reactor can be kept low, which combined with a high Thiele modulus for oxidative coupling makes dual function catalysis possible. Using a reforming core diameter of approximately 15–40μm, the steam reforming and oxidative coupling reaction rates could be effectively tuned to achieve autothermal operation, while the methane conversion was enhanced from 34 to 48% at optimum C2 production rates. In addition, it was shown that the temperature profiles in the reactor can be strongly reduced by employing the dual function catalyst and that the use of axial oxygen membrane flux profiles enables the use of a single particle configuration to approach autothermal operation in the entire reactor.

A Multi-Wavelength Simultaneous Study of the Composition of the Halley Family Comet 8P/Tuttle

We report on simultaneous optical and infrared observations of the Halley Family comet 8P/Tuttle performed with the ESO Very Large Telescope. Such multi-wavelength and coordinated observations are a good example of what can be done to support space missions. From high resolution optical spectroscopy of the CN (0,0) 388 nm and NH2 (0,9,0) 610 nm bands using UVES at UT2 we determined 12C/13C = 90 ± 10 and 14N/15N = 150 ± 20 in CN and we derived a nuclear spin temperature of NH3 of 29 ± 1 K. These values are similar to those found in Oort-Cloud and Jupiter Family comets. From low resolution long slit spectroscopy with FORS1 at UT2 we determined the CN, C3 and C2 production rates and the parent and daughter scale lengths up to 5.2 105 km tailward. From high resolution IR spectroscopy with CRIRES at UT1 we measured simultaneously the production rates and mixing ratios of H2O, HCN, C2H2, CH4, C2H6, and CH3OH.

Gas and dust in Comet C/2000 WM1 during its closest approach to Earth: Optical imaging and long-slit spectroscopy

We have investigated the dust and gas coma of the comet C/2000 WM1 (LINEAR) during Dec. 2−4, 2001, its closest approach to Earth. The gaseous coma is slightly asymmetric in CN and C2, indicating the presence of some structures which are detected either by applying enhancement techniques or studying the azimuthal distribution of material. This asymmetry is due to the existence of a double jet in a direction almost perpendicular to the Sun-comet direction. These structures are not clearly detected in C3. The morphological analysis of the dust coma shows a faint, short and narrow sunward structure, besides the dust tail in a position angle of ∼60 ◦ . CN is produced at a rate of 2.43 × 10 26 s −1 with lifetimes that can be considered as representative of HCN being the parent species. In the case of C2, the use of usual lifetimes (τp ,τ d) requires a modified two-steps photolytic model to account for the pronounced flatness of the column density profile in the inner coma. Usual steady-state model gives rise to a QC2 = 3.11 × 10 26 s −1 . Other gas species as C3 and NH2 have been also detected from the spectrum, although with a lower S/N. The computed production rates are 3.6 × 10 24 s −1 and 2.15 × 10 26 s −1 , respectively. Determination of the CN and C2 production rates by means of the Haser modeling (Haser 1957) with customary scalelengths indicates that C/2000 WM1 is a C2-enriched comet at 1.178 AU heliocentric distance. The dust production rate, parameterised by A(θ)f ρ ,i s∼300−400 cm when measured in two continuum regions, centered at 4845 and 6840 A, whilst this value is slightly lower in Bessel R and Gunn i (∼230 cm). Thus, the gas-to-dust mass ratio is in the order of 6.3, a relatively gassy comet. The surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in east−west direction from the long-slit spectrum, can be well fitted with m ≈ 1i n logB − log ρ representation. On the other hand, the study of the dust coma as imaged with the broadband and narrowband filters reveals that variations in the size and/or composition of the grains might be occurring while traveling outward. This fact is manifested as a sharp decrease of Afvs. ρ at 700 ≤ ρ ≤ 15 000 km. The fit of these Afprofiles vs. ρ points to the existence of two different grain populations with their own scattering properties and scalelengths (i.e. lifetimes). These pronounced variations of Afvs. ρ are accompanied by clear dust color gradients in the 2D maps, unlike the dust color vs. ρ as derived from the spectrum. Since spectroscopic measurements are only obtained in east−west direction and with a very long exposure time, the effect of color variations inside the coma due to disintegrating grains is difficult to detect.

Polarised and unpolarised charmonium production at higher orders in ν

We study the unpolarised and polarised hadro-production of charmonium in non-relativistic QCD (NRQCD) at low transverse momentum, including sufficiently higher orders in the relative velocity, ν, so as to study the ratio of χ c1 and χ c2 production rates.

Aperture Polarimetry And Photometry Of Comet Hale-Bopp

We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7 m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used. The C2 and C3 production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets. The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49° the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are most likely caused by an over-abundance of small or/and absorbing dust particles in the coma.

Spectroscopy of comet P/Schaumasse

Column density profiles for CN, C3, C2 and NH have been determined from a long-slit CCD spectrum of periodic comet P/Schaumasse (1992x). Comparisons of these profiles with Haser models indicate that the ratios of the CN, C3 and C2 production rates are typical for a short-period comet. Although the scale lengths for NH and its parent species are uncertain, the results indicate that the production rate for NH is much greater than for either C2 or CN.

Evaluation of gas production rates in some comets

Emission fluxes of CN, C2 and C3 carbon-bearing molecular species observed in the coma of comets Bennett (1969i ≡ 1970II), West (1975n ≡ 1976VI), P Halley (1982i), Hartley-Good (19851) and Bradfield (1987s) are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C2 and C3 production rates on the heliocentric distance and the possible correlations among these radicals is studied and briefly discussed.

Gas Production Rates in Comets

Emission fluxes of CN, C2and C3carbon- bearing molecular species observed in the coma of comets Bennett (1969i ≡ 1970II), West (1975n = 1976VI), P/Halley (1982i), Hartley-Good (1985l) and Bradfield (1987s) are analysed in the framework of Haser model. CN, C2and C3production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C2and C3production rates on the heliocentric distance and the possible correlations among these radicals is studied and briefly discussed.

Molecular emission from the head of Comet Okazaki-Levy-Rudenko (1989r)

Spectrophotometric scans of the head of Comet Okazaki-Levy-Rudenko during three nights in October 1989 are presented. An estimate of the CN and C2 production rates and column densities have been made.

An analysis of the spectrophotometric and photometric observations of comets 1967II, 1968I, 1968V, and 1968VI

Emission fluxes of CN, C2 and C3 carbon-bearing molecular species observed in the coma of comets Rudnicki (1967II), Ikeya-Seki (1968I), Whitaker-Thomas (1968V), and Honda (1968VI) are analysed in the framework of Haser model. CN, C3 and C3 production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C2 and C3 production rates on the heliocentric distance and the possible correlations among these radicals is studied. It is shown that comets Ikeya-Seki (1968I) and Honda (1968VI) have the same mean color indices (B-V) and (U-B).

Haser model CN, C2 and C3 production rates in some comets

Emission fluxes of CN, C2 and C3 species observed in the coma of some comets are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies and dependence of CN, C2 and C3 production rates on the heliocentric distance is studied. Evidence for a burst type activity around January 15, 1974 in the post-perihelion period of comet Kohoutek (1973f) is observed.

Another look at abundance correlations among comets

Abundance correlations among comets are examined using three different data sets. The C2 and C3 production rates prove to be well correlated with CN production rates, but large scatter exists. It is concluded that a 'normal' comet is one for which Q(C3)/Q(CN) = 0.19 + or - 0.13 and Q(C2)/Q(CN) = 1.46 + or 0.68.

Narrowband photometry of Comet P/Stephan-Oterma and the backscattering properties of cometary grains

The results of narrowband filter photometry of Comet P/Stephan-Oterma are presented. For heliocentric distances between 1.95 and 1.58 AU prior to perihelion, OH, CN, and C2 production rates varied approximately as r to the -5th power. Following perihelion, an even steeper dependence of production rate on heliocentric distance was observed for these species. A pronounced brightening of the comet's continuum near opposition is interpreted in terms of preferential backscattering by the cometary dust. While Comet P/Stephan-Oterma is one of the dustier comets observed, it shows no indication of compositional anomalies in either the gas or the dust.