Ortho-para Hydrogen Conversion Research Articles

Ortho-para conversion of hydrogen in copper as origin of time-dependent heat leaks

It is shown that H2 precipitated into bubbles in copper undergoes ortho-para conversion. This conversion, at a rate of about 2%/h, and the hydrogen content are detected quantitatively by the time-dependent heat release associated with them. The heating is comparable to the time-dependent heat leak observed in copper nuclear refrigerators. The amount of H2 necessary to explain the data lies between 10 and 100 ppm. Due to its much smaller conversion rate and the smaller energy conversion, the heat released from D2 in copper is essentially time independent fort<100 h and is up to two orders of magnitude smaller than the one for H2.

Magnetocatalytic effect on rare earth oxides

The rates of ortho–para hydrogen conversion on some self-supported rare earth oxides have been measured at room temperature. The change of the rates by application of a weak external magnetic field has been determined. Only on Pr2O3 and Yb2O3 a small decrease of the rates could be observed. On the basis of two coupled proton spins under the influence of a magnetic field—composed of an extrinsic and an intrinsic part—a simple theory is given which explains the occurrence, the sign and also the field dependence of the magnetocatalytic effect. It is found that the sign depends upon the mutual orientation of the H2 molecules and the magnetic moments of the active sites. Finally, the theory allows us to deduce the well-known formula for the ortho–para hydrogen conversion rate given by Wigner.

Certain relationships in predicting the activity of oxide catalysts

1. In the model reactions of low-temperature hydrogenation of ethylene and double-bond migration in butenes, we have carried out a systematic investigation of the catalytic properties of the rare earths and the oxides of first-row and second-row transition elements and subgroups I and IIIb of the periodic table, as well as type Y zeolite in the Na, Cr, Nd, and La forms, and CeO2-Nd2O3 solid solutions. The data available in the literature on these reactions have been analyzed critically and refined, as well as data on the ortho-para conversion and homomolecular exchange of hydrogen and isotope deuterium exchange in hydrocarbons. 2. For the first time, we have established experimentally and justified theoretically the extremal dependence of specific catalytic activity of oxides on the calcination temperature. Activity of oxide systems is manifested only after partial dehydroxylation and desorption of acidic impurities (CO2, NO2, etc.). At the Tamman temperature, deactivation is observed. 3. In the reactions investigated, the low-temperature activity of oxides with a valence of the element equal to the number of the group increases from top to bottom and from right to left in the groups and periods of the periodic table. For oxides of a given element, the activity decreases with increasing degree of oxidation of the metal. It is suggested that the active centers are of a basic nature. 4. All oxide catalysts for low-temperature hydrogenation are also active, under similar conditions, in double-bond migration in olefins, homomolecular exchange and ortho-para conversion of hydrogen, and isotope deuterium exchange in hydrocarbons; the mechanism of catalyst action is similar in all of these reactions.

Quantum formulation of a magneto-catalytic reaction

A general quantum formulation of an ortho-para hydrogen conversion reaction on dilute paramagnetic catalysts by means of tensorial algebra is developed. An efficiency operator describing the tensorial structure of the conversion rate is built, and important selection rules pertaining to the tensorial ranks are derived. Taking into account the time and spatial symmetries of the system, an expansion of the density operator over some multipole operators is performed. A magnetic field allows one to separate and thus to measure the relative influence of the magnetic dipoles and electric quadrupoles. It is concluded that the important field acceleration, observed by Selwood, corresponds to dipole highly polarized states. These states may be described by an alignment of the ${\mathrm{H}}_{2}$ nuclear spins along the electronic spins of chromium catalysts, or along the ${\mathrm{H}}_{2}$ rotational angular momentum when rare-earth catalysts are used.

Interpretation of ortho–para hydrogen conversion

The interpretation of statistics of the nuclear spin states in the ortho–para hydrogen conversion by hydrogen atoms is considered. It is shown that the observable rate coefficient (which is the sum of the forward and reverse rate coefficients for ortho→para conversion) equals the distinguishable-atom rate coefficient in the classical limit. In addition to spin-1/2 nuclei, we also consider spin-1 nuclei and spin-0 nuclei.

Magnetic effects on the ortho-parahydrogen conversion over α-Cr 2O 3, CoO, and MnO

Catalyzed ortho-parahydrogen conversion rates were measured on two antiferromagnetic oxides, α-Cr 2 O 3 and CoO over a range of temperature and both with and without an extrinsic magnetic field. A few measurements were made on MnO. The conversion mechanism was, in all cases, nondissociative. Results for the three catalysts, while qualitatively similar, differed in detail. A correlation between the zero field conversion rate and the extrinsic field rate suggests a relation to the paramagnetic-antiferromagnetic phase transition in catalysts of high surface site density. A positive extrinsic field effect was observed only at temperatures above the Neel point ( T N ). Negative effects were found, in some cases, both above and below T N . The latter results were characterized by a maximum rate decrease just below T N and by a rate saturation effect that was a function of applied field strength. The several effects observed suggest that, on high site density surfaces, two or more mechanisms are operative in an extrinsic field. These, and other related results, show that the current state of conversion theory over magnetic surfaces is still incomplete.

Diffusion and ortho-para conversion in solid hydrogen

The ortho-para conversion in solid hydrogen is investigated at 12, 4.2, and 1.57 K in long-term experiments up to 1200 h. The results are discussed in the light of the influence of molecular diffusion on the conversion rate. At 4.2 and 1.57 K, the conversion rate shows that the diffusion coefficient is $D\ensuremath{\approx}{10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. This value is much larger than those predicted from an extrapolation of experimental results at higher temperatures and from recent theoretical predictions. At 1.57 K, an increase of the conversion rate with decreasing $o\ensuremath{-}{\mathrm{H}}_{2}$ concentration is observed. Calculations show this increase is due to clustering of $o\ensuremath{-}{\mathrm{H}}_{2}$ molecules caused by electric quadrupole-quadrupole interactions between them. The estimated diffusion rate is consistent with that found previously from the rate of clustering.

The catalyzed o-pH2 conversion and magnetocatalytic effects on EuO and CrO2

Abstract Catalyzed ortho-parahydrogen conversion rates have been measured on both insulating and conducting types of europium monoxide over the temperature range 65–400 K. There was no rate anomaly in the neighborhood of the ferromagnetic-paramagnetic phase transition. A positive extrinsic field magnetocatalytic effect was observed in the paramagnetic phase but little effect was found below the Curie point. Chromium dioxide proved to be too reactive for measurements to be made near the Curie point. Catalytic activity was found, but no extrinsic field effect at room temperature. The theoretical implications of these results are discussed.

The activation of yttria, lutetia, and ytterbia for the ortho-parahydrogen conversion

Abstract The diamagnetic oxides Y2O3 and Lu2O3 may be activated for the p-H2 conversion by heating them in H2 between 823 and 973 K. This pretreatment appears to generate the ions Y2+ and Lu2+, respectively. From the quantity of water formed during the activation it is possible to estimate the surface concentration of paramagnetic sites. By combining this information with the specific conversion rates it is possible to obtain the net conversion rate per active site. The results are in agreement, to about one order, with recent theoretical calculations of the absolute rates. From conversion rate studies over Lu2O3 and Yb2O3 and from extrinsic field studies, it is concluded that heating Yb2O3 in H2, and possibly other rare earths in the C crystallographic form, causes surface reactions resulting in a 4fn5d0 → 4fn5d1 electronic change rather than 4fn5d0 → 4fn+15d0.

Comparison of the Catalytic Activities of Metal Oxides for Hydrogen-Deuterium Equilibration and Ortho- Parahydrogen Conversion

Abstract The corrected activities, rates divided by square of spin magnatic moment of a metal ion, in ortho-parahydrogen conversion on first series transition metal oxides at 77°C were shown to give a very similar activity pattern to that obtain in hydrogen-deuterium equilibrtation.

Catalytic activity of organic semiconductors. Hydrogen conversion on triphenylene-alkali metal ion radical salts

An extensive study was made of the kinetics and the mechanism of the orthoparahydrogen conversion, the deuterium exchange reaction, and the hydrogen-deuterium equilibration over solid triphenylene-alkali metal ion radical salts (K-, Rb-, and Cs- salts). Two species, mono- and dinegative anion salts, were stoichiometrically prepared. Two different reaction paths, one through the chemisorption process and the other via the exchange process, were observed. With simultaneous measurements of the rate of hydrogen adsorption and also of the time course of ortho-parahydrogen conversion on the salt catalysts, a novel kinetic analysis, based on the two processes, was presented. In comparing these processes, the chemisorption process was found to be predominant in every reaction over the complexes in question. Further, it was observed that the catalytic activity depends strongly on the valency of anions and on the sort of alkali metal; the rate constant of the dianion salt was much larger than that of the monoanion salt, and the activity increased in the order of K, Rb, and Cs.

Parahydrogen conversion and magnetocatalytic effects over yttria and lutetia

Yttria (Y 2O 3) and lutetia (Lu 2O 3) pretreated in hydrogen above 550 °C become effective catalysts for the ortho-parahydrogen conversion by the magnetic mechanism. The source of activity in each case appears to be the paramagnetic 2+ ion, Y 2+ or Lu 2+. This view is supported by electron spin resonance measurements. The rate of conversion is strongly influenced by a magnetic field of a few oersteds. Over lutetia a field of 40 Oe applied to the catalyst decreases the rate by 42%, and the presence of the Earth's field may readily be demonstrated. Some recent theoretical work is consistent with these magnetocatalytic results.

Orientation Effect of the Surface Magnetization of Ferromagnetic Catalysts on Ortho-Para Hydrogen Conversion

It is a well-known result that an external magnetic field may enhance catalytic activity. A tentative interpretation of this effect in the case of ortho-para hydrogen conversion is given. When the energy of activation is of magnetic origin, the conversion rate is found to be related to the structure of the ferromagnetic domains (or to the magnetic phase) at the surface. It is found that this rate is 16 times greater when the surface magnetization is parallel than when it is perpendicular to the surface plane. More generally, in catalytic reactions the interaction symmetry may explain the effect of an external magnetic field on these reactions.

Theoretical Rates and Correlation Functions in Ortho-ParaH2Conversion on Paramagnetic Surfaces

A theoretical study of ortho-para hydrogen conversion on paramagnetic surfaces is given. In the absorbed layer, hydrogen molecules are assumed to behave like plane rotators, and their translation at the surface is described by means of a two-dimensional diffusion equation. The spatial dependence of the dipolar matrix elements is found to be important only in the close vicinity of surface impurities. Ortho-para time-correlation functions are computed, and their behavior is compared to the nuclear relaxation case. The total conversion probability is calculated, and two extreme cases are distinguished: (i) The ortho-para conversion energy goes to the thermal bath and the conversion rate is proportional to the diffusion constant and (ii) the ortho-para conversion energy goes to the spin impurity system. In contrast to Leffler's results, the best catalysts are found to be those which have the shortest impurity relaxation times.

A NMR study of the spin conversion in solid methane

The proton magnetization of solid methane is studied during the conversion of the spin modifications, as a function of temperature (4.2 to 2.0 K), oxygen concentration (12 ppm to 4.1 %) and frequency (25.6 and 8.95 MHz). Kinetics of the conversion is discussed and some comparison with the ortho-para conversion in hydrogen is made.

Studies of the hydrogen held by solids. XX. The chromatographic separation of the isotopic and allotropic hydrogens on alumina and fluorided alumina

Chromatographic separation factors and retention times have been determined for the hydrogen allotropes and isotopes over pseudoboehmite, ν + γ alumina, and fluorided alumina columns. Measurements were made as a function of the extent of dehydroxylation of the surface, pulse size, and the fluoride level. With all preparations, the retention times and separation factors decreased with increasing pulse volume. With pseudoboehmite, the retention times of ortho and para-H 2 increased rapidly as the catalyst was dehydroxylated and converted into the transition aluminas. Separation factors increased with retention times for pretreatment temperatures up to about 350°C, where the ortho-para separation collapsed as the allotropic conversion reaction became faster than the separation. A related, but somewhat different, behavior was exhibited by fluorided aluminas and the transition alumina after storage. With these materials the retention times and separation factors remained nearly constant up to a critical pretreatment temperature, where the separation collapsed; at still higher pretreatment temperatures the retention times increased sharply. With fluorided aluminas the effects of the ortho-para conversion reaction and the sharp increase in retention time set in at lower pretreatment temperatures, the higher the fluoride content. The hydrogen isotopes were used to extend the measurements to higher pretreatment temperatures where the allotropes could not be separated. With HD the retention times continued to increase as the catalyst was dehydroxylated to the point where the test pulse was virtually irreversibly chemisorbed by the column; separation factors were also increased substantially. The separation factor data were used to estimate lower limits of the magnitude of the electric field emanating from the strongest sites. A maximum value of 2.67 volts/Å was calculated. Selective poisoning with CO 2 was used to identify the sites used in the separations with those which effect the ortho-para hydrogen conversion.

Catalytic activity of organic semiconductors and enzymes.

The catalytic activity and reactivity of the charge-transfer complexes and the ion radical salts have been investigated by means of the ortho-parahydrogen conversion, the deuterized reaction and the hydrogen-deuterium equilibration. The predominant reaction on the aromatics-alkali metal ion salt occurred through a chemisorption mechanism. These findings were confirmed by the electrical conductivity and also the photoemission measurements of unexposed and hydrogen-exposed ion salts. Hydrogenase activity in the dry state is also discussed.

Hydrogen reaction mechanisms on TiO 2

A kinetic study of the ortho-para hydrogen conversion and the hydrogen-deuterium equilibration reaction was made on the anatase form of TiO 2, over the temperature range 93 ° to 773 °K. The products of the conversion reaction were analyzed by microthermal conductivity and those of the equilibration reaction by mass spectrometry. Three distinct temperature regions of activity were found and the results are discussed in terms of a low-temperature physical mechanism, a high-temperature chemical mechanism, and the nature of the surface sites on TiO 2.

Kinetic Studies of the Catalytic Ortho-Parahydrogen Conversion and Hydrogen-Deuterium Equilibration on γ-Alumina

Abstract Kinetic studies of the catalytic ortho-parahydrogen conversion and hydrogen-deuterium equilibration on highly purified γ-alumina were conducted in the range of temperature from 77 to 221°K and in the range of pressure from 10 to 120 mmHg. The rate equation obtained can be well explained by a mechanism which involves the reaction between an adsorbed atom and a molecule. The true activation energy, E, and the heat of adsorption of the reactive molecule, Q, were estimated as E=0.51–0.80 and Q=0.51–0.72 kcal/mol for the conversion (77–95°K), and E=4.6 and Q=1.9 kcal/mol for the equilibration (145–221°K). The conversion at temperatures below 90°K was found to proceed mostly via the magnetic mechanism. A thermal desorption study of adsorbed ethylene showed that there are three types of adsorption on γ-alumina with different activation energies of desorption, and that these species occupy a total of only a few hundredths of the surface. It was concluded that the most strongly-adsorbed ethylene blocks the reactions via the chemical mechanism, while the most weakly-adsorbed one retards the conversion via the magnetic mechanism. The nature of active sites for the reactions was discussed by taking the results of the ESR measurement into consideration.

Hydrogen-deuterium equilibration and ortho-parahydrogen conversion on molecular sieves

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHydrogen-deuterium equilibration and ortho-parahydrogen conversion on molecular sievesJohn Turkevich and Stanislaw CiborowskiCite this: J. Phys. Chem. 1967, 71, 10, 3208–3217Publication Date (Print):September 1, 1967Publication History Published online1 May 2002Published inissue 1 September 1967https://doi.org/10.1021/j100869a012RIGHTS & PERMISSIONSArticle Views158Altmetric-Citations15LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts