Ortho-hydrogen conversion

The range of observations concerning the direct comparison of the catalytic activity of nickel and rich in nickel alloys with their respective hydride phases has been further extended on reactions of a more complicated nature such as para-ortho hydrogen conversion and ethylene hydrogenation. 

Other transformations supplied by these enzymes are para-ortho-hydrogen conversion, and the exchange reaction between H2 and protons of water.409-412 The hydrogenase enzymes found in various microorganisms are very different in their protein structure and in the types of electron carrier they use. 

Fiq. 3. Para-ortho hydrogen conversion on nickel foil annealed at different temperatures (225° to 500°C.) according to Cremer and Kerber (19) Arrhenius plot. 

The interpretation of the C.E. by a superimposition of reactions occurring at different active surface centers is compatible with the fact that many multicomponent catalysts exhibit a C.E. but no C.E. is found when very pure substances have been subjected to different thermal pretreatments (17). This implies the possibility that many active centers are due to impurities and that their numbers may change with the pretreatment of the catalyst, e.g., by means of aggregation, volatilization, etc. As an illustration, data for the decomposition of N2O on MgO, prepared from synthetic and from natural magnesites, and data for the para-ortho hydrogen conversion on pure metals and on alloys are presented in Tables II and III. 

In Fig. 1, the rate of para to ortho hydrogen conversion over charcoal is shown as a function of temperature, and is seen to pass through a definite minimum... 

One enzyme reaction has been detected at extremely low hydration. Yagi et al. (1969) found that hydrogenase lyophilized at 10 mm pressure catalyzed the para-hydrogen-ortho-hydrogen conversion. 

The direction of the effect is the same as that for the same reaction on copper (55) and on nickel (60) and for the para-ortho hydrogen conversion on alumina (110), but it is not reasonable to search for a similar explanation, a destruction of a hydrogen-containing active center. The dose for half-destruction by neutrons of the activity of zinc oxide was about 1 x 10 ev/gm and of alumina (110) about 10 ev/gm. On the metals with a particles, it was even higher since the corresponding doses were 4 x 10 and 2 X 10 ev/cm for nickel and copper with the depth affected only about 10" cm. It is clear that the much smaller dose to zinc oxide could not have a major effect on the activity if it had to act by removing surface groups. 

Table 11-5 Catalyst and rate data for ortho-hydrogen conversion on NiOlAI2O3...

So the question should never be (nor has it ever been) one of choosing between all catalytic chemists studying ortho-para hydrogen conversion, molecular orbitals and the like, or all catalytic chemists studying fuel synthesis and exhaust catalysts a healthy society is a judiciously balanced society, and the concern for relevance is one for a shift toward greater dedication in the direction of the most vital needs for the survival and health of the kinetic system of human society. 

The conversion of ortho-hydrogen to para-hydrogen is slow in the absence of a catalyst. Therefore, as one cools room-temperature hydrogen to low temperatures, the ortho. para ratio remains at 3 1, and entropy is present that results from the mixing of these two different types of hydrogen. 

The ortho-para conversion of molecular hydrogen is catalyzed by NiO. A supported catalyst is available with a specific surface area of 305 m2/g and a void volume of 0.484 cm3/g. A spherical catalyst pellet has an apparent density of 1.33 g/cm3 and a diameter of 0.5 cm. If the system is not far from equilibrium, an apparent first-order rate constant (kr) can be defined in the following manner. 

The enthalpy of copper at nitrogen temperature is H77K = 6 J/g, so the total entropy of the sphere will be about 6 x 106 J. The time needed to cool from 77 K down to 4K is of the order of 4h. The total helium consumption from room temperature to 4.2 K would be about 6001. The temperatures reached in a test run are reported in Table 16.2. The expected final sphere temperature is about 20 mK. A comparison of MiniGRAIL and Nautilus cool down is made in Table 16.2. The high power leak on the sphere has been attributed to a time-dependent heat leak caused by the ortho-para conversion (see Section 2.2) of molecular hydrogen present in the copper of the sphere (see Fig. 16.5) (the Nautilus bar instead is made by Al). A similar problem has been found in the cool down of the CUORICINO Frame (see Section 16.6). 

Fig. 16.5. An overview of the minimum temperature of the different elements of the system. An estimate is made on the heat flows Q due to conduction between the different stages that are all connected with stainless steel rods or tubes. The total heat leak on the mixing chamber is estimated to be 45pW. This heat leak decreases in time and comes from the sphere and copper masses. We will see further on that this can be explained by ortho-para conversion of 70 ppm hydrogen impurities in the copper (courtesy of Leiden Cryogenics).

One of them is the ortho-para conversion of hydrogen trapped in the frame copper during the production process (see ref. [100-102] and Section 2.2). 

The effects of an uncompensated electron are (1) to split the molecule s spectral lines into doublets, or in the case of certain diradicals, into triplets, (2) to make the molecule paramagnetic, (3) to catalyze the conversion of para and ortho hydrogen molecules, and (4) to cause paramagnetic resonance absorption. 

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

The theoretical minimum work for hydrogen liquefaction depends on the pressure of the hydrogen feed, the rate of ortho-para conversion and the temperature difference between ambient temperature and the temperature of the liquid hydrogen. The following formula is valid for ambient input and output pressures ... 

Ortho-para deuterium, 27 25, 50 Ortho-para hydrogen conversion, 27 23 Oscillatory catalytic reactions, 37 213-215, 271-272 see also Platinum catalytic CO oxidation on Pt(l 11) and Pt(llO) surfaces COj formation, 37 216-217 kinetic oscillation mechanism, 37 220-228... 

EPR has been observed and studied in porous carbons by numerous authors 178-182). The carbons studied have been prepared by pyrolysis of organic material such as dextrose 180), coal 181), and natural gas or oils 181,182). Porous carbons are of considerable technological importance and show catalytic activity for the ortho-parahydrogen conversion, the hydrogen-deuterium reaction, and many reactions of inorganic complex ions 156). Relationships between the characteristics of the EPR absorption and the catalytic activity of porous carbons for the o-p Hj and Hj-D reactions have been demonstrated by Turkevich and Laroche 183). 

The effect of temperature on the first-order rate constant of the ortho-para hydrogen conversion, given by the Arrhenius equation k — was determined for the series of Pd-Au alloys from 0 to... 

The catalyzed ortho-para hydrogen conversion rate may be measured in either a flow, or a static, reactor. The former is the more convenient, the latter is generally used for obtaining absolute rates. Both methods have been used to study the extrinsic field effect, but most of the data have been obtained by the flow method. 

---