Berner, H.

Ensslen, K.

Oesterreicher, H.

Schlapbach, L.

RERO DOC Digital Library

Zeitschrift fur Physikalische Chemie Neue Folge, Bd. 132, S. 75-84 (1982)© by Akademische Verlagsgesellschaft, Wiesbaden 1982The Formation of ThRh Hydride and the Synthesisof Methane by its Reaction with COByH. Berner, H. Oesterreicher1, K. EnsslenFakultat fiir Physik, Universitat Konstanz, D-7750 Konstanz, West GermanyandL. SchlapbachLaboratorium fiir Festkorperphysik, ETH, CH-8093 Zurich, Switzerland(Received July 26, 1982)Metal hydride / Surface segregation / Catalysis / MethanationThe compound ThRh forms ThRhH3A under 1 bar H2 pressure at room temperature.Thermodynamic analysis yields AH° =—58.7kJ/mole H2 and AS" =—128 J/K mole H2. Amethanation reaction is observed when ThRhH3[ is exposed to CO. A preferential reaction ofCO with the hydrogen stored in the metal hydride rather than with the hydrogen in the ambientgas phase is shown by use of hydrogen-isotopes. XPS experiments of ThRh and ThRhHzdemonstrate surface segregation into Th02 and Rh clusters, when exposed to 02 or CO.Die intermetallische Verbindung ThRh bildet bei einem H2-Gleichgewichtsdruck von 1 barund Zimmertemperatur das Hydrid ThRhH3dessen Bildungsenthalpie AH" = —58.7 kJ/molH2 und Bildungsentropie AS° = -128J/K mol H2 betragt. Wird ThRhH3 , einer CO-Atmosphare ausgesetzt, ist die Entstehung von Methan nachzuweisen. Mit Hilfe von Wasser-stoffisotopen wird gezeigt, daB bei dieser Reaktion der Wasserstoff, der im Metallhydridgespeichert ist, gegeniiber dem Wasserstoff aus der Gasphase bevorzugt eingebaut wird. XPS-Untersuchungen von ThRh und ThRhHz zeigen bei Zugabe von 02 bzw. CO eine Oberflachen-zersetzung, bei der Th02- und Rh-Teilchen entstehen.IntroductionMetals and intermetallic compounds consisting of rare earths or actinidesin combination with transition metals can absorb large amounts of hydrogen.1 Guest Professor, permanent address at UC-San Diego, La Jolla where work is supported bya grant from the Department of Energy, Basic Energy Sciences76 H. Berner, H. Oesterreicher, K. Ensslen and L. SchlapbachOne step of this process is the dissociation of the H2 molecule at the surface.As shown by the example of LaNi5 [1] a segregation of the rare earth elementat the surface takes place, binding the oxidizing impurities and keeping thetransition metal in the metallic state. In this way sites are created, which areable to dissociate molecules. On the other hand these compounds can deliverlarge quantities of atomic hydrogen from the bulk. They therefore should begood candidates as catalysts for hydrogenation reactions.Wallace et al. [2—4] investigated various compounds of this type in theirunhydrided state with regard to their catalytic activity in the NH3 and CH4synthesis. They found, that under the synthesis gas CO and H2 the com-pounds decompose into crystallites consisting of the transition metal andof an oxide of the rare earth/actinide which showed at least in one case higheractivity than a conventional oxide supported catalyst of the same elements.Soga et al. [5] studied the hydrogenation of ethylene over LaNi5 and found,that the rate of hydrogenation over the hydride was larger than over theunhydrided material. Oesterreicher et al. [6] showed the catalytic formationof water, when hydrides of intermetallic compounds were exposed to air.In this work we studied the CH4 synthesis from carbon monoxide overThRh hydride. We choose ThRh as a model compound type "rare earth oractinide/transition metal" for several reasons:a) the material is expected to form a hydride, which is even stable attemperatures of 200-300 ° C.b) Rhodium could become catalytically active in clustersize on thesurface, if the expected segregation of Thorium occurs.c) Thorium is reported [4] to form a more active support than La, U or Zr.In addition to a study of the hydriding behaviour of ThRh and thereaction of ThRh hydride with CO, XPS was used to help understanding ofthe surface processes.Experimental details and results1. MaterialThRh was prepared by melting the components together on a water cooled copper hearth ofan arc furnace under argon atmosphere. The button was flipped and remelted several times toimprove homogeneity.The orthorhombic crystal structure [7] was verified by analysis of X-ray powder diagrams(Guinier camera, Cu-A'a radiation). There was also evidence of Th02 presence, but not of Th orRh.2. HydridingThRh was hydrided in the reactor of a high-pressure stainless steel apparatus with variousreservoirs of known volume and facilities to work at different pressure ranges. The temperaturewas monitored by a NiCr —Ni thermocouple in the reactor bed. For the variation of thetemperature, an external heater was used.The Formation of ThRh Hydride and the Synthesis of Methane 77The sample was exposed to H2 gas (99.999%), additionally purified from 02 and H20 bychemisorption and molecular sieve filtration, respectively, accomplished by passing the gasthrough a commercially available cartridge (Oxisorb, Messer Griessheim). The amount ofabsorbed hydrogen was calculated from the pressure drop. The first reaction with H2 (28 bar)occurred spontaneously at room temperature without previous activation. At this temperatureand 1 bar equilibrium H2 pressure the hydride ThRhH3 , is formed. The pressure-compositionisotherms, shown in Fig. 1 were measured for four temperatures by monitoring the equilibriumH2 pressure starting from a point, where ThRh is in the hydride phase and pumping out knownquantities of hydrogen gas. Three of these desorption isotherms show plateaus, associated withthe transformation of the hydride (/?) phase into the solid solution (a) phase. Thermodynamicparameters derived from a logarithmic plot of plateau pressure vs. reciprocal temperature areAH" =-58.7kJ/mole H2 and AS" = -128J/K molr H2. The X-ray pattern of the ThRhhydride could not be indexed by enlarging the orthorhombic cell of ThRh, nor by the presence ofThH2 or Th4H15.3. Catalysis3.1. ApparatusThe catalysis experiments were performed in the hydriding apparatus byuse of corresponding parts of it as a single pass reactor (volume 5 cm3) or as a78 H. Berner, H. Oesterreicher, K. Ensslen and L. Schlapbachclosed system. This contained several chambers for filling in known amountsof different gases. In addition to CO (99.97%), H2 (99.999%) and D2(99.4 %) required for the CH4 synthesis, Ar (99.997 %) was used as a referencegas to check changes in the homogeneity of the gas mixture and in the partialpressures of the components.A quadrupole mass spectrometer (Balzers, QMS) built together with aseparate high vacuum station was used. The components of the gas werequantitatively analysed by considering the known ratios of singly and doublyionized and fragmentary molecules and by also including a factor for theprobability of ionization, which is different for different gases [8,9]. Since thepeak at the atomic mass unity (amu) 16 is composed not only of CH4 but alsoof CO and H20, the peak at amu 15 (CH3-fragment) was used to detect thepresence of CH4.3.2. ProcedureA ThRh piece of about 0.75 g was hydrided at room temperature, cooledto—50 °C and after removing the gaseous hydrogen, exposed to CO and Ar.Then the reactor was heated to 100 ° C and part of the hydrogen in the hydrideallowed to desorb. This temperature was held for about 12 h while the gaseswere circulated in the closed system, driven by convection, to reachequilibrium and homogeneity. This was confirmed by observing constanttotal pressure and obtaining the same ratios of the partial pressures as thoseratios determined from the filled in quantities. Either the closed system wasused during the whole experiment or the reactor was switched over to a singlepass reactor and the gases passed over the ThRh hydride at a constant feedrate of 1 ml gas/min (standard pressure). This rate was limited by the massspectrometer, which was run at 1 • 10~4mbar. At the end of each singleexperiment the reactor was evacuated, the gas further analysed by massspectrometry and the hydride was outgassed at 200 °C for about 1 h. Thisprocedure was repeated several times with the same ThRh sample.3.3. ResultsIn all experiments with exposing ThRh hydride to CO, CH4 was the onlydetectable product. The presence of H20 was not detected either during theexperiment or during evacuation at the end of the experiment. Further aprogressive decrease of the hydrogen storage capacity was observed. Thisfact, together with the absence of oxygen containing gaseous products andthe recorded pressure decrease during CH4 synthesis suggest, that the ThRhsample became more and more oxidized.In each successive experiment the time and the temperature, at which thebeginning of the CH4 synthesis could be detected, was lower than in theprevious experiment. We took the detection of 1 % of CH4 relative to the totalThe Formation of ThRh Hydride and the Synthesis of Methane 79 300-L, 200-•- 100-0,100t/1<UJ LUo =>en -JLUD_ Ui50 HvH20.750.60QMS-I 0.150o£-I 0.30 ffi<i i i—i T—i-—r0 10 20 30 40 50 60 70 80TIME (min)Fig. 2. Rate of CH4 formation from CO over ThRhHz during increase of the temperature. H2evolves from the hydride. Ar is used as a "feed rate" referenceamount of the analysed gas as a measure for the onset. In the virginhydrogenation cycle of ThRh the onset is found after 12 h at 100"C plus 3.5 hat 200 ° C. In the 8th catalysis cycle it could be already seen after 12 h at 95 ° C.The total pressure was 1.1 and 1.2 bar, respectively.We studied more detailed the influence of the desorption of the hydrogenfrom the metal hydride on the CH4 formation. In Fig. 2 the gas mixture andtemperature course in a single pass experiment are shown. A temperatureincrease leads to increased hydrogen desorption, initiating instantaneously ahigher rate of CH4 formation compared with the fraction of argon, which is areference of the feed rate during desorption. To show the preference in themethane synthesis between the hydrogen evolved from the hydride and theambient H2 gas, an isotope experiment was carried out. ThRh was hydridedand after removing most of the gaseous hydrogen, exposed to carbonmonoxide and deuterium in the closed system. During evacuation of thereactor the mass spectrum showed peaks at amu 2, 3 and 4 in the ratio 7:6:8arising from H2, HD and D2 and peaks at amu 12 to 20, which refer to severalC—H—D compounds listed in Table 1. Although the presence of more D2gas than H2 gas is evident, CH4 and CH3D are dominant over CD4. Fromthese results we conclude, that in forming methane, hydrogen from thehydride is preferred over hydrogen from the gas phase.After 7 experiments with CO and 10 hydriding cycles, the ThRh piece haddisintegrated to powder and had increased its weight by 7%. X-raydiffraction analysis showed Th02, ThRh3 and Rh. This sample was thenexposed to oxygen at 200 ° C for 12 h and the result of the subsequent catalysis80 H. Berner, H. Oesterreicher, K. Ensslen and L. SchlapbachTable 1. Distribution of reaction products of CO and D2 over ThRhHz at 171 °C_J_I_I_I_CH* CH3D CH2D2 CHD3 CD<0 50 100 150 200 250 300TIME (min)Fig. 3. Formation of CH4 and C02 from CO and H2 over the oxidized ThRh sample duringincrease of the temperature. H20 is also formed (see text). Ar is used as a "feed rate" referenceexperiment is shown in Fig. 3. Hydrogen was admitted in the gaseous state,because the material did not take up any more hydrogen. The decrease of COcombined with the increase in CH4 implies, that CH4 is formed from CO andnot from C02, which was also produced. Water was detected in largeramounts during evacuation and therefore not marked in the figure. TheX-ray diffraction analysis showed that the material consists of Th02 and Rh.The last experiment carried out without the ThRh sample or itsdecomposed products confirmed, that no products at reaction conditionswere formed by experimental artefacts.The Formation of ThRh Hydride and the Synthesis of Methane SI4. Surface analysisThe ThRh surface was studied by X-ray photoemission spectroscopy witha VG-Escalab-Spectrometer using Mg-Ka radiation (hv = 1253.6eV; Au4fV2 at 83.9 eV, FWHM 1.2 eV). The ThRh was cleft under UHV conditions(2—4- 10~10mbar) and after analysing the clean surface the samples wereexposed in situ to increasing doses of 02 and CO, respectively. Thephotoelectron energy distribution curves of the core levels Th 4/7/2i5/2,Rh 3J5/2 3/2, O 1 s1/2 and C 1 s1/2 were measured to evaluate the chemicalstate and the concentration ratios within the escape depth of the photo-electrons (~20A). The chemical state was determined by comparison toknown spectra. The concentration ratio was calculated from the integratedpeak height divided by theoretical cross sections [10].In Fig. 4 the spectra of cleft ThRh before and after oxygen treatment areshown. The clean sample contains metallic Th [11,12] and metallic Rh[13,14]. With increasing amounts of oxygen, increasing intensity of thechemically shifted Th 4/peaks is observed. This is explained by formation ofTh02 [12] in agreement with the increase and position of the oxygen peak.The Rh peak shifts 0.2 eV to lower binding energies, which could be explainedby the formation of small Rh clusters with a high amount of top surfaceatoms. The C spectra do not change during exposure to 02. The con-centration Th : Rh changes from 1.6 :1 of the cleft sample to 2.7 :1 after 100 L02 (1 L = 1 Langmuir = 1 • 10~6Torr s). Therefore 02 exposure leads toenrichment of Th at the surface in the form of Th02. The Rh remainsmetallic.330 335 340 345 350 305 310 280 285 290 530 535BINDING ENERGY (eV| [Ep = 0]-Fig. 4. XPS spectra of cleft ThRh exposed to 02 at 20 C: (a) cleft sample, Th : Rh = 1.6 :1;(b) exposed tot L 02, Th : Rh = 2.1 :1; (c) exposed to 10LO2,Th:Rh = 2.5:1; (d) exposed to100 L 02, Th:Rh = 2.7:182 H. Berner, H. Oesterreicher, K. Ensslen and L. SchlapbachFig. 5. XPS spectra of cleft ThRh exposed to CO at 200 C: (a) cleft sample, Th : Rh = 1.5 :1;(b) exposed to 10 L CO, Th : Rh = 1.6 :1; (c) exposed to 1 000 L CO, Th : Rh = 1.8 : 1Further the spectra ofThRh were recorded before and after CO exposureat room temperature and at 200° C. Some of them are shown in Fig. 5. Thesame effect—enrichment of Th as ThOz and metallic Rh—as after 02exposure is seen. However, it takes 1000 L CO to produce the same amount ofTh02 as with only 1 L 02. The C spectra show new small contributions atenergies between 282 and 285 eV, especially at 200 °C, which could originatefrom dissociated CO [15]. The ratio C: O however changes from 1 :0.5 for thecleft sample to 1 :1.2 after 1000 L CO, so that not all of the oxygen can bederived from dissociated CO. To analyse the amount of surface coverage byoxygen, which diffuses from the bulk to the surface, we followed the surfaceconcentration of O and C of a freshly cleft sample at 200 °C over 2h. Thecarbon peak did not increase at all. The oxygen peak increased 40 % over thatin the clean surface, where the (Th + Rh) :0 ratio was 3.2 :1.The cleft ThRh was hydrided in a separate chamber of the UHV-system at20 bar starting at room temperature and cooled down to —100° C to avoidstrong H2 desorption. The hydride was exposed to CO and afterwardswarmed up to remove all hydrogen. The powdered hydride had the lowest ofall measured contents of impurities C and O as seen in Fig. 6. CO exposure ofthat partially hydrided sample shows no further difference as compared to theunhydrided material.DiscussionThe derived thermodynamic properties of ThRh hydride, AH0 =—58.7 kJ/mole H2 and AS° = —128 J/K • mole H2, are comparable to thoseof similar hydrides such as ThNi and ThCo, which are reported in theThe Formation of ThRh Hydride and the Synthesis of Methane 83330 335 340 345 350/305 310 280 265 290 530 535BINDING ENERGY I eV )-*>Fig. 6. XPS spectra of cleft ThRh and of ThRhHz exposed to CO: (a) cleft sample at 20°C,Th:Rh= 1.6:1; (b) hydrided, cooled to -100°C, Th:Rh= 1.4:1; (c) hydride exposed to10 L CO, Th:Rh = 1.5:1; (d) hydride exposed to 100 L CO, Th:Rh = 1.5:1; (c) hydride ex-posed to 1000 L CO, Th:Rh= 1.6:1; (/) hydride exposed to 10000 L CO, Th: Rh = 1.8:1;(g) hydride warmed up to 20 C, Th:Rh = 2.5:1; (h) completely dehydrided,Th:Rh = 2.4:1literature [16], Thus ThRh hydride is stable enough to be handled withoutstrong desorption at temperatures up to 200 °C. This temperature range isnecessary to study CH4 synthesis from CO over the metal hydride.With the enrichment of Th as Th02 on the surface and the formation ofmetallic Rh, when exposed to 02, ThRh has the expected surface propertiesanticipated from similar experiments with LaNi5 [1], where oxidized La andmetallic Ni are found in the ratio 1 :1 on the surface.When ThRh hydride is exposed to CO, CH4 is formed below tempera-tures of 200°C. The hydrogen evolving from the hydride is preferred informing CH4, compared to the hydrogen in the gas phase, as shown in theisotope experiment. This fact makes evident that the origin and state of thehydrogen is very important. This preference appears to be caused by the largesupply of atomic hydrogen by the hydride. In addition new clean, i.e.catalytically active surfaces may be created during hydriding and dehydridingof the intermetallic compound by progressive disintegration or segregation.However the oxygen of the carbon monoxide is used subsequently foroxidation of Th, although a "reducing atmosphere" is formed by dehydrid-ing. But if the CH4 synthesis from CO would work only on the basis ofoxidation of Th, the completely oxidized sample would not show any CH484 H. Berner, H. Oesterreicher, K. Ensslen and L. Schlapbachsynthesis. This is not the case. The X-ray pattern shows, that Rh crystalliteshave been formed. These cristallites together with Th02 can also produceCH4 and might be considered as thoria supported Rh catalyst. The formationof CH4 from CO and H2 over the severely by CO oxidized sample started atthe lowest temperature of all experiments. This extremely low value was95 °C.AcknowledgementsWe would like to thank Professor E. Bucher for support and interest inthis work and Mr. G. G. Baumann for help with the mass spectrometricanalysis.References1. H. C. Siegmann, L. Schlapbach, Phys. Rev. Lett. 40 (1978) 972.-L. Schlapbach, A. Seiler,F. Stucki and H. C. Siegmann, J. Less-Common Met. 73 (1980) 145.2. T. Takeshita, W. E. Wallace and R. S. Craig, J. Catal. 44 (1976) 236.-V. T. Coon, T.Takeshita, W. E. Wallace and R. S. Craig, J. Phys. Chem. 80 (1976) 1878.3. H. Imamura and W. E. Wallace, J. Catal. 65 (1980) 127.4. A. Elattar, W. E. Wallace and R. S. Craig, Adv. Chem. 178 (1979) 7.5. K. Soga, H. Imamura and S. Ikeda, J. Phys. Chem. 81 (1977) 1762.6. H. Oesterreicher, K. Ensslen and E. Bucher, Appl. Phys. 22 (1980) 303.-H. Oesterreicherand F. Spada, Mater. Res. Bull. 15 (1980) 477.7. Shunk, Constitution of Binary Alloys, Supplement Nr. 2 McGraw-Hill, New York 1969.8. Index of Mass Spectral Data, AMD 11, ASTM.9. Balzers, Physikalische Grundlagen zur Parlialdruckanalyse (14.9.1973).10. J. H. Scofield, J. Electron Spectrosc. 8 (1976) 129.11. S. B. Nornes and R. G. Meisenheimer, Surf. Sci. 88 (1979) 191.12. J. C. Fuggle, A. F. Burr, L. M. Watson, D. J. Fabian and W. Lang, J. Phys. F: 4(1974) 335.13. R. Nyholm and N. Martensson, J. Phys. C: 13 (1980) L279.14. T. L. Barr, J. Phys. Chem. 82 (1978) 1801.15. J. C. Fuggle, in: Handbook of X-ray and ultraviolet photoelectron spectroscopy (D. BriggsEd.), Heyden (1977).16. H. H. van Mai, Proefschrift (1976).

The Formation of ThRh Hydride and the Synthesis of Methane by its Reaction with CO

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library