Meleorilics R Planelaty Science 33,243-251 (1998) 0 Meteoritical Society, 1998. Printed in USA.

Formation of carbides and hydrocarbons in chondritic interplanetary dust particles: A laboratory study

JORDI LLORCA1.2* AND IGNASI CASANOVAlJ

'Institut d'Estudis Espacials de Catalunya, Edifici Nexus, Gran Capith, 2-4. E-08034 Barcelona, Spain 2Departament de Quimica Inorgtinica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain

3ETSECCPB, Universitat Polittcnica de Catalunya, Modul C-I, E-08034 Barcelona, Spain *Correspondence author's e-mail address: jllorca@kripto.qui.ub.es

(Received 1997 June 11; accepted in revised form 1997 September 28) (Part of a series ofpapers dedicated to the memory of Paul Barringer)

Abstract-The reaction between kamacite grains and H2 + CO gas mixture has been tested in the laboratory under experimental conditions presumed for interplanetary dust particle (IDP) formation in a nebular-type environment (H2:CO = 250: 1 ; 5 x 10-4 atm total pressure, and 473 K). Carbon deposition, hydrocarbon pro- duction in the C1-C4 range, and the formation of an €-carbide phase occur when well-defined model FeNi bcc alloy (kamacite) particles are exposed to a mixture of H2 + CO during 103 h. These results strongly sup- port the idea that gas-solid reactions in the solar nebula during CO hydrogenation represent a plausible scenario for the formation of carbides and carbonaceous materials in IDPs, as well as for the production of hydro- carbons through Fischer-Tropsch-type reactions.

INTRODUCTION

Chondritic interplanetary dust particles (IDPs), broadly classified as chondritic porous (CP) and chondritic smooth (CS) (Brownlee et al., I982), have been extensively studied and characterized over the past two decades with increasingly sophisticated analytical tech- niques. The elemental composition of the CP particles is similar to carbonaceous chondrites, including C, but they have a characteristic cluster-of-grapes morphology distinct from any known meteoritic materials (Brownlee, 1985). Their fluffy texture, high C content, and high atmospheric entry velocities inferred from He retention are traditionally associated with cometary dust grains (Bradley and Brownlee, 1986; Sandford and Bradley, 1989); similarities between CP particles and dust grains from comet Halley have been reported as well (Jessberger e f al., 1988; Brownlee et al., 1987). Most CP particles are highly heterogeneous and appear to be mixtures of com- ponents out of chemical equilibrium with each other, which suggests that such particles have not undergone substantial processing or al- teration since aggregation and, therefore, represent very primitive material.

Iron-nickel carbides, filamentous C and carbonaceous rims on grains have been identified in numerous CP particles (Fraundorf, 1981; Bradley and Brownlee, 1983; Christoffersen and Buseck, 1983; Bradley et al., 1984; Bradley, 1994). The mineralogies and compositions of these phases are not well known, and the existence of different types of hydrocarbons has been tentatively suggested (Sandford, 1987; McKeegan e f al., 1985; Allamandola et al., 1987; Clemett e f al., 1993). All these C-containing compounds, which are believed to have a primitive origin and not to be derived from atmos- pheric entry (Christoffersen and Buseck, 1983), are typical byprod- ucts of catalytic reactions between a C-rich gas (e.g. , CO) and the active surfaces of FeNi metal grains (e.g. , Hindermann et al., 1993, and references therein). In fact, the growth of C layers on Fe cata- lysts from reactions involving CO always involves the formation of Fe carbide. One subset of reactions of this type is the Fischer- Tropsch synthesis, as suggested for the formation of organic com- pounds in chondritic meteorites (Hayatsu and Anders, 1981) and of carbonaceous material in C-rich aggregates in type-3 ordinary chon- drites (Brearley, 1990). Some authors, however, have proposed a parent body origin through hydrothermal alteration for the latter

243

case (Krot et al., 1997). On the other hand, some workers have suggested that FeNi carbides in CP IDPs formed during nebular carburization of FeNi metal catalyst grains, followed by deposition of C on the catalyst surface (Christoffersen and Buseck, 1983; Bradley et al., 1984; MacKinnon and Rietmeijer, 1987).

The preparation of FeNi model catalysts of well-defined struc- ture and the investigation of their chemical and physical properties under nebular conditions are of fundamental importance for the un- derstanding of carburization processes in IDPs and the origin of their carbonaceous phases. However, experimental conditions of labora- tory studies mentioned above (1 to 10 atm total pressure and H2:CO from 1: 1 to 20: 1) differ notably from those prevailing in the nebula

atm and H2:CO of -103:l; Anders et al., 1974) and the relevance of these results to astrophysical environments is question- able.

This paper presents the results of detailed laboratory studies car- ried out to simulate the interaction between FeNi metal particles (in the form of silica-supported bcc FeNi alloy), and a gas phase con- sisting of a H2+CO mixture under nebular-type conditions: 5 x lo4 atm total pressure, 473 K and a H2:CO ratio of 250: 1. The FeNi carbide formation, hydrocarbon synthesis, and carbonaceous deposi- tion on catalyst grains are studied by means of in situ infrared spec- troscopy (FTIR), mass spectrometry (MS) of products evolved, transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Our main results suggest that carbide forma- tion was possible in the solar nebula through interaction of nebular gas with FeNi metal grains, and that carburization was accompanied by the simultaneous synthesis of hydrocarbons and nongraphitic C deposition on FeNi metal grains. Several associations of metal- carbon in CP IDPs may be explained in these terms.

EXPERIMENTAL PROCEDURE

to

Sample Preparation

Catalyst samples were prepared in situ and manipulated under strictly controlled conditions on a glass vacuum line fitted with greaseless stopcocks and capable of <lo4 mbar. Special greaseless vacuum Pyrex@ infrared cells with CaF2 windows, which allowed thermal treatment, were used. Amorphous silica was choosen as a support in order to avoid interactions with the metal. since it is

244 J. Llorca and I. Casanova

chemically inert to Fe- and Ni-based systems (Benziger and Robert, 1982; Jiang et al., 1985). Self-supporting pressed wafers (-10 mg) of Degussa aerosil silica (surface area of 200 m2g-I) were placed into the infrared cells and cleaned from organic contaminants as de- scribed by Santos et al. (1983). This procedure involves high-tem- perature treatment in 02 and under vacuum, followed by exposure to water vapor at 380 K. Silica wafers were then partially dehydroxi- lated by treatment under high vacuum mbar) at 473 K for 16 h and impregnated with a gaseous mixture of Fe(CO)5 and Ni(C0)4 to a final pressure of 0.1 mbar, which represents a metal content of 2 x 10" mol, or 1.2 wt% metal loading. In order to obtain alloy particles free from residual contaminants, Fe(CO), and Ni(C0)4 were used as precursors for the preparation of model FeNi metal particles. Conventional preparation procedures of Fe-Ni catalysts do not allow a good control of both bulk and surface chemical compositions (Raupp and Delgass, 1979a; Unmuth et al., 1980a; Jiang et al., 1984; Nagorny and Bubert, 1987; Matsuyama et al., 1986; Mizushima et al., 1988; Cooper and Frost, 1990; Boellaard et al., 1994; Van de Loosdrecht et al., 1995). The carbonyl precursors were purified by vacuum transfer and thoroughly degassed prior to FTIR studies. Following impregnation, the carbonyl precursors were thermally decomposed at 473 K under dry deoxygenated Ar. After complete decarbonylation of samples, which was monitored by FTIR, several pulses of H2 (200 mbar) were admited into the infrared cell at 473 K and subsequently evacuated to ensure alloy formation. Characterization of the alloy was performed in situ by FTIR spec- troscopy (using CO as a probe molecule) and by TEM and XPS after removal of the sample from the infrared cell under controlled conditions. The metal loading of the samples were determined by ICP atomic absorption on a Jobin-Yvon JY38-VHR instrument, after dissolution in HF/HN03/HCI.

Catalytic Test

Reactions were carried out in situ in the infrared cells at 5 x lo4 atm (20.1 x lo4) total pressure and 473 K (20.5 K) with a H2/CO ratio of 250 (k I) . All gases were of high purity grade, as checked by MS, and used without further purification. During the experi- ments, interaction between the catalyst and the gas phase and prod- ucts evolved was monitored by MS and FTIR. After 1000 h of reaction, samples were removed from the infrared cell under con- trolled atmosphere and studied by TEM and XPS in order to eval- uate carbide formation and carbon deposition. Instrumental Techniques

Infrared spectra were recorded on a Nicolet 520 Fourier trans- form spectrophotometer by co-adding 100 scans at a spectral resolu- tion of 2 cm-I. All the infrared spectroscopic measurements were obtained at room temperature, which may result in the trapping of gaseous products on the catalyst. Infrared absorption spectra of CO molecules bonded to the catalyst surface were obtained by computer substraction of the initially recorded Si02 reference spectrum from the spectrum of interest. Mass spectra were obtained in the mass range of 1-200 with a Bakers VG-200 spectrometer connected to a high-vacuum device that allowed direct sampling from the infrared cell. Samples for TEM and XPS were passivated in situ before re- moval from the infrared cell by introducing 0 2 diluted in Ar, follow- ing the procedure described by Shroff and Datye (1996). Lack of adequate passivation can cause transformation of the carbide phase to an oxide phase (magnetite) as a result of atmospheric exposure. Metal particle size and individual particle composition were measured with a Hitachi H 800-MT scanning transmission electron micro-

scope (STEM) operated at 150 kV and coupled with a Kevex energy dispersive x-ray (EDX) analyzer. The x-rays emitted upon electron irradiation of specimens with an electron beam -2 nm in diameter were acquired in the x-ray energy range 0-10 keV. Electron diffrac- tion studies were carried out using a Hitachi H 800-NA transmission electron microscope under convergent beam mode (CBED) at 200 kV with a 2-5 nm probe. Combined high-resolution transmission electron microscopy (HRTEM) and EDX spectrometry were carried out with a Philips Ch4-30 electron microscope working at 300 kV with a 0.20 nm point-to-point resolution and equipped with a Lynk analytical system. The magnification of the HRTEM images and electron diffraction patterns were calibrated with pure Fe foils under the same electron-optical conditions. Gold grids with a holey-carbon- film were used in all cases. The grids were dipped into the pulver- ized sample and the excess sample was shaken off. No solvents were used at any stage of the process to avoid hydrocarbon contamina- tion. X-ray photoelectron spectroscopy (XPS) was used to charac- terize the surfaces of catalysts. X-ray photoelectron spectra were recorded with a Perkin Elmer PHI-5500 spectrometer equipped with a Al x-ray source and a hemispherical electron analyzer. The x-ray source was operated at 12.4 kV. Binding energies were referred to the Si 2p line at 103.4 eV. The accuracy of the binding energy was within 0.1 eV.

RESULTS Characterization of Model Iron-Nickel Particles

When the mixture of Fe(CO)5 and Ni(C0k is impregnated onto the silica support, a simultaneous slight decrease in the intensity of the 3745 cm-l YOH absorption band, corresponding to free silanol groups ([Si-01-H), and a broad band centered at 3640 cm-' are observed. According to Lamb et al. (l988), these spectral features are indicative of H interaction between the silica surface and the metal carbonyl groups. On the other hand, the impregnation of the carbonyl precursors gives rise to two absorption bands in the vCo region centered at 1990 and 2061 cm-l (Fig. 1, spectra a and b). The interaction between Fe(CO)5 and Ni(C0)4 with silica is very weak (Jackson and Trusheim, 1982) and, consequently, the wavenumbers of the strongest CO stretching bands observed for silica-supported Fe(CO)5 and Ni(C0)4 are comparable to the values of free Fe(C0)S and Ni(C0)4 at 1988 and 2057 cm-I, respectively.

Thermal decomposition under Ar at 473 K causes the complete loss of carbonyl ligands. A simultaneous increase of the 3745 cm-I VOH band intensity and the decrease of the 3640 cm-I band intensity after thermal treatment are indicative of loss of H bonding with the metal carbonyl groups and regeneration of silanol groups. The ab- sence of vc0 bands afler thermal decomposition at 473 K evidences the complete loss of the carbonyl groups (Fig. I , spectrum c). Re- moval of CO prior to H2 treatment of the sample is necessary in order to avoid the incorporation of C into the metal phase during alloy formation.

When CO is admited into the cell (as a probe molecule) after re- moval of the carbonyl ligands through thermal decomposition under Ar and H2 treatment at 473 K, a single broad absorption band arises in the YCO region at 2029 cm-l (Fig. 1, spectrum d). The fact that only one band appears and that no absorption bands are seen at wavenumbers corresponding to silica-supported Fe(CO)5 and Ni(C0)4 is indicative of FeNi alloy formation. Similar infrared spectra have been obtained for other supported FeNi alloys. Boellaard et al. (1994) have found, on a Fe:Ni = 1 : 1 sample after exposure to CO, a single band centered at 2043-2027 cm-I, depending on surface coverage.

Formation of carbides and hydrocarbons in chondritic interplanetary dust particles 245

C

I , 21 00 2000 1900

Wavenumbers (cm-1) FIG. 1. Infrared spectra in the vc0 region of the silica support blank (a) and subsequent treatment involving (step 1) exposure to 0.1 mbar of a mixture of [Fe(CO),] and mi(CO),] (b); (step 2) thermal decomposition under Ar at 473 K (c); and (step 3) admision of 30 mbar of CO into the infrared cell (d).

Cho and Schulman (1 964) studied the infrared absorption by CO on a series of silica-supported FeNi alloys. For an alloy composition Fe95NiS, a CO absorption band was found at -2035 cm-I, which is in good agreement with our results.

The resulting silica-supported FeNi bimetallic particles (heretofore referred to as FeNi/Si02) are well dispersed onto the support (Fig. 2). The mean particle size deter- mined by point counting on a photograph depicting over >500 particles is very homogeneous, 20 f 3 nm. Energy dispersive x-ray analysis were performed on multiple indi- vidual metal particles. In all cases, bimetallic composition was found with a Fe/Ni ratio corresponding to the global value determined by means of ICP-AA chemical analysis of 5.04 WWO Ni. Maximum deviation was within 2% of the measured value. Electron diffraction evidences the exis- tence of a single bcc phase with a cell parameter of 2.8 A, which is in good agreement with reported values for kam- acite with 5-10 wt% Ni content in CP interplanetary dust particles (Bradley, 1988, 1994).

X-ray photoelectron spectra of the resulting FeNi/Si02 model catalyst was recorded for Fe 2p312, Ni 2p3n and C Is signals. The binding energies obtained for the Fe 2~312 and Ni 2~312 signals, 707.2 and 852.6 eV, respectively (Ta- ble I ) , are indicative of complete surface reduction of the catalyst particles. From their intensities, referred to the Si 2p signal, a surface ratio of FeMi = 19.5 is found, which

corresponds to a value of 4.88 wt% Ni on the surface. This value is in excellent agreement with the bulk composition of the alloy par- ticles, as determined by EDX and chemical analysis, thus indicating that no preferred metal segregation exists towards the particle surface. Carburization Studies

Upon reaction of the H2 + CO mixture with the FeNi/SiOz cata- lyst, prepared in situ in the same infrared cell, one absorption band centered at 2020 cmd is observed in the YCO region (Fig. 3, spec- trum a), which can be attributed to CO molecularly bonded to FeNi metal. This value for vc0 is lower than the one reported above for the same sample at 2029 cm-l (Fig. I , spectrum d) due to CO surface coverage, &o. Under reaction conditions, 8 ~ 0 equals 0.05, causing a shift of the CO absorption band to lower wavenumbers. As the reaction proceeds, this band vanishes progressively (Fig. 3, spectra b to d), evidencing the disappearance of metallic Fe and Ni on the sur- face of the metal particles due to formation of FeNi carbide. After a contact time of 1000 h, the band at 2020 cm-I has decreased its in- tensity by -80%. A similar decrease in the intensity of the vCo ab- sorption band when alumina-supported Fe samples are exposed to H2 + CO has been reported by Perrichon et al. (1 984), Pijolat et al. (1987), and Boellaard et al. (1 996).

The surface of the resulting sample after 1000 h of reaction has been investigated by x-ray photoelectron spectroscopy (Table I).

TABLE 1. X-ray photoelectron spectroscopy data of silica-supported FeNi alloy model particles prior to and after ex osure to a H,:CO = 250: 1 gas mixture at 5 x lo4 atm and 473 K for 10 P h.

Binding energies Atomic ratios

Si 2p Fe 2p,, Ni 2p,,, C Is FeMi C/(Fe +Ni)

Before reaction 103.4 707.2 852.6 286.8 19.5 0.002 Afterreaction 103.4 707.5 852.8 286.6 21.2 0.47

283.4

FIG. 2. Transmission electron microscope images corresponding to silica supported bcc FeNi alloy model particles. (a) Low-magnification micrograph showing excellent dis- persion and particle size homogeneity; (b) representative particle showing cubic sym- metry; (c) electron dimaction pattern obtained in convergent-beam mode corresponding to particle depicted in (b). The particle is oriented along [loo]. Planes close to origin are (01 1) and (017).

246 J. Llorca and I. Casanova

I I I

21 00 2000 1900 Wavenumbers (cm-1)

FIG. 3. Infrared spectra in the vc0 region recorded during exposure of silica-supported bcc FeNi alloy model particles to a H,:CO = 250:l gas mixture at 5 x lo4 bar and 473 K. (a) t = 0; (b) t = 250 h; (c) t = 500 h; (d) t = 1000 h. The intensity decrease of the CO absorption band is attributed to carbide formation.

The C Is binding energies at 286.6 and 283.4 eV are at- tributed to the formation of carbonaceous deposits as well as adsorbed hydrocarbon products and carbidic carbon, respectively (Kuivila et al., 1988, 1989). After the surface C is etched away by Ar ion sputtering, only the signal at 283.4 eV corresponding to carbide carbon persists. Fe 2p3n and Ni 2p3n binding energies of 707.5 and 852.8 eV, re- spectively, have been measured, representing an increase of 0 . 2 4 . 3 eV relative to the Corresponding Fe 2p,,, and Ni 2p,,, binding energies for the reduced FeNi alloy particles. Similar reproducible shifts have been recorded in other Fe and Ni core levels. This shift, coupled with the appear- ance of the low binding energy C peak, is attributed to the formation of a carbide phase within the surface region of the catalyst. The exact chemical state of the surface Fe can- not be determined unambiguously since the binding ener- gies of the various carbide phases are very similar (Kuivila et a/ . , 1988). Shifts of +O.3 eV relative to reduced Fe have been reported by Kuivila et al. (1988, 1989) over supported and unsupported Fe after H2 + CO treatment at 473-578 K and 1-7 atm with a H2:CO ratio of 3 : l . Such shifts in binding energy values have been attributed to carbide for- mation, and in some cases, total carburization of the cata- lysts has been evidenced by Mossbauer spectroscopy on the same samples (Kuivila et a / . , 1988).

High-resolution transmission electron microscopy of the sample exposed to carburizing conditions shows lattice fringes of the carbide phase of -2 A and an accompanying

layer of surface C (Fig. 4). While the fringes by themselves do not uniquely identify the carbide phase, a combination of the characteri- zation techniques (HRTEM, electron diffraction and XPS) lead us to conclude that the particles present in our sample are Fe-Ni carbide. The inset diffraction pattern obtained from a single carbide crystal- lite shows spots corresponding to €-carbide; however, it is difficult to define a precise stoichiometry due to the lack of relevant data for metal carbides containing both Fe and Ni. Supercell reflections are commonly seen as a result of an ordered arrangement of C atoms in the carbide lattice. The carbide particles are encapsulated by a sec- ond phase with thickness ranging from 4 to 18 nm (Fig. 4). Com- bined HRTEM-EDX and XPS results suggest that the encapsulating material is a poorly crystalline C phase, developed during carburiza- tion of the sample. Given that the maximum thickness of a C over- layer due to the passivation procedure in TEM sample preparation is, at most, only a few nanometers, it can be concluded that the C layer observed on the sample is partly due to C deposition during carbide phase formation.

Hydrocarbon Production

Carbide formation in our sample is accompanied by the progres- sive production of hydrocarbons, as proved by the characteristic infrared asymmetric and symmetric CH stretching vibrations of methyl-CH, and methylene -CH,- groups in the 3000-2800 cm-I range. The hydrocarbon C-H stretch region at different stages of the experiment is shown in Fig. 5. Minor contamination on the surface of the catalyst from the initial exposure to the H2 + CO gas mixture has been detected (Fig. 5, spectrum a). As the reaction proceeds (Fig. 5, spectra b to d), a set of well-developed bands arise at 2962, 2929,2874 and 2852 cm-I. Bands at 2962 and 2874 cm-I are inter- preted as derived from methyl groups (expected values at 2960 and 2876 cm-1); whereas, those at 2929 and 2852 cm-' are assigned to

FIG. 4. Transmission electron microscope images corresponding to silica supported bcc FeNi alloy model particles after 1000 h of exposure to a H,:CO = 250: 1 gas mixture at 5 x lo4 bar and 473 K. (a) Lattice fringe image of a representative FeNi particle containing €-carbide rimmed by a poorly ordered carbonaceous phase; (b) convergent- beam electron diffraction pattern of area labelled c corresponding to hexagonal FeNi c- carbide oriented along [ i l O O ] . Planes close to origin are (1 130) (d (, , jo) = 2.3 A) and (0002) (d(,,,,,,*) = 2.1 A). (OOOr) ( I = odd) reflections are due to multiple diffraction effects.

Formation of carbides and hydrocarbons in chondritic interplanetary dust particles 241

b

I ' I

3200 3000 2800 Wavenumbers (cm-1 )

FIG. 5. lnfrared spectra in the vCH region recorded during exposure of silica-supported bcc FeNi alloy model particles to a H,:CO = 250:l gas mixture at 5 x lo4 bar and 473 K. (a) t = 0; (b) t = 250 h; (c) t = 500 h; (d) t = 1000 h. Strong absorption bands at 3017, 2962, 2929,2874 and 2852 cm-I are attributed to methane, and methyl- and methylene-bearing species adsorbed on the particles' surface.

methylene groups (expected values at 2925 and 2855 cm-I) of hydro- carbon chains. The 3016 cm-' band is assigned to methane adsorbed on the catalyst surface (methane v3Q branch at 3017 cm-I). It is interesting to note that bands corresponding to methyl groups become more intense than those of methylene groups as the reaction pro- ceeds, reflecting increasing hydrogenation of adsorbed intermediate products. On the other hand, after 500 h of reaction, CH4 is first identified on the catalyst surface (Fig. 5 , spectrum c); and after 1000 h, some CH4 exists also in the gas phase of the cell, as inferred from the rotational structure at wavenumbers >3000 cm-I (Fig. 5, spec- trum d). Infrared spectra recorded after exposure of a bare silica support (blank) to the Hz + CO gas mixture under the same experi- mental conditions exhibit no absorption bands in the VCH region. This proves that hydrocarbon formation in our experiments requires the presence of FeNi particles.

Mass spectrometry of the products evolved after 1000 h of reac- tion confirms the presence of hydrocarbons in the gas phase. Both saturated and unsaturated molecules have been identified in the CI- C4 range (Table 2). In addition to hydrocarbons, water also has been detected as a constituent of the reaction mixture. Formation of COz by water gas shift reaction or CO disproportionation has not been detected. Under these conditions, a total conversion value of 0.01% relative to C in CO is obtained.

DISCUSSION

Carbide Formation in Interplanetary Dust Particles

It is well known that Fe catalysts in all forms (precipitated, fused, and supported) are converted to one or more carbides under typical

TABLE 2. Hydrocarbon abundance in the gas phase of the reaction vessel after 1 O3 h of exposure of the FeNilSiO, sample to a H,:CO = 250: 1 gas mixture at 5 x lo4 atm and 473 K.

C"", abundance ("A)

90 2.1

'ZH6 5.0 C3H6 0.5

C4H10 0.4

CH4 CZH,

C3H8 2.0

commercial Fischer-Tropsch synthesis conditions. The working catalysts usually consist of a mixture of metallic, carbide, and oxide phases. Although the catalytic role of carbides remains a controver- sial subject, the fact that carbides have an important effect on the structure and the properties of the catalyst surface cannot be dis- puted. Mixtures of c-Fe2C, ~'-Fe2,2C and X-Fe5Cz (Hagg carbide) are formed during synthesis at 1 atm and -525 K (Bianchi et al., 1983; Bukur et al., 1995a), while cohenite (O-Fe3C) appears at temperatures >625 K (Loktev et aL, 1972; Niemantsverdriet et al., 1980). The combination of carbides that are formed is controlled by reaction conditions and the nature of the Fe catalysts (Nieman- tsverdriet et nl., 1980; Jung and Thomson, 1992). The carbides may be arranged in the following order of increasing stability: c'-Fe2,2C < c-Fe2C < x-Fe5C2 < fl-Fe3C. The phase change from one carbide to another requires only small changes in Fe contents and C stoi- chiometry, and phase identification for small supported particles, as in the case for IDPs, is often difficult. On the other hand, nickel carbide (Ni3C) is formed in a CO atmosphere at temperatures be- tween 440 and 525 K (Galwey, 1962), but it is less stable than iron carbides and decomposes readily at 625 K (Unmuth et al., 1980b). Nickel carbide and iron carbide are completely soluble in one another (Goldschmidt, 1967) and the resulting mixed-metal carbides are sta- bilized to -775 K (Unmuth et al., 1980b), depending on their Ni contents.

The carbides so far unambigously identified in IDPs are c-(FeNihC (Christoffersen and Buseck, 1983; Bradley et al., 1984) and O-(FeNihC (Fraundorf, 1981 ; Bradley et al., 1984). The existence of other carbide phases also has been suggested (Bradley et al., 1984) but not unambiguously determined. While ~ - ( F e N i ) ~ c is generally interpreted as a product of FeNi carburization in IDPs during solar nebula residence (Mackinnon and Rietmeijer, 1987; Bradley et al., 1984), microstructural studies performed on fl-(FeNi)$ in the CP interplanetary dust particle named Oz (Fraundorf, 1981) have re- vealed that the observed cohenite rim was not the result of solid state diffusion of C into the FeNi metal host but instead was deposited from an 0-depleted gas phase onto a previously solidified metal sur- face.

Therefore taking into account carbide relative stability and in- ferred maximum temperatures of C-rich aggregates in IDPs (Mac- Kinnon and Rietmeijer, 1987; Bradley, 1994), c'-(FeNi)2,$ and c-(FeNi)zC represent the carbides that might be formed in CP IDPs via carburization of kamacite precursor grains in a nebular environ- ment. Results from our electron diffraction studies show unambig- uous evidence of the presence of ordered hcp ~ - ( F e N i ) ~ c carbide in our experiments (Fig. 4), a carbide phase that has also been positive- ly identified in CP IDPs (Christoffersen and Buseck, 1983; Bradley et al., 1984). Therefore, this work provides experimental evidence

248 J. Llorca and I. Casanova

that carbides in IDPs could be formed efficiently by carburization of metal precursors by H2 + CO gas in the solar nebula. The apparent absence of e'-(FeNi)2,2C carbide phase in our experiments may be attributed either to the temperature used in the catalytic test, 473 K, or its higher instability under low CO/H2 ratios, a result which would generally apply to the solar nebula, where the prevailing nominal CON2 ratio was lower than our value of H2:C0 = 250:l. The presence of other carbide phases in IDPs, like those reported by Bradley et al. (1984), may be attributed to subsequent processing of the e-(FeNi)zC carbide precursor at higher temperatures during metamorphic events. The temperature of transformation from one carbide to another may depend on a variety of factors, including the concentration of impurities and heating mechanism. On the other hand, decarburization under H2 of the eFe2C phase proceeds at tem- peratures >-575 K (Goodwin and Parravano, 1978) at 1 atm. Al- though no experiments have been addressed to study decarburization of f-(FeNi)lC, which is expected to be more reactive than e-Fe2C, it appears that under nebular conditions decarburization reactions were not efficient. In fact, carburization is promoted by the pres- ence of H2, which maintains a well-reduced surface of the metal and avoids excessive C deposition.

In relation to the role played by Ni on the carburization reaction, Unmuth et nl. (1980b) have demonstrated that the rate of carbide formation on a 3Fe: INiiSiO2 sample is enhanced relative to Fe/SiO2 at 1 atm and H2:C0 ratio of 3:1, thus indicating that Ni has an im- portant promoter effect on the reaction. More specifically, it is known thatfcc and bcc FeNi alloys have different C solubility and diffusiv- ity properties (Matsuyama et al., 1986). These factors, together with crystal structure, play a key role in carburization processes. Both alloys cfcc and bcc) carburize under H2 + CO mixtures; however, the Fe-rich alloy (bcc) appears to promote the rapid diffusion of C re- quired to generate carbides (Rodriguez ef al., 1997). Raupp and Delgass (1 979a,b,c) have studied the carburization of FeNi/SiO2 catalysts under mixtures of H2:CO = 3:l and observed that the Ni- poor phase carburized readily, whereas the Ni-rich phase remained uncarburized. We believe that kamacite grains may be transformed to carbide from H2 + CO mixtures in a nebular environment more easily than other metal phases present in IDPs.

Carbonaceous Deposits

Noncrystalline carbon-bearing phases are prominent in CP IDPs (Fraundorf, 198 1 ; Fraundorf et al., 1983). Although particles differ from one another in texture and composition, submicron crystals in them often appear to be coated with, or embedded in, a low atomic weight amorphous material, most of it being probably carbonaceous, indigenous to the aggregates. We have found that the formation of this amorphous material may also be explained in terms of interaction between H2 + CO mixtures and metal surfaces during carburization reactions in heterogeneous phase. In fact, we consider that all the transformations obtained in our experiments, namely carbide forma- tion, growth of carbonaceous layers and synthesis of hydrocarbons, are linked to the same reaction pathway.

Let us consider the so-called "competition model" for the car- burization reaction, first proposed by Niemantsverdriet and Van der Kraan (1981), which best explains most experimental observations. Surface metal atoms are viewed as active sites, and the formation of hydrocarbons, carbonaceous deposits and bulk carbides involves a common carbon intermediate. Initially, CO dissociates fast over the metal surface to yield adsorbed C and 0 atoms (Dwyer and Somor- jai, 1978). At low COM2 ratios, 0 is removed by reaction with H2

to yield H20 (Dry et al., 1972). During the early stages of the inter- action between metal and C, diffusion of C into the bulk particle is rapid, lowering the surface mean concentration. However as car- burization proceeds, active C on the surface increases leading to the formation of amorphous carbonaceous deposits that, in turn, may lead to hydrocarbon production by hydrogenation reactions. The amount of deposited C increases with increasing CO/H2 ratio, tem- perature and time, which also control the chemical state of the car- bonaceous layer (Bonze1 and Krebs, 1980; Rodriguez et d., 1993) and, consequently, hydrocarbon production efficiency. Total hydro- genation of nongraphitic carbonaceous layers produces methane, but partial hydrogenation to C1 species such as MCH, MCH2, M2CH2, or MCH3 (M = Fe, Ni) may also occur (King, 1980). These inter- mediate species may participate then in chain growth to yield higher hydrocarbons through a mechanism that is not fully established yet (Biloen et al., 1979; Joyner, 1977). However, an excess of amor- phous layers or their reconstruction to graphitic layers may hinder the difussion of the reactants to the catalyst surface, and eventually causing the deactivation of the catalyst. This phenomenon becomes significant at high reaction temperatures, high CO/H2 ratios or after long periods of time.

In a nebular environment typical of IDP formation where, pre- sumably, temperatures and CO/H2 ratios were low, formation of graphitic C through carburization of metal precursors in CP IDPs was unlikely to occur. In fact, most of the carbonaceous layers surrounding metallic particles in IDPs are indeed poorly ordered in nature, and those which show evidence of graphitization could be explained in terms of secondary events, such as thermal metamor- phism or atmospheric entry (Bradley, 1994). In this context, the presence of nongraphitic C over our model particles after H2 + CO exposure is inferred from XPS bands at 286.6 and 283.4 eV (Table 1) and TEM studies (Fig. 4); no graphitic C has been detected. Hydrocarbon Synthesis During Carburization

drocarbons can be generally represented as: The overall CO hydrogenation reaction for production of hy-

nCO + { (m + 2n)/2}H2 2 C,H, + nH20

where C,H, is a general representation for alkanes and alkenes. It can be seen that, for every C atom appearing in a hydrocarbon mol- ecule, l mole water is produced. High conversions, generally >5%, can produce sufficient water to promote the oxidation of surface carbides to oxide phases (Kuivila et al., 1989; Jung and Thompson, 1993). The conversion level obtained in our experiments, 0.01%, is too low for carbide oxidation, and no oxide phase has been observed by means of XP spectroscopy or TEM techniques. Carbon mon- oxide hydrogenation reactions yielding alcohols and other organics and reactions yielding C02 instead of H 2 0 such as:

2nC0 + (m/2)H2 2 C,H, + nCO2

were probably much less favorable at the low pressure and CO/H2 ratios prevailing in the solar nebula. Only hydrocarbons (Table 1) and water have been detected as reaction products in our experi- ments. Moreover, IR spectroscopy shows no evidence of hydroxy carbene, formyl species or any other 0-bearing intermediates ad- sorbed on the FeNi/Si02 sample at any stage of the reaction. Other than CO, the surface species observed during reaction contain only C and H.

The distribution of hydrocarbons obtained under our experimen- tal conditions over the FeNi/Si02 sample (Table 1) deserves some

Formation of carbides and hydrocarbons in chondritic interplanetary dust particles 249

additional comments. Methane is produced as a major product (selectivity of goo/,), the other products being ethylene, ethane, pro- pylene, propane and butane. On the other hand, alkenes represent a considerable fraction of products obtained in the C2+ range, with an alkene/alkane ratio of -0.35. The fact that methane is formed preferentially at low C0/H2 ratios is not surprising, but the presence of higher alkanes and alkenes under the same conditions was unex- pected. We attribute this particular product distribution to the speci- fic activity of the FeNi model particles surface during carburization.

It is well known that on Ni-based systems, hydrogenation of adsorbed C to yield methane is rapid because H dissociates very easily, but chain growth is not important (Wentrcek et al., 1976). Galwey (1962) demonstrated that on heating pure nickel carbide in excess H2 at 525 K, methane and ethane were formed in the ratio 1:0.05, and no other products were detected. In contrast, hydro- genation of adsorbed C coating Fe-based systems yields a broad dis- tribution of products with a wide range of molecular weights, including alkenes (Kolbel and Tillmetz, 1974). Iron-based catalysts promote the formation of longer chain hydrocarbons, although sur- face H concentration favours methanation and chain termination reactions at low CO/H2 ratios (Bukur et al., 1995b). Krebs et al. (1 979) investigated the hydrogenation of CO on a clean Fe foil at a pressure of 1 atm and temperature of 465 K with a H2:CO ratio of 1OO:l and found that the reaction products were predominantly methane and alkenes in the range of C1-C4. Infrared studies of CO and H2 adsorbed on silica-supported Fe performed by Heal et al. (1976, 1978) shows that at pressures of 2 x atm and H2:C0 ratios of 10: 1, C1-C) hydrocarbon formation occurs at temperatures >450 K. On the other hand, it has been reported that incorporation of small amounts of Ni can produce significant modifications in the catalytic behaviour of supported Fe for the conversion of H2 + CO mixtures (Raupp and Delgass, 1979a,b,c; Arai et al., 1984; Jiang et al., 1985; Rodriguez et al., 1997). Arai et al. (1984) and Ishihara et al. (1987) showed that the activity of silica-supported bimetallic cata-lysts of FeNi in the H2 + CO reaction was greater than those of pure metal catalysts, and that the selectivity of the alloy catalysts relative to pure metals was shifted to higher molecular hydro- carbons. Raupp and Delgass ( 1 979a,b,c) investigated the formation of carbides on silica-supported Fe and FeNi catalysts and concluded that alloying Fe with Ni makes the iron carbide less stable and enhance the activity for the CO hydrogenation reaction. In addition to producing major changes in the conversion and selectivity pattern compared to that exhibited by the single metal components, FeNi alloys have been found to be also very active catalysts for the growth of surface C (Raupp and Delgass, 1979b; Rodriguez et al., 1997).

Therefore, it can be concluded that CO hydrogenation over kamacite particles is enhanced relatively to pure Fe, and that prod- uct distribution over kamacite particles is shifted toward higher hy- drocarbons. This represents a point that may be important when considering solar nebula gas-grain chemistry of C. Our infrared experiments demonstrate that FeNi alloy model particles carburize under nebular conditions (Fig. 3) and, simultaneously, hydrocarbons are produced over the resulting metal surface (Fig. 5 ) . We conclude that the simultaneous formation of methane and higher hydro- carbons over kamacite grains was possible in the solar nebula at the same time that carbides and carbonaceous layers developed onto the metal precursor. We cannot extrapolate our results to reaction times longer than 1 O3 h since the poisoning effect of excessive carbonace- ous deposition on the catalyst active sites is difficult to evaluate. On the other hand, although H2S in the solar nebula can induce partial

sulfidization of metallic phases that may eventually prevent metal- catalysed reactions of hydrocarbons (e.g., Fegley and Prinn, 1988), the positive identification of S-free metal particles in IDPs suggests that, at least, some of them were not affected by such poisoning.

From our experiments, we cannot infer the precise sequence of steps for the H2 + CO reaction over the FeNVSi02 system, but it seems likely that the kinetics may be understood in terms of a single expression like r = k(CO)X(H2)Y, which is widely used in most Fischer-Tropsch studies. At low conversion levels and low CO/H2 ratios, the rate of the reaction may be controlled by the partial pressure of CO alone. However, it should be noted that the rate of hydrocarbon production is directly proportional to the amount of easy hydrogenatable carbon regardless of whether CO is present or not in the gas phase (Krebs and Bonzel, 1980). In other words, the surface of the active metal particles is covered mostly by a C inter- mediate, whose hydrogenation represents the rate-determining step in hydrocarbon synthesis reactions. This issue has strong conse- quences when considering nebular-type scenarios, where the low Pco (10-6-10-* atm; 2 x 10-6 atm in our experiments) and absence of catalytic surfaces, may kinetically inhibit hydrogenation to hy- drocarbon. However, if hydrocarbon production took place on active surface C deposited on metal particles through carburization processes, then the reaction was probably accelerated. As an example, Krebs and Bonzel (1980) have reported that over polycrystalline Fe and at 560 K, hydrocarbons are produced by hydrogenation of carbonaceous layers at PCo = 1 0-5 atm in comparable amount as that obtained with a reaction between H2 and molecular CO at Pc- = 1 0-3 atm. Thus, it appears that hydrocarbon production from H2 + CO mixtures in the solar nebula was probably enhanced by the presence of FeNi carbide phases and incipient carbonaceous deposits.

CONCLUSIONS Preparation of well-defined model FeNi alloy particles from

carbonyl precursors is a feasible methodology for the study of the catalytic properties of kamacite grains in IDPs during solid-gas phase reactions with a C-bearing gas in a nebular environment. The main results of this work can be summarized as follows:

(1) Exposure of silica-supported FeNi model particles to a H2 + CO gas mixture under nebular conditions (5 x lo4 atm, 473 K, H2:C0 = 250:l) causes the partial carburization of the alloy parti- cles. Under these conditions, the formation of €-carbide phase is reported. It has been demonstrated experimentally that gas-solid reactions in the solar nebula during CO hydrogenation processes represent a plausible scenario for the formation of carbides in IDPs.

(2) During carburization, nongraphitic poorly crystalline carbo- naceous layers develop, encapsulating the carbide phase. Carbon deposits and the carbide phase are similar in nature to those reported in C-rich aggregates in CP IDPs. There is strong evidence that at least some carbonaceous materials in IDPs were produced by hydro- genation of CO in heterogeneous phase over metallic phases under nebular conditions, an old hypothesis never experimentally tested before.

(3) In addition to carbide formation and C deposition, hydro- carbons in the CI-C4 range (selectivity of methane being 900/) are produced by Fischer-Tropsch mechanisms. It appears that formation of hydrocarbons, carbides and carbonaceous deposits are simul- taneous and related processes, which are enhanced by Ni-bearing metal grains (kamacite) relatively to pure Fe.

Ongoing experiments of carburization and Fischer-Tropsch-type reactions, with emphasis on reaction parameters and presence of HIS

250 J. Llorca and I. Casanova

in particular, are targeted towards better understanding the mechanism of metal-catalyzed C O hydrogenation reactions in a nebular environ- ment and in order to evaluate the extent of poisoning effects.

Acknowledgments-J. LI. would like to thank A. J. Brearley, R. H. Jones and J . I . Papike (University of New Mexico) for many useful discussions prior to the beginning of this work. We thank two anonymous reviewers for their very positive comments, and Scott Sandford for handling the manuscript. We also appreciate motivation and insightful comments by J. Or6 (Uni- versity of Houston). I. C. acknowledges support from CIIUT (Generalitat de Catalunya) through a research contract PQS-97. This work was partially funded by research grants from DGICYT AMB96-0953 and PB95-0780-A

Editorial handling: S. Sandford

(IC) and MAT96-0859-CO2 (JII).

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