Spectroscopic and structural characterization of the formation of olefin metathesis initiating sites on unsupported β-Mo₂C

Abstract

Reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS) measurements were used to follow the preparation of surface alkylidenes on β-Mo₂C. Treating the molybdenum carbide surface with acrolein and acetone respectively formed propylidiene and 2-propylidene groups. The propylidiene functionalised surface was found to be active for a ring opening insertion reaction with 1,3,5,7-cyclooctatetraene (COT). Helium ion microscopy (HIM) was used to determine the topological features of the surface. The surface region giving the XPS and RAIRS data is composed of large grains of several microns in diameter. In turn, the surface of these grains shows finer structure in the form of small grains suggesting that the olefin metathesis reactivity is related to the morphology of the sample.

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1. Introduction

An increased industrial demand for propene,¹ and potential applications in the processing of renewable raw materials has lead to a strong renewal of interest in the development of heterogeneous olefin metathesis catalysis.^2–9 The catalysts are typically prepared by dispersing Re, W or Mo oxides on high surface area supports.¹⁰ For example, tungsten oxide on silica is used to produce propene from a mixture of ethene and 2-butene in the olefin conversion technology licensed by ABB Lummus Inc.⁴ Recently, Bao et al.^3,5–8 reported a series of measurements on Mo aluminosilicate metathesis catalysts. In this paper, we present surface spectroscopy and HIM data for olefin metathesis related chemistry on molybdenum carbide. The first observation of olefin-metathesis on pure metal carbides was made in a surface science study by Oudghiri-Hassani and co-workers.¹¹ In subsequent studies Siaj et al. used RAIRS, TPD and XPS measurements to explore olefin-metathesis surface processes on bulk β-Mo₂C.^12,13

As described by the Chauvin mechanism,¹⁴olefin metathesis proceeds through the formation of metal carbene initiating and propagating species. On contact with the reactant, catalyst precursors generate metal carbene initiating sites. Direct evidence of metathesis active sites formed on metallic surfaces are limited to β-Mo₂C,^11–13MoAl¹⁵ and Ru.^16–18 Interestingly, these systems involve Mo or Ru, the metals most associated with the development of homogeneous olefin metathesis.¹⁴ Active Ru nanoparticles and polycrystalline films may be prepared by using diazo compounds to generate surface alkylidene groups. Active polycrystalline molybdenum carbide foils may be prepared through carbonyl bond breaking in chemisorbed aldehydes and ketones to yield surface alkylidene and oxo groups.^19–22 To date, this type of surface chemistry has not been reported for single crystal metal carbide samples. In this study, HIM imaging is used to characterize the foil as a first step towards determining if the carbide structure plays an important role in the olefin metathesis related surface chemistry.

2. Experimental

The molybdenum carbide sample was prepared by carburizing molybdenum foil in CH₄/H₂ using the temperature programmed technique.^23,24 The formation of bulk β-Mo₂C was verified using X-ray diffraction. The sample was mounted on the cold finger of an ultrahigh vacuum manipulator by wrapping a Ta foil against the back of the carbide and around the front edges. This method of holding the sample left approximately 90% of the surface free for analysis using surface science techniques. The tantalum foil was shaped so as to join it to current feedthroughs via two narrow strips. The copper feedthroughs served to cool the sample to approximately 100 K, using liquid nitrogen in the cold finger, and to resistively heat the sample to approximately 1400 K. Traces of sulfur contamination were removed using argon ion sputtering as verified by XPS measurements. XPS was also used to establish the stoichiometry of the surface region as Mo₂C. Holding the sample at 600 K and exposing it to propene minimized oxygen contamination. Excess carbon at the surface was removed by annealing to 1385 K. The sample was heated to 1300 K as a final cleaning step prior to each surface science experiment. Helium ion microscopy studies were performed to determine the morphology of the surface. The Carl Zeiss Orion Plus HIM was operated at an accelerating voltage of 30.8 kV and an imaging current of 13 pA. These settings provided a sufficiently strong backscattered ion signal for imaging.

All of the surface spectroscopy measurements were performed in a UHV chamber (base pressure approximately 2 × 10⁻¹¹ Torr). Dosing of gases via leak valves was performed in the 10⁻⁸–10⁻⁷ Torr range, using uncorrected gauge readings, and is expressed as Langmuirs (1 L = 1 × 10⁻⁶ Torr.s). Acetone, acrolein and 1,3,5,7-cyclooctatetraene were purified by successive freeze-pump-thaw cycles prior to dosing and their purity was checked in situ using the quadrupole mass spectrometer. RAIRS spectra were measured with the sample held at 100 K by averaging 800 sample scans and using the clean surface as a reference. Data recorded using InSb and narrow band MCT detectors are presented separately. All of the experiments were repeated multiple times to confirm results. TPD spectra were measured by placing the sample face at approximately 1 mm from the entrance of the shielded quadrupole mass spectrometer. The temperature was controlled electronically and set to 0.5 K/s.

3. Results and discussion

3.1 Surface 2-propylidene

Two routes to form surface carbene groups on molybdenum carbide were explored using acetone and acrolein. Data for acetone will be presented first. Reference RAIRS spectra (Fig. 1) were obtained for molecular adsorption at 100 K. Monolayer saturation is observed at approximately 3 L after which the carbonyl band characteristic of condensed acetone grows in at 1716 cm⁻¹.

Fig. 1 RAIRS data for acetone on β-Mo₂C at 100 K. Saturation of the chemisorbed monolayer occurs at an exposure of ∼3 L as shown by onset of intense bands due to condensed acetone. The 33 L spectrum is characteristic of condensed acetone.

The chemisorption layer ν(CO) band, at 1660 cm⁻¹, is characteristic of an η¹(O) state, as also reported^25–27 for acetone on Pt(111). The molecularly chemisorbed layer also displays bands at 1424, 1367, 1239 and 1109 cm⁻¹ that are assigned to δ_a(CH₃), δ_s(CH₃), ν_a(C–C–C) and ρ(CH₃) modes, respectively.^25–28 As shown in Fig. 2, the η¹(O) state can be isolated by heating the sample to 200 K while annealing to 300 K leads to the loss of the ν(CO) band. When compared to acetone on Pt-group metals^25–28 the molybdenum carbide sample displays a distinct surface chemistry in that carbonyl bond scission occurs to form surface 2-propylidene and oxo groups on annealing to the 300–500 K range. This may be seen in Fig. 2 by noting that while the ν(CO) band is eliminated, bands characteristic of the CH₃CCH₃ moiety are retained. By reference to data for other carbonyl molecules on β-Mo₂C^19–22 the two new bands which grow in at 982, 1120 cm⁻¹ are assigned to the oxo (Mo=O) vibration and a vibration with Mo=C stretching character, respectively.

Fig. 2 RAIRS spectra for 30 L acetone on β-Mo₂C measured at 100 K following annealing to the indicated temperatures.

XPS C(1s) data for acetone are presented in Fig. 3 where they are compared with reference surface sensitive synchrotron-XPS spectra²⁹ for clean β-Mo₂C and for the surface following decomposition of 10 L ethene at 600 K (spectra 3 a, b). While the clean surface displays a single peak at 282.8 eV the ethene treated surface is characterized by a peak at 283.4 eV corresponding to excess surface carbon.²⁹ Multilayer acetone (spectrum c) displays methyl and carbonyl C(1s) peaks at 285.3 and 288.2 eV, respectively. Annealing the multilayer to 400 K leads to two new peaks at 284.8 and 285.8 eV also with an intensity ratio of 2 : 1. The latter peaks are attributed to the methyl groups and tentatively to the sp²carbon of 2-propylidene, respectively. Although the binding energy attributed to the sp²carbon is surprisingly high, the assignment is supported by previous XPS data for other alkylidenes on β-Mo₂C.¹⁹

Fig. 3 High-resolution C(1s) data recorded for 20 L acetone on β-Mo₂C (c) and for acetone annealed to 400 K (d). Reference spectra, discussed in ref. 29, are shown for the clean carbide (a) and for the surface treated with 10 L ethene at 600 K (b).

3.2 Surface propylidiene

RAIRS data for acrolein on β-Mo₂C are displayed in Fig. 4 and 5. The chemisorbed monolayer is saturated at an exposure of 2 L (Fig. 4). Condensed acrolein is characterized by a carbonyl band at 1699 cm⁻¹. Molecularly chemisorbed acrolein, isolated at 200 K (Fig. 5), displays bands at 1635, 1425, 1175, 1015 and 887 cm⁻¹ in good agreement with the HREELS spectrum for acrolein on Pt(111).^30,31 The feature at 1635 cm⁻¹ is due to either the ν(CO) stretching vibration of η¹(O) chemisorbed acrolein or to the ν(C=C) stretching vibration, or to an overlap of both. The bands at 887, 1175 and 1425 cm⁻¹ may be assigned to ρ(CH₂), ν(C–C) and δ(CH₂) modes, respectively.^30,31Annealing to 500 K leads to a number of new bands that we attribute to the formation of surface propylidiene and oxo groups. The Mo=O band is observed at 990 cm⁻¹. The band at 1625 cm⁻¹ is assigned to the ν(C=C) vibration of the vinyl alkylidene. The band at 1517 cm⁻¹ is tentatively assigned to a mixed ν(C=C) + ν(Mo=C) mode. The ν(C–C) mode is observed at 1155 cm⁻¹. These peaks are also present at 300 K, but with different relative intensities, consistent with dissociation through an η²(C,O) intermediate. CH stretching region spectra for propylidiene formed from acrolein on β-Mo₂C are published elsewhere.¹²

Fig. 4 RAIRS spectra of acrolein adsorbed on β-Mo₂C at 100 K.

Fig. 5 RAIRS data for (a) molecular acrolein on β-Mo₂C at 200 K and for the surface groups formed on annealing the sample to (b) 300 K and (c) 500 K.

The thermal stability of the propylidiene surface species was studied by XPS and TPD. Surface propylidiene is characterized by a peak at 284.3 eV and a smaller peak at 285.8 eV (Fig. 6d). As described above for acetone, the latter peak is tentatively attributed to the carbon atom that forms a double bond to the metal. Annealing to higher temperatures leads to a transfer of C(1s) intensity to the peak at 283.4 eV. This is consistent with the deposition of carbon and C_xH_y fragments on the carbide surface through the progressive decomposition of the alkylidene species.^19,32 However, a fraction of the propylidiene surface groups remain stable to 900 K consistent with previous reports of the anomalously high thermal stability of alkylidenes on the molybdenum carbide surface.^11,19,32 Complete removal of the alkylidenes from the surface occurs at 920 K as shown by the recombinative desorption peak shown in Fig. 7. The low temperature peak is due to desorption from the condensed layer. The structure in the H₂ desorption spectrum in the 800–1200 K region is attributed to competition between recombinative desorption of acrolein and decomposition of propylidiene to progressively more dehydrogenated C_xH_y surface species.³²

Fig. 6 X-ray photoelectron C(1s) spectra for acrolein (a, b) on β-Mo₂C as a function of exposure at 100 K and (c–f) as a function of annealing temperature for the 20 L exposed sample.

Fig. 7 Temperature programmed desorption data obtained following 20 L acrolein exposure to β-Mo₂C at 100 K. The spectrum shows data for H₂ and molecular acrolein.

3.3 Interaction with 1,3,5,7-cyclooctatetraene

The propylidiene functionalized surface, prepared at 400 K, was exposed to 1,3,5,7-cyclooctatetraene (COT) in order to probe for ring opening metathesis polymerization (ROMP) activity. Reference RAIRS spectra (not shown) taken at 100 K show only two peaks, a CH stretching vibration at 3009 cm⁻¹ and a ring distortion mode at 802 cm⁻¹ in rough agreement with previous low resolution HREELS measurements on different metal surfaces.^33,34 C(1s) measurements for the adsorption of COT on β-Mo₂C at temperatures above 200 K show a single peak at 283.8 eV due to molecular cracking (spectrum not shown). In contrast, difference spectra show that exposure of a propylidiene-modified surface to COT at 300 K leads to the growth of a C(1s) feature at 284.3 eV (Fig. 8). The difference spectra are relative to the propylidiene functionalized surface (spectrum 6d). The feature at 284.3 eV is attributed to a ring opening insertion of COT at surface propylidiene sites.

Fig. 8 Difference C(1s) spectra taken as a function of exposure of propylidiene functionalized β-Mo₂C to 1,3,5,7-cyclooctatetraene (COT) at 300 K. The difference spectra are with respect to a propylidiene functionalized surface (spectrum 6d).

This conclusion is supported by RAIRS measurements (Fig. 9) showing that the interaction of COT with the propylidiene functionalized surface leads to a new band at 3033 cm⁻¹. The solution phase infrared spectra of both trans, trans-1,3,5,7-octatetraene and all-trans-1,3,5,7,9-decapentaene³⁵ display strong bands at ∼3030 and ∼3009 cm⁻¹. The increase in C(1s) intensity at 283.4 eV (Fig. 8) shows that COT also undergoes decomposition on the functionalized surface. Neither the XPS nor the RAIRS results show evidence for multiple insertions of COT, and it is likely that an optimal surface coverage of initiating akylidenes is required for chain growth. The surface coverage of propylidiene used in the present experiments was estimated as 0.32 of a monolayer using established XPS methods.^36,37 The monolayer is defined by modelling the entire surface as the (0001) face and assuming full occupancy of two Mo sites per unit cell. This approximate surface coverage is in very good agreement with a previous study of cyclopentylidene on the same β-Mo₂C sample.¹⁹

Fig. 9 RAIRS data for the interaction of 1,3,5,7-cyclooctatetraene (COT) with a propylidiene functionalized β-Mo₂C surface. The alkylidene functionalized surface (a) was prepared by chemisorbing (20 L) acrolein and annealing to 400 K. The interaction with COT was studied at 300 K.

3.4 Surface morphology

As described above, the sample used in the study was prepared by completely transforming a polycrystalline molybdenum foil into β-Mo₂C. The RAIRS and XPS measurements provide an averaged signal over the central region of the front of the sample. Neither of these two techniques is extremely sensitive. For example, it is unlikely that alkylidenes located only at a small fraction of active sites would furnish the RAIRS signal observed for the 500 K annealed samples (Fig. 2 and 5). Under the assumption that the alkylidene species are distributed over the surface, helium ion microscopy (HIM) measurements were performed to probe the morphology of the region giving the XPS and RAIRS data.

Fig. 10 presents two typical images of the sample showing both the topological features (Fig. 10a) and contrast due to channelling (Fig. 10b). It is apparent that the carbide surface is composed of large grains typically 5–10 microns in diameter. The channelling contrast in particular is important as it is dependent on the orientation of the grain with the beam. When the beam is oriented along a direction parallel to a crystallographic direction the ions travel further into the sample. As a consequence of enhanced channelling, less backscattered ions are able to be collected by the detector and the grain appears dark.³⁸ Selecting an area on the surface, Fig. 11a is a HIM image of the area at 0 degrees while Fig. 11b shows that tilting the sample causes the the contrast to vary. Local minima of the intensity values indicate when a channelling condition has been achieved. Comparing the curves in Fig. 11b we observe that while all of the large grains exhibit signs of channelling the minima appear at different intensities. This implies that the region labelled ‘B’, for example, is a surface of low areal density while the region labelled ‘C’, displaying the highest backscattered yield, is indicative of a densely packed plane.

Fig. 10 Helium ion microscopy (HIM) images of a region of the β-Mo₂C surface representative of the area giving the RAIRS and XPS data shown above. (a) SE2 image of the active carbide surface showing the topological features of the sample. (b) Backscattered ion image of the same area showing channelling contrast from the grains. (c) High magnification SE2 image of the surface showing an example of the surface nanostructure.

Fig. 11 Helium ion microscopy data. (a) Backscattered ion image of the active carbide surface showing the channelling contrast in the sample. (b) Normalized intensity from the regions labelled A–E. The local minima indicate a channelling condition. The large particle on the right of the image was used for image registration.

On closer inspection, the surfaces of the large grains show additional structure (Fig. 10c) in the form of much smaller grains. The overall morphology of the surface is akin to a compacted powder where the smallest particles are nano-sized. The surface morphology of the sample may play a role in the demanding carbonyl bond dissociation step inherent to alkylidene formation. We assume that this step only requires isolated metal surface atoms, as single tungsten atom complexes can form oxo-W-alkylidene products through carbonyl bond scission in direct analogy to the surface reaction.³⁹ Careful studies of α-Mo₂C(0001) single crystals⁴⁰ show that it is possible to prepare Mo- and C-terminated surfaces. Studies on carbide surface layers on Mo single crystals show that an annealing step at 1200 K is required to drive carbon from the surface into interstitial sites so as to generate a surface with a reactivity characteristic of the bulk carbide.^41,42 We propose that the activity of the highly structured carbide foil arises from the presence of unsaturated Mo atoms in the outermost surface layer. On the basis of these results, the targeted design of nanostructured films is expected to play a key role in exploring the impressive catalytic and surface reactivity properties of molybdenum carbide.^23,42–46

4. Conclusions

Surface vibrational spectroscopy and XPS data show the dissociative adsorption of acetone and acrolein on β-Mo₂C to yield 2-propylidene and propylidiene surface alkylidene groups. The propylidiene functionalized surface displays activity towards ring opening insertion of 1,3,5,7-cyclooctatetraene (COT). Helium ion microscopy characterization of the area of the sample probed by the spectroscopic techniques shows that the surface is composed primarily of large grains, and that the surface of these grains show additional structure in the form of smaller grains typically less than 200 nm in width. The combination of spectroscopy and microscopy data suggests that the observed olefin metathesis related reactivity of the carbide sample is related to its surface morphology.

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