Origins of the High Thermal Stability of Alkylidene Groups on the Surface of β-Mo₂C

Abstract

Surface alkylidenes can be formed on β-Mo 2 C through the selective carbonyl bond scission of ketones and aldehydes. Spectroscopic studies show that alkylidene groups remain intact on the carbide surface to above 900 K under ultrahigh vacuum conditions. A rationalization for the anomalously high thermal stability is presented on the basis of surface analysis studies. It is shown that the relatively high reactivity of the clean carbide surface permits both carbonyl bond scission and other less selective processes involving CC and CH bond cleavage. The combined decomposition channels lead to the deposition of excess carbon and highcoordination oxygen, resulting in an inert surface on which alkylidenes are thermally stable. Removal of surface carbon, through CO desorption and carbon diffusion, occurs at high temperatures leading to a newly reactive surface. Alkylidenes trapped at low temperatures on the intrinsically passivated surface can survive until the clean surface is partially restored at high temperatures.

Preliminary study of the surface reactivity of 2D α‐Mo2C crystallites

“…Hence, we rule out 2methylfuran desorption as being a significant process in the present system. In summary, we tentatively attribute the XPS signal to a mixture of h 2 (C,O) states (Figure 1b), surface alkylidenes (Figure 1c), and hydrocarbon fragments resulting from more extensive decomposition, as previously reported for cyclic ketones on bulk b-Mo 2 C by Oudghiri-Hassani et al [37] High-resolution XPS studies and reflectance absorbance infrared spectroscopy experiments are needed to describe the complex furfural/2D-a-Mo 2 C/Cu system with more molecular detail.…”

“…The spectrum of the as-received sample displays a weak peak at 283.2 eV and a strong broad peak centered at 285.0 eV. The low binding energy peak is readily attributed to the carbide by reference to previous work from our group, [36,37] and data reported by Bauer et al and Gleichweit et al [38,39] Our previous studies found that b-Mo 2 C is characterized by a C(1s) binding energy of 282.8 eV and that b-Mo 2 C bearing excess C on the surface (prepared by exposure at 600 K to a small dose of propene) displays a binding energy of 283.2 eV. Bauer et al and Gleichweit et al reported that monolayer carbide on Mo(110) formed at 1200 K by exposure to C 2 H 4 displays a binding energy of 283.8 eV, while bulk carbide prepared by high exposures to ethylene shows a binding energy of 283.0 eV.…”

“…The atomic Mo:C ratio was found to be 1.5:1.0 and 1.1:1.0 at take‐off angles of 0 and 75°, respectively. The excess carbon is possibly the reason why the measured C(1s) is 283.2 eV rather than 282.8 eV, as seen for clean β‐Mo 2 C. As mentioned above, excess carbon produced by propene decomposition on β‐Mo 2 C is characterized by a binding energy of 283.2 eV …”

“…The excess carbon is possibly the reason why the measured C(1s) is 283.2 eV rather than 282.8 eV, as seen for clean b-Mo 2 C. As mentioned above, excess carbon produced by propene decomposition on b-Mo 2 C is characterized by a binding energy of 283.2 eV. [36,37] However, it should be emphasized that the present XPS studies are not high-resolution measurements such as can be performed at synchrotron installations. Hence, for example, the data do not support comment on possible binding energy shifts related to the dimensionality and shapes of the crystallites or to the presence of the Cu support and resulting Mo 2 C/Cu interfaces.…”

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

“…This is consistent with the deposition of carbon and C x H 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.…”