Effect of Surface Orientation on Methanol Adsorption and Thermally Induced Structural Transformations on Copper Surfaces

Roey Ben David,Adva Ben Yaacov,Baran Eren

doi:10.1021/acs.jpcc.0c10278

Roey Ben David, Adva Ben Yaacov + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcc.0c10278

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Abstract

Molecular adsorption of methanol on Cu(111), Cu(100), and Cu(110) surfaces in the 90–200 K temperature range is studied by a combination of infrared (IR) spectroscopy, thermal desorption analysis, and low-energy electron diffraction. Our results reveal the occurrence of a structural transformation in the H-bonded methanol assembly following heating from 90 to 120 K and demonstrate the effect of Cu surface orientation on the produced H-bonded structures, as well as thermal stability and ordering. At 90 K, the IR spectra indicate that similar H-bonded structures, presumably linear chains, are formed on all three surfaces. However, on Cu(111) the chains are assembled in ordered domains, whereas on Cu(100) and Cu(110) the chains are disordered. Heating to 120–130 K causes prominent changes in the IR spectrum of methanol on all surfaces, but there are significant differences between Cu(111) and the two other surfaces. We believe that such differences originate from different H-bonded methanol structures obtained on each surface after thermal annealing. On Cu(111) we suggest that cyclic structures (probably hexamers) are prevalent, whereas on Cu(100) and Cu(110) both cyclic and chain structures may coexist. The H-bonded structures produced at 120 K exhibit no long-range order on all three surfaces and show stronger adsorption (higher desorption temperate) on Cu(111). The higher stability and ordering (only at low temperatures) of adsorbed methanol on Cu(111) are attributed to the matching between the geometry of H-bonded methanol clusters and the surface structure.

Highlights

Adsorption is the first step of any surface reaction
Bonding and van der Waals interactions were taken into account, much higher adsorption energies of −0.74 and −0.67 to −0.75 eV were calculated for Cu(110)[35] and Cu(111),[20] respectively. These findings demonstrate the importance of H-bonding in methanol physisorption, and may explain the differences in the thermal desorption spectroscopy (TDS)
In the case of Cu(111), we suggest that the modification in the O−H band may be associated with the structural transformation from H-bonded methanol chains to cyclic hexamer clusters observed in previous scanning tunneling microscopy (STM) studies.[18,20]

Summary

INTRODUCTION

Adsorption is the first step of any surface reaction. Methanol adsorption on Cu is especially interesting because Cu-based catalysts are used in many reactions that involve methanol either as a reactant or as a product. That, depending on the surface coverage, two major H-bonded clusters exist: linear chains and cyclic hexamers (the latter can be both planar and buckled) For Cu(111), in line with previous studies,[18,20] our results suggest a transformation from linear H-bonded methanol chains into cyclic clusters, most probably hexamers, as a result of thermal annealing slightly below the desorption temperature. The thermal desorption profiles of methanol from Cu(100) and Cu(110) contain an additional peak compared to that of Cu(111), which suggests that both linear and cyclic structures may coexist on these two surfaces These results highlight the role of the surface orientation in determining the H-bonded structures of methanol, as well as the thermal stability and ordering of these structures

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES