Reduced molybdenum oxide MoOx as a model catalyst for methanol photosynthesis from carbon dioxide and water

I.V. Silaev, S.A. Khubezhov, I.V. Tvauri, A.G. Kaloeva, D.D. Kibizov, A.G. Ramonova, G.S. Grigorkina, Z.S. Demeev, A.P. Bliev, D. Sekiba, S. Ogura, K. Fukutani, T.T. Magkoev
North Ossetian State University,
Russian Federation

Keywords: catalysis, photocatalysis, oxide surfaces, methanol synthesis, carbon dioxide, water, surface science

Summary:

Corresponding author, e-mail: t_magkoev@mail.ru (Tamerlan Magkoev) The work deals with the design of molybdenum oxide MoO2 with a specific density of anionic vacancies for methanol synthesis from carbon dioxide and water. Surface sensitive studies were carried out in ultra-high vacuum chamber (base pressure: 2 x 10-10 Torr) by means of atomic force microscopy (AFM), reflection-absorption infrared spectroscopy (RAIRS), X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) using Al K (1486.6 eV) and He II (40.8 eV) irradiation, respectively [1]. Molybdenum oxide film was grown on Mo(110) bulk crystal by its oxidation in an oxygen ambient at partial pressure of 10-6 Torr and at a substrate temperature ranging from 1000 to 1300 K. Thin surface oxide MoO2(010) film grows at a substrate temperature of 1300 K, while thicker three-dimensional film grows after reducing the substrate temperature to 1000 K. Thick film develops into a periodically faceted surface with nanowirelike structures composed of MoO2 (021) and (02-1) faces [2]. To reduce the as-grown MoO2 oxide it was bombarded by Ar ions at energy of 1 keV and current density of 10 mA/cm2 to produce oxygen vacancies of approximately of 1/4 of their initial concentration in surface region. After cooling the sample down to 90 K the carbon dioxide was admitted into the vacuum chamber up to 10-8 Torr, followed by subsequent inlet of water until the base pressure reached 10-6 Torr. Further, the sample was irradiated by laser light from pulsed ArF eximer laser (photon energy: 6.4 eV, fluence: 4 mJ/cm2, pulse duration 9 ns, repetition rate: 10 Hz). Ultraviolet photoelectron spectra provide an evidence of photoinduced formation of methanol from carbon dioxide and water on the surface of anion deficient molybdenum oxide film. The possible mechanism is photostimulated dissociation of H2O into H+ and OH-, CO2 bond photoactivation into CO2- with formation of formate species via CO2- + H+ = CHOO-, which is hydrogenated further to dioxymethylene CH2O2 and subsequently to formaldehyde CH2O and methoxy O–CH3, forming finally methanol. Thus, MoO2 with submonolayer concentration of anionic vacancies (c.a. 25%) is and efficient model photocatalyst for methanol synthesis from carbon dioxide and water. The work was supported by The Ministry of Education and Science of Russian Federation (Goszadanie SOGU). The reported study was funded by RFBR according to projects # 16-52-50023-JaF, 16-02-00138-a. References [1] I.V. Tvauri, B.E. Gergieva, V.D. Magkoeva, G.S. Grigorkina, A.P. Bliev, O.G. Ashkhotov, V.A. Sozaev, K. Fukutani, T.T. Magkoev, Solid State Commun. 213 (2015) 42. [2] K. Radican, N. Berdunov, G. Manai, I. V. Shvets, Phys. Rev. B75 (2007) 155434 (1-7).