Low-Temperature Reductive Coupling of Formaldehyde on Rutile TiO₂(110)

Abstract

The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO 2 (110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (V O ) defect site. V O -bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two V O -bound formaldehyde molecules can couple and form Tibound species, which desorbs above ∼215 K. This coupling reaction heals both V O sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty V O sites upon dissociation of the C−O bond in V O -bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction products on the surface can only account for a half of the consumption of the initial V O coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposure between 215 and 300 K. ■ INTRODUCTIONChemical reactions of aldehyde on reducible metal oxides are important in numerous catalytic reactions in the applications related to the degradation of toxic compounds, air purification, and chemical synthesis. 1−3 For instance, reductive coupling of carbonyl species has proven to be a versatile strategy for the functionalization of vicinal carbon atoms in organic molecules. 3−5 Adsorption and reaction of various oxygenated hydrocarbons on well-defined surfaces have been extensively studied to provide unparalleled understanding of the reaction mechanisms. 6−10 Production and conversion of simple C1 molecules such as methane, methanol, formaldehyde and formic acid are involved in many of these reactions. Hence the reactivity of formaldehyde has been studied on many metal 11−17 and metal oxide surfaces 18−24 . On metal surfaces, formaldehyde generally dehydrogenates to form CO via various intermediates such as formyl, methoxy, and formate. The coupling of aldehydes to form alkenes has been identified on TiO 2 , 6,9,18,23 UO 2 , 20 V(100), 11 and Mo(110). 12 On these surfaces, the coupling reaction proceeds through the formation of diolates via C−C coupling of two aldehydes. 6,9,12,18,20 A distinct low-temperature pathway was determined on V(100), which occurs via the coupling of methylene groups formed upon dissociation of the C−O bond in adsorbed formaldehyde. 11 On reduced TiO 2 (110) surfaces, several ensemble-averaged experimental studies reported the C−C coupling of CH 2 O forming a diolate (−OCH 2 CH 2 O−) species, followed by ejection of the olefin at high temperature (∼600 K). 18,23 There is currently a controversy regarding the role of the defects including the bridging-bonded oxygen (O b ) vacancy (V O ) and Ti interstitial in this coupling reaction. 6,18,...

Photoinduced decomposition of formaldehyde on rutile TiO2(100)-(1×1)

“…But it is considerably different from the phenomenon observed on rutile TiO 2 (110) [19,21,22], rutile TiO 2 (100)-(2×1) [29], and the reduced anatase TiO 2 (100)-(1×4) surfaces [27]. On these surfaces [19,21,22,27,29], two CH 2 O molecules can be coupled to form C 2 H 4 , and surface O v sites are identified as the reactive sites for the formation of C 2 H 4 . In our experiments, the rutile TiO 2 (100)-(1×1) surface used is annealed in >4×10 −7 Torr O 2 , and nearly no surface O v sites exist on the rutile TiO 2 (100)-(1×1) surface after sample preparation.…”

“…As irradiation time increases, a peak at ∼580 K appears and increases in intensity. Based on previous works [17,19,21,22], this peak is due to desorption of C 2 H 4 product, which is formed via the carbon-carbon coupling of two CH 2 O moleules. Clearly, C 2 H 4 product is another source for the m/z=28 signal at 580 K. This is very similar to the observation of CH 2 O photodecomposition on the rutile TiO 2 (110) surface.…”

“…When TiO 2 surfaces contain bridging oxygen vacancy (O v ) sites, CH 2 O can adsorb at O v sites [20,23,29]. Further heating may result in the formation of other products via the carbon-carbon coupling reactions of two CH 2 O molecules, such as ethylene (C 2 H 4 ) [19,[21][22][23]29]. In addition, the possibility of forming paraformaldehyde chains on rutile TiO 2 (110) has also been reported by Wöll and coworkers [25,30].…”

“…The adsorption and reactions of CH 2 O on TiO 2 surfaces have been extensively studied both experimentally and theoretically [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. It has been well-established that CH 2 O can adsorb on TiO 2 surfaces in two different configurations [19, 21-23, 25, 28, 29].…”

The Mechanism of Adsorption, Diffusion, and Photocatalytic Reaction of Organic Molecules on TiO2 Revealed by Means of On-Site Scanning Tunneling Microscopy Observations

“…HCHO preferably adsorbed on the O vac sites, forming O vac -bound species and the intact HCHO diffused along the Ti row and then diffused to nearby O vac sites upon C-O bond dissociation. Afterward, this HCHO coupled with another HCHO located across the Ti row and desorbed as an ethylene molecule [55]. TMA diffusion tends to be forbidden at RT due to its chemical bond with the surface.…”