Control of Reaction Selectivity via Surface Oxygen Coverage:  Thermal Decomposition of Azomethane on Rh(111)

Abstract

The thermal decomposition of azomethane on clean and oxygen-covered Rh(111) has been investigated using a combination of temperature-programmed reaction, high-resolution electron energy loss and Fourier transform infrared spectroscopies. On clean Rh(111), azomethane adsorbs molecularly in the trans conformation and is stable on the surface up to 250 K. Above 300 K azomethane reacts exclusively by nitrogen-nitrogen bond scission, yielding gaseous HCN, H 2 , N 2 , and C 2 N 2 . HCN is evolved in four peaks between 300 and 600 K. Adsorbed CN reacts via two competing pathways: recombination to cyanogen and dissociation to adsorbed N, which recombines to gaseous N 2 above 600 K. Adsorbed oxygen inhibits NdN bond breaking, leading to the carbon-nitrogen bond dissociation products CO, CO 2 , and formaldehyde. On Rh(111)-p(2×1)-O (θ O ) 0.5 monolayers), ∼77% of the azomethane reacts via carbon-nitrogen bond scission. The C-N bond breaking is proposed to occur via adsorbed CH 2 dNsNsCH 3 , based on vibrational and temperatureprogrammed reaction data. Subsequent carbon-nitrogen bond cleavage yields gaseous N 2 and is proposed to be accompanied by formation of transient CH 2 and CH 3 , which add to surface oxygen. Methyl would add to oxygen to form methoxy, which rapidly dehydrogenates to produce CO, CO 2 , and H 2 O. Gaseous formaldehyde is proposed to form from addition of CH 2 to oxygen followed by rapid desorption. The variation in the product distributions with oxygen coverage indicates a subtle interplay between the inhibition of nitrogen-nitrogen and C-H bond cleavage by oxygen and is cast in a general framework for hydrocarbon oxidation on Rh surfaces.

Thermal and UV photo-induced decomposition of azomethane on Rh(111)

“…Attempts were made to idenresults. The same product distribution was observed by Bol et al14 In our case, the detailed coverage-dependent TPD measurements reveal the following picture.…”

“…A recent high-resolution electron energy loss spectroscopy (HREELS) study found that azomethane adsorbs molecularly in the trans-conÐguration on Rh(111) at 100 K, and dissociates exclusively via NÈN bond scission above 300 K.14 Preadsorbed oxygen inhibits N2N bond breaking, leading to the CÈN bond dissociation. 14 The present paper reports on the use of combined RAIRS (with its better resolution) and TPD to acquire more information on the adsorption modes at 90 K, and on the bond rearrangement on a clean surface in the low-temperature range. It was found that UV light illumination alters the adsorption mode and chemistry of azomethane on Rh(111).…”

Adsorption of azomethane on Cu(110) and Cu(111) surfaces

“…In the study of azomethane on Pt(1 1 1), Jentz et al [22] proposed that the molecule was bonded to the surface via the nitrogen lone pairs. The N±N Applied Surface Science 169±170 (2001) 153±159 scission of adsorbed azomethane was suggested to proceed through isomerization to cis-azomethane upon adsorption at 84 K, and then tautomerization to form formaldehyde methylhydrazone (CH 3 -NHNCH 2 ) when annealed to 200 K. Similarly, it was reported by Bol et al [16] that azomethane isomerized to the cis-isomer upon adsorption via the nitrogen lone pairs on Mo(1 1 0) at 100 K.…”

Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001)

“…On Rh(111), ν­(CN), δ­(NH 2 ), and ν s (NH 2 ) peaks of CNH 2 from H 3 CNH 2 decomposition were observed with RAIRS at 1311, 1562, and 3367 cm –1 , respectively . In the same study, CNH 2 was ruled out as the species responsible for peaks at 1380 and 1564 cm –1 observed with HREELS after heating azomethane to 350 K . In contrast, a later RAIRS study of azomethane on Rh(111) did attribute a 1564 cm –1 peak to δ­(NH 2 ) of CNH 2 .…”