J. G. Serafin

Surface science and the silver-catalyzed epoxidation of ethylene : an industrial perspective

Abstract Key surface science studies relating to the silver-catalyzed epoxidation of ethylene are reviewed from the perspective of industrial catalyst development. Fundamental studies using primarily Ag(111) and Ag(110) single crystals have provided evidence for the key role of atomic rather than molecular oxygen in both the epoxidation and combustion reactions. Model studies employing higher alkenes are also discussed as they reveal the importance of C–H bond reactivity in the combustion pathways. The influence of alkali and halide promoter species via electronic and geometric effects is discussed. Recent STM and in situ Raman studies of oxygen, chlorine and CO 2 adsorption on Ag surfaces showing dynamic silver restructuring and the identification of reactive surface species are highlighted as an area that will further the understanding of the epoxidation reaction by providing key structural information. Such information would not only benefit computational modelling efforts, but could also lead to more rational catalyst development processes.

DECOUPLING OF VIBRATIONAL MODES AS A STRUCTURAL TOOL : COVERAGE-INDUCED REORIENTATION OF METHOXIDE ON MO(110)

The structure and reactivity of methoxide adsorbed on Mo(110) was investigated using temperature programmed reaction, x‐ray photoelectron, high resolution electron energy loss and infrared reflection absorption spectroscopies. Methanol decomposes through a methoxy surface intermediate on Mo(110), with dehydrogenation and carbon–oxygen bond scission occurring at ∼400 K. The structure of the methoxy moiety is dependent on coverage, by reference to data obtained using surface infrared spectroscopy in combination with selective isotopic labeling. We demonstrate that methoxy exhibits C3v symmetry, i.e., the C–O bond is normal to the surface, at coverages below 0.17 ML. However, the C–O axis begins to tilt towards the surface at higher coverages, so that at saturation coverage (0.25 ML), two distinct methoxy species with Cs symmetry are observed with an average tilt angle of 25°±15° from the surface normal. In addition, we conclusively show that the intense features at ∼2910 cm−1 in the infrared spectrum of ads...

Bonding and reactivity of acetonitrile on W(100)‐(5×1)‐C

The structures of absorbed acetonitrile and the surface intermediate resulting from the formation of an N–H bond and the cleavage of one C–H bond on W(100)‐(5×1)‐C under ultrahigh vacuum conditions have been investigated using high resolution electron energy loss and x‐ray photoelectron spectroscopies. At temperatures below 200 K, a weakly bound molecular state and a species with a strongly perturbed C–N bond are present on the surface. The molecular state coordinates to a single tungsten atom via the nitrile nitrogen and undergoes competing desorption and reaction below 400 K. Acetonitrile is hydrogenated at the nitrile nitrogen and dehydrogenated at the methyl carbon to form a CH2CNH surface intermediate in the temperature range of 200–400 K in the competing reaction. The C–N stretching frequency for the RCNH intermediate is approximately 1400 cm−1 corresponding to a weakening of the C–N bond proposed to result from bonding of both the nitrile carbon and nitrogen to the surface. The RCNH intermediate re...

Bonding and adsorption structure of CO on W(100)‐(5×1)–C

The adsorption structure of carbon monoxide on W(100)‐(5×1)–C has been characterized using high‐resolution electron energy loss spectroscopy and near edge x‐ray absorption fine structure measurements. A single C–O stretch of 2100 cm−1 is observed at a surface temperature of 130 K. The C–O bond vector is determined to be normal to the surface on the basis of near edge x‐ray absorption fine structure measurements. The position in energy of the C(1s)→σ* resonance in CO was measured to be 304.2 eV. The empirical relation between the σ* resonance energy and bond length yields a bond length of 1.12 A. All spectroscopic results are consistent with CO bound atop a single tungsten atom, normal to the surface plane with minimal C–O bond weakening. These results correlate with the known difference in the reactivity of the W(100)‐(5×1)–C surface compared to clean W(100). The increased barrier for CO dissociation on the W(100)‐(5×1)–C surface is explained on the basis of reduced backdonation of electron density from t...

Isotopic labeling as a tool to establish intramolecular vibrational coupling: The reaction of 2‐propanol on Mo(110)

The reactions of 2‐propanol on Mo(110) were investigated using temperature programmed reaction, high resolution electron energy loss, and x‐ray photoelectron spectroscopies. 2‐Propanol forms 2‐propoxide upon adsorption at 120 K on Mo(110). The 2‐propoxide intermediate deoxygenates via selective γ C–H bond scission to eliminate propene as well as C–O bond hydrogenolysis to form trace amounts of propane. The C–O bond of 2‐propoxide is estimated to be nearly perpendicular to the surface. Selective isotopic labeling was used to establish the coupling between the C–O stretch and modes associated with the hydrocarbon framework. The degree of coupling was strongly affected by bonding to the surface, primarily due to weakening of the C–O bond when 2‐propoxide is bound to Mo(110). Selective isotopic labeling was, therefore, essential in making vibrational assignments and in identifying key reaction steps. Only a small kinetic isotope effect was observed during reaction of (CD3)(CH3)CHOH, consistent with a substant...

Carbon-oxygen bond strength as a control of reaction kinetics: Phenol on Mo(110)

Abstract The reaction of phenol on Mo(110) has been studied using temperature programmed reaction and X-ray photoelectron spectroscopies. After desorption of multilayers and a weakly bound molecular species, decomposition produces the only reaction products observed: gaseous dihydrogen, surface carbon and surface oxygen. The O-H bond cleaves first at temperatures below 360 K to form surface phenoxide (C 6 H 5 O-), followed by C-H bond activation commencing at 370 K. C-O bonds are cleaved in the temperature range of 370 to 450 K. After annealing to 300 K, multiple species are detected on the surface by X-ray photoelectron spectroscopy. The cleavage of C-H bonds in the same temperature regime as C-O bonds is thought to lead to selective decomposition of phenol on Mo(110). The reaction of phenol is contrasted to that of a sulfur-containing analogue, benzenethiol, on the Mo(110) surface. The stability of the phenoxide intermediate with respect to carbon-heteroatom bond cleavage is greater than that of the corresponding phenyl thiolate formed from benzenethiol. Comparison of the reaction of phenol and benzenethiol demonstrates the importance of C - X ( X = O , S ) bond strength in determining the reactivity and selectivity of these molecules.