Experimental studies of cyclic oxygenates on metal surfaces: Effects of ring structure on reactivity.

摘要

Surface science experiments have been performed to study a series of related cyclic oxygenate species on catalytic metal surfaces. Specifically, the thermal chemistry of 2(5H)-furanone (25HF), gamma-butyrolactone (GBL), 2,5-dihydrofuran (2,5-DHF) and 2,3-dihydrofuran (2,3-DHF) has been studied following adsorption on Pd(111) and Pt(111) single crystals. High resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD) were used as primary techniques to study each molecule on the catalytic metal surfaces.;The thermal surface chemistry of 25HF on palladium (111) and platinum (111) was studied first. After adsorbing 25HF on each surface at 140 K, increasing the temperature above 300 K resulted in opening and decomposition of the furanone ring. On both surfaces, 25HF undergoes decarbonylation and dehydrogenation to form CO and H2 as the principal desorption products. A key difference between Pd(111) and Pt(111) reactivity is the relatively high amount of CO2 produced from Pt(111), suggesting that 25HF decomposition proceeds in part through an additional surface intermediate on Pt(111). HREELS provides further indications that the reactions proceed through distinct pathways. On Pd(111), direct decarbonylation to surface CO and ethylidyne is observed. On Pt(111), two reaction pathways are proposed. One pathway is similar to the reaction pathway for Pd(111) and produces CO during TPD, and the other proceeds through an intermediate that retains the OCO functional group and results in CO2 as a desorption product.;The adsorption and thermal chemistry of GBL on the (111) surface of Pd and Pt was investigated next. GBL differs in its structure from 25HF in that there are no C=C double bonds, allowing for the determination of the role of saturated and unsaturated bonds in ring-opening chemistry. HREELS results indicate that GBL adsorbs at 160 K on both surfaces through its oxygenate functionality. On Pd(111), adsorbed GBL undergoes ring opening and decarbonylation by 273 K to produce adsorbed CO and surface hydrocarbon species. On Pt(111), little dissociation is observed using HREELS, with almost all of the GBL simply desorbing. TPD results are consistent with decarbonylation and subsequent dehydrogenation reactions on Pd(111), although small amounts of CO2 are also detected. TPD results from Pt(111) indicate that a small proportion of adsorbed GBL (perhaps on defect sites) does undergo ring-opening to produce CO, CO2, and H2. These results suggest that the primary dissociation pathway for GBL on Pd(111) is through O-C scission at the carbonyl position. Through comparisons with previously published studies of cyclic oxygenates, these results also demonstrate how ring strain and functionality affect the ring-opening rate and mechanism.;Next, 2,3-DHF and 2,5-DHF were studied on Pd(111) to identify how the chemistry of unsaturated cyclic ethers compares to that of the unsaturated cyclic esters discussed above. The results, paired with earlier computational results, indicate that 2,3-DHF and 2,5-DHF both adsorb on Pd(111) primarily via their respective olefin functional groups at low temperature (170 K). Both molecules undergo dehydrogenation by 248 K to form furan, which is detected in TPD as a major product of 2,3- and 2,5-DHF dehydrogenation, desorbing above 320 K. The furan intermediate can undergo decomposition to form C 3H3 and CO, eventually producing CO and hydrogen as decomposition products. In addition, benzene resulting from the combination of C3H 3 intermediate fragments is detected as a desorption product from both species, at about 520 K. A key difference between the two species is that 2,3-DHF can hydrogenate to produce tetrahydrofuran at about 330 K, whereas 2,5-DHF is more likely to dehydrogenate, producing furan at about 320 K.;Experimental results from work performed with a series of cyclic oxygenate species can be used to understand general trends regarding stability and reactivity of cyclic oxygenate species relating to various aspects of the ring structure, including the presence of an olefin group, ring size, and the identity of the oxygenate functional group, among others. Some of the significant conclusions of this work include: (1) Cyclic oxygenate molecules that contain an unsaturated C=C functional group prefer to adsorb on Pd(111) or Pt(111) primarily through their olefin. Saturated species adsorb more weakly through their oxygenate functionality. (2) Adsorption through an olefin group can serve to stabilize the ring structure against ringopening. The unsaturated species studied ring-open around 300 to 323 K, whereas the saturated species ring-open by 273 K or below. (3) A high degree of ring strain (such as for a 3-membered epoxide ring) leads to a low activation energy for ring-opening. A more stable ring (such as a 5-membered lactone) requires more thermal energy in order to undergo ring-opening. An understanding of the effects of ring structural characteristics may in turn lead to an improved ability to rationally design catalysts for high performance in a wide array of heterogeneous catalysis applications.