Xingcai Su

High pressure catalytic processes studied by infrared-visible sum frequency generation

The recent development of infrared-visible sum frequency generation (SFG), a surface-specific vibrational spectroscopy, has helped bridge the pressure gap between studies of heterogeneous catalysis under high vacuum and atmospheric pressure. This is achieved by in situ monitoring of surface species at high pressure via their SFG vibrational spectra and correlating the results with the simultaneously measured reaction rate using gas chromatography. Examples of systems studied include olefin hydrogenation and carbon monoxide oxidation over the (111) crystalline face of platinum. In these examples, the studies succeed in revealing the molecular details of the surface reactions. Identification of key intermediates and their concentrations has made it possible for the first time to calculate turn over rates per active surface species rather than just per exposed surface metal atom. In all cases, the key intermediate of the reaction is not detectable on the surface in UHV under similar temperatures.

The first measurement of an absolute surface concentration of reaction intermediates in ethylene hydrogenation

The first measurement of a turnover rate with respect to surface intermediate concentration in a high pressure heterogeneous catalytic reaction is reported. By using infrared-visible sum frequency generation to study the hydrogenation of ethylene on Pt(111), it was found that the surface concentration of π-bonded ethylene, the key reaction intermediate, represented approximately 4% of a monolayer. Thus the absolute turnover rate per surface adsorbed ethylene molecule is 25 times faster than the rate measured per platinum atom. To explain these results, we propose a model of weakly adsorbed ethylene intermediates reacting on atop sites.

1,3- and 1,4-cyclohexadiene reaction intermediates in cyclohexene hydrogenation and dehydrogenation on Pt(111) crystal surface: a combined reaction kinetics and surface vibrational spectroscopy study using sum frequency generation

Abstract The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) surface were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under ultrahigh vacuum (UHV) and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures up to 590 Torr. Under UHV, cyclohexene on Pt(111) undergoes a change from π/σ-bonded, σ-bonded, and c-C6H9 surface species to adsorbed benzene when the surface was heated. A site-blocking effect was observed at saturation coverage of cyclohexene and caused the dehydrogenation to shift to higher surface temperature. At high pressures, however, none of the species observed in UHV condition were seen. 1,4-cyclohexadiene (CHD) was found to be the major species on the surface at 295 K even in the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300–400 K with 1,3-CHD on the surface as revealed by SFG. Further increasing surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate with both 1,3-CHD and 1,4-CHD detectable on the surface simultaneously. Monitoring the reaction kinetics and the chemical nature of surface species together allows us to postulate a reaction mechanism: cyclohexene hydrogenates to cyclohexane via a 1,3-CHD intermediate, and dehydrogenates to benzene through both 1,4-CHD and 1,3-CHD intermediates. Both 1,3- and 1,4-CHD dehydrogenate to benzene at sufficiently high temperature on Pt(111).

Cyclohexene dehydrogenation and hydrogenation on Pt(111) monitored by SFG surface vibrational spectroscopy: different reaction mechanisms at high pressures and in vacuum

The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) crystal surfaces were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under vacuum and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures from 30 up to ~600 Torr. At high pressures, the gas composition and turnover rate (TOR) were measured by gas chromatography. In vacuum, cyclohexene on Pt(111) undergoes a change from π/σ‐bonded, σ‐bonded cyclohexene and c‐C6H9 surface species to adsorbed benzene when the surface was heated from 130 to 330 K. A site‐blocking effect was observed at saturation coverage of cyclohexene that caused dehydrogenation to shift to somewhat higher surface temperature. At high pressures, however, none of the species observed in vacuum conditions were detectable. 1,4‐cyclohexadiene (1,4‐CHD) was found to be the major species on the surface at 295 K, even with the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300 and 400 K with 1,3‐cyclohexadiene (1,3‐CHD) on the surface, as revealed by SFG. Further increasing the surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate and both 1,3‐CHD and 1,4‐CHD were present on the surface simultaneously. The simultaneous observation of the reaction kinetic data and the chemical nature of surface species allows us to postulate a reaction mechanism at high pressures: cyclohexene hydrogenates to cyclohexane via a 1,3‐CHD intermediate and dehydrogenates to benzene through both 1,4‐CHD and 1,3‐CHD intermediates. Isomerisation of the 1,4‐CHD and 1,3‐CHD surface species is negligible.

The hydrogenation and dehydrogenation of isobutene on Pt(111) monitored by IR–visible sum frequency generation and gas chromotography

The hydrogenation of isobutene (2-methylpropene) has been monitored near ambient pressure over a Pt(111) single crystal at 295 K by SFG vibrational spectroscopy. During hydrogenation tert-butyl groups and π-bonded isobutene species were the dominant surface species detected. Only small amounts of dehydrogenated species such as isobutylidyne could be observed unless the reaction conditions were very hydrogen poor. The hydrogenation rate for isobutene was at least one order of magnitude lower than for but-1-ene and cis-but-2-ene. Vacuum studies indicated that the rate of tert-butyl hydrogenation was much slower than that of isobutyl, which may force isobutane production to proceed from π-bonded isobutene through the slow kinetic step of isobutyl formation. Vacuum studies of isobutene dehydrogenation show evidence for a stable intermediate during the dehydrogenation of di-σ-bonded isobutene to isobutylidyne.

The surface chemistry of 1,3-cyclohexadiene and 1,4-cyclohexadiene on Pt(111) studied by surface vibrational spectroscopy with sum frequency generation

The structure and reactions of 1,3-cyclohexadiene and 1,4-cyclohexadiene on Pt(111) has been studied by sum frequency generation at a pressure of 10−10–1 Torr. It was found that 1,4-cyclohexadiene adsorbs flat on the surface at low temperature and dehydrogenates to adsorbed benzene at about 300 K. 1,3-Cyclohexadiene undergoes a rearrangement to 1,4-cyclohexadiene through a 3–5 hydrogen shift which competes with the dehydrogenation reaction pathway at low temperature (<290 K) and dehydrogenates to benzene at higher temperature. This competing reaction behavior has also been observed at 1 Torr pressure of 1,3-cyclohexadiene on Pt(111) at 285 K.

Molecular surface studies of adsorption and catalytic reaction on crystal (Pt, Rh) surfaces under high pressure conditions (atmospheres) using sum frequency generation (SFG) – surface vibrational spectroscopy and scanning tunneling microscopy (STM)

Sum frequency generation (SFG) – surface vibrational spectroscopy and the scanning tunneling microscope (STM) have been used to study adsorption and catalyzed surface reactions at high pressures and temperatures using (111) crystal surfaces of platinum and rhodium. The two techniques and the reaction chambers that were constructed to make these studies possible are described. STM and SFG studies of CO at high pressures reveal the high mobility of metal atoms, metal surface reconstruction, ordering in the adsorbed molecular layer, and new binding states for the molecule. CO oxidation occurs at high turnover rates on Pt(111). Different adsorbed species are observed above and below the ignition temperature. Some inhibit the reaction, and others are reaction intermediates since their surface concentration is proportional to the reaction rate. The dehydrogenation of cyclohexene on Pt(100) and Pt(111) proceeds through a 1,3‐cyclohexadiene surface intermediate. The higher dehydrogenation rate is related to the higher surface concentration of these molecules on the (100) crystal face.

SFG and STM Studies of the Pt(111) Crystal Face at Atmospheric CO and Oxygen Pressures: Preparation of Platinum Nano-Cluster Arrays[Sum Frequency Generation]

Studies of crystal surfaces at high pressures (ca. 1 atm) and during catalytic reactions become possible by the application of sum frequency generation (SFG) vibrational spectroscopy and scanning tunnelling microscopy (STM). We present the SFG spectra of CO on Pt(111) as a function of pressure and show evidence for the reversible formation of carbonyl clusters with a CO/Pt ratio of > 1 and for an incommensurate CO layer at high pressures. These species turn over rapidly to produce CO2 during CO oxidation. STM studies show significant reconstruction of Pt(110) in both CO and O2, while the surface structure of the Pt(111) crystal face exhibits only minor changes. Platinum nanocluster arrays in the 3–100 nm range have been produced by electron beam lithography. These cluster systems produced on different oxide supports can be used as model catalysts.

Surface-induced ferroelectric ice

Summary form only given. Can ice be ferroelectric! This question has long attracted much attention. In crystalline ice, water molecules are held together by tetrahedralhydrogen bonding. The molecular orientations at the lattice points should obey the ice rules, which require that each molecule donate two protons to two of the attached water molecules and accept two protons from the other two. We used infrared-visible sum-frequency generation (SFG) spectroscopy as a probe in our experiment. As a second-order nonlinear optical process, SFG is forbidden in a medium with inversion symmetry. In its application to an ice film, the spectrum could be weak if the water molecules in the film are non-polar oriented.