Zongfang Wu

Photocatalytic Cross-Coupling of Methanol and Formaldehyde on a Rutile TiO2(110) Surface

The photocatalytic oxidation of methanol on a rutile TiO2(110) surface was studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The combined TDS and XPS results unambiguously identify methyl formate as the product in addition to formaldehyde. By monitoring the evolution of various surface species during the photocatalytic oxidation of methanol on TiO2(110), XPS results give direct spectroscopic evidence for the formation of methyl formate as the product of photocatalytic cross-coupling of chemisorbed formaldehyde with chemisorbed methoxy species and clearly demonstrate that the photocatalytic dissociation of chemisorbed methanol to methoxy species occurs and contributes to the photocatalytic oxidation of methanol. These results not only greatly broaden and deepen the fundamental understanding of photochemistry of methanol on the TiO2 surface but also demonstrate a novel green and benign photocatalytic route for the synthesis of esters directly from alcohols or from alcohols and aldehydes.

Active hydrogen species on TiO2 for photocatalytic H2 production

Photocatalytic H2 production over TiO2 has attracted tremendous attention and achieved great progress, but the active hydrogen species is still unknown. Employing a rutile TiO2(110) surface as a model catalyst we report here for the first time the direct observation of photocatalytic H2 production under ultrahigh vacuum conditions during UV-light irradiation at 115 K and the identification of negatively-charged hydride-type H-Ti species as the corresponding photoactive surface species by means of thermal desorption spectroscopy, photon-stimulated desorption spectroscopy, X-ray photoelectron spectroscopy and DFT calculations. The formation and stability of H-Ti species are closely related to available surplus electrons on the rutile TiO2(110) surface that can be created by the formation of surface BBO vacancies or by the formation of surface hydroxyls via the adsorption of atomic H or molecular H2 on O sites. The photocatalytic H2 production from H-Ti species is hole-mediated and co-existing water exerts a negative effect on this process.

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface.

Exploring reactions of methanol on TiO2 surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO2(001)-(1×4) surface. The methanol-to-dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold-coordinated Ti(4+) sites (Ti(4c)), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO2 facets. Surface chemistry of methanol on the anatase TiO2(001)-(1×4) surface is seldom affected by co-chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO2 surfaces but also show that TiO2 with a high density of Ti(4c) sites is a potentially active and selective catalyst for the important methanol-to-DME reaction.

Reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts revealed by an FeO(111)/Pt(111) inverse model catalyst

We have employed XPS and TDS to study the adsorption and surface reactions of H2O, CO and HCOOH on an FeO(111)/Pt(111) inverse model catalyst. The FeO(111)-Pt(111) interface of the FeO(111)/Pt(111) inverse model catalyst exposes coordination-unsaturated Fe(II) cations (Fe(II)CUS) and the Fe(II)CUS cations are capable of modifying the reactivity of neighbouring Pt sites. Water facilely dissociates on the Fe(II)CUS cations at the FeO(111)-Pt(111) interface to form hydroxyls that react to form both water and H2 upon heating. Hydroxyls on the Fe(II)CUS cations can react with CO(a) on the neighbouring Pt(111) sites to produce CO2 at low temperatures. Hydroxyls act as the co-catalyst in the CO oxidation by hydroxyls to CO2 (PROX reaction), while they act as one of the reactants in the CO oxidation by hydroxyls to CO2 and H2 (WGS reaction), and the recombinative reaction of hydroxyls to produce H2 is the rate-limiting step in the WGS reaction. A comparison of reaction behaviors between the interfacial CO(a) + OH reaction and the formate decomposition reaction suggest that formate is the likely surface intermediate of the CO(a) + OH reaction. These results provide some solid experimental evidence for the associative reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts.

Reaction heat-driven CO2 desorption during CO oxidation on Au(997) at low temperatures

Adsorption and reaction of CO and CO2 were studied on oxygen-covered Au(997) surfaces by means of temperatureprogrammed desorption/reaction spectroscopy. Oxygen atoms (O(a)) on Au(997) enhances the CO2 adsorption and stabilizes the adsorbed CO2(a), and the stabilization effect also depends on the CO2(a) coverage and involved Au sites. CO2(a) desorption is the rate-limiting step for the CO+O(a) reaction to produce CO2 on Au(997) at 105 K and exhibits complex behaviors, including the desorption of CO2(a) upon CO exposures at 105 K and the desorption of O(a)-stabilized CO2(a) at elevated temperatures. The desorption of CO2(a) from the surface upon CO exposures at 105 K to produce gaseous CO2 depends on the surface reaction extent and involves the reaction heat-driven CO2(a) desorption channel. CO+O(a) reaction proceeds more easily with weakly-bound oxygen adatoms at the (111) terraces than strongly-bound oxygen adatoms at the (111) steps. These results reveal complex rate-limiting CO2(a) desorption behaviors during CO+O(a) reaction on Au surfaces at low temperatures which provide novel information on the fundamental understanding of Au catalysis.

Self-Anticoking of a Cobalt Surface by Subsurface Oxygen in the Fischer-Tropsch Synthesis

Understanding the fundamental processes taking place on Co surfaces during the Fischer-Tropsch (FT) synthesis is of great interest and importance. We herein report a self-anticoking mechanism of a cobalt surface by subsurface oxygen. The active carbidic carbon species for FT synthesis tends to transform into the inactive graphitic carbon species on clean Co(0001) and poisons the Co surface. Subsurface atomic oxygen on Co(0001) can stabilize the active carbidic carbon species and quench the transformation process. These results reveal, to the best of our knowledge, for the first time the reactivity of various surface species on Co surfaces that dynamically maintain a delicate balance to enhance the long-term stability of Co catalysts during FT synthesis.

Oxidation of formic acid on stepped Au(997) surface

The adsorption and reaction of formic acid (HCOOH) on clean and atomic oxygen-covered Au(997) surfaces were studied by temperature-programmed desorption/reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). At 105 K, HCOOH molecularly adsorbs on clean Au(997) and interacts more strongly with low-coordinated Au atoms at (111) step sites than with those at (111) terrace sites. On an atomic oxygen-covered Au(997) surface, HCOOH reacts with oxygen atoms to form HCOO and OH at 105 K. Upon subsequent heating, surface reactions occur among adsorbed HCOO, OH, and atomic oxygen and produce CO 2 , H 2 O, and HCOOH between 250 and 400 K. The Au(111) steps bind surface adsorbates more strongly than the Au(111) terraces and exhibit larger barriers for HCOO(a) oxidation reactions. The surface reactions also depend on the relative coverages of co-existing surface species. Our results elucidate the elementary surface reactions between formic acid and oxygen adatoms on Au surfaces and highlight the effects of the coordination number of the Au atoms on the Au catalysis.