O-covered Surface Research Articles

Interaction of H2S with perfect and O-covered Pd(100) surface: A first-principles study

Using density functional theory (DFT) calculations, we investigated the dissociative mechanism of H2S adsorption and its dissociation on both clean and oxygen covered Pd (100) surface. For adsorption mechanism, the site preference was considered for H2S and HS monomers on both surfaces. The results suggest that H2S is energetically adsorbed on high symmetry sites, particularly the hollow site, on clean surface and top site on O-covered Pd (100) surface. Chemisorption of HS is stronger than H2S at the favorable hollow site on clean surface and O-covered surface. The energy barriers for S–H bond-breaking during the first H2S dehydrogenation are 0.35, 0.07, 0.32, and 0.16 eV, while for second dehydrogenation are 0.32 and 0.30 eV, respectively. On the oxygen-covered surface, the H2S adsorption site transitions from bridge to the top site and their energy barrier for first-order decomposition is 0.05 eV, significantly lesser than that observed on a clean surface. To examine more, the electronic densities of states as well as d-band center model were used to characterize the interaction of adsorbed H2S with the surfaces. Overall, our findings show that H2S decomposition on these surfaces is exothermic and relatively easy, but the presence of surface oxygen hinders the breaking process of H–S bond due to kinetic and thermodynamic factors.

Atomistic Simulation Study of Impacts of Surface Carrier Scatterings on Carrier Transport in Pt Nanosheets

The understanding of carrier transport in metal nanostructures is indispensable for the development of nanoelectronics. In particular, Pt nanostructures have been intensively studied to realize gas sensors based on adsorbate-induced surface electron scattering. Conventionally, electron scattering at the surface of metal nanostructures has been phenomenologically described by a single specularity parameter. In this work, surface electron scattering was quantitatively studied through molecular dynamics simulations, followed by density functional nonequilibrium Green’s function calculations. Although the extracted specularity parameters qualitatively agreed with empirically treated diffusive scattering at the O-covered Pt surface and specular scattering at the H-covered Pt surface, our atomistic calculation revealed an increase in resistivity owing to H adsorption on thin Pt(111) nanosheets.

Detection of a real heterogeneous catalyst with an inactive oxygen-covered surface: Au/Li4Ti5O12

In the investigation of the catalytic mechanism, O-covered transition-metal oxide surfaces with concealed transition-metal cations are occasionally employed as model catalyst support structures. These are the most extreme models for investigating the importance of O species on the support surface. There are few cases in which a catalytic metal is supported on O-covered surfaces, and its catalytic activity is confirmed. In this study, we confirmed that the most exposed surface of Li4Ti5O12 is the O-covered (Ti-sublayer) (111) surface, and we prepared and evaluated a real catalyst using Li4Ti5O12 as the support. Au nanoparticles were used as the catalytic metal, and the interface orientation was determined to be Au(111)//Li4Ti4O12(111). Furthermore, the interface comprising the inactive O-covered (Ti-sublayer) surfaces of transition-metal oxides exhibited catalytic activity. These are important for examining the catalytic mechanism using the surface O species of the support by theoretical calculations, thereby bridging the gap between theory and experiments.

Stepwise dehydrogenation of ammonia on Fcc-Co surfaces: A DFT study

The stepwise dehydrogenation of ammonia on clean and O-covered Co surfaces have been studied by performing density functional theory (DFT) calculations. It is found that the interaction of species NHx (x=0–3) with the Co surfaces become stronger with its further dehydrogenation, and oxygen atom not only strengthens ammonia-substrate interaction but also facilitates ammonia dissociation. Specifically, pre-adsorbed O atom significantly promotes the stepwise dehydrogenation of ammonia on Co(110), giving rise to N atom strongly binding with the surface. In contrast, the dissociation of NH appears to be the rate-determining step on O-covered Co(111) and Co(100) surfaces, due to the high energy barriers. And present results demonstrate that the species N and NH produced in ammonia dehydrogenation are likely responsible for cobalt catalyst deactivation in the excess of oxygen atom.

Energetics of elementary reaction steps relevant for CO oxidation: CO and O2 adsorption on model Pd nanoparticles and Pd(111)

The energetics of elementary surface processes relevant for CO oxidation, particularly CO and 02 adsorption, were investigated by a direct calorimetric method on model Pd nanoparticles and on the extended Pd(111) single crystal surface. The focus of this study lies on a detailed understanding of how a nanometer scale confinement of matter affects the binding strength of gaseous adsorbates. We report adsorption energies and sticking coefficients of CO and 02 measured as a function of the adsorbate surface coverage both on pristine and O-covered Pd surfaces. The reduced dimensions of the Pd substrate were found to affect the binding strength of the adsorbates in two principle ways: (i) via the change of the local adsorption environment that can result e.g. in stronger adsorbate bonding at the particle's low coordinated surface sites and (ii) via the contraction of the Pd lattice in small clusters and a concomitant weakening of chemisorptive interaction. Particularly for 02 adsorption, the change of the adsorption site from a three-fold hollow on Pd(111) to the edge site on Pd nanoparticles (approximately 4 nm sized on average) was found to result in a strong increase of the Pd-O bond strength. In contrast, CO adsorbs weaker on Pd nanoparticles as compared to the extended Pd(111) surface. In total, the binding energies of adsorbates on Pd and with this their surface coverages turn out to depend in a non-monotonic way on the particular structure of Pd surfaces, including the local structure of the adsorption site as well as the global properties of the small clusters arising e.g. from the lattice contraction.

Searching for active binary rutile oxide catalyst for water splitting from first principles

Water electrolysis is an important route to large-scale hydrogen production using renewable energy, in which the oxygen evolution reaction (OER: 2H(2)O → O(2) + 4H(+) + 4e(-)) causes the largest energy loss in traditional electrocatalysts involving Ru-Ir mixed oxides. Following our previous mechanistic studies on the OER on RuO(2)(110) (J. Am. Chem. Soc. 2010, 132, 18214), this work aims to provide further insight into the key parameters relevant to the activity of OER catalysts by investigating a group of rutile-type binary metal oxides, including RuNiO(2), RuCoO(2), RuRhO(2), RuIrO(2) and OsIrO(2). Two key aspects are focused on, namely the surface O coverage at the relevant potential conditions and the kinetics of H(2)O activation on the O-covered surfaces. The O coverage for all the oxides investigated here is found to be 1 ML at the concerned potential (1.23 V) with all the exposed metal cations being covered by terminal O atoms. The calculated free energy barrier for the H(2)O dissociation on the O covered surfaces varies significantly on different surfaces. The highest OER activity occurs at RuCoO(2) and RuNiO(2) oxides with a predicted activity about 500 times higher than pure RuO(2). On these oxides, the surface bridging O near the terminal O atom has a high activity for accepting the H during H(2)O splitting. It is concluded that while the differential adsorption energy of the terminal O atom influences the OER activity to the largest extent, the OER activity can still be tuned by modifying the electronic structure of surface bridging O.

Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces

Temperature-programmed desorption (TPD) and Density Functional Theory (DFT) were used to investigate the reactions of oxametallacycles derived from ethylene oxide on clean and oxygen-covered Ag(110) surfaces. Ethylene oxide ring-opens following adsorption at 250 K on both clean and O-covered Ag(110) to form a stable oxametallacycle. On the clean Ag(110) surface, the oxametallacycle reacts to reform the parent epoxide at 280 K during TPD, while the aldehyde isomer, acetaldehyde, is observed at higher oxametallacycle coverages. In the presence of coadsorbed oxygen atoms, a portion of the oxametallacycles dissociate to release ethylene. However, of those that react to form oxygen-containing products, the fraction forming ethylene oxide is similar to that on the clean surface. The acetaldehyde product of oxametallacycle reactions combusts via formation of acetate species; the acetates react to form CO2 at temperatures as low as 360 K on the O-covered surface. No evidence was observed for other combustion channels. This work provides experimental evidence for the connection of oxametallacycles to combustion via acetaldehyde formation as well as to ring-closure to form ethylene oxide.

Fingering instabilities on reaction fronts in the CO oxidation reaction on Pt(100)

Fingering instabilities arising from local perturbations to planar reaction fronts in the CO oxidation reaction on Pt(100) are presented. CO oxidation represents a heterogeneous nonlinear system with the necessary kinetic and diffusive transport properties to support the development of fingered wave fronts. External forcing was utilized to create CO wave fronts on an otherwise monostable, O-covered surface, which, upon destabilization, gave rise to fingers of adsorbed CO extending into the O adlayer ahead of the reaction front. Finger spreading and tip-splitting were observed as the finger pattern evolved towards an intrinsic wavelength, independent of the length of the reaction front, calculated to be approximately 40 μm. Our data also show the presence of a shielding process, where at wavelengths less than twice the observed intrinsic value, additional fingers were created on the reaction front through a tip-splitting bifurcation of an existing finger. At wavelengths greater than twice the intrinsic value, additional fingers formed in the troughs between adjacent fingers, apparently unaffected by the presence of the larger surrounding fingers.

Adsorption and Dissociation of CO as Well as CHx Coupling and Hydrogenation on the Clean and Oxygen Pre-covered Co(0001) Surfaces

Spin-polarized density functional theory calculations were performed to investigate the oxidation of the Co(0001) surface and the effects of surface O on CO adsorption and dissociation as well as CHx coupling and hydrogenation. Under the realistic Fischer−Tropsch synthesis conditions (493 K, PH2O/PH2 = 1−1.5), Co(0001) can be oxidized by H2O forming a 1/4 monolayer O-covered surface, while subsurface oxidation or high oxidized surface (1/2 monolayer) is thermodynamically not possible. Compared to clean Co(0001), the oxygen pre-covered surface lowers the ability for CO adsorption and activation, elevates the CO dissociation barrier, and favors CO2 formation, as well as elevates the CH/CH coupling barrier and favors the CH hydrogenation thermodynamically. Consequently, the catalytic activity is reduced, and the monomeric building block for chain growth changes from CH to CH2. Interestingly, the formation of CO2 will refresh the surface. Therefore, we suggest that either controlling H2O content to lower the ...

Catalytic reduction of N2O by H2 over well-characterized Pt surfaces

A 0.65% Pt/SiO2 catalyst has been prepared using an ion exchange technique and extensively characterized prior to being used for continuous catalytic N2O reduction by H2 at very low temperatures, such as 363 K. The supported Pt with a high dispersion of 92% gave no presence of O atoms remaining on an H-covered Pts, based on in situ DRIFTS spectra of CO adsorbed on Pts after either N2O decomposition at 363 K or subsequent exposure to H2 for more than 1 h; thus the residual uptake gravimetrically observed even after the hydrogen titration on an O-covered surface is associated with H2O produced by introducing H2 at 363 K onto the oxidized Pts. Dissociative N2O adsorption at 363 K on Pts was not inhibited by the H2O(ad) on the silica surface but not on Pts, as acquired by IR peaks at 3,437 and 1,641 cm−1, in very consistent with the same hydrogen coverage, established via H2-N2O titration on a reduced Pts, as that revealed upon the titration reaction with a fully wet surface on which all bands and their position in IR spectra for CO are very similar to that obtained after H2 titration on a reduced Pts. Based on the characterization using chemisorption and in situ DRIFTS and TPD measurements, the complete loss in the rate of N2O decomposition at 363 K after a certain on-stream hour, depending significantly on N2O concentrations used, is due to self-poisoning by the strong chemisorption of O atoms on Pts, while the presence of H2 as a reductant could readily catalyze continuous N2O reduction at 363 K that is a greatly lower temperature than that reported earlier in the literature.

Reactivity to sulphur of clean and pre-oxidised Cu(1 1 1) surfaces

The reactivity of clean and pre-oxidised Cu(1 1 1) surfaces exposed to sulphur (H 2S) has been studied at room temperature by Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. On the clean surface, the sulphur-saturated surface structure is dominated by the 4 1 3 5 or so-called “zigzag” superstructure. It is shown that a single orientation domain is favoured by the slight misorientation (∼2°) of the surface with respect to the (1 1 1) plane. Scanning tunneling microscopy measurements also revealed two minority structures. Pre-oxidation 4 1 3 8 - O / Cu ( 1 1 1 ) structure was performed by exposure to 1.5 × 10 4 L of O 2 at 300 °C. Under exposure to H 2S (1 × 10 −7 mbar) at room temperature, the oxygen is totally substituted by sulphur. Once initiated, sulphur adsorption seems to propagate to cover the whole surface on the O-covered surface faster than on the clean Cu(1 1 1). At saturation by adsorbed sulphur, the surface is completely covered by the ( 7 × 7 ) R 19.1 superstructure of highest coverage. This enhanced uptake of sulphur is assigned to the surface reconstruction of the copper surface induced by the pre-oxidation, causing a stronger reactivity of the Cu atoms released by the decomposition of the oxide.

Local Tunneling Barrier Height Studies of Thermally Treated CO- and O-covered Pt(100) Surfaces

CO- and O-covered Pt(100) surfaces after thermal treatment have been studied by scanning tunneling microscopy (STM) and local tunneling barrier height (LBH) imaging. The coexistence of the reconstructed (hex) and reconstruction-deconstructed (1×1) domains with similar well-defined shapes is observed for both surfaces. However, the LBH images show that the local work function of the (1×1) domains is lower than that of the hex domains in the case of the CO-covered surface, but higher in the case of the O-covered surface. This feature is considered due to the electric dipole moment induced by O2 adsorption being higher than that induced by CO adsorption.

Adsorption of 1,3-butadiene on supported and promoted silver catalysts

After exposure to 1,3-butadiene, unpromoted Ag/ α-Al 2O 3 and Ag/ α-Al 2O 3 promoted with Cs and Cl were characterized using DRIFTS. Weakly adsorbed molecular butadiene was the dominant species for all samples regardless of the adsorption temperature. After purging alumina with He, no IR absorption features remained with alumina, whereas spectra showed that butadiene adsorbed irreversibly at 300 K via one of its CC bonds on either the reduced or O-covered unpromoted Ag surface which led to a surface species resembling 1,2-butadiene, and on the O-covered Ag surface, crotonaldehyde and carbonaceous species were also present. The formation of these carbonaceous species on an O-covered Ag catalyst implied that chemisorbed oxygen facilitates additional CH bond breaking. On Cs-promoted Ag/ α-Al 2O 3 covered with a monolayer of oxygen following reduction at 673 K, a dehydrogenated 1,2-butadiene-type surface species appeared to exist at 300 K in addition to other carbonaceous species. Isotope studies revealed that 1,3-butadiene adsorbs through both CC bonds on a Cs-promoted Ag catalyst and may exist as an oxametallacycle intermediate requiring two types of sites, i.e., an unpromoted Ag site and a Cs-promoted Ag site containing surface oxygen. In the presence of Cl, 1,3-butadiene was weakly adsorbed on the Ag surface at 300 K through a single CC bond regardless of the presence or absence of chemisorbed oxygen. DRIFT spectra obtained under epoxidation reaction conditions at 473 K and 1 atm indicated that 1,3-butadiene dehydrogenated on alumina alone to give carbonaceous species, whereas carboxylate compounds were the principal species on both unpromoted and promoted Ag catalysts, although crotonaldehyde was also observed. No epoxybutene was detected.

Adsorption of NO on promoted Ag/ α-Al 2O 3 catalysts

Nitric oxide was used to probe Ag/ α-Al 2O 3 epoxidation catalysts. NO dissociated on reduced silver catalysts to oxidize metallic silver sites and desorb N 2O at 300 K. Uptake measurements and DRIFTS results showed that AgO sites were the principal surface species formed after NO adsorption at 300 K on reduced, unpromoted Ag catalysts, as indicated by the formation of gas-phase N 2O as well as a value of unity for the NO/O uptake ratio. The formation of nitrite species was time dependent and a weak band due to chelating nitrito was observed after long exposure times for the unpromoted catalyst after reduction at 473 K. On the unpromoted, O-covered Ag surface, NO adsorption produced a band at 1396 cm −1 which was assigned to a nitrate species. Bands at 1238 and 1345 cm −1, assigned to chelating nitrito and nitrate species, respectively, were detected after NO adsorption on Cs-promoted Ag surfaces reduced at 473 K or covered with chemisorbed oxygen following reduction at 673 K. Only the 1238 cm −1 band was observed after NO adsorption on the Cs-promoted catalyst following reduction at 673 K, and this band was the only absorption feature after NO adsorption on a Ag surface promoted with both Cs and Cl, regardless of the pretreatment. In the presence of Cs, all results indicated that the chemistry of oxygen at the Ag surface was different and more oxygen existed on the surface. The presence of surface oxygen enhanced NO x adsorption, and the presence of both Cl and Cs may decrease the surface concentration of nucleophilic oxygen, which forms nitrate species. After O 2 and NO coadsorption on unpromoted Ag, the O 2 desorption peak was shifted 100 K higher to 660 K. When NO was adsorbed in the presence of gas-phase O 2, bulk Ag nitrate was a dominant species.

Influence of external stress on surface reaction dynamics.

In exploring the dynamics of dissociative chemisorption of O2 on clean and O-covered Cu(001), we demonstrate that the chemisorption is very sensitive to uniaxial tensile stress within an elastic limit. The stress enhances the dissociative chemisorption on clean Cu(001) when the translational energy of incident O2 is below 250 meV, and suppresses it when the energy is above 250 meV. In the case of an oxygen-covered Cu(001)- (2squareroot2 x squareroot2) surface, the dissociative adsorption probability of O2 decreases as the stress increases. The effect of external stress on the O2 dissociation dynamics is different between clean and O-covered surfaces.

Determination of Metal Dispersion and Surface Composition in Supported Cu–Pt Catalysts

A combination of adsorption and surface reaction probes has been used to characterize SiO2-supported Cu–Pt catalysts. CO chemisorption at 300 K on the reduced catalyst measured only surface Pt atoms (Pts). Dissociative N2O adsorption at 363 K produced a monolayer of oxygen on the Pts atoms and converted the Cu surface atoms (Cus) to a surface “Cu2O” phase. The hydrogen titration reaction on this O-covered surface at 210 K occurred only with oxygen chemisorbed on Pts sites with no H spillover to the Cu2O surface, thus providing a second measure of Pts atoms. In contrast, this titration reaction at 300 K provided evidence for H interaction with this Cus–O–Cus phase via a spillover process. CO adsorption on this O-covered surface created by N2O dissociation resulted in CO adsorption on the Cu+1 sites and titration of the chemisorbed oxygen on the Pts atoms according to the two equations Cus+1+CO(g)→Cus+1−CO,Pts−O+2CO(g)→Pts−CO+CO2(g). Thus the total number of surface metal atoms (Cus+Pts) can be measured if there is no CO2 adsorption. The difference between this number and that for Pts atoms gives the number of Cus atoms. The oxygen uptakes via N2O dissociation as well as the net weight changes during the CO titration reaction predicted by these surface compositions were in agreement with values measured gravimetrically and with information deduced from X-ray diffraction measurements. DRIFT spectra of the reduced bimetallic catalyst showed only one band at 2061 cm−1 for CO on reduced Pts atoms, whereas CO adsorbed at 300 K after exposure of this catalyst to N2O again produced this band plus another near 2125 cm−1, with the latter band indicative of CO adsorbed on Cu+1 sites. No evidence for residual oxidized Pts atoms was observed, in agreement with the latter equation above.

Surface Reactions of Ethyl Groups on Clean and O-Modified Ru(001)

The surface reactions of C2H5 species produced by thermal and UV photon-induced dissociation of C2H5I have been followed by means of temperature-programmed desorption and X-ray photoelectron spectroscopy. Cleavage of the C−I bond begins at 130 K on the clean surface. The primary products of thermal dissociation are adsorbed C2H5 and I. C2H5 groups take part in hydrogenation/dehydrogenation reactions forming C2H6 in gas-phase and adsorbed ethylidyne (CCH3) on the surface. Preadsorbed O(a) exerts a significant stabilization influence on the dissociation and the desorption of the parent molecule. In its presence the amount of CCH3 decreases and C2H4 appears in the desorbing products. In addition, oxygen atoms react with C2H5 to give diethyl ether and, at higher coverage, acetaldehyde. The decomposition of CCH3(a) on the O-presaturated sample produces carbidic deposits (C, CHx), which react with oxygen to form CO. UV illumination enhances the dissociation of the C−I bond and consequently the formation of CCH3(a) on clean Ru(001), but the product distribution on an O-covered surface is not affected by irradiation.

Effects of Cesium and Chlorine on Butadiene Adsorption on Ag/α-Al2O3 Catalysts

Integral heats of adsorption, Qad, were measured at 300 K for butadiene on unpromoted and promoted Ag/α–Al2O3 catalysts with average Ag particle sizes in the range of 250–350 nm. The promoted catalysts mimic those used commercially and were prepared using either CsNO3 to study the effect of cesium or CsCl to study the combined effect of cesium and chlorine, and the Cs content was maintained at either 400 ppm or 1200 ppm. Prior to measurement of either the butadiene uptake or the corresponding energy change, the catalysts were subjected to a pretreatment involving calcination at 523 K for 2 h followed by reduction in H2 at 473 K for 1 h. The butadiene measurements were made on either a clean reduced or an O-covered Ag surface, with the latter containing either a monolayer or a half-monolayer of oxygen after adsorption at 443 or 300 K, respectively. Butadiene adsorption on reduced Ag surfaces was almost all reversible, but the very low irreversible uptakes gave Qad values ranging from 130 to 250 kcal/mol, thus indicating that more than molecular adsorption was occurring. For the unpromoted catalyst, the amount of irreversibly adsorbed butadiene increased as the extent of the surface precovered by oxygen increased, and apparent Qad values for irreversibly adsorbed butadiene on the O-covered surfaces were much lower compared to those for the reduced surface, with the lowest Qad value of 13 kcal/mol occurring for the surface that was half covered by oxygen at 300 K. The amount of irreversibly adsorbed butadiene on O-covered surfaces decreased only slightly and was the same when the catalyst was promoted with 400 ppm of cesium via either CsNO3 or CsCl; however, Qad increased to 84 kcal/mol when CsNO3 was used, but decreased to 33 kcal/mol with CsCl, compared to 64 kcal/mol for the unpromoted sample. Both the amount and the Qad value for irreversibly adsorbed butadiene on O-covered Ag surfaces dropped when the amount of Cs was increased to 1200 ppm, and with the CsCl-promoted catalyst almost all the butadiene was reversibly adsorbed. Adsorption of 1-butene on the unpromoted Ag catalyst indicated lower uptakes compared to butadiene, but much higher heats of adsorption. This implies that certain aspects of the bonding of 1-butene and butadiene on the supported catalyst are different. Differences in the adsorption of 1-butene and butadiene on unsupported Ag were less apparent, possibly due to the low uptakes of butadiene on the low-surface-area Ag powder. Both the high Qad values and the frequent exothermic tail during calorimetric measurements appear to be associated with butadiene dissociation or dimerization reactions on the Ag surface.

Dissociative N2O Adsorption on Supported Pt

The oxygen monolayer coverage obtained by N2O adsorption/decomposition at 363 K on a 0.81% Pt/SiO2 catalyst was compared to those acquired by H2, O2, and CO chemisorption. It was equal to the hydrogen monolayer coverage, but about 8% higher than the O coverage acquired by dissociative O2 chemisorption at either 300 or 363 K, and about 20% greater than irreversible CO coverage. Titration of the O-covered Pt surface obtained via N2O decomposition gave a dispersion value in excellent agreement with that based on H chemisorption, whereas the dispersion value based on titration of an oxygen monolayer obtained by dissociative O2 chemisorption was slightly lower, as expected. Thus the appropriate stoichiometry for N2O adsorption is Pts+N2O(g)→Pts−O+N2(g). Diffuse reflectance IR Fourier transform (DRIFT) spectra of CO chemisorbed on either clean or O-covered SiO2-supported Pt surfaces gave one predominant peak at 2073±3 cm−1 for linearly adsorbed CO and indicated a small extent of bridge-bonded CO. CO adsorption on an O-covered surface also gave a band at 2188 cm−1, indicative of a small residual amount of oxidized Pt; however, H2 titration completely removed all surface oxygen. The CO adsorption process is adequately described by Pts−O+2CO(g)→Pts−CO+CO2(g). Similar measurements with a commercial 3% Pt/C catalyst revealed that a large, irreversible O2 uptake occurred on the carbon surface at either 300 or 363 K; however, the amount of titratable oxygen at 300 K was similar to, but slightly higher than, the amount of chemisorbed hydrogen. The oxygen coverage established by N2O decomposition was titrated and found to be slightly lower than the H coverage. DRIFT spectra of CO adsorbed on the Pt/C catalyst gave a single band whose maximum varied from 2019 to 2047 cm−1, depending on the pretreatment.

Ir deposition by surface combustion of (MeCp)Ir(COD): An experimental simulation of metallorganic chemical vapor deposition

The decomposition of (5-methylcyclopentadienyl)(1,5-cyclooctadiene) iridium, (MeCp)Ir(COD), has been studied on an O-covered Rh surface by isothermal and temperature-programmed desorption and isothermal reaction spectroscopy using time-of-flight mass spectrometry, and by Auger electron spectroscopy. After dosing at 100 K, monolayer molecularly adsorbed (MeCp)Ir(COD) desorbs at 325 K competing with decomposition to form adsorbed MeCp and COD. We infer that these react with adsorbed oxygen to form O-containing organic surface-bound fragments that decompose to desorb only CO 2, CO and H 2O since no other O-containing products were detected in the 3D spectra. While dosing both (MeCp)Ir(COD) and O 2, pure Ir film growth occurs above 500 K provided there is a large excess of O 2. When dosing these two reactants at 450 K, O, C, H and Ir all accumulate to give a slowly growing film that slowly evolves CO but negligible H 2, H 2O or CO 2.