Function Of CO Pressure Research Articles

Role of Carbon Monoxide in Catalyst Reconstruction for CO Hydrogenation: First-Principles Study of the Composition, Structure, and Stability of Cu/Co(101̅2) as a Function of CO Pressure

CoCu-based catalysts are promising candidates for the large-scale application of CO hydrogenation to higher alcohols with varying hydrocarbon chain length. To mimic the Co@Cu core–shell structure of nanosized CoCu particles, we choose a Cu/Co(1012)-oriented slab and find, in agreement with chemical imaging results, that the slab surface is always Cu-terminated, with Co underneath. Using DFT calculations, we observe major surface atom exchange in the presence of adsorbed CO, with up to 50% of the Cu atoms being replaced by Co in the straight-chain steps of the slab surface. Co atom exchange beyond 50% is not observed. More specifically, this work is accomplished by scanning the configurational space of adsorbed CO, surface Co, and surface Cu and then identifying minimum-energy surface configurations. Phase diagrams are also constructed to determine the thermodynamic driving force imposed in the presence of adsorbed CO. The inclusion of surface phonon modes is shown to ensure the correctness of the calcula...

Dynamic fluxionality and enhanced CO adsorption in the presence of coadsorbed H2O on free gold cluster cations

This paper presents mass spectrometry measurements of the saturated adsorption of CO in the presence of coadsorbed H2O on gas phase gold cluster cations, Aun+, n=3–20, stored in a quadrupole ion trap. Initial mass spectra obtained at 150K for specific cluster ion sizes as a function of CO pressure and reaction time, indicate increased CO saturation levels correlated with the coadsorption of background H2O vapor. Subsequent to these low temperature experiments, measurements were made of CO and H2O coadsorbed on Au6+ as a function of reaction time at 300K. These mass spectra indicate that the reaction rate at constant CO pressure increases by an order of magnitude for a constant H2O pressure. First-principles density-functional theory calculations in conjunction with the above measurements allowed identification of energy barriers that control dynamic structural fluxionality between adsorption complexes that depends strongly on preadsorbed water. The calculations revealed that in the presence of H2O the energy barrier for the transition state between ground-state triangular and the incomplete hexagonal isomers of the [Au6(CO)3(H2O)2]+ complex is reduced to ∼0eV and the exothermicity is increased by 0.43eV. The theoretical results also identified kinetic pathways exhibiting a transition of the incomplete hexagonal isomer of [Au6(ih)(CO)3(H2O)2]+ to the final saturated complex, Au6(ih)(CO)4+. The energetics and kinetic pathway calculations are consistent with increased formation rates of Au6(CO)4+ as observed in mass spectra. The insights gained from these theoretical results not only explain measurements of the CO saturated adsorption on Au6+ in the presence of water, but also assist in rationalizing coadsorption results obtained over the broader range of cluster size at 150K.

Kinetics and mechanism of cyclohexene hydrocarbomethoxylation catalyzed by a Pd(II) complex

The kinetics of cyclohexene hydrocarbomethoxylation catalyzed by the Pd(PPh3)2Cl2-PPh3-p-toluenesulfonic acid (TSA) is reported. The reaction is first-order with respect to cyclohexene and TSA and of order 0.5 with respect to Pd(PPh3)2Cl2. The reaction rate as a function of CO pressure or methanol or PPh3 concentration passes through an extremum. The chloride anion inhibits the reaction. A mechanism involving cationic hydride complexes as intermediates is suggested. A rate equation is set up by the quasi-steady-state treatment of experimental data.

Bistability and oscillations in CO oxidation studied with scanning tunnelling microscopy inside a reactor

This article focuses on the microscopic origin of oscillations in the rate of CO oxidation at atmospheric pressures on Pt group metals. We start by briefly reviewing several existing models for oscillatory CO oxidation on such surfaces at pressures ranging from ultrahigh vacuum to several bar. We describe our observations for this reaction on Pt(1 1 0) and Pd(1 0 0) at atmospheric pressures, obtained with a scanning tunnelling microscope inside a microflow reactor. The combination of STM movies and the simultaneously determined partial pressures and reaction rate reveals that by switching between a CO-rich flow and an O 2-rich flow, and vice versa, we can reversibly oxidize and reduce the Pt and Pd surfaces. We find that on both surfaces the formation of the oxide, seen in the STM images, is accompanied by a stepwise increase in the reaction rate and a change in the reaction mechanism. The switching between the reduced metal and the surface oxide exhibits significant hysteresis as a function of CO pressure and for both Pt and Pd there exists a CO pressure window in which both the metal and the oxide state are stable. This bistabililty leads to spontaneous reaction oscillations on Pd(1 0 0). Under our atmospheric conditions these oscillations are not due to the ‘familiar’ bistability in Langmuir–Hinshelwood kinetics, in contrast with common ‘wisdom’. Instead, the high-pressure reaction oscillations reflect the periodic switching between the low-reactivity metallic surface and the high-reactivity oxide surface. Our results show further that effects that are held responsible for low-pressure reaction oscillations are not relevant at atmospheric pressure.

CO Adsorption on Au(110)−(1 × 2): An IRAS Investigation

IRAS studies of CO adsorption on the Au(110)−(1 × 2) surface were performed as a function of CO pressure and sample temperature. It was found that with increasing coverage, the CO IRAS absorption shifts red from 2118 to 2108 cm-1. Clausius−Clapeyron data analysis yielded a heat of CO adsorption (−ΔHads) of 10.9 kcal/mol at low coverages and 7.8 kcal/mol at coverages greater than 19%.

CO-induced restructuring of Pt(110)-(1×2): Bridging the pressure gap with high-pressure scanning tunneling microscopy

We present an extensive investigation of CO-induced structural transformations occurring on the reconstructed Pt(110)-(1×2) surface while bridging the so-called pressure gap between surface science and industrial catalysis. The structural changes are followed on the atomic scale as a function of CO pressure over 12 orders of magnitude, up to 1 bar, by the use of a novel high-pressure scanning tunneling microscope (HP-STM). The transition between the low-coverage and saturation-coverage structures is found to proceed through local displacements of substrate Pt atoms. The structural transformations of the Pt surface as observed by STM can be explained within a very simple picture governed by the gain in CO binding energy when CO binds to low-coordinated metal atoms.

Sum Frequency Generation Study of CO Adsorption on Palladium Model Catalysts

In the present contribution CO adsorption on palladium aggregates was examined with IR-visible sum frequency generation vibrational spectroscopy. Supported Pd nanoparticles of different mean size and surface roughness were prepared by electron beam evaporation on an ordered Al2O3 thin film formed on NiAl(110). SFG spectra were acquired for a wide range of CO pressures (from 1 × 10—7 to 200 mbar) at 190 to 300 K. At low pressure, evidence for a particle structure dependence of the CO adsorption geometry was found, with on-top CO sites significantly occupied on defective particles. On the other hand, under a high pressure regime, discrepancies between rough and smooth well-faceted particles were mostly reduced, with the relative fraction of on-top CO nearly equal for both cases. As a reference model, adsorption on well-defined Pd(111) single crystals was monitored as a function of CO pressure up to 1 bar and contrasted to the work done on aggregates. Under ultrahigh vacuum the SFG spectra on large Pd particles closely resembled the Pd(111) response. However, for pressures higher than 1 mbar at 190 K twofold bridge bonded sites were no longer populated on the Pd(111) surface while the bridge bonded occupancy was significant on Pd aggregates for all pressures studied.

The binding of CO to nickel clusters. I. Determination of saturation coverages

The reactions of small nickel clusters Nin (n=3–60) with carbon monoxide are studied in a gas-phase flow-tube reactor. Cluster coverage is determined as a function of CO pressure at temperatures between −160 and 20 °C. The reactions are kinetically controlled under these conditions, but the CO uptake is characterized by a transition from a fast kinetics to a slow kinetics process. Sticking probabilities in the fast kinetics region are near unity at low temperature. The coverage at the transition is consistent with random filling of the cluster surface with van der Waals CO molecules in a standing up configuration with the carbon end toward the metal. Higher coverages in most cases are a consequence of changes in nickel cluster structure to more open ones having larger surface areas.

Pressure dependence (10-8–1000 mbar) of the vibrational spectra of CO chemisorbed on polycrystalline platinum studied by infrared–visible sum-frequency generation

Picosecond infrared–visible sum-frequency generation (SFG) surface vibrational spectroscopy was applied for insitu monitoring of chemisorbed CO on a polycrystalline platinum foil at room temperature. The dependence of the SFG spectra on the CO gas-phase pressure was investigated in the range pCO=10-8–1000 mbar. From the measured SFG spectra, frequencies of the CO vibrationally resonant contribution were determined as a function of CO pressure. At low CO pressures (10-8–10 mbar) a single vibrational band with center frequency in the range 2091–2099 cm-1 was obtained which is characteristic of stretching vibrations of CO terminally adsorbed on Pt atoms of low-index (100)-, (110)- and (111)-type planes. At higher pressure (pCO50 mbar) the appearance of new “low-frequency’' CO surface species was observed which dominate the SFG spectra at CO pressures above 300 mbar. The low-frequency spectral feature was found to be completely reversible and reproducible with variation of the CO pressure. The pressure-dependent change in the SFG spectra suggests formation of Pt–(CO)2 platinum carbonyl binary complexes at surface structures resulting from a reversible CO adsorbate-induced displacive reconstruction of the Pt substrate at high CO pressures.

Kinetic Analysis of the Decomposition of Nitrous Oxide over ZSM-5 Catalysts

A detailed comparative kinetic analysis has been made of the N2O decomposition over Co-, Fe-, and Cu-ZSM-5. The effect of partial pressure of N2O, O2, CO, and NO, the space time, and temperature have been investigated. The decomposition is first order in N2O over Fe- and Co-ZSM-5 and has a slightly lower order over the Cu sample. Inhibition of oxygen observed for Cu-ZSM-5 is absent for Co and for Fe at the lower temperatures. N2O destruction is enhanced by CO for all catalysts and by NO for Fe-ZSM-5 only. In the presence of NO, NO2is produced over Fe- and Co-ZSM-5. Over Cu-ZSM-5 the enhancement by CO passes through a maximum as a function of CO pressure due to the strong adsorption at reduced sites. A detailed kinetic model that accounts quantitatively for the observed dependencies and which deviates from the classical model for oxidic systems is advanced. Estimates of the maximum turnover rates for the various model steps range from 10−4to 1 s−1.

96/01851 Mathematical modeling of swirling flames of pulverized coal: What can combustion engineers expect from modeling?

This article focuses on the microscopic origin of oscillations in the rate of CO oxidation at atmospheric pressures on Pt group metals. We start by briefly reviewing several existing models for oscillatory CO oxidation on such surfaces at pressures ranging from ultrahigh vacuum to several bar. We describe our observations for this reaction on Pt(1 1 0) and Pd(1 0 0) at atmospheric pressures, obtained with a scanning tunnelling microscope inside a microflow reactor. The combination of STM movies and the simultaneously determined partial pressures and reaction rate reveals that by switching between a CO-rich flow and an O2-rich flow, and vice versa, we can reversibly oxidize and reduce the Pt and Pd surfaces. We find that on both surfaces the formation of the oxide, seen in the STM images, is accompanied by a stepwise increase in the reaction rate and a change in the reaction mechanism. The switching between the reduced metal and the surface oxide exhibits significant hysteresis as a function of CO pressure and for both Pt and Pd there exists a CO pressure window in which both the metal and the oxide state are stable. This bistabililty leads to spontaneous reaction oscillations on Pd(1 0 0). Under our atmospheric conditions these oscillations are not due to the ‘familiar’ bistability in Langmuir–Hinshelwood kinetics, in contrast with common ‘wisdom’. Instead, the high-pressure reaction oscillations reflect the periodic switching between the low-reactivity metallic surface and the high-reactivity oxide surface. Our results show further that effects that are held responsible for low-pressure reaction oscillations are not relevant at atmospheric pressure.

Co adsorption on Al‐Zr at room temperature

A study of CO adsorption on room temperature Al–Zr has been done using Auger electron spectroscopy and gas pressure measurements. The atomic surface composition of clean Al–Zr surfaces prepared in UHV by Ar+ sputtering was measured as a function of CO pressure and exposure time. The adsorbed C and O have the same rate constants and surface concentrations for exposures less than 2 Langmuir. Above this exposure quasi‐saturation occurs. The measured sticking probability of CO on Al–Zr is constant, S = 0.7±0.3, for fractional coverage ϑ≲0.5. Above this point S decreased rapidly. The effect of surface roughness on the apparent sticking coefficient has been calculated by a Monte Carlo simulation. Reactivation of Al–Zr by heat treatment to 700 °C has also been studied.

ChemInform Abstract: ULTRAVIOLET SPECTRUM AND REACTION KINETICS OF THE FORMYL RADICAL

A transient ultraviolet absorption attributed to the formyl radical, HCO, was produced by the flash photolysis of water vapor in the presence of CO. For CO saturated with H/sub 2/O at a total pressure of 1 atm, the formation of HCO is complete within approx. 60 ..mu..s, after which it undergoes second-order decay, with k/epsilon(230 nm) = 1.5 +- 0.2) x 10/sup 7/ cm s/sup -1/. During the relatively slow formation reaction (H + CO(+M) ..-->.. HCO), the competing reaction H + HCO ..-->.. H/sub 2/ + CO is significant. By employing the H/sub 2/O + CH/sub 4/ system as an actinometer to determine the amount of H/sub 2/O dissociated by the flash (discussed elsewhere) and by computer modeling the observed formation and decay of HCO as a function of CO pressure, the following absolute rate constants were obtained: k/sub 3/(H + CO(+M) ..-->.. HCO) = (3.6 +- 0.9) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CO, (5.8 +- 0.6) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CH/sub 4/, and (3.8 +- 0.4) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = H/sub 2/, k/sub 4/(HCO + HCO ..-->.. products) = (1.4 +-more » 0.3) x 10/sup 10/ M/sup -1/ s/sup -1/, and k/sub 5/(H +- HCO ..-->.. products) = (6.9 +- 1.7) x 10/sup 10/ M/sup -1/ s/sup -1/. The molar extinction coefficient epsilon(HCO, 230 nm) = 941 +- 171 M/sup -1/ cm/sup -1/.« less

Ultraviolet spectrum and reaction kinetics of the formyl radical

A transient ultraviolet absorption attributed to the formyl radical, HCO, was produced by the flash photolysis of water vapor in the presence of CO. For CO saturated with H/sub 2/O at a total pressure of 1 atm, the formation of HCO is complete within approx. 60 ..mu..s, after which it undergoes second-order decay, with k/epsilon(230 nm) = 1.5 +- 0.2) x 10/sup 7/ cm s/sup -1/. During the relatively slow formation reaction (H + CO(+M) ..-->.. HCO), the competing reaction H + HCO ..-->.. H/sub 2/ + CO is significant. By employing the H/sub 2/O + CH/sub 4/ system as an actinometer to determine the amount of H/sub 2/O dissociated by the flash (discussed elsewhere) and by computer modeling the observed formation and decay of HCO as a function of CO pressure, the following absolute rate constants were obtained: k/sub 3/(H + CO(+M) ..-->.. HCO) = (3.6 +- 0.9) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CO, (5.8 +- 0.6) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = CH/sub 4/, and (3.8 +- 0.4) x 10/sup 7/ M/sup -2/ s/sup -1/ for M = H/sub 2/, k/sub 4/(HCO + HCO ..-->.. products) = (1.4 +-more » 0.3) x 10/sup 10/ M/sup -1/ s/sup -1/, and k/sub 5/(H +- HCO ..-->.. products) = (6.9 +- 1.7) x 10/sup 10/ M/sup -1/ s/sup -1/. The molar extinction coefficient epsilon(HCO, 230 nm) = 941 +- 171 M/sup -1/ cm/sup -1/.« less

The reactivity and auger chemical shift of oxygen adsorbed on platinum

The reaction of carbon monoxide with oxygen chemisorbcd on polycrystalline platinum has been studied using Auger spectroscopy. Two types of chemisorbed oxygen are distinguished on the basis of Auger electron chemical shifts and reactivity towards carbon monoxide. When the substrate is below 800 K, a single very reactive type of chemisorbed oxygen is formed. Above 800 K a new species begins to form which is characterized by an Auger chemical shift of about 6 eV and by low reactivity. The decay of the oxygen Auger signal using several fixed pressures of carbon monoxide was measured. The reaction is first order in carbon monoxide pressure but no clear decision can be made about the order with respect to oxygen coverage. With the reaction CO + 1 20 2 → CO 2 operating at steady-state, the oxygen coverage was measured as a function of CO pressure. In the region 363–600 K, the steady state oxygen coverage began to decline measurably when p CO p O 2 reached 0.1. When p CO > p O 2 the oxygen coverage became immeasurably small. A simple model is used to relate these phenomena to observed carbon dioxide production rates.

The chemisorption of carbon monoxide on the iridium (III) surface

The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.