Elucidation of CO2 Formation Mechanism in CO + NO Reaction on Pd(111) and Pd(110) Surfaces Using IR Chemiluminescence Method

The infrared (IR) chemiluminescence technique was applied to steady-state CO oxidation by NO on Pd(111) and Pd(110). From a comparison of IR emission spectra of CO₂ between the CO + NO and CO + O₂ reactions, it was found that the vibrational energy states of CO₂ in the CO + NO reaction were similar to those in the CO + O₂ reaction. This indicates that the reaction path of CO₂ formation in CO + NO is the same as that in CO + O₂, although the vibrational states are very dependent on the surface structure.     

Kinetics and mechanism of NO reduction with CO on Ir surfaces

The adsorption and thermal reactivity of NO and CO and the kinetics of the NO reduction with CO on Ir surfaces were studied using X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and temperature programmed desorption. The NO adsorption and dissociation activity was strongly dependent on the Ir surface structure. The NO dissociation activity of the Ir planes decreased in the order (100) > (211) ? (111). In contrast, the type of the CO adsorption site was independent of the Ir surface structure. The activity of Ir(111) for N₂ and CO₂ production from the NO + CO reaction was low compared with the activities of Ir(100) and Ir(211). The kinetic data for an Ir/SiO₂ powder catalyst were similar to data obtained for Ir(211). The order of the turnover frequencies for N₂ and CO₂ formation for the Ir planes was in good agreement with the order for NO dissociation activity, and this agreement indicates that the catalytic activity for NO reduction was dependent on NO dissociation. A kinetic study of the elementary steps indicated that the rate-limiting step for NO reduction with CO was the NO dissociation step.     

First-principles analysis of the hydrogenation of carbon monoxide over palladium

The reaction paths for the hydrogenation of CO to methanol over Pd_x (x = 1–4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies for the most favorable binding modes for CO, H₂, HCO, H₃CO, CH₃OH, C, O and H on a Pd₁₉ model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH₃O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (ζ²μ³), and the di-σ sites, respectively. At higher surface coverages, however, the atop ζ¹ (C) and the π modes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the ζ¹ (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation was found to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the ζ¹ (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd–H bond. The barrier for this path was computed to be +78 kJ/mol on the Pd₁₉ cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling of H and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

X-ray photoelectron spectroscopy as a tool for in-situ study of the mechanisms of heterogeneous catalytic reactions

An ESCALAB High Pressure photoelectron spectrometer specially designed by Vacuum Generators Co. (UK) for in-situ measurements at high pressures has been modified for adsorption and catalytic experiments. New construction of a gas cell, which serves to create a high-pressure zone, allows us to measure photoemission spectra in-situ at pressures up to 0.2–0.5 mbar and to use a sample holder with independent heating of a sample. This makes it possible to measure temperature programmed desorption and reaction spectra and, as a consequence, to characterize the catalytic properties simultaneously with the acquisition of X-ray photoelectron spectroscopy (XPS) data. Capabilities of the modified spectrometer are demonstrated by the examples of studies of CO and methanol adsorption on Pd(111) single crystal and methanol oxidation over Cu_poly.The XPS study of high-pressure adsorption of CO on perfect and sputtered Pd(111) showed that raising the pressure has the same effect as decreasing the temperature. The CO coverage is increased due to adsorption of CO to weakly bound (on-top) state, which appears in addition to more strongly bound three-fold and bridged species. No indications of CO dissociation or carbonyl formation were found under the given experimental conditions (high pressure and sputtering-induced defects), provided that the CO gas was sufficiently clean.Two pathways of methanol decomposition to CH_{x, ads} (x3) and to CO_ads was unambiguously identified, with their contribution being dependent on P and T. The decomposition to carbon-originated species had a small contribution in UHV conditions at room temperature, but became more significant at higher pressures and temperatures.Analysis of the distribution of the reaction products and surface species during methanol oxidation over copper in the sub-millibar pressure range showed that an increase in catalytic activity observed on heating the sample from 420 to 670K is accompanied by transformation of the adlayer composition. At low temperatures, the surface is covered by methoxy- and, to a lesser extent, formate-groups. The active surface (T > 520 K) is metallic copper with two adsorbed oxygen species. The possible nature of these species is discussed.     

Surface reactions on inverse model catalysts: CO adsorption and CO hydrogenation on vanadia- and ceria-modified surfaces of rhodium and palladium

Reducible transition metal oxides are well-known promoters of the hydrogenation of CO on noble metal surfaces. In this study the promotional effect of vanadia and ceria adlayers on Rh and Pd surfaces was investigated with emphasis on the effect of the oxidation state on CO adsorption and catalytic activity. Inverse supported catalysts were prepared by UHV deposition of V and Ce on the noble metal surface (Rh(111), Pd(111) or Rh foil). After oxidation and specified reduction, the reaction kinetics on polycrystalline Rh was measured at atmospheric pressure, and the molecular and dissociative chemisorption of CO on Rh(111) and Pd(111) and the methanation kinetics on Rh(111) were investigated by molecular beam techniques. On Rh(111), the probability of CO dissociation and the reaction rate are enhanced by submonolayer VO _x deposits. Local pressures between 10^-2 and 1 mbar are sufficient to drive the methanation at 573 K with measurable amounts of products, accompanied by significant restructuring of the catalyst surface. Although the reaction on Rh is generally promoted by small quantities of vanadia and ceria, the reaction rates depend strongly on the extent and temperature of hydrogen reduction. The observed increase of the reaction rate by reduction up to 673 K can be correlated to concomitant changes of the structure and composition of the VO _x deposits. If the reduction temperature is raised above 673 K, metallic V is partially dissolved in the bulk, and the resulting V/Rh subsurface alloy exhibits a particularly high activity. Contrary to vanadia, ceria islands on Rh promote the initial reaction only after a low-temperature reduction, but the activity decreases after reduction above 573 K.     

Fabrication of an Effusive Molecular Beam Instrument for Surface Reaction Kinetics—CO Oxidation and NO Reduction on Pd(111) Surfaces

A simple molecular beam instrument (MBI) was fabricated for measuring the fundamental parameters in catalysis such as, sticking coefficient, transient and steady state kinetics and reaction mechanism of gas/vapor phase reactions on metal surfaces. Important aspects of MBI fabrication are given in detail. Nitric oxide (NO) decomposition and NO reduction with carbon monoxide (CO) on Pd(111) surfaces were studied. Interesting results were observed for the above reactions and they support the efficiency of the MBI to derive the fundamental parameters of adsorption and catalysis. Sustenance of CO oxidation at 400 K is dependent mostly on the absence of CO-poisoning; apparently, CO + O recombination is the rate determining step ≤400 K. NO adsorption measurements on Pd(111) surface clearly indicating a typical precursor kinetics. Displacement of the chemisorbed CO by NO on Pd(111) surfaces was observed directly with NO + CO beams in the transient kinetics. It is also relatively easy to identify the rate-determining step directly from the MBI data and the same was demonstrated for the above reactions.     

Adsorbed Atoms and Molecules Destined for a Reaction

The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn₂ dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si₄ cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H₂ were studied. N₂ desorbing from Pd(110) by the recombination of N atoms obeys cos⁶ – cos⁷ but the N₂ produced by a catalytic reaction of NO with H₂ obeys cos. In contrast, the N₂ desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos⁴⁶( – 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces.     

CO and NO adsorption for the IR characterization of Fe2+ cations in ferrierite: An efficient catalyst for NOx SCR with NH3 as studied by operando IR spectroscopy

Iron was introduced by ionic exchange inside the FER structure in order to yield a Fe-FER series with increasing metal loading. Characterization of the Fe²⁺ cations by adsorption of CO at liquid nitrogen temperature followed by infrared spectroscopy allowed to identify three distinct sites for iron. The most abundant iron species are located on easily accessible sites of the FER structure, whereas high metal loading is required to observe more confined Fe²⁺ species. According to the CO adsorption results, the main iron species appears to be coordinatively unsaturated whereas isotopic labelling upon NO adsorption indicates that two distinct iron sites almost give rise to the same mononitrosyl infrared signature. Studying the catalyst upon interaction with NO and O₂ in operando conditions leads to the observation of these mononitrosyl species who behave as reaction intermediates for the NO oxidation into NO₂. All our Fe-FER samples presenting these mononitrosyl complexes are active not only in NO-to-NO₂ reaction but also in the NOx selective catalytic reduction with ammonia. The effects of both NH₃ and SO₂ as adsorption competitor during the low temperature NH₃-SCR are also discussed.     

Low-Temperature Combustion of CH4 over CeO2–MO x Solid Solution (M = Zr4+, La3+, Ca2+, or Mg2+) Promoted Pd/γ–Al2O3 Catalysts

The catalytic behavior of Pd (2 wt%) catalysts supported on γ-Al₂O₃ and promoted with CeO₂ ? MO _x (M = Zr⁴⁺, La³⁺, Ca²⁺, or Mg²⁺) solid solution was investigated for methane combustion. The results demonstrated that Pd/γ-Al₂O₃CeO₂ MO _x catalysts can be effective for the low-temperature catalytic combustion of methane and are comparable in activity to other conventional catalysts for this reaction. The XPS and XRD results indicated that an enhanced mobility of lattice oxygen induced by the perturbation of Ce–O lattice was responsible for an increased catalytic performance during oxidation reaction. The most active sites in the catalyst system involve contacts between Pd and the CeO₂–MO _x mixed oxide component. Meanwhile, pre-treatment conditions have significant effect on the catalytic activity in methane combustion.     

Pd(111) versus Pd–Au(111) in carbon monoxide oxidation under elevated pressures

The oxidation of CO on Pd(111) and Pd₇₀Au₃₀(111) has been studied under pressures upto 100 Torr. Gold is found to decrease the surface activity by inhibiting oxygen dissociation. For a sufficient conversion time depending on the CO coverage and the surface identity, a dramatic boost of activity occurs. This is ascribed to a switch from CO-induced inhibition of O₂ adsorption to a regime determined by CO adsorption. The other kinetic features are explained by oxidation of palladium and adsorption-induced restructuring of the surfaces.     

Marked effect of In,Pb and Ce addition upon the reduction of NO by CO over SiO2 supported Pd catalysts

Addition effect of In, Pb and Ce on the NO–CO reaction over SiO₂ supported Pd catalysts was studied, using a closed gas circulation system as well as an in situ infrared spectroscopy. Formation of intermetallic compounds was observed in the cases of Pd–In/SiO₂ and Pd–Pb/SiO₂ catalysts, which caused the drastic enhancement of the reaction rate of N₂O formation. The infrared analyses revealed the weakening of adsorption strength of CO on Pd metal by the formation of intermetallic compounds, which is the main reason for the enhancement of the reaction rate. In the case of Pd–Ce/SiO₂ catalysts, SMSI-like interaction between Pd and CeO₂ would be important for the enhancement effect.     

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

Rh<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Pd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Nanoparticle Composition Dependence in CO Oxidation by NO

Abstract  

Bimetallic 15 nm Pd-core Rh-shell Rh_1−x Pd _x nanoparticle catalysts have been synthesized and studied in CO oxidation by NO. The catalysts exhibited composition-dependent activity enhancement (synergy) in CO oxidation in high NO pressures. The observed synergetic effect is attributed to the favorable adsorption of CO on Pd in NO-rich conditions. The Pd-rich bimetallic catalysts deactivated after many hours of oxidation of CO by NO. After catalyst deactivation, product formation was proportional to the Rh molar fraction within the bimetallic nanoparticles. The deactivated catalysts were regenerated by heating the sample in UHV. This regeneration suggests that the deactivation was caused by the adsorption of nitrogen atoms on Pd sites.     

The promotional effect of Cr on catalytic activity of Pt/ZSM-35 for H2-SCR in excess oxygen

Selective catalytic reduction of NO by hydrogen was studied over Cr modified Pt/ZSM-35 catalysts. The preparation process greatly influenced catalytic activity and sample prepared by co-impregnation method exhibits the best activity. In situ DRIFT studies revealed that on Pt–Cr/ZSM-35, (1) new Pt-NO^δ+ and NO species adsorbed on Pt were detected upon NO + O₂ adsorption; (2) much more ammonia species were formed under reaction condition. Cr addition not only enhanced the adsorption of NO_x but also promoted the formation of surface NH₄⁺ species, which should be the origin of promotional effect of Cr on Pt/ZSM-35 for H₂-SCR reaction.     

Quantitative analyses of oxygen release/storage and CO2 adsorption on ceria and Pt–Rh/ceria

The CO/O₂ and CO₂ pulse experiments were carried out to acquire useful information about oxygen release/storage and CO₂ adsorption on ceria and Pt–Rh/ceria. In the CO pulse experiments at 500 °C, ca. 60% of CO uptake was released as CO₂ while the rest of CO uptake was retained as carbon residuals on the surfaces of both samples. The carbon residuals could be removed when O₂ was provided. In the CO₂ pulse experiments, the adsorption of CO₂ was found to relate to the temperatures and the oxidation states of surface cerium. The reduced Ce³⁺ sites (O vacancies) were responsible for the adsorption of CO₂ at the temperature of 500 °C. In addition, the molar ratios of CO₂ adsorption to O vacancies (38–39%) were in agreement with the ratios of carbon residuals to CO uptake ( ca. 40%) measured in the CO pulse experiments. Quantitative analyses of oxygen release/storage and CO₂ adsorption implied that in the process of oxygen release, carbon residuals were possibly in the form of a carbonate-like species due to the adsorption of CO₂ onto the reduced Ce³⁺ sites.     

Adsorbed species and reaction rates for NO-CO-O2 over Rh(111)

We have studied the NO-CO-O₂ reaction over a Rh(111) catalyst by monitoring the reaction products (CO₂, N₂O, and N₂) and the infrared (IR) intensity of surface CO and NO at various partial pressures of NO, CO and O₂, and sample temperatures. The selectivity for N₂O formation, apparent activation energy for product formation, and NO consumption rate during NO-CO-O₂ are identical to those measured during the NO-CO reaction. The IR measurements show that during NO-CO-O₂ the same two adsorbed species, NO at 1640 cm^-1 and linear CO at ~2040 cm^-1, are present in the same surface concentrations as during NO-CO. For this reason the NO-CO-O₂ kinetics are dominated by the NO-CO kinetics, the NO consumption is rate limited by dissociation of adsorbed NO, and the N₂O selectivity is dominated by surface NO coverage. In contrast, O₂ consumption is adsorption rate limited with the NO-CO adsorption-desorption equilibrium controlling the vacant sites required to dissociatively adsorb O₂. These kinetic and IR data of the CO-NO-O₂ reaction and our interpretation of them agree with previous studies over supported Rh catalysts and thus confirm the previously proposed explanation. From RAIRS and kinetic data we estimate the rate constant for the CO+O→CO₂ elementary step. The pre-exponential factor for this rate is 2×10¹⁰ s^-1, a factor of 50 smaller than previous estimates. This rate constant is important to the NO-CO-O₂ kinetics because it affects O coverage, which, under certain conditions, inhibits NO consumption. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO Oxidation and the CO/NO Reaction on Pd(110) Studied Using “Fast” XPS and a Molecular Beam Reactor

We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO–NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal “light-off” phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O₂ dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2CO_a + O_2g + 2S → 2CO_2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. “Fast XPS”, carried out in situ during the course of the reaction, shows domination of the surface by CO_a below 390 K and domination by O_a above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO–NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. “Light-off” is dictated by the onset of the dissociation of NO_a, which occurs at ～400 K. Once N_a and O_a are formed, N₂O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing O_a as CO_2g. Again, the reaction self-accelerates and there is a rapid change of surface coverage from NO_a to O_a at ～450 K. The overall self acceleration is due to the following overall reaction, 2NO_a + CO_g + S → N₂O_g + CO_2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by O_a and to the reduced coverages of the molecular species.     

Vibrationally Excited CO2 Formed in CO + NO Reaction on Pd(110) Surface in High Surface Temperature Range

The infrared chemiluminescence spectra of CO₂ formed during steady-state CO+NO reaction over Pd(110) indicated that the temperature of the bending vibrational mode was much higher than that of the antisymmetric one at higher surface temperatures such as 800–850 K. Especially, in the high temperature range, more vibrationally excited CO₂ was formed from CO+NO reaction than CO+O₂ reaction. On the basis of the result, we propose the model structure of reaction intermediates for CO₂ formation in CO+NO reaction, which is different from that in CO+O₂ reaction.     

TiO2 Supported Nano-Au Catalysts Prepared Via Solvated Metal Atom Impregnation for Low–Temperature CO Oxidation

The Ce _x Ti_1?x O₂ mixed oxides at different mole ratios (x=0.1–1.0) were prepared by co-precipitation of TiCl₄ and Ce(NO₃)₃. The structural and reductive properties of the Ce _x Ti_{1? x} O₂ were affected by calcination temperature. At x=0.1–0.3, CeTi₂O₆ phase was formed and mainly as amorphous after calcination at 650°C. At x=0.3, only CeTi₂O₆ was formed after calcination at 750°C and CeTi₂O₆ crystallized completely after calcination at 800°C. TPR analyses showed that the amount of H₂ consumption by Ce _x Ti_1?xO₂ (650°C) (except x=0.1) was greater than that by single CeO₂, and the valence of CeO₂was the lowest (+3.18) at x=0.3. CuO/Ce_0.3Ti_0.7O₂ was prepared by the impregnation method and catalytic properties were examined by means of a GC micro-reactor NO+CO reaction system, BET, TPR, XRD, XPS and NO-TPD. It was found that CuO/Ce_0.3Ti_0.7O₂ calcined at 650°C had the highest activity in NO+CO reaction with 100% NO conversion at reaction temperature of 300°C, and at 650°C Ce_0.3Ti_0.7O₂just began to crystallize. The catalytic activities were largely affected by the pre-treatment conditions. At low reduction temperature (100°C), CuO species was difficult to reduce. When high degree of reductions took place, both CuO species and Ce_0.3Ti_0.7O₂ reduced and thus a part of CuO species on the support surface would be covered. The XPS and NO-TPD analyses showed that CuO/Ce_0.3Ti_0.7O₂ had four NO absorption centers (Cu⁺, Cu²⁺(I), Cu²⁺(II) and Ce³⁺). The CuO species involving in NO+CO reaction included Cu²⁺(I) and Cu⁺, and CeO₂ species (Ce³⁺ and Ce⁴⁺).     

PdZn Surface Alloys as Models of Methanol Steam Reforming Catalysts: Molecular Studies by LEED,XPS, TPD and PM-IRAS

The formation and stability of PdZn/Pd(111) surface alloys have been studied, with emphasis on their interaction with CO, methanol and D₂O, applying complementary techniques such as low energy electron diffraction, X-ray photoelectron spectroscopy, temperature programmed desorption (TPD), and polarization–modulation infrared reflection absorption spectroscopy. PdZn surface alloys represent well-suited model systems for technological methanol steam reforming (MSR) catalysts. It could be shown that upon Zn deposition on Pd(111) at or below room temperature non-interacting Zn layers are formed first, that subsequently transform to PdZn surface alloys upon annealing above 473 K. At annealing temperatures above approximately 623 K the surface alloy starts to decompose, finally restoring the clean Pd(111) surface. TPD spectra reveal that methanol was decomposing to a significant amount on Pd(111), yielding CO and CH_x (apart from H₂), a process that did not occur on the PdZn surface alloys (i.e. methanol desorbed molecularly). This difference in part explains the improved catalytic properties (selectivity and stability) of PdZn catalysts for the MSR reaction.