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.     

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.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Understanding the Catalytic Conversion of Automobile Exhaust Emissions Using Model Catalysts: CO + NO Reaction on Pd(111)

The CO + NO reaction is one of the profoundly important reactions that take place on Pd-based industrial three-way catalysts (TWC). In this review, we discuss results from polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and conventional IRAS experiments on CO adsorption, NO adsorption and the CO + NO reaction on a Pd(111) model catalyst surface within a wide range of pressures (10^?6–450 Torr) and temperatures (80–650 K). It will be shown that these studies allow for a detailed understanding of the adsorption behavior of these species as well as the nature of the products that are formed during their reaction under realistic catalytic conditions. CO adsorption experiments on Pd(111) at elevated pressures reveal that CO overlayers exhibit similar adsorption structures as found for ultrahigh vacuum (UHV) conditions. On the other hand, in the case of the CO + NO reaction on Pd(111), the pressure dependent formation of isocyanate containing species' was observed. The importance of this observation and its effects on the improvement of the catalytic NO _x abatement is discussed. The kinetics of the CO + NO reaction on Pd(111) were also investigated and the factors affecting its selectivity are addressed.     

NO x Reduction by Ethanol on Pd/Sulphated Zirconia

The NO reduction by ethanol was studied on palladium catalyst supported on sulphated zirconia. Temperature programmed desorption of NO and ethanol (TPD) and temperature programmed surface reaction (TPSR) analyses as well as catalytic tests in reducing and oxidizing conditions (O₂ presence), besides diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed the formation of intermediate species during the reaction, such as ethoxy species that reacted forming ethylene. Besides dehydrogenate formed adsorbed acetate species, which than decompose and/or react with hydroxyls of the support. The sulphated zirconia support increased the acid sites with the formation of strong Brönsted sites, favoring the formation of ethoxy species. Acetate species also react with NO adsorbed on Pd forming N₂, N₂O, CO and CO₂. The excess of O₂ favored ethanol oxidation to CO₂, consequently less ethanol was available to react with NO _x .     

Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

The dynamics of CO oxidation on Pt(110) studied by infrared chemiluminescence of the product CO2: effect of CO coverage

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on a Pt(110)(1×2) surface. The vibrational and the rotational states of CO₂ formed on the reconstructed Pt(110)(1×2) surface are more excited than those on the terrace Pt(111) surface. The vibrational state of the product CO₂ strongly depends on the CO coverage: the vibrational temperature (T_V) of the product CO₂ becomes higher, as the coverage of CO increases. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C₂H₄ over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C₂H₄ were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al₂O₃ for CO or C₂H₄ oxidation. For NO decomposition, the H₂-reduced Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C₂H₄, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al₂O₃ has been reduced to zero valence during reduction by H₂ at 450°C, but in Co/γ-Al₂O₃ only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl₂O₄-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl₂O₄ which cannot be reduced by H₂ at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al₂O₃ has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al₂O₃. This difference mainly results from the distorting tetrahedrally- coordinated Co²⁺ ions which have lower coordination symmetry than Co⁰ in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al₂O₃ makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co⁰ is the main active phase. For NO decomposition and selective reduction, Co⁰ is also catalytically active, but it can be oxidized into Co₃O₄ by oxygen at high reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Positive Effect of Water Vapor on CO Oxidation at Low Temperature over Pd/CeO2–TiO2 Catalyst

CO oxidation at low temperature over Pd/CeO₂–TiO₂ catalyst was carried out in the feed containing different contents of water vapor (H₂O). A positive effect of H₂O was observed on the catalytic performance of Pd/CeO₂–TiO₂ in CO oxidation at low temperature. The extent of this effect depends on the content of H₂O in the feed; with a H₂O content being 2.5 vol%, the catalyst Pd/CeO₂–TiO₂ exhibits the highest stability (longest life time for CO oxidation at 80 °C). The results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction (TPReaction) reaction illustrated that H₂O in the feed supplies sufficient OH groups in the presence of O₂, which can react with adsorbed CO on Pd species to produce CO₂. Moreover, H₂O may also enhance the adsorption of CO and suppress the formation of some carbonate species.     

Carbon incorporation and deactivation of MgO(0 0 1) supported Pd nanoparticles during CO oxidation

We have investigated the structure and composition of the model catalyst system Pd/MgO(0 0 1) during oxidation, CO reduction and CO oxidation at near atmospheric pressures by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy and spectroscopy techniques. From the in situ X-ray experiments, we find: (a) the Pd nanoparticles with 9 nm in diameter transform into epitaxial PdO above 10⁻¹ mbar O₂ pressure at 570 K, (b) the oxidation process can be reverted by CO exposure, recovering Pd nanoparticles in their initial orientation, and (c) during CO oxidation in a mixture of 50 mbar O₂ and 50 mbar CO a new phase is evolving with lattice constant close to the MgO substrate value, which we assign to expanded Pd nanoparticles forming upon carbon incorporation. Ex situ transmission electron microscopy and different spectroscopy techniques uncover the CO₂ induced growth of a disordered overlayer containing C, Mg and O, which forms during CO oxidation and leads to an overgrowth of Pd nanoparticles thereby deactivating the catalyst.     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

The positive effect of hydrogen on the reaction of nitric oxide with carbon monoxide over platinum and rhodium catalysts

The effect of adding 330–4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200–250°C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO₂ rather than react with hydrogen to form water. This reaction is described by CO + NO +3/2H₂ CO₂ + NH₃ and accounts for 50–100% of the CO₂ formed with Pt/Al₂O₃ and 20–50% with Rh/Al₂O₃. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H₂ present.     

Methane oxidation to acetic acid catalyzed by Pd2+ cations in the presence of oxygen

Synthesis of acetic acid from methane catalyzed by Pd²⁺ cations dissolved in sulfuric acid was investigated to determine the effects of reaction conditions and the mechanism. Acetic acid yield was found to be a strong function of CH₄ and O₂ partial pressures. High O₂/CH₄ ratio and high total pressure delivered the highest yield of acetic acid (14.2 turnovers of Pd²⁺) and the highest retention of Pd²⁺ in solution (96%). Byproducts were sulfur containing compounds (most notably methyl bisulfate) and CO_x, but the acetic acid selectivity was maximized (82%) by lowering the reaction temperature. Methane is activated by Pd(OSO₃H)₂, forming (CH₃)Pd(OSO₃H). CO, generated from the oxidation of methyl bisulfate, inserts into the CH₃Pd bond creating a (CH₃CO)Pd(OSO₃H) species. Reaction of this complex with H₂SO₄ produces acetic acid. Pd²⁺ is reduced to Pd⁰ during the oxidation of methyl bisulfate or CO, and Pd⁰ is reoxidized to Pd²⁺ by H₂SO₄ and O₂.     

Mechanism of NO reduction by CO over Pt/SBA-15

The mechanism of the CO + NO reaction catalyzed by Pt/SBA-15 was studied via independent investigations of CO oxidation and NO disproportionation. Below 400 °C, both CO + O₂ and CO + NO reactions approach 100 % conversion, while the catalyst shows negligible activity for NO disproportionation. These results suggest that CO oxidation by atomic oxygen arising from NO dissociation is not a major route for CO₂ formation in the CO + NO reaction. In situ IR spectra reveal the formation of isocyanates (NCO⁻) adsorbed on silica. Their surface concentration changes with the extent of the CO + NO reaction. A mechanism is proposed in which isocyanates are reaction intermediates.     

Influence of OSC Material on the Behaviour of Metallic Pd Catalysts in C2H4 and CO Oxidation as Well as in NO Reduction

The interaction of CO, C₂H₄, O₂, and NO reaction gas compounds over the metallic Pd/Al₂O₃ and Pd/OSC/Al₂O₃ monoliths was investigated in order to understand the behaviour of OSC material in the oxidation and reduction reactions. FT-IR gas analyser was used for the analysis of the product gas composition. Several activity experiments carried out with dissimilar feedstreams have revealed that the Ce _x Zr_1–x O₂ mixed oxide is an oxygen storage compound, which promotes CO and C₂H₄ oxidation as well as NO reduction in particular at low temperatures.     

Effect of metal dispersion on CO hydrogenation over Pd/HZSM-5 catalysts

Pd/HZSM-5 catalysts prepared by ion-exchange method using Pd(NH₃) ₄ ²⁺ were calcined and reduced at different temperatures to provide different metal dispersions. The effect of Pd dispersion on CO adsorption characteristics and acidity were observed through FT-IR study. Methanol and dimethyl ether were the main products in CO hydrogénation over Pd/HZSM-5 catalyst with small Pd particles on which CO was weakly adsorbed, while the selectivity to methane increased with metal sizes.     

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO₂ catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k₁ of the CO oxidation reaction on PdO, and that of k ₂ ^app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO₂ pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO₂. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.     

Reaction of Formic Acid on Zn-Modified Pd(111)

The decomposition of formic acid on Zn/Pd(111) was studied using Temperature Programmed Desorption and High Resolution Electron Energy Loss Spectroscopy. On Pd(111), HCOOH decomposes via both dehydration and dehydrogenation pathways to produce CO, CO₂, H₂ and H₂O. Small amounts of Zn (<0.1 mL) incorporated the Pd(111) surface were found to increase the stability of formate species and alter their decomposition selectivity to favor dehydrogenation, resulting in an increase in CO₂ production. This difference in reactivity appears to be caused by relatively long range electronic interactions between surface Pd and Zn atoms and may be important in Pd/ZnO methanol steam reforming catalysts which exhibit high selectivities to CO₂ and H₂.