The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Simulation of kinetics of nitrogen desorption from Rh(111)

Associative desorption of N atoms from the Rh(111) surface is simulated in the framework of the lattice-gas model. The Arrhenius parameters and nearest-neighbour lateral interaction employed to describe the measured thermal desorption spectra are as follows:v=10¹³ s^–1,E _d=40 kcal/mol, and ₁=1.7 kcal/mol. The results obtained are used to clarify the role of nitrogen desorption in the NO + CO reaction on Rh(111) atT=400–700 K andP _NOP _CO0.01 atm.     

Hysteresis and oscillations in the selectivity during the NO-H2 reaction over Rh(533)

The NO-H₂ reaction over Rh(533) shows oscillatory behaviour at H₂-rich mixtures in the 10^–6 mbar pressure regime around 470 K. The selectivity changes periodically in time: the rate of N₂ formation is out of phase with the NH₃ and H₂O formation rates. Accumulation of atomic N plays a central role in the oscillating behaviour. A comparison will be made with the NO-H₂ reaction over Pt(100).     

Oscillatory behaviour of the reduction of NO by H2 over Rh

The NO reduction by H₂ on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andP _NO = –1.5 ×10^–1 Torr andP _H2 =1×10^-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H₂ and NO-NH₃ reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.     

Chemically resolved dynamical imaging of catalytic reactions on composite surfaces

The catalytic reduction of NO by hydrogen is investigated at (T = 650 K and (p≈10^-6 mbar on a microstructured Rh/Pt(100) surface consisting of Pt(100) domains surrounded by a 600 Åthick Rh film. Synchrotron radiation scanning photoemission microscopy (SPEM), using photons focused into a spot of less than 0.2 μm diameter, is employed as a spatially and chemically resolving in situ technique. The chemical waves which arise in the bistable system NO+H₂/Rh are imaged with SPEM monitoring the N 1s and O 1s photoelectrons. The reaction fronts initiate transitions from an inactive oxygen-covered surface (Θ_O≈0.25 ML) to a reactive nitrogen-covered surface (Θ_N≈0.06 ML). At the Pt/Rh interface, synergetic effects can be observed: the chemical waves on the Rh film nucleate preferentially at the Pt/Rh interface. This nucleation is poisoned by carbon contamination on the Pt area but is prevented in the vicinity of the Pt/Rh interface by the adjacent clean Rh film. No segregation of Pt to the surface was observed for the 600 Å thick Rh film.     

Electrochemically Induced Oscillations of C2H4 Oxidation Over Thin Sputtered Rh Catalyst Films

The electrochemically promoted induction of self-sustained catalytic rate and potential oscillations during C₂H₄ oxidation was studied over sputtered Rh thin (40 nm catalyst films interfaced with ZrO₂ (8 mol% Y₂O₃). The reaction rate oscillates simultaneously with the catalyst potential, and always in the opposite direction. The electrochemically induced oscillatory rate is typically 60 times larger than the open-circuit catalytic rate and 1000 times larger than the electrochemical rate of O²⁻ supply to the catalyst. The phenomenon is completely reversible and only observed under anodic polarization where the rate oscillates between the values corresponding to metallic Rh and surface Rh₂O₃. The oscillations are caused by the electrochemically controlled backspillover of O²⁻ to the catalyst surface and the concomitant, via repulsive lateral interactions, decomposition of surface rhodium oxide followed by surface reoxidation to Rh₂O₃ by gaseous O₂.     

Structure sensitivity for NO dissociation on palladium and rhodium surfaces

We present a comparative density-functional theory study of the chemisorption and the dissociation of the NO molecule on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium. The energetic and kinetic properties of the reaction pathways are reported. The structure sensitivity is correlated to the catalytic activity, which can be linked to the calculated dissociation rate constants at 300 K: Rh (100)⩾terrace Rh (511)>step Rh (511)>step Pd (511)>Rh (111)>Pd (100)⩾terrace Pd (511)>Pd (111). The effect of the steps on the activity is found to be clearly favorable for the Pd (511) surface and unfavorable for the Rh (511) surface. The reaction barriers are correlated to the stability of the final states and the geometries of the molecular precursor states.     

CO dissociation and oxidation on small supported rhodium particles: SSIMS and TPR study

The dissociation and oxidation of carbon monoxide on small rhodium particles prepared by vapour deposition of Rh on either MgO or alumina substrate has been investigated by means of static secondary ion mass spectrometry (SSIMS), and temperature programmed reaction (TPR). The intensity ratios Rh _n C⁺/Rh⁺ _n measured by SSIMS, have been used to monitor the build-up of surface carbon concentration. It was shown that a part of the CO molecularly adsorbed on clean particles undergoes dissociation during heating. The dissociation is more important for smaller particles. This behaviour is explained in terms of increase of CO dissociation probability in the case of CO adsorption near structural irregularities of a surface (edges, corners, steps). During the reaction of CO oxidation the intermediate carbon formation, which is more pronounced for smaller particles, is observed. The temperature dependent carbon concentration exhibits a maximum resulting from a counterbalance between CO dissociation first and carbon reaction with adsorbed oxygen consequently.     

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.     

A comparison of the NO–CO reaction over Rh(100), Rh(110) and Rh(111)

Reaction rates and product selectivities were measured over the Rh(100) surface as a function of temperature, and CO and NO partial pressures. These results are compared with our prior studies of the NO–CO reaction on the Rh(111) and Rh(110) surfaces. The only products detected for all three surfaces were CO₂, N₂O, and N₂. Furthermore, for the Rh(100) surface we have found a significant change in the apparent activation energy (E _a) with reaction temperature. For the Rh(100) surface it was found that the E _a can change by a factor of 2.3 in the temperature range investigated here, from 528 to 700 K, with the lower values obtained at higher temperatures. In contrast, E _a's were found to remain constant over the same temperature range for the Rh(110) and Rh(111) surfaces. The results observed for the Rh(100) surface suggest that reaction kinetics are dominated by variations in NO coverages. At low temperatures, the surface is fully saturated with NO, and dissociation is limited by the availability of vacancy sites through NO desorption. At high temperatures, the surface is still primarily covered with NO, however, the number of vacancy sites has increased substantially. In this case, we propose that the apparent activation energy is now reflecting NO dissociation kinetics rather than those for NO desorption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared spectroscopic detection of Rh(1) by CO in supported Rh catalyst

The detection limit of Rh(1) in the Rh/Al₂O₃ catalyst in a form of Rh¹(CO)₂ was determined by FTIR spectroscopy. It was demonstrated that at least 0.5 g Rh, corresponding to 0.005 wt% of Rh, can be identified in this way. During synthesis gas conversion the predominant surface species is Rh _x -CO, but a detectable amount of Rh(1) exists on the catalyst up to 473 K.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Surface intermediates in the catalytic reaction of NO+H2 on Rh studied by method of interacting bonds calculations

The semi-empirical method of interacting bonds was used to clarify the mechanism of oscillatory behavior of the catalytic system (NO+H₂)/Rh. Various rhodium planes and surface defect regions were characterized by the strength of the nitrogen bond to the surface, the stability of the adsorbed NH_n species (n=0, 1, 2, 3), and the reactivity of NH_n species towards hydrogen. Calculations admit the earlier suggested reaction mechanism, which attributes the surface wave propagation to the intermediate formation of NH_a species. The activity of the rhodium surface in oscillations is expected to increase in a row of planes: (100)<(111)<(335). The activity of Rh(335) single crystal in the reaction rate oscillations is probably governed by the presence (in contrast to ideal terraces) of gradient and broad range of the reaction intermediate properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of a potassium overlayer on the reaction pathway of CH2 and C2H5 on Rh(111)

The effect of potassium on the reaction pathways of adsorbed CH₂ and C₂H₅ species on Rh(111) was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TDS). Hydrocarbon fragments were produced by thermal and photo-induced dissociation of the corresponding iodo compounds. Potassium adatoms markedly stabilized the adsorbed CH₂ and converted it into C₂H₄, the formation of which was not observed for K-free Rh(111). New routes of the surface reactions of C₂H₅ have been also opened in the presence of potassium, namely its transformation into butane and butene.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

Molecular surface studies of adsorption and catalytic reaction on crystal (Pt,Rh) surfaces under high pressure conditions (atmospheres) using sum frequency generation (SFG) – surface vibrational spectroscopy and scanning tunneling microscopy (STM)

Sum frequency generation (SFG) – surface vibrational spectroscopy and the scanning tunneling microscope (STM) have been used to study adsorption and catalyzed surface reactions at high pressures and temperatures using (111) crystal surfaces of platinum and rhodium. The two techniques and the reaction chambers that were constructed to make these studies possible are described. STM and SFG studies of CO at high pressures reveal the high mobility of metal atoms, metal surface reconstruction, ordering in the adsorbed molecular layer, and new binding states for the molecule. CO oxidation occurs at high turnover rates on Pt(111). Different adsorbed species are observed above and below the ignition temperature. Some inhibit the reaction, and others are reaction intermediates since their surface concentration is proportional to the reaction rate. The dehydrogenation of cyclohexene on Pt(100) and Pt(111) proceeds through a 1,3‐cyclohexadiene surface intermediate. The higher dehydrogenation rate is related to the higher surface concentration of these molecules on the (100) crystal face. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

Conditioning of Rh/α-Al2O3 catalysts for H2 production via CH4 partial oxidation at high space velocity

Experimental evidence and literature indications suggest that the process of methane partial oxidation over Rh catalysts is structure sensitive. Crystal phases and Rh cluster size are thus expected to affect the final catalytic performance. In this work, it is observed that outstanding performances are obtained when the as-prepared catalysts are conditioned through repeated runs at increasing temperature and O₂/CH₄ = 0.56. Catalysts slowly activate, that is CH₄ conversion and synthesis gas selectivity progressively grow with time on stream. On the basis of TPO and CH₄ decomposition measurements, this phenomenon is herein explained as the result of a surface reconstruction driven by the repeated exposition to the reaction at high temperature; it is thought that such reconstruction tends to eliminate defect sites and disfavors C-deposition reactions (extremely fast over steps and kinks). Conditioning with O₂-enriched feed streams makes conditioning faster, since the accumulation of surface C-species is suppressed; however, the catalyst is eventually less active than a catalyst conditioned with standard feed mixtures. As an alternative, accumulation of carbon can be suppressed and surface reconstruction proceeds faster if the catalyst is directly exposed to the reaction at high temperature for several hours.     

CO oxidation on a model Cu/Rh(100) catalyst

The reaction between carbon monoxide and molecular oxygen on a model Cu/Rh(100) bimetallic catalyst was studied at 455 K using a CO/O₂ = 2 reactant gas mixture at a total pressure of 10.0 Torr. A maximum in the initial activity was observed at a Cu coverage of 1.3 monolayers. However, the Cu overlayers were found to be unstable at the reaction conditions employed in that the Cu films interact strongly with surface oxygen to form three-dimensional CuO_x clusters. The morphological modifications were found to influence markedly the catalytic properties of the surface. However, the initial catalytic activity and surface morphology of the Cu films could be restored by flashing the sample to > 750 K.