R.M. van Hardeveld

Deposition of inorganic salts from solution on flat substrates by spin-coating: theory, quantification and application to model catalysts

The theory of spin-coating is applied to predict the amount of inorganic material that is deposited from a solution on a flat substrate on the basis of concentration, density, viscosity and evaporation rate of the solution and the spin speed applied during spin-coating. Measurements by Rutherford backscattering spectrometry (RBS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) confirm the validity of the theory. Applications of the method in the preparation of model catalysts are discussed.

The adsorption of NH3 on Rh(111)

The adsorption of NH a on Rh(lll) has been investigated by temperature programmed desorption, work function measurements, low energy electron diffraction and secondary ion mass spectrometry. TPD indicates the existence of three distinct desorption states; el-, ~2- and 13-NHa with peak maxima at 320, 155 and 130 K, respectively. For the most strongly chemisorbed state, ~I-NH3, the desorption energy is 81.5 kJ mol-k A (2 x 2) LEED pattern is observed after complete filling of the ~1- and ~2-NH3 states, which is attributed to saturation of the first adsorption layer, corresponding to an NH3 coverage of 0.25 ML. The sticking coefficient for NH 3 adsorption at 120 K, is on the order of unity and independent of coverage during filling of the first adsorption layer. NH 3 adsorption causes a significant work function decrease of 2.4 eV below the value of clean Rh(lll). The initial work function decrease corresponds to an average dipole per NH 3 molecule of 1.9 D, i.e. higher than that of the NH3 molecule in the gas phase. SIMS spectra of NH a on Rh(lll) contain Rh(NH3) + as the predominant ammonia-derived duster ion. The intensity of the Rh(NH3) + duster ion behaves strongly non-linear with the NH3 coverage and indicates some changes in the NH3 adlayer at a coverage of ~0.12 ML.

Cyanide intermediates in catalytic reduction of NO by C2H4 on rhodium (111)

Temperature programmed reaction spectroscopy and secondary ion mass spectrometry (SIMS) have been applied to study reactions of NO and C2H4 coadsorbed on Rh(111). As expected, H2O, CO2, and N2 are the main products at low C2H4 coverages, but at higher coverages H2, HCN, CO, and NO, and even some C2N2 evolve as well. Static SIMS indicates the formation of a large supply of adsorbed CN species, part of which desorbs as HCN, while the remainder decomposes and is responsible for delayed formation of N2. For the highest C2H4 coverages, the majority of the nitrogen atoms in the initially adsorbed NO desorbs as HCN.

Surface reactions of nitrogen oxide on rhodium (100), adsorption, dissociation and desorption

Abstract Dissociation of NO on Rh(100), and desorption of NO and N 2 from Rh(100) have been investigated by temperature-programmed desorption and static secondary ion mass spectrometry. Comparison is made with the dissociation of NO on Rh(111).

Formation of NH3 and N2 from atomic nitrogen and hydrogen on rhodium (111)

Reactions of adsorbed N atoms on Rh(111) to N2 and NH3 were studied with temperature programmed desorption, temperature programmed reaction spectroscopy, and static secondary ion mass spectrometry. For N-atom coverages below ≈0.15 monolayers, desorption of N2 follows simple second-order kinetics, but at higher coverages the desorption traces broaden to higher temperatures. Hydrogenation to NH3 can be described by a stepwise addition of H atoms to Nads in which the reaction from NH2,ads+Hads to NH3,ads determines the rate. The activation energy for the rate determining step is 76 kJ/mol. The desorption of NH3 from Rh(111) was studied separately. The kinetic parameters for desorption at low NH3 coverage are 81 kJ/mol and 1013 s−1, but the rate of desorption increases rapidly with increasing NH3 coverage. It is argued that the remarkable coverage dependence of the desorption rate is unlikely to be caused by lateral repulsive interactions but may be due to a coverage dependence of the pre-exponential factor.

Surface reactions of nitrogen oxide and ethylene on rhodium (111)

Temperature programming of NO and C2H2 coadsorbed on Rh(111) gives rise to the desorption of a number of gases. Where H2, H2O, CO2 and N2 are the main products at low C2H2 coverages, significant amounts of HCN, CO and NO evolve at higher C2H4 coverages. Static SIMS indicates the formation of a large supply of adsorbed CN species, part of which desorbs as HCN, while the remainder decomposes and is responsible for delayed formation of N2. For the highest C2H4 coverages the majority of the initially adsorbed NO desorbs as HCN.

Role of surface diffusion in the ordering of adsorbed molecules: dynamic Monte Carlo simulations of NO on Rh(111)

The saturation coverage of molecules adsorbed on metal surfaces is often seen to increase with temperature of adsorption, and may be accompanied by the ordering of the molecules into periodic structures at higher temperatures. The case of NO on Rh(111) presents a specific example of this behavior. Modelling the adsorption process by means of Monte Carlo simulations in which diffusion and lateral interaction are considered indicates that both the increase of the saturation coverage and the ordering with increasing adsorption temperature are in agreement with an enhanced mobility of the adsorbed molecules.

Kinetics of elementary surface reactions studied by static secondary ion mass spectrometry and temperature programmed reaction spectroscopy

Abstract The application of static secondary ion mass spectrometry (SIMS) in combination with temperature programmed desorption (TPD) or reaction spectroscopy in surface chemistry on metal single crystal surfaces is illustrated with reactions that are relevant in the context of automotive exhaust catalysis on rhodium. In unravelling the complex mechanism of surface reactions between NO and ethylene, TPD detects the evolution of products in the gas phase while secondary ion mass spectrometry (SIMS) monitors processes on the surface in substantial detail. The possibility to derive kinetic parameters of elementary surface reactions from SIMS data is demonstrated with studies on the dissociation of NO and the formation of CN species from N and C atoms.