Time-resolved Infrared Reflection Absorption Spectroscopy Research Articles

The CO oxidation kinetics on supported Pd model catalysts: A molecular beam/in situ time-resolved infrared reflection absorption spectroscopy study

Combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum (UHV) conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ IR spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurements in combination with time-resolved IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been proposed previously. Instead, we show that the transient behavior can be semiquantitatively simulated on the basis of a simple kinetic model considering a homogeneous surface, and accounting for the inhibition of the dissociative adsorption of O2 at high CO coverage. Moreover, it is discussed how the inherent heterogeneity of the supported particle system can additionally enhance the observed effect.

In situ observation of the exchange reaction of formate with molecular formic acid on Ni(110)

The exchange reaction of adsorbed formate on Ni(110) with formic acid in gas phase was studied by the time-resolved infrared reflection absorption spectroscopy (TR-IRAS). It was found that the pre-adsorbed deuterated formate (DCOO(a)) was changed to HCOO(a) in the presence of HCOOD(g) at the temperature region from 240 to 300 K. In such a temperature region, the formate was stable on the surface in vacuum and the decomposition of formate was not observed. It was suggested that the pre-adsorbed DCOO(a) was desorbed as DCOOD accepting deuterium atom from HCOOD. The activation energy of the exchange reaction was estimated to be 23±2 kJ mol −1. The reaction order with respect to formic acid pressure was 0.3±0.1, while that as to the coverage of DCOO(a), was unity. Taking into account all the experimental results, the exchange of pre-adsorbed formate with post-exposed formic acid is suggested to proceed via forming a hydrogen-bonded intermediate complex on the formate's site, because all the sites were already occupied by formate. The small activation energy of the exchange reaction is discussed on the basis of `adsorption–assisted desorption'. We propose that when desorption and adsorption are coupled, the activation energy of the exchange reaction should be described as a sum of the potential energy profiles of a desorbing and an adsorbing molecule.

Exchange Reaction of Adsorbed Formate with Gaseous Formic Acid on Ni(110) Studied by Time-Resolved Fourier Transform Infrared Reflection Absorption Spectroscopy

The dynamic behavior of adsorbed formate on Ni(110) in the presence of gaseous formic acid was studied by time-resolved infrared reflection absorption spectroscopy. The preadsorbed formate (DCOO(a)) was replaced by the postexposed formic acid (HCOOD) to form HCOO(a) on Ni(110) in the temperature region between 230 and 300 K, where the formate was stable under vacuum and no decomposition of formate was observed. It was confirmed that the preadsorbed DCOO(a) desorbed as DCOOD accompanied by formation of HCOO(a) via the deuterium atom transfer from HCOOD.

Current oscillations during the oxidation of formic acid on Pt(100) as studied by in situ time-resolved infrared reflection absorption spectroscopy

Current oscillations in the electrochemical oxidation of formic acid on a Pt(100) single crystal electrode were studied by time-resolved infrared reflection absorption spectroscopy (IRAS) in a dilute perchloric acid aqueous solution. In situ IRAS measurements were made possible by improving functional designs in an spectroelectrochemical cell. Two different kinds of on-top CO absorptions were observed in the spectra while the oscillations were taking place. A higher frequency band at 2060 cm −1 was observed at the bottom of the oscillation spikes (so-called inactive states) and the lower one at 2050 cm −1, at the top of the spikes (active states). On the other hand, the amount of CO 2 production (which was derived from organic intermediates), remained constant irrespective of the spikes in the oscillations. These results do not support the traditional view that the oxidative removal and accumulation of adsorbed CO species are the origin of the oscillations, rather they support a modified view that they are caused by the adsorption and desorption of OH species in the presence of adsorbed CO on the electrode.

In Situ Observation of the Dehydration of Formate on Ni(110)

The dehydration of formate adsorbed on Ni(110) has been studied by the direct observation of the CO(a) produced on the surface by means of time-resolved infrared reflection absorption spectroscopy (TR-IRAS). By the quantitative analysis of the temperature-programmed desorption and TR-IRAS results, it was found that the dehydration proceeds simultaneously with the dehydrogenation. The primary kinetic isotope effect (kH/kD) using HCOO(a) and DCOO(a) was observed not only in the dehydrogenation but also in the dehydration (kH/kD = 3.3 at 300 K). This result suggests that the rate-determining step (RDS) is the C−H bond dissociation in both processes. These suggestions let us propose a possible mechanism of the dehydration of formate on Ni(110); the RDS of the dehydration is the C−H bond dissociation which is the same as that of the dehydrogenation, and the produced CO2(a) is the precursor of the production of CO(a) and O(a).

Temporal and local reduction of adsorption potential energy under gas phase: CO on Ni(100) and Pt(111)

Direct observation of dynamical equilibrium between preadsorbed CO and gas-phase CO is carried out by means of time-resolved infrared reflection absorption spectroscopy. For CO molecules on Ni(100) and on Pt(111) surfaces, the temporal bimolecular CO cluster, adsorbed at nearby sites, exhibits repulsive interaction resulting in the enhancement in the desorption rate. Effective cluster formation may correlate with the accommodation time of an adsorbed molecule on a certain site, i.e., the hopping rate of the individual adsorbed molecules on the surface.

Broken symmetry of adsorbed methane and self-limiting photoinduced dissociation on Pt(111)

Adsorbed states of methane on Pt(111) and photoinduced processes by ArF laser irradiation (193 nm) have been studied by infrared reflection absorption spectroscopy (IRAS). The symmetry of first layer methane is degraded from T d to C 3v (or lower symmetry), but that of overlayer methane maintains T d symmetry. The overlayer and first layer methane molecules desorb from Pt(111) at ∼40 and ∼70 K, respectively. The first layer C 3v CH 4 is photodissociated into CH 3 and H species. These reaction products modify the surface in such a way as to make the remaining first-layer CH 4 molecules photochemically inactive, and thus the reaction becomes self-limiting. The photoreaction kinetics have been studied with time-resolved IRAS, and the total cross-sections are estimated to be σ = 2.6 × 10 −19 cm 2 for CH 4 and σ = 1.7 × 10 −19 cm 2 for CD 4.

Direct observation of isothermal adsorption and desorption processes of CO on the Ni(100) surface

Isothermal adsorption and desorption processes of CO on the Ni(100) surface have been investigated using time-resolved infrared reflection absorption spectroscopy at several temperatures from 310 to 415 K. The following information on the kinetics of CO adsorption and desorption is obtained by direct observation of the adsorbed CO. The initial sticking coefficient is found to be constant within this temperature range. The desorption energy and the preexponential factor are determined to be 127 kJ mol −1 and 1.6 × 10 14 s −1, respectively.