Adsorption-assisted Desorption Research Articles

Unravelling Irreversible Adsorbate Thermodynamics through Adsorption-Assisted Desorption.

Strongly bound surface species like alkylamines adsorbed on the Brønsted acid site of aluminosilicate zeolites exhibit negligible rates of molecular desorption, preventing them from achieving an equilibrated state on experimentally relevant time scales that limit the measurement of their adsorption thermodynamics. Through adsorption-assisted desorption, whereby distinct alkylamines facilitate desorption from Brønsted acid sites, we demonstrate that equilibrated states are achieved. Breakthrough adsorption measurements reveal that while 2-butylammonium on a Brønsted acid site is irreversibly adsorbed, it readily undergoes molecular desorption when exposed to a distinct alkylamine like 2-propanamine. As a result, two-adsorbate equilibrium was achieved when the Brønsted acid sites of aluminosilicate zeolites were exposed to a binary vapor-phase alkylamine mixture. By varying relative vapor-phase partial pressures and temperatures, we demonstrate the ability to experimentally measure the adsorption enthalpy and entropy of alkylammonium adsorbates on mostly isolated Brønsted acid sites in H-ZSM-5 (Si/Al = 140). A multiadsorbate Langmuir isotherm was found to quantitatively describe the coadsorption of alkylamines varying in size and basicity over a wide range of conditions through which the relative adsorption enthalpy and entropy of alkylamines were measured. Across a homologous family of sec-alkylamines (C3-C5) adsorbed on isolated Brønsted acid sites, a fixed contribution to the enthalpy (19 ± 4 kJ mol CH2-1) and entropy (25 ± 4 J mol CH2-1 K-1) of adsorption per methylene unit was found to exist, likely resulting from electrostatic interactions between the alkyl chain and the surrounding pore environment.

On the reactivity of carbonate species on a Pt/CeO 2 catalyst under various reaction atmospheres: Application of the isotopic exchange technique

The reactivity and stability of the carbonate species formed at the surface of a Pt/CeO 2 catalyst during the reverse was-gas-shift (RWGS) reaction was investigated by in situ diffuse reflectance (DRIFT) FT-IR spectroscopy at 523 K. Like most oxide materials presenting some basic character, the surface of the sample (not yet exposed to the RWGS stream) exhibited a significant concentration of carbonate species, mostly of the polydentate-type. A fraction of these carbonates was reduced by H 2 at 573 K, leading to the formation of Pt-bound surface carbonyls. Additional carbonates (mostly bidentate species) were formed under RWGS conditions (1% CO 2 + 4% H 2 in Ar at 523 K), along with formate species and more carbonyl groups. These additional carbonates, like the polydentate species, were essentially stable under Ar and, surprisingly, under H 2/Ar. On the contrary, all carbonate species were readily reacted and/or exchanged under the RWGS feed, as evidenced by steady-state isotopic transient kinetic analysis (SSITKA), and under CO 2 and O 2 mixtures. It is concluded that the bonding strength of the carbonates and, as a consequence, the reactivity of those depends on the nature of the feed. The possibility of ceria over-reduction is discussed as being a factor that would increase carbonate stability and result in surface poisoning. The possibility of adsorption-assisted desorption is also presented. The data reported here clearly show the need to use operando or steady-state techniques (such as SSITKA) to determine the reactivity of surface species in actual catalysis conditions, as misleading conclusions can otherwise be drawn on reaction mechanisms.

Modelling of H/D exchange over Pd

Abstract The isotopic exchange between hydrogen and deuterium was studied over a Pd/alumina catalyst. A model was proposed, including the adsorption and dissociation of hydrogen and deuterium. An adsorption-assisted desorption mechanism was also included and turned out to play a minor role in the H 2 /D 2 exchange but have to be considered in the reaction between deuterium and water. A comparison between the experimental data and the simulation revealed that the model could describe the observed dissociation and isotope exchange reaction.

Investigation of NO Reduction by H 2 on Pd Monolith with Transient and Isotopic Exchange Techniques : I. H 2/D 2 Exchange with H 2O and NH 3

Isotope exchange of hydrogen atoms in water, ammonia, and hydrogen with deuterium, as well as adsorption of ammonia and water on an alumina-supported Pd monolith, were studied with transient step-response experiments. The investigation provided new insights into reaction mechanisms. Deuterium underwent an exchange reaction with hydrogen in ammonia and water. The step-response experiments yielded information about the surface reaction mechanisms, indicating adsorption-assisted desorption of hydrogen, decomposition of ammonia, and stepwise formation of deuterated water and ammonia. Detailed reaction pathways explaining isotope exchange of the hydrogen atoms in water, ammonia, and hydrogen with deuterium were proposed. The work forms the basis for the understanding of reaction mechanisms between nitrogen oxide and hydrogen, where ammonia is formed on Pd as a by-product.

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.

Readsorption and Adsorption-Assisted Desorption of CO2 on Basic Solids

The desorption of 13CO2 from the surface of basic oxides based on modified MgO was examined at conditions of chemical equilibrium using a 13CO2/12CO2 isotopic switch method. 13CO2 desorption rates increase with increasing concentration of 12CO2 in the gas phase and with increasing carrier gas flow rate, both during isotopic switch and transient temperature-programmed desorption of CO2. These effects are caused by a decrease in the probability of 13CO2 readsorption and not by faster desorption steps assisted by gas-phase 12CO2. This isotopic switch method allows the determination of desorption rates and of the density of sites that can bind CO2 reversibly at conditions typical of steady-state catalytic reactions. These measurements require, however, that we include in data analysis protocols any effects of readsorption processes.

Adsorption and desorption kinetics of CO on Ir(111): bridging the pressure gap

Rates of desorption of CO chemisorbed on Ir(111) were investigated using time-resolved Fourier-transform IR reflection-absorption spectroscopy in combination with an isotopic jump method. Data for desorption into vacuum and desorption in the presence of an ambient CO background pressure were measured at surface temperatures from 390 to 500 K and for a wide range of CO pressures from 1 × 10 −8 to 1 × 10 −5mbar, extending the range of control parameters substantially compared to all other previously reported investigations. The rates of desorption in both cases are essentially identical, and the existence of the frequently discussed phenomenon of “adsorption-assisted desorption” can be excluded for CO on Ir(111).

Chemisorption of CO on the Ir(111) Surface: Does Adsorption-Assisted Desorption Occur?

Chemisorption of CO on the Ir(111) Surface: Does Adsorption-Assisted Desorption Occur?

A New-Type Adsorption Isotherm of CO on Transition Metal Surfaces with Adsorption-Assisted Processes

Abstract A new adsorption isotherm is proposed, taking the adsorption-assisted desorption into consideration for the first time. The new adsorption isotherm is in reasonable agreement with the experimental data.

ChemInform Abstract: Chemical Dynamic Behavior at Solid Surfaces

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Chemical Dynamic Behavior at Solid Surfaces

The orthodox approaches to elucidate the mechanisms of chemical reactions on solid surfaces have been discussed. Since catalysis is a dynamic process, it should be elucidated only through dynamic approaches. The importance of in situ studies of catalytic reactions and “isotope jump method” have been emphasized by the author for the first time, and some of the examples, such as “adsorption-assisted desorption” and isotope exchange reactions of carbon monoxide on ruthenium single crystal surfaces, are explained. Some methods to identify catalytic active sites at atomic scale in the future have been imagined.

Competition between ethyne, ethene and carbon monoxide for the active sites during hydrogenation at transient conditions over supported metal catalysts

The influence of carbon monoxide on ethene hydrogenation over Pd/α-Al2O3 (Rh/Al 2O 3, Pt/Al 2O 3) was found to be temporarily eliminated after a short pulse of ethyne, propadiene or butadiene. These compounds were found to rapidly accelerate the very slow thermal desorption of carbon monoxide, an effect of the same kind as the so-called adsorption-assisted desorption and cross-desorption. The same phenomenon was valid for the reverse process, i.e. the increased desorption of ethyne after a step of carbon monoxide. The activation energy of the increased desorption of carbon monoxide was found to be low.

Adsorption-Assisted Desorption and Cross Desorption of CO and H2 on Nickel Polycrystalline Surfaces

Abstract The rates of adsorption and desorption during co-adsorption process of CO and H2 on the Ni polycrystalline surface were measured. The desorption rate of CO(a) was enhanced by the presence of CO(g) or H2(g) in the gas phase. The desorption rate of adsorbed hydrogen was accelerated by CO(g) but not by H2(g).

Dynamic behavior in adsorbed layers on a solid surface

The rate of desorption of molecules adsorbed on solid surfaces has been generally accepted to be dependent only on the amount of adsorbed species at constant temperature. However, it was discovered that the rate of desorption of carbon monoxide was markedly dependent upon the CO pressure in the ambient gas, which is called “adsorption-assisted desorption” (AAD). It was also confirmed that the rate of desorption of hydrogen was also accelerated by the presence of ambient CO gas molecules, but not by the presence of hydrogen gas. The influence of sulfur coverage of a nickel surface was examined, which revealed that the presence of sulfur suppressed the AAD effect. A new concept of AAD is proposed and its mechanism is discussed.

“Adsorption-assisted desorption” — Reality or fiction?

The term “adsorption-assisted desorption” means that desorption of one type molecules is accelerated by adsorption of another type molecules. This term was introduced by T. Yamada and co-workers [Surface Sci. 133 (1983) 533]. They have observed that 12C 18O desorption from Pd is accelerated by adsorption of 12C 16O. In the present paper, we discuss the accuracy of determination of the 12C 18O desorption rate in the above-mentioned experiment. Arguments are presented for the fact that “the adsorption-assisted desorption” may be considered as artifact.

“Adsorption-assisted desorption” — Reality or fiction?

The term “adsorption-assisted desorption” means that desorption of one type molecules is accelerated by adsorption of another type molecules. This term was introduced by T. Yamada and co-workers [Surface Sci. 133 (1983) 533]. They have observed that 12C 18O desorption from Pd is accelerated by adsorption of 12C 16O. In the present paper, we discuss the accuracy of determination of the 12C 18O desorption rate in the above-mentioned experiment. Arguments are presented for the fact that “ the adsorption-assisted desorption” may be considered as artifact.

Reply to “adsorption-assisted desorption — Reality or fiction?” by V.P. Zhdanov

Reply to “adsorption-assisted desorption — Reality or fiction?” by V.P. Zhdanov

Adsorption-desorption kinetics of carbon monoxide on palladium polycrystalline surfaces

The kinetics of adsorption and desorption of carbon monoxide on palladium polycrystalline surfaces is examined by measuring the “net” adsorption rate and the “net” desorption rate while the adsorption is actually proceeding. The net adsorption rate is proportional to the pressure of CO gas and is almost independent of the surface coverage of CO below 0.5. The initial sticking probability is 0.88, which stays constant in the temperature range between 339 and 466 K. The net desorption rate includes the thermal desorption rate and the adsorption-assisted desorption rate which is an increasing function of the gas-phase pressure of CO. The adsorption-assisted desorption rate increases continuously with the temperature of palladium with an activation energy of 4.9 kcal mol −1.