Temperature-programmed Desorption Experiments Research Articles

An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

A unified transient kinetic model which can predict the adsorption, desorption and oxidation kinetics of NH3 over hydrothermally aged Cu-chabazite was developed. The model takes into account the variation of fractional coverages of NH3 storage sites due to hydrothermal aging. In order to determine the fractional coverage of these sites, the catalyst was aged for various times at a certain temperature followed by NH3 adsorption, desorption and temperature-programmed desorption (TPD) experiments. TPD profiles were deconvoluted mainly into three peaks with centres at 317, 456 and 526 °C, respectively. Hydrothermal aging resulted in the progressive increase in the intensity of the peak at 317 °C and decrease in the intensity of the peaks at 456 and 526 °C, along with decreased NH3 oxidation at high temperatures. A model for hydrothermal aging kinetics of the fractional coverage of storage sites was developed using three reactions with appropriate rate expressions with parameters regressed from experimental data. The model was then incorporated into a multi-site kinetic model for the degreened Cu-Chabazite by the addition of aging reactions on each storage site. The effects of both aging time and temperature on the kinetics NH3 adsorption, desorption and oxidation were successfully predicted in the 155-540 °C range. This study is the first step towards the development of a hydrothermal aging-unified kinetic model of NH3-Selective Catalytic Reduction over Cu-chabazite.

Simulated temperature programmed desorption experiments for nanoceria powders

Density functional theory calculations (DFT), coupled with microkinetic modelling, have been used to simulate Temperature Programmed Desorption (TPD) experiments for calcined ceria nanopowders with the aim to gain insight into the chemistry governing their high redox activity. Our simulations consider two main nanoparticle models. One is a perfect ceria octahedron supercharged with adsorbed oxygen molecules turned into superoxide ions, as has previously been used to explain the enhanced oxygen storage capacity (OSC) in nanoceria. The other model is a variant where we have introduced oxygen vacancies under ridge Ce ions, thereby reducing their coordination numbers to five. The results from our microkinetic modelling suggest that including such five-coordinated Ce adsorption sites results in a TPD spectrum that better matches the experimental counterpart in terms of both peak position and width. In addition, this new structural model allows for the co-existence of Ce3+ ions, superoxide ions and O2 molecules, as seen in experiments in the literature.

Quantification of Formate and Oxygen Coverages on Cu Under Industrial Methanol Synthesis Conditions

A method for quantifying the surface coverage of adsorbate species for reactions at high pressure and high temperature is presented. Methanol was synthesized from CO2/CO/H2 (3/29/68) at 523 K and 50 bar on Raney Cu, which yielded a turnover frequency (TOF) per Cu surface atom of (6.22 ± 1.04) × 10–3 s–1. Rapid quenching by submerging the catalytic reactor in ice water during operation allowed the formate surface coverage on Cu (θCu-HCOO) at reaction conditions to be determined in a subsequent temperature programmed desorption experiment. This yielded θCu-HCOO = 0.071 ± 0.012 ML and a corresponding TOF per adsorbed HCOO of 0.088 ± 0.021 s–1. CO2 was essentially observed to be the exclusive source of methanol formation over Raney Cu, as the CO in the syngas could be replaced by N2 without impact on the methanol formation rate. Based on this observation and the considerable support in the literature for a CO2-pathway via surface formate on Cu the TOF per surface formate species should represent the true rate of the catalytic cycle in Cu-catalyzed methanol synthesis. Temperature programmed hydrogenation of Raney Cu and unsupported Cu after quenching during high-pressure operation did not indicate a presence of oxygen species (O or OH) on the working Cu surface.

Formation of complex molecules in translucent clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via “nonenergetic” processing of C2H2ice

Context.Complex organic molecules (COMs) have been identified toward high- and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C2HnO, such as acetaldehyde (CH3CHO), vinyl alcohol (CH2CHOH), ketene (CH2CO), and ethanol (CH3CH2OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts.Aims.In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H2O ice under translucent cloud conditions (1 ≤AV≤ 5 andnH~ 103cm−3). We focus on the interactions of C2H2with H-atoms and OH-radicals, which are produced along the H2O formation sequence on grain surfaces at 10 K.Methods.Ultra-high vacuum experiments were performed to study the surface chemistry observed during C2H2+ O2+ H codeposition, where O2was used for the in situ generation of OH-radicals. These C2H2experiments were extended by a set of similar experiments involving acetaldehyde (CH3CHO) – an abundant product of C2H2+ O2+ H codeposition. Reflection absorption infrared spectroscopy was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer was used as a complementary analytical tool. The IR and QMS spectral assignments were further confirmed in isotope labeling experiments using18O2.Results.The investigated 10 K surface chemistry of C2H2with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C2H4) and ethane (C2H6), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C2H2, acting as a molecular backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides so far an experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed.

TPD characterization of Al-OD-Si sites at the interface of bilayer Al0.42Si0.58O2/Ru(0001) thin-films.

Temperature programmed desorption (TPD) has been used to isolate and investigate water interactions unique to bilayer Al0.42Si0.58O2/Ru(0001) via comparison with identical water TPD experiments on a polymorphically equivalent bilayer SiO2/Ru(0001) sample. Relative to SiO2, the aluminosilicate shows evidence of additional desorption processes leading to TPD peaks near 400, 500, and 600 K, with the highest temperature feature exhibiting excellent agreement to previous results documenting infrared monitored loss of Al-OD-Si sites from nominally identical samples. The second order desorption energy estimated from this peak is ~1.6 eV, which is considerably lower than that reported for conventional zeolites, but potentially on par with desorption features reported from other minerals within the feldspar family. Quantification of the peak intensity suggests a ~5% likelihood for formation during water condensation and subsequent desorption. The lower temperature features have been assigned to desorption of molecular water bound at the excess hydroxyl sites. Each hydroxyl appears to bind 2 additional water molecules, which then desorb in a stepwise sequential process. The same low temperature TPD features are also apparent after creating silanol groups on (Al-free) SiO2/Ru(0001) via electron bombardment of supported ice layers, indicating similar water-OH-film interactions independent of specific OH site type.

Mercury forms and their transformation in pyrite under weathering

Pyrite is considered to be the major carrier of mercury in coal. Here, the chemical characteristics of two natural pyrite samples of different weathering degrees were characterized by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). Thermal stability of Hg was also analyzed via temperature programmed desorption experiment (TPD). Characteristic ions such as S−, Fe+, FeS−, and FeS2−were detected on the surface of fresh pyrite. The release temperature of Hg ranged between 180°C and 300°C, and the characteristic peak of black HgS was recorded. In addition, abundant Fe2O3−, FeSO−, SO4−, and HSO4−were detected on the surface of weathered pyrite, and the release temperature of Hg therein was mainly distributed at 260°C to 380°C and 520°C to 600°C, corresponding to the characteristic peaks of red HgS and HgSO4, respectively. The results show that pyrite is acidified during weathering and that Hg forms in pyrite are transformed from the original state (HgS) to HgSO4.

Zeolite Beta Doped with La, Fe, and Pd as a Hydrocarbon Trap

Hydrocarbon trapping is a technique of great relevance, since a substantial part of hydrocarbon emissions from engines are released from engines before the catalyst has reached the temperature for efficient conversion of the hydrocarbons. In this work, the influence of doping zeolite beta (BEA) with Fe, Pd, and La on the storage and release of propene and toluene is studied. Five monolith samples were prepared; Fe/BEA, La/BEA, Pd/BEA, Pd/Fe/BEA, and Pd/La/BEA using incipient wetness impregnation, and the corresponding powder samples were used for catalyst characterization by Inductively coupled plasma sector field mass spectrometry (ICP-SFMS), Temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and Scanning transmission electron microscopy with Energy dispersive X-ray analysis (STEM-EDX). The hydrocarbon trapping ability of the samples was quantified using Temperature-programmed desorption (TPD) of propene and toluene, and in situ Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results from the TPD experiments show that the addition of Pd and La to the zeolite affected the release patterns of the stored hydrocarbons on the trapping material in a positive way. The in situ DRIFTS results indicate that these elements provide H-BEA with additional sites for the storage of hydrocarbons. Furthermore, EDX-mapping showed that the La and Pd are located in close connection.

On-Surface Synthesis of Nonmetal Porphyrins.

We report the on-surface synthesis of a nonmetal porphyrin, namely, silicon tetraphenylporphyrin (Si-TPP), by the deposition of atomic silicon onto a free-base TPP layer on a Ag(100) surface under ultrahigh vacuum (UHV) conditions. Scanning tunneling microscopy provides insights into the self-assembly of the TPP molecules before and after Si insertion. Silicon coordinates with all four nitrogen atoms of the TPP macrocycle and interacts with a silver atom of the substrate as confirmed by scanning tunneling spectroscopy, X-ray photoelectron spectroscopy, and complementary density functional theory calculations. The Si-TPP complex presents a saddle-shaped conformation that is stable under STM manipulation. Our study shows how protocols established for the on-surface metalation of tetrapyrroles can be adopted to achieve nonmetal porphyrins. Complementary experiments yielding Si-TPP and Ge-TPP on Ag(111) highlight the applicability to different main group elements and supports. The success of our nonmetal porphyrin synthesis procedure is further corroborated by a temperature-programmed desorption experiment, revealing the desorption of Ge-TPP. This extension of interfacial complex formation beyond metal elements opens promising prospects for new tetrapyrrole architectures with distinct properties and functionalities.

On the reversible deactivation of cobalt ferrite spinel nanoparticles applied in selective 2-propanol oxidation

CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies.

AMn2O4 (A=Cu, Ni and Zn) sorbents coupling high adsorption and regeneration performance for elemental mercury removal from syngas

AMn2O4 (A= Cu, Ni and Zn) spinel sorbents synthesized by a low-temperature sol-gel auto-combustion method were for the first time used to eliminate elemental mercury (Hg0) from syngas. CuMn2O4 sorbent exhibits the highest Hg0 adsorption performance under simulated syngas, higher than 95 % Hg0 capture efficiency is obtained at 200 °C. Adsorption-regeneration experiments demonstrate that the regenerability of CuMn2O4 is excellent. The influences of syngas compositions on Hg0 elimination over CuMn2O4 were examined. H2S plays the most important role in Hg0 removal from syngas, which can be adsorbed on CuMn2O4 surface and transformed into active sulfur species to react with Hg0 to form surface-bonded HgS. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments prove that HgS is formed on the spent sorbent. The surface Cu2+ cations and chemisorbed/lattice oxygens participate in Hg0 adsorption and transformation. HCl promotes Hg0 removal by forming surface active chlorine species. H2, CO, and H2O inhibit Hg0 removal under N2 atmosphere. However, they exhibit no obvious effect on Hg0 removal with the assistance of H2S. The excellent Hg0 capture performance, good regenerability and H2O resistance of CuMn2O4 make it to be a very promising sorbent for Hg0 removal from syngas at higher temperature.

Role of oligomer structures in the surface chemistry of amidinate metal complexes used for atomic layer deposition of thin films

Abstract

Resonance Energy Transfer from Alkylbenzene Substrates to Naphthalene and Biphenyl Overlayers on Al2O3

In an ultrahigh vacuum chamber, an alkylbenzene was deposited from the vapor phase onto a cryogenically cooled Al2O3 surface. An additional layer with a fluorophore of naphthalene or methylnaphthalene was deposited, and the bilayer was optically pumped. Vapor-deposited naphthalene and methyl-substituted naphthalene molecules on the surface of Al2O3 are amorphously arranged and exhibit a characteristic excimeric fluorescence that is spectrally broad, featureless, and red-shifted relative to the monomer fluorescence. When the bilayer was heated in a temperature-programmed desorption (TPD) experiment, the ordering in the alkylbenzene substrate caused the overlayer of naphthalenes to crystalize due to heteroepitaxy. Finally, the two layers mixed when the surface temperature was nearly at the desorption temperature and resonance energy transfer was observed from the alkylbenzene to naphthalene and methylnaphthalenes when the alkylbenzene percolated through the naphthalene and methylnaphthalene adlayers. The wa...

Plasma-assisted CO2 methanation: effects on the low-temperature activity of an Ni-Ce catalyst and reaction performance.

Ni–Ce three-dimensional material with macropore diameter of 146.6 ± 8.4 nm was synthesized and used as a methanation catalyst. Firstly, H2 reduction of the catalyst was conducted in the thermal fixed bed and plasma reactor, respectively, then X-ray diffraction (XRD) and CO2 temperature programmed desorption experiments on the two reduced samples were carried out to reveal the plasma effect on the catalyst's physico-chemical properties. It was found that plasma reduction created more abundant basic sites for CO2 adsorption, in particular the medium basic sites were even doubled compared with the thermal-reduced catalysts. The plasma-reduced catalyst exhibited excellent low-temperature activity, ca 50–60°C lower than the thermal catalyst (the maximum CO2 conversion point). Based on the optimum reduced catalyst, plasma effect in the reactor level was further investigated under high gas hour space velocity of approximately 50 000 h−1. The plasma reactor showed higher CO2 conversion capacity and efficiency than the thermal reactor.

Adsorptive Desulfurization of Liquid Fuels at Elevated Temperatures Using Metal Exchanged Zeolite Y

It has long been known that the ability of adsorptive methods to remove sulfur from transportation fuels at room temperature is limited by the competition of aromatic hydrocarbons and organosulfur compounds for the active sites of the adsorbent. In an effort to overcome these limitations, we studied adsorptive desulfurization at temperatures substantially higher than those considered by previous investigators. Na-Y and Cu-exchanged Na-Y (CuNa-Y) zeolites were used to remove sulfur from model fuels and a JP-8 fuel at temperatures up to 180 °C. Batch desulfurization and temperature-programmed desorption experiments with model fuels containing 3-methyl-benzothiophene (3-MBT) and dodecane showed that 3-MBT removal was strongly dependent on treatment temperature and involved weak physisorption bonds with Na, weak interactions with Cu sites at 30 or 80 °C, and strong chemisorption (S–Cu bonding) at 130 or 180 °C. Overcoming the competition that favors the adsorption of aromatics at low temperatures, the formati...

Entrapment of CO in CO2 Ice

Abstract Planet atmosphere and hydrosphere compositions are fundamentally set by accretion of volatiles, and therefore by the division of volatiles between gas and solids in planet-forming disks. For hyper-volatiles such as CO, this division is regulated by volatile sublimation energies, and by the ability of other ice components to entrap. Water ice is known for its ability to trap CO and other volatile species. In this study we explore whether another common interstellar and cometary ice component, CO2, is able to trap CO as well. We measure entrapment of CO molecules in CO2 ice through temperature-programmed desorption experiments on CO2:CO ice mixtures. We find that CO2 ice traps CO with a typical efficiency of 40%–60% of the initially deposited CO molecules for a range of ice thicknesses between 7 and 50 monolayers, and ice mixture ratios between 1:1 and 9:1. The entrapment efficiency increases with ice thickness and CO dilution. We also run analogous H2O:CO experiments and find that under comparable experimental conditions, CO2 ice entraps CO more efficiently than H2O ice up to the onset of CO2 desorption at ∼70 K. We speculate that this may be due to different ice restructuring dynamics in H2O and CO2 ices around the CO desorption temperature. Importantly, in planet-forming disks, the ability of CO2 to entrap CO may change the expected division between gas and solids for CO and other hyper-volatiles exterior to the CO2 snowline.

Adsorption Characteristics of Gas Molecules (H2O, CO2, CO, CH4, and H2) on CaO-Based Catalysts during Biomass Thermal Conversion with in Situ CO2 Capture

Biomass thermochemical conversion with in situ CO2 capture is a promising technology in the production of high-quality gas. The adsorption competition mechanism of gas molecules (H2O, CO2, CO, CH4, and H2) on CaO-based catalyst surfaces was studied using density functional theory (DFT) and experimental methods. The adsorption characteristics of CO2 on CaO and 10 wt % Ni/CaO (100) surfaces were investigated in a temperature range of 550–700 °C. The adsorption energies were increased and then weakened, reaching their maximum at 650 °C. The simulation results were verified by CO2 temperature-programmed desorption (CO2-TPD) experiments. By the density of states and Mulliken population analysis, CaO doped with Ni caused a change in the electronic structure of the Osurf atom and decreased the C–O bond stability. The molecular competition mechanism on the CaO-based catalyst surface was identified by DFT simulation. As a result, the adsorption energies decreased in the following order: H2O > CO2 > CO > CH4 > H2. The increase of CO2 adsorption energy on the 10 wt % Ni/CaO surface, compared with the CaO surface, was the largest among those of the studied molecules, and its value increased from 1.45 eV to 1.81 eV. Therefore, the 10 wt % Ni/CaO catalyst is conducive to in situ CO2 capture in biomass pyrolysis.

Design, Synthesis, and Characterization of Metal–Organic Frameworks for Enhanced Sorption of Chemical Warfare Agent Simulants

Metal–organic frameworks (MOFs) and specifically the UiO family of MOFs have been extensively studied for the adsorption and degradation of chemical warfare agents (CWAs) and their simulants. We present a combined experimental and computational study of the adsorption of dimethyl methylphosphonate (DMMP), a CWA adsorption simulant, in functionalized UiO-67. We have used density functional theory (DFT) to design functionalized MOFs having a range of binding energies for DMMP. We have selected three different functionalized MOFs for experimental synthesis and characterization from a total of eight studied with DFT. These three MOFs were identified as having the weakest, intermediate, and strongest binding energies for DMMP of the set, as predicted by our DFT calculations. We find that the order of predicted binding energies agrees with data from temperature-programmed desorption experiments. Moreover, the values of the binding energies are also in good agreement. This serves as a proof of concept that ab in...

Selective Catalytic Reduction of NOx over V2O5-WO3-TiO2 SCR Catalysts—A Study at Elevated Pressure for Maritime Pre-turbine SCR Configuration

The selective catalytic reduction (SCR) of NOx using NH3 was studied at pressures up to 5 bar over a vanadium-based SCR catalyst (~1 wt% V2O5 and 10 wt% WO3/TiO2), relevant for the installation of SCR reactors upstream of the turbocharger at marine engines. Experiments were performed using both granulated catalyst in a lab-scale fixed-bed reactor and a monolith catalyst in a bench-scale setup. The residence time across the catalytic bed was kept constant, by increasing the (normalized (0 °C, 1 atm)) volumetric flow rate proportionally to the pressure. The results show that for the granulated catalyst, the NOx conversion was independent of the pressure, indicating that the SCR kinetics are not affected by the increased pressure up to 5 bar. NH3 temperature-programmed desorption experiments showed that the catalyst NH3 adsorption increased with more than 30% when the pressure was increased from 1 bar to 4.5 bar. On the other hand, when the adsorption temperature was increased from 150 to 300 °C, the adsorption capacity decreased by approximately 60% independent on the pressure. The SCR reaction was unaffected by the increased NH3 uptake caused by the increased pressure, because only a certain fraction of the sites ( $$ {\theta}_{N{H}_3}^{\ast } $$ = 0.14) was found to be active in the SCR reaction, and these are filled up at lower NH3 partial pressure than the total number of sites. Experiments using a monolithic catalyst showed that at temperatures above 250 °C, the NOx conversion was lower at an increased pressure (3.1 bar) when the residence time was held constant. This decrease was ascribed to increased internal and external diffusion limitations at the elevated pressure.

NOx Oxidation and Storage Properties of a Ruddlesden-Popper-Type Sr3Fe2O7-δ-Layered Perovskite Catalyst.

The development of NOx-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NOx oxidation and storage reactions proceed over Pt-free Sr3Fe2O7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO3-δ and Sr3Fe2O7-δ, are studied as NOx-trapping catalysts. Sr3Fe2O7-δ shows higher NOx storage capacity than SrFeO3-δ; its activity is comparable to that of Pt/Ba/Al2O3 calcined at 1273 K. NOx temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NOx-trapping ability of Sr3Fe2O7-δ over SrFeO3-δ. In addition, NO temperature-programmed reactions and O2 temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350-400 K. The reduction performance of trapped NOx on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H2 as the reductant. Pd/Sr3Fe2O7-δ shows significantly high NOx removal efficiency over the entirety of each lean-rich period. Modifying Sr3Fe2O7-δ with Pd is also effective for NOx storage in the presence of H2O and CO2 and the regeneration of the catalyst following SOx sorption. Sr3Fe2O7-δ, with both NOx adsorption and NO oxidation capabilities, acts as a Pt-free NOx-trapping catalyst, exhibiting both high NOx storage capacity and high thermal tolerance.

Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption

Abstract The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H2 formation at relatively high temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and the desorption energetics of H2 molecules on an amorphous diamond-like carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the H2 coverage-dependent activation energies for H2 desorption are determined. The activation energies decrease with increasing H2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at relatively high temperatures.