Dissociative Adsorption Of CH4 Research Articles

Nature of the Active Sites on Ni/CeO2 Catalysts for Methane Conversions.

Effective catalysts for the direct conversion of methane to methanol and for methane’s dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO2(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO2(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C–H bond cleavage during the dissociative adsorption of CH4 as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.

Comparing the performance of density functionals in describing the adsorption of atoms and small molecules on Ni(111)

Reaction networks calculated using density functional theory (DFT) play a crucial role in understanding and predicting heterogeneous catalyst performance. However, results obtained using different density functionals can vary significantly. Accordingly, it is important to assess the accuracy of available functionals in capturing adsorption energetics derived from single-crystal adsorption microcalorimetry experiments. Here, we evaluate how different density functionals describe the molecular and dissociative adsorption of CH3I, the adsorption of CH3, I, and H, and the dissociative adsorption of CH4 on Ni(111). First, we identify energetically-preferred adsorbed states for each adsorbate at three surface coverages and calculate the corresponding enthalpies of adsorption at 160 K. Then, we compare the results to experimental measurements published in the literature. For each adsorption process we find at least one functional that is quantitatively accurate within experimental error, but no functional is accurate (within experimental error) for all adsorption processes studied. Even when assuming an additional DFT error of ±20 kJ/mol on top of the experimental error, only the PBE-D3 and RPBE-D3 functionals are accurate for all considered adsorption systems. These results indicate that quantitative agreement between density functional theory calculations and experimental measurements is both system- and functional-dependent. Importantly, single-crystal adsorption microcalorimetry experiments will continue to play an important role for benchmarking density functionals.

Hydrotalcite-derived Co/Mg(Al)O as a stable and coke-resistant catalyst for low-temperature carbon dioxide reforming of methane

The Co catalyst using hydrotalcite-derived Mg(Al)O as support shows promising catalytic activity for low-temperature CH4-CO2 reforming. Under a wide range of reaction conditions, that is T = 773–1023 K, CH4/CO2/N2 = 25/25/50–50/25/25, WHSV = 60,000–120,000 mL h−1 g−1, and TOS = 25–200 h, the Co catalyst exhibits stable activity and high coke resistance. The performance is superior to the Ni counterpart which readily deactivates at 773 K due to severe coke deposition. The initial coking rate on the Co catalyst was estimated to be about 1/20 times that on the Ni catalyst. The kinetic study suggests that the dissociative adsorption of CH4 is favored on the Ni catalyst, whereas the Co catalyst has high affinity for CO2 adsorption. The results of CH4-TPSR and direct CH4 decomposition experiments further reveal that the Co catalyst is much less active for carbon formation through CH4 decomposition. It is deduced that a combination of lower activity for CH4 dissociation/decomposition and higher affinity for CO2 adsorption as well as a stronger metal-support interaction is responsible for the higher coke resistance of the Co catalyst.

A re-examination of the catalyst activation and temperature hysteresis in methane combustion on Pt/Al2O3

Methane combustion is an industrially important reaction that has been extensively studied. Supported Pt catalysts on Al2O3 exhibit excellent activity in this reaction owing to their CH bond activation capability. Pt/Al2O3 catalyst remains almost inactive until the conversion increases abruptly at the ignition temperature (Tign). The temperature hysteresis behavior has been reported, where after ignition, as the temperature is lowered below Tign, the catalyst continues to exhibit high activity. The activation has been previously attributed to the removal of adsorbed oxygen on Pt sites at Tign, while the hysteresis has been explained by the local temperature elevation caused by the reaction heat. In this work, this behavior was re-examined at a small scale in a fixed-bed flow reactor over a range of pre-treatment and reaction mixture conditions. Low-temperature activation was achieved by pre-treating the catalyst with CH4. Carefully designed control experiments yielded observations that cannot be rationalized by the existing theory, suggesting that it is incomplete. Based on these new results, we propose a more comprehensive theory describing the active sites in the oxidation reaction as methane-derived adsorbed carbon active sites, and their formation as the controlling factor in the catalyst activation. The hysteresis can be better explained by the enhanced stability of active sites under reaction conditions. In situ infrared spectroscopy and mass spectrometry studies were conducted in a flow cell, providing strong evidence supporting the theory. The dissociative adsorption of CH4 was studied under a variety of pre-treatment conditions, and proved to be responsible for the active site formation. The fundamental understanding obtained in this work contributes significant insights to the understanding of reaction mechanism.

Unexpected Mechanistic Variants in the Thermal Gas-Phase Activation of Methane

In this review gas-phase studies conducted (mostly) at the Berlin laboratory of the authors are presented. The focus will be on describing mechanistic variants we (and others) came across recently in investigating the thermal activation of methane in the gas phase under idealized conditions. Typical examples include the discussion of those hydrogen-atom-transfer processes that do not follow the well-established conventional pathways in which oxyl radicals play a decisive role. This is the case when the spin is located at a metal center, as in [Al2O2]•+, and the C–H bond cleavage follows a proton-coupled electron-transfer mechanism. Also, examples will be presented in which a high spin density at a bridging oxygen atom can be generated by judicious “doping” of the cluster oxides. Further, the particular role Lewis-acidic sites play in the methane activation by closed-shell metal-oxide ions will be highlighted. Then, aspects of the dissociative adsorption of CH4 on rather small cluster ions will be analyzed...

Theoretical study on the dissociative adsorption of CH4 on Pd-doped Ni surfaces

Density functional theory (DFT) was employed to predict the most stable structure of Pd-doped Ni(111), Ni(100), and Ni(211) surfaces, and the activity for CH4 dissociation on pure and Pd-doped Ni surfaces. We predict that the thermodynamically most stable structures are the surface-doped Pd/Ni surfaces, where a surface Ni atom is replaced by a Pd atom; subsurface-doped Pd/Ni surfaces are thermodynamically unstable. Of the surface-adsorbed Pd/Ni surfaces, only the Pd/Ni(211) surface is thermodynamically stable. From the calculated adsorption energies of CH4 dissociation intermediates (CH4, CH3, CH, C and H) on surface-doped Pd/Ni surfaces, we find Pd-doping to reduce the adsorption energy for all species except for CH4. In addition, from the calculated activation barriers for CH4 and CH dissociations, we predict CH4 and CH to dissociate predominately on Ni(211) and Pd/Ni(211) step surfaces, followed by the broad Ni(100) and Pd/Ni(100) surfaces. Pd-doping raises the activation barriers for CH4 and CH dissociations. For the most active Ni(211) surface, Pd-doping causes the CH dissociation step to have a higher activation barrier than the CH4 dissociation step, which changes the rate limiting step, and helps reduce carbon deposition.

Photoinduced Radical Processes on the Spinel (MgAl2O4) Surface Involving Methane, Ammonia, and Methane/Ammonia

The present study explored photoinduced radical processes caused by interaction of CH(4) and NH(3) with a photoexcited surface of a complex metal oxide: magnesium-aluminum spinel (MgAl(2)O(4); MAS). UV irradiation of MAS in vacuo yielded V-type color centers as evidenced by the 360 nm band in difference diffuse reflectance spectra. Interaction of these H-bearing molecules with photogenerated surface-active hole states (O(S)(-)•) yielded radical species which on recombination produced more complex molecules (including heteroatomic species) relative to the initial molecules. For the MAS/CH(4) system, photoinduced dissociative adsorption of CH(4) on surface-active hole centers produced •CH(3) radicals that recombined to yield CH(3)CH(3). For MAS/NH(3), a similar dissociative adsorption process led to formation of •NH(2) radicals with formation of NH(2)NH(2) as an intermediate product; continued UV irradiation ultimately yielded N(2). For the mixed MAS/CH(4)/NH(3) system, however, interaction of adsorbed NH(3) and CH(4) on the UV-activated surface of MAS yielded •NH(2) and •CH(3) radicals, respectively, which produced CH(3)-NH(2) followed by loss of the remaining hydrogens to form a surface-adsorbed cyanide, CN(S), species. Recombination of photochemically produced radicals released sufficient energy to re-excite the solid spinel, generating new surface-active sites and a flash luminescence (emission decay time at 520 nm, τ ~ 6 s for the MAS/NH(3) case) referred to as the PhICL effect.

Adsorption and co-adsorption of CH3 and H on flat and defective nickel (111) surfaces

We use a periodic density functional theory (DFT) code to study the adsorption of CH3 and H, as well as their co-adsorption on a Ni(111) surface with and without Ni ad-atom, at a surface coverage of 0.25 monolayer (ML). We systematically investigate the site preference for CH3 and H. Then we combine CH3 and H in many co-adsorbed configurations on both surfaces. Methyl and hydrogen adsorption on a flat Ni(111) surface favours the hollow site over the top site. The presence of a Ni ad-atom stabilizes the adsorption of CH3 better than a flat surface, while hydrogen is more stable on a flat Ni(111) surface. When H and CH3 are co-adsorbed at nearest Ni neighbours on the (111) surface, their interaction is always repulsive. However, the dissociative adsorption of CH4 is stabilised when the fragments are infinitely separated. For the co-adsorbed fragments CH3 and H, in the presence of an ad-atom, the repulsive interaction is lowered, so that the dissociative form of CH4 is locally stable.

The Mechanistic Study of Methane Reforming with Carbon Dioxide on Ni/α-Al2O3

The reactions of oxidized and reduced 6 wt % NiO/α-Al2O3 with H2, CH4, CO2, O2, and their mixtures are studied in flow and pulse regimes using a setup equipped with a differential scanning calorimeter DSC-111 and a system for chromatographic analysis. It is shown that treatment with hydrogen at 700°С results in the partial reduction of NiO to Ni. Methane practically does not react with oxidized Ni/α-Al2O3 but it does react actively with the reduced catalyst to form H2 and surface carbon. The latter is capable of reacting with lattice oxygen of Ni/α-Al2O3 (slowly) and with adsorbed oxygen (rapidly). Carbon dioxide also reacts with surface carbon to form CO (rapidly) and with metallic Ni to yield CO and NiO (slowly). Thus, the main route of methane reforming with carbon dioxide on Ni/α-Al2O3 is the dissociative adsorption of CH4 to form surface carbon and H2 and the reaction of this carbon with CO2 resulting in the formation of CO by the reverse Boudouard reaction. Side routes are the interaction of the products of methane chemisorption with catalyst oxygen and the dissociative adsorption of CO2 on metallic nickel. A competitive reaction of surface carbon with adsorbed oxygen results in a decrease in the CO2 conversion in methane reforming with carbon dioxide. Therefore, the presence of gaseous oxygen in the reacting mixture decelerates methane reforming (catalyst poisoning by oxygen).

Methane Reforming with Carbon Dioxide on the Co/α-Al2O3 Catalyst: The Formation, State, and Transformations of Surface Carbon

The interactions of oxidized and reduced Co/α-Al2O3 (4 wt % CoO) with H2, CH4, CO2, and O2 and their mixtures are studied in flow and pulse regimes using a setup involving a DSC-111 differential scanning calorimeter and a system for chromatographic analyses. It is shown that treatment with hydrogen at 700°C results in the partial reduction of cobalt oxide to Co. Methane poorly reacts with the oxidized catalyst but readily reacts with the reduced catalyst to form H2 and surface carbon. The initial surface carbon transforms into other forms, which block the cobalt surface to different extents and differ in the heats of reaction with CO2. Carbon dioxide may react with the surface carbon to form CO (rapidly) and with metallic Co to form CO and CoO (slowly). Thus, the main route of methane reforming with carbon dioxide on Co/α-Al2O3 is the dissociative adsorption of CH4 to form surface carbon and H2 and the reaction of surface carbon with CO2 to form CO via the reverse Boudouard reaction.

Theoretical calculations of dissociative adsorption of CH4 on an Ir(111) surface.

The activation energy for chemisorption of CH(4) at an Ir(111) surface is determined using density functional theory combined with an estimate of the long range dispersion interaction. The results are found to be in good agreement with published results of bulb and beam experiments analyzed with a precursor model. A surprisingly large surface relaxation is found where an Ir surface atom is displaced outwards by as much as 0.6 A. A strongly bound molecular state at kinks and adatoms involving eta(2)-H,H bonding was also found.

Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points

An improved way of estimating the local tangent in the nudged elastic band method for finding minimum energy paths is presented. In systems where the force along the minimum energy path is large compared to the restoring force perpendicular to the path and when many images of the system are included in the elastic band, kinks can develop and prevent the band from converging to the minimum energy path. We show how the kinks arise and present an improved way of estimating the local tangent which solves the problem. The task of finding an accurate energy and configuration for the saddle point is also discussed and examples given where a complementary method, the dimer method, is used to efficiently converge to the saddle point. Both methods only require the first derivative of the energy and can, therefore, easily be applied in plane wave based density-functional theory calculations. Examples are given from studies of the exchange diffusion mechanism in a Si crystal, Al addimer formation on the Al(100) surface, and dissociative adsorption of CH4 on an Ir(111) surface.

A molecular-beam study of methane dissociative adsorption on oxygen-precovered Pt{110}(1×2)

Molecular-beam techniques have been used to determine the effect of oxygen adatoms on the dissociative adsorption of CH4 at a surface temperature of 400K. The initial sticking probability, s0, is always smaller than that for the clean surface in the energy range studied, 90meV≤Et≤500meV and at oxygen precoverages up to one monlayer. As the internal energy of the incident CH4 is increased, s0 for the clean and oxygen-precovered surfaces converges; oxygen blocks fewer surface sites to methane dissociative adsorption. Carbon adatoms, formed from the dissociative adsorption of CH4, react readily with Oa to form CO.

Factors Affecting the Catalytic Activity of Pd/ZrO2for the Combustion of Methane

The catalytic combustion of CH4has been investigated over Pd/ZrO2with the aim of identifying how the state of oxidation of the Pd affects the catalytic activity, particularly at low temperatures. It is found that fully reduced Pd is inactive and that the activity increases at 533 K until a plateau in activity is achieved when 6–7 ML of PdO have formed over the surface of the Pd crystallites. The maximum activity is identical to that for fully oxidized Pd. Examination of the PdO formed under reaction conditions and during oxidation in O2at 533 K reveals some differences. Temperature-programmed reduction in CH4and Raman spectroscopy demonstrate that the oxide formed under reaction conditions is largely crystalline PdO, whereas the oxide formed in O2is a mixture of amorphous and crystalline PdO. The crystalline form of PdO is found to reduce more readily than the amorphous form. It is observed that the low-temperature CH4combustion activity of PdO can be enhanced by producing a small amount of metallic Pd on the surface of PdO. The metallic Pd is more effective than PdO for the dissociative adsorption of CH4. It is hypothesized that the products of CH4adsorption on Pd, Hsand CHx,s(x=3–1), rapidly diffuse to the Pd/PdO interface and then reduce the PdO to Pd. The effects of metallic Pd cannot be sustained, though, under steady-state conditions, because the metallic Pd is oxidized back to PdO.

Kinetics, intermediates and mechanism for the CO2-reforming of methane on supported nickel catalysts

The nature and the reactivity of an intermediate hydrocarbon species, CHx, ads, have been determined for the CO2-reforming of CH4 over supported Ni catalysts by using pulse surface reaction rate analysis (PSRA). When a small amount of CH4 pulse was injected into flowing gas mixture of CO2 and He, it was immediately adsorbed on the catalyst and then gradually reacted with CO2 to CO and H2. Analysis of the dynamic behaviour of the produced CO enabled us to determine the first-order rate constant for the reaction between adsorbed CH4 and CO2. It was found from the dynamics of the H2 produced that two steps were responsible for H2 production: the dissociative adsorption of CH4 to produce (4 –x)/2 H2 and CHx, ads and the simultaneous production of x/2 H2 together with CO from the CHx, ads species. Separation of the amount of H2 between these two steps led us to determine the average number of hydrogen atoms involved in the hydrogen-deficient adsorbed hydrocarbon species, i.e. 2.7 for Ni/MgO, 2.4 for Ni/Al2O3, 1.9 for Ni/TiO2 and 1.0 for Ni/SiO2. The reactivity of the intermediate species was not correlated with the number of hydrogen atoms involved. The highest activity on Ni/TiO2 and almost the same activity on Ni/MgO, Ni/Al2O3 and Ni/SiO2 were observed both for the pulse reaction and for the steady-state reaction, suggesting that the reaction of CHx, ads with CO2(or Oads) is the rate-determining step in CO2-reforming.