Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2011"

Hydrogen bonds and van der waals forces in ice at ambient and high pressures.

Abstract: The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.

Parallel solution of partial symmetric eigenvalue problems from electronic structure calculations

Abstract: The computation of selected eigenvalues and eigenvectors of a symmetric (Hermitian) matrix is an important subtask in many contexts, for example in electronic structure calculations. If a significant portion of the eigensystem is required then typically direct eigensolvers are used. The central three steps are: reduce the matrix to tridiagonal form, compute the eigenpairs of the tridiagonal matrix, and transform the eigenvectors back. To better utilize memory hierarchies, the reduction may be effected in two stages: full to banded, and banded to tridiagonal. Then the back transformation of the eigenvectors also involves two stages. For large problems, the eigensystem calculations can be the computational bottleneck, in particular with large numbers of processors. In this paper we discuss variants of the tridiagonal-to-banded back transformation, improving the parallel efficiency for large numbers of processors as well as the per-processor utilization. We also modify the divide-and-conquer algorithm for symmetric tridiagonal matrices such that it can compute a subset of the eigenpairs at reduced cost. The effectiveness of our modifications is demonstrated with numerical experiments.

Beyond the random-phase approximation for the electron correlation energy: the importance of single excitations.

Abstract: The random-phase approximation (RPA) for the electron correlation energy, combined with the exact-exchange (EX) energy, represents the state-of-the-art exchange-correlation functional within density-functional theory. However, the standard RPA practice---evaluating both the EX and the RPA correlation energies using Kohn-Sham (KS) orbitals from local or semilocal exchange-correlation functionals---leads to a systematic underbinding of molecules and solids. Here we demonstrate that this behavior can be corrected by adding a ``single excitation'' contribution, so far not included in the standard RPA scheme. A similar improvement can also be achieved by replacing the non-self-consistent EX total energy by the corresponding self-consistent Hartree-Fock total energy, while retaining the RPA correlation energy evaluated using KS orbitals. Both schemes achieve chemical accuracy for a standard benchmark set of noncovalent intermolecular interactions.

Pore structure and surface area of silica SBA-15: influence of washing and scale-up.

Abstract: The removal of the surfactant (EO(20)PO(70)EO(20)) by washing before final calcination is a critical step in the synthesis of silica SBA-15 In contrast to washing with pure water or ethanol, washing with water and ethanol may, depending on the quantity of solvent used, alter the homogeneity and order of the pores, but also lead to an increase of the surface area of SBA-15 A reduction of solvent volume and a controlled washing protocol allow the synthesis of high surface area SBA-15 materials with a narrow monomodal pore size distribution For larger batch sizes the influence of the quantity of solvent on the quality of the SBA-15 is reduced

Coupling of excitons and defect states in boron-nitride nanostructures

Abstract: The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the “free” exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra.

Van der Waals interactions in ionic and semiconductor solids.

Abstract: van der Waals (vdW) energy corrected density-functional theory [Phys. Rev. Lett. 102, 073005 (2009)] is applied to study the cohesive properties of ionic and semiconductor solids (C, Si, Ge, GaAs, NaCl, and MgO). The required polarizability and dispersion coefficients are calculated using the dielectric function obtained from time-dependent density-functional theory. Coefficients for "atoms in the solid" are then calculated from the Hirshfeld partitioning of the electron density. It is shown that the Clausius-Mossotti equation that relates the polarizability and the dielectric function is accurate even for covalently-bonded semiconductors. We find an overall improvement in the cohesive properties of Si, Ge, GaAs, NaCl, and MgO, when vdW interactions are included on top of the Perdew-Burke-Ernzerhof or Heyd-Scuseria-Ernzerhof functionals. The relevance of our findings for other solids is discussed.

Unraveling the stability of polypeptide helices: critical role of van der Waals interactions.

Abstract: Folding and unfolding processes are important for the functional capability of polypeptides and proteins. In contrast with a physiological environment (solvated or condensed phases), an in vacuo study provides well-defined "clean room" conditions to analyze the intramolecular interactions that largely control the structure, stability, and folding or unfolding dynamics. Here we show that a proper consideration of van der Waals (vdW) dispersion forces in density-functional theory (DFT) is essential, and a recently developed DFT+vdW approach enables long time-scale ab initio molecular dynamics simulations at an accuracy close to "gold standard" quantum-chemical calculations. The results show that the inclusion of vdW interactions qualitatively changes the conformational landscape of alanine polypeptides, and greatly enhances the thermal stability of helical structures, in agreement with gas-phase experiments.

Electronic structure and magnetic properties of the graphene/Fe/Ni(111) intercalation-like system

Abstract: The electronic structure and magnetic properties of the graphene/Fe/Ni(111) system were investigated via combination of the density functional theory calculations and electron-spectroscopy methods. This system was prepared via intercalation of thin Fe layers (1 ML) underneath graphene on Ni(111) and its inert properties were verified by means of photoelectron spectroscopy. Intercalation of iron in the space between graphene and Ni(111) changes drastically the magnetic response from the graphene layer that is explained by the formation of the highly spin-polarized 3dz2 quantum-well state in the thin iron layer.

Electronic structure and magnetic properties of the graphene/Fe/Ni(111) intercalation-like system

Abstract: The electronic structure and magnetic properties of the graphene/Fe/Ni(111) system were investigated via combination of the density functional theory calculations and electron-spectroscopy methods. This system was prepared via intercalation of thin Fe layer (1 ML) underneath graphene on Ni(111) and its inert properties were verified by means of photoelectron spectroscopy. Intercalation of iron in the space between graphene and Ni(111) changes drastically the magnetic response from the graphene layer that is explained by the formation of the highly spin-polarized $3d_{z^2}$ quantum-well state in the thin iron layer.

State- and conformer-selected beams of aligned and oriented molecules for ultrafast diffraction studies

Abstract: The manipulation of the motion of neutral molecules with electric or magnetic fields has seen tremendous progress over the last decade. Recently, these techniques have been extended to the manipulation of large and complex molecules. In this article we introduce experimental approaches to the manipulation of large molecules, i.e., the deflection, focusing and deceleration using electric fields. We detail how these methods can be exploited to spatially separate quantum states and how to select individual conformers of complex molecules. We briefly describe mixed-field orientation experiments made possible by the quantum-state selection. Moreover, we provide an outlook on ultrafast diffraction experiments using these highly controlled samples.

Ultrafast electron dynamics in the charge density wave material TbTe3

Abstract: Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques—such as angle-resolved photoemission spectroscopy (ARPES)—probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe3, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures.

An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

Abstract: A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical/thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques andin situreflectivity measurements and (ii) the adsorption/reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005‐1.5 × 10 15 moleculescm −2 s −1 and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190‐3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJcm −2 ) corresponds to the detection limit for adsorption of less than 1.5 × 10 12 CO moleculescm −2 or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10 15 surface Pt atomscm −2 ). The absolute accuracy in energy is within ∼7%‐9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K. © 2011 American Institute of Physics.

Infrared-induced reactivity of n2o on small gas-phase rhodium clusters

Abstract: Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, RhnN2O+ (n = 4−8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N2O has been observed to result in the decomposition of the N2O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N2O decomposition mechanisms are given based on DFT calculations using exchange-cor...

Adsorption energetics of CO on supported Pd nanoparticles as a function of particle size by single crystal microcalorimetry.

Abstract: The heat of adsorption and sticking probability of CO on well-defined Pd nanoparticles were measured as a function of particle size using single crystal adsorption microcalorimetry. Pd particles of different average sizes ranging from 120 to 4900 atoms per particle (or from 1.8 to 8 nm) and Pd(111) were used that were supported on a model in situ grown Fe3O4/Pt(111) oxide film. To precisely quantify the adsorption energies, the reflectivities of the investigated model surfaces were measured as a function of the thickness of the Fe3O4 oxide layer and the amount of deposited Pd. A substantial decrease of the binding energy of CO was found with decreasing particle size. Initial heat of adsorption obtained on the virtually adsorbate-free surface was observed to be reduced by about 20–40 kJ mol−1 on the smallest 1.8 nm sized Pd particles as compared to the larger Pd clusters and the extended Pd(111) single crystal surface. This effect is discussed in terms of the size-dependent properties of the Pd nanoparticles. The CO adsorption kinetics indicates a strong enhancement of the adsorbate flux onto the metal particles due to a capture zone effect, which involves trapping of adsorbates on the support and diffusion to metal clusters. The CO adsorption rate was found to be enhanced by a factor of ∼8 for the smallest 1.8 nm sized particles and by ∼1.4 for the particles of 7–8 nm size.

Electronic structure of dye-sensitized TiO2 clusters from many-body perturbation theory

Abstract: The development of new types of solar cells is driven by the need for clean and sustainable energy. In this respect dye-sensitized solar cells (DSC) are considered as a promising route for departing from the traditional solid state cells. The physical insight provided by computational modeling may help develop improved DSCs. To this end, it is important to obtain an accurate description of the electronic structure, including the fundamental gaps and level alignment at the dye-TiO2 interface. This requires a treatment beyond ground-state density functional theory (DFT). We present a many-body perturbation theory study, within the G0W0 approximation, of two of the crystalline phases of dye-sensitized TiO2 clusters, reported by Benedict and Coppens, [J. Am. Chem. Soc. 132, 2938 (2010)]. We obtain geometries in good agreement with the experiment by using DFT with the Tkatchenko-Scheffler van der Waals correction. We demonstrate that even when DFT gives a good description of the valence spectrum and a qualitatively correct picture of the electronic structure of the dye-TiO2 interface, G0W0 calculations yield more valuable quantitative information regarding the fundamental gaps and level alignment. In addition, we systematically investigate the issues pertaining to G0W0 calculations, namely: (i) convergence with respect to the number of basis functions, (ii) dependence on the mean-field starting point, and (iii) the validity of the assumption that the DFT wave function is a good approximation to the quasiparticle wave function. We show how these issues are manifested for dye molecules and for dye-sensitized TiO2 clusters.

Quantum reflection of He2 several nanometers above a grating surface.

Abstract: Quantum reflection allows an atom or molecule to be reflected from a solid before it reaches the region where it would encounter the repulsive potential of the surface. We observed nondestructive scattering of the helium dimer (He(2)), which has a binding energy of 10(-7) electron volt, from a solid reflection grating. We scattered a beam containing the dimer as well as atomic helium and larger clusters, but could differentiate the dimer by its diffraction angle. Helium dimers are quantum reflected tens of nanometers above the surface, where the surface-induced forces are too weak to dissociate the fragile bond.

Scattering of Stark-decelerated OH radicals with rare gas atoms

Abstract: We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X2Π3/2, J = 3/2, f) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D2 as a function of the collision energy between ∼70 cm−1 and 400 cm−1. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by coupled channel scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006)], Ar [L. Scharfenberg, J. Klos, P.J. Dagdigian, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D2 [M. Kirste, L. Scharfenberg, J. Klos, F. Lique, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.

Accumulation of Stark-decelerated NH molecules in a magnetic trap

Abstract: Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ− ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground-state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.

Crossed beam scattering experiments with optimized energy resolution

Abstract: Crossed molecular beam scattering experiments in which the energy of the collision is varied can reveal valuable insight into the collision dynamics. The energy resolution that can be obtained depends mainly on the velocity and angular spreads of the molecular beams; often, these are too broad to resolve narrow features in the cross sections like scattering resonances. The collision energy resolution can be greatly improved by making appropriate choices for the beam velocities and the beam intersection angle. This method works particularly well for situations in which one of the beams has a narrow velocity spread, and we here discuss the implications of this method for crossed beam scattering experiments with Stark-decelerated beams.

Lithium as a Modifier for Morphology and Defect Structure of Porous Magnesium Oxide Materials Prepared by Gel Combustion Synthesis

Abstract: Defect rich MgO nanocrystals arranged in a hierarchic three-dimensional pore network were synthesized by using gel combustion synthesis (GCS). By adding Li to the combustion precursor, Li-induced changes in the morphology and defect structure of MgO could be studied systematically. At low Li loadings (up to 1 wt %), the three-dimensional pore network was resistant to temperatures up to 800 °C, even though the primary MgO nanoparticles had changed their morphology from on average 8 nm size {100} terminated nanocubes to up to 250 nm large complex polyhedral, exposing more and more {111} facets. At higher Li loadings, the primary MgO particles grow even further, to up to 500 nm, causing the three-dimensional pore network to collapse. After describing the GCS method, detailed structural characterizations of all of the materials synthesized were conducted by means of XRD, BET and pore size analysis, and electron microscopy. IR and thermogravimetric mass spectroscopy (TG-MS) in combination with XRD were used to investigate the formation and decomposition of carbonate species during synthesis and calcination. Diffuse reflectance UV/Vis (DR-UV/Vis) spectroscopy was used to characterize surface defects, such as low coordinated O2− ions at edges, corners, and kinks of the MgO surface. Bulk defects were studied by using electron paramagnetic resonance (EPR) spectroscopy. Morphology and defect concentration of the Li/MgO materials were found to be strongly dependent on the fuel-to-oxidizer ratio used in the combustion synthesis, the Li concentration, and the calcination atmosphere.

Probing the 4f states of ceria by tunneling spectroscopy.

Abstract: Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce4+ ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between −1.0 and −1.5 eV below EFermi in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce3+ ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce3+ ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system.

Rotational-state-specific guiding of large molecules

Abstract: A beam of polar molecules can be focused and transported through an ac electric quadrupole guide. At a given ac frequency, the transmission of the guide depends on the mass-to-dipole-moment (m/μ) ratio of the molecular quantum state. Here we present a detailed characterization of the m/μ selector, using a pulsed beam of benzonitrile (C6H5CN) molecules in combination with rotational quantum state resolved detection. The arrival time distribution as well as the transverse velocity distribution of the molecules exiting the selector are measured as a function of ac frequency. The μ/Δμ resolution of the selector can be controlled by the applied ac waveforms and a value of up to 20 can be obtained with the present setup. This is sufficient to exclusively transmit benzonitrile molecules in quantum states with the same m/μ value as the absolute ground state. The operation characteristics of the m/μ selector are in quantitative agreement with the outcome of trajectory simulations.

Catalytic CO Oxidation on Individual (110) Domains of a Polycrystalline Pt Foil: Local Reaction Kinetics by PEEM

Abstract: The reaction kinetics of catalytic CO oxidation on individual grains of a polycrystalline Pt foil has been studied simultaneously by photoemission electron microscopy (PEEM) and mass spectroscopy (MS), in the pressure range ~10−5 mbar. By processing the video-PEEM images of ongoing catalytic reaction, the kinetic transitions were tracked for individual [110]-oriented domains. The obtained local kinetic phase diagrams were contrasted to those obtained from global MS activity measurements. These data and the observation of reaction front propagation on different Pt(110) domains indicate a quasi-independent behaviour of the crystallographic domains. The observed front propagation velocities and the degree of their anisotropy on Pt foil corroborate earlier observations on Pt(110) single crystals, confirming our concept of using Pt foil to monitor and compare different surface terminations in parallel.

Coexistence of negatively and positively buckled isomers on n+-doped Si(111) − 2 × 1.

Abstract: A long-standing puzzle regarding the $\mathrm{Si}(111)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface has been solved. The surface energy gap previously determined by photoemission on heavily $n$-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

Otto Stern (1888–1969): The founding father of experimental atomic physics

Abstract: We review the work and life of Otto Stern who developed the molecular beam technique and with its aid laid the foundations of experimental atomic physics. Among the key results of his research are: the experimental test of the Maxwell-Boltzmann distribution of molecular velocities (1920), experimental demonstration of space quantization of angular momentum (1922), diffraction of matter waves comprised of atoms and molecules by crystals (1931) and the determination of the magnetic dipole moments of the proton and deuteron (1933).

Rotational-state-specific guiding of large molecules

Abstract: A beam of polar molecules can be focused and transported through an ac electric quadrupole guide. At a given ac frequency, the transmission of the guide depends on the mass-to-dipole-moment (m/\textmu) ratio of the molecular quantum state. Here we present a detailed characterization of the m/\textmu selector, using a pulsed beam of benzonitrile (C$_6$H$_5$CN) molecules in combination with rotational quantum state resolved detection. The arrival time distribution as well as the transverse velocity distribution of the molecules exiting the selector are measured as a function of ac frequency. The \textmu/$\Delta$\textmu resolution of the selector can be controlled by the applied ac waveforms and a value of up to 20 can be obtained with the present setup. This is sufficient to exclusively transmit molecules in the absolute ground state of benzonitrile, or rather in quantum states that have the same m/\textmu value as the ground state. The operation characteristics of the m/\textmu selector are in quantitative agreement with the outcome of trajectory simulations.

Methanol adsorption on V2O3(0001)

Abstract: Well ordered V2O3(0001) layers may be grown on Au(111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V2O3(0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V2O3(0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol → methoxy + H reaction.