Catalytic Reactions Of Methanol And Formaldehyde On Transition Metal Surfaces: A First Principles Study

Due to the high catalytic activity and many other excellent properties such ashigh resistance to temperature, oxidation and corrosion, transition metals have beenwidely used as catalysts in various chemical reactions. In recent years, the adsorptionand transformation of methanol on transition metal surfaces have attracted muchattention for its unique advantages in hydrogen production and fuel cell. People havegreat expectations for its application prospect. On the other hand, formaldehyde is oneof the dominant indoor pollutants and bad for health, so the adsorption andtransformation of which on catalyst surfaces have become an important issue forcontrolling indoor air quality. Additionally, as an important intermediate or productduring the catalytic reaction of methanol, formaldehyde is a precursor for manypolymers in industry, so understanding its coupling reaction has significantly practicalmeaning.Due to the wide application in areas of industry and environment, the adsorptionand dissociation of methanol and formaldehyde on various metal surfaces haveattracted large number of studies. The results show that the activity of adsorbedmethanol/formaldehyde is strongly dependent on the nature of metal surface and thesurface oxygen species （O/OH） have an important effect on the reaction mechanism.Although the state-of-the-art experimental surface science techniques can providesome valuable information on the atomic level, many fundamental questions such asthe details of the reaction path, the catalytic selectivity, and the stability of theintermediates and products are elusive for the complexity of the heterogeneouscatalytic reaction and the limitation of the experimental methods. Meanwhile, computational chemistry has a fast development in hardware or software and the firstprinples method based on the density functional theory has been widely applied in thestudies of surface catalysis. The microkenetic method combined with theoretical andexperimental data has also been developed rapidly along with the application ofnumerical methods and computer technology. In this paper, the catalytic reaction ofmethanol and formaldehyde on transition metal surfaces were studied by usingperiodic density functional theory and microkenetic method, and the effect of surfaceoxygen species （O/OH） on the reaction mechanisms were discussed. The studies andobtained results are summarized as follows:1. Decomposition and Oxidation of Methanol on Ir（111）: Iridium is anexcellent catalyst in some industrial catalytic processes for its high low-temperatureoxidation activity and selectivity. A recent experiment shows that on clean Ir（111） thedissociation of CH₃OH leads to CO and H₂at very low temperature, while theexistence of oxygen species on the surface has an important effect on the surfacereaction mechanism and allows for different reaction pathways. To make clear thedecomposition mechanism of methanol on Ir（111） and the effect of oxygen species,we studied the adsorption, decomposition, and oxidation of methanol on Ir（111） basedon periodic density functional calculations and microkinetic modeling. Eachelementary step in the methanol decomposition reaction on clean Ir（111） via O H,C H, and C O bond scissions was considered. The formation mechanisms of CO,CO₂, H₂O, and CH_x（x=13） were elucidated. The results show that the desorption anddecomposition of methanol are competitive on clean surface and the presence of O orOH has larger effect on some specific reaction steps. The surface-assisteddecomposition of methanol mainly follows two competitive dehydrogenationpathways initialed with O–H and C–H bond scissions, respectively, i.e.CH₃OHâ†'CH₃Oâ†'HCHOâ†'CHOâ†'CO andCH₃OHâ†'CH₂OHâ†'CHOHâ†'CHOâ†'CO. The predosed O enhances thedehydrogenation of CH₃OH into CH₃O, while surface is slightly more active towardthe C H bond breaking of CH₃O than O and OH. HCHO would like to dehydrogenateinto CHO assisted by surface or OH, followed by OH-assisted dehydrogenation into CO. CO combines with O to yield CO₂. However, if the surface O coverage is higher,CO₂could be formed via the oxidation pathway of HCHO, i.e.,HCHOâ†'H₂CO₂â†'HCO₂â†'CO₂.2. Decomposition of Methanol on Clean and Oxygen-predosed V（100）: Alarge number of studies on reactions of CH₃OH with various pure metal surfaces showthat the activity of adsorbed CH₃OH is strongly dependent on the nature of metalsurface. Recently, the reaction of CH₃OH on clean and oxygen predosed V（100）surfaces was studied in experiment. However, it is not clear how the oxygen on thesurface affects the reaction mechanism and no virtual theoretical study has beenperformed about the microscopic surface chemistry of CH₃OH on the V（100） surface.It is highly desirable to characterize the mechanistic details of the adsorption anddecomposition processes of CH₃OH on V（100） for searching better and more effectivecatalysts. In the present study, the decomposition of CH₃OH on clean andoxygen-predosed V（100） surfaces was studied on the basis of periodic densityfunctional calculations and microkinetic modeling. The results indicate that the O Hbond scission of CH₃OH is thermodynamically and kinetically favorable on cleanV（100） while the C H and C O bond scissions are unlikely to occur at lowtemperature and, as a result, CH₃O is the major intermediate in the decompositionprocess. The C O bond scission of CH₃O to form CH₃is much easier than the C Hbond scission to form HCHO. Hydrogenation of CH₃by the surface hydrogen fromdissociating CH₃OH and CH₃O is responsible for the desorption of CH₄at low andhigh temperatures, respectively. HCHO further undergoes decomposition or/andcoupling to form CO or/and C₂H₄. When oxygen is pre-adsorbed on the surface at lowcoverage, the O H bond scission of CH₃OH is virtually not affected, while thecleavages of the C O and C H bonds from CH₃O are inhibited in different degrees,leading to the decrease in the ratio of CH₄produced at the low temperature relative tothat at the high temperature. All products are delayed to higher temperature. Theresults are in good agreement with experimental observations.3. Formaldehyde Decomposition and Coupling on V（100）: The adsorption and decomposition of formaldehyde have been extensively studied on different metal andmetal oxide surfaces, and efforts have mainly focused on surfaces involving latetransition metals. A recent report about the catalytic reaction of formaldehyde onV（100） shows that, both C₂H₄and CH₄are formed in two temperature regimes.Although the possible reaction mechanisms for the formation of C₂H₄are proposed,many fundamental issues are still elusive, such as the stability and site selectivity ofthe available surface species （HCHO, C₂H₄, CO, OCH₂CH₂O） on V（100）, the detailedreaction paths for the formation of C₂H₄and CH₄in two temperature regions, and howthe on-surface oxygen affects the reaction processes, etc. In addition, theCH₄-formation mechanism is unclear experimentally. So in this work, we studied thedecomposition of formaldehyde （HCHO） and possible pathways for the formation ofC₂H₄and CH₄on clean and oxygen-predosed V（100） surfaces by periodic densityfunctional theory. It is shown that both C-H and C-O bond scissions of HCHO arethermodynamically and kinetically favorable on clean V（100）. Three reactionpathways for the formation of C₂H₄and two for the formation of CH₄weredetermined. Our results suggest that the preferred pathway for C₂H₄formation atlow-temperature is the coupling of two methylenes （CH₂） produced by an early C=Odissociation step at lower O coverage; while as the increase of the on-surface Ocoverage, this path is suppressed whereas the direct coupling of HCHO to formintermediate OCH₂CH₂O is favored at high temperature. For the formation of CH₄,different mechanisms are also identified corresponding to the two reaction regions.The low-temperature reaction likely occurs via successive hydrogenation of CH₂while the high-temperature reaction may proceed via the CH₃O intermediate formedby hydrogenation of HCHO firstly. The present calculations show that the oxygendeposited on the V（100） surface contributes to the shifting of the mechanisms in low-and high-temperature regions, in line with the experimental results.