The adsorption and reaction of CO on a monolayer carbide and a bulk carbide, prepared on Mo(110), was studied with synchrotron-based XPS, TPD, and density-functional calculations using slab models. In the experiments on the monolayer carbide, we find two CO species at 140 K, with a saturation coverage of ∼0.7 ML, while on the bulk carbide, Mo2C, three molecular adsorption states are found, showing a similar total coverage of ∼0.7 ML at saturation. In addition, CO partly dissociates on both surfaces (monolayer carbide: 7%, bulk carbide: 15%). The calculations on the monolayer carbide show that the adsorption of CO on Mo sites is most stable. At increased coverages, several different adsorption sites on the monolayer carbide become possible. From the core level shifts, an assignment to the experimentally found species becomes available. Upon heating, we find on both carbides the competing processes of desorption, interconversion of different CO species, and dissociation of CO. The detailed quantitative analysis of these processes shows that desorption and dissociation to atomic oxygen and carbon is completed at ∼400 K on the monolayer carbide and ∼450 K on the bulk carbide; in both cases, about 35% (0.25 ML) of the initially adsorbed CO decomposes upon heating. Above 800 K, atomic carbon and oxygen desorb associatively, and at 1200 K the carbide surfaces are restored.

Controlled patterning of graphene is an important task towards device fabrication and thus is the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2–30 keV) and doses (50–800 mC m−2) either reduction of GO or formation of permanent lattice defects occurs. This result reflects a step towards controlled FEBIP processing of oxo-G.

Nanocluster arrays on graphene are suitable model systems for catalysis. We use this model system to study sulfur poisoning, which is a major deactivation process of heterogeneous catalysts. Using high-resolution X-ray photoelectron spectroscopy, we investigated the adsorption and desorption of CO on sulfur-poisoned graphene-supported palladium nanoparticles. We find that sulfur blocks CO adsorption sites, with hollow sites being most affected. From the unchanged desorption temperatures, we conclude that the Pd–CO bond strength is not altered by co-adsorbed sulfur. The degree of site blocking compared to the amount of sulfur indicates that the palladium nanoparticle surface is dominated by (111) and (100) facets.

AbstractDetailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time-dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) we show that a time scale of hours (t≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF-GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430–680 K and an O2 partial pressure of 1 mbar.

This perspective summarizes results of in situ adsorption and reaction experiments on graphene-supported nanoparticles: the particular aim is to point out similarities and differences between studies on “traditional” single crystal studies and the more complex, more realistic nanoparticles. It is shown that the use of quantitative X-ray photoelectron spectroscopy allows for gaining a detailed insight even into these complex systems, thereby facilitating a further step into bridging the materials gap from fundamental science to applied sciences. The use of graphene as a substrate gives intriguing new possibilities, as the template effect of graphene can lead to a very narrow size distribution of the clusters, while graphene itself is chemically innocent, thereby making side processes such as spill over and reverse spill over less likely. The systems discussed range from extremely well studied systems such as the adsorption and reaction of CO on a Pt(111) surface, to stepped surfaces and finally to nanocluster arrays supported on a graphene support. Also, the important chemistry of sulfur, being a strong catalyst poison, on such systems will be discussed. While the adsorption behavior on nanoclusters is strongly reminiscent of the adsorption on stepped surfaces, a strong increase in the reactivity of nanoparticles systems is found.

•NAP XPS investigations of the oxidation of liquid platinum/gallium alloys•Surface depletion of Pt in metallic Pt/Ga alloys•Surface enrichment of Pt in gallium oxide filmsThe formation of surface Ga2O3 films on liquid samples of Ga, and Pt–Ga alloys with 0.7 and 1.8 at.% Pt was examined using near-ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS). Thickness, composition and growth of the oxide films were deduced as a function of temperature and Pt content of the alloys, in ultra-high vacuum and at oxygen pressures of 3 × 10− 7, 3 × 10− 3 and 1 mbar. We examined oxide layers up to a thickness of 37 Å. Different growth modes were found for oxidation at low and high pressures. The formed Ga2O3 oxide films showed an increased Pt content, while the pristine GaPt alloy showed a surface depletion of Pt at the examined temperatures. Upon growth of Ga2O3 on Pt/Ga alloys a linear increase of Pt content was observed, due to the incorporation of 3.6 at.% Pt in the Ga2O3. The Pt content in Ga2O3, at the examined temperatures and bulk Pt concentrations is found to be independent of pressure, temperature and the nominal Pt content of the metallic alloy.

•Argon intercalation between nickel and graphene•Preimplantation of argon for synthesis•Model for the shape of the argon structures under grapheneWe report on the intercalation of graphene grown on a Ni(111) crystal with argon. Argon is implanted in the Ni(111) crystal by ion bombardment before graphene growth, and diffuses to the surface during the growth of graphene at elevated temperatures. Graphene acts as an atomically thin barrier and keeps the argon underneath. We investigated this system with high resolution X-ray photoelectron spectroscopy. From our experiments we determined the mean quantities of argon under graphene. From our analysis, a simple model to determine the pressure under the graphene layer is presented. In our measurements, we find an increased thermal stability of the intercalated graphene as compared to non-intercalated graphene on Ni(111).

We have studied the dehydrogenation of the liquid organic hydrogen carrier (LOHC) dicyclohexylmethane (DCHM) to diphenylmethane (DPM) and its side reactions on a Pd(111) single crystal surface. The adsorption and thermal evolution of both DPM and DCHM was measured in situ in ultrahigh vacuum (UHV) using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (HR-XPS). We found that after deposition at 170 K, the hydrogen-lean DPM undergoes C-H bond scission at the methylene bridge at 200 K and, starting at 360 K, complete dehydrogenation of the phenyl rings occurs. Above 600 K, atomic carbon incorporates into the Pd bulk. For the hydrogen-rich DCHM, the first stable dehydrogenation intermediate, a double π-allylic species, forms already at 190 K. Until 340 K, further dehydrogenation of the phenyl rings and of the methylene bridge occurs, yielding the same intermediate that is formed upon heating of DPM to this temperature, that is, DPM dehydrogenated at the methylene bridge. The onset for the complete dehydrogenation of this intermediate occurs at a much higher temperature than after adsorption of DPM. This behavior is mainly attributed to coadsorbed hydrogen from DCHM dehydrogenation. The results are discussed in comparison to our previous study of DPM and DCHM on Pt(111) revealing strong material dependencies.

Nanocluster arrays on graphene (Gr) are intriguing model systems for catalysis. We studied the adsorption and oxidation of CO on Pt/Gr/Rh(111) with synchrotron-based high-resolution X-ray photoelectron spectroscopy. On the nanoclusters, CO is found to adsorb at three different sites: namely, on-top, bridge, and step. The C 1s spectra exhibit remarkable similarities to those on the stepped Pt(355) surface. Similar to the case for stepped Pt surfaces, a clear preference for the adsorption on the step sites is found, while the preference for the adsorption on the on-top site over the bridge site on the terraces is much less pronounced in comparison to that on Pt single crystals. Temperature-programmed X-ray photoelectron spectroscopy revealed an enhanced binding energy for the cluster step sites, similar to the situation on stepped Pt surfaces. The oxidation of CO on graphene-supported Pt nanoclusters follows a pseudo-first-order rate law. Applying an Arrhenius analysis, we found an activation energy of 13 ± 4 kJ/mol, which is much smaller than that on Pt(111), due to the more reactive step and kink sites on the nanoclusters.Keywords: CO oxidation; graphene; HR-XPS; nanoclusters; platinum

•Hydrogenation of nitrogen-doped graphene•Functional groups on graphene•Chemical reactivity of NH groups on grapheneWe studied the hydrogenation and dehydrogenation of nitrogen-doped graphene (NDG) by in situ high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS (TPXPS). Nitrogen-doped graphene was prepared by low energy nitrogen implantation in pristine graphene on Ni(111). Hydrogenation of NDG was performed by exposure to atomic hydrogen. Upon hydrogenation the XP spectra in the C 1s region reveal one new peak, shifted to lower binding energies as compared to graphene, which is associated with newly formed CH groups. In the N 1s region two new peaks, shifted to higher binding energies are observed; these are associated with hydrogenated pyridinic and graphitic nitrogen. TPXPS spectra reveal a different thermal stability of the two hydrogenated nitrogen species, while the C–H groups of graphene show no significant changes compared to undoped hydrogenated graphene.

We investigated the surface reaction of the liquid organic hydrogen carrier dicyclohexylmethane (DCHM) on Pt(111) in ultrahigh vacuum by high-resolution X-ray photoelectron spectroscopy, temperature-programmed desorption, near-edge X-ray absorption fine structure, and infrared reflection–absorption spectroscopy. Additionally, the hydrogen-lean molecule diphenylmethane and the relevant molecular fragments of DCHM, methylcyclohexane, and toluene were studied to elucidate the reaction steps of DCHM. We find dehydrogenation of DCHM in the range of 200–260 K, to form a double-sided π-allylic species coadsorbed with hydrogen. Subsequently, ∼30% of the molecules desorb, and for ∼70%, one of the π-allyls reacts to a phenyl group between 260 and 330 K, accompanied by associative hydrogen desorption. Above 360 K, the second π-allylic species is dehydrogenated to a phenyl ring. This is accompanied by C–H bond scission at the methylene group, which is an unwanted decomposition step in the hydrogen storage cycle, as it alters the original hydrogen carrier DCHM. Above 450 K, we find further decomposition steps which we assign to C–H abstraction at the phenyl rings.

Atomic sulfur and its oxides are common catalyst poisons and intriguing research subjects. Recently, graphene-supported nanoclusters were introduced as suitable model catalysts. We investigated the adsorption and reaction of SO2 on graphene-supported Pt nanocluster arrays with high-resolution X-ray photoelectron spectroscopy. SO2 adsorbs in two geometries—perpendicular and parallel to the surface—on both cluster facets and steps. Further insight is gained from the comparison of our results to previous data of SO2 on Pt(111) and two stepped single crystal surfaces—Pt(322) and Pt(355)—with (100) and (111) steps, respectively. We find a remarkable similarity to the adsorption situation on Pt(322). However, thermal evolution experiments revealed several similarities to both Pt(322) and Pt(355), showing that the Pt nanoclusters exhibit a mixture of (100) and (111) steps.

Graphene grown on Rh(111) was used as a template for the growth of Pd nanoclusters. Using high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy, we studied the deposition of Pd on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster growth mode. The formation of clusters with 3 nm diameter was confirmed by low-temperature scanning tunneling microscopy measurements. The investigation of the thermal stability of the Pd particles showed three characteristic temperature regimes: Up to 550 K restructuring of the particles takes place, between 550 and 750 K the clusters coalesce into larger agglomerates, and finally between 750 and 900 K Pd intercalates between the graphene layer and the Rh surface.

The adsorption of CO on graphene-supported Pd nanoparticles was studied in situ with high-resolution synchrotron-based X-ray photoelectron spectroscopy. At 150 K, CO adsorbs mainly in bridge and 3-fold-hollow sites. The nanoparticles are considered as a mixture of low-index facets. The variation of the amount of deposited Pd revealed identical CO adsorption behavior for all investigated cases, confirming a similar average cluster size over a wide range of Pd coverages. The desorption characteristics were studied with temperature-programmed XPS. The observed desorption maxima at 230 and 430 K are in good agreement with temperature-programmed desorption data on stepped Pd single crystals. At 500 K, CO is completely desorbed from the Pd clusters. The adsorption and desorption of CO are found to be not fully reversible as the Pd particles undergo restructuring upon heating.

Carbon dioxide (CO2) absorption by the amine-functionalized ionic liquid (IL) dihydroxyethyldimethylammonium taurinate at 310 K was studied using surface- and bulk-sensitive experimental techniques. From near-ambient pressure X-ray photoelectron spectroscopy at 0.9 mbar CO2, the amount of captured CO2 per mole of IL in the near-surface region is quantified to ∼0.58 mol, with ∼0.15 mol in form of carbamate dianions and ∼0.43 mol in form of carbamic acid. From isothermal uptake experiments combined with infrared spectroscopy, CO2 is found to be bound in the bulk as carbamate (with nominally 0.5 mol of CO2 bound per 1 mol of IL) up to ∼2.5 bar CO2, and as carbamic acid (with nominally 1 mol CO2 bound per 1 mol IL) at higher pressures. We attribute the fact that at low pressures carbamic acid is the dominating species in the near-surface region, while only carbamate is formed in the bulk, to differences in solvation in the outermost IL layers as compared to the bulk situation.

The growth and oxidation of graphene supported on Rh(111) was studied in situ by high-resolution X-ray photoelectron spectroscopy. By variation of propene pressure and surface temperature the optimum growth conditions were identified, yielding graphene with low defect density. Oxidation of graphene was studied at temperatures between 600 and 1000 K, at an oxygen pressure of ∼2 × 10−6 mbar. The oxidation follows sigmoidal reaction kinetics. In the beginning, the reaction rate is limited by the number of defects, which represent the active sites for oxygen dissociation. After an induction period, the reaction rate increases and graphene is rapidly removed from the surface by oxidation. For graphene with a high defect density we found that the oxidation is faster. In general, a reduction of the induction period and a faster oxidation occur at higher temperatures.

The growth and oxidation of graphene supported on Rh(111) was studied in situ by high-resolution X-ray photoelectron spectroscopy. By variation of propene pressure and surface temperature the optimum growth conditions were identified, yielding graphene with low defect density. Oxidation of graphene was studied at temperatures between 600 and 1000 K, at an oxygen pressure of ∼2 × 10−6 mbar. The oxidation follows sigmoidal reaction kinetics. In the beginning, the reaction rate is limited by the number of defects, which represent the active sites for oxygen dissociation. After an induction period, the reaction rate increases and graphene is rapidly removed from the surface by oxidation. For graphene with a high defect density we found that the oxidation is faster. In general, a reduction of the induction period and a faster oxidation occur at higher temperatures.

Controlled patterning of graphene is an important task towards device fabrication and thus is the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2–30 keV) and doses (50–800 mC m−2) either reduction of GO or formation of permanent lattice defects occurs. This result reflects a step towards controlled FEBIP processing of oxo-G.