Performing oxidation reactions at low temperatures using earth-abundant materials is crucial for advancing solutions for sustainable chemistry. CO oxidation serves as a benchmark reaction to characterize oxidation and to advance fundamental concepts in surface chemistry. While there are several examples of CO oxidation occurring on metal oxides at low temperatures, from 300 K to ∼200 K, reactivity in the cryogenic temperature regime typically requires a metal nanoparticle on a metal oxide. Here, we show oxygen atoms on the (111) facet of Cu₂O react with CO to form CO₂ at temperatures below 100 K. Combining spectroscopic experimental evidence with calculations, we propose a low barrier path for CO oxidation at reconstructed surface sites on Cu₂O(111). This finding is a rare example of an earth-abundant metal oxide, in this case copper, that can provide highly reactive multifunctional sites, enabling both adsorption and reaction fundamental steps toward the efficient heterogeneous oxidation of chemicals.

Tar is an undesirable byproduct of biomass gasification, which can be removed through catalytic reforming to syngas components. Iron is a promising, abundant alternative to highly active but toxic nickel catalysts. The results observed so far in catalytic studies with iron have been mixed. In this paper, the decomposition of naphthalene, a representative model compound of tar, was studied on the catalytic Fe(110) surface using temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), and X-ray photoelectron spectroscopy (XPS). Napthalene adsorption, dehydrogenation and the formation of surface carbon were investigated, as well as the influence of oxygen. In comparison with previous studies on Ni(111), a similar dehydrogenation activity was found for Fe(110) with two main H2 TPD peaks at 450 and 550 K. The reaction of naphthalene on Fe(110) resulted in the predominant formation of carbidic and atomically adsorbed carbon on the surface, which did not dissolve into the bulk even at high temperatures. A moderately carbon-covered surface was shown to still be active toward naphthalene decomposition. Similarly to Ni(111), large amounts of oxygen inhibited the reaction but, at low oxygen doses, very high hydrogen yields were observed, suggesting that Fe(110) could be a valid alternative for tar decomposition.

There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5√3 × 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature.

Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO.

Methanol decomposition on Ni(111) surfaces has been studied in the presence and absence of oxygen using temperature-programmed desorption and temperature-dependent sum frequency generation spectroscopy. Under both conditions the C–H and O–H bonds break, forming carbon monoxide and atomic hydrogen on the surface. No C–O bond scission was observed, limiting the number of reaction pathways. The O–H bonds break first (>150 K), forming surface methoxy, followed by C–H bond breakage (>250 K). All atomic hydrogen desorbs from the surface as H₂ through H+H recombinative desorption. H₂ desorbs at a higher temperature in the presence of oxygen (>300 K) than the absence of oxygen (>250 K) as the oxygen on the surface stabilizes the H atoms, forming surface hydroxide (OH). The surface oxygen also appears to stabilize the O–H and C–H bonds, leading to slightly higher dissociation temperatures. The CO molecules occupy both the bridge sites and the top sites of the Ni atoms as surface H appears to force the CO molecules to the top sites. There is a slight blueshift in the C–O bond vibration for both the O covered and O free surfaces due to CO being more mobile. On the O free surface, the C–O peak width broadens as low-frequency modes are activated. Finally, CO desorbs between 350 and 400 K.

Ethanol dehydrogenation on metal oxides such as Cu2O is an important reaction for the production of renewable energy by fuel cells both via the production of H2 fuel and applied in direct alcohol fuel cells. To better understand this reaction we studied the adsorption, dissociation and desorption of ethanol on Cu2O(111) and (100) surfaces using high-resolution photoelectron spectroscopy (PES), vibrational sum frequency generation spectroscopy (SFG), and temperature programmed desorption (TPD) accompanied by density functional theory (DFT) calculations. On Cu2O(100) the first layer consists primarily of dissociatively adsorbed ethoxy. Second and third layers of ethanol physisorb at low temperature and desorb below 200 K. On the Cu2O(111) surface, adsorption is mixed as ethoxy, ethanol and the products following C-C cleavage, CHx and OCHx, are found in the first layer. Upon heating, products following both C-C and C-O bond breaking are observed on both surfaces and continued heating accentuates the molecular cracking. C-O cleavage occurs more on the (100) surface, whereas on the Cu2O(111) C-C cleavage dominates and occurs at lower temperatures than on the (100) surface. The increased ability of Cu2O(111) to crack ethanol is explained by the varied surface structure including both surface oxygen, electron rich O-vacancies and Cu.

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.

We describe how the unique temporal and spectral characteristics of X-ray free-electron lasers (XFEL) can be utilized to follow chemical transformations in heterogeneous catalysis in real time. We highlight the systematic study of CO oxidation on Ru(0001), which we initiate either using a femtosecond pulse from an optical laser or by activating only the oxygen atoms using a THz pulse. We find that CO is promoted into an entropy-controlled precursor state prior to desorbing when the surface is heated in the absence of oxygen, whereas in the presence of oxygen, CO desorbs directly into the gas phase. We monitor the activation of atomic oxygen explicitly by the reduced split between bonding and antibonding orbitals as the oxygen comes out of the strongly bound hollow position. Applying these novel XFEL techniques to the full oxidation reaction resulted in the surprising observation of a significant fraction of the reactants at the transition state through the electronic signature of the new bond formation.

Adsorption and desorption of methanol on the (111) and (100) surfaces of Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120-620 K, in combination with density functional theory calculations and sum frequency generation spectroscopy. The bare (100) surface exhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (root 3 x root 3)R30 degrees-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surface oxygen.

An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit that it has been in use for a long time in fossil feedstock catalytic steam reforming, understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature-programmed desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may coexist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation, and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.