Low Ni Coverage Research Articles

Revealing the effect of metal-support interactions at the Ni/In2O3(111) interface on the selective CO2 hydrogenation

In2O3-supported Ni catalysts exhibit remarkable catalytic activity and selectivity in CO2 hydrogenation to methanol, but the underlying mechanisms and metal-oxide interactions during the reaction remain elusive. Herein, we investigate the Ni-In2O3 interaction by physical vapor deposition of Ni onto well-defined In2O3(111) thin films. In-situ near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to probe the CO2 hydrogenation processes on these Ni/In2O3(111) model systems. Our results reveal that the small Ni clusters supported on the In2O3(111) surface at low Ni coverages exhibit cationic states. The chemical bonding and associated electron transfer at the Ni/In2O3(111) interface play crucial roles in the activation of H2 and CO2. Importantly, reaction intermediates (CO3*, OH, and HCOO*) are readily formed and desorbed under CO2 hydrogenation conditions. Our study highlights the significance of metal-support interactions on the selectivity of CO2 hydrogenation. These findings provide valuable insights into the rational design of advanced In2O3-based catalysts for CO2 hydrogenation.

CO desorption from nickel-decorated muscovite mica

Abstract Adsorption and desorption of CO from Ni nanostructures deposited on muscovite mica substrates have been studied by TPD and characterized by XPS and AFM. In agreement with previous reports it is found that CO does not adsorb on the bare mica substrate. The desorption spectra depend strongly on the Ni coverage on the mica substrate. Three coverage regimes have been found; for low Ni coverage of approximately 0.2 monolayers a dominant desorption peak is found near 400 K. For medium coverage of approximately 0.6 monolayers three distinct peaks appear in the temperature region from 250 to 550 K. For higher Ni coverage of approximately 3 monolayers a single peak is observed near 550 K. The high temperature peak was assigned to desorption from terrace sites, the medium temperature peak was assigned to step sites, and the low temperature peak was argued to stem from lateral interaction between adsorbed CO species and possible sites with low coordination, e.g. kink sites. Desorption parameters have been extracted from the low temperature part of the desorption spectra. Anomalous low values for the vibrational prefactors were obtained in the case of low Ni coverage. This is argued to be due to dissociation of CO.

CO adsorption on nanoislands: Ni on Au(111).

The adsorption behavior of CO on Ni islands grown on Au(111) was studied with a combination of temperature programmed desorption, Fourier transform infrared spectroscopy, and surface resistivity measurements. The Au(111) herringbone reconstruction provides a template for the growth of ordered Ni islands, with evidence for the presence of strain and Au atoms within the islands. The islands grow radially until θNi ≈ 0.3 ML, after which subsequent Ni atoms contribute primarily to a second layer. We study saturated CO adsorption at 227 K over a range of Ni island sizes and find layer-dependent adsorption properties. For single-layer islands at low Ni coverage, CO adsorbs primarily in the atop position and desorbs at lower temperatures than on pure Ni, with a saturation CO coverage of about 0.5 CO/Ni. As second layer Ni grows, saturated CO coverages on the Ni approach unity, with higher desorption temperatures, but still with primarily atop CO. Based on previous studies, we propose that in the first Ni layer, ligand effects from the Au substrate and possibly Au in the islands and strain due to the Ni/Au lattice mismatch affect the Ni-CO bonds. CO adsorption behavior on the two-layer islands is qualitatively explained by a decrease in Au nearest neighbors and the presence of a more expanded/corrugated structure.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. I. Photoelectron emission microscopy.

Chemical waves that arise in the H2 + O2 reaction on a bimetallic Rh(111)/Ni surface have been studied in the 10-6 and 10-5 mbar range at T = 773 K with photoelectron emission microscopy (PEEM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Nickel coverages of 0.3, 0.6, and 1.0 monolayers were investigated. Coadsorbed with some oxygen, Ni starts to penetrate the Rh bulk region substantially only beyond 900 K. In the 10-5 mbar range, chemical waves are characterized by three distinct gray levels in PEEM. This number reduces to only two levels in the 10-6 mbar range. In situ LEED showed the periodic appearance of a (n × 1) (n = 8, 10) pattern during chemical waves which was assigned to a 2D-Ni oxide. With in situ AES, one observes that the bright phase in PEEM correlates with a high Ni coverage and the dark phase with a low Ni coverage.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen.

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

Evidence for the interfacial reaction between Ni adatoms and H-Si(001) surface

Hydrogen termination of Si surfaces (H-Si) does not stop the interfacial reaction between Ni adatoms and H-Si(001) surface at room temperature. At low Ni coverage of 0.1% (equivalent to 0.02 ML), X-ray photoelectron spectroscopy (XPS) reveals a binding energy shift of Ni 2p 3/2 to 854.0 eV, which corresponds to the formation a NiSi-like environment. As the coverage of Ni increases, the Ni 2p 3/2 eventually shifts to 852.8 eV, indicative of formation of metallic Ni. XPS intensity vs Ni coverage analysis reveals a growth process akin to pseudo-layer-by-layer growth mode, thereby suggesting the formation of bulk-Si(001)/NiSi-like/Ni-rich-silicide-like/metallic Ni structure as growth proceeds. For Ni coverage not more that 33% (equivalent to 12.68 ML), Ni remains protected by the silicide environment and no oxidation of Ni to form Ni-oxides was observed even after exposing the samples to air for 400 days. For samples with Ni coverage above 41%, oxidation of Ni is observed by presence of NiO and NiSiO 3 peaks at 854.5 and 857.0 eV, respectively. The current studies suggest that there is reaction between Ni adatoms and Si at the growth front on H-Si(001) surfaces upon Ni deposition at room temperature and hydrogen termination does not suppress the interface reaction between Ni and Si.

Metal overlayers on organic functional groups of self-assembled monolayers: VIII. X-ray photoelectron spectroscopy of the Ni/COOH interface

The interaction of vacuum deposited Ni with the COOH organic functional groups of mercaptoundecanoic acid [HS(CH2)10COOH] self-assembled monolayers formed on an Au substrate has been characterized with in situ x-ray photoelectron spectroscopy. Steady loss and complete disappearance of the hydroxyl component of O 1s peak at 532.8 eV provides evidence for the formation of a Ni–O bond at the Ni/COOH interface. Further evidence for interfacial compound formation is provided by a binding energy shift in the high binding energy component of the Ni 2p3/2 peak from 854.8 to 854.4 eV for Ni coverages below ∼0.2 nm. These results are consistent with the donation of electrons from the deposited Ni to the COOH oxygens at low Ni coverages. The absence of the characteristic satellite feature in the Ni 2p peak excludes the possibility that a full electron charge is donated to each COOH organic functional group as a bidentate complex. However, the data are consistent with a two step reaction mechanism in which Ni initially reacts weakly with the oxygens in COOH below one monolayer coverage and then forms a complex above this coverage.

Magnetic properties of ultrathin films of Ni/Cu(001) probed by soft-x-ray magnetic circular dichroism (abstract)

The magnetic properties of MBE-grown ultrathin Ni films on a Cu(001) substrate are investigated using soft-x-ray magnetic circular dichroism (SXMCD) spectroscopic technique. These measurements are carried out by recording Ni L2,3 absorption spectra with highly circularly polarized soft x rays recently obtained at the AT&T Bell Laboratories Dragon synchrotron radiation beamline.1 For low Ni coverages, no MCD effect along the 〈100〉 direction is observed at room temperature, while for intermediate coverages, the MCD effect becomes appreciable and increases to its bulk value at ∼25 ML. With only the assumption that the unoccupied density of states of Ni lie mainly in the minority bands, the amount of the MCD effect can be related to the magnetic moment. The thickness and temperature dependence of the magnetic moment obtained from our SXMCD measurements are compared with those of Ni/Cu(111) obtained from magneto-optical data.2 The advantages and differences between the SXMCD and MOKE techniques are discussed. The novel SXMCD technique allows for element specific magnetic characterization of a variety of interesting structural architectures including 2 D films, dilute impurities, and disordered matrices.

Impact collision ion scattering spectrometry studies of thin metal overlayers on Si(111) surfaces

The technique of impact-collision ion scattering spectrometry (ICISS) was used to determine the in-plane geometries of the (a) Si(111)−(√3×√3) Ag and (b) Si(111)− Ni surfaces. For Si(111)−(√3×√3) Ag, comparison of experimental data with computer simulation leads to a substitutional trimer (ST) model in which Si honeycombs sit on top of Ag trimers occupying substitutional sites. The separation of the Si and Ag layers is 0.7±0.3 Å and the SiAg bond length is 2.66±0.3 Å. We also observed type-A and type-B arrangements of the Si and Ag atoms on the surface, with type-B being type-A rotated 180° about the surface normal. Our results are consistent with reported scanning tunneling microscopy images and also with a missing top layer (MTL) model proposed from photoemission and x-ray photoelectron diffraction studies. For Si(111)− Ni, the agreement between computer simulations and experimental data indicates that at extremely low Ni coverages, ∼0.1 ML, Ni atoms diffuse into the Si substrate at room temperature and form regions of predominantly type-B NiSi 2.

Properties of monolayer and multilayer Ni films on the Ru(0001) surface

The adsorption, growth and chemisorptive properties of ultra-thin Ni films ( < 10 monolayers (ML)) on the Ru(0001) surface were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD) and work function (φ) measurements. LEED studies show that submonolayer coverages of Ni at 100 K are dispersed and disordered, but form well ordered 2D islands pseudomorphic to the Ru(0001) substrate upon annealing to approximately 700 K. Above 1 ML, a layer-by-layer growth pattern is observed after annealing to approximately 700 K, but these layers show evidence of periodic strain and misfit dislocations. Annealing films thicker than 1 ML to 1100 K results in the formation of 3D islands leaving a large portion of the Ru(0001) surface covered only by the single pseudomorphic Ni layer. The work function change, Δφ, varies linearly with Ni coverage, reaching −410 meV at 1 ML. With additional Ni deposition, Δφ rises to −260 meV which is the difference in work function between bulk Ru(0001) and bulk Ni(111) surfaces. First-order desorption kinetics are observed for thermal desorption of Ni up to a coverage of 4 ML; coverages greater than 4 ML exhibit near-zero-order desorption kinetics. The activation energy for desorption increases from 76k cal/mol at low Ni coverages to 91 kcal/mol at 5 ML coverage. CO desorbs at a higher temperature from the pseudomorphic Ni ML than from either Ni(111) or Ru(0001) indicating considerable electronic perturbation of the Ni monolayer.

Impact-collision ion-scattering-spectrometry study of Ni layers deposited on Si(111) at room temperature.

Overlayers of Ni, ranging in thickness from 0.1 to 34 A\r{}, were deposited on a Si(111) surface. Surface structure and composition were monitored during the film growth using impact-collision ion-scattering spectrometry. Results indicate that at extremely low Ni coverages, \ensuremath{\sim}0.1 A\r{}, Ni atoms diffuse into the Si substrate and form regions of predominantly type-B ${\mathrm{NiSi}}_{2}$. Further Ni deposition results in the lateral growth of silicide islands enriched with Si on the surface. At 12 A\r{} coverage the islands coalesce, then grow in thickness until 30 A\r{} Ni has been deposited, at which point a pure-Ni film begins to form.

CO Adsorption suppression due to charge transfer in the Ni–SiOx–n-Si(111) system at low Ni coverage

No CO adsorption was observed in the He I u.v. photoelectron spectra at low Ni coverage on SiOx–n-Si(111) where charge transfer has been concluded to occur, whereas normal CO adsorption was observed at low Ni coverage on SiOx–p-Si(111) or at high Ni coverage on SiOx–n-Si(111); these results show that charge transfer occurring in a narrow region at low Ni coverage is responsible for the suppression of CO adsorption.