Abstract
Nickel is an active catalyst for hydrogenation and re-forming reactions, with the reactions showing a strong dependence on the surface exposed. Here, we describe the mixed hydroxyl–water phases formed during water dissociation on Ni(110) using scanning tunneling microscopy and low-current low-energy electron diffraction. Water dissociation starts between 150 and 180 K as the H-bond structure evolves from linear one-dimensional (1D) chains of intact water into a two-dimensional (2D) network containing short rows of face-sharing hexagonal rings. As further water desorbs, the hexagonal rows adopt a local (2 × 3) arrangement, forming small, disordered domains separated by strain relief features. Decomposition of this phase occurs near 220 K to form linear 1D structures consisting of flat, zigzag water chains, with each water stabilized by donating one H to hydroxyl to form a branched chain structure. The OH–H2O chains repel each other, with the saturation layer ordering into a (2 0, 1 4) structure that decomposes to OH near 245 K as further water desorbs. The structure of the mixed OH/H2O phases is discussed and contrasted with those found on the related Cu(110) surface, with the differences attributed to strain in the 2D H-bond network caused by the short Ni lattice spacing and strong bond to OH/H2O.
Highlights
We explore the dissociation of a water film on Ni(110) using low-temperature scanning tunneling microscopy (STM)
Thermal desorption spectroscopy is combined with low-current low-energy electron diffraction (LEED) to determine the lateral order of different structures formed without influence from electron-induced restructuring,[42] allowing us to relate the STM measurements to previous experimental studies.[9,15,16,24−32] We find that above 150 K, dissociation occurs in parallel with water desorption to form first a disordered 2D water/hydroxyl network, with a variable OH/H2O composition, and an ordered array of 1D (OH + H2O) chains
Water desorption peaks appear at very similar temperatures for H2O and D2O, but the shape of the at K (A2) peak is noticeably different between the two isotopes, being slightly broader for D2O, while at low coverage, the A1 peak is smaller for D2O than for H2O
Summary
Nickel is an important catalyst in re-forming reactions,[1−3] finding application in practical catalysts for water dissociation and re-forming.[4,5] The reactivity of both metals is extremely face dependent, with water remaining intact on the close-packed faces[6−8] but dissociating at moderate temperatures on more open surfaces[9,10] as the water binding energy increases.[4,5,11−13] The open (110) surface is the most widely studied reactive face, with early reports suggesting copper and nickel forming similar water structures.[14−16] the two metals differ considerably in their reactivity and lattice spacing, and more recent studies indicate significant differences in their interaction with water. Water adsorbs and desorbs below room temperature on Cu(110) without dissociating,[17] forming one-dimensional (1D) chains of face-sharing pentamers that aggregate into a two-dimensional (2D) network only at high coverage.[18] The behavior on Ni(110), where water has a higher binding energy, is quite different. Instead of forming cyclic, face-sharing water rings, water instead forms linear, twocoordinate zigzag chains along the close-packed Ni rows, maximizing bonding to Ni at the expense of a reduced H-bond coordination.[19]. Cu(110) surfaces dissociate water only when it is adsorbed above 255 K,10 but hydroxyl can be formed by reaction with preadsorbed O atoms at low temperature.[14,20,21] By using the O surface reaction to form known amounts of hydroxyl, three different partially dissociated phases can be characterized on Cu(110). Thermal desorption spectra of water from Ni(110) (Figure 1) are similar to those obtained for H2O/OH on Cu(110), with three desorption peaks appearing at temperatures above the water multilayer peak.[9,23,29] The hightemperature peak is due to OH disproportionation to form O and water,[16,23,24] similar to Cu(110),[33] but it is not known if the two lower temperature desorption features correspond to Received: September 24, 2020 Revised: October 3, 2020 Published: October 15, 2020
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