Nickel Supported on Mesoporous Zirconium Oxide by Atomic Layer Deposition: Initial Fixed-Bed Reactor Study

Atomic layer deposition (ALD) is gaining attention as a catalyst preparation method able to produce metal (oxide, sulphide, etc.) nanoparticles of uniform size down to single atoms. This work reports our initial experiments to support nickel on mesoporous zirconia. Nickel (2,2,6,6-tetramethyl-3,5-heptanedionate)2 [Ni(thd)2] was reacted in a fixed-bed ALD reactor with zirconia, characterised with BET surface area of 72 m2/g and mean pore size of 14 nm. According to X-ray fluorescence measurements, the average nickel loading on the top part of the support bed was on the order of 1 wt%, corresponding to circa one nickel atom per square nanometre. Cross-sectional scanning electron microscopy combined with energy-dispersive spectroscopy confirmed that in the top part of the fixed support bed, nickel was distributed throughout the zirconia particles. X-ray photoelectron spectroscopy indicated the nickel oxidation state to be two. Organic thd ligands remained complete on the surface after the Ni(thd)2 reaction with zirconia, as followed with diffuse reflectance infrared Fourier transform spectroscopy. The ligands could be fully removed by oxidation at 400 °C. These initial results indicate that nickel catalysts on zirconia can likely be made by ALD. Before catalytic testing, in addition to increasing the nickel loading by repeated ALD cycles, optimization of the process parameters is required to ensure uniform distribution of nickel throughout the support bed and within the zirconia particles.

6 powders (diameter 2 cm) was used with the associated filter to hold up to ca. 5 g of support. 119 The particle bed height was over one centimetre (accurate height not measured). The Ni(thd)2 120 reactant was placed in an open glass boat within the reactor and sublimated at 140 °C, operat-121 ed under a moderate vacuum of 0.6-4 mbar (pressure measured after the support bed). Nitro-122 gen (>99.99999 %, generated with a Parker HPN2-5000 from air, with less than 10 ppm of 123 oxygen) was used as a carrier and purging gas, with constant flow rate of 400 sccm. The sup-124 port was stabilised at 400 °C for 3 h in a stream of nitrogen. After this, the temperature was 125 stabilised to the desired reaction temperature (200 °C) and the reactant vapour was led down-126 wards through the fixed support bed for 3 h. After the reaction, the sample was purged with 127 nitrogen at the reaction temperature for 2 h. At the end of the run, the reactor was cooled in 128 nitrogen flow close to room temperature before unloading. Samples were taken from the top 129 part of the support bed and mixing the rest of the material as one sample. The samples were 130 stored in a desiccator. The chemical composition and nickel loading of the prepared materials were measured semi-138 quantitatively by X-ray fluorescence (XRF) using a PANalytical AxiosMax Wavelength Dis-139 persive X-ray Fluorescence Spectrometer (WD-XRF). The device was equipped with a scin-140 tillation detector and a rhodium tube, which operated at 60 kV with a current of 50 mA. The 141 samples (100-500 mg) in powder form were placed on a supporting thin film using XRF sam-142 ple cup (32 mm width). 143 144

X-ray Photoelectron Spectroscopy 145
The X-ray photoelectron spectroscopy (XPS) measurements were made using Kratos Axis 146 Ultra system, equipped with a monochromatic AlKα X-ray source. All measurements were 147 performed with 0.3 mm x 0.7 mm analysis area and the charge neutraliser on. A wide scan 148 was performed with 80 eV pass energy and 1 eV energy step. High resolution scans were per-149 formed with 20 eV pass energy, 0.1 eV steps size for 5 min for the C 1s, Zr 3d and O 1s and 150 for 20 min for Ni 2p. The energy calibration was made using the adventitious carbon C1s 151 component at 284.8 eV. All deconvolutions were made with CasaXPS using GL(30) peaks 152 (product of 30% Lorenztian and 70% Gaussian). Information depth in XPS is roughly ten 153 atomic layers. 154 155

Scanning Electron Microscopy and Energy-dispersive X-ray Spectrometry 156
Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) examina-157 tion was carried out using Tescan Mira3 scanning electron microscope fitted with a Thermo Sci-158 entific energy-dispersive X-ray spectrometer. The EDS system was equipped with silicon drift 159 detector (SDD). In sample preparation, mesoporous Ni(thd)2-modified zirconia particles were 160 mounted in epoxy resin utilizing vacuum impregnation. The cured mounts were ground and pol-161 ished to expose cross-sections of the particles at the face of specimen. Subsequently, specimens 162 8 were coated with carbon to prevent charging under the electron beam. In the SEM and EDS exam-163 ination, electron accelerating voltage of 15 keV was used. First, qualitative elemental analysis was 164 performed to identify elements present in the specimen. Secondly, EDS line scans were performed 165 across a selected Ni(thd)2-modified zirconia particle. The length of the line was 600 m including 166 100 measurement points. Integration of 40 scans was utilised to improve precision of the meas-167 urement. Estimated detection limit of EDS is 0.1-0.3 wt-%. for 3 h, followed by cooling down to 30° C. This was done to reduce moisture, which had 190 been transferred within the sample to the in situ cell through ambient air. Next, the oxidation 191 of the surface species was studied by feeding 10% O2/N2 (synthetic air 99.99%) to the cham-192 ber at 30 °C followed by increasing the temperature stepwise (steps of 25 °C) to 500 °C.  ing the stepwise heating of the sample, spectra (4 cm -1 resolution, wavenumber range 4000-194 1000 cm -1 , 100 scans) were recorded every 25 °C, i.e., approximately every 4 minutes. 195 The Ni 2p region shows the 2p3/2 peak at 855.5 eV and the 2p1/2 peak at 873.2 eV. Both 244 peaks have a satellite roughly 6 eV above the main peak. We also measured pure Ni(thd)2 for 245 reference, and noticed that the shape of the spectrum is similar, although the intensities of the 246 satellites compared to the main peak are higher in the Ni(thd)2-modified zirconia samples. 247

Results and Discussion
The satellite intensity in the Ni(thd)2 increased when the material was left in air for one night 248 (not shown). We expect this change is due to exposure to humidity. Deconvolution of the Ni 249 spectrum was not performed, but we compared the Ni spectra against reference spectra of NiO 250 and Ni(OH)2 [58]. NiO reference shows two components in the 2p3/2 peak around 855.5 eV 251 separated by 1.7 eV not visible in our data. The Ni(OH)2 reference shows one main peak at 252 855.5 eV and a satellite 6 eV above that, resembling our data. However, the Ni(OH)2 peaks 253 reported by [58] are not sufficient to reproduce our data. This indicates slightly different envi-254 ronment for Ni atoms than in Ni(OH)2 or NiO but their oxidation state seems to be two.

SEM-EDS 264
Initial EDS results showed the presence of Ni in the studied sample. The results concerning Ni 265 distribution across a zirconia particle are presented in Figure 4. Figure   The zirconia support was measured as a reference and the spectrum at 30 °C was recorded 301 after heating in N2 at 200 °C for 2 hours (spectrum A in Figure 6). The spectrum showed 302 peaks at 3776 cm -1 and 3671 cm -1 , and a small shoulder between these two bands at 3734 cm -303 1 . The peaks at 3776 cm -1 and 3671 cm -1 can be assigned to terminal and tribridged OH 304 groups [59]. The small shoulder at 3734 cm -1 is likely indicating the existence of bibridged 305 OH groups [59]. Small bands observed between 1600 and 1000 cm -1 can be assigned to 306 residual carbonate groups trapped inside the zirconia bulk [60]. The spectrum of the zirconia 307 support (spectrum A in Figure 6) also showed moisture on the sample that was expected due 308 to the pretreatment at low temperature (200 °C). The OH groups have been reported to have 309 more intense peaks when calcined at 600 °C for 2 hours in air flow [59].

349
This article reports our first efforts to support nickel on mesoporous high-surface-area zirco-350 nia by ALD for catalytic purposes. A nickel loading of approximately 1 wt-% was obtained 351 by the Ni(thd)2 reaction at 200 °C using a commercial fixed-bed powder ALD reactor. The 352 corresponding surface loading on zirconia (with BET surface area of 72 m 2 /g and mean pore 353 diameter of 14 nm) was on the order of 1 Ni/nm 2 . Full saturation throughout the support bed 354 was not yet attained in this initial work. According to XPS, all nickel had oxidation state two. 355 According to SEM-EDS cross-sectional observation, at the top part of the fixed particle bed, 356 nickel was observed throughout the zirconia particle. Organic thd ligands remained complete 357 on the surface after the Ni(thd)2 reaction with zirconia, as followed with DRIFT spectroscopy. 358 The first ALD cycle was completed by oxidation, which removed the remaining organic lig-359 ands at approximately 400 °C and re-created OH groups on the surface. 360 361 To use the Ni/zirconia materials as catalysts, it is advisable to ensure full saturation through-362 out the support bed and within the zirconia particles. Further optimization work is needed to 363 ensure saturation and increase the nickel loading before catalytic testing. 364 365