Submonolayer Is Enough: Switching Reaction Channels on Pt/SiO2 by Atomic Layer Deposition

Abstract

The reaction mechanism of CO2 with H2 is studied on platinum nanoparticles supported on fumed silica. It is found that platinum nanoparticle size, reaction temperature, and metal oxide promoters play important roles in determining the reaction rate and the mechanism of forming surface carbonyl species. Metal oxide promoters consist of submonolayer titanium oxide or aluminum oxide overcoated onto the catalysts by atomic layer deposition (ALD). These alter the CO formation rate, influence the adsorption and desorption behavior, and switch the surface reaction channel from the Eley−Rideal to Langmuir−Hinshelwood mechanism due to an enhancement of CO2 affinity to the metal−metal oxide interface. At the temperatures relevant for catalytic turnover, ALD overcoating significantly increases catalytic activity in CO2 hydrogenation to CH4 and CO, while the identity of the oxide overcoat helps control product selectivity. ■ INTRODUCTION The gas phase reaction CO2 + H → CO + OH has been extensively studied via semiclassical and quantum dynamical simulations on multidimensional potential energy surfaces. Key findings of the theoretical studies are that there is an “early” barrier leading to a HOCO intermediate and a “late” barrier for scission of the O−C bond. By analogy to the theoretical framework from the gas phase reaction, hydrogenassisted dissociation of carbon dioxide may be responsible for carbonyl formation on Pt surfaces during CO2 + H2 reactions, given almost barrierless dissociation of hydrogen molecules on Pt (111) at room temperature. Overall, this process can proceed via a Langmuir−Hinshelwood (L−H) mechanism or Eley−Rideal (E−R) mechanism (Scheme 1). In a L−H mechanism, the reaction proceeds as Pt−H + CO2(ads) → Pt−CO + OH(ads). Because of the positive adsorption energy of CO2 on Pt (111), 22−24 this mechanism is likely to occur only at the metal/metal oxide interface, as suggested by experimental and theoretical studies. For the E−R mechanism, a direct gas−solid reaction pathway is proposed: (1) Pt−H + CO2(g)→ Pt...CO* + OH(ads), (2) Pt...CO* → Pt−CO (∗ represents a hot intermediate). The Eley−Rideal mechanism has been proposed for CO2 hydrogenation on Cu (111) surfaces and Cu supported on SiO2, where formate is observed as a product. Because of its weak adsorption on copper and SiO2 surfaces, gaseous CO2 is proposed to react directly with Received: June 6, 2021 Revised: August 8, 2021 Scheme 1. Mechanism of CO2 + /2H2 → CO + OH on Pt Article pubs.acs.org/JPCC © XXXX American Chemical Society A https://doi.org/10.1021/acs.jpcc.1c04972 J. Phys. Chem. C XXXX, XXX, XXX−XXX D ow nl oa de d vi a N O R T H W E ST E R N U N IV o n A ug us t 2 6, 2 02 1 at 1 6: 18 :1 2 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. surface hydrogen atoms. Arguably, a precursor-mediated reaction could also occur, where a mobile, physisorbed CO2 molecule reacts with Pt−H to form HOCO and onward to CO(ads). This third mechanism could be viewed as an L−H mechanism where the surface CO2 acts like a 2-D gas at the surface temperature. For these three different reaction mechanisms, different reaction cross sections are expected, especially when a “late” barrier is present. Moreover, the fate of the reaction products from these mechanisms would be expected to differ due to their unique translational/vibrational energy distributions. Therefore, it is expected to be difficult to switch between different reaction channels due to complexities in the reaction dynamics on the multidimensional potential energy surfaces. Atomic layer deposition (ALD) has an excellent ability to prepare nanostructures with nanometer/subnanometer precision, which carries advantages for preparing and modifying catalysts at an atomic level. With precise control over coverages, nanoparticle sizes, and the ability to introduce specific adsorption sites, it is of interest to determine whether ALD layers may modulate reaction channels in a desirable way. In this study, we demonstrate that a submonolayer overcoating of TiO2 or Al2O3 on Pt/SiO2, achieved by ALD, is adequate to change the reaction mechanism for surface carbonyl formation in the CO2 + H2 reaction, which ultimately influences the catalytic activity and selectivity at elevated temperatures. ■ EXPERIMENTAL METHODS Fumed SiO2 nanoparticles (AEROSIL OX 50, 50 m /g), were used for the support, as received. Nonporous fumed silica was specifically chosen to minimize transport limitations and to minimize the adsorption of CO2 and formation of carbonates on the support. Pt nanoparticles (NPs) were prepared on fumed SiO2 via ALD using MeCpPtMe3 and ozone following our previously published procedure. Pt loading and the Pt NP sizes were tuned by changing the number of ALD cycles, and Pt NPs samples prepared with 1, 3, and 10 cycles of ALD are named 1c-Pt/SiO2, 3c-Pt/SiO2, and 10c-Pt/SiO2, respectively. The three materials had average NP sizes of 1.3, 1.6, and 4.3 nm, respectively, (see Figures S1 and S2) as determined (see below) by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Total Pt surface areas were estimated as 0.004, 0.025, and 0.103 m/g, respectively. TiO2 and Al2O3 were deposited onto Pt/SiO2 by ALD by using one cycle of titanium isopropoxide (TTIP) and trimethylaluminum (TMA) as the metal precursors, respectively. The O precursor was deionized H2O in both cases, and a deposition temperature of 150 °C was used. By inductively coupled plasma (ICP) elemental analysis, these conditions deposited 4.0 Al atoms/nm and 2.5 Ti atoms/nm, consistent with prior reports. For overcoating on 10c-Pt/SiO2, this gives total atomic ratios of Al:Pt = 1.85:1 and Ti:Pt = 1.17:1. Because the SiO2 surface area is much greater than the Pt surface area (50 m/g vs >0.1 m/g), the vast majority of the Al2O3 or TiO2 is deposited on the SiO2 support. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed by using a Thermo Nicolet 6700 FTIR in the REACT core facility at Northwestern University. For a typical experiment, glass wool was loaded into a Praying Mantis IR cell, and 15−20 mg of Pt/SiO2 was packed on top of the wool. For a typical run, the catalysts were initially raised to ∼250 °C in flowing (30 sccm) 5% O2/Ar (UHP) to remove any residual surface carbonyls. IR spectra were measured continuously until the spectrum stabilized, indicating the removal of adsorbed CO. Spectra were recorded continuously every 9 s. Procedures for a typical experimental sequence were as follows: (1) the cell was equilibrated to the desired temperature in flowing Ar (UHP, 100 sccm), and a background spectrum was obtained; (2) 5 sccm CO2 was added to the Ar; (3) 30 sccm of 5% H2/Ar was added to the CO2/Ar; (4) the gas was switched to 1% CO/N2 (30 sccm); and (5) the gas flow was switched back to 100 sccm Ar. The catalytic activity for CO2 hydrogenation was determined in a separate experiment using a static, batch reactor employing transmission IR spectroscopy to detect the gas phase reaction products, as described elsewhere. TEM samples were prepared by drop-casting a Pt−SiO2/ isopropyl alcohol dispersion onto holey carbon TEM grids. Grids were air-dried and then exposed to UV radiation in a vacuum for 10 min at 40% power in a ZONESEM Cleaner (Hitachi, Japan) to reduce carbon contamination arising from the dispersion. HAADF-STEM was performed on a JEOL ARM300F Grand ARM at an accelerating voltage of 300 kV (Figure S1). Collection angles in the range 90−220 mrad were used. HAADF is an incoherent imaging technique, measuring the intensity (I) of electrons scattered to an annular detector at each pixel. The brightness of an image is proportional to the mass and the thickness of the specimen. The high collection angle means phase and diffraction contrast are generally excluded, making image interpretation relatively straightforward. Particle sizes were measured by using 50 particles from across several images in Digital Micrograph (Gatan, USA) by determining the onset of contrast changes around a particle in a HAADF image (Figure S2). Pt is much heavier than Si and O and appears much brighter in a HAADF image. ■ RESULTS AND DISCUSSION CO2 + H2 on 1c-, 3c-, and 10c-Pt/SiO2 at 25 °C. The temporal evolution of the surface carbonyl species (Pt−CO) from the reaction of CO2 + H2 was monitored, in situ, via DRIFTS at 25 °C for the three different sizes of Pt NPs. As described in the Experimental Methods section, the reactant gas was introduced in the following order: (1) CO2/Ar → (2) H2/CO2/Ar → (3) CO/N2 → (4) Ar, as indicated at the top of the panels in Figures 1a−c. Identical dosing procedures and durations were used in each time-dependent DRIFTS study. Figures 1a−c provide a visual depiction of the IR spectra vs time with the absorbance color-coded as shown in the scale at the right of each panel. Note that the absorbance range increases in the sequence 1(a) < 1(b) < 1(c), corresponding to the amount of CO detected. Upon CO2 dosing at room temperature, there are no detected bands in the 1500−1900 cm−1 region of the DRIFTS spectra that would correspond to surface carbonate, bicarbonate, or formate species, confirming the weak interaction of CO2 with both Pt nanoparticles and fumed SiO2 support. Adding H2 to CO2/ Ar did not produce any notable surface carbonyls or other surface species, regardless of the size of Pt NPs. Switching to CO(g) revealed the presence of previously unoccupied CO adsorption sites on Pt via the infrared absorption features between 2075 and 2090 cm−1, assigned to linear-bound CO. CO2 + H2 on 1c-, 3c-, and 10c-Pt/SiO2 at Increasing Temperature. Increasing the surface temperature is found to The Journal of Physical Chemistry C pubs.acs.org/JPCC Article https://doi.org/10.1021/acs.jpcc.1c04972 J. Phys. Chem. C XXXX, XXX, XXX−XXX B promote the formation of surface