Room-Temperature CO2 Hydrogenation to Methanol over Air-Stable hcp-PdMo Intermetallic Catalyst

CO2 hydrogenation to methanol is one of the most promising routes to CO2 utilization. However, difficulty in CO2 activation at low temperature, catalyst stability, catalyst preparation, and product separation are obstacles to the realization of a practical hydrogenation process under mild conditions. Here, we report a PdMo intermetallic catalyst for low-temperature CO2 hydrogenation. This catalyst can be synthesized by the facile ammonolysis of an oxide precursor and exhibits excellent stability in air and the reaction atmosphere and significantly enhances the catalytic activity for CO2 hydrogenation to methanol and CO compared with a Pd catalyst. A turnover frequency of 0.15 h–1 was achieved for methanol synthesis at 0.9 MPa and 25 °C, which is comparable to or higher than that of the state-of-the-art heterogeneous catalysts under higher-pressure conditions (4–5 MPa).


Preparation of catalysts
h-PdMo, h-PdMo/Mo 2 N, and Mo 2 N catalysts were prepared via ammonolysis of an oxide precursor. Oxide precursors were prepared by the Pechini method using (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (81-83% MoO 3 basis, Aldrich) and Pd(CH 3 COO) 2 (98%, TCI) as Mo and Pd sources, respectively. Mo and Pd sources in predetermined ratios and citric acid at twice the amount of the total metal ions were dissolved in 6% aqueous HNO 3 at room temperature. The solution mixture was placed in a heating mantle at 80 °C, stirred, and evaporated until a transparent gel formed. The temperature was then raised to 200 °C and the gel was converted to an amorphous precursor, which was calcined at 500 °C for 2 h in air to obtain the oxide precursor. Finally, ammonolysis of the oxide precursor was performed at 700 °C in a flow of NH 3 (10 mL min −1 ) for 12 h. The resultant material with Pd/Mo=1.08 is referred to as h-PdMo, whereas that with Pd/Mo<1.08 is named as h-PdMo/Mo 2 N because PdMo intermetallic nanoparticles are formed on Mo 2 N as shown in Figures 2a-e. Mo 2 N was prepared from Mo-oxide without Pd by the same procedure as h-PdMo. To examine the effect of Pd precursor, h-PdMo/Mo 2 N (PdMo=0.05) was also synthesized by using Pd(NH 3 ) 4 Cl 2 ·H 2 O (98%, Aldrich) as the Pd source. For Pd/Mo 2 N catalyst, Pd(acac) 2 (99%, Aldrich) was used as a Pd precursor, which was mixed with Mo 2 N in an agate mortar. The mixture was then heated at 300 °C in a flow of H 2 (10 mL min −1 ) for 2 h with a heating rate of 2 °C min −1 .
The copper-based methanol synthesis catalyst (Cu/ZnO/Al 2 O 3 pellets, Alfa Aesar) was obtained as a commercially available product. Cu/ZnO/Al 2 O 3 pellets were hand-milled in an agate mortar and used as a powder. All catalysts were reduced by H 2 at 300 °C for 2 h with a heating rate of 2 °C min −1 before the CO 2 hydrogenation reaction.

Catalyst characterization
The crystal structure was identified using X-ray diffraction (XRD; MiniFlex600, Rigaku or D2 PHASER, Bruker) with Cu Kα radiation (λ = 0.15418 nm). The composition of the PdMo catalyst (Pd/Mo = 1.08) was determined as the average of measurements taken at 50 random points using an electron probe micro analyzer (EPMA; JXA-8530F, Jeol). The amount of Mo and Pd in the catalysts was estimated from inductively coupled plasma atomic emission spectroscopy (ICP-AES; ICPS-8100, Shimadzu) measurements. The morphology and elemental distribution of a single particle of catalyst were evaluated using field-emission scanning electron microscopy (FE-SEM; JSM-7600F, Jeol) with energy-dispersive X-ray spectroscopy (EDX).
Temperature-programmed desorption (TPD) of N 2 was conducted by heating (10 °C min −1 ) a sample in an Ar stream, and the desorbed gas was monitored with a mass spectrometer (BELMass, MicrotracBEL). TPD of CO was performed using the same instrument as TPD of N 2 . Prior to measurements, the samples (ca. 30 mg) were reduced under an H 2 flow (10 mL min −1 ) at 300 °C for 2 h. After cooling to room temperature with flowing Ar, CO adsorption was conducted in a stream of 10 vol% CO/He at 35 °C.
Then, the sample was heated (5 °C min −1 ) in a stream of Ar and the desorbed gas was monitored with a mass spectrometer (BELMass, MicrotracBEL).
Temperature-programmed reaction (TPR) of H 2 was conducted by heating ( hydrogenation, the samples were reduced by H 2 at 300 °C for 2 h. After the sample was cooled to room temperature, a mixed gas (CO 2 :H 2 = 1:3, 20 mL min −1 ) was supplied to the chamber, and measurements were conducted at room temperature.

Supplementary Note 1: Stabilization of hcp-PdMo intermetallic phase
According to the equilibrium diagram of the Pd-Mo system, an intermetallic compound with the hexagonal close-packed (hcp) structure exists in the range of 50-60 at% Pd. 1 The XRD pattern for this phase (ICSD No. 105061) is very close to that of the present sample (Pd = 52 at%) ( Figure S1). In the XRD pattern of this sample, peaks except for the 002 plane are shifted to lower angle, which indicates distortion and expansion of the lattice due to the insertion of anions into the interstitial sites, as confirmed by compositional analysis (Table S1). HAADF-STEM observation revealed that Pd and Mo were distributed in alternating layers perpendicular to the C-axis direction (Figure 1b and S2). Such an ordered structure is considered to be responsible for the anisotropic lattice expansion. The PdMo phase containing anions is referred to as h-PdMo and is distinguished from the PdMo intermetallic without anions. The PdMo intermetallic generally decomposes into Mo (body-centered cubic (bcc) structure) and Pd (face-centered cubic (fcc) structure) below 1450 °C. Therefore, it is impossible to synthesize or stabilize this phase at lower temperatures. In our experiments, we speculate that the insertion of anions into the interstitial sites during the ammonolysis process enabled the synthesis and stabilization of this phase at lower temperatures (≤700 °C). N 2 desorption temperature from h-PdMo was lower than that from Mo 2 N ( Figure S3), which suggests that nitrogen is surrounded by not only Mo but also Pd.
When the anions were desorbed by TPD, the h-PdMo sample decomposed into Mo and Pd, as shown in Figure S4.          Figure S15 GC-MS spectra of (a) 13 CH 3 OH obtained from 13 CO 2 hydrogenation, and (b) 12 CH 3 OH as a reference. Hydrogenation of 13 CO 2 (purity 99%) to 13 CH 3 OH was conducted using a 25 mL stainless steel autoclave equipped with a manometer. The autoclave was flushed five times with H 2 , and then the mixture of 13 CO 2 and H 2 gases (total pressure: 1 MPa, 13 CO 2 :H 2 = 1:3) was introduced into the reaction system. The reaction was performed with h-PdMo catalyst (ca. 400 mg) at 35 °C for 24 h. After the reaction, the pressurized gases were introduced into the water, and the 13 CH 3 OH trapped in the water was analyzed by GC-MS (GCMS-QP2020 NX, Shimadzu).   Selectivity of x species (%) = 100 × F x,out / (F methanol,out + F CO,out + F CH4,out ) (1) where F x,out is the outlet flow rate of products (ml min     [11] Pd/SiO 2 250 2 0.06 † [11] Pd/AC 250 2 0.11 † [11] * TOF calculated from the rate of methanol synthesis divided by the total metal sites. The total metal sites was estimated from the BET surface area, assuming that all surfaces are metal atoms (Pd:Mo = 6:4). † TOF calculated from the rate of methanol synthesis divided by total metal (Ir, Cu, or Pd) sites. ‡ TOF calculated based on a method in the literature. 9 The highest and lowest TOFs were calculated on the basis of the amount of exposed sulfur vacancies or Mo atoms. The average value of the highest and lowest TOFs was then calculated as the TOF of the FL-MoS 2 catalyst.
where F CO2,in is the inlet flow rate of CO 2 (ml min −1 ) and F x,out is the outlet flow rate of products (ml min −1 ). The parentheses indicate the CO 2 conversion determined by eq 3.
CO 2 conversion (%) = 100 × (F CO2,in − F CO2,out ) / F CO2,in The values of the CO 2 conversion obtained from both equations are comparable, and the mass balances for carbon generally fulfilled.