Study on the Effect of K Promoter on the CO₂ Hydrogenation Performance of CoCu/ZrMgOx Catalysts

After a gel was formed, it was transferred to an oven at 100 oC and dried overnight. The dried catalyst was thoroughly ground into a powder, transferred to a muffle furnace, and calcined at 500 oC for 3 h.Catalyst characteration: The crystal structures of various catalyst samples were characterized by powder X-ray diffraction (XRD). XRD measurements were performed on a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation (40 kV，40 mA) at a scanning rate of 2o min-1 over a 2θ range of 5o -80o.The specific surface areas and pore size distributions of the different catalysts were determined by nitrogen adsorption at -196 oC using a Micromeritics TriStar II 3020 instrument.

The samples were first degassed at 250 oC for 8 h. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method using the adsorption data in the relative pressure (p/p0) range of 0.05-0.25, while the pore volumes and pore size distributions were determined by the t-plot method and the Barrett–Joyner–Halenda (BJH) method, respectively.CO2 or CO temperature-programmed desorption (CO2-TPD or CO-TPD) experiments were performed on a Micromeritics AutoChem II 2920 instrument according to the following procedure. 50 mg of catalyst was loaded into a U-shaped reactor tube and pretreated at 300 oC in a He flow for 1 h.

After the sample was cooled to 30 oC, CO2 or CO was introduced into the tube to achieve saturated adsorption. Subsequently, the catalyst was purged with a He flow for 30 min. Finally, the temperature was raised from 30 to 450 oC at a rate of 10 oC ∙min-1, and the desorbed CO2 or CO was monitored by a thermal conductivity detector (TCD).Hydrogen–deuterium (H2–D2) exchange reactions were carried out in a micro fixed-bed reactor. 100 mg of CoCu/ZrMgOx or CoCuK2/ZrMgOₓ catalyst was loaded into a U-shaped reactor tube and pretreated at 500 oC under a H₂ flow (30 mL∙min-1) for 2 h.

After cooling to 30 oC in an Ar flow, H2 (10 mL∙min-1) and D₂ (10 mL∙min-1) were introduced into the reactor, with Ar as the carrier gas (60 mL∙min-1), and the temperature was raised from room temperature to 500 oC at a rate of 5 oC ∙min-1. The products in the effluent were analyzed by an online mass spectrometer (Hiden Analytical HPR-20 R&D).In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded on a Bruker Vertex 80 spectrometer.

Approximately 30 mg of K0 or K2 sample was first pretreated in a H2 flow (30 mL∙min-1) at 500 oC for 2 h. After cooling to 30 oC in the H2 flow, a background spectrum was recorded. Subsequently, H2 (30 mL∙min-1) and CO2 (10 mL∙min-1) were introduced, and the temperature was raised from 30 to 500 oC at a rate of 4.7 oC ∙min-1.

IR spectra were collected at intervals of 20 oC during the heating process at a resolution of 4 cm-1. For time-dependent in situ IR measurements at the same temperature, the same procedure was adopted: the sample was first pretreated at 500 oC C in a 30 mL∙min-1 H2 flow for 2 h, and after cooling to 450 oC, a background spectrum was collected. Then H2 (30 mL∙min-1) and CO2 (10 mL∙min-1) were introduced into the in situ reaction cell, and IR spectra were obtained by accumulating 60 scans at a resolution of 4 cm-1.

By comparing the IR absorption band intensities of the intermediate species formed on each catalyst (normalized by sample mass), their relative amounts could be roughly estimated; the higher the normalized IR band intensity, the greater the amount of intermediates formed.Catalyst evaluation: he catalytic performance of CoCuKy/ZrMgOx catalysts with different K contents in the reverse water-gas shift (RWGS) reaction was evaluated in a fixed-bed reactor at constant pressure.

A fixed amount of catalyst was placed in the middle of a stainless-steel reactor tube with an inner diameter of 10 mm and then reduced at 500 oC for 2 h under a H2 flow of 30 mL∙min-1. After the temperature was lowered to 400 oC, a mixture of H2/CO2 with a molar ratio of 3:1 was fed into the reactor at a total flow rate of 40 mL∙min-1 and a gas hourly space velocity (GHSV) of 12000 mL∙g-1∙h-1. The gaseous products were analyzed online by an Agilent 7890A gas chromatograph equipped with one thermal conductivity detector (TCD), two flame ionization detectors (FID), and two capillary columns (J&W 127-7031, 30 m×530 μm×0.25 μm; Agilent 19095P-S25, 0 m×530 μm×15 μm).