Noble metal-promoted IN2O3 catalyst for the hydrogenation of CO2 to methanol

Abstract

Method to prepare a catalyst for use in a process for the synthesis of methanol, comprising indium oxide in the form of In.sub.2O.sub.3, and at least one additional metal selected from a noble metal; and in that the average particle size of said noble metal phase is, preferably at least 0.05 nm, and less than 5 nm as determined by STEM-EDX, characterized in that the catalyst is prepared by co-precipitation of a saline solution at a pH above 8.5 comprising an indium salt and a salt of the at least one additional metal selected from a noble metal and optionally further comprising a salt of the at least one alkaline earth metal.

Claims

1. A Method to prepare a catalyst for use in a process for the synthesis of methanol, comprising indium oxide in the form of In.sub.2O.sub.3, and at least one additional metal selected from a noble metal; and in that the average particle size of said noble metal phase is, at least 0.05 nm, and less than 5 nm as determined by STEM-EDX, characterized in that the catalyst is prepared by co-precipitation of a saline solution at a pH above 8.5 comprising an indium salt and a salt of the at least one additional metal selected from a noble metal and optionally further comprising a salt of the at least one alkaline earth metal; comprising a further calcination step of the catalyst, wherein the calcined catalyst obtained comprised from 0.01 to 1.0 wt. % of the additional noble metal based on the total weight of the calcined catalyst and wherein the catalyst content of indium oxide in the form of In.sub.2O.sub.3 based on the calcined catalyst is ranging from 60 to 99.99 wt. %.

2. The method according to claim 1, characterized in that the co-precipitation is performed at a pH above 9; and at a temperature of at least 293 K (19.85 C.).

3. The method according to claim 1, characterized in that the catalyst is a calcined catalyst, and in that the method comprises a step of calcination of the catalyst performed at a temperature of at least 473 K (199.85 C.).

4. The method according to claim 1, characterized in that said catalyst further comprises at least one alkaline earth metal.

5. The method according to claim 1, characterized in that said catalyst further comprises at least one alkaline earth metal being incorporated simultaneously with said indium salt and a said salt of the at least one additional metal selected from a noble metal at the co-precipitation stage.

6. The method according to claim 1 characterized in that said In.sub.2O.sub.3 is present in the form of particles having an average crystal size of less than 20 nm as determined by XRD.

7. The method according to claim 1 characterized in that said In.sub.2O.sub.3 is present in the form of particles having an average crystal size of less than 10 nm as determined by XRD.

8. The method according to claim 1, characterized in that said at least one additional metal is a noble metal selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), platinum (Pt), copper (Cu), gold (Au), iridium (Ir), and any combination thereof.

9. The method according to claim 1, characterized in that said at least one additional metal is a noble metal selected palladium (Pd) and/or platinum (Pt).

10. The method according to claim 1, characterized in that the average particle size of the noble metal phase obtained on said catalyst is less than 4 nm as determined by STEM-EDX.

11. The method according to claim 1, characterized in that the average particle size of the noble metal phase obtained on said catalyst is less than 2 nm as determined by STEM-EDX.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the X-ray diffraction (XRD) patterns of PdIn.sub.2O.sub.3 samples prepared by dry impregnation and co-precipitation in fresh form and after use in the reaction.

(2) FIG. 2 shows the results of the analysis by scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (STEM-EDX) of PdIn.sub.2O.sub.3 samples prepared by dry impregnation and co-precipitation in fresh form and after use in the reaction.

(3) FIG. 3 shows the X-ray photoelectron (XPS) spectra of the materials prepared by co-precipitation and dry impregnation in fresh form and after use for 16 h in the reaction.

(4) FIG. 4 shows the evolution of the space-time yield of methanol over time-on-stream for the materials prepared by co-precipitation and dry impregnation.

(5) FIG. 5 is an extended stability test under optimized reaction conditions of the catalyst prepared by co-precipitation.

(6) FIG. 6 depict the temperature-programmed reduction with hydrogen (H.sub.2-TPD) analysis that was performed at 5.0 MPa.

DETAILED DESCRIPTION OF THE INVENTION

(7) As used herein the generic term catalyst refers to both a bulk and a supported catalyst. A bulk catalyst is a catalyst containing the additional metal (i.e., the alkali and/or the noble metal) without its support (in this case the indium oxide). A co-precipitated catalyst is a catalyst wherein the active phase is intimately mixed with the support, in contrast with spray deposition techniques and impregnation techniques wherein the active phase is deposited on the support. Although impregnation is one of the easiest methods for producing a catalyst, it has been found that the homogeneity of product, especially for high metal loading, and the reproducibility of this process are better when a co-precipitation strategy is applied. The nature of the interaction of the material components is different between impregnated material and co-precipitated materials.

(8) In methanol synthesis according to the invention, a feed gas composed of hydrogen gas and carbon oxides (CO.sub.2 alone or a mixture of CO.sub.2 and CO gases) are caused to interact on an indium oxide-based catalyst produced by co-precipitation with a noble metal.

(9) The noble metals are metals resistant to corrosion and oxidation and are selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), platinum (Pt), copper (Cu), gold (Au) and iridium (Ir). In a preferred embodiment of the invention, the catalyst is devoid of gold (Au).

(10) The alkaline earth metals are selected from beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

(11) Catalyst and Preparation of the Catalyst

(12) The present invention contemplates the use of a catalyst in a process for the synthesis of methanol, wherein the catalyst comprises indium oxide in the form of In.sub.2O.sub.3, at least one additional noble metal selected from a noble metal and optionally at least one alkaline earth metal; further wherein the average particle size of the one or more noble metal phase is, preferably at least 0.05 nm, less than 5 nm as determined by STEM-EDX.

(13) According to the invention, the fresh catalyst exhibits the additional noble metal in oxidized form after synthesis and in reduced form under reaction conditions as determined by XPS.

(14) In a preferred embodiment, the catalyst comprises indium oxide in the form of In.sub.2O.sub.3, at least one additional noble metal selected from a noble metal, and optionally at least one alkaline earth metal; further wherein the average particle size of the one or more noble metal phase is less, preferably at least 0.05 nm, than 5 nm as determined by STEM-EDX; In.sub.2O.sub.3 is present in the form of particles having an average crystal size of less than 20 nm as determined by XRD.

(15) In an embodiment, the In.sub.2O.sub.3 is present in the form of particles having an average crystal size of less than 20 nm as determined by XRD; preferably, the average crystal size of In.sub.2O.sub.3 is less than 15 nm; more preferably less than 12 nm; and even more preferably, less than 10 nm.

(16) The catalyst is a catalyst prepared by co-precipitation, an average particle size of less than 5 nm, preferably less than 2 nm, for both the noble-metal and the optional alkaline-earth metal after deactivation of the catalyst, allow to differentiate it from the catalysts wherein the noble metal is deposited on the support by impregnation or deposition techniques. Thus, is an embodiment, the noble metal average particle size after use of the catalyst is less, preferably at least 0.05 nm, than 5 nm as determined by STEM-EDX. To the contrary, when the catalyst is prepared by impregnation or deposition techniques the noble metal particle will agglomerate under reaction conditions, and therefore the noble metal average particle size will be larger after use than before use.

(17) According to an embodiment, the catalyst exhibits an initial signal in temperature-programmed reduction with H.sub.2 at 5.0 MPa of less than 300 K.

(18) Based on the nature of the synthesis strategy as well as XRD, XPS, and STEM-EDX analyses, it is conceivable that the noble metal is homogeneously distributed (possibly even atomically) through the bulk of In.sub.2O.sub.3 for the material prepared by co-precipitation, whereas it is present in the form of highly dispersed clusters for materials prepared by impregnation or other deposition techniques. STEM-EDX analyses is preferred to determine the size of crystals and/or particles lower than 10 nm.

(19) In a preferred embodiment, the average size of the noble metal phase is less, preferably at least 0.05 nm, than 5 nm as determined by STEM-EDX, preferably less than 4 nm, more preferably less than 2 nm.

(20) In an embodiment, the at least one noble metal is selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), platinum (Pt), copper (Cu), gold (Au), iridium (Ir), and any combination thereof; preferably a noble metal selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), platinum (Pt), copper (Cu), and any combination thereof; more preferably, a noble metal selected from ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), platinum (Pt), and any combination thereof; even more preferably, a noble metal selected from palladium (Pd) and/or platinum (Pt), and most preferably the noble metal is palladium (Pd)

(21) The catalyst is preferably a calcined catalyst. This feature can be evidenced by the loss of the indium hydroxide form and formation of the pattern attributed to an indium oxide form observed in the X-ray diffraction.

(22) In an embodiment, the catalyst is a calcined catalyst and comprises from 0.01 to 10 wt. % of the at least one additional noble metal based on the total weight of the calcined catalyst.

(23) With preference, the catalyst is a calcined catalyst and comprises at least 0.1 wt. % of the at least one additional noble metal based on the total weight of the calcined catalyst, preferably at least 0.3 wt. %, more preferably at least 0.5 wt. %, even more preferably at least 0.6 wt. %, and most preferably at least 0.7 wt. %.

(24) With preference, the catalyst is a calcined catalyst and comprises at most 10.0 wt. % of the at least one additional noble metal based on the total weight of the calcined catalyst, preferably at most 7.0 wt. %, more preferably at most 5.0 wt. %, even more preferably at most 2.0 wt. %, and most preferably at most 1.0 wt. %.

(25) When present, the at least one alkaline earth metal is selected from beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), and any combination thereof.

(26) With preference, the catalyst is a calcined catalyst and comprises at least 0.1 wt. % of at least one alkaline earth metal based on the total weight of the calcined catalyst, preferably at least 0.3 wt. %, more preferably at least 0.5 wt. %, even more preferably at least 0.6 wt. %, and most preferably at least 0.7 wt. %.

(27) With preference, the catalyst is a calcined catalyst and comprises at most 10.0 wt. % of at least one alkaline earth metal based on the total weight of the calcined catalyst, preferably at most 7.0 wt. %, more preferably at most 5.0 wt. %, even more preferably at most 2.0 wt. %, and most preferably at most 1.0 wt. %.

(28) In an embodiment, the catalyst is a calcined catalyst and the indium oxide content in the form of In.sub.2O.sub.3 based on the calcined catalyst is ranging from 50 to 99.99 wt. %, preferably from 93 to 99.5 wt. %.

(29) With preference, the catalyst is a calcined catalyst and comprises at least 60 wt. % of indium oxide based on the total weight of the calcined catalyst, preferably at least 70 wt. %, more preferably at least 80 wt. %, even more preferably at least 90 wt. %, and most preferably at least 95 wt. %.

(30) With preference, the catalyst is a calcined catalyst and comprises at most 99 wt. % of indium oxide based on the total weight of the calcined catalyst, preferably at most 98.5 wt. %, more preferably at most 98 wt. %, and even more preferably at most 97.5 wt. %.

(31) According to the invention, the catalyst is prepared by co-precipitation performed at a pH above 8.5 of a saline solution comprising an indium salt and a salt of the at least an additional metal selected from a noble metal and optionally further comprising a salt of the at least one alkaline earth metal.

(32) With preference, the catalyst is prepared by co-precipitation of a saline solution comprising In(NO.sub.3).sub.3.xH.sub.2O and a salt of the at least additional metal selected from a noble metal and optionally further comprising a salt of the at least one alkaline earth metal.

(33) In a preferred embodiment the noble metal is palladium. With preference, the salt is Pd(NO.sub.3).sub.2.

(34) With preference, the co-precipitation is above 9; and at a temperature of at least 293 K (19.85 C.).

(35) In a preferred embodiment, the catalyst is a calcined catalyst, and the method comprises a step of calcination of the catalyst performed after the co-precipitation step. The calcination is preferably performed at a temperature of at least 473 K, with preference of at least 573 K.

(36) Hydrogenation of Carbon Dioxide to Methanol

(37) In methanol synthesis according to the invention, a feed gas composed of hydrogen gas and carbon oxides (CO.sub.2 alone or a mixture of CO.sub.2 and CO gases) are caused to interact on an indium oxide-based catalyst.

(38) The invention provides a process for methanol synthesis comprising the following steps: providing a feed stream comprising hydrogen and carbon oxides selected from carbon dioxide or a mixture of carbon dioxide and carbon monoxide, wherein carbon dioxide represents from 1 to 50 mol % of the total molar content of the feed stream, carbon monoxide is contained from 0 to 85 mol % of the total molar content, and H.sub.2 is comprised from 5 to 99 mol % of the total molar content of the feed stream; providing a catalyst comprising indium oxide in the form of In.sub.2O.sub.3, and at least one additional metal selected from a noble metal; and in that the average particle size of the one or more noble metal phase is, preferably at least 0.05 nm, less than 5 nm as determined by STEM-EDX; putting in contact said feed stream with said catalyst at a reaction temperature of at least 373 K (99.85 C.) and under a pressure of at least 0.5 MPa; and recovering the methanol from the effluents by a separation process.

(39) The process can be carried out in a gaseous phase or in a liquid phase. The solvent that can be used for the reaction in liquid phase includes hydrocarbons and other solvents which are preferably insoluble or only sparingly soluble in water. Preferably, the process is carried out in a gaseous phase.

(40) Prior to reaction the catalyst is activated in situ by raising the temperature to at least 553 K (279.15 C.) in a flow of a gas feed stream for activation selected from inert gases, hydrogen, carbon monoxide, carbon dioxide, or a mixture thereof, preferably the gas feed stream for activation is or comprises an inert gas, more preferably the gas feed stream for activation is or comprises nitrogen.

(41) The process is carried out in a reactor comprising: lines to introduce a feed stream to the reactor and remove products from the reactor; a device for heating the reactor; a temperature sensor and controller for detecting and controlling the temperature of the reactor at a reaction temperature chosen between 373 K (99.85 C.) and 773 K (499.85 C.) flow controllers to control the rate of the feed stream to the reactor; and a pressure controller to control the reactor pressure in order to set it at a pressure of at least 0.5 MPa.

(42) In accordance to the invention, the feed stream comprises hydrogen and carbon oxides selected from carbon dioxide (CO.sub.2) or a mixture of carbon dioxide and carbon monoxide. However, in a preferred embodiment, the feed stream comprises hydrogen and carbon dioxide.

(43) When the feed stream comprises hydrogen and a mixture of carbon dioxide and carbon monoxide, the feed stream can be CO-rich or CO.sub.2-rich. In accordance to the invention, CO.sub.2-rich feed stream contains more than 30 mol % of CO.sub.2 based on the total molar content of the carbon oxide. In a preferred embodiment of the invention, the feed stream is CO.sub.2-rich.

(44) The feed stream comprises CO.sub.2 and H.sub.2, or H.sub.2 and a mixture of CO.sub.2 and CO, preferably the feed stream may also comprise a further gaseous component such as an inert gas. The inert gas is for example nitrogen.

(45) In a preferred embodiment, the molar ratio of hydrogen to carbon dioxide in the feed stream is at least 1:1, preferably at least 3:1, more preferably at least 4:1, even more preferably at least 6:1; and/or the molar ratio of hydrogen to carbon dioxide in the feed stream is at most 12:1.

(46) In a preferred embodiment, the feed stream contains hydrogen and carbon oxides selected from carbon dioxide or a mixture of carbon dioxide and carbon monoxide and the feed stream comprises at least 10 mol % of H.sub.2 based on the total molar content of the feed stream, preferably at least 20 mol %, more preferably at least 30 mol %.

(47) In a preferred embodiment the feed stream contains hydrogen and carbon oxides selected from carbon dioxide or a mixture of carbon dioxide and carbon monoxide and the feed stream comprises at most 90 mol % of H.sub.2 based on the total molar content of the feed stream, preferably at most 80 mol %, more preferably at most 70 mol %, even more preferably at most 60 mol %.

(48) In a preferred embodiment, the process is carried out at a reaction temperature of at least 463 K (189.85 C.), preferably of at least 563 K (289.85 C.), more preferably of at least 663 K (389.85 C.).

(49) In another preferred embodiment, the pressure is at least 1 MPa, preferably at least 2 MPa, more preferably at least 3 MPa, even more preferably at least 4 MPa and most preferably at least 5 MPa.

(50) In a preferred embodiment, the weight hourly space velocity (WHSV) is in the range of 1,000 to 100,000 cubic centimeters at standard temperature and pressure (STP) of reactant gases per gram of catalyst charged to the reactor per hour, preferably 2,000 to 50,000 cm.sup.3.sub.STP g.sub.cat h.sup.1, more preferably 5,000 to 40,000 cm.sup.3.sub.STP g.sub.cat h.sup.1, and more preferably 15,000 to 40,000 cm.sup.3.sub.STP g.sub.cat h.sup.1.

(51) In a preferred embodiment the process can be carried out with a stable performance with respect to activity and selectivity during more than 100 h, preferably more than 1,000 h, more preferably more than 10,000 h, and even more preferably more than 100,000 h without the need of reactivation or replacement of the catalyst.

(52) In an embodiment, the process is carried out in a fixed-bed or in a fluidized-bed reactor comprising at least one catalytic bed. Such reactors are well-known from the person skilled in the art and for instance described in EP2257366 or in U.S. Pat. No. 7,279,138.

Test Methods and Definitions

(53) Activity for methanol synthesis reaction is determined using a home-made fixed-bed reactor set-up, which has been described in detail previously (M.S. Frei et al. J. Catal., 2018, 361, 313-321). Briefly, it comprises a high-pressure continuous-flow fixed-bed reactor with an inner diameter of 2.1 mm surrounded by an electric furnace. The reactor was loaded with 100 mg of catalyst with a particle size of 100-125 m, which was held in place by a bed of quartz wool and was heated from ambient temperature to 553 K (5 K min.sup.1) at 5 MPa under a He flow of 20 cm.sup.3.sub.STP g.sub.cat min.sup.1. After 3 h, the gas flow was switched to the reactant mixture (40 cm.sup.3.sub.STP min.sup.1) comprising H.sub.2 and CO.sub.2 (Messer, 99.997% and 99.999%, respectively) in a molar ratio of 4:1. A constant flow (2.5 cm.sup.3.sub.STP min.sup.1) of 20 mol % CH.sub.4 in He (Messer, both 99.999%) was added to the effluent stream to serve as an internal standard. The effluent stream was sampled every 12 min and analyzed by an online gas chromatograph (GC, Agilent 7890A), equipped with two parallel columns (Agilent GS Gaspro and Agilent DB-1) connected to a flame ionization detector (FID) and a thermal conductivity detector (TCD), to determine the mol % content of the reactants H.sub.2, CO.sub.2, and CO in the feed stream and the mol % content of the reactants and the methanol product in the outlet stream.

(54) For each compound i, the response factor F.sub.i respective to the internal standard (CH.sub.4) was calculated by the following equation:

(55) $F_i=\frac{A_i/{\stackrel{.}{n}}_i^{in}}{A_{C{H}_4}/{\stackrel{.}{n}}_{C{H}_4}^{in}}$

(56) where A.sub.i is the integrated area determined for compound i by the GC and {dot over (n)}.sub.i.sup.in corresponds to its known adjusted molar flowrate. Each response factor was calculated as the average of 5 calibration points around the expected concentration of the respective analyte, i.

(57) Upon reaction the unknown effluent molar flowrate {dot over (n)}.sub.i.sup.out was determined by the following equation:

(58) ${\stackrel{.}{n}}_i^{\mathrm{out}}=\frac{A_i{F}_i}{A_{C{H}_4}}{\stackrel{.}{n}}_{C{H}_4}^{in}$

(59) CO.sub.2 conversion (X.sub.CO2), methanol selectivity (S.sub.MeOH) and yield (Y.sub.MeOH) in percent and methanol space-time yield (STY.sub.MeOH) were calculated applying the following equations:

(60) $X_{C{O}_2}=\frac{{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{in}-{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{\mathrm{out}}}{{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{in}}100S_{MeOH}=\frac{{\stackrel{.}{n}}_{MeOH}^{in}-{\stackrel{.}{n}}_{MeOH}^{\mathrm{out}}}{{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{in}-{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{\mathrm{out}}}100$$Y_{MeOH}=X_{C{O}_2}{S}_{MeOH}$${\mathrm{STY}}_{MeOH}=\frac{{\stackrel{.}{n}}_{MeOH}^{in}-{\stackrel{.}{n}}_{MeOH}^{\mathrm{out}}}{W_{\mathrm{cat}}}{M}_{MeOH}$

(61) where W.sub.cat is the weight of the loaded catalyst and M.sub.MeOH is the molar weight of methanol (32.04 g mol.sup.1).

(62) Data reported correspond to the average of the 4 measurements preceding a specific time-on-stream, or to the average of 7 measurements collected during each individual condition when temperature or gas flows were altered. The carbon loss in percent was determined for each experiment according to equation 4 and was found to be always less than 3%.

(63) ${\mathrm{.Math.}}_C=\frac{{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{\mathrm{out}}-{\stackrel{.}{n}}_{MeOH}^{\mathrm{out}}-{\stackrel{.}{n}}_{\mathrm{CO}}^{\mathrm{out}}}{{\stackrel{.}{n}}_{{\mathrm{CO}}_2}^{in}+{\stackrel{.}{n}}_{MeOH}^{in}}100$

(64) The absence of intra- and extraparticle diffusion limitations was corroborated by the fulfillment of the Weisz-Prater and Carberry criteria.

(65) Powder XRD analysis was performed using a PANalytical X'Pert Pro MPD instrument, utilizing CuK radiation (=0.1541 nm), an angular step size of 0.05 2 and a counting time of 12 seconds per step. The average crystal size of In.sub.2O.sub.3 was estimated from the (222) reflection applying the Scherrer equation.

(66) XPS analysis was performed in a Physical Electronics Instruments Quantum 2000 spectrometer using monochromatic Al K radiation generated from an electron beam operated at 15 kV and 32.3 W. The spectra were collected under ultra-high vacuum conditions (residual pressure=510.sup.8 Pa) at a pass energy of 46.95 eV. All spectra were referenced to the C 1s peak at 284.8 eV. Although samples were extracted from the reactor in inert atmosphere, the design of the instrument made a brief (<2 min) exposure to air upon sample introduction unavoidable.

(67) STEM-EDX measurements were performed using a Talos F200X instrument operated at 200 kV and equipped with a FEI SuperX detector.

(68) The metal composition of the calcined samples was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a Horiba Ultra 2 instrument equipped with photomultiplier tube detector. Prior to analysis, the catalysts were dissolved in aqua regia and the resulting solutions were diluted with twice-distilled water

(69) Specific surface area and pore volume were determined from the sorption isotherm of N2 at 77 K using a Micromeritics TriStar II analyzer. The Brunauer-Emmett-Teller (BET) method was applied for calculating the specific surface area according to ASTM D3663-03 and the volume of gas adsorbed at saturation pressure was used to determine the pore volume.

(70) Temperature-programmed reduction with H.sub.2 (H.sub.2-TPR) was carried out at the reaction pressure (5.0 MPa) in a Micromeritics AutoChem HP II analyser. 100 mg of catalyst was used for each analysis. A drying step in 100 cm.sup.3.sub.STP min.sup.1 Argon was carried out at 0.1 MPa between 303-393 K, at a heating rate of 5 K min.sup.1 and a hold time of 60 min at the final temperature. Thereafter, the temperature was lowered to 183 K at a rate of 5 K min.sup.1 and reduction with 5% H.sub.2 in Argon at a flow rate of 50 cm.sup.3.sub.STP min.sup.1 was carried out between 183-1103 K, with a heating rate of 5 K min.sup.1, at a pressure of 5.0 MPa, and a hold time of 30 min at the final temperature.

EXAMPLES

(71) The advantages of the present invention are illustrated in the following examples. However, it is understood that the invention is by no means limited to these specific examples.

Example 1Catalyst Synthesis

(72) Materials were prepared through a co-precipitation method. Various metals (Pd, Pt, Cu, Ag, Ru, Os) were precipitated with indium to enhance the intermixing among In.sub.2O.sub.3 and the noble metal in the working catalyst. The noble metal loading ranged from 0 to 5 wt. % based on the total weight of the calcined catalyst.

(73) Co-Precipitation (CP):

(74) An example of a catalyst containing 0.75 wt. % Pd is as follows: In(NO.sub.3).sub.3.xH.sub.2O (3.48 g, Sigma-Aldrich, 99.99%, x=6.9) and Pd(NO.sub.3).sub.2.xH.sub.2O (34.8 mg, Sigma-Aldrich, >99.99% metals basis, x=5.5) were dissolved in deionized water (50 cm.sup.3) in a round-bottomed flask. In a second vessel, a Na.sub.2CO.sub.3 solution was prepared by hydrolyzing Na.sub.2CO.sub.3 (10.0 g) in deionized water (100 cm.sup.3). 38.8 cm.sup.3 of the Na.sub.2CO.sub.3 solution were added dropwise (3 cm.sup.3 min.sup.1) to the solution of metal nitrates under magnetic stirring at ambient temperature to reach a pH value of 9.2. The resulting slurry was aged for 60 min. After adding deionized water (50 cm.sup.3), the precipitate was separated by high-pressure filtration, washed with deionized water (3 times, 500 cm.sup.3 each time), dried in a vacuum oven (1.5 kPa, 323 K, 1.5 h), and calcined in static air (573 K, 3 h, 2 K min.sup.1).

(75) For comparison, materials were also prepared through spray deposition and dry impregnation methods.

(76) Spray Deposition (SD):

(77) Palladium (nominal loading=0.5, 1, and 2 wt. %) was deposited onto In.sub.2O.sub.3 by means of spray deposition in a Bchi Mini Spray Dryer B-290. After priming the spray dryer with deionized water, a slurry containing 10 cm.sup.3 of deionized water, 1 g of In.sub.2O.sub.3, and 0.059, 0.119, or 0.179 g of a palladium nitrate solution (8.5 wt. % Pd in diluted nitric acid, ABCR-Chemicals) was fed to the dryer. The following parameters were set on the instrument: aspiration=80% (ca. 28 m.sup.3 h.sup.1), spray gas (air) flow=0.6 m.sup.3 h.sup.1 at 0.6 MPa, pump=10% (ca. 1 cm.sup.3 min.sup.1), inlet temperature=593 K, and nozzle cleaner=0. The sample was unloaded from the collector and calcined for 3 h at 373 K (2 K min.sup.1) in static air.

(78) Dry Impregnation (DI):

(79) A 25-cm.sup.3 round-bottomed flask was loaded with Pd(NO.sub.3).sub.2.xH.sub.2O (23.5 mg) and deionized water (0.39 g). In.sub.2O.sub.3 (1.00 g) and 5 stainless steel spheres (radius=3 mm) were added and the flask was rotated (ca. 45 rpm) using a Bchi R-114 rotation evaporator at ambient temperature and pressure. After 12 h, the pressure was lowered to 2 kPa and the temperature raised to 333 K for 1 h to allow the evaporation of the solvent. The thus obtained samples were calcined for 3 h at 373 K (2 K min.sup.1) in static air.

(80) Characterization data of the products are provided in Table 1 and FIGS. 1-3. FIG. 1: XRD of DI and CP samples of indium oxide with 0.75 wt. % Pd. Fresh catalysts are indistinguishable using XRD due to the overlap of peaks of the respective components and the small size of the Pd particles. The used materials correspond to the catalysts extracted from the reactor after the tests depicted in FIG. 4. The DI sample produced a reflection specific to Pd, which is absent for the used CP material.

(81) FIG. 2: STEM-EDX of the same samples as in FIG. 1 evidencing very high dispersion of the noble metal in both materials prior to use in the reaction. However, the used CP sample appears unaltered, while severe sintering (i.e., increase of particle size) is observed for the oxide as well as the noble metal phase in the DI material.

(82) FIG. 3: XPS Pd 3d core-level spectra of the CP and DI catalysts prior to and after use for 16 h in the reaction. Prior to the reaction, the CP material features oxidic Pd species which are quickly reduced to Pd.sup.0 upon exposure to the reaction environment. After this initial activation, the Pd 3d signal remains unaltered in shape and intensity for further 15 h on stream, as expected based on the observed stability of this material. Pd is almost three times more abundant on the surface of the catalyst prepared by DI (1.1 at. %) as compared to the CP sample (0.4 at. %). Since both materials possess the same bulk metal loading (0.75 wt. %), it is deduced that a significant quantity of Pd is incorporated in the bulk of the oxide crystals in this solid.

Example 2Catalyst Testing

(83) The reactor was loaded with 50 mg of catalyst with a crystal size of 100-125 m, which was diluted in 50 mg of TiO.sub.2 (100-125 m, Sigma-Aldrich, >99.9%) and held in place by a bed of quartz wool and heated from ambient temperature to 553 K (5 K min.sup.1) at 0.5 MPa under a He flow of 20 cm.sup.3.sub.STP min.sup.1. After 3 h at 553 K, the pressure was raised to 5 MPa in the same stream, which typically took 20 min. Then, the gas flow was switched to the reactant mixture (40 cm.sup.3.sub.STP min.sup.1) corresponding to a weight hourly space velocity (WHSV) of 48,000 h.sup.1, with a H.sub.2:CO.sub.2 ratio of 4. The effluent stream was sampled after 1 h on stream and then every 12 min and at least 7 measurements were averaged under each set of reaction conditions. 2.5 cm.sup.3.sub.STP min.sup.1 of a 20 mol % CH.sub.4 in He was used as an internal standard by injecting a constant flow after the reactor outlet. Tests were carried out up to 500 h time-on-stream). The results are provided in Table 2 and in FIG. 4.

(84) The presence of any noble metal produced an increase in productivity. However, the choice of said metal is important. The productivity is increased to a much more significant extent only when Pt or Pd are employed. Concerning Pd-promoted catalysts, only materials prepared by the CP technique retained their performance, while materials prepared by deposition or impregnation methods rapidly deactivated (see Table 2).

(85) TABLE-US-00001 TABLE 1 Characterization data of selected catalysts. Samples discussed in FIG. 1-5 are marked in bold. Nominal ICP-OES promoter N.sub.2 sorption promoter loading V.sub.pore S.sub.BET loading Support Promoter [wt. %] Synthesis [cm.sup.3 g.sup.1] [m.sup.2 g.sup.1] [wt. %] In.sub.2O.sub.3 0.37 125 In.sub.2O.sub.3 Pd 0.25 CP 0.40 147 0.31 In.sub.2O.sub.3 Pd 0.75 CP 0.51 174 0.74 In.sub.2O.sub.3 Pd 1.5 CP 0.56 149 1.45 In.sub.2O.sub.3 Pd 3.5 CP 0.53 158 3.36 In.sub.2O.sub.3 Pd 0.25 DI 0.36 127 0.25 In.sub.2O.sub.3 Pd 0.75 DI 0.35 131 0.73 In.sub.2O.sub.3 Pd 3.5 DI 0.26 113 3.43 In.sub.2O.sub.3 Pd 0.75 SD 0.40 131 0.63 ZrO.sub.2 0.45 110 ZrO.sub.2 Pd 0.75 DI 0.32 102 0.73 ZrO.sub.2 Pd 0.75 CP 0.50 195 0.69 TiO.sub.2 0.15 59 TiO.sub.2 Pd 0.75 DI 0.12 58 0.72 TiO.sub.2 Pd 0.75 CP 0.09 24 0.68 In.sub.2O.sub.3 Ag 0.75 CP 0.40 146 0.71 In.sub.2O.sub.3 Ru 0.75 CP 0.38 134 0.78 In.sub.2O.sub.3 Cu 0.75 CP 0.43 157 0.75 In.sub.2O.sub.3 Pt 0.75 CP 0.44 150 0.74

(86) TABLE-US-00002 TABLE 2 Catalytic performance in the direct hydrogenation of CO.sub.2 to methanol of selected catalysts. Samples discussed in FIG. 1-5 are marked in bold. Conditions applied in all tests: 553 K, 5 MPa, molar H.sub.2:CO.sub.2 = 4. ICP-OES Initial Promoter STY.sub.MeOH loading WHSV X.sub.CO2 S.sub.MeOH (and after 16 h) Support Promoter Synthesis [wt. %] [cm.sup.3.sub.STP g.sub.cat h.sup.1] [%] [%] [g.sub.MeOH g.sub.cat.sup.1 h.sup.1 In.sub.2O.sub.3 24,000 2.3 90 0.16 (0.15) In.sub.2O.sub.3 Pd CP 0.31 24,000 8.2 85 0.48 (0.47) In.sub.2O.sub.3 Pd CP 0.74 24,000 11.5 78 0.66 (0.61) In.sub.2O.sub.3 Pd CP 0.74 48,000 9.7 75 1.01 (1.00) In.sub.2O.sub.3 Pd CP 1.45 24,000 12.3 72 0.64 (0.61) In.sub.2O.sub.3 Pd CP 3.36 24,000 5.4 89 0.33 (0.29) In.sub.2O.sub.3 Pd DI 0.25 24,000 2.2 78 0.18 (0.15) In.sub.2O.sub.3 Pd DI 0.73 24,000 9.6 76 0.64 (0.43) In.sub.2O.sub.3 Pd DI 3.43 24,000 10.1 74 0.63 (0.33) In.sub.2O.sub.3 Pd SD 0.63 24,000 9.8 78 0.61 (0.39) ZrO.sub.2 24,000 0 0 (0) ZrO.sub.2 Pd DI 0.73 24,000 7.3 29 0.15 (0.13) ZrO.sub.2 Pd CP 0.69 24,000 2.1 10 0.02 (0.02) TiO.sub.2 24,000 0 0 (0) TiO.sub.2 Pd DI 0.72 24,000 7.2 7 0.01 (0) TiO.sub.2 Pd CP 0.68 24,000 0.9 5 0.01 (0.01) In.sub.2O.sub.3 Ag CP 0.71 24,000 3.3 82 0.18 (0.17) In.sub.2O.sub.3 Ru CP 0.78 24,000 4.0 84 0.23 (0.22) In.sub.2O.sub.3 Cu CP 0.75 24,000 3.1 32 0.18 (0.17) In.sub.2O.sub.3 Pt CP 0.74 24,000 4.4 89 0.27 (0.25)

Example 3Catalyst Differentiation

(87) Two samples of catalysts have been prepared, one by co-precipitation (CP) and the other one by deposition impregnation (DI). H.sub.2-TPD was performed at 5.0 MPa (FIG. 6) resulting in a temperature of around 313 K for the material prepared by DI versus 293 K for the material prepared by CP.