**6. Summary**

The latest development in sulfur-tolerant water gas shift catalysts and its application in low/lean steam/gas ratio technology (LSGRT) in industrial operation were comprehensively reported in this work. LGSRT possessed many improvements as compared with the traditional high steam/gas ratio technology. The LSGRT catalyst does not need extra steam adding in the WGS process; it just requires adding liquid water directly to the raw syngas stream to effectively control the hot spot and reduce the energy input. This can also help to control the WGS reaction depth, as well as the suppression of methanation side reactions.

Catalysts based on Mo–Co/alkali/Al2O3 composition were successfully developed and applied to the LSGRT water gas shift process, and a series of industrial samples (CATs) used under different conditions and lifetime stage were collected, laboratory tested and characterized with the techniques of XRD, SEM and Laser-Raman.

From the above works, many important findings were made: 1) The CO conversion of Mo–Co/ alkali/Al2O3 catalyst (CATs in the above discussions) gradually fell after 1–4 years time on stream under the conditions of LSGRT while the difference between medium temperature (350 ◦C) and high temperature (450 ◦C) catalytic performances became less as the catalyst was being used. 2) Caking, water-soaking and sintering led to negative effects on the catalytic performance of LSGRT catalysts, whereas sintering gives the most severe damage to catalyst in terms of changes in catalyst crystal structure and surface properties as reflected by the characterizations. 3) Cl and As could be poisonous for LSGRT catalysts and were supposed to disturb the catalyst sulfurization. On the other hand, water-soaking might also lead to a similar result. 4) New spectroscopy evidence was found in the characterizations of served and deactivated LSGRT SWGS catalysts which together with the above experimental results could be a good reference for researchers and industry.

## **7. Expanded Discussion**

Even though LSGRT SWGS catalysts have been successfully employed in many important WGS plants in the past years, there is still an urgen<sup>t</sup> demand for the continuous study and more tests, especially in the real field conditions.

One would argue how the 'accidentally deactivated' samples discussed in this work could be representative for researchers and field experts; besides, these deactivation approaches might be random and therefore, the results only teach less. To make sure our results are representative and referential, one would apply their WGS catalyst under a similar condition, i.e., reaction with a lower steam/gas ratio; the accidental deactivations described in this work have encompassed as many originally unexpected deactivations as the catalyst inventers (part of our authors) could record, since the LSGRT SWGS catalysts were extensively applied:1) LSGRT SWGS catalyst in one plant could be accidentally caked, water-soaked, sintered, Cl poisoned or As poisoned; however, ye<sup>t</sup> no other major reason was reported as newly found accidental deactivation. 2) One type of LSGRT SWGS catalyst accident could occur several times, or in di fferent plants; however, these changes are significant deactivations. Although the exact mechanism of these accidental deactivations is not completely clear, we have given as much reliable explanations (e.g., a general reason for caking could be the high-pressure hydrothermal condition) as the current work could support and we do appeal for more researchers seeking future explorations.

Another discussion could be over a long time period in an environment that is lacking in the normal control which one would use in a lab setting, to which extent we could assure that something besides the operating conditions did not a ffect the LSGRT SWGS catalytic performance. This question points to not only WGS catalysts but also all other industrially employed catalysts, e.g., zeolites, as the amplification of catalytic performance from a laboratory reaction to industrial plant application does bring in many uncertainties. However, the potential risks could be e ffectively prohibited and controlled in many ways, such as rational catalytic design, long-term laboratory experiments, pilot test, the real monitoring of plants and more importantly, a continuous study on employed catalysts to further improve its performance [33,34]. While the practical WGS reaction occurring in a plant is a heterogenous, multi-direction process with several side reactions, e.g., methanation, it is first and foremost a chemical phenomenon that can be e ffectively controlled with the help of plant-integrated monitoring systems [18,21]. The above accidental deactivations may have not been found by the previous WGS laboratory studies, but observed and recorded by the practical plant monitoring; in this work, their significance on real WGS performance was firstly reported with a preliminary evaluation on the causes, which is of grea<sup>t</sup> importance to prohibit future deactivations and helps to e ffectively control plant operating conditions.

**Author Contributions:** For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used "conceptualization, B.L. and Q.Z.; methodology, B.L., Q.Z. and T.X.; software, X.Z., J.Z. and F.W.; validation, L.Z. and Z.W.; formal analysis, B.L., L.Z., Z.W. and X.Z.; investigation, B.L., L.Z., Z.W., J.Z., Q.Z., and X.Z.; resources, J.Z. and Q.Z.; data curation, B.L., L.Z., Z.W., J.Z. and Q.Z.; writing—original draft preparation, B.L., T.X. and Q.Z.; writing—review and editing, B.L., Q.Z., Z.Z., J.G., and T.X.; visualization, B.L., X.Z. and F.W.; supervision, Q.Z., H.A., Z.Z., J.G. and T.X.; project administration, Q.Z. and J.G.; funding acquisition, B.L., Q.Z., H.A. and T.X.

**Funding:** The work was supported by the National natural science foundation of China youth program (NSFC code 21808241) and Shandong Provincial Key Research and Development Plan (code 2017CXGC1111).

**Acknowledgments:**We appreciate all the reviewers for their very useful and constructive comments. More appreciations are given to our group members at the China University of Petroleum (Beijing, China), and the oversea colleagues at the University of Oxford (Oxford, UK) and KACST (Riyadh, Kingdom of Saudi Arabia) for their contributions to the experiments and manuscript. The Industrial Engineering Laboratory of Sulfur Tolerant Water Gas Shift Catalyst subjected to China Petroleum and Chemical Industry Federation (CPCIF) is part of the Qingdao Lianxin Catalyst Company (China). The state Key Laboratory of Advanced Materials for Smart Sensing is a sub-department of General Research Institute for Nonferrous Metals (China). The industrial LSGRT SWGS catalysts were originally invented by Qingdao Lianxin Catalyst Company (China). Industrially served CATs and the as-prepared CATs were provided by the company. More importantly, they also kindly offered the practical plant running data (as the LSGRT SWGS catalyst supplier of those plants, Qingdao Lianxin Catalyst Company supervised and monitored the catalyst loadings as well as the practical plant runnings as part of their product services; data were collected for R&D purposes). (J. Zhang and Q. Zong) from Qingdao Lianxin Catalyst Company also took important roles in the experiments. People (F. Wei) from the General Research Institute for Nonferrous Metals (Beijing, China) helped in the SEM.

**Conflicts of Interest:** The authors have no commercial interest for the reported results.
